Myelinated nerve cell bodies in the dorsal horn of the monkey (Saimiri sciureus)

Fine structure of the superficial layers of the viper optic tectum. A Golgi and electron-microscopic study

The superficial layers of the viper optic tectum, which receive fibers from he retina, were studied using both light and electron microscopes. The optic fibers layer, or stratum opticum, is composed of 200 to 250 tight fascicles containing thin fibers, nearly all of which are myelinated. The main optic terminal layers, the stratum griseum et fibrosum superficiale, the greatest part of the cellular population is composed of small vertically oriented neurons and horizontal nerve cells, many of which are probably local circuit neurons. The neuropil of the stratum griseum et fibrosum superficiale is made up of small nerve elements, including three types of profiles containing synaptic vesicles; 1) boutons with pleiomorphic synaptic vesicles (P), representing over 47% of the total population of profiles containing synaptic vesicles and comprising three subgroups (P1, P2, and P3); 2) boutons with spheroidal synaptic vesicles (S), forming more than 29% of the total populations of profiles containing synaptic vesicles and comprising two categories, S1 and S2 (S2, the more numerous, represents the optic boutons, which make up 22% of the total populations of profiles containing synaptic vesicles); and 3) dendrites with pleiomorphic vesicles, accounting for approximately 23% of the total populations of profiles containing synaptic vesicles. A study of synaptic patterns revealed a large number of serial synapses and a lesser number of triplets or triadic synapses. The presynaptic components are boutons containing spheroidal (S1, S2) or pleiomorphic (P1, P2, P3) synaptic vesicles. The intermediate profile was always a dendrite with synaptic vesicles which frequently belonged to the small neurons of the stratum griseum et fibrosum superficiale. Comparison of the present results with other recent data shows that the synaptic circuitry in the optic tectum of Vipera aspis closely resembles the pattern observed in the optic tectum of other vertebrates, ranging form fish to mammals. However, quantitative differences exist, especially with regard to the proportion of dendrites containing synaptic vesicles. Their number seems to be higher in sauropsidians than in mammals, particularly in primates.

Distribution and morphology of myelinated perikarya and dendrites in the olfactory bulb of primates

Numerous myelinated perikarya occur in different layers of the olfactory bulbs of a chimpanzee and two species of New World primates, that is, the squirrel and the Cebus monkey. It appears that somata of all established neuron categories, except for the mitral cells, can become ensheathed in myelin. Myelinated dendritic segments are found in the periglomerular region and in the external plexiform layer; tufted and periglomerular cells most likely to give rise to these myelinated dendrites. The myelin sheath is predominantly of the compact C.N.S. type. Perikaryal and dendritic myelin often ends in typical feet of glial cytoplasm. The termination site of dendritic myelin is a preferred site of synaptic contacts. Myelinated profiles are more numerous in the two monkey species than in the chimpanzee.

The ventral dendritic arbor of marginal (lamina I) neurons in the adult primate spinal cord

The ventral (gelatinosal) dendritic arbor of marginal neurons taken from lumbosacral cord was examined in Golgi preparations from adult Macaque monkey and adult Squirrel monkey where neurons were filled with horseradish peroxidase (HRP) via secondary diffusion. Based on morphological criteria two types of marginal neurons were found which give rise to a substantial ventral dendritic arbor. These dendrites penetrate the depth of lamina II and some extend well into lamina III. The cell bodies are medium to large in size and confined to the medial half of lamina I. Marginal neurons located in the lateral half of the marginal layer lack gelatinosal dendrites and are characterized instead by numerous interstitial dendrites.

The habenular nuclei of the elasmobranch "Scyllium stellare": myelinated perikarya

Some habenular cells of the elasmobranch, Scyllium stellare, have a perikaryon completely enveloped by myelin of the compact type. They are found only in the left habenula and not in the right. The myelin sheath accompanies the proximal segment of the process arising from the perikaryon, in a node of Ranvier-like pattern. The identity of these myelinated cells and the functional significance of their myelination are unknown.

The fine structure of myelinated nerve cell bodies in the bulbus olfactorius of man

In the bulbus olfactorius of man numerous myelinated nerve cell bodies occur in the stratum plexiforme internum and stratum granulosum internum. In many respects they resemble the neighbouring granule cells: small chromatin clumps border on more than half of the circumference of the nucleus, the thin cytoplasmic rim contains abundant polysomes and sometimes pigment complexes with numerous light vacuoles, the cells often show a process which extends up to the stratum glomerulosum, the perikarya are devoid of synaptic contacts whereas the proximal segment of the peripheral processes display rare contacts. The myelin sheath varies in thickness consisting of 2 to 24 lamellae with distances between the major dense lines ranging from 9.3 to 11.3 nm. The myelin sheath may enclose the cell body completely or partially and accompany the proximal segment of the process arising from the perikaryon. On partially enveloped perikarya, the myelin lamellae end in formations like those of the node of Ranvier, though often less regularly. Within the compact myelin sheath all of its lamellae may be distended for a short distance by glial cytoplasm as in the Schmidt-Lanterman incisures of peripheral nerve fibres. Adjacent to the outermost myelin lamella myelinated axons and cell bodies, tentatively identified as oligodendrocytes, as well as granule cells may be closely joined.

Myelinated granule cell bodies in the cerebellum of the monkey (Saimiri sciureus)

Electronmicrographs of both the vermis and hemisphere of the cerebellum of the squirrel monkey (Saimiri sciureus) show numerous granule cell bodies partially or completely surrounded by myelin. The myelin is of the compact type and consists of 1 to 13 lamellae. In several cases of partially ensheathed cells the myelin is clearly derived from extensions of myelin sheaths that surround small caliber axons. Either all or only the outer lamellae surrounding the axon contribute to the extensions. In the first instance the myelin buckles at its mesaxon pole and the resulting doubled flap extends for a variable distance along the cell surface.