Ultrastructure of mouse egg cylinders developed in vitro.

Abstract

AbstractMouse blastocysts were grown in vitro and the ultrastructure of resulting egg cylinders was compared with the ultrastructure of egg cylinders isolated from uterus. Egg cylinders grown in vitro had two or three germ layers and were attached to the bottom of the dish through the layer of trophoblastic cells. The entoderm was composed of two cell types intermixed haphazardly: (1) One type had numerous microvilli on the free surface and a large number of dense bodies, autophagic vacuoles, big and small vacuoles and myelin figures in the cytoplasm. (2) The other type had rich, rough endoplasmic reticulum with wide cisternae. The cisternae were filled with fine, granular material similar to that found as a thick membrane separating the entodermal layer from the rest of the embryo. The first type of cell was ultrastructurally similar to cells of the visceral entoderm of egg cylinders isolated from uterus and the second type was similar to cells of the parietal entoderm. The thick basement membrane observed in embryos grown in vitro was similar to Reichert's membrane. Parietal entoderm and Reichert's membrane were never found as separate structures in embryos grown in vitro. It is probable that the entodermal cells in blastocysts differentiate in vitro into both parietal and visceral entodermal cells within the same layer. Mesodermal and ectodermal cells of vitral embryos were similar to such cells in the egg cylinders isolated from the uterus. The cytoplasm was filled with free ribosomes in the form of polysomes and also contained a few profiles of endoplasmic reticulum, well developed mitochondria and Golgi apparatus. Intercellular spaces between mesodermal cells were large and the cells were not attached to one another. Ectodermal cells were tightly packed and inter‐connected with numerous desmosomes. The ultrastructure of egg cylinders in vitro and in vivo was similar. Observed differences might be caused by culture conditions and some of them probably represented the changing pattern of differentiation in vitro.