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The horizontal organization of stellate cell dendrites in layer IV of the visual cortex of tree shrews
Neuroscience Vol. 4. pp. 889 to 896 Pergamon Press Ltd 1979. Printed in Great
Britain
THE HORIZONTAL ORGANIZATION OF STELLATE DENDRITES IN LAYER IV OF THE VISUAL CORTEX OF TREE SHREWS
CELL
E. E. GEISWT, JR.’ and R. W. GUILLFBY~ Department of Anatomy and Neuroscience Program, University of Wisconsin, Madison, WI 53706, U.S.A. Abstract-Within layer IV of the visual cortex (area 17) of the tree shrew essentially all of the neurons are stellate cells. The majority (approximately !30%) are small and have spiny dendrites while the rest are larger with smooth dendrites. The dendritic arbors are all confined within the borders of layer IV and show a remarkable tendency, particularly marked for the small cells, to be flattened
in a plane parallel to the pia. The dendritic arbors can be regarded as discs. More than half of the arbors occupy less than one-sixth of the thickness of layer IV: about 90”/,occupy less than one-third of the thickness. The highly oriented arrangement of the dendritic arbors is related to the laminar organization of the geniculocortical terminals in layer IV. It appears that most of the stellate cells can receive their direct input from only one or two of the six geniculate layers. Therefore, interactions several retino-geniculo-cortical pathways must occur at intracortical levels.
GENICULO-CORTICAL pathways show two types of organization of their cortical terminals: a laminar and a columnar pattern. In the macaque monkey, the laminar pattern can be seen by comparing the cortical distribution of axons arising from the parvocellular and the magnocellular geniculate laminae. Each component ends within a distinct cortical layer or group of sub-layers (HUBEL & WIEZZL, 1972). Each component also shows a columnar organization; the terminals of geniculate neurons innervated by one eye alternate with the terminals of neurons innervated by the other eye (HUBEL & WJPSEL,1972). In contrast to this, in tree shrews, the laminar pattern appears to be the sole, or dominant type of organization of geniculocortical terminals, and the columnar organization (SKEEN,HUMPHREY,NORTON& HALL, 1978) is probably introduced at intracortical levels. There is no evidence that distinct ocular dominance columns are formed in the visual cortex of tree shrews (HARTING,DIAMOND& HALL, 1973;CASAGRANDE & HARTING,1975; HUBEL,1975; HUMPHREY,ALBANO& NORTON, 1977). Instead, each geniculate lamina or group of laminae is made up of cells whose axons have a distinct terminal zone within a sublamina of layer IV of the visual cortex. The discrete pattern of cortical lamination found in the tree shrew may thus represent an extreme variant of geniculocortical organization. It has not been studied beyond the termination of the geniculocortical axons. The arrangement of the cells that receive ’ Present address: University of Wisconsin, Department of Psychology, Madison, WI 53706, U.S.A. 2 Present address: University of Chicago, Department of Pharmacological and Physiological Sciences, 947 East 58th Street, Chicago, IL 60637, U.S.A. 889
between the
this highly laminated input is of obvious interest, for it is in the arrangement and connections of these cells
that one can expect to see morphological signs of interactions that may occur between the several apparently distinct laminar geniculocortical pathways of the tree shrew. This study was undertaken to reveal the morphological arrangement of the cells that lie within layer IV of the tree shrew visual cortex. EXPERIMENTAL
PROCEDURES
In the present study 14 tree shrews (Tupaia glis) were used. The brains of these animals were stained by a variety of Golgi methods: ten of the brains were stained by
B&e&erg’s modification of the Golgi-Kopsch method (BRA~~N~~RG, GUGLIELMO~TI & SNX, 1%7), three of the brains were stained by a modification of the Golgi-Cox method (RAM~N-MOLIN~R, 1970)and one brain was stained by the rapid Golgi method (VALVERDE,1970). The brains were embedded in celloidin and sectioned at 1OOpm. All of the brains were sectioned in the coronal plane, except one (EG 6). The right hemisphere of EG 6 was sectioned in the parasagittal plane and the left hemisphere was sectioned tangential to the pial surface of area 17. Cells were drawn routinely at 400x. For the purpose of illustration some cells were also drawn at 1009 x . From the neurons impregnated in layer IV a group of cells was selected and drawn for the purpose of determining the extent of their dendritic fields. All of the cells selected were taken from coronal sections in the region of the striate cortex that lies on the dorsal surface of the occipital lobe, where layer IV is approximately flat. This is the binocular portion of the cortex (see K44s, HALL, KILLACKEY & DLWOND,1972).All cells included in the sample had secondary dendrites that were not cut by the microtome. No other criteria of selection were used. All suitable cells from each section were drawn and the borders of layer IV were indicated on the drawings. The borders of layer IV were defined by a variety of
x90
E. F.
C~HSPRT. JR. and
K.
W. G,~II.IER\
(b)
AI
_.
I
_.
-.A
FIG. 1. Figure to show the measurements made after the celts had been drawn and the borders of layer IV marked. A is the distance from the l&IV border to the approximate center of the cell body, and B is the distance from the III-IV border to the IV-Y border. The ratio AjB k a measure of the relative depth of the cell body within-layer IV. C is the longest axis of the dendritic field and D is the width of the dendritic field perpendicular to C. The C/D ratio is a me%sure of the degree of elongation of the dendritic fieId of the &I. E is the total spread of the dendrites parpendicufar to the pial surface. The ratio E/B describes the relative vertical extent of layer IV which ic occupied by the dendrites of the cell. 8 is the an&? between the longest axis of the dendritic field and the borders of layer IV. The measurements ofe which indicate the direction of the de&it& field orientation have been assigned to one of four groups: +_15’ from the horizontal, +lY from the vertical,-15 45’ from the horjzonta~. and 45 75’ from the htirizontal.
