An analysis of the condensation process during chondrogenesis in the embryonic chick hind limb.

An analysis has been made of the pre-cartilaginous condensation stage in the development of the femur and tibia/fibula skeletal blastemata of the embryonic chick hind limb. Light microscopy serial sections were used to 'map' the mesenchymal cell condensations of both myogenic and chondrogenic anlagen in the limb-bud from stages 22 to 26 (Hamburger & Hamilton, 1951). Cell counts reveal that an increase in mesenchymal cell number per unit area occurs in the central chondrogenic locus at stage 24 (4 1/2 days) prior to matrix formation. Electron microscopy, using a simultaneous double fixation with osmium and glutaraldehyde, reveals that the pre-chondrogenic cells are characterized by large areas of close surface contact between adjacent cells, as compared with the extensive intercellular spaces associated with undifferentiated mesenchymal cells. The results are discussed and related to other investigations of in vivo chondrogenesis and to analyses of cellular events during in vitro chondrogenesis. These observations are consistent with the theory that condensations are formed by a process of aggregation rather than by localized increased mitosis.