BIOL 225 Lecture Notes - Lecture 47: Zona Pellucida, Spindle Apparatus, Acrosome Reaction
14 May 2018
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Chapter 47 →Notes
Animal Development
• Fertilization→ sperm must dissolve or penetrate any protective layer
surrounding the egg to reach the plasma membrane
• Molecules on the sperm surface bind to receptors on the egg surface, helping
ensure that a sperm of the same species fertilizes the egg→ changes at the
surface of the egg prevent polyspermy (entry of multiple sperm nuclei into
egg)
Fertilization in Mammals
• Fertilize eggs internally
• Secretion in female reproductive tract provide moist environment for sperm
and bring about changes in sperm mobility and structure
• After these changes occur does sperm have the capacity to fertilize an egg
• Capacitation occurs during the first 6 hours after the sperm enter the female
reproductive tract
• Supporting cells of developing follicle surround the mammalian egg and
remain with it during and after ovulation→ sperm travels through this layer
of follicle cells b4 it reaches the zona pellucida (the extracellular matrix of
the egg)
• Zona pellucida→ component that functions as a receptor for sperm
• Binding of a sperm to this receptor induces an acrosomal reaction→
facilitating sperm passage through the zona pellucida that binds with the egg
plasma membrane→ at this point the two cells fuse
• Sperm binding→ triggers changes that lead to a cortial reaction (release of
cortical granules to the outside of the cell)
• These enzymes catalyze changes in the zona pellucida, which then functions
as the slow block to polyspermy
• Egg and sperm fuse→ whole sperm is taken into the egg
• Envelopes of both haploid nuclei have dispersed, the sperm and egg
chromosomes are organized onto a single mitotic spindle
• After the 1st division is there a true diploid nucleus with a nuclear membrane
Cleavage
• Once fertilization complete→ rapid cell divisons that characterize the
cleavage stage of early development
• During cleavage→ cell cycle consists primarily of the S (DNA synthesis) and
M (mitosis) phases
• Cells basically skip G1 and G2 (gap) phases→ little or no protein synthesis
occurs
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