methods. In many of our Golgi preparations the borders of layer IV could be seen quite clearly because the neuronal perikarya had been faintly stained and the region of densest ceil pack& which corresponds to layer IV, was apparent. All of the sections from the brains impregnated by the Goigi-Cox method were count~tai~~ according to the method de+aibed by RAMNJHOLMER (1970) -and some of the Go&i-Kopsch impregnated sections were also counte~tain~~(~E~RT & UPDYK+ 1973. However, even in the non-countmstained sections. the borders cif layer IV could be i&r&i&d using phase microscopy, since the dense cell packing of the neuronS within layer IV is &a&y seen in this way. Where we ha% checked the border determinations made on the non-counterstained sections by sub-
sequent counterstaining, we have contjrmed the ear&r decisions precisely. Since many of the layer IV stellate cetis are flattened in the plane aI the lam&a, we made a u&m&r of measufe$ that would represent the shape and the arientatian~of the dendritic arbors and related this trr the position of the c&k. Figur;e 1 shows the way iii which the &s. were anaIyzed. Computation of the relative depth of the celi body within lay&r IV (ratio A/B in Fig. la) depended upoh a decision-r&r&g the center of the p&&arya. %I.+ the perikarya Fe.small relative to the depth of layer IV and siuce~~most ti~fdore or less reufided. thiS measuie was reasonably a__ccurate.The measux.C (Fig- lb) was taken as the greatest diameter of the dendritic field as seen in
Stellate cells in area 17 of tree shrews
891
FIG. 2. A composite drawing of neurons from layer IV of the tree shrew striate cortex. Four of the cells are type A neurons (A) and one is a type B neuron (B). This is one of the more elongated type B neurons having an elongation factor of 2.6. The solid lines represent the borders of layer IV. a two-dimensional projection of the cell, and D was measured at right angles to this. The measures E and E/B are explained in the legend to Fig. 1. While there are possi-
bilities for errors in the determination of the measures, these errors are small in relation to the measuies obtained. The measure are used to illustrate the pattern of dendritic arborization that tends to characterize layer IV and, since they are not presented as absolute measures, the errors that may have been introduced into the measurement will not be considered further. We regarded any cell with an elongation factor (C/D) greater than 1.5 as being elongated, and only cells that met this criterion of elongation were classified as to their orientation (see Fig. 1).
RESULTS
The cytoarchitectonic structure of the tree shrew’s striate cortex (area 17) has been described by DuMOND,SNYDER, KILLACKEY, JANE & HALL (1970), by KAASet al. (1972) and by HARTING et al. (1973). The striate cortex is a well-defined area, with a prominent layer IV which is composed of tightly packed small neurons. This dense layer IV is easily recognizable in Nissl preparations and it stops abruptly at the border of area 17. The striate cortex can be divided into a binocular and a monocular portion (Kales et al., 1972), with the binocular portion extending over the dorsal surface and onto the medial wall, and the monocular portion lying on the ventral aspect of the tree shrew’s occipital lobe. Types of neuron in layer IV
Almost all of the neurons within layer IV are stel-
late cells. In Nissl preparations, layer IV consists of closely packed, small, rounded cell bodies. Of the hundreds of neurons seen in our Golgi preparations only one was a pyramidal cell. The dendrites in layer IV belong mainly to the stellate cells that chafacterize this layer and these dendrites show a predominantly horizontal arrangement, which will be considered in detail below. Dendrites of other neurons also pass into layer IV. The apical dendrites of the layer V and VI pyramidal cells traverse the layer. These dendrites are spinous as they pass vertically through layer IV. Large stellate cells within layer V also send dendrites up into layer IV. In addition, immediately above layer IV there are some relatively large flattened stellate cells. These cells have horizontally arranged dendrites that lie just above layer IV, and occasionally a portion of one of these dendrites enters the dorsal part of layer IV. Some of the pyramidal cells in layer 111 also have a basal dendrite that enters layer IV, but there are relatively few of these. Within layer IV two types of stellate cell are recognizable. The type A cells (see Fig. 2) have relatively small perikarya, and they have three to four primary dendrites which radiate from the cell body giving off branches that extend away from the cell body. Type B cells have larger perikarya and have five or six primary dendrites. These dendrites are smooth and curve around the cell in graceful arches (see Fig. 2). In general, dendrites of type A cells are well covered by spines, and where we have seen a smooth type A dendrite we could not exclude the possibility that this was due to poor staining.
892
E. E.
GEISERT,
JR. and K. W. GLIILLERY
Distribution of‘ dendrites of stellate ceils
Dendritic field @’ individucd steik~
All of the stellate cells in layer IV seen in the present material had dendrites that were confined within the borders of layer IV. The degree to which the dendrites of layer IV cells stay within the boundaries of layer IV is most strikingly demonstrated by the impregnated neurons in the monocular portion of the striate cortex, where layer IV is especially thin, In this region all of the stellate ceil dendrites lie within layer IV, and the dendritic fields of these cells are remarkably flattened. Although the monocular segment shows this relationship most strikingly, one finds that stellate cells are flattened in ail parts of layer IV of the striate cortex, the binocular as well as the monocular parts. Frontal or sagittal sections show slender elongated dendritic arborizations parallel to the piai surface. while sections cut parallel to the pial surface show dendrites that radiate away from the ceil body. forming a roughly circular arborization. That is. the stellate cells have disc-shaped dendritic arborizations. and the discs are piled closely one above the other. lying in the plane of layer IV. In order to define the distribution of the dendrites in a quantitative manner. and in order to relate it to the arrangement of the geniculocortical a&rents, a number of measurements have been made. A total of 186 ceils, selected according to the criteria given in Experimental Procedures, have been classified and measured. Of these ceils only 15 were the large type B cells, the rest being type A cells. Since in a Nissl section one also finds that the majority of the cells in layer IV are small and that only a few scattered cells have the large perikarya typical of type B cells, it is probable that the Golgi method was not strongly selective as regards the impregnation of either the A cells or the B cells. Of the neurons of layer IV that were sampled, the majority (89Q had elongated dendritic fields. (C/D 2 1.5, see Fig. 1). and more of the A cells than of the B ceils showed the elongation. Thus. 158 of the 171 type A ceils (92”a) had elongation factors of 1.5 or greater. while only 7 of the 15 type B cells showed this elongation. When one considers higher elongation factors, one finds that 145 A ceils (85:; of the A ceils) and 3 of the B cells had an elongation factor of 2.0 or greater, while as many as 111 of the A cells (657,“) had an elongation factor of 3.0 or greater. That is, the majority of the dendritic fields of type A steilate ceils are discs with a diameter more than three times their height.. When one looks at the orientation of these discs, it is evident, as indicated above, that most are oriented ‘horizontally’, that is, paraiiei to the pia. The majority of the ceils having an elongation factor greater than 1.5 were oriented within 15” of the horizontal plane (141 out of 165 or 85%). Of the 111 cells that had an elongation factor of 3.0 or greater, 103 cells (93?J were oriented within 15’ of the horizontal plane.
