Cell cycle analysis of facial mesenchyme in the chick embryo

Abstract

ABSTRACT The spatial distribution and developmental fate of quiescent and/or slow cycling cell populations of the primary palate were studied employing label-dilution techniques. -day-old chick embryos were labelled sequentially for 12 h with [3H]thymidine and then chased with cold thymidine. The embryos were reincubated to continue development and were subsequently sacrificed at intervals from the end of labelling at 4 days to 14 days of incubation (10 days after labelling) and processed for autoradiography. Retention of label was used as the assay for identification of quiescent and/or slow-cycling cells. Grain density over the nuclei of labelled cells was determined in the maxillary process and the roof of the stomodeum. Cells with a label density at later time points comparable to that found in cells immediately after the labelling period were defined as label-retaining cells, i.e. those which had become quiescent or had significantly altered generation times. The location and developmental fate of these cell populations were confirmed using slides containing adjacent sections stained with Nuclear Fast Red, Alcian Blue, and Tartrazine. The results demonstrated an association between label-retaining cells and chondrogenic differentiation in the roof of the stomodeum. Subpopulations of label-retaining cells (quiescent and/or slow cycling) which we believe to be prechondroblasts appeared in the chondrogenic region of the roof of the stomodeum prior to, or coincident with, cartilage formation. The retention of label, as evidenced by comparison of nuclear grain counts at the end of the labelling period with subsequent time points, indicated that a cell cycle block may have occurred in the prechondroblastic cell population. The block lasted until these cells expressed the chondrogenic phenotype, after which they resumed cell division.