Transcript 11.4: Reproduction

11.4: Reproduction
11.4.1
spermatids
Sertoli cell
Developing
spermatozoa
Leydig cell
Germinal
epithelium cells
(spermatogonium)
Each time a spermatogonium divides it may either undergo mitosis, to maintain the number of
spermatogonia, or spermatogenesis.
Spermatogenesis is the production of spermatids by the process of meiosis.
Spermatogenesis begins with meiosis I to produce two haploid cells.
Each haploid cell divides again during meiosis II to produce a total of 4 spermatids.
Spermatids undergo major rearrangements of their cellular components as they differentiate
into sperm.
Sertoli cells regulate the process of spermatogenesis and nourish the developing sperm.
As the spermatids develop they migrate from the outer edge of the seminiferous tubule to the
lumen, the central cavity of the tubule.
Once mature, sperm are released into the lumen (at a rate of several million per day) and
pushed to the epididymis where they acquire motility.
11.4.3
Spermatogensis:
LH (secreted by pituitary gland): Stimulates
the secretion of testosterone by the testes
FSH (secreted by pituitary gland):
Stimulates the first division of meiosis
Testosterone (secreted by Leydig cell):
Stimulates the second division of meiosis
and cells to develop into sperm
Primary
follicle
Germinal
epuithelium
Secondary
oocyte
Secondary oocyte
Mature follicle
Primary follicles
Secondary oocyte
inside mature follicle
Polar bodies
Gametogenesis
Spermatogenesis
Oogenesis
Gametogenesis
Spermatogenesis
• Mitosis in germinal
epithelium cells
(spermatogonium cells)
Oogenesis
• Mitosis in germinal
epithelium cells
(oogonium cells)
Gametogenesis
Spermatogenesis
• Mitosis in germinal
epithelium cells
(spermatogonium cells)
Oogenesis
• Mitosis in germinal
epithelium cells
(oogonium cells)
Gametogenesis
Spermatogenesis
• Mitosis in germinal
epithelium cells
(spermatogonium cells)
• Some cells remain as
germinal epithelium cells
and some undergo meiosis
Oogenesis
• Mitosis in germinal
epithelium cells
(oogonium cells)
• Some cells remain as
germinal epithelium cells
and some undergo meiosis
Gametogenesis
Spermatogenesis
• Mitosis in germinal
epithelium cells
(spermatogonium cells)
• Some cells remain as
germinal epithelium cells
and some undergo meiosis
• Cell growth before meiosis
Oogenesis
• Mitosis in germinal
epithelium cells
(oogonium cells)
• Some cells remain as
germinal epithelium cells
and some undergo meiosis
• Cell growth before meiosis
Gametogenesis
Spermatogenesis
• Mitosis in germinal
epithelium cells
(spermatogonium cells)
• Some cells remain as
germinal epithelium cells
and some undergo meiosis
• Cell growth before meiosis
• 2 divisions of meiosis to
produce spermatids [n]
(not differentiated into
spermatozoa yet)
Oogenesis
• Mitosis in germinal epithelium
cells
(oogonium cells)
• Some cells remain as germinal
epithelium cells and some
undergo meiosis
• Cell growth before meiosis
• 2 divisions of meiosis to
produce eggs [n]
Gametogenesis
Spermatogenesis
• Commences at puberty
• Meiosis is continuous
Oogenesis
• Commencing in fetal
development , a girl is born
with approx 400,000
developing egg cells
arrested in Prophase I
(meiosis is not continuous)
surrounded by a layer of
follicle cells forming the
primary follicle.
Gametogenesis
Spermatogenesis
• Meiosis is continuous
Oogenesis
• Meiosis is not continuous.
• During puberty, once a month,
6-12 primary follicles develop
forming two haploid nuclei [n].
Meiosis is arrested in Prophase
II. Now the developing egg cell
is called a secondary oocyte.
As follicle grows it develops a
large fluid filled cavity, now it
is called a mature follice. Only
one mature follicle develops
per month.
Gametogenesis
Spermatogenesis
• Four viable spermatids
produced per meiosis
Oogenesis
• One viable egg produced
per meiosis and three polar
bodies.
Gametogenesis
Spermatogenesis
• Four viable spermatids
produced per meiosis
• Equal division of cytoplasm
Oogenesis
• One viable egg produced
per meiosis and three polar
bodies.
• Unequal division of
cytoplasm
Gametogenesis
Spermatogenesis
• Four viable spermatids
produced per meiosis
• Equal division of cytoplasm
• Millions produced daily
Oogenesis
• One viable egg produced
per meiosis and three polar
bodies.
