An ultrastructural analysis of the developing enteric nervous system of the guinea-pig small intestine

Summary The developing enteric nervous system of the guinea-pig has been analysed ultrastructurally. In addition, electron microscope autoradiography, following incubation with tritiated 5-hydroxytryptamine (³H]5-HT) or tritiated norepinephrine (³H]NE) was used to locate the developing axons of enteric serotoninergic and adrenergic neurons respectively. Observations have been correlated with previous studies of the development of the various types of enteric neuron and the onset of intestinal neuromuscular function. Prior to 25 days of gestation no neurons can be recognized morphologically. Neurons first appear at 25 days' gestation, together with a primitive neuropil in neural islands within the outer gut mesenchyme. Ganglion cell precursors are primitive at first and resemble the cells in the surrounding mesenchyme. Growth cones are abundant but there are no terminal varicosities or synapses. The circular muscle also begins to form at this time. At 32 days' gestation the longitudinal layer of smooth muscle can be discerned and, within the myenteric plexus, terminal axonal varicosities appear containing small (about 50 nm in diameter) electron-lucent synaptic vesicles. The submucosal plexus appears to be derived from neurons and neurites that reach it from the earlier-developing myenteric plexus. The submucosal plexus can be recognized at 38 days of gestation but is not well developed until day 42. Synapses on ganglion cell somata first appear in the myenteric plexus on gestational day 38 and are numerous on day 42 when the first axo-dendritic synapses can be seen. Between days 42 and 48 the developing neural tissue and growing smooth muscle cells interdigitate but after day 48, the plexus becomes ensheathed by supporting cells and connective tissue and this interdigitation is lost. Prior to day 48 most varicosities contain small electron-lucent synaptic vesicles; however, after this time a variety of terminals appears. Between days 48 and 53 of gestation evidence of degenerating neuronal processes is common, indicating that cell death may occur. Electron microscopic autoradiography with ³H]5-HT reveals labelling of axons in the neuropil of the myenteric plexus at day 32 of gestation. Some primitive appearing cell bodies, however, are also labelled and these cells seem to be entering the myenteric plexus from the surrounding mesenchyme. After 42 days of gestation ³H]5-HT labels only axons of both nerve plexuses. Often, labelled terminals are apposed to ganglion cells or dendrites. In contrast, significant labelling by ³H]NE is not found until gestational day 48. Axons are labelled by ³H]NE and these tend to be located at the interface between the myenteric plexus and the surrounding connective tissue.