The cellular basis of cartilage morphogenesis in embryonic chick limbs.

Cartilagineous long bone rudiments, of the chick embryo, were used as a model for the mammalian epiphyseal growth plate. Both contain 3 zones of chondrocytes which, therefore allows the cartilege rudiment to be considered as an expanded growth plate. The involvement of each zone in the growth of the rudiment was determined at the cellular level, by counting cell numbers in histological sections of the ulna. Studies in the cell kinetics of the rudiments, in vitro, demonstrated that cell division was confined, mainly to the zone of rounded cells, with little or no division observed in the zones of cell flattening and hypertrophy.
It is proposed that the morphogenesis of the early cartilage long bone rudiment is influenced by the structure of it's surrounding perichondrium, rather than by propert intrinsic to the constituent chondrocytes. The perichondrium is thought to exert its influence though a process termed 'directed dilation', whereby circumferential expansion is resisted and longtitudinal growth is favoured. Ultrastructural examinations show that the perichondrium of a long bone rudiment has a variable structure: distinct at the diaphysis and loose at the epiphysis. By contrast, the perichondrium surrounding heckel's cartilage, which has only one type of chondrocyte, appears to have a uniform structure.
Evidence for the lack of intrinsic property determining morphogenesis comes from the observation that chondrocytes, from various cartilage elements, behave identically in vitro, underappropriate conditions. In addition, contrary to reports that that the expression of a cartilage phenotype is dependant on high cell densities and histogenic interactions, it is proposed that the maintenance of a rounded cell configuration is sufficient to elicit phenotypic expression.
Experimental evidence suggests that the 3 zones of cells are set up by combination of interactions with the perichondrium and by a signal specific to the cartilage matrix.

A Thesis submitted for the degree of Doctor of Philiosophy.