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11.4 notes
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Review
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Sperm and ovum are required for fertilization and sexual reproduction to occur.
These gametes are produced by the processes spermatogenesis and oogenesis.
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Spermatogenesis
o process of producing sperm through meiosis. It occurs in the testis, which
consist of narrow tubes called seminiferous tubules separated by cells called
interstitial cells
o
o Near the outer wall of the tubules are germinal epithelial cells called
spermatogonia. At any time a spermatogonia may be undergoing mitosis or
meiosis.
o
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o Spermatogonia undergo meiosis in order to produce spermatozoa which are
motile sperm. They also under mitosis to replenish cells that have developed
into spermatozoa.
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CELL
TYPE
CELL #
CHROMOSOMES (n = #
of chromosomes)
PROCESS CELL ENTERS
Spermatogonia
1
46 – 2n
Mitosis
Primary Spermatocyte
2
46 – 2n
Meiosis I
Secondary Spermatocyte
2
23 - n
Meiosis II
Spermatid
4
23 - n
Spermatogenesis
Spermatozoa
4
23 - n
o While undergoing meiosis and developing in the seminiferous tube,
spermatids are nursed (provided nutrients) by Sertoli cells.
 When they have fully matured and formed the flagella/acrosome
(head) they release from the Sertoli cells into the tubule lumen and
are swept away into the epididymis.
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o Seminiferous Tubule Diagram
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Oogenesis
o the process that produces egg cells in the ovaries.
o Production begins during the fourth to fifth month of pregnancy and is
completed by the seventh month.
o As a result, females are born having already produced their egg cells.
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CELL
TYPE
CHROMOSOMES
PROCESS CELL ENTERS
Oogonium
2n
Mitosis
Primary
Oocyte
2n
Meiosis I
Secondary Oocyte
1n
Meiosis II
Ovum
1n
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SPERMATOGENESIS
OOGENESIS
Millions of sperm
produced per day
One secondary oocyte produced per menstrual cy
4 gametes produced for each spermatogonium
1 gamete produced for
each oogonium
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Very small haploids
Very large haploids
Occurs in testes (gonads)
Occurs in ovaries (gonads)
Spermatozoa released during ejaculation
Oocyte released during ovulation
Haploid nucleus from meiosis
Haploid nucleus from meiosis
Continues all through
life from puberty
Starts at puberty and
ends at menopause
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Cytoplasm
o Sperm only contain the resources necessary to move and penetrate the egg.
During differentiation, most of the cytoplasm is eliminated. As a result, the
size of egg cytoplasm is significantly larger.
# of Gametes
o Ovulation occurs only once per menstrual cycle in humans, which typically
results in one egg being released. In contrast, testes produce sperm
continuously after puberty, millions per day.
Fertilization
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Polyspermy
o results when multiple sperm fertilize an egg, which is fatal. As a result, there
are several mechanisms to prevent this.
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In animals, fertilization can take place inside or outside the body.
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o Aquatic animals can release gametes directly into water where fertilization
occurs. Risks include predation and susceptibility to environmental factors.
o Terrestrial animals utilize internal fertilization, which prevents gametes from
drying out. After fertilization, embryos are protected.
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Development
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Pregnancy
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Some tissue from the developing embryo is used to create extra-embryonic sacs,
including the amniotic sac.
It is filled with amniotic fluid in which the fetus floats.
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Functions of the sac/fluid include:
o Acting as a cushion to blunt force
o Providing environment for free-movement
o Providing excellent thermal stability (fluid is mainly water)
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^^positive feedback loop. Very rare in the body
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After the baby has been delivered, the afterbirth is discharged from the
mother. This consists of the placenta and fetal membranes that once
surrounded the fetus.
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Gestation time is the period of time from conception until birth. This time
varies in mammals, but in humans the average is 38 weeks.
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