Patterns of cell division, cell death and chondrogenesis in cultured aggregates of normal and talpid³ mutant chick limb mesenchyme cells

Aggregates were prepared from dissociated mesenchyme cells obtained from normal and talpid³ mutant chick limb buds at stage 26 and were maintained for 4 days in culture. They were shown by autoradiographic techniques to consist initially of populations of uniformly dedifferentiated cells within which chondrogenesis was initiated between 1 and 2 days, leading to the formation of areas of precartilage in the interior of the aggregates.

Measurements of cell population density, cell death and cell division were made in precartilage and non-cartilage regions on sections prepared from normal and mutant aggregates fixed at 1-day intervals and were related to the pattern of chondrogenesis.

Non-cartilage areas consisted of cells surrounding the precartilage areas and extended to the surface of the aggregate; these cells showed no special pattern or histochemical reaction. Precartilage areas consisted of one or more ‘condensations’, comprising cells arranged in concentric rings around a central cell or group of cells, characterized by uptake of [³⁵S]sulphate and taking up alcian blue stain in the intercellular matrix. Chondrogenesis was initiated at the condensation foci and spread centrifugally.

Condensations were arranged in a simple pattern, roughly equidistantly from each other and never at the surface of the aggregate. The shape and arrangement of the cells comprising them suggested that they were formed by a process of aggregation towards the condensation foci. The relation of these observations to events in the intact limb bud developing in vivo is discussed.