The fine structure of neurons and other elements in the nervous system of the giant African land snailArchachatina marginata

Abstract

Much of the fine structure of the neurons, interstitial cells, blood vessels, collagen, and muscle fibres resembles that of similar elements in other species. In this preliminary report attention is paid to those features which characterize the structure of the neurons and interstitial cells. Within each group there are morphologically distinct cell types. The giant neurons (> 100 [mu]m) are distinguished from smaller neurons by the irregular shapes of their nuclei, and by the extensive penetration of their perikarya by processes of surrounding sheath cells. A substance having a structure similar to that of glycogen is present both in the sheath cells and in the neuronal cytoplasm. The sheath cells may, therefore, be functioning as "nurse" cells. The sheath cells form one group of interstitial cells. Two other groups are defined in accordance with their spatial relationships to the neurons. The glial cells are distributed between the sheath cells, where they may form a mass of glial tissue, and they are present in the walls of the blood vessels. The cytoplasm is extended into processes and in the region of the nucleus appears to be extremely active. It is characterized particularly by an abundance of the Golgi complex, small granular and large laminated electron-dense inclusions, and the particulate substance which may be glycogen. Structures which resemble the electron-dense inclusions of glial cells are found in the cytoplasm of neurons. The third group of interstitial cells consists of supporting cells which surround nerve fibres. Large fibres and particularly the neurite are deeply penetrated by these cells and the phenomenon appears to be associated with the formation of collateral nerve fibres. Dense fibrils permeate the cytoplasm and desmosomes are present between the cells, which, therefore, appear to form a resilient framework around the nerve fibres. Nerve fibres in the neuropil and nerves contain a complex array of vesicular and granular inclusions. These comprise small clear vesicles (350 to 600 A), electron-dense granules (600 to 2000 A) and a range of granular vesicles which have a structure intermediate in character between that of the clear vesicles and the dense granules. The dense granules in the nerve fibres are identical with those which are present in the perikarya and which have their origin in the cisternae of the Golgi complex. A dense substance accumulates between the membranes and is concentrated in the form of a granule towards one end of a cisterna, from which it is pinched off by constriction of the membranes. This is probably the origin of the granules which collect in the nerve fibres. The vesicular and granular elements in the fibres are never completely segregated, although clusters occur in which one type predominates. Occasionally, in regions where the clear vesicles are prevalent, these may be associated with a thickened membrane which is apposed to a similar thickening of an adjacent nerve fibre. The whole structure closely resembles synapses which have been described in other animals. The nature of the substances located in the vesicles and granules has not been determined.