• Unequal division of
cytoplasm
• One egg cell on average
every 28 days
Gametogenesis
Spermatogenesis
• Released during ejaculation
Oogenesis
• Released on about day 14 of
the menstrual cycle
Gametogenesis
Spermatogenesis
• Released during ejaculation
Oogenesis
• Released on about day 14 of
the menstrual cycle
Gametogenesis
Spermatogenesis
• Released during ejaculation
Oogenesis
• Released on about day 14 of
the menstrual cycle
11.4.6
Corona radiata
Zona pellucida
nucleus
Cortical granules
Polar body
Polar body in orange
11.4.7
Epididymis:
*testicle fluids are removed and the sperm
concentrated
*sperm mature here and develop the ability to swim
Seminal vesicles:
*adds nutrients that include fructose sugar for
respiration
*mucus to protect sperm in the cell
Prostate:
*adds fluids that neutralise the vaginal acids
11.4.8
Oogenesis
Spermatogenesis
Location
Testes
Ovaries
Onset
Production begins at puberty
Production begins in a female fetus
Duration
Meiosis 1 and 2 are continuous
Meiosis stops for many years
Final products
(1 diploid cell and) 4 sperm cells
(1 diploid cell ànd) 1 egg 3 polar bodies
Quantity
Several million sperm per day
One egg per month
Mobility
Self-propelling/ have flagellum
Do not have flagellum
Size
Sperm is much smaller than egg
Egg is much larger than sperm
11.4.9
1. Attracted by a chemical signal,
sperm swim up the uterus and into the
oviduct.
11.4.9
2. Sperm wriggle
through corona radiata
cells
11.4.9
2. When the first sperm contacts the zona pellucida it triggers the
acrosome reaction, in which the contents of the acrosome are released.
11.4.9
3. Proteases from the acrosome digest the zona pellucida enabling the sperm
to reach the plasma membrane of the egg.
11.4.9
4. When 1 sperm finally contacts the egg, the cell membranes of egg and
sperm fuse, and the sperm head is drawn into the cytoplasm of the egg.
5. As the sperm enters it triggers two changes to the egg. First, it triggers the
cortical reaction which causes vesicles near the surface of the egg to harden the
zona pellucida thereby preventing additional sperm from entering.
11.4.9
6. Second, it causes the egg to undergo a second meiotic division, producing a
haploid gamete and a second polar body.
11.4.10
11.4.10
11.4.11
1. Fertilisation occurs in the oviduct. After fertilisation, the
egg continues its passage along the oviduct, where the
first cell divisions (mitosis) take place.
11.4.11
2. The first division occurs about 36 hours after fertilisation.
About 24 hours later, the 2 cells divide to form 4 cells.
11.4.11
3. At 4 days a solid ball of cells called the morula has
formed: it is no larger than the original egg.
11.4.11
4. By about 7 days there are more than 100 cells arranged
around a central cavity; a stage called the blastocyst.
11.4.11
5. When the blastocyst reaches the uterus it becomes
implanted in the endometrial lining, where it absorbs nutrients
and thickens.
11.4.11
6. The developing embryo extends fingerlike projections
into the uterus that develop into part of the placenta.
11.4.12
1. After an embryo implants in the uterine wall, tissues of
the uterus and fetus form the placenta.
2. The placenta has 2 major functions: 1) production of estrogen & progesterone, and
2) exchange of molecules between maternal & fetal blood.
Progesterone and estrogen are needed to prevent menstruation and build up the wall
of the uterus.
11.4.12
3. The placenta begins to form when the fetus develops a villus,
a finger-like growth into the uterus.
Blastocyst implanting
4. The number of villi increases steadily to meet the needs of the growing fetus.
Maternal blood flows out of capillaries into inter-villous spaces surrounding each
villus.
Fetal capillaries are very close to the surface of each villus (within 5 µm of the
maternal blood).
11.4.12
5. The cells separating fetal and maternal blood form a selectively permeable
barrier known as the placental barrier.
Microvilli project from each villus to increase surface area, which allows rapid
diffusion of molecules.
11.4.12
6. Nutrients and O2 diffuse from maternal blood to fetal blood, and carbon dioxide
diffuses from fetal blood to maternal blood.
11.4.12
7. Fetal blood flows toward the placenta in the umbilical artery, and away from
the placenta in the umbilical vein.
11.3.4
Materials are exchanged between
the maternal and fetal blood in the
placents
11.4.13
The fetus is supported by and protected by the amniotic fluid and
amniotic sac
11.4.15 End of pregnancy
1. End of gestation:
progesterone levels
fall.
2. With fall of
progesterone, the
pituitary gland secretes
the hormone oxytocin.
3. The oxytocin causes the smooth muscle in the walls of
the uterus to contract and labour has begun
3. This in turn further stimulates the pituitary of the mother
to release more oxytocin. POSITIVE FEEDBACK
4. The strength and frequency of the myometrial contractions is
further increased.
5. The strength and frequency of the myometrial
contractions is further increased.
6. In turn this further stimulates more oxytocin production
7. The process builds with stronger and stronger contractions
final the child passes though the cervix and vagina to be born.
Contractions continue for a further period until the placenta is
delivered (after birth).