A freeze‐fracture study of the guinea pig yolk sac epithelium

Abstract

The yolk sac epithelium functions in endocytic absorption of macromolecules from the uterine lumen and in maintaining permeability barriers between the maternal (uterine) compartment and underlying fetal compartments. In this study, cell membranes and intercellular junctions of the guinea pig yolk sac were examined using the freeze-fracture technique. Intramembranous particle (IMP) distribution and size were examined in microvilli, intermicrovillous membrane, and endocytic pits. IMPs on the P-fracture face were not evenly distributed within these membrane domains and the IMPs were of several sizes. In addition, short filamentous strands sometimes bridged between the cytoplasm and the E-face of endocytic pit membranes. These results indicated that the regions of yolk sac cell membrane involved in endocytosis did not have unusual numbers or distributions of IMPs compared to other membrane regions. The intercellular junctions between endoderm cells consisted of zonulae occludentes, gap junctions, and desmosomes. The zonula occludens usually consisted of four or five interconnected strands or ridges, and were elaborately developed where three cells abutted one another. Gap junctions were infrequent, small, and often associated with the zonula occludens. Desmosomes were observed both as typical macular structures and as more extensive, elongated structures. These results confirm previous thin-section studies of yolk sac indicating the presence of tight junctions. However, the extent of development of tight junctions does not correlate with the regard absence of a potential difference across yolk sac epithelium. The presence of gap junctions does not confirm results of previous freeze-fracture studies of other yolk sacs.

Transmission of Substances through the Exocoelomic Cavity after Introduction into the Vitelline Circulation by Cannulation: What Route?

“…The lack of lanthanum nitrate on the apical surfaces of these endodermal cells is confirmatory of the fact that these substrates do not become available to the apical surfaces of these cells where uptake of substances occurs [Jollie and Triche, 1971;Hollie, 1986]. As has been demonstrated in the guinea pig [King, 1982], rat, hamster and mouse [Carpenter and Dishaw, 1979] and other mammalian species, for example the monkey [Bremer et al, 1985], the endodermal cells of the yolk sac separated at their apical surfaces by impermeable junctional complexes comprising zonulae occludentes and nexuses. Since these junctional complexes prevent substances from gaining access to the basolateral surfaces of the cells from the apical surfaces, they probably also prevent any solutions between the basolateral surfaces of contiguous cells from reaching the apical surface.…”

Dinâmica da inversão do saco vitelino em preás (Galea spixii Wagler, 1831)

“…Como consequência, o endoderma visceral passa a formar uma interface com o tecido materno, tornando-se exposto as secreções produzidas pelas glându-las uterinas (King 1982, Mossman 1987, Jollie 1990.…”