VENTRAL ECTODERMAL RIDGE AND VENTRAL ECTODERMAL GROOVE - 2 DISTINCT MORPHOLOGICAL FEATURES IN THE DEVELOPING RAT EMBRYO TAIL

The ventral ectodermal ridge (VER) is a thickening of the surface ecto derm on the ventral side of the embryonic tail which resembles the api cal ectodermal ridge of the limb bud. The morphological characteristic s of the ventral part of the embryo tail were investigated in 10.5- to 14-day rat embryos by light microscopy of serial semithin sections an d by scanning and transmission electron microscopy. In 10.5- to 11.5-d ay embryos the thickening of the ventral surface ectoderm includes the complete ventral midline of the tail and can be divided into two part s. The posterior part is elevated and represents the ventral ectoderma l ridge. The anterior part is, in contrast to the ridge, concave, and we have termed it the ventral ectodermal groove (VEG). The cloacal mem brane is located at its anterior end. Contacts between the VER and the mesenchymal cells are visible until an intact basal lamina is formed at 11.5 days. Similarly, the VEG is connected by elongated cell proces ses with the ventral part of the tail gut. Gap junctions are present b etween the apical parts of ridge and groove cells. The VEG flattens an d disappears in 12-day embryos. At this stage the ridge is at its maxi mum height, simultaneously undergoing extensive cell death. The VER is no longer visible in 14-day rat embryos.