King B F
Anat Rec. 1982 Feb;202(2):221-30. doi: 10.1002/ar.1092020207.
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.
卵黄囊上皮细胞具有从子宫腔中进行大分子的内吞吸收以及维持母体（子宫）腔室与下方胎儿腔室之间渗透屏障的功能。在本研究中，使用冷冻蚀刻技术检查了豚鼠卵黄囊的细胞膜和细胞间连接。在微绒毛、微绒毛间膜和内吞小窝中检查了膜内颗粒（IMP）的分布和大小。P面断裂面上的IMP在这些膜区域内分布不均匀，且IMP有几种大小。此外，短丝状链有时在内吞小窝膜的细胞质和E面之间桥接。这些结果表明，与其他膜区域相比，卵黄囊细胞膜参与内吞作用的区域在IMP数量或分布上并无异常。内胚层细胞之间的细胞间连接由紧密连接、缝隙连接和桥粒组成。紧密连接通常由四或五条相互连接的链或嵴组成，在三个细胞相互邻接的地方发育得很精细。缝隙连接不常见、较小，且常与紧密连接相关。桥粒既表现为典型的斑状结构，也表现为更广泛、细长的结构。这些结果证实了先前对卵黄囊的超薄切片研究表明存在紧密连接。然而，紧密连接的发育程度与卵黄囊上皮细胞跨膜电位差的缺乏情况并无关联。缝隙连接的存在与先前对其他卵黄囊的冷冻蚀刻研究结果不符。
