This article reviews the current state of knowledge concerning morphological and physiological mechanisms important to growth and differentiation of the mammalian blastocyst between compaction and implantation. Morphological processes occur in conjunction with major changes in transport systems that control the movement of substances into and out of the embryo. Compaction is a morphological development that is associated with the formation of an outer squamous epithelium, the trophectoderm, which regulates the composition of the medium bathing the presumptive embryo (the inner cell mass). Implantation involves the interaction of two epithelia, the adhesion between the trophectoderm and the maternal endometrium. Before adhesion, the blastocyst lies free in the uterine fluid and exchanges occur between this fluid and the embryo. Apposition of these epithelia is brought about in part by expansion of the blastocyst and removal of the uterine fluid. Blastocyst physiology is an inherently important field because vectorial transport system development and the genes that regulate it can be studied.
The effects of inositol-1,4,5-trisphosphate (IP3) and of diacylglycerol (DAG) and its analogues on the membrane potential of eggs from the sea urchin Strongylocentrotus purpuratus were examined. Injection of IP3 into eggs resulted in a change in membrane potential that was similar in magnitude and time course to the fertilization potential elicited by sperm attachment. In low-calcium seawater, IP3 injection elicited a partial response. DAG and its analogues phorbol myristyl acetate and 1-oleoyl-2-acetylglycerol did not affect membrane potential either when applied by perfusion or when injected. The results indicate that IP3, but not DAG or its analogues, may be involved in the generation of the fertilization potential triggered by the interaction of sperm with sea urchin eggs.
Amiloride-sensitive Na+ channels were localized in semithin frozen sections of rat renal medullary collecting ducts, using polyclonal antibodies directed against purified bovine kidney Na+ channel protein. The apical plasma membrane of collecting duct principal cells was heavily stained by indirect immunofluorescence, whereas intercalated cells were negative. Basolateral plasma membranes of both cell types were unstained, as were subapical vesicles in the cytoplasm of these cells. In the thick ascending limb of Henle, some scattered granular fluorescence was seen in the cytoplasm and close to the apical pole of epithelial cells, suggesting the presence of antigenic sites associated with some membrane domains in these cells. No staining was detected in thin limbs of Henle, or in proximal tubules in the outer medulla. These results show that amiloride-sensitive sodium channels are located predominantly on the apical plasma membrane of medullary collecting duct principal cells, the cells that are involved in Na+ homeostasis in this region of the kidney.
Summary. Transtrophectodermal 3-0-methyl glucose (3-0MG) transport in the rabbit blastocyst at Days 6 and 7 post coitum was investigated to understand better how the trophectoderm can regulate inner cell mass growth by controlling substrate availability. 3-0MG rapidly traversed the trophectoderm and displayed saturation kinetics (Km = 4\m=.\3 \ m=+-\ 0\m=.\5 mM, Vmax =79 \m=+-\ 3\m=.\8 nmol.cm\p=n-\2). The flux of 3-0MG was inhibited nearly 95% by 10\p=n-\4M-phloretin, and only 15% by 10\p=n-\4 M-phlorizin. Furthermore, 3\ x =r eq-\ OMG influx was inhibited by cytochalasin B (5 \g=m\M) and was unaffected by removal of sodium. The transport system had a high specificity for 2-deoxy-D-glucose and glucose, and a very low specificity for fructose and 4-\g=a\-methylglucoside. Western blots probed with a polyclonal antibody to the human erythrocyte glucose transport protein and also with a polyclonal antibody to the C-terminus of the glucose transport protein of the rat brain revealed a broad band with a molecular weight of 55 000. Using immunogold labelling techniques, Na +-independent glucose transporters were localized to both the apical and basolateral borders of the trophectodermal cell. These results suggest that the mechanism in the trophectoderm responsible for transport of glucose is similar to other sodium-independent glucose transport systems. In addition, 3-0MG influx was unaffected by short-term incubation with progesterone, the progesterone antagonist mifepristone (RU-486), PGF-2\g=a\,PGE-2, insulin, or cAMP. Day-7 p.c. embryos also transported hexoses by a similar system because the influx rate and the phlorizin/ phloretin sensitivity were the same as in the Day-6 p.c. embryo.
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