Changes of ribonucleoproteic structures of embryonic epidermal cell nuclei during differentiation and maturation

The nuclear structure of the epiblast and skin cells was studied during the development of the chick embryo (Gallus domesticus). Ribonucleoproteic structures (RNPs) and chromatin were characterized with preferential staining techniques. The changes of the frequency of a type of RNP particle, the perichromatin granule, from gastrulating embryos to newly hatched chick, were analyzed by electron microscopic quantitative stereology. The changes of nuclear and nucleolar volume were estimated using the light microscope. The differentiation of ectoderm from epiblast is characterized by a significant increase in the nucleolar volume and in the frequency of perichromatin granules. These features suggest that ectoderm differentiation involves a general increment of mRNA and rRNA transcription. At the beginning the synthesis of a sulfhydryl‐rich protein, which is coincident with the thickening of the bilayered epithelium to a multilayered epithelium, a large increase of the numerical density of perichromatin granules not accompanied by any modification of the nucleolar volume was found. The same changes characterize the acquisition of epidermal mature cell features immediately after hatching. These findings as well as previous results concerning the differentiation of central nervous system and striated muscle suggest that the increases of the number of perichromatin granules take place during the acquisition of new cytophysiological states related to functional maturation and not to early stages of cell differentiation.