Embryonal tissue of rat's septum and hippocampus developing in the anterior eye chamber for three to four months was investigated. Electron microscopic analysis of axonal processes and synaptic boutons is presented in this paper. Dense neuropil of the grafts includes myelinated and unmyelinated fibers. Some of them have definite structural features of peripheral fibers; there are also some transitory forms, combining the traits of peripheral and central axons. Synapses of presumed monoaminergic nature and other unidentified synaptic contacts with the neurons are formed by peripheral fibers entering the grafts from the iris. The majority of axons belongs to the neuronal elements of the grafts themselves. Distribution of the synapses upon neuronal somata and dendritic trees seems to be quite normal. Synapses of symmetric type are present mainly upon neuronal bodies and dendritic shafts; synapses with dendritic and somatic spines are asymmetric. At the same time some unusual features are observed: the presence of the axonal growth cones, long ribbon-like axonal terminals, forming numerous serial synapses with elements of neuropil or neuronal bodies, pseudo-glomerular synaptic contacts encapsulated by glial processes. Increased pinocytotic and microphagocytotic activity between various elements of neuropil is present within the grafts. Spinular complexes, which are often encountered in interneuronal contacts, possibly also participate in processes of exocytosis-endocytosis. Some ultrastructural characteristics of the grafted tissue indicate to its incomplete maturation which may result from deficit of normal afferentation or from absence of some more general developmental factors in the intraocular grafts.
Animals Anterior Chamber