An apparent somato-somatic synaptic structure in the pineal gland of the rat

V. Ii’. Hopsu and A. U. Arstila

484

REFERENCES 1. BLAIR, P. V., OD.~, T., GREEN, D. E. and ~~ERN.~XDEZ-MOR~N, H., Biochem. 2, 756 (1963). 2. FERNANDEZ-MORAX, H., ODA, T., BLAIR, P. V. and GREEN, D. E., .I. CelZ Riot. 22, 63 (1964). Naturforsch. Ges. Ziirich 98, 20 3. FREY-WYSSLING, A. and STEINMASN, E:., Vierteljuhresschr. (1953).

4. GREEN, D. E., Proceedings of the 5th. International Congress of Biochemistry. Vol. IX, p. 9. Pergamon Press, London and New York, 1963. Translated by T. Oda, Protein, Nucleic Acid and Enzyme 9, 154 (1962). 5. GREEN, D. E. and ODA, T., J. Biochem. (Tokyo) 49, 742 (1961). 6. GREES, D. E., TZAGOLOFF, A. and ODA, T., Proceedings of the 1st International of Cellular Chemistry, Ohtsu, Japan, 1963. In press. 7. HUZISIGE, H., Unpublished. 8. JACOEY, G. and PERNER, E., Flora 150, 209 (1961). 9. KAHS, A. and v. WETTSTEIN, D., J. Ultrastract. Res. 5, 557 (1961). 10. OD.~, T., Symposia Cell. Chem. 13, 11 (1963). 11. ODA, T. and Homo, T., Exptt Cell Res. 34, 414 (1964). 12. ODA, T. and NISHI, Y., J. Electron Microscopy 12, 290 (1963). 13. PARK, R. B. and BIGGENS, J., Science 144, 1009 (1964). 14. PARK, R. B. and Pos, N. G., J. Mol. Riot. 6, 105 (1963). 15. WESSELS, J. H. C., Proc. Roy. Sot. London R, 157, 345 (1963).

AN APPARENT SOMATO-SOMATIC IN THE PINEAL GLAND V. Department

K.

HOPSU of Anatomy, Received

and

SYNAPTIC OF THE

A. U.

University September

Moscow, 1961. into
Symposium

STRUCTURE RAT

ARSTILA of Turku,

Finland

28, 1964

M.mYtypes

of cell membrane specializations have been described in the cells of the nervous system [2, 3, 41. Those described electronmicrosco$cally as synapses are characterized usually by thickenings of the cell membranes, especially at the post-synaptic side, accumulations of synaptic vesicles near the presynaptic membrane or around a presynaptic dense projection and presence of numerous mitochondria in the presynaptic cytoplasm. In spite of many differences in the ultrastructural details of synapses in various parts of the nervous system there is agreement concerning the location of the synapses in the nervous cells. On the basis of the location of the synapFig. 1.-A synaptic contact between two type I pineal cells. Note two projections vesicles and the vicinity of the nuclei of the contacting cells. x 18,000. Fig. 2.-Three synaptic projections surrounded by vesicles. Note the vicinity in the presynaptic cell (left). )i 16,500. Fig. 3.-A synapse like in Fig. 2 except that now the nucleus of the post-synaptic x 35,000. Fig. 4-5.-Synaptic projections and vesicles on both sides of the opposing x 35,000. Experimental

Cell Research

37

surrounded of the

by

nucleus

cell is visible. cell membranes.

