Philippe Sengel

Early regionalization of the somitic mesoderm as studied by the development of the axial skeleton of the chick embryo

Abstract The excision of a portion of unsegmented somitic mesoderm from the thoracic region has no or little effect on the development of vertebrae and ribs. Its orthotopic replacement by thoracic somitic mesoderm obtained from another embryo does not cause important deficiencies of the rib basket. But its replacement by nonsomitic tissue (neural tube, midgut, somatic mesoderm) not only causes the formation of a costal gap within the thoracic region, but also leads to the production of a defective spine on the operated side. Segmented or unsegmented somitic mesoderm from the cervical region, implanted in place of unsegmented thoracic somitic mesoderm, differentiates according to its origin: no ribs develop in the host embryo at the operation level. Conversely, segmented or unsegmented somitic mesoderm from the thoracic region, implanted in place of unsegmented cervical somitic mesoderm, also differentiates according to its origin, giving rise to supernumerary ribs in front of the hosts rib basket. These extra ribs, however, always lack a sternal component. It is concluded that the level-specific morphogenetic capacity of the cervical and thoracic somitic mesoderm is determined before metamerization occurs. Despite this early determination, the unsegmented somitic mesoderm is endowed with regulative properties.

Peridermal cell patterning in the feather-forming skin of the chick embryo

Abstract The shape, distribution, and orientation of peridermal cells were examined in the dorsolumbar skin of 7 1 2 - day chick embryos. Most feather rudiments of middorsal and lateral rows showed a marked cephalocaudal polarity. A similar polarity was found in the prospective rudiments of skin areas lateral to the last-formed row. On the cranial slope and apex of rudiments, cells are convex and predominantly elongated at right angles with respect to the cephalocaudal axis, whereas on the caudal slope, most cells are flat, polygonal, surrounded by a border-line ridge, and oriented predominantly with their long axis parallel to the cephalocaudal axis. The significance of this pattern is discussed in view of the fact that the epidermis is the determinant tissue in feather orientation.

Effect of cytochalasin B on the evagination in vitro of leg imaginal discs

Abstract Cytochalasin B (1 μg/ml) completely inhibited the evagination of isolated leg imaginal discs cultured in vitro in a synthetic medium (ME) containing α-ecdysone (3 μg/ml). In discs precultured for 6 hr in medium ME without the drug, then transferred to cytochalasin B-containing medium, continuation of evagination was stopped immediately. The inhibition of evagination was completely reversible, provided pretreatment with cytochalasin B did not exceed 8 hr. Results are discussed in view of what is known on the effect of cytochalasin B on other developmental systems. Findings are compatible with the primary action of the drug being an alteration of cell surface properties, thus bringing to light the importance of these properties in the course of normal imaginal disc evagination.