In an effort to study the molecular basis of the determination processes of the mammalian germ cell lineage, we have tried to isolate a mouse gene homolog to vasa, which plays an essential role as a maternal determining factor for the formation of Drosophila germ cell precursors. By reverse transcriptase PCRs of mouse primordial germ cell cDNAs using family-specific primers, we obtained a gene (Mvh) encoding a DEAD-family protein that showed a much higher degree of similarity with the product of the Drosophila vasa gene (vas) than previously reported mouse genes. In adult tissues, Mvh tnscripts were exclusively detected in testicular germ cells, in which Mvh protein was found to be localized in cytoplasm of spermatocytes and round spermatids including a perinuclear granule. The protein was also expressed in germ cells colonized in embryonic gonads but was not detected in pluripotential embryonic cells such as stem cells and germ cells. These results suggest the possibility that the Mvh protein may play an important role in the determination events of mouse germ cells as in the case of Drosophila vasa.In mice, germ cell precursors, termed primordial germ cells (PGCs), are generated in the epiblast. They consist of putative pluripotential cells that can be first identified as a small cluster of alkaline phosphatase-positive cells in the extraembryonic mesoderm at 7.25 days postcoitum (dpc). Then they move down to the embryonic mesoderm at the posterior end of the primitive streak (8.0 dpc), migrate through the hindgut endoderm, and colonize the developing genital ridges (10.5-11.5 dpc), in which they are destined to form functional germ cells (1, 2). Despite these detailed morphological observations, molecular mechanisms regulating this developmental pathway still remain unclear.Unlike mammals, it has been well established that several maternal factors are involved in germ cell determination of many animal species. In particular, in Drosophila it has been shown that the pole plasm localized at the posterior pole ofthe oocyte contains determining substances for the abdomen and the germ lineage. After fertilization, only the nuclei that migrate into the pole plasm are destined to form the germ-line progenitors (pole cells). Genetic identification ofgenes whose function is required for pole cell formation has revealed at least eight maternally active genes, cappucino, spire, staufen, oskar, vasa, valois, mago-nashi, and tudor (3, 4). Among these genes, vasa (vas) is one of the best characterized. Homozygous mutant vasa females produce no eggs. In ectopic formation of pole cells induced by mislocalization of oskar mRNA to the anterior pole of the oocyte, only vasa and tudor were required for ectopic pole cell formation (5-10). The gene vas encodes a DEAD (Asp-Glu-Ala-Asp)-family protein of putative RNA helicases, which is found to be present as a component of both the polar granules at the posterior end of the oocyte and the nuage structure in the germ cells, and zygotic expression is also restricted to the ge...

The present study used a molecular approach toward understanding the mechanism of hormone- and region-dependent remodeling of the small intestine during metamorphosis of Xenopus laevis. A protein spot was noticed on a two-dimensional polyacrylamide gel as a protein whose expression was metamorphic stage- and region-dependent. The protein was identified as the Xenopus homolog (Xcalbindin) of chick calbindin D28k. Xcalbindin expression in the intestine was restricted to absorptive cells in the posterior part, being detectable at stages 49-61, not detectable at stages 62-63, detectable again at stages 64-66, and finally becoming undetectable in the adult. During spontaneous metamorphosis, the level of Xcalbindin mRNA was significantly increased between stages 57 and 58, dramatically reduced at stage 59, and the mRNA was undetectable from stages 60-63, after which it was weakly re-expressed until the end of metamorphosis. Such up- and down-regulation of Xcalbindin mRNA was induced precociously by exogenous thyroid hormone. These results indicated that Xcalbindin is a specific marker of the differentiated absorptive cells of the intestine. Immunohistochemistry with specific antibodies against Xcalbindin demonstrated that precursor cells of adult intestinal epithelial cells expressed Xcalbindin. Considering these results, the origin of adult intestinal epithelial cells was discussed.

The skin of an adult frog of Xenopus laevis was characterized by the reactivity of 20 lectins. The lectins were classified into six groups in their binding to the epidermal cells: Lycopersicon esculentum lectin (LEL)‐type which was positive for all epidermal cells; Pisum sativum agglutinin (PSA)‐type for stratum germinativum; succinylated wheat germ agglutinin (sWGA)‐type for strata spinosum, granulosum and corneum; Dolichos biflorus agglutinin (DBA)‐type for strata germinativum and spinosum; peanut agglutinin (PNA)‐type for stratum spinosum; and Ulex europaeus agglutinin (UEA‐I)‐type for strata granulosum and corneum. PSA and sWGA were utilized as markers of mitotically active germinative cells and the differentiated cells of the epidermis, respectively, to describe the metamorphic conversion of larval epidermal cells to adult type. PSA stained all epidermal cells of tadpoles before metamorphic climax. At the end of metamorphosis, PSA‐positive cells were restricted to cells in the basal layer of body epidermis while all the tail epidermis remained PSA‐positive. The other cell marker, sWGA, only stained apical cells in tadpole epidermis. During the metamorphic climax, sWGA‐positive cells appeared in the cells beneath the stratum corneum of the body region, but not in the tail region. The present study demonstrates that PSA and sWGA are useful to investigate metamorphic changes in tadpole epidermal cells.

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