Apparent

somato-somatic

synaptic

structure

in pineal

gland

Experimental

Cell

485

Research

37

486

V. K. Hopsu

and A. U. Arstila

ses they are classified as axo-dendritic, axo-somatic or axo-axonic while other types of contacts and membrane fusions are also described which may mediate electrotonic transmission. In a recent report [I] concerning the ultrastructure of the rat pineal gland two different cell types of nervous origin were characterized and typical synaptic structures were demonstrated between them. This report presents some further observations on the structures in question. Material and methods.-The pineal glands of Long-Evans rats were fixed in 1 per cent phosphate buffered 0~0, for 2 hr, embedded in Epon 812 and stained with uranyl acetate and lead citrate. The sections were cut with Porter-Blum ultramicrotome and photographed with Siemens Elmiskop I electron microscope. Results.-The basic structures of the synapses in question can be seen in Figs. 1 to 5. All have a clear straight or curved dense projection surrounded by numerous small vesicles. The synaptic cleft cannot be shown to be clearly different from the intercellular space elsewhere. The postsynaptic membrane does not show any thickening or increased density of staining. In Fig. 1 two small projections are seen in the right hand cell, clearly located in the cell perikaryon or soma between two nucleated pineal cells. This location of the synaptic structure justifies the synapse to be called somato-somatic. The side of the projection is generally identified as presynaptic [4]. Following this classification the cell on the right is presynaptic and that on the left postsynaptic. Both cells are morphologically identical type I cells in our classification. Similar structures were, however, seen also between the type I and type II cells, both of which are considered to be of nervous origin. They were never found between these cells and S.C. fat-laden cells considered to be of glial origin. Fig. 2 presents altogether three projections with surrounding vesicles situated just beneath a cell nucleus. Again the presynaptic side cannot be considered to present any cell process but the perikaryon or soma of the cell. This contact must be taken either as somato-somatic or somato-dendritic depending on the identification of the postsynaptic side. In Fig. 3, on the other hand, the nucleus is very near the synaptic structure on the postsynaptic side. This structure is to be considered either as somato-somatic or axo-somatic. Figs. 4 and 5 demonstrate that a projection with vesicles may also be present at both sides of the cell membranes opposite to each other. In these cases morphological classification of the contacting area in terms of presynaptic or postsynaptic entities is not as clearcut because both of them have the typical presynaptic structures. This kind of arrangement was seen fairly frequently between two perikaryons as well as between cell processes. Discussion.--PMembrane connected presynaptic dense projections and numerous synaptic vesicles are the structures typical to synapses described. They lack postsynaptic membrane thickenings and formed material in the synaptic cleft. They remind of the type II synapses described by Gray [3] in the rat cerebral cortex (axosomatic contacts) which, however, had some postsynaptic membrane thickening. From the figures it is evident that the location of the synaptic structures in the cell differs basically from those described so far. Unlike in other parts of the nervous system a structure considered typical of the presynaptic side of the contact can be identified in the perikaryon and in the immediate vicinity of the nervous cell nucleus as well as in the cell processes of the pineal cells. Similarly the postsynaptic side was idenfindings tified either as a cell perikaryon or a cell process. These morphological would suggest that synaptic transmission may occur directly from one nervous Experimental

Cell Research

37

Les caracttkistiques

ultrastructurales

du ihymus a lapkiodepkrinatale

487

cell body to the next. The fact that similar synaptic structures are located opposite to each other in two neighbouring cells would suggest that transmission may take place in either direction. Synaptic vesicles are considered as carriers of transmitter substance in the synapses. Therefore, the presence of the vesicles would suggest that the transmission in these contacts is neurohumorous rather than electrotonic. The physiological significance of these contacts cannot be deduced from morphological grounds alone, but besides the possible excitatory synaptic function they could be inhibitory, since it has been suggested that synapses close to the origin of the axon are inhibitory [5]. The fact that similar synaptic structures have not been described in other parts of the central nervous system may be due to the fact that glial cells separate the nerve cells more effectively and thus this kind of contact is either impossible or very seldom existing. It is also possible that they cannot exist in normal nerve cells but only in the modified nervous cells of the pineal gland. This work has been partly carried out using the facilities scope Laboratory, University of Helsinki.

of the Electronmicro-

REFERENCES I.

2. 3.

ARSTILA, A. U. and HOPSU, V. K., Ann. Acod. Sci. COLONNIER, M. and GUILLERY, R. W., Z. Zellforsch. GRAY, E. G., J. Anat., Lond. 93, 420 (1959).

4. -ibid. 5. RGTZLAFF,

97, 101 (1963). E., J. Comp. Neural.

Fenn. Series A. V. No. 113 (1964). 62, 333 (1964).

107, 209 (1957).

LES CARACTl?RISTIQUES

ULTRASTRUCTURALES

A LA PtiRIODE

DU

THYMUS

PI?RINATALE

J. IZARD Laboratoires

de microscopic blectronique et d’embryologie Fact& de Midwine, Toulouse, France Rqu

!e 6 octobre

gtWrale,

1964

L ES experiences de thymectomie chez les mammiferes nouveau-n& dcmontrent que, au debut de la vie, le thymus joue un role fondamental dans la formation des lymphocytes du sang, de la rate et des ganglions lymphatiques [2]. Parallelement, le thymus dquipe les formations lymphoi’des en cellules (( immunologiquement competentes D et controle ainsi le developpement de l’immunite de greffe et de l’immunite serique [2, 61. Y a-t-il, B l’echelle ultrastructurale, des differences caracteristiques entre le thymus d’un animal adulte et celui d’un nouveau-m! ou dun f&us vers la fin de la gestation ? En d’autres termes, y a-t-i1 a la periode perinatale un critere ultrastructural du role lymphopoietique et immunologique du thymus ? Experimental

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37