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Reproductive System

Chapter 26 – Day 3

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Gametogenesis Production of gametes = haploid “sexual reproduction”

cells Testes = Spermatogenesis

Ovaries = Oogenesis

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Spermatogenesis

Fig. 26.6

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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

Fig. 26.5

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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

Fig. 26.7

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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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Spermatogenesis

Fig. 26.5

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Spermatogenesis

Fig. 26.12

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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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Follicle maturation

Fig. 26.16

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Follicle maturation

Fig. 26.16

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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

Fig. 26.15

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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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Oogenesis

Fig. 26.26 – 1 – also look at 26.26-2

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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

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Sex & Sexual Dysfunction Excitement phase = “foreplay”

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Oogenesis

Fig. 26.15