A note on the neurosecretory cells and their axonal pathways in the earthworm, Pheretima posthuma

GI;NERAL

AND

COMF.\R.ITITV:

de c&ways

BNDOCRISOLOGP

9, 241-244

(1967)

on the Neurosecretory in the

Ceils

Earthworm,

Phereiim

G. S. DUGRA

Received

May

The method developed earlier (Dogra and Tandan, 1964) for the study of the neurosecretory systems of invertebrates and vertebrates in situ has been found extremely helpful in bringing out information which histological studies have failed to reveal. Among such, the ontogenetic fate of the larval neurosecretory cells of the dipterous Ay Sarcophaga ruficornis and the role of the aorta wall as the st#orage-andl*eleaae organ of the hemipteran Ranatm &oE.gntn have already been reported (Dogra and Tandan, 1965, 1966, 1967). During the investigation of neurosecretory phenomena in the earthworm Pheretima ~osthu?na, with paraldehyde-fuchsin (PF) , perfbrmic acid-Victoria blue (PAVB) and paraldehyde-thionin (PATh) used for in situ t#echniques (Dogra and Tandan, 1964) ; one more feature hitherto not clearly demonstrated, has been observed and is briefly reported here. In P. posthunzcr,, the neurosecretory system eomprises the neurosecretory cells and their axons. There are three types of neurosecretory cell (a, b, and c) distributed in the nervous system. The a-cells stain purple with PF, greenish-blue wit’h PAVB, and blue with PATh. The a-cell is characterized by the presence of a large and oval pcrikaryon, a comparatively small nucleus, and neurosecretory material (NSM) in the form of distinct aggregates (Fig. 1). These a-cells occur in large numbers in the supraesophageal ganglion (Fig. 2) in two groups: of 50-55 cells each (Figs. 1 and 3) in the subesophageal ganglion, and 6#-8 cells in every ganglion of the nerve cord. The b-

24. 1967

cells stain light purple or brick-red with PF, light’-blue with PATh, and show a negative response to PAVB. They out-. number a-cells in the supraesophageal gauglion and the ganglia of the nerve cord, but are less numerous then th.e a-cells in the subesophageal ganglion. The b-cells WE” small in size and oval in shape wi-th a relatively large nucleus; they contain NSM iii the form of fine granules, Aggregates of thc~ material are uscally absent. The e-cells ?~TC negative to the basic stains of the tc~hniques employed and are: therefore, noi revealed in the bulk-stained preparations In sections of PF and PAThi they arr’ stained orange with Ealmi’s counterstain mixture, while with PAVB, the cells are stained with safranin. The e-cells are smallest in size; they are roullti iI1 shape and are characterize6 by the ngranu'im cytoplasm which gives a i’ia!ky appenranW. These cells are scarce in t’he supraesophageal and subesophageal ganglia, b-It, thc~ outnumber a- and b-cells in the ;lerve-eorri ganglia. In the presence of t~liree types of neurosccretory cells, the neurosecretory system of P. posthuma iz similar to t,hat’ oi other oligochacte worms (Charnia~-Ex-(.~ot,ton and Kleinhoiz, 1964). The axons of the a- and b-cells xw visible when the perikarya of these cells are loaded heavily with NSlLI, but the axons of the c-cells aYe diEicult to follombecause they merge with the neumpik which takes up identical stains. In the supraesophageal ganglion, owing to the compact arrangement of the cells in severai tiers, it is difficult to follow the cnmplctc 241

242

G.

FIG. 1. Magnified view ing axons (Ax) of a-cells

2. Whole

mount

FIG. 3. Whole of P. posthuma

mount showing

FIG.

S.

