4/20/08 reproductive system chapter 26 – day 3. 4/20/08 gametogenesis production of gametes =...
TRANSCRIPT
4/20/08
Gametogenesis Production of gametes = haploid “sexual reproduction”
cells Testes = Spermatogenesis
Ovaries = Oogenesis
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Spermatogenesis Takes place in the seminiferous tubules
Between the tubules there is areolar tissue♦With interstitial cells = cells of Leydig
♦These cells produce adrogens including testosterone
Testosterone stimulates spermatogenesis
Seminiferous tubules have several layers of cells around a central lumen which is filled with fluid in the center
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Spermatogenesis The outermost cells (away from the lumen) =
spermatogonia go through cell division (mitosis) Making more cells with identical chromosome numbers These undergo further development to become primary
spermatocytes Second layer of cells (primary spermatocytes) begin
meiosis♦Making 2 cells with ½ the number of chromosomes = meiosis I
♦These are now secondary spermatocytes, = duplicated chrom.
♦Another division (meiosis II) occurs to yield 4 cells from the previous 2: these are the spermatids = immature haploid gametes
Cell division is complete, but sperm formation (spermiogenesis) needs to take place to produce actual sperm
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Spermatogenesis Within the spermatocytes & spermatids are
sustentacular cells ♦These cells help maintain the composition of the fluid = high
levels of hormones
•There is actually a blood-testes barrier so that blood hormone level changes will not alter spermatogenesis
♦Respond to testosterone to stimulate spermatogenesis
♦Support spermiogenesis by providing proper nutrients
♦Secrete inhibin – inhibits spermatogenesis
Spermiogenesis: one spermatid becomes a spermatozoa
Spermiogenesis takes place near the lumen – sperm/spermatozoa are released into the lumen
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Spermatozoon Acrosomal cap contains enzymes for
fertilization ♦These are released only upon contact with the egg
Head – nucleus with chromosomes (DNA) Middle Piece – mitochondria for energy Tail – flagellum for movement
Need energy for flagellar movement♦Comes from mitochondria
Need sugars (glucose) to make energy♦Comes from semen
Less content in sperm = better mobility (fast swimmers)
Fig. 26.8
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Oogenesis The ovaries are the site for oogenesis Oogenesis begins before birth in females Ovaries contain oogonia these go through mitosis to
make primary oocytes
Development is suspended until puberty
Many get disintegrated – a fixed number remain Every month after puberty - the development of one
primary oocyte takes place
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Oogenesis – Follicle Maturation In the ovary the primary oocytes are clustered near the
tunica albuginea Each oocyte is surrounded by a layer of follicular cells =
primordial follicle At puberty the female body secretes FSH (follicle
stimulating hormone) in cycles♦FSH activates a group of primordial follicles
♦Follicular cells enlarge
♦oocyte enlarges
♦Cells around the follicle produce estrogens
Few proceed to the next step… Become secondary follicle which, after further
enlargement becomes a tertiary follicle
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Oogenesis – Oocyte development Now the follicle has expanded a central chamber
(antrum) with follicular fluid Primary oocyte is suspended in meiosis I
♦LH (leutenizing hormone) levels increase
♦The oocyte completes meiosis I
♦This yields a secondary oocyte & a polar body
These are suspended in meiosis II until fertilization
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Oogenesis - Ovulation The secondary oocyte is released from the tertiary
follicle The corona radiata stays attached to the ovary Fluid currents push the secondary oocyte into the uterine
tube After the oocyte is released the follicle develops into the
corpus luteum♦These enlarged cells = endocrine cells
♦They make progestins (progesterone) and some level of estrogens (not nearly as much as during ovulation)
This stimulates the maturation of the uterine wall Approximately 12 days later the corpus luteum is
destroyed if the egg is not fertilized
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Hormonal Changes in Female Time of oocyte release depends on several hormones Gonadotropin releasing hormone (GnRH)
♦Secreted by the hypothalamus
♦Triggers the adenohypophysis to secrete Follicle Stimulating Hormone (FSH) & Leutenizing Hormone (LH)
♦Frequency of GnRH pulses determines which hormones are secreted
The start of the ovarian cycle = follicular phase♦GnRH is released slowly – triggers FSH
♦FSH stimulates the development of the primordial follicle into the primary follicle
♦The primary follicle begins secreting estrogen •The primary follicle can also secrete inhibin, which would inhibit FSH
production having the opposite effect
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Hormonal Changes in Female Increasing levels of estrogen and a higher frequency of
GnRH pulses stimulates secretion of Leutenizing hormone (LH)
High LH levels promote the release of the secondary oocyte from the tertiary follicle = OVULATION
Promotes the production of the corpus luteum Begins the Luteal Phase The corpus luteum secretes progesterone & low levels
of estrogen LH & FSH levels are low Completes buildup of endometrium This is maintained for 10-14 days (days 11-17 of a
womans cycle, she is most fertile
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Hormonal Changes in Female If there is no fertilization then the corpus luteum
degenerates to the corpus albicans♦Estrogen and progesterone production ends
♦The functional zone of the endometrium sloughs off – leading to menstruation (days 28-2 of cycle)
♦More GnRH is secreted & the pituitary gland secretes more FSH
If the egg IS fertilized it gets implanted in the endometrium & embryo development begins
Embryo and placenta release human chorionic gonadotropin (HCG) which keeps the corpus luteum from disintegrating – thus there is continuous secretion of estrogen & progesterone
If the levels of estrogen & progesterone decrease, this can lead to a miscarriage
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Hormonal Changes in Male GnRH released at a steady rate
♦Stimulates the release of FSH & LH from the pituitary
LH triggers testosterone production which affects ♦Spermatogenesis
♦Development of primary sex characteristics
♦Muscle growth
FSH triggers spermatogenesis
Inhibin, secreted by sustentacular (Sertoli) cells, slows down FSH production to control the rate of spermatogenesis
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Hormonal Changes in Male Testosterone is a very important hormone in males Deficiency in testosterone production:
♦All male reproductive organs decrease in size
♦Sex drive ↓
♦Penile erection ↓
♦Volume of ejaculate ↓
Castration♦There is no testosterone = loss of sex drive
♦Erection might still be possible
♦Gradual loss of facial hair & muscle tone
Prepubertal castration♦Male will lack facial hair, will have fatty deposits in breasts &
smooth skin
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Sex & Sexual Dysfunction Sexual intercourse is described by different terms:
♦Technical, colloquial, and clinical
♦Clinical = coitus
♦For legal purposes – intercourse = penetration of the penis into the vagina with or without ejaculation
Events of Coitus: There are 3 phases:
1.Excitement
2.Plateau
3.Orgasm/Ejaculation