Our nexl problem was &oconsider the approximate fraction of the total thickness of layer TV that could be reached by the deudrites of one steiiate ceil and. thus. to relate the morphology ‘if rhe stell&- cells to their highly l~inated input ti‘each genicuiate iamina sends axon terminals to ;t distinct sub-lam& of layer IV (and this possibility 1%cctnsidercd in morr detail in the Discussion) then, cince the six pniculate laminae appear to share the avaii~~~le layer IV reerritory on 3 more or less equal basis ~HARTIW L':d. 1973). the terminals from each gzniculate layer will occupy approximately one-sixth QE the thickness of layer IV. We will argue later that the fraction for some layers may be greater. huf 11 is unlikeiy io he very much less. We calculated the fraction I.)!’iayer IV that was occupied by the dendrites of each &I in the sample, and determined how many of fhc dendritic tiborizations occupied one-sixth or less ~1’the :.ertical thickness of layer IV (see Fig. 1). Of the-B cells, 4 (approx. 7_5”;,)had dendritic arbors occupying one-sixth OFless of the thickness of layer IV. and 10 (669~) had arbors occupying less than one-third of the thickness of layer IV. The type A ceils showed a much more marked limitation to thin portions of layer IV. A Mai uf 02 of the A ceils (541,) occupied one-sixth or less of the thickness of layer IV, and t5: of &e A cells 19?‘,,i had dendrites that spread into one-third or less -of the total tarnina IV thickness. Thatai is. there are 2 great many steliate cells in layer IV of the striate cortex whose input is likely to come predominantly l’rom one or two genicuiale laminae. Positims
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of i~d~~id~~istetl&e c&h m Buyer- TV
The final step in the analysis was to determine-hou the shape (the elongation factor& the vertical extent of the dendritic field, and its orientation were related to the position of the ceil body within layer IV; it is necessary to point out that the.cell body does net necessarily lie close to the center of the dendritic field. although it is commtm for it to be fairly close. Figure 3 shows the relationship bdwee:en.the &ongation factor and the position of the prikaryon throughout the depth of layer IV. i”eiis with an eiongation factor greater than 1.5 arc shown as lines. The orientation of these lines indictits the orientation of the dendritic arbors. The marked iendency for a hortzontai orientation of dendrites iq. evident from this figure, and it is noticeable that the most marked evidence for a horizontal orientation is seen at the edges of layer IV, as though a major limitation upon the shape of the dendritic arbors was one which tends to keep dendrites confined within layer IV. Thus. the cells with the highest eion@on factors a~ aii horizontally oriented and tend TVlie at the edges of layer IV; cells with a non-horizontal orientation tend to be away from the edges of layer IV. It is worth pointing out that some of these: cells in the middle of jayer IV span much of th& total thickness of’ the
893
Stellate cells in area 17 of tree. shrews lo-
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FIG.3. A plot of the elongation factor (C/D) vs the relative depth (A/R) of the cells in layer IV. The dots represent cells which were not considered elongated (C/D < 1.5). The remaining cells with elongated dendritic fields are represented by bars. The angle of the bar reflects the orientation of the dendritic field (e). For abbreviations see Fig. 1.
layer. These are a minority, but may play an important role in interrelating the several geniculocortical inputs to layer IV. Figure 4 shows the proportion of the total thickness of layer IV occupied by each dendritic arbor and plots this against the relative position within layer IV. There is some tendency in this figure for the broadest cells to be away from the edges of layer IV, and for the B cells (shown as boxes) also to stay away from the edges. The marked tendency for most of the cells to occupy only a small fraction of the total layer IV thickness is shown strikingly in this figure. DISCUSSION Laminar arrangement of ajerents izations
and dendritic arbor-
In the tree shrew, the thalamocortical afferents to the visual cortex show a highly laminated arrangement within layer IV and this organization is recapitulated in the laminar arrangement of dendritic aborixations of layer IV cells. The functional significance of this apparently parallel 1amina.r arrangement of a&rents and cortical receptive arbors is not altogether clear. The individual geniculate laminae represent synaptic relays that are
probably distinctive in terms of function and that are certainly distinctive in terms of their input from either the left or the right eye. Thus, in the monkey and the cat there is strong evidence in favor of the view that different geniculate laminae represent relays for functionally distinct but separate pathways from the eyes to the cortex (e.g. Wnso~, ROWE& STONE,1976; DREHER, FUKADA& RODDXK,1976; SHERMAN, WILSON,KAAS& WEBB,1976; and SCHILLER& MALPELI, 1978). Corresponding evidence for such a separation of functions in the geniculate laminae of the tree shrew is not available (see SHERMAN, NORTON& CASAGRANDE, 1975; and CASAGRANTJE, GUILLERY &
HART~NG,1978) but this is probably because the evidence is difficult to obtain from the very narrow geniculate layers of the tree shrew, not because there is no such separation of functions. In contrast to this, the evidence is very strong that in the tree shrew two geniculate layers (1 and 5) receive input from the ipsilateral eye, while the other layers (2, 3, 4 and 6) receive input from the contralateral eye (CASAGRANDE & HARTING,1975; CASAGRANDE et al., 1978). Afferents to individual stellate cells. The majority
of the stellate cells in layer IV appear to receive their primary direct geniculate inputs from one, or perhaps two geniculate laminae. The evidence from degeneration studies is strong that axons from geniculate
E. E. GEISERT.
and R. W.
JR.
GI:II.LL:R~
1
l (1
b
b
l
.
plot of the vertical extent of layer IV dendritic fields over the total thickness of layer IV (E/B) against the relative depth in layer IV (A/B). The dots represent type A cells and the open
FIG. 4. A
boxes are the type B dells. For abbreviations see Fig. 1.
