Quantitative electron microscopic autoradiography was used to detect and characterize endoneurial sites of lipid synthesis in mouse sciatic nerve. Six tritiated phospholipid precursors (choline, serine, methionine, inositol, glycerol, and ethanolamine) and a protein precursor (proline) were individually injected into exposed nerves and after 2 h the mice were perfused with buffered aldehyde. The labeled segments of nerve were prepared for autoradiography with procedures that selectively remove nonincorporated precursors and other aqueous metabolites, while preserving nerve lipids (and proteins). At both the light and electron microscope levels, the major site of phospholipid and protein synthesis was the crescent-shaped perinuclear cytoplasm of myelinating Schwann cells. Other internodal Schwann cell cytoplasm, including that in surface channels, Schmidt-Lanterman incisures, and paranodal regions, was less well labeled than the perinuclear region. Newly formed proteins were selectively located in the Schwann cell nucleus. Lipid and protein formation was also detected in unmyelinated fiber bundles and in endoneurial and perineurial cells. Tritiated inositol was selectively incorporated into phospholipids in both myelinated axons and unmyelinated fibers. Like inositol, glycerol incorporation appeared particularly active in unmyelinated fibers. Quantitative autoradiographic analyses substantiated the following points: myelinating Schwann cells dominate phospholipid and protein synthesis, myelinated axons selectively incorporate tritiated inositol, phospholipid precursors label myelin sheaths and myelinated axons better than proline.
The human secondary yolk sac persists for a short but critical period of development during early organogenesis. Although it is established that the yolk sac is the site of origin of several cell lines, a locus of hematopoiesis and secretes several proteins, the yolk sac may have other as yet less well defined functions. Here we focus on ultrastructural evidence of the role of the human yolk sac as an organ of exchange with the extraembryonic coelom and the yolk-sac cavity. Yolk sacs of 6 to 10 weeks developmental age were fixed and processed for light microscopy, TEM and SEM by standard procedures. Numerous microvilli, coated and uncoated vesicles, and lysosomes in the mesothelial layer together suggest a high capacity for pinocytosis of coelomic fluid and lysosomal digestion of internalized substrate. It is not known whether this process is a significant nutritional source for the yolk sac and embryo as it is in the rodent. Both SEM and TEM observations provide evidence of cilia on dispersed cells in the endodermal layer lining the yolk sac cavity. To our knowledge, there is only one other brief report of cilia on the human yolk sac.
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