T3 treatment increases mitosis, then bax expression and apoptosis in the optic lobe of the chick embryo.

We investigated T3 effects on cell proliferation and apoptosis in the optic lobe of the chick embryo between embryonic days (E) 6 and 11. Injection of T3 into the yolk increased [3H]thymidine incorporation between E7 and E9. This increased mitosis was followed by altered timing and degree of apoptosis during E9-11. In T3-treated embryos a marked increase in apoptosis occurred on E10, coincident with increased levels of mRNA encoding Bax, a pro-apoptotic protein. By E11, the overall morphology and total number of cells in each layer of the optic lobe were not different in control and treated embryos. Thus, although T3 transiently increases cell number, a homeostatic mechanism enters into play re-adjusting the balance between cellular proliferation and cell death.