The distribution of lens differentiation capacity in the head ectoderm of chick embryos

The distribution of lens self-differentiation capacity in the head ectoderm of chick embryos during stages 4-13 (defined by Hamburger and Hamilton) was investigated. Different fragments of head ectoderm were cultivated in organ culture in vitro. Lens cells and lentoids formed in explants were identified by the indirect immunofluorescence technique with monospecific rabbit antisera to chick lens proteins: alpha-, beta-, and delta-crystallins. It was established that besides presumptive lens ectoderm, the lateral head ectoderm and ectoderm of stomodeal area of stages 10-11 chick embryos differentiated into lens cells when separated and cultured alone. The cultivation of presumptive lens ectoderm resulted in lens fiber differentiation in most explants. In the explants of lateral head ectoderm and those of the ectoderm of the stomodeal area, the lentoids and groups of cells containing crystallins were also found in most cases. In some, all cells of the explants taken from these areas differentiated into lens tissue. These results show that in the chick embryos at stages 10-11, capacity for lens self-differentiation is distributed over a large continuous area of the head ectoderm. The potential seems to arise during the process of gastrulation. The head ectoblast of stage 4-5 chick embryos can achieve at least three differentiation in vitro--lens, epidermal-like, and neutral. In the chick head ectoderm, at the beginning of neurulation two areas can be distinguished according to their capacity for self-differentiation. The area situated in front of the head fold develops into lens and epidermal-like tissues in vitro, whereas the area between the head fold and Hensen's node develops into neural tissue. In the course of further development, lens potential was restricted to the presumptive lens ectoderm. Most of the head ectoderm develops in normogenesis in several directions without any traits of lens differentiation. The formation of Rathke's pouch (adanohypaphysis anlage) by cells which have synthesized beta-crystallins is the only known case (besides the lens anlage) in which an expression of lens potential of head ectoderm occurs.