Germ Line Mechanics--And Unfinished Business

Abstract

Primordial germ cells are usually made early in the development of an organism. These are the mother of all stem cells that are necessary for propagation of the species, yet use highly diverse mechanisms between organisms. How they are specified, and when and where they form, are central to developmental biology. Using diverse organisms to study this development is illuminating for understanding the mechanics these cells use in this essential function and for identifying the breadth of evolutionary changes that have occurred between species. This essay emphasizes how echinoderms may contribute to the patchwork quilt of our understanding of germ line formation during embryogenesis.

Keywords: Gametes; Germ line; Nanos; Sea urchin; Vasa.

Figure 1

Diagram of the development of a sea urchin. Early development yields Vasa-positive cells (shown in red, beginning with a uniform Vasa positive egg and early embryo). At the 32-cell stage, the sMics are uniquely Vasa-positive. These cells move into the coelom during gastrulation, segregate into the left and right coelomic pouches, and expand to contribute to the germ cells of the adult, and likely also to some somatic cells of the rudiment. (from Wessel at al., 2014)

Figure 2

The sMics are dispersible for development but required for gamete development. A: Method of sMic removal with a glass pipette. The resulting embryos develop into a larva (B–E), form an adult rudiment (D, E, arrow in E), and metamorphos into an adult (F) that does not produce gametes. Scale bars, 80 μm. (from Yajima and Wessel, 2011)

Figure 3. Vasa in perinuclear granules throughout the embryo

The S. purpuratus Vasa open reading frame is sufficient for specific small micromere enrichment of Vasa protein (left column), whereas deletion of 141 N-terminal amino acids causes broad Vasa expression throughout the embryo in perinuclear granules. Scale bar = 50 μm. (from Gustafson et al., 2011)