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The present study examined the tissue-expression of MRP8 in human placenta using a biotinylated DNA-probe for in situ hybridization. During the first and second trimesters high level and synchronous expression of MRP8 was detected in cytotrophoblasts (Langhans' cells), placental-tissue macrophages (Hofbauer cells), fibroblast-like cells, endothelial cells and monocytic lineages in the foetal capillaries. The highest expression was seen in large and oval-shaped cytotrophoblasts and stromal-cell populations at around 8-11 weeks. At term placentas had low level MRP8 expression chiefly in the myelomonocytic lineages in foetal blood vessels. The peripheral monocytes in the maternal space also expressed MRP8 at high levels during the first and second trimesters, which subsequently decreased at term. We suggest three hypotheses based on these results; (1) The initial expression of MRP8 may occur in two cell lineages of extra-embryonic and intra-embryonic origin in the first two trimesters; (2) the cytotrophoblasts, placental-tissue macrophages and fibroblasts may play important roles in the production of placental hormones and the immuno-regulation of foetal acceptance; and (3) MRP8-expression may be synchronously inhibited once the trophoblasts and stromal cell-constituents have differentiated in the chorionic villi.

Immunohistochemistry revealed initial expression of the stage-specific glycoprotein, GP68, in various mesenchymal tissue substructures of mouse embryos. During the 11-15th days of gestation, GP68 was localized in the primitive meninges, chondroblasts and perichondrium of pre-cartilaginous vertebral bodies and ribs, connective tissue cells of the dermis, the epicardium and endocardium of the heart, the epimysium and perimysium of skeleton musclature, and the basement membranes of splanchnic organs. Double staining for laminin expression indicated coincidental expression in identical tissue substructures. However, laminin was expressed in days 10-18 embryos and the neonate. Therefore, GP68 is coincidentally expressed with laminin in mesenchymal tissues between the 11th and 15th day of gestation, and may play a role as a laminin-associated protein. In the light of these results, a hypothesis concerning the relationship between these two proteins and the mechanisms of non-integrin laminin-associated proteins during normal embryogenesis is discussed further.

Vitellogenin has been localized at the electron microscopical level in the liver of the cockerel using a colloidal gold technique. White leghorn cockerels were treated with 17 beta-oestradiol to induce vitellogenesis. Pieces of liver were removed from control and experimental birds on the 4th and 8th days following hormone treatment, and embedded in Lowicryl K4M. Vitellogenin was isolated from the plasma of oestradiol-treated cockerels, and the antibody to it elicited in rabbits and made vitellogenin-specific by affinity chromatography on lipovitellin-Sepharose columns. At the light microscopical level, the intensity of immunohistochemical staining was considerably above background levels in oestradiol-treated cockerels. At the electron microscopical level, gold particles indicating antigenic sites of vitellogenin were largely confined to the rough endoplasmic reticulum, Golgi apparatus, immature and lysosomes and phagosomes within hepatocytes and sinusoidal cells respectively. These observations strongly suggest that the intracellular pathway of vitellogenin secretion in chicken hepatocytes under the experimental conditions studied involves external stimuli and secretory vacuoles. The labelling of lysosomes may reflect catabolic turnover (crinophagy).