Genetic Heterogeneity and the Establishment of Tissue Systems During Development

Abstract

The significance of trait expression at the tissue level during morphogenesis has already been demonstrated for the case of the early morphogenetic process of primary embryonic induction. The presumptive neuroectoderm is heterogeneous and is made of two cell populations. Both electron-dense and electron-transparent cells were obtained from this tissue from cynopus pyrrhogaster in the gastrula stage. The electron-dense cells degenerated while the electron-transparent cells developed into various mesodermal structures when explants of presumptive ectoderm were treated with mesodermal inductors. Dissociated cells of the presumptive ectoderm could be divided into two groups with the aid of electrophoresis: (1) cells which were able to react to a neuralizing inductor and which perished after the addition of mesodermalizing agent, and (2) cells with the opposite reaction to such treatment. It was demonstrated that the acquired competence of the dissociated cells of the presumptive ectoderm is correlated with the altered sensitivity to the inductor (see review Ignatijeva, 1967). It should be noted that there is a significant loss of cells during morphogenetic processes, and, therefore, a successive replacement of cell populations during ontogenesis. This process occurs in various tissues such as nerve (Hughes, 1968), muscle (Saunders et al., 1957), and others (Berdishev, 1968). Cell loss has also been described as part of the differentiation of insect ima- ginal discs (Fristrom, L969; Michinoma and Kaji, 1970).