Surgical manipulation of the developing chick embryo in ovo after the 4th day of incubation.

Surgical manipulations were performed on the eyes of the developing chick embryo between 6 1/2-12 days of incubation at stages when operations had to be performed through the shell and the extra-embryonic membranes. A sufficient proportion of the embryos survived a postoperative incubation period of 6-10 days to analyse the effects of surgical intervention on development.

Experimental evidence for autonomous action of the periodic albinism (ap) gene within developing retinal pigment cells and melanophores of Xenopus laevis.

Genes which affect pigment elaboration may do so by autonomous action within the developing pigment cells or by way of tissue interactions leading to pigment cell differentiation. The site of action of the periodic albinism (ap) gene was investigated by substituting presumptive neural ectoderm of gastrulae of one genotype with uncommitted ectoderm of different genotype. Retinal pigment cells and melanophores arising from such grafts were found to differentiate according to their own genotype in spite of having spent their entire developmental history in tissues of different genotype. This finding demonstrates autonomous action of the ap gene within pigment cell derivatives and does not support recent proposals that the ap gene is involved in inductive interactions leading to melanogenesis. Experiments in which portions of presumptive dorsal mesoderm, implanted in gastrulae of different genotype, induced secondary pigment cells of host phenotype further support the proposal that the ap effect on pigment cells is not mediated by inductive interactions.

The control of mealanoblast differentiation in the periodic albino mutant of Xenopus.

Studies on the incidence of melanophores in older ventral trunk tissues and in isolated regions of periodic albino embryos of Xenopus suggest that melanin granule formation in mutant melanoblasts depends on an environmental contribution which arises at stage 43 in the endodermal tissues.

An analysis of pigment cell development in the periodic albino mutant of Xenopus.

The periodic al,bino mutant (apap) of Xenopus in which the development of melanophores is impaired, is further reported here to possess an aberrant pattern of iridophore differentiation. The development of mutant and wild-type neural crest explants isolated in vesicles derived from tissues from identical and different genotypes was examined to determine if the mutant effect resides in the pigment cells or is mediated by the environmental tissues. Mutant melanophores and iridophores cultured in either mutant or wild-type tissues exhibited mutant patterns of differentiation. Wild-type pigment cells cultured in both wild-type and mutant tissues exhibited wild-type patterns of differentiation. Hence the mutation affects the capacities of melanoblasts and iridoblasts to differentiate but not the ability of the environmental tissues to support pigment cell differentiation.

Melanoblast-tissue interactions and the development of pigment pattern in Xenopus larvae.

The melanophores of larval Xenopus laevis are disparately distributed on the hypomere in that the upper region (UHT) is densely pigmented, the median region (MHT) is moderately pigmented, and the lower region (LHT) is unpigmented. The roles of the melanoblasts and their tissue environment in determining the melanophore pattern was investigated by heterotopic transplantation of hypomeric tissues, culture of neural crest explants in vesicles derived from hypomeric tissues and radioactive marking of neural crest cells. Somite-situated grafts of UHT, MHT and LHT were found to possess melanophore densities similar to those exhibited by such hypomeric tissues when in their normal situation. The number and distribution of trunk melanophores in 'crestless' second host larvae bearing grafts of UHT, MHT and LHT transferred from the somites of primary host embryos indicated that (a) many melanoblasts entered all transplants during neural crest migration in the primary host: subsequently, a small number of melanoblasts were lost from transplants of UHT, a greater number from transplants of MHT and almost all from transplants of LHT; (b) almost all melanoblasts migrated out from transplants of MHT and LHT and entered the tissues of the 'crestless' host, whereas a considerable number of melanoblasts remained in the transplant when it was formed from UHT. Grafts of UHT placed mid-ventrally in the hypomere failed to exhibit melanophores. Vesicles of (a) UHT + MHT and (b) LHT containing neural crest tissue possessed similar numbers of melanophores. Vesicles of LHT differed from those of UHT + MHT in that melanophores were densely aggregated in the implanted neural tissues. Following radioactive marking of neural crest cells labelled nuclei were found on the dorsal ridges of the somites, the surfaces of the neural tube and notochord and in the mesoderm of the upper hypomere and the fin, but were absent from the lateral surfaces of the somites. These results showed that the melanophore pattern in larval Xenopus depended upon melanoblast-tissue interactions, which influenced the migration, rather than the differentiation, proliferation or destruction, of melanoblasts and suggested that tissue selection by migrating melanoblasts enabled these cells to distribute themselves in embryonic tissues in accordance with a hierarchy of melanoblast-tissue affinities. Melanoblast-tissue affinities appeared to be related to the adhesiveness of mesodermal cells: melanoblast extensibility appeared to facilitate exploration of the surrounding tissues. The formation of pigment pattern in larval Xenopus appeared to depend upon the interaction between the melanoblast population pressure and melanoblast-tissue affinities. The present results and those of other workers on amphibian pigmentation were used to construct a model capable of accounting for species-specific differences in larval amphibian pigment patterns, in terms of interactions between species-specific differences in melanoblast-tissue affinities and melanoblast population pressure.