Analysis of nuclear ribonucleoproteic structures during notochordal cell differentiation and maturation in chick embryos

The ultrastructure of notochordal cells and the quantitative changes of nuc lear mRNA-containing particles were studied in several stages of the develo pment of the chick embryo. The modifications in the frequency of perichroma tin granules (PCG) were analyzed in embryos at 24 hr to 10 days of incubati on (stages 6-36 of Hamburger and Hamilton). The ultrastructural and morphom etric data show that notochordal cells undergo changes that can be systemat ized in four periods. Very early notochordal cells (stages 6-11), are chara cterized by the presence of large nucleoli and abundant PCG, traits probabl y related to the frequent mitotic division and the expression of inductive signals reported in numerous papers. During the second period (stages 16-21 ) the number of PCG and the size of the nucleolus decrease. These changes a re coincident with the beginning of vacuolization. In the third period (sta ges 21-30), the notochordal cells undergo a second cytodifferentiation char acterized by a large increase of cytoplasmic vacuolization and secretion of materials that thicken the perichordal sheath. During this period, the nuc leolus becomes smaller and the number of PCG increases. Similar features we re previously described during functional maturation of embryonic neurons a nd striated fibers at synaptogenesis, and epidermal cells. The fourth perio d, beginning at stage 30, is characterized by the decrease of the density o f PCG and of the nucleolar volume and corresponds to cessation of mitosis a nd cell degeneration. Anat Rec 259:113-123, 2000. (C) 2000 Wiley-Liss, Inc.