TYPE-II COLLAGEN DISTRIBUTION DURING CRANIAL DEVELOPMENT IN XENOPUS-LAEVIS

Epithelially expressed type II collagen is thought to play a prominent role in the embryonic patterning and differentiation of the vertebrat e skull, primarily on the basis of data derived from amniotes. We desc ribe the spatiotemporal distribution of type II collagen in the embryo nic head of the African clawed frog, Xenopus laevis, using whole-mount and serial-section immunohistochemical analysis. We studied embryos s panning Nieuwkoop and Faber (1967) stages 21-39, a period including cr anial neural crest cell migration and ending immediately before the on set of neurocranial chondrogenesis. Xenopus displays a transient expre ssion of type II collagen beginning at least as early as stage 21; sta ining is most intense and widespread at stages 33/34 and 35/36 and sub sequently diminishes. Collagen-positive areas include the ventrolatera l surface of the brain, sensory vesicles, notochord, oropharynx, and i ntegument. This expression pattern is similar, but not identical, to t hat reported for the mouse and two bird species (Japanese quail, domes tic fowl); thus epithelially expressed type II collagen appears to be a phylogenetically widespread feature of vertebrate cranial developmen t. Consistent with the proposed role of type II collagen in mediating neurocranial differentiation, most collagen-positive areas lie adjacen t to subsequent sites gf chondrogenesis in the neurocranium but not th e visceral skeleton. However, much of the collagen is expressed after the migration of cranial neural crest, including presumptive chondroge nic crest, seemingly too late to pattern the neurocranium by entrapmen t of these migrating cells.