Evolution of Xenopus endodermal cells cultured on different extracellular matrix components. Identification of primordial germ cells

"A narrow bridge home": The dorsal mesentery in primordial germ cell migration

Specification of primordial germ cells (PGCs) in all vertebrates takes place in extragonadal sites. This requires migration of PGCs through embryonic tissues towards the genital ridges by both passive and active types of migration. Commonly, colonization in the genital ridges follows migration of the PGCs along the thin tissue of the dorsal mesentery. Here we review the anatomy of the dorsal mesentery, the role it plays in migration of PGCs, and the interactions of PGCs with different cell types, extracellular matrix and signaling pathways that are all essential for attraction and orientation of PGCs along the dorsal mesentery towards the gonad anlage.

Developmental regulation of locomotive activity in Xenopus primordial germ cells

Primordial germ cells (PGCs) arise in the early embryo and migrate toward the future gonad through species-specific pathways. They are assumed to change their migration properties dependent on their own genetic program and/or environmental cues, though information concerning the developmental change in PGC motility is limited. First, we re-examined the distribution of PGCs in the endodermal region of Xenopus embryos at various stages by using an antibody against Xenopus Daz-like protein, and found four stages of migration, namely clustering, dispersing, directionally migrating and re-aggregating. Next, we isolated living PGCs at each stage and directly examined their morphology and locomotive activity in cell cultures. PGCs at the clustering stage were round in shape with small blebs and showed little motility. PGCs in both the dispersing and the directionally migrating stages alternated between the locomotive phase with an elongated morphology and the pausing phase with a rugged morphology. The locomotive activity of the elongated PGCs was accompanied by the persistent formation of a large bleb at the leading front. The duration of the locomotive phase was shortened gradually with the transition from the dispersing stage to the directionally migrating stage. At the re-aggregating stage, PGCs became round in shape and showed no motility. Thus, we directly showed that the locomotive activity of PGCs changes dynamically depending upon the migrating stage. We also showed that the locomotion and blebbing of the PGCs required F-actin, myosin II activity and RhoA/Rho-associated protein kinase (ROCK) signaling.

Evidence for fibronectin degradation by calpain II

Recent work supports the hypothesis that calpain II can be exteriorized. Indeed, this cysteine calcium-dependent proteinase was shown to be intercellularly, and, more particularly, associated to extracellular matrix components. Thereby, calpain II could be involved in hydrolysis of pericellular matrix components such as fibronectin, which is known to play an important role in cellular differentiation. Our in vitro studies provide evidence that fibronectin is a potential substrate for calpain II. On cultured cells, our findings show that calpain II is able, on the one hand, to cleave the fibrillar network of fibronectin secreted by fibroblasts, and, on the other, to decrease dramatically the fibronectin amount secreted by myoblasts just before fusion. Moreover, following this treatment, myoblasts become spherical due to the cleavage of this attachment factor. However, these cells, plated on an appropriate substrate are still able to differentiate. Our results suggest that calpain II is indeed involved in myoblast fusion via the fibronectin cleavage since it is well established that myogenic lineages lose this glycoprotein at the time of fusion.