Frogs: Frog gastrulation begins when a group of cells on the dorsal side of the blastula begins to invaginate. This forms a crease along the region where the gray crescent formed. Cells continue to move from the embryo surface into the embryo by involution. These cells become the endoderm and mesoderm. Cells on the embryo surface will form the ectoderm. The newly formed cavity is called the archenteron. This opens through the blastopore, which will become the anus
Chicks: Prior to gastrulation, the embryo is composed of an upper and lower layer, the epiblast and hypoblast, respectively. During gastrulation, epiblast cells move toward the midline of the blastoderm and then into the embryo toward the yolk. The midline thickens and is called the primitive streak. The hypoblast cells contribute to the sac that surrounds the yolk and a connection between the yolk and the embryo, but do not contribute to the embryo itself.
Humans: Human eggs have very little yolk. A blastocyst is the human equivalent of the blastula. The inner cell mass is a cluster of cells at one end of the blastocyst. The trophoblast is the outer epithelial layer of the blastocyst and does not contribute to the embryo, but instead initiates implantation. Following implantation, the trophoblast continues to expand, and a set of extraembryonic membranes is formed. These enclose specialized structures outside of the embryo. Gastrulation involves the inward movement from the epiblast through a primitive streak, similar to the chick embryo. After gastrulation, the embryonic germ layers have formed.
Ciliary function is essential for proper specification of cell fate in the human embryo. Monocilia (stationary primary cilia) jut from the surfaces of nearly all cells, one per cell. Motile cilia are restricted to cells that propel fluid over their surfaces, such as epithelial cells of airways. Motile cilia are also found on sperm, as flagella that propel movement. Monocilia act as antennae on the cell surface, receiving signals from multiple signaling proteins, such as Sonic hedgehog. When monocilia are defective, signaling is disrupted. Insight into the role of motile cilia in development comes from identification of Kartagener's syndrome, a set of medical conditions that often appear together. These include immotile sperm, infections of nasal sinuses and bronchi, and situs inversus, a reversal of normal left-right asymmetry. All of the associated conditions result from a defect that makes cilia immotile. 12th EditionElaine Nicpon Marieb 13th EditionTerry R. Martin 9th EditionLauralee Sherwood 8th EditionElaine N. Marieb, Katja Hoehn