laminae 1 and 2 terminate in distinct sublaminae of cortical layer IV, lamina 1 having terminations closer to the III/IV border than lamina 2 (HARTINGer al.. 1973). That is, stellate cells confined to the most dorsal parts of layer IV receive inputs from the ipsilateral eye through geniculate larrina 1. The evidence from transneuronal transport of tritiated amino acids suggests an overlap of terminals from geniculate laminae 1 and 2 (CASAGRANDE& HARTING, 1975; HUBEL, 1975), but since this method may produce an artifactual appearance of overlap through imerlaminar leakage at the geniculate level or by leakage between cortical sublaminae, it is more reasonable to take the evidence from degeneration studies at its face value. Degeneration and transneuronal transport studies both suggest that there is a significant intermediate zone of cortical layer IV receiving a crossed ihput and no uncrossed a&rents. The evidence is less clear regarding terminations in the deeper parts of cortical layer IV, and there may be some binocular overlap in this region. Comparison of morphological and electrophysiologicai studies. On the basis a$ the primary geniculocorti-
cal connections, one would expect to ftnd a ma&&y of stellate cells in layer IV which are monocularly driven. HUMPIWEYer al. (1977)have recently shown
that some of tire neurons within layer IY of the tree shrew striate cortex are driven only by the contralateral eye. However, the majority of the celis in the striate cortex of the tree-shrew are~binocularly driven, including the neurons of layer IV. These electrophysiological observations can be reconciled with the morphological relationships in one of twoways. One can argue that since the stellate~celif of layer IV are extremely small, the microelectrodes may have tended to select the B cells over the- A &Is, and since the B cells generally have dendrites that spread further in a vertical direction, they probably do receive prey dominantly binocular inputs. Alternatively, the binocular responses recorded from the layer IV @late cells may be produced by intracortical connections. Intracortical connectivity
The first stage of the visual pathway within area 17 of the tree shrew thus involves a_ rather distinct cortical su&&ar ~arrangement. A similar, but clearer, horizor@I, or laminar connectivity -pattern has been demonstrated in old world rdonkeys, wfrere axons fiorn~ th& parvocellular and magnocellular genicoaical layers culate lam&ae terminate in (Huae~ Bt Wm 1972; 1977) and where the cortical cells in receipt of these separate inputs also appear
Stellate cells in area 17 of tree shrews
to be distinguishable in terms of their intracortical connections (LUND, 1973; LUND & BOOTIU3,1975). In the monkey, however, a system of ocular dominance columns is also recognizable within each of the cortical layers, so that the dual organization of cortical connectivity, columnar and laminar, is established already by the geniculocortical inputs. In the tree shrew, in contrast, the geniculocortical inputs are entirely or predominantly laminar. The columnar organization in the tree shrew, which has recently been demonstrated by SKEENet al. (1978) for orientation selectivity, is probably established entirely by intracortical mechanisms. In the Golgi-stained material available to us it was not possible to follow the intracortical connectivity patterns to a significant extent. Relatively few axons were stained, and most of those that were impregnated could not be followed to their terminal arbors before they left the plane of section. In addition, the sublaminar arrangement of layer IV is not distinguishable on the basis of cytoarchitectonic criteria, as it is in the monkey. The relatively homogeneous appearance of layer IV in the tree shrew suggests that intracortical pathways may be kept less clearly distinct from each other in the tree shrew than they are in the monkey. We have noted two tendencies in the dendritic organization of layer IV stellate cells. One is for the dendrites of all cells to stay within the borders of layer IV and the other is for the flattened dendritic arbors of most of the cells to be limited to a relatively small fraction of layer IV. These probably represent two separate developmental factors, since the flattening in all parts of layer IV is greater than that needed to limit the arbors to layer IV itself, and since there is some indication that the flattening is especially marked at the borders of layer IV. The flattening would appear to limit many cells to a region innervated by only one geniculate lamina. However, while there is also a significant overlap of dendrites between the cortical sublaminae there is essentially no overlap at the upper and lower borders of layer IV.
hinated
arrangement of a&rents
895 in other regions
Finally, it is of some interest to compare the laminar arrangement of inputs to visual cortex with laminar arrangements seen in other structures, such as the cornu ammonis or the fascia dentata (BLACKSMD, 1956; RAISMAN,COWAN & POWELL, 1965; HJORTH-SIMONSEN & JEUNE, 1972). In these other structures a highly ordered, laminated set of inputs establishes contacts upon different segments of the dendritic arbors of one relatively uniform set of postsynaptic elements. There may be local specializations corresponding to particular layers of the input (e.g. HAMLYN,1963), but the overall scheme that emerges is of many inputs converging upon one cell type. The intracortical integration here must be studied to a large extent in terms of the properties of the single post-synaptic cell receiving the multiple inputs. In contrast to this, the several separate inputs that reach the visual cortex from the lateral geniculate nucleus impinge upon separate post-synaptic stellate elements, and the cortical integration is most reasonably thought of in terms of circuits originating from these stellate cells. It is possible that in the visual cortex of the tree shrew there are pyramidal cells whose apical dendrites, as they pass through the sublayers of lamina IV, receive some inputs from dl the sublayers, but the evidence so far available for other species suggests that this type of connection, if it occurs, is not dominant (GAREY& POWELL,1971; PETERS & FELDMAN, 1976). If there are cells with multiple, distinct, laminated inputs in the visual cortex, comparable to the hippocampal pyramids, then these probably receive input primarily from intrinsic interneurons, not from extrinsic afferents as in the hippocampal formation. Acknowledgements-We thank ELAINELANGERfor assisting in the preparation of the histological material and illustrations and CHARLOTTE fiSUK for typing the manuscript. This study was supported by Grants numbers NS 06662, NS 14283 and EY 00962 from the United States Public Health Service.
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V. A., GUILLERY R. W. & HARTINGJ. K. (1978) Differential effects of monocular deprivation seen in different layers of the lateral geniculate nucleus. J. camp. Neural. 179, 469-486. CASAGRANDE V. A. & HARTINGJ. K. (1975) Transneuronal transport of tritiated fucose and proline in the visual pathways of tree shrew (Tupaia glis). Brain Res. 96, 367-372. DIAMOND I. T., SNYDERM., KILLACKEY H., JANEJ. & HALLW. D. (1970) Thaiamo~rtical projections in the tree shrew (Tupaia glis). J. cony. Neural. 139, 273-306. CASAGRANDE
DREHERB., FUKADAY. Bt RODIECK R. W. (1976) Identification, classification, and anatomical segregation of cells with X-like and Y-like properties in the lateral geniculate nucleus of old world primates. J. Physiol., Land. Us, 433-452. GAREYL. J. & POWELL T. P. S. (1971) An experimental study of the termination of the lateral geniculocortical pathway in the cat and monkey. Proc. R. Sot. B 179,41-63. GEI~~RTE. E. 8c UPDYKEB. V. (1977) Chemical stabilization of Golgi silver chromate impregnations. Stain Technol. 52, 137-141.
