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BIOLOGY 205/SECTION 7 DEVELOPMENT-LILJEGREN Lecture 3 Fertilization 1. Long range signals + cell:cell contact both influence fertilization. a. Some sperm respond to chemotactic signals made by eggs to find them. Sperm sense diffusible chemical cues from egg e.g. resact , a 14 aa peptide (species specific) b. Cell:cell contact - sperm and egg mediated by specific transmembrane glycoproteins . Two steps: sperm/egg shell and sperm/egg plasma membrane c. Contact of the sperm with the egg shell (zona pellucida) starts the ACROSOMAL REACTION i. Sperm has specialized acrosomal vesicle containing enzymes that processes the jelly layer/eggshell to allow direct contact and fusion between the Sperm/egg plasma membranes ii. Sea urchins —interactions between bindin (sperm) and bindin receptor (eggshell)- regulate species specificity. Prerequisite to fusion with the egg membrane. iii. Mammals= ZP3 receptor (sperm): ZP3 (eggshell) d. Sperm-egg contact leads to signaling pathways in both the egg & sperm and to the initiation of development. This initiates changes in the egg -- the changes are remarkable for their rapidity, and result in a BLOCK TO POLYSPERMY , that prevents other sperm from fertilizing the egg. These blocks are very important since polyspermy can be disasterous! (ie. unequal chromosomal distribution which would cause death or abnormal development). i. FAST BLOCK to POLYSPERMY : rapid membrane depolarization -- >prevents sperm fusion and rapidly triggering signaling pathway throughout the egg. Ie. seawater high Na+ outside cell, low inside, resting membrane potential is –70mV (inside of cell negatively charged with respect to exterior) so small influx of sodium ion from the seawater causes membrane potential to shift to a positive level (+20mV). Sperm can’t fuse when membrane have a positive membrane potential. This effect is transient (~1minute). ii. SLOW BLOCK TO POLYSPERMY: removes sperm bound to egg shell through the exocytosis of cortical granules. (15,000 per sea urchin egg!). Proteases released by the granules clip off the bindin receptor and any sperm attached to it. In mammals, this process modifies the
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This note was uploaded on 05/26/2011 for the course BIO 205 taught by Professor Reed during the Spring '11 term at UNC.

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