Chapter 47 Animal Development. Overview: A Body-Building Plan for Animals. From egg to organism, an animal’s form develops gradually. . The question of how a zygote becomes an animal has been asked for centuries.
In other species, the differences between cells are due to their location in the developing embryo.
Concept 47.1 After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis
Fertilization in terrestrial animals, including mammals, is generally internal.
The binding of the sperm cell to the egg triggers changes within the egg, leading to a cortical reaction, the release of enzymes from cortical granules to the outside via exocytosis.
Yolk is most concentrated at the vegetal pole and least concentrated at the animal pole.
In frogs, the first two cleavages are vertical and result in four blastomeres of equal size.
Animals with less yolk (such as the sea urchin) also have an animal-vegetal axis.
The blastomeres sort into upper and lower layers, the epiblast and the hypoblast.
Gastrulation in the chick is similar to frog gastrulation in that it involves cells moving from the surface of the embryo to an interior location.
Unique to vertebrate embryos is a band of cells called the neural crest, which develops along the border where the neural tube pinches off from the ectoderm.
As organogenesis progresses, morphogenesis and cell differentiation refine the organs that form from the three germ layers.
The trophoblast, the outer epithelium of the blastocyst, secretes enzymes that break down the endometrium to facilitate implantation of the blastocyst.
The fourth extraembryonic membrane, the allantois, is incorporated into the umbilical cord, where it forms blood vessels that transport oxygen and nutrients from the placenta to the embryo and rid the embryo of carbon dioxide and nitrogenous wastes.
Concept 47.2 Morphogenesis in animals involves specific changes in cell shape, position, and adhesion
Morphogenesis is a major aspect of development in plants and animals, but only in animals does it involve cell movement.
Cell crawling is also involved in convergent extension, a type of morphogenetic movement in which the cells of a tissue layer rearrange themselves so the sheet converges and extends, becoming narrower but longer.
In frog gastrulation, fibronectin fibers line the roof of the blastocoel.
Concept 47.3 The developmental fate of cells depends on their history and on inductive signals
Development requires the timely differentiation of cells in specific locations.
First, during early cleavage divisions, embryonic cells must somehow become different from one another.
Second, once initial cell asymmetries are set up, subsequent interactions among the embryonic cells influence their fate, usually by causing changes in gene expression.
The cells of the mammalian embryo remain totipotent until the 16-cell stage, when they become arranged into the precursors of the trophoblast and inner cell mass of the blastocyst.
A growth factor called bone morphogenetic protein 4 (BMP-4) is active exclusively in cells on the ventral side of the amphibian gastrula.
Many inductions involve a sequence of inductive steps that progressively determine the fate of cells.
One limb-bud organizer region is the apical ectodermal ridge (AER), a thickened area of ectoderm at the tip of the bud.
The second major limb-bud organizer region is the zone of polarizing activity (ZPA), a block of mesodermal tissue located underneath the ectoderm where the posterior side of the bud is attached to the body.
We can conclude from these experiments that pattern formation requires cells to receive and interpret environmental cues that vary with location.