Chapter 12—Male Reproductive System

Chapter 12—Male Reproductive System

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Said an ovum one night to a sperm, “You’re a very attractive young germ.

Come join me, my sweet, let our nuclei meet

And in nine months we’ll both come to term.”

--Isaac Asimov

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Ch. 12-- Study Guide 1. Critically read:

– (A) pages pp. 239-247 right before Sexual Differentiation section;

– (B) Negative feedback regulators (pp. 254-255) right before prepuberty subsection

2. Comprehend Terminology (the text in bold/italic)

3. Study and understand the text and corresponding figures.

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12.1. Morphology of the testes— Leydig cells and

seminiferous tubules

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§ Testes (Fig. 12.2, x)-ADimension-- Oval organ; 3 cm (anterior to

posterior) x 2.5 cm (wide) x 4 cm (long)

Each testis is surrounded by two tunics (C.T.): (From outermost and moving in)

1.Its anterior and lateral surfaces are covered by tunica vaginalis

2.Tunica albuginea– testis itself has a white fibrous capsule

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§ Testicular Thermoregulation• Sperm cannot be produced at core body

temperature (too warm; 37 degrees Celsius): – Pampiniform plexus = near testicular artery,

a network of veins– forming countercurrent heat exchanger that cools arterial blood entering testis by 1-2 degree Celsius (Fig. x, y)

• When too cold: muscle contraction to:– Pull testes close to body– Contract and scrotum becomes taut– wrinkles skin reducing surface area of scrotum

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§ Testes (Fig. 12.1, 12.4)-B3. Septa divide testes into 250-300 wedge-

shaped lobules containing seminiferous tubules (where sperm are produced); each tubule lined with a thick germinal epithelium:– Several layers of germ cells for sperm production– Tall Sertoli (sustentacular) cells; function?

4. Between the seminiferous tubules are interstitial (Leydig) cells, the source of testosteroneFig. 27.10 and X

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See next slide for details of germ and sustentacular cells

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Lumen of seminiferous tubule

Spermatozoon

Spermatids

SecondaryspermatocytePrimaryspermatocyte

Tight junctions --see next slide

Spermatogonium

Sertoli cell

Stages of sperm maturation:

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12.1. Morphology of the testes— Spermatogenesis

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§ Spermatogenesis--A

1. Def. of spermatogenesis– The production of sperm cells through a series of mitotic and meiotic cell divisions• Location?• How long does it take?• Microscopic examination– two important

cell types (see next slide)

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§ Spermatogenesis--B

2. Two important cell types in seminiferous tubulesA. Germ cells– • In various stages of sperm development,

such as spermatogonia, primary spermatocytes, secondary spermatocytes

B. Sustentacular (Sertoli) cells– these cells provide crucial support for spermatogenesis

Figure x

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§ Spermatogenesis--C

3.Three major stages—A. Mitotic proliferation—• Spermatogonia located in the outermost

layer of the seminiferous tubule, outside the blood-testis barrier (BTB)

• One of the daughter cells (Type A spermatogonium) remain at the outer edge of the tubule; importance?

• The other daughter cell (Type B spermatogonium) starts moving toward lumen forming 4 identical primary spermatocytes (2N) 12-17

§ Spermatogenesis--D

B. Meiosis—• Each primary spermatocyte (2N) must pass

through BTB (tight junction) and ultimately yield 4 spermatids (1N)

C. Spermiogenesis—• Spermatids become extremely

specialized and motile spermatozoa

• Sperm travel lightly

Figure w, x, y, z

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Meiosis

A

B

C

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SpermatogenesisStages:

Mitosis

Meiosis

Spermio-genesis

Spermatids

Secondaryspermatocyte

Primaryspermatocyte

Spermatozoa

First meioticdivision

Second meioticdivision

One daughter cell remainsat the outer edge of theseminiferous tubule tomaintain the germ cell line

One daughter cell movestoward the lumen to produce spermatozoa

Chromosomes:46; 2n (diploid number;single strands)

46; 2n(diploid number;single strands)

46; 2n (diploid number;doubled strands)

23; n(haploid number;Single strands)

23; n(haploid number;double strands)

23; n(haploid number;single strands)

Spermatogonia

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Figure 27.16

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12.1. Morphology of the testes— Male reproductive

tract

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§ Sperm’s journey (male reproductive tract)

1. Testes– sperm-producing organs; inside the scrotum (skin-covered sac)

2. Routes (spermatic ducts) the sperm travel: A-Testes B-Efferent ductules C-Epididymis D-Ductus deferens E-Ejaculatory duct F-Urethra Exterior

Fig. x

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Urinary bladder

DF

Glans penis A

C

3. Bulbourethral gland

E

1. Seminal vesicle

Ureter2. Prostate gland

B

Penis

Anterior/posterior view?

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12.2. control of testicular function— Leydig cells

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§ Leydig cells1. Principal role of Leydig cells– synthesize

testosterone (& estrogen) in response to LH

2. In the fetus-- initially Leydig cells depend on chorionic gonadotropin; later on LH

3. After birth– Leydig cells regress and die until puberty

4. Initial steps of testosterone formation– similar to adrenal cortex; from cholesterol to pregnenolone

5. Four other enzymes convert pregnenolone (21 C) to testosterone (19 C) (Fig. 12.5) 12-26

Testosterone

Pregnenolone

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§ Germinal epithelium (Sertoli cells)1. Principal role of germinal epithelium

(Sertoli cells)– produce sperm

2. Sertoli cells– only cells known to express FSH receptors in human males; also have testosterone receptors

3. FSH, LH, and testosterone all play vital roles in spermatogenesis. How so for LH and testosterone?

Fig. 12.6 12-28

CREB– cyclic AMP response element binding protein

LH

FSH

Leydig cell

Sertoli cell

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12.3. Testosterone

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§ Secretion and metabolism of T1. Testosterone (T) is the principal androgen

secreted by the mature testis. In that 5% is from adrenal cortex.

2. Aging– no sharp drop in T (unlike estrogen in the postmenopausal woman)

3. In blood 98% of T binds to binding proteins — albumin, sex hormone-binding globulin (SHBG also called TeBG; having higher affinity for T), and ABP (androgen binding protein) in Sertoli cells

4. The other 2% of T can diffuse out of the blood

5. Liver is principal site of degradation of T; T also can convert to other steroids (Fig. 12.7)

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§ Mechanism of action by T1. Nuclear receptors-- Testosterone (T) often

converts to 5alpha-dihydrotestosterone before binding to their nuclear receptors (Fig. 12.8)

2. T also may bind to membrane receptors-- either directly or through the bind of the sex hormone binding globulin

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§ Effects of testosterone1. Testosterone promotes -- growth, differentiation,

and function of accessory organs of reproduction.

2. Maintenance of normal reproductive function in the adult depends on continued T secretion.

3. T stimulates transport and delivery of sperm

4. On sexual characteristics– during early adolescence, T stimulates growth of pubic hair, axillary, and facial hair; lowers the pitch of the voice

5. T stimulates secretion of the hormone erythropoietin; T increases sexual drive (libido) in both men and women .

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12.4. Negative feedback regulators

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§ hypothalamus-pituitary-testis1. Hypothalamus GnRH– stimulates secretion of

both LH and FSH

2. Testosterone, which is secreted in response to LH, acts as a feedback regulator of LH.

3. FSH stimulates the Sertoli cells to synthesize and secrete inhibin (glycoprotein), which regulates FSH secretion.

Fig. 12. 17

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Stimulation

Inhibition

Stimulation

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