HAMLYNL. H. (1963) An electron microscopic study of pyramidal ncuronh in Amman’s horn t,i the rabbit. .! .4nrrr. 97, 189~201, HARTINGJ. K.. DIAMONDI. ‘f. & HAI.L W. (‘. t 197.1)Anterogradc degeneration study of the ~ortlcal projections of the lateral geniculate and pulvinar nuclei in the irre \hraw I TI~~U;C~ q/r,). 6. LfV”/,. Mcurd. 150, iY?--439. HJORTH-SIMONSEN A. &,JF.I!NI 8. 119721Origin and terminatir)n ,>f the hippocampal perforant p:& in the rat studied by silver impregnation. J. camp. Neurd. 144. 215 232. HUBELD. H. (19751 An autoradiographic study of the regime-cortical protrctiom In Iltr iret \hrr\\ {TUP~~~Uglr.~). ~r~i,, Re.\. 96, 41-50. HUBELD. H. & WIESF.LT. W. f 19721 Laminar ,rnd columnar distrlbutlon uf geniculo-cortic,d lihers in the macaque monkey. J. camp. Neurd 146, 421 -450 HUBEL D. H. & WIESEI.T. N. 119771Functional architecture of macaque monkey visual cortex. b’rf7c R. S,lc. tl 1%. 1 59. HUMPHREYA. I... ALBANOJ. E. & NORTONT‘. 7. (lY77i Organbation ol’ ocular dominance ii1 tree \hrcw striate cortex. Bruin Res. 134, 225 236 KAAS J. H., HAI.L W. C.. KII.U(.I(FY H. & DIAMO~I)I. T. (19721Visual cortex of the tree shrew (?icpic~g/i,<):Plrchitectonrc subdivisions and representalion of the visual ticld. Hrclin RCY. 42, 491 4%. LAEMLEL. K. f 1968) Retinal prqjections of 7Lplricl q/is. Brtrirl R&L.. Er~l. 1. 473. 499. LUND J. S. (1973) Organization of neurons m the visual cortex. area 17 of the monkey ~Mu~~;c~,tmulurtn). J. c,,mp. Neural. 147, 455-495. LUND J. S. & BOOTHER. G. (1975) Interlaminar connections and pyramidal neuron organization in the visual cortex. area 17, of the macaque monkey. J. crmp. ~iewd. 159. 30$334. PETERSA. & FELDMANM. L. (1976) The projection of the lateral geniculate nucleus to area 17 ot the cerebral cortex- ~1. General description. J. Nt,uroc~~rol.5, 63 84 RAISMANG.. COWAN W. M. & POWELI T. I’ S (1965) The extrinsic an’erents, commissural and assocration fibers of the hippocampus. Bruit1 88, 963 -996. RAM~N-MOLINER E. (1970) The Golgi Cox technique. In t’ontcwponrn Rr.wwt~l~ hferkods its Mennmn~iorn~ (eds NAUTA W. J. H. & EBBESSON S. 0. E.). pp. 32 55. Springer. New York. SCHILLERP. H. & MALPFI.IJ. G. 11978) Functional specificity of lateral geniculate nucleus iaminae of the Rhesus monkey. J. Nruroph!sicjl. 41, 7% 797. SHERMANS. M., NORTONT. ‘T. & CASAGKANDF V. A. (1975) X- and Y-cells in the dorsal laterni geniculate nucleus of the tree shrew (Tupuicr yli.5). Bruin Rtx 93, 152.~157. SWERMAN S. M., WILSONJ. R.. KAASJ. H. & WEBHS. V. (1976) X- and Y-cells in the dorsal later-d geniculate nucleus of the owl monkey (.4otrrs rritirgufus). Scierlct~,*V.I. 192, 475.-477. SKEEN L. C.. HUMPHREY A. I_.. NOIUO~ T. T. & HALL W. C. (1978) Deoxygiucose mapping of the orientation eoiumn system in the striate cortex of the tree shrew (Tuirpcricc y/is). Bruin Rus. 142, 538-545. VALVERDE F. (1970) The Golgi method: A tool for comparative structural analysis. In Cornzrmpr~rc~+RuseurcLh M&hods in Neuroanatom~ (eds NAUTA W. J. H. 8 EB~ESS~~S. 0. E.). pp. 12-31. Springer. New York WILSONP. D.. ROWEM. H. & STONE J. (1976) Properties of relay cells in cat’s Lateral geaiculate tmckus: A comparison of W-cells with X- and Y-cells. J. Nr~rrc>plr!.vio/.39. 1193m1109.
Britain
THE HORIZONTAL ORGANIZATION OF STELLATE DENDRITES IN LAYER IV OF THE VISUAL CORTEX OF TREE SHREWS
CELL
E. E. GEISWT, JR.’ and R. W. GUILLFBY~ Department of Anatomy and Neuroscience Program, University of Wisconsin, Madison, WI 53706, U.S.A. Abstract-Within layer IV of the visual cortex (area 17) of the tree shrew essentially all of the neurons are stellate cells. The majority (approximately !30%) are small and have spiny dendrites while the rest are larger with smooth dendrites. The dendritic arbors are all confined within the borders of layer IV and show a remarkable tendency, particularly marked for the small cells, to be flattened
in a plane parallel to the pia. The dendritic arbors can be regarded as discs. More than half of the arbors occupy less than one-sixth of the thickness of layer IV: about 90”/,occupy less than one-third of the thickness. The highly oriented arrangement of the dendritic arbors is related to the laminar organization of the geniculocortical terminals in layer IV. It appears that most of the stellate cells can receive their direct input from only one or two of the six geniculate layers. Therefore, interactions several retino-geniculo-cortical pathways must occur at intracortical levels.
GENICULO-CORTICAL pathways show two types of organization of their cortical terminals: a laminar and a columnar pattern. In the macaque monkey, the laminar pattern can be seen by comparing the cortical distribution of axons arising from the parvocellular and the magnocellular geniculate laminae. Each component ends within a distinct cortical layer or group of sub-layers (HUBEL & WIEZZL, 1972). Each component also shows a columnar organization; the terminals of geniculate neurons innervated by one eye alternate with the terminals of neurons innervated by the other eye (HUBEL & WJPSEL,1972). In contrast to this, in tree shrews, the laminar pattern appears to be the sole, or dominant type of organization of geniculocortical terminals, and the columnar organization (SKEEN,HUMPHREY,NORTON& HALL, 1978) is probably introduced at intracortical levels. There is no evidence that distinct ocular dominance columns are formed in the visual cortex of tree shrews (HARTING,DIAMOND& HALL, 1973;CASAGRANDE & HARTING,1975; HUBEL,1975; HUMPHREY,ALBANO& NORTON, 1977). Instead, each geniculate lamina or group of laminae is made up of cells whose axons have a distinct terminal zone within a sublamina of layer IV of the visual cortex. The discrete pattern of cortical lamination found in the tree shrew may thus represent an extreme variant of geniculocortical organization. It has not been studied beyond the termination of the geniculocortical axons. The arrangement of the cells that receive ’ Present address: University of Wisconsin, Department of Psychology, Madison, WI 53706, U.S.A. 2 Present address: University of Chicago, Department of Pharmacological and Physiological Sciences, 947 East 58th Street, Chicago, IL 60637, U.S.A. 889
between the
this highly laminated input is of obvious interest, for it is in the arrangement and connections of these cells
that one can expect to see morphological signs of interactions that may occur between the several apparently distinct laminar geniculocortical pathways of the tree shrew. This study was undertaken to reveal the morphological arrangement of the cells that lie within layer IV of the tree shrew visual cortex. EXPERIMENTAL
PROCEDURES
In the present study 14 tree shrews (Tupaia glis) were used. The brains of these animals were stained by a variety of Golgi methods: ten of the brains were stained by
B&e&erg’s modification of the Golgi-Kopsch method (BRA~~N~~RG, GUGLIELMO~TI & SNX, 1%7), three of the brains were stained by a modification of the Golgi-Cox method (RAM~N-MOLIN~R, 1970)and one brain was stained by the rapid Golgi method (VALVERDE,1970). The brains were embedded in celloidin and sectioned at 1OOpm. All of the brains were sectioned in the coronal plane, except one (EG 6). The right hemisphere of EG 6 was sectioned in the parasagittal plane and the left hemisphere was sectioned tangential to the pial surface of area 17. Cells were drawn routinely at 400x. For the purpose of illustration some cells were also drawn at 1009 x . From the neurons impregnated in layer IV a group of cells was selected and drawn for the purpose of determining the extent of their dendritic fields. All of the cells selected were taken from coronal sections in the region of the striate cortex that lies on the dorsal surface of the occipital lobe, where layer IV is approximately flat. This is the binocular portion of the cortex (see K44s, HALL, KILLACKEY & DLWOND,1972).All cells included in the sample had secondary dendrites that were not cut by the microtome. No other criteria of selection were used. All suitable cells from each section were drawn and the borders of layer IV were indicated on the drawings. The borders of layer IV were defined by a variety of
x90
E. F.
C~HSPRT. JR. and
K.
