The development of the perichondrium in the avian ulna.

The development of the perichondrium surrounding the ulna of chick embryos was examined between stages 26 and 34 (Hamburger & Hamilton, 1952), i.e. 5.5-8 d of incubation. The first appearance of a barrier between cartilage and adjacent tissue was observed at stage 27 when cells were arranged circumferentially around the central flattened core, but no true perichondrium was seen until chondrocyte hypertrophy began at stage 30. At this stage, cells in the barrier became elongated parallel to the long axis of the rudiment and overlapped. With further cell hypertrophy, the perichondrium consolidated and by stage 31 a distinct, compact structure, several cells thick and exhibiting cell-cell contacts had formed around the hypertrophic cell region. By stage 32, overlapping perichondrial cells could be seen surrounding the flattened cell region and this structure consolidated as cell hypertrophy spread from the centre towards the ends of the rudiment. The rounded cell zone at the epiphyses never formed a distinct perichondrium during the time course of this study. This differential structure of a compact perichondrium surrounding the hypertrophic cell region, a loose perichondrium surrounding the rounded cell region and an intermediate perichondrium surrounding the flattened cell region may facilitate the morphogenesis of the rudiment by acting as a constraining sheath restricting radial expansion but favouring longitudinal expansion.