Vogt (1922 a,b) recognized the capacity of the marginal zone to constrict and close the blastopore, and the dorsal marginal zone can autonomously undergo the narrowing (convergence) and lengthening (extension) ( Spemann, 1902 O. There are several reasons to suspect that convergent extension may play a major and perhaps dominant role in gastrulation of Xenopus and perhaps of other amphibians as well. The role of convergent extension in gastrulation of other amphibians and other metazoans and its significance to related problems in early development are discussed.Īlthough convergence and extension has largely been ignored in recent studies, with certain exceptions ( Schechtman, 1942 Ikushima & Maruyama, 1971 Phillips, 1984 Keller, 1984), the classical literature shows it to be important (see Keller, 1985). Removal of the entire blastocoel roof of the early gastrula, including all or part of the NIMZ, shows that convergent extension of the IMZ alone can bring about its involution and blastopore closure. Direct time-lapse video and cine micrography of deep cells in cultured explants show that convergent extension involves radial and circumferential intercalation.
Labelling of small populations of these cells with a cell lineage tracer shows that convergent extension involves intercalation of deep cells to form a longer, narrower array. Tissue recombination experiments show that the deep cells of the dorsal IMZ bring about convergent extension. Concurrently, the corresponding region of the overlying non-involuting marginal zone (NIMZ) begins a complementary convergent extension, but at a greater rate, which spreads vegetally to occupy surface area vacated by the IMZ. The dorsal involuting marginal zone (IMZ) undergoes convergence (narrowing) and extension (lengthening) after its involution, beginning at the midgastrula stage and continuing through neurulation, such that it simultaneously extends posteriorly across the yolk plug and narrows the blastoporal circumference. Microsurgical manipulation and explantation studies, analysed by time-lapse video and cine microscopy, shows that the dorsal circumblastoporal region contains two regions which show either autonomous or semiautonomous convergent extension. A review of recent and classical results shows that only the last accounts for the bulk of the tissue displacement of gastrulation, including spreading of the marginal zone toward the blastopore, involution of the marginal zone, and closure of the blastopore. The processes thought to function in Xenopus gastrulation include bottle cell formation, migration of cells on the roof of the blastocoel, and autonomous convergent extension of the circumblastoporal region.
0 kommentar(er)
