Pregnancy and Birth
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fertilization
the union of a sperm with an egg to form a zygote
fertilization typically occurs in
the ampulla of a Fallopian tube
semen is deposited into the vagina as a liquid, but
fibrinogenase converts fibrinogen into a fibrin clot within one minute to prevent semen from leaking out of the vagina, and also to protect the sperm from the acidity of the vagina
after about 20 minutes,
vaginal acidity in neutralized and sperm can survive so semen clot liquefies
of the several hundred million sperm ejaculated into vagina,
fewer than 1% survive
the cervix contains a maze of
folds and outpocketings that can easily trap sperm
shortly before ovulation,
cervical mucus becomes thinner to facilitate passage of sperm through cervix
approx one million sperm survive
journey through cervix
the uterine cavity is filled with
a watery fluid through which sperm can easily pass
only a few thousand sperm survive to enter the Fallopian tubes and
they move through the Fallopian tubes assisted by ciliary action
only 50 to 100 sperm will actually reach
a secondary oocyte
sperm appear to be drawn toward
an oocyte by some type of chemical communication, but before they can fertilize an oocyte they must undergo capacitation
capacitation probably removes
a protective coating from head of a sperm that blocks release of acrosome enzymes needed for penetration of secondary oocyte
when sperm approach a secondary oocyte,
small openings form in the acrosome which allows enzymes to escape and digest a path for sperm to enter into the secondary oocyte
the secondary oocyte responds to sperm penetration by
blocking entry of more than one sperm, which prevents polyspermy from occuring
cortical granules inside membrane of
secondary oocyte are released
secondary oocyte completes
second meiotic division to produce ovum and polar body
the sperm fuses with the ovum to produce
a zygote and the chromosomes unite, which restores the diploid condition
secondary oocyte will degenerate within 24 hours of
ovulation unless it is fertilized
sperm can survive in female reproductive tract for
7 days so fertilization can occur during a one week “window of opportunity” around time of ovulation
if two oocytes are released during ovulation and each gets penetrated by a separate sperm,
two eggs may be fertilized and dizygotic (fraternal) twins result
approx 30 hours after fertilization,
the zygote divides by mitosis and forms two identical daughter cells
mitotic divisions continue to occur every
16 to 20 hours, which produces a collection of blastomeres that form a solid morula
these divisions occur quite rapidly and blastomeres
get progressively smaller, morula remains same size as original zygote