DOGRA

of one group of a-ceils of the subesophageal ganglion convergin, = in a small area (arrow). PF. x410.

of the

cerebral

ganglion

of one group of cystine-rich that all a-cells are positive

of P. posthuma

showing

of P. posthzma

a- and

neurosecretory cells of the to PAVB. PAVB. x320.

b-cells.

subesophageal

PF.

show-

~85

ganglion

NEUROSECRETORY

CELLS

route of t,he axons. But the course of an axon in the region of the subesophageal ganglion is easily traceable (AX, Fig. l), because there are fewer neurones in one or two tiers. However, their dorso-ventral arrangement makes it difficult to photograph

FIG. 4. Magnified view of two a-cells of the subesophageal ganglion of P. posth~naa showing axons. PF. X850.

them under one focus. The axons are short, usually stout, and straight (Fig. 4). The proximal portions of the axons, near the perikarya, are enlarged into a conical or pear-shaped structure which can be delimited as the ‘neck’ (arrow, Fig. I), and

FIG.

of XSM

IX

P.

~08thZlWla

243

the bulb-like distal portions are designated the axonal bulbs (arrows, Fig. 5). Pn the subesophageal ganglion, axons of each group converge in a small area (arrow, Fig. 1) in the vicinity of the blood capillaries where they probably release their neuroeecretory contents. This area appears to be like an elementary neurohaemai organ, a possibility suggested earlier by Clark (1963) for polychaete and ohgochaete brains. In the axons, as well as in their cell bodies, the NSM is in a continuous column which is more compact at the bulbous end (arrow, Fig. 5). On the basis of histological observations, it is difficuit to interpret the disposition of material in the neurones. However, it appears that owing to differences in the rates of synthesis and release, the 1JSM has been accumulated momentarily in the cell. This interpretation is in agreement with the conclusions drawn by Clark (1963). According to him: ‘The presence of stainable material in annelid neurosecretory cells indicates that the rat<: of synthesis of the material has outpaced its rate of release and that accumulation of it in the cell may be caused by a change in either rate.” On the other hand, Highnam and Lusis (1962j believe that. when thi amount of stainable material in a neurosecretory system is large, the material is accumulat.ing and the system is inactive, Many workers have postulated the release of KSM through the axonal buIh (Bern! 1962; and Scharrer and Scharrrr,

5. Oil immersion photograph of the area shown by arrow in Fig. 1. Note the large accumular,ion in the axonal bulbs (arrows). x750.

244

G.

S.

1963), but the axonal bulbs have not been as clearly substantiated by photomicrographs as they are in Fig. 4. The study of recent literature shows that there is no consensus of opinion regarding the release of NSM through the axonal bulbs (Bern, 1962: and Scharrer and Scharrer. 1963). Photomicrographic demonstration ’ of this mechanism is probably difficult in animals where a well-developed neurohaemal organ is present, since the intermingling of a number of axons makes them obscure, as does the deposition of large amounts of NSM in the glands. The limitations of the light microscope prevent the study of the mechanism of release in P. posthuma, but it is strongly believed that ultrastructural studies of the isolated neurones and their axonal bulbs in oligochaete worms may provide more convincing evidence of the process of NSM release through the axonal bulbs. Thus the present report demonstrates the axonal bulbs and the pathway of a single axon.

DOGRA

The author is grateful to Professor R. B. Clark, The University Newcastle upon Tyne, for critically reading the manuscript and for making valuable suggestions, and to Dr. B. K. Tandan, The University Lucknow, for providing laboratory facilities. REFERENCES DOGRA, DOGRA,

G. S. (1967). J. Morphol. 121, 223. G. S., AND TANDAN, B. K. (1964). &uarl.

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Microscop. sci. 105, 455. Doc1.4,

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S.,

AND

perientia 21, 216. DOGR~, G. s., AND Roy.

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CHARNIAUX-COTTON,

(1964).

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B. K. (1965). Ex-

B. K. (1966). hoc. 41, 57. H., .~ND KLEINHOLZ, L. H. 4, 135.

TANDAN,

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R. B. (1963). J. Endocrinol. 26, 18. HIGHNAM, K. C., AND LUSIS, 0. (1962). Quart. J. Microscop.xci. 103, 73. BERN, H. A. (1962). Gen. Comp. Endocrinol. Suppl. 1, 117. SCHARRER, E., AND SCHARRER, B. (1963). “Neuroendocrinology.” Columbia University Press, New York, p. 289. CLARK,