Chapter 47 Preformation vs epigenesis Preformation embryo development

Chapter 47

Preformation vs. epigenesis • Preformation: embryo development is miniature versions of adult like Russian nesting dolls • Epigenesis: relative formless egg develops into adult

Fertilization in mammals • • • Acrosomal Reaction Sperm reaches egg; acrosome releases hydrolytic enzymes by exocytosis. Enzymes activate cytoskeletal elements that elongate the end structure and penetrate the jelly coat of the egg. Tip of acrosome has proteins that bind to receptors on the vitelline layer just exterior to the plamsa membrane. In some animals, there is a “lock and key” recognition to assure that they are only fertilized by the proper species. Egg and sperm plasma membranes fuse. This causes an electrical response in the egg plamsa membrane. This depolarization prevents other sperm from penetrating the egg (fast block to polyspermy) Fusion also causes vessicles inside egg to exocytosis contents which catalyze a hardening of the extracellular matrix of the cell thus acting as the slow block to polyspermy) In mammals, fusion of egg and sperm nuclei does not occur until after the first mitotic division.

• Apoptosis: programmed cell death; necessary for proper development • accounts for the lack of webbing between our fingers and toes (which is their in embryos) • totipotent: a cell that has retained the potential to form all parts of the animal • determination: progressive restriction of a cell’s developmental potential. • Induction: the ability of one group of cells to influence the development of an adjacent groups of cells • Tissue-specific proteins: proteins found only in a certain type of cell • Genomic equivalence: all cells contain the same genes • Maternal effect genes: phenotypes expressed by these genes are exclusively the product of the mother’s contribution; usually mutations in genes that are necessary for the egg polarity to be set up properly. • Homeotic genes: genes that determine the anatomical identify of different segments of an individual.