MORPHOLOGY OF INCIPIENT MESODERM FORMATION IN THE RABBIT EMBRYO - A LIGHT-MICROSCOPIC AND RETROSPECTIVE ELECTRON-MICROSCOPIC STUDY

Mesoderm formation is a hallmark of vertebrate gastrulation and, at th e same time, one of the prime examples for epithelio-mesenchyma transf ormation. Recent advances in experimental embryology and molecular bio logy have clarified the role of growth factors and genes in this proce ss; however, its microscopic anatomy in higher vertebrates is still fa r from clear. Therefore, the present study describes the morphology of mesoderm formation in the rabbit embryo, a species which may be repre sentative for both the avian and the mammalian embryo in this respect. Serial semithin sections were correlated with topographical landmarks in surface views of embryonic discs at 6.4, 6.5, and 6.6 days post co nceptionem, and selected semithin sections were reembedded for ultrast ructural analysis. Mesoderm cells are shown to be generated by ingress ion of bottle-shaped epiblast cells in the area of the posterior node and the primitive streak. Here, basal endocytotic pits and absence or discontinuity of the basal lamina are taken as suggestive evidence for specific removal of extracellular matrix material. Within the bottle- shaped cells most organelles are concentrated in a narrow apical neck which will subsequently constitute the 'trailing end' of the ingressin g mesoderm cells. These features support the assumption that most prin ciples of epithelio-mesenchymal transformation seen during primary mes enchyme formation in the sea urchin also apply to mesoderm formation i n vertebrates. However, transient tripartite zonula adherens-type junc tions are formed apically between ingressing mesoderm cells and the ne ighboring epiblast cells. They are interpreted here as being responsib le for maintaining supracellular integrity of the embryonic disc durin g the shedding of mesoderm cells in the amniote embryo.