Primordial Germ Cells in Anuran Embryos: Their Movement and Its Guidance

Research over the greater part of the last century has considerably increased our knowledge of cell movement. In particular, studies of cells in vitro, where they are amenable to microscopic study and biochemical manipulation, have made great progress in the recognition of biological polymers involved in the contractile basis of cell movement (Abercrombie et al. 1971, Goldman et al. 1975, Isenberg et al. 1976, Izzard and Lochner 1976, Lazarides 1975, 1976, Pollack et al. 1975, Weber 1976, Wessells et al. 1973). Studies on the mechanism of cell movement in vivo, however, are complicated by the fact that rather crude optical methods have to be used, due to the opacity of most living material. The only reliable information comes from work on the few examples of motile cells which, either by natural coincidence or ingenious manipulation, are amenable to study with good optics, or with the electron microscope. These include the invagination of primary mesenchyme cells during sea-urchin gastrulation (Gustafson and Wolpert 1961), the movement of fibroblasts over the developing cornea in chick embryos (Bard and Hay 1975), deep cell movement during epiboly in Fundulus embryos (Trinkaus 1973), invaginating dorsal lip cells of amphibian gastrulae (Kubota and Durston 1978), and healing of wounds in amphibian epithelium (Radice 1978). (Studies on the movement of cells in vivo are reviewed in Trinkaus 1976.) Despite this mass of work, we are still ignorant of the way in which movement of cells during embryogenesis is instigated, its detailed mechanism, and its guidance along specific pathways. This problem is of no small importance. The differentiation of many organs in the body is associated with cell movement, which may take several forms, such as (a) movement of whole sheets of cells, as in epipoly; (b) movements of cords, or clumps of cells, as in the invasion by epithelial cells of the surrounding mesenchyme during gland and duct formation; or (c) movement of individual cells, as with neural crest cells and primordial germ cells. All of these movements are guided in some way, and when the guidance mechanisms fail, congenital malformation of the resultant offspring becomes inevitable. We have been studying the mechanisms of embryonic cell movement and its guidance, using as an example the migration of primordial germ cells (PGCs) to the site of gonad formation in the anuran amphibian Xenopus laevis (Heasman and Wylie 1978, Wylie and Heasman 1976, Wylie and Roos 1976, 1979). We chose this particular example for three reasons: (a) PGCs are very large and easy to identify during their migration; (b) they appear to migrate as individuals, rather than as a sheet of cells; and (c) they move along a clearly defined path, which is accessible to a certain amount of observation and amenable to experimental manipulation.