CHANGES OF RIBONUCLEOPROTEIC STRUCTURES OF EMBRYONIC EPIDERMAL-CELL NUCLEI DURING DIFFERENTIATION AND MATURATION

The nuclear structure of the epiblast and skin cells was studied durin g the development of the chick embryo (Gallus domesticus). Ribonucleop roteic structures (RNPs) and chromatin were characterized with prefere ntial staining techniques. The changes of the frequency of a type of R NP particle, the perichromatin granule, from gastrulating embryos to n ewly hatched chick, were analyzed by electron microscopic quantitative stereology. The changes of nuclear and nucleolar volume were estimate d using the light microscope. The differentiation of ectoderm from epi blast is characterized by a significant increase in the nucleolar volu me and in the frequency of perichromatin granules. These features sugg est that ectoderm differentiation involves a general increment of mRNA and rRNA transcription. At the beginning the synthesis of a sulfhydry l-rich protein, which is coincident with the thickening of the bilayer ed epithelium to a multilayered epithelium, a large increase of the nu merical density of perichromatin granules not accompanied by any modif ication of the nucleolar volume was found. The same changes characteri ze the acquisition of epidermal mature cell features immediately after hatching. These findings as well as previous results concerning the d ifferentiation of central nervous system and striated muscle suggest t hat the increases of the number of perichromatin granules take place d uring the acquisition of new cytophysiological states related to funct ional maturation and not to early stages of cell differentiation.