Ventral ectodermal ridge and ventral ectodermal groove: two distinct morphological features in the developing rat embryo tail.

The ventral ectodermal ridge (VER) is a thickening of the surface ectoderm on the ventral side of the embryonic tail which resembles the apical ectodermal ridge of the limb bud. The morphological characteristics 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 and by scanning and transmission electron microscopy. In 10.5- to 11.5-day embryos the thickening of the ventral surface ectoderm includes the complete ventral midline of the tail and can be divided into two parts. The posterior part is elevated and represents the ventral ectodermal ridge. The anterior part is, in contrast to the ridge, concave, and we have termed it the ventral ectodermal groove (VEG). The cloacal membrane 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 processes with the ventral part of the tail gut. Gap junctions are present between the apical parts of ridge and groove cells. The VEG flattens and disappears in 12-day embryos. At this stage the ridge is at its maximum height, simultaneously undergoing extensive cell death. The VER is no longer visible in 14-day rat embryos.