Morphological and histochemical events during first bone formation in embryonic chick limbs.

Staged embryos from White Leghorn chicken eggs were used to assemble a detailed morphological, cellular and molecular picture of the complex events of first-bone formation. To provide these details, light and electron microscopic, histochemical and immunocytochemical techniques were used to establish a temporal sequence for long bone development in chick wing and leg from Hamburger-Hamilton stage 29 through stage 35. Three distinctive cell regions can be morphologically identified by stage 28 (leg) or 29 (wing) at the mid-diaphysis. These regions are: 1. an outer grouping of loose mesenchymal and myogenic cells, 2. an osteoprogenitor layer which will later divide to maintain this progenitor layer in a brickwork or stacked configuration and to produce round, tightly packed osteoblasts, and 3. a core (rod) of cartilage. First bone is laid down just outside the cartilage core, initially as a layer of Type I collagen-rich osteoid which later becomes mineralized. Vascular elements then come to reside above this mineral layer, and osteoid is laid down between vascular elements and eventually above them to form a second layer of trabecular bone. As this radial formation of layers of bone is progressing, so too is the proximal and distal expansion of the first bone forming process. A model is presented which considers that chondrogenic and osteogenic cell commitment occur simultaneously in early limb development and that it is the expression of the osteogenic phenotype which governs the boundaries of cartilage development. Importantly, the vasculature plays a key role in the patterning of bone formation well before it enters the cartilaginous core at stage 35 and participates in the erosion of the core. While this report is restricted to events occurring through stage 35, it relies on data presented in a companion report detailing later bone development and remodeling (Pechak et al; Bone 1986) and emphasizes that the cartilage model does not provide the scaffolding for bone but rather defines the marrow space.