Nematode Gastrulation: Having a BLASTocoel! Pradeep M. Joshi, Joel H. Rothman  Current Biology  Volume 15, Issue 13, Pages R495-R498 (July 2005) DOI: 10.1016/j.cub.2005.06.030 Copyright © 2005 Elsevier Ltd Terms and Conditions

Figure 1 Proposed model for a spatiotemporal shift in developmental events leading to the unusual mode of gastrulation in derived nematodes. (A) Similarity between the blastula stages (16 cell stage) of the sea urchin L. variegatus and the freshwater nematode T. diversipapillatus, showing a clear blastocoel (asterisk). The image of the sea urchin was adapted from Jeff Hardin’s Sea Urchin embryology tutorial (http://worms.zoology.wisc.edu/urchins/SUcleavage_stages.html). (T. diversipapillatus image reproduced with permission from [2]; L. variegatus image courtesy of Charles Ettensohn and Jeff Hardin.) (B) In T. diversipapillatus embryos, internalization (assumed here to be invagination) initiates with the apical constriction of a group of cells located anteriorly (left in the cartoon) at the 64 cell stage, resulting in gastrulation reminiscent of that in other protostomes. In more derived nematodes, exemplified by C. elegans, it is postulated that a change in the position of prospective endoderm cells shifts the gastrulation cleft posteroventrally. A heterochronic shift in the developmental program causes endoderm specification to begin at the seven-cell stage, endowing endoderm precursor cells (red nuclei) with the ability to invaginate earlier (28 cell stage) and in the absence of a prominent blastocoel. Current Biology 2005 15, R495-R498DOI: (10.1016/j.cub.2005.06.030) Copyright © 2005 Elsevier Ltd Terms and Conditions