33.Internal fertilization in mammals and humans. Blockage of the polyspermy. In vitro fertilization.
Covers where human fertilization occurs in the uterine tube, how sperm travel and become ready to penetrate the egg through capacitation and the acrosome reaction, and how external fertilization in fishes and amphibians differs. Explains polyspermy and the fast block, with its charge-based mechanism in sea urchins and other species, then the slow cortical reaction that hardens the zona pellucida in mammals. Includes two pictures and a list of species examples.
Topic 33 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 6 sections, about 475 words (3 min of reading), with 8 flashcards.
In this note
Internal fertilization in mammals and humans. Blockage of the polyspermy. In vitro fertilization.
Fertilization (Internal – Human)
Examples in Vertebrates
Polyspermy & Its Prevention
Fast Block to Polyspermy
Slow Block to Polyspermy (Cortical Reaction)
Fertilization (Internal – Human)
Fertilization is the fusion of male and female gametes, and in humans it typically occurs in the ampullary region of the uterine tube — the widest part of the tube, located near the ovary.
Sperm can remain viable in the female reproductive tract for several days. However, only ~1% of ejaculatory sperm even enter the cervix, where they can survive for many hours.
Once inside the tract, sperm move toward the uterine tube mainly through muscular contractions of the uterus and oviduct, not through their own propulsion. The journey from the cervix to the oviduct may take as little as 30 minutes or as long as 6 days.
After reaching the isthmus, sperm temporarily become less motile and pause. At ovulation, signals such as chemoattractants from the cumulus cells surrounding the egg re-activate the sperm, guiding them to the ampulla, where fertilization usually takes place.
Before a sperm can fertilize an oocyte, it must undergo:
Capacitation
Acrosome reaction
These steps allow penetration of the egg’s outer coverings.
External Fertilization
Occurs when sperm and egg fuse outside the bodies of the parents.
Typical in aquatic environments where water acts as a medium for sperm movement.
Sperm are adapted for mobility — usually flagellated and streamlined.
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