A Ventral Ectodermal Ridge of the Tail in Mouse Embryos

Abstract

THE limb buds of mammalian and avian embryos show a thickening of the ectoderm on the margin of the foot plates, the so-called apical ectodermal ridge, and it is known that this ridge plays an important part in the growth and differentiation of the limbs^1,2. It has recently been discovered that a similar thickening of the ectoderm is present on the ventral side of the tail in 10- and 11-day old mouse embryos. In 10-day old embryos (crown-rump length 3.4–4.2 mm. ; lens invaginations still widely open), there occurs a very conspicuous zone of columnar epithelium on the ventral aspect which extends from the very tip of the tail in a proximal direction for roughly 250 micra ; the exact length of the zone is difficult to determine as it gradually disappears proximally. The accompanying photomicrographs (Fig. 1) are four transverse sections through the tail of such an embryo (crown-rump length 4.1 mm. ; Bouin fixation ; hæmatoxylin and eosin ; 7.5 micra ; × 125) at intervals of about 68 micra ; (a) is the most distal and (d) the most proximal of these sections. The columnar zone of epithelium is obvious in the first three sections but has disappeared in the fourth. In 11-day old embryos (crown-rump length 5.3–5.7 mm.), the structure is rather less conspicuous, and in one out of three such embryos it occupied a position somewhat proximal to the tail tip. In 12-day old embryos the structure has virtually disappeared.