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Tissue recombinants of embryonic urogenital sinus mesenchyme (UGM) and epithelium of the urinary bladder (urothelium, BLE) of adult rats and mice were grown for 3-30 d in male syngeneic hosts. Short-term in vivo growth indicated that prostatic morphogenesis is initiated as focal outgrowths from the basal aspect of the adult urothelium. The solid epithelial buds elongate, branch, and subsequently canalize, forming prostatic acini. After 30 d of growth in the male hosts, prostatic acini exhibit secretory activity. The marked changes in urothelial morphology induced by the UGM are accompanied by the expression of fine-structural features indicative of secretory function (rough endoplasmic reticulum, Golgi apparatus,.and secretory granules). During this process, urothelial cells express prostatic histochemical markers (alkaline phosphatase, nonspecific esterase, glycosaminoglycans) and prostate-specific antigens. The expression within BLE of prostatic characteristics is associated with the loss of urothelial characteristics. These data indicate that adult urothelial cells retain a responsiveness to embryonic mesenchymal inductors. Furthermore, mesenchyme-induced changes in urothelial cytodifferentiation appear to be coupled to changes in functional activity.

Development of male and female accessory sexual glands is described in terms of the respective roles of epithelium and mesenchyme. During embryonic and neonatal periods mesenchyme alone exhibits androgen receptor activity (nuclear androgen binding sites) and is the actual target and mediator of the morphogenetic effects of androgens upon the epithelium. Mesenchyme induces specific patterns of epithelial morphogenesis, cytodifferentiation, and probably also specifies the functional (biochemical) activities of the epithelium. Mesenchymal influence upon expression of epithelial characteristics occurs in the perinatal period during morphogenesis, but also plays an important role in adulthood by maintaining favorable conditions for maintenance of epithelial morphology and function. Morphogenetic processes in adult hormone-dependent organs are though to be mediated by stromal cells.

The splenic macrophages of rats and mice were studied by light and fluorescence microscopy to determine their phagocytotic uptake of carbon and neutral polysaccharide (Fic-F), and their lysosomal enzyme activities. In rats, the large macrophages of the marginal zone (MZ) showed a moderate to strong acid phosphatase activity, and took up most of the Fic-F, even though they showed a weak phagocytotic activity to carbon particles. Red-pulp macrophages, however, ingested a large quantity of carbon particles, and are considered to be the major scavengers in the rat spleen. In contrast, the MZ macrophages in the mouse spleen were the major scavengers and showed a vigorous uptake of both carbon and Fic-F. In rats, the marginal metallophils (MM), located at the outer border of the periarterial lymphatic sheath and boundary between the MZ bridging channel and surrounding tissue, ingested Fic-F, whereas those located around the follicular area did not. In mice, on the other hand, the MM never ingested Fic-F. Lightly carbon-ladened small cells were constantly seen in the MZ of both rats and mice. They showed little acid phosphatase activity and did not ingest Fic-F. They were also present in the blood circulation.

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