Vasculogenesis and the induction of skeletogenic condensations in the avian eye.

Blood vessels form via two distinct mechanisms: vasculogenesis, the formation of new blood vessels; and angiogenesis, the remodeling of preexisting blood vessels to form mature vasculature. Little research, however, focuses on the relationship between blood vessels and skeletogenic condensations, a key step in bone formation. Here, the development of the scleral ossicles in the chick begins with the induction of a neural crest-derived condensation at HH Stages 35 and 36 by overlying papillae in a 1:1 pattern. These papillae, which are epithelial thickenings of the conjunctiva, begin to form at HH Stage 30, following a distinct pattern. Nothing is currently known about their induction, or patterning. As the first papilla always forms above the ciliary artery, we mapped blood vessel development in the eye between HH Stages 28 and 36.5 using camera lucida drawings, fluorescence microscopy, and histology. Our results show that a blood vessel meshwork begins to form de novo once the ring of conjunctival papillae is complete (HH Stages 34 through 36) suggesting no direct correlation between these two events. We also observe an avascular zone beneath each conjunctival papilla, which is first visible at HH Stage 35, coinciding with the onset of induction of the skeletogenic condensations. Importantly, our findings suggest that remodeling of the vasculature and development of the avascular zones occurs at the same time as induction, but prior to the presence of the skeletogenic condensations of the intramembranous bones; this process is dissimilar to that documented for endochondral ossification in avian limb buds.