Cell population growth regulates dorsalization and caudalization during chick morphogenesis and programmed cell death in lens fibres.

The chick embryo ectoblast was examined for a possible relationship between the state of neural competence and cell population growth. It was found that although ectoblast cells with doubling times ranging between 5 to 20 h exhibit neural competence, the extent of neutralization induced by the Hensen's node depends on the duration of the cell cycle; the longer the doubling time of the competent ectoblast, the stronger the induction and the greater the induced neural tissue. Neural induction in the competent ectoblast occurs in at least two steps: the first lasts for 1-2 h of direct contact with the inducing Hensen's node graft; a contact for another 2 h with even a non-inducing post-nodal fragment is essential to consolidate neutralization. Hensen's node graft induces mitotic activity in the competent ectoblast in contact. Teratogens which inhibit cell population growth, development and blastoderm expansion in chick embryo gastrula cause concomitant caudalization of the embryonic axis. We confirm Yamada's hypothesis that dorsalization is under positive mitogenic control, whereas caudalization is controlled by a negative cell cycle regulation. Reverse transcripts of chick gastrula mRNA were cloned in pBR322. Colony hybridization with cDNA made against chicken yolk RNA showed positive clones. Thus chicken yolk contains maternal mRNAs. cDNA made against mRNA extracted from stage 10 foreheads was hybridized with RNA from stage 1 to 13 embryos, 19 day lens and egg yolk. The hybridization signal, which was low between stages 1 to 7, increased between stages 10-13 and decreased thereafter. Forehead cDNA also hybridized to yolk RNA. Thus, maternal RNA sequences are present in the early chick embryo. During lens development, epithelial cells retain proliferative activity and their progeny reaching a stationary phase join the fibre area and contribute to the growth of fibre cells. The rate of transfer from epithelium to fibre regulates the rate of programmed cell death of the non-dividing differentiated lens fibre cells.