[Effects of vinblastine on the morphogenesis of the chick embryo limb. Histological aspects]

Vinblastine was injected into the amniotic cavity of 6-5-day-old chick embryos. Acropods were fixed 0, 2, 3-5, 6, 12, 18, 24, 36 and 48 h after treatment and 1 mum thick sections were prepared from the region of digit IV and interdigit III-IV. Cell counts were mainly performed in a distal zone (see Fig. 2) comprising the ectodermal apical ridge, the distal non-ridge ectoderm and the distal underlying mesoderm. In control non-treated embryos, the mitotic index does not vary significantly in either of the three tissues between 6-5 and 8 days except for a temporary increase at 7 days in the mesoderm. In treated embryos, the mitotic index increases rapidly in the non-ridge ectoderm and in the mesoderm to a maximum of 44 and 35% respectively, 18 h after injection, after which it decreases almost as rapidly to a level of about 15% 36 h after injection. In the apical ridge ectoderm, the increase of the mitotic index is much slower and reaches its maximum value of about 30% only 36 h after injection. This suggests that most of the cells participating in the AER do not arise within the ridge but are generated in more proximal zones of the ectoderm. Due to mitotic arrest, the number of mesodermal cells is thus decreased to approximately half the normal value within 36 h and the AER disappears earlier than normal. These alterations are the direct cause of the hypophalangy observed at the morphological level. A particular distribution pattern of arrested mitoses is revealed: in the digital as well as in the interdigital mesoderm, the majority of blocked metaphases is located in a dorsal and a ventral wing-shaped subectodermal 500 mum long area whose thickness is maximal at the level of the marginal sinus and gradually decreases in proximal direction. In the central-part of the mesoderm blocked mitoses are relatively rare. The present results are discussed in view of the recent literature on the morphogenetic processes of limb development.