Ultrastructural changes in the interhaemal membrane and junctional zone of the murine chorioallantoic placenta across gestation

The mouse is an extremely useful experimental model for the study of human disease owing to the ease of genetic and physiological manipulation. A more detailed knowledge of murine placental development will, we hope, increase our understanding of the pathogenesis of placentally related complications of human pregnancy. The murine placenta consists of two main fetally derived compartments: the labyrinthine zone and the junctional zone. Exchange in the labyrinthine zone takes place across an interhaemal membrane comprising an outer layer of cytotrophoblast cells and two inner layers of syncytial trophoblast. The cytotrophoblast layer thins as gestation advances, and in addition becomes highly perforated after embryonic day (E)12.5. Furthermore, as gestation advances cytotrophoblast nuclear volume and DNA content increase, suggesting the formation of labyrinthine trophoblast giant cells. The syncytial layers become increasingly microvillous, enlarging the surface area for exchange. Separate basement membranes support the syncytium and the fetal capillary endothelium throughout gestation, although these appear to fuse where the capillaries are closely approximated to the trophoblast. The junctional zone consists of two principal trophoblast cell types, spongiotrophoblasts and invasive glycogen cells, yet the functions of each remain elusive. Spongiotrophoblasts vary in their appearance even when not fully differentiated, but a striking feature is the extensive endoplasmic reticulum of the more mature cells. Early glycogen cells are distinguished by the presence of electron-dense glycogen granules, and large amounts of surrounding extracellular matrix. Later the accumulations of glycogen granules occupy almost all the cytoplasm and there are few organelles. This is the first study to use both scanning and transmission electron microscopy in an ultrastructural description of murine placental development and is complementary to contemporary genetic investigations

Fate of the nucleolar vacuole during resumption of cell cycle in pea cotyledonary buds

Meristematic cells of pea cotyledonary buds blocked in G(0-1) state contain a small nucleolus with a large central clear area surrounded by a fibrillar rim. The nucleolar structure varies according to the cell cycle from the G(0-1)-blocked state until the first mitoses occurring between 24 and 27 h after removal of the main stem. In order to better identify and understand the role of the central area in the nucleolar function, its content was investigated by cytochemical and terminal deoxynucleotidyl transferase-immunogold methods. The central area showed the characteristics of a vacuole commonly constituted of the condensed chromatin, ribonucleoprotein granules, and lack of argyrophilic proteins. 3 h alter decapitation, a thickening of the fibrillar rim occurred, accompanied by an increase of granules in the vacuole. After 6 h, the unique vacuole broke up into two to four small vacuoles in which the granules are more abundant. After 12 h the nucleolus acquired compact structure with few minute vacuoles dispersed over the fibrillar component. During the whole cell cycle, the condensed chromatin is always observed in the vacuole. Our findings suggest that the appearance of the vacuoles is subsequent to the output of preribosomes from nucleolus. These vacuoles might play a role in condensation and decondensation of the chromatin