W. G,~II.IER\
(b)
AI
_.
I
_.
-.A
FIG. 1. Figure to show the measurements made after the celts had been drawn and the borders of layer IV marked. A is the distance from the l&IV border to the approximate center of the cell body, and B is the distance from the III-IV border to the IV-Y border. The ratio AjB k a measure of the relative depth of the cell body within-layer IV. C is the longest axis of the dendritic field and D is the width of the dendritic field perpendicular to C. The C/D ratio is a me%sure of the degree of elongation of the dendritic fieId of the &I. E is the total spread of the dendrites parpendicufar to the pial surface. The ratio E/B describes the relative vertical extent of layer IV which ic occupied by the dendrites of the cell. 8 is the an&? between the longest axis of the dendritic field and the borders of layer IV. The measurements ofe which indicate the direction of the de&it& field orientation have been assigned to one of four groups: +_15’ from the horizontal, +lY from the vertical,-15 45’ from the horjzonta~. and 45 75’ from the htirizontal.
methods. In many of our Golgi preparations the borders of layer IV could be seen quite clearly because the neuronal perikarya had been faintly stained and the region of densest ceil pack& which corresponds to layer IV, was apparent. All of the sections from the brains impregnated by the Goigi-Cox method were count~tai~~ according to the method de+aibed by RAMNJHOLMER (1970) -and some of the Go&i-Kopsch impregnated sections were also counte~tain~~(~E~RT & UPDYK+ 1973. However, even in the non-countmstained sections. the borders cif layer IV could be i&r&i&d using phase microscopy, since the dense cell packing of the neuronS within layer IV is &a&y seen in this way. Where we ha% checked the border determinations made on the non-counterstained sections by sub-
sequent counterstaining, we have contjrmed the ear&r decisions precisely. Since many of the layer IV stellate cetis are flattened in the plane aI the lam&a, we made a u&m&r of measufe$ that would represent the shape and the arientatian~of the dendritic arbors and related this trr the position of the c&k. Figur;e 1 shows the way iii which the &s. were anaIyzed. Computation of the relative depth of the celi body within lay&r IV (ratio A/B in Fig. la) depended upoh a decision-r&r&g the center of the p&&arya. %I.+ the perikarya Fe.small relative to the depth of layer IV and siuce~~most ti~fdore or less reufided. thiS measuie was reasonably a__ccurate.The measux.C (Fig- lb) was taken as the greatest diameter of the dendritic field as seen in
Stellate cells in area 17 of tree shrews
891
FIG. 2. A composite drawing of neurons from layer IV of the tree shrew striate cortex. Four of the cells are type A neurons (A) and one is a type B neuron (B). This is one of the more elongated type B neurons having an elongation factor of 2.6. The solid lines represent the borders of layer IV. a two-dimensional projection of the cell, and D was measured at right angles to this. The measures E and E/B are explained in the legend to Fig. 1. While there are possi-
bilities for errors in the determination of the measures, these errors are small in relation to the measuies obtained. The measure are used to illustrate the pattern of dendritic arborization that tends to characterize layer IV and, since they are not presented as absolute measures, the errors that may have been introduced into the measurement will not be considered further. We regarded any cell with an elongation factor (C/D) greater than 1.5 as being elongated, and only cells that met this criterion of elongation were classified as to their orientation (see Fig. 1).
RESULTS
The cytoarchitectonic structure of the tree shrew’s striate cortex (area 17) has been described by DuMOND,SNYDER, KILLACKEY, JANE & HALL (1970), by KAASet al. (1972) and by HARTING et al. (1973). The striate cortex is a well-defined area, with a prominent layer IV which is composed of tightly packed small neurons. This dense layer IV is easily recognizable in Nissl preparations and it stops abruptly at the border of area 17. The striate cortex can be divided into a binocular and a monocular portion (Kales et al., 1972), with the binocular portion extending over the dorsal surface and onto the medial wall, and the monocular portion lying on the ventral aspect of the tree shrew’s occipital lobe. Types of neuron in layer IV
Almost all of the neurons within layer IV are stel-
late cells. In Nissl preparations, layer IV consists of closely packed, small, rounded cell bodies. Of the hundreds of neurons seen in our Golgi preparations only one was a pyramidal cell. The dendrites in layer IV belong mainly to the stellate cells that chafacterize this layer and these dendrites show a predominantly horizontal arrangement, which will be considered in detail below. Dendrites of other neurons also pass into layer IV. The apical dendrites of the layer V and VI pyramidal cells traverse the layer. These dendrites are spinous as they pass vertically through layer IV. Large stellate cells within layer V also send dendrites up into layer IV. In addition, immediately above layer IV there are some relatively large flattened stellate cells. These cells have horizontally arranged dendrites that lie just above layer IV, and occasionally a portion of one of these dendrites enters the dorsal part of layer IV. Some of the pyramidal cells in layer 111 also have a basal dendrite that enters layer IV, but there are relatively few of these. Within layer IV two types of stellate cell are recognizable. The type A cells (see Fig. 2) have relatively small perikarya, and they have three to four primary dendrites which radiate from the cell body giving off branches that extend away from the cell body. Type B cells have larger perikarya and have five or six primary dendrites. These dendrites are smooth and curve around the cell in graceful arches (see Fig. 2). In general, dendrites of type A cells are well covered by spines, and where we have seen a smooth type A dendrite we could not exclude the possibility that this was due to poor staining.
