Download Chapter 17, Part A (Male Reproduction)

The Male Reproductive System
Figure 17.1!
1!
Scrotum
Supports and protects testes
Two sacs divided by CT + smooth muscle
septum
•  A. Dartos muscle = smooth muscle
Wrinkles skin of scrotum
•  B. Cremaster muscle
Raises and lowers testes
a) Temperature change or sexual
arousal
b) Maintain temp about 1°C < Tbody
2!
1
Male Reproductive System
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! Martini 2004!
3!
Testes!
Paired gonads (singular = testis)
Heterocrine glands
•  Produce sperm
•  Produce male sex hormones = androgens
Develop in embryonic abdomen near kidneys
•  Descend through inguinal canal (about 7th
month)
4!
2
Seminiferous Tubules
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Figure 17.3!
Sertoli cell
5!
Seminiferous Tubules
Spermatogenesis occurs here
Contains spermatogenic cells and sustentacular
cells
Processes occurring here:
1.  Mitosis
2. Meiosis