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J. Anat. (1978), 127, 1, pp. 169-180 With 16 figures Printed in Great Britain

The fine structural organization of the cuneate nucleus in the monkey (Macaca fascicularis) C. Y. WEN, W. C. WONG AND C. K. TAN

Department of Anatomy, Faculty of Medicine, University of Singapore, Sepoy Lines, Singapore 3

(Accepted 10 September 1977) INTRODUCTION

The cuneate nucleus of various mammals has been the subject of several investigations. Its structure has been studied with the light microscope in the rat (Basbaum, 1972; Basbaum & Hand, 1973), cat (Kuypers & Tuerk, 1964) and monkey (Ferraro & Barrera, 1935; Chang & Ruch, 1947; Biedenbach & Towe, 1971; Biedenbach, 1972). But reports dealing with its fine structure are relatively few and accounts are only available in the rat (Valverde, 1966; Tan & Lieberman, 1974) and cat (Walberg, 1965, 1966). The present study is an attempt to provide some knowledge of the fine structural organization of the cuneate nucleus in the monkey. MATERIALS AND METHODS

Three adult male monkeys weighing 1-5-2 kg were used for this study. Each animal was anaesthetized by an intraperitoneal injection of 0 5 ml of Sagatal (sodium pentobarbital 60 mg/ml) per kg body weight. Artificial respiration with oxygen through a tracheostomy was done before thoracotomy. 1000 units of heparin and 2 ml of 1 % sodium nitrite per kg body weight were given by intracardiac injection. After 3-5 minutes the animal was rapidly perfused through the left cardiac ventricle with 200 ml of Ringer's solution at pH 7 3-7A4, followed by 500 ml of dilute fixative (1 % paraformaldehyde + 1 -25 00 glutaraldehyde) and then 1000 ml of concentrated fixative (4 % paraformaldehyde + 5 0 glutaraldehyde). The total perfusion time was usually about 25 minutes. After perfusion the animal was decapitated and the head immersed in concentrated fixative and kept overnight at 4 'C. The caudal part of the medulla, including the upper cervical cord, was carefully dissected away. Transverse slices about 1 mm thick were cut with a razor blade. From these slices portions of the cuneate nucleus were trimmed under a stereomicroscope and placed in ice-cold 0 1 M cacodylate buffer at pH 7-4 containing 5 % sucrose. After four changes of the buffer at 10 minute intervals the slices were post-fixed with 1 % ice-cold osmium tetroxide for 2 hours. The tissue slices were then dehydrated through an ascending series of acetone and embedded in an Epon mixture. Thick sections were stained with 1 % methylene blue. Ultrathin sections were doubly stained with uranyl acetate and lead citrate and examined in either a Hitachi HS-8 or a Philips 301 electron microscope.

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C. Y. WEN, W. C. WONG AND C. K. TAN RESULTS

Neurons The profiles of the cell bodies of neurons in the cuneate nucleus of the monkey varied in shape and size, some being round, others oval, elongated or polygonal. The lengths of their short axes ranged from 4 to 22 ,m and that of their long axes from 9 to 37,um. Since each neuronal profile represented only a very thin segment of the whole cell, it was not possible to classify cells according to overall shape and size. Consequently only neuronal profiles that contained nucleoli have been used for the present classification, which is based on nuclear morphology and the content of cytoplasmic organelles.

Group L. Neurons with a relatively regular nucleus and a prominent rough endoplasmic reticulum Neurons in this category were round, oval or polygonal. The nucleus, which appeared round or oval with a relatively regular nuclear envelope, was usually centrally placed (Fig. 1). The cytoplasm formed a large proportion of the neuronal profile and was rich in organelles. A prominent Golgi apparatus was usually located in the perinuclear region and the rough endoplasmic reticulum (RER) was arranged in clumps. Mitochondria, microtubules, neurofilaments, smooth endoplasmic reticulum, and electron-dense bodies of various shapes and sizes were found scattered in the cytoplasm. This type of neuronal profile was the least commonly encountered. Group I. Neurons with a slightly irregular nucleus Neurons in this group were the most commonly observed in the cuneate nucleus. The shape of such neuronal profiles was highly variable. The nucleus, which was slightly irregular and indented, was usually eccentrically placed (Fig. 2). In some neuronal profiles the Golgi apparatus was well-developed and some of the RER was arranged in small stacks. In other profiles the Golgi apparatus was less developed and the RER consisted of only single strands randomly dispersed in the cytoplasm. Group III. Neurons with a highly indented nucleus The neurons in this group, which were also highly variable in shape, were less commonly seen than those of Group II. The characteristic feature was the deeply indented nucleus (Fig. 3). The cytoplasm was scanty and formed only a thin rim around the nucleus. Such cells were poor in cytoplasmic organelles. Axons and axon terminals In the cuneate neuropil, myelinated and unmyelinated axons of various calibre intermingled with axon terminals, dendrites and glial elements. The axon terminals contained round or flattened vesicles. Profiles of axon terminals with round vesicles varied in shape and size, but their contours were usually irregular (Figs. 4, 5). The diameters of 370 profiles were measured. Their short diameters ranged from 0'3 to 3-4 ,um (mean 1-3 /,m ± S.D. 0X6 ,um) and their long diameters from 0 4 to 12-5 ,um (mean 24 ,um ± S.D. 1-5 /,m). The round vesicles, which ranged from 40 to 60 nm in diameter, were evenly distributed in most terminals (Fig. 5). In some terminals, however, they formed clusters near the active zones. In the smaller terminals these vesicles appeared to be more densely packed. The number of mitochondria in each terminal varied from none

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