Neural induction and differentiation with minimal numbers of cells

Abstract

Summary Four series of explantation experiments have been carried out in order to test the extent to which limiting the numbers of either ectoderm or mesoderm cells imposes a limit to neural induction and differentiation in Xenopus embryos. Normal-sized pieces of dorsal lip mesoderm combined with fewer than ten ectoderm cells produce only 20.9% neural tissue, whereas with normal-sized pieces of ectoderm they produce 97.6% neuralization, with higher percentages of brain and neural tube than with the small numbers of ectoderm cells (series 1: Table 1). No neural differentiation at all was obtained when fewer than ten ectoderm cells from the neural plate region of a late gastrula were explanted. In explants of larger pieces, both the total incidence of neuralization and the proportions of brain and neural tube, rather than merely neuroid formations, were higher, the larger the piece (series 2: Table 2). As much as 80.9% neuralization was achieved when fewer than ten mesoderm cells were combined with normal-sized pieces of ectoderm, but in most cases there were only neuroid formations (series 3: Table III). When groups of fewer than 10 ectoderm cells, which had been pretreated by contact with dorsal lip in culture for 3 hours, were combined with larger pieces of noninduced ectoderm, 38% showed neuralization. These were mostly neuroid (series 4: Table IV). It is concluded that for success of neuralization an adequate number of ectoderm cells, but not of mesoderm is critical. The quality of neural differentiation apparently depends on some second-stage interaction, subsequent to the induction process. This second-stage reaction would repay further study.