892
E. E.
GEISERT,
JR. and K. W. GLIILLERY
Distribution of‘ dendrites of stellate ceils
Dendritic field @’ individucd steik~
All of the stellate cells in layer IV seen in the present material had dendrites that were confined within the borders of layer IV. The degree to which the dendrites of layer IV cells stay within the boundaries of layer IV is most strikingly demonstrated by the impregnated neurons in the monocular portion of the striate cortex, where layer IV is especially thin, In this region all of the stellate ceil dendrites lie within layer IV, and the dendritic fields of these cells are remarkably flattened. Although the monocular segment shows this relationship most strikingly, one finds that stellate cells are flattened in ail parts of layer IV of the striate cortex, the binocular as well as the monocular parts. Frontal or sagittal sections show slender elongated dendritic arborizations parallel to the piai surface. while sections cut parallel to the pial surface show dendrites that radiate away from the ceil body. forming a roughly circular arborization. That is. the stellate cells have disc-shaped dendritic arborizations. and the discs are piled closely one above the other. lying in the plane of layer IV. In order to define the distribution of the dendrites in a quantitative manner. and in order to relate it to the arrangement of the geniculocortical a&rents, a number of measurements have been made. A total of 186 ceils, selected according to the criteria given in Experimental Procedures, have been classified and measured. Of these ceils only 15 were the large type B cells, the rest being type A cells. Since in a Nissl section one also finds that the majority of the cells in layer IV are small and that only a few scattered cells have the large perikarya typical of type B cells, it is probable that the Golgi method was not strongly selective as regards the impregnation of either the A cells or the B cells. Of the neurons of layer IV that were sampled, the majority (89Q had elongated dendritic fields. (C/D 2 1.5, see Fig. 1). and more of the A cells than of the B ceils showed the elongation. Thus. 158 of the 171 type A ceils (92”a) had elongation factors of 1.5 or greater. while only 7 of the 15 type B cells showed this elongation. When one considers higher elongation factors, one finds that 145 A ceils (85:; of the A ceils) and 3 of the B cells had an elongation factor of 2.0 or greater, while as many as 111 of the A cells (657,“) had an elongation factor of 3.0 or greater. That is, the majority of the dendritic fields of type A steilate ceils are discs with a diameter more than three times their height.. When one looks at the orientation of these discs, it is evident, as indicated above, that most are oriented ‘horizontally’, that is, paraiiei to the pia. The majority of the ceils having an elongation factor greater than 1.5 were oriented within 15” of the horizontal plane (141 out of 165 or 85%). Of the 111 cells that had an elongation factor of 3.0 or greater, 103 cells (93?J were oriented within 15’ of the horizontal plane.
Our nexl problem was &oconsider the approximate fraction of the total thickness of layer TV that could be reached by the deudrites of one steiiate ceil and. thus. to relate the morphology ‘if rhe stell&- cells to their highly l~inated input ti‘each genicuiate iamina sends axon terminals to ;t distinct sub-lam& of layer IV (and this possibility 1%cctnsidercd in morr detail in the Discussion) then, cince the six pniculate laminae appear to share the avaii~~~le layer IV reerritory on 3 more or less equal basis ~HARTIW L':d. 1973). the terminals from each gzniculate layer will occupy approximately one-sixth QE the thickness of layer IV. We will argue later that the fraction for some layers may be greater. huf 11 is unlikeiy io he very much less. We calculated the fraction I.)!’iayer IV that was occupied by the dendrites of each &I in the sample, and determined how many of fhc dendritic tiborizations occupied one-sixth or less ~1’the :.ertical thickness of layer IV (see Fig. 1). Of the-B cells, 4 (approx. 7_5”;,)had dendritic arbors occupying one-sixth OFless of the thickness of layer IV. and 10 (669~) had arbors occupying less than one-third of the thickness of layer IV. The type A ceils showed a much more marked limitation to thin portions of layer IV. A Mai uf 02 of the A ceils (541,) occupied one-sixth or less of the thickness of layer IV, and t5: of &e A cells 19?‘,,i had dendrites that spread into one-third or less -of the total tarnina IV thickness. Thatai is. there are 2 great many steliate cells in layer IV of the striate cortex whose input is likely to come predominantly l’rom one or two genicuiale laminae. Positims
zeih
of i~d~~id~~istetl&e c&h m Buyer- TV
The final step in the analysis was to determine-hou the shape (the elongation factor& the vertical extent of the dendritic field, and its orientation were related to the position of the ceil body within layer IV; it is necessary to point out that the.cell body does net necessarily lie close to the center of the dendritic field. although it is commtm for it to be fairly close. Figure 3 shows the relationship bdwee:en.the &ongation factor and the position of the prikaryon throughout the depth of layer IV. i”eiis with an eiongation factor greater than 1.5 arc shown as lines. The orientation of these lines indictits the orientation of the dendritic arbors. The marked iendency for a hortzontai orientation of dendrites iq. evident from this figure, and it is noticeable that the most marked evidence for a horizontal orientation is seen at the edges of layer IV, as though a major limitation upon the shape of the dendritic arbors was one which tends to keep dendrites confined within layer IV. Thus. the cells with the highest eion@on factors a~ aii horizontally oriented and tend TVlie at the edges of layer IV; cells with a non-horizontal orientation tend to be away from the edges of layer IV. It is worth pointing out that some of these: cells in the middle of jayer IV span much of th& total thickness of’ the
893
Stellate cells in area 17 of tree. shrews lo-
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FIG.3. A plot of the elongation factor (C/D) vs the relative depth (A/R) of the cells in layer IV. The dots represent cells which were not considered elongated (C/D < 1.5). The remaining cells with elongated dendritic fields are represented by bars. The angle of the bar reflects the orientation of the dendritic field (e). For abbreviations see Fig. 1.
layer. These are a minority, but may play an important role in interrelating the several geniculocortical inputs to layer IV. Figure 4 shows the proportion of the total thickness of layer IV occupied by each dendritic arbor and plots this against the relative position within layer IV. There is some tendency in this figure for the broadest cells to be away from the edges of layer IV, and for the B cells (shown as boxes) also to stay away from the edges. The marked tendency for most of the cells to occupy only a small fraction of the total layer IV thickness is shown strikingly in this figure. DISCUSSION Laminar arrangement of ajerents izations
and dendritic arbor-
In the tree shrew, the thalamocortical afferents to the visual cortex show a highly laminated arrangement within layer IV and this organization is recapitulated in the laminar arrangement of dendritic aborixations of layer IV cells. The functional significance of this apparently parallel 1amina.r arrangement of a&rents and cortical receptive arbors is not altogether clear. The individual geniculate laminae represent synaptic relays that are
probably distinctive in terms of function and that are certainly distinctive in terms of their input from either the left or the right eye. Thus, in the monkey and the cat there is strong evidence in favor of the view that different geniculate laminae represent relays for functionally distinct but separate pathways from the eyes to the cortex (e.g. Wnso~, ROWE& STONE,1976; DREHER, FUKADA& RODDXK,1976; SHERMAN, WILSON,KAAS& WEBB,1976; and SCHILLER& MALPELI, 1978). Corresponding evidence for such a separation of functions in the geniculate laminae of the tree shrew is not available (see SHERMAN, NORTON& CASAGRANDE, 1975; and CASAGRANTJE, GUILLERY &
HART~NG,1978) but this is probably because the evidence is difficult to obtain from the very narrow geniculate layers of the tree shrew, not because there is no such separation of functions. In contrast to this, the evidence is very strong that in the tree shrew two geniculate layers (1 and 5) receive input from the ipsilateral eye, while the other layers (2, 3, 4 and 6) receive input from the contralateral eye (CASAGRANDE & HARTING,1975; CASAGRANDE et al., 1978). Afferents to individual stellate cells. The majority
of the stellate cells in layer IV appear to receive their primary direct geniculate inputs from one, or perhaps two geniculate laminae. The evidence from degeneration studies is strong that axons from geniculate
E. E. GEISERT.
and R. W.
