The initial stage of fertilization is the attachment of the sperm head to the zona pellucida of the egg. A successful fertilization restores the full complement of 46 chromosomes and subsequently initiates the development of an embryo. Fertilization involves several steps. Recognition of the egg by the sperm occurs first. The next step is the regulation of sperm entry into the egg. A series of key molecular events, collectively called polyspermy block, prevent multiple sperm from entering the egg. Coupled with fertilization is the completion of the second meiotic division of the egg, which extrudes the second polar body. At this point, the male and female pronuclei unite, followed by initiation of the first mitotic cell division (Fig. 39.1).
The zona pellucida contains specific glycoproteins that serve as sperm receptors. They selectively prevent the fusion of inappropriate sperm cells (e.g., from a different species) with the egg. Contact between the sperm and egg triggers the acrosome reaction, which is required for sperm penetration. Sperm proteolytic enzymes are released that dissolve the matrices of the cumulus (granulosa) cells surrounding the egg, enabling the sperm to move through this densely packed group of cells. The sperm penetrates the zona pellucida, aided by proteolytic enzymes and the propulsive force of the tail; this process may take up to 30 minutes. After entering the perivitelline space, the sperm head becomes anchored to the membrane surface of the egg, and microvilli protruding from the oolemma (plasma membrane of the egg) extend and clasp the sperm. The oolemma engulfs the sperm, and eventually, the whole head and then the tail are incorporated into the ooplasm.
Shortly after the sperm enters the egg, cortical granules, which are lysosome-like organelles located underneath the oolemma, are released. The cortical granules fuse with the oolemma. Fusion starts at the point of sperm attachment and propagates over the entire egg surface. The content of the granules is released into the perivitelline space and diffuses into the zona pellucida, inducing the zona reaction, which is characterized by sperm receptor inactivation and a hardening of the zona. Consequently, once the first spermatozoon triggers the zona reaction, other sperm cannot penetrate the zona, and therefore, polyspermia is prevented.
An increase in intracellular calcium initiated by sperm incorporation into the egg triggers the next event, which is the activation of the egg for completion of the second mei-otic division. The chromosomes of the egg separate and half of the chromatin is extruded with the small second polar body. The remaining haploid nucleus with its 23 chromosomes is transformed into a female pronucleus. Soon after being incorporated into the ooplasm, the nuclear envelope of the sperm disintegrates,- the male pronucleus is formed and increases 4 to 5 times in size. The two pronuclei, which are visible 2 to 3 hours after the entry of the sperm into the egg, are moved to the center of the cell by contractions of microtubules and microfilaments. Replication of the haploid chromosomes begins in both pronuclei. Pores are formed in their nuclear membranes, and the pronuclei fuse. The zygote (fertilized egg) then enters the first mitotic division (cleavage) producing two unequal sized cells called blas-tomeres within 24 to 36 hours after fertilization. Development proceeds with four-cell and eight-cell embryos and a morula, still in the oviduct, forming at approximately 48, 72, and 96 hours, respectively. The morula enters the uterine cavity at around 4 days after fertilization, and subsequently, a blastocyst develops at approximately 6 days after fertiliza-
Granulosa (cumulus cells)
First polar body
Granulosa (cumulus cells)
Spindle of first mitotic division
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.