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The formation of the definitive excretory system (nephridium and bladder complex) in Hirudo medicinalis during the last two thirds of embryonic development was observed with light- and electron microscopy, immunocytochemistry, and nuclear labeling. In jawed leeches, two excretory systems develop and function successively. The nephridia of the cryptolarva are associated with the larval sac and persist until the definitive nephridia are sufficiently developed to be functional. Development of the definitive excretory system begins with the differentiation of the (ectodermal) bladder and urethra. The cells from which they arise incorporate bacteria and are thereby recognizable at day 8. The (mesodermal) urine-forming tissues of the nephridium (canalicular cells and central canal cells) appear a day later. By day 17, the nephridia are in contact with the outlet region and structurally able to function. Each nephridium is individually innervated by a peripheral neuron, the nephridial nerve cell, which expresses FMR Famide-like immunoreactivity and begins growing into the nephridium on day 11. Organogenesis of the leech nephridium is compared with the formation of excretory organs in other species. The temporal correlation of innervation and the development of the transporting cells is discussed.

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An analysis of the ultrastructure of the tube feet of three species of sea urchins (Strongylocentrotus franciscanus, Arbacia lixula and Echinus esculentus) revealed that the smooth muscle, although known to be cholinoceptive, receives no motor innervation. The muscle fibers are attached to a double layer of circular and longitudinal connective tissue which surrounds the muscle layer and contains numerous bundles of collagen fibers. On its outside, the connective tissue cylinder is invested by a basal lamina of the outer epithelium to which numerous nerve terminals are attached. These are part of a nerve plexus which surrounds the connective tissue cylinder. The plexus itself is an extension of a longitudinal nerve that extends the whole length of the tube foot. It is composed of axons, but nerve cell bodies and synapses are conspicuously lacking, suggesting that the axons and terminals derive from cells of the radial nerve. Processes of the epithelial cells penetrate the nerve plexus and attach to the basal lamina. There is no evidence that the epithelial cells function as sensory cells. On the basis of supporting evidence it is suggested that the transmitter released by the nerve terminals diffuses to the muscle cells over a distance of several microns and in doing so affects the mechanical properties of the connective tissue.

The region at and around the site of optic nerve transeetion (ONS) in goldfish, topologically the equivalent of the glial seal' in mammals, is reported to remain free of astrocytes over weeks, but its cellular constituents are unknown.To leam what type of cell occupies the site of injury and thus provides support for the rapidly regenerating retinal growth cones, immunostaining experiments at the light micro scopic level and electron microscopic examinations were undertaken. Between 2 and 30 days after ONS, an area up to 150 f1m wide at the transection site exhibits intense anti-fibronectin immunoreactivity. This site contained cells and processes with ultrastructural characteristics of fibroblasts and abundant collagen fibrils. Moreover, on fibroblast cultures derived from regenerating optic nerves, retinal axons grew to considerable density in vitro. Since fibroblasts are constituents of the interfascicular spaces and outer nerve sheath of the normal goldfish optic nerve, the present data imply that fibroblasts of either source migrate into the.lesion. Judging from fibronectin immunostaining they remain there during the passage of regenerat ing axons, and thus may provide physical and perhaps molecular support for axon growth. The fibroblasts are again restricted to interfascicular spaces after restoration of the astrocytic glia limitans around regenerated fascicles. .