JR.
GI:II.LL:R~
1
l (1
b
b
l
.
plot of the vertical extent of layer IV dendritic fields over the total thickness of layer IV (E/B) against the relative depth in layer IV (A/B). The dots represent type A cells and the open
FIG. 4. A
boxes are the type B dells. For abbreviations see Fig. 1.
laminae 1 and 2 terminate in distinct sublaminae of cortical layer IV, lamina 1 having terminations closer to the III/IV border than lamina 2 (HARTINGer al.. 1973). That is, stellate cells confined to the most dorsal parts of layer IV receive inputs from the ipsilateral eye through geniculate larrina 1. The evidence from transneuronal transport of tritiated amino acids suggests an overlap of terminals from geniculate laminae 1 and 2 (CASAGRANDE& HARTING, 1975; HUBEL, 1975), but since this method may produce an artifactual appearance of overlap through imerlaminar leakage at the geniculate level or by leakage between cortical sublaminae, it is more reasonable to take the evidence from degeneration studies at its face value. Degeneration and transneuronal transport studies both suggest that there is a significant intermediate zone of cortical layer IV receiving a crossed ihput and no uncrossed a&rents. The evidence is less clear regarding terminations in the deeper parts of cortical layer IV, and there may be some binocular overlap in this region. Comparison of morphological and electrophysiologicai studies. On the basis a$ the primary geniculocorti-
cal connections, one would expect to ftnd a ma&&y of stellate cells in layer IV which are monocularly driven. HUMPIWEYer al. (1977)have recently shown
that some of tire neurons within layer IY of the tree shrew striate cortex are driven only by the contralateral eye. However, the majority of the celis in the striate cortex of the tree-shrew are~binocularly driven, including the neurons of layer IV. These electrophysiological observations can be reconciled with the morphological relationships in one of twoways. One can argue that since the stellate~celif of layer IV are extremely small, the microelectrodes may have tended to select the B cells over the- A &Is, and since the B cells generally have dendrites that spread further in a vertical direction, they probably do receive prey dominantly binocular inputs. Alternatively, the binocular responses recorded from the layer IV @late cells may be produced by intracortical connections. Intracortical connectivity
The first stage of the visual pathway within area 17 of the tree shrew thus involves a_ rather distinct cortical su&&ar ~arrangement. A similar, but clearer, horizor@I, or laminar connectivity -pattern has been demonstrated in old world rdonkeys, wfrere axons fiorn~ th& parvocellular and magnocellular genicoaical layers culate lam&ae terminate in (Huae~ Bt Wm 1972; 1977) and where the cortical cells in receipt of these separate inputs also appear
Stellate cells in area 17 of tree shrews
to be distinguishable in terms of their intracortical connections (LUND, 1973; LUND & BOOTIU3,1975). In the monkey, however, a system of ocular dominance columns is also recognizable within each of the cortical layers, so that the dual organization of cortical connectivity, columnar and laminar, is established already by the geniculocortical inputs. In the tree shrew, in contrast, the geniculocortical inputs are entirely or predominantly laminar. The columnar organization in the tree shrew, which has recently been demonstrated by SKEENet al. (1978) for orientation selectivity, is probably established entirely by intracortical mechanisms. In the Golgi-stained material available to us it was not possible to follow the intracortical connectivity patterns to a significant extent. Relatively few axons were stained, and most of those that were impregnated could not be followed to their terminal arbors before they left the plane of section. In addition, the sublaminar arrangement of layer IV is not distinguishable on the basis of cytoarchitectonic criteria, as it is in the monkey. The relatively homogeneous appearance of layer IV in the tree shrew suggests that intracortical pathways may be kept less clearly distinct from each other in the tree shrew than they are in the monkey. We have noted two tendencies in the dendritic organization of layer IV stellate cells. One is for the dendrites of all cells to stay within the borders of layer IV and the other is for the flattened dendritic arbors of most of the cells to be limited to a relatively small fraction of layer IV. These probably represent two separate developmental factors, since the flattening in all parts of layer IV is greater than that needed to limit the arbors to layer IV itself, and since there is some indication that the flattening is especially marked at the borders of layer IV. The flattening would appear to limit many cells to a region innervated by only one geniculate lamina. However, while there is also a significant overlap of dendrites between the cortical sublaminae there is essentially no overlap at the upper and lower borders of layer IV.
hinated
arrangement of a&rents
895 in other regions
Finally, it is of some interest to compare the laminar arrangement of inputs to visual cortex with laminar arrangements seen in other structures, such as the cornu ammonis or the fascia dentata (BLACKSMD, 1956; RAISMAN,COWAN & POWELL, 1965; HJORTH-SIMONSEN & JEUNE, 1972). In these other structures a highly ordered, laminated set of inputs establishes contacts upon different segments of the dendritic arbors of one relatively uniform set of postsynaptic elements. There may be local specializations corresponding to particular layers of the input (e.g. HAMLYN,1963), but the overall scheme that emerges is of many inputs converging upon one cell type. The intracortical integration here must be studied to a large extent in terms of the properties of the single post-synaptic cell receiving the multiple inputs. In contrast to this, the several separate inputs that reach the visual cortex from the lateral geniculate nucleus impinge upon separate post-synaptic stellate elements, and the cortical integration is most reasonably thought of in terms of circuits originating from these stellate cells. It is possible that in the visual cortex of the tree shrew there are pyramidal cells whose apical dendrites, as they pass through the sublayers of lamina IV, receive some inputs from dl the sublayers, but the evidence so far available for other species suggests that this type of connection, if it occurs, is not dominant (GAREY& POWELL,1971; PETERS & FELDMAN, 1976). If there are cells with multiple, distinct, laminated inputs in the visual cortex, comparable to the hippocampal pyramids, then these probably receive input primarily from intrinsic interneurons, not from extrinsic afferents as in the hippocampal formation. Acknowledgements-We thank ELAINELANGERfor assisting in the preparation of the histological material and illustrations and CHARLOTTE fiSUK for typing the manuscript. This study was supported by Grants numbers NS 06662, NS 14283 and EY 00962 from the United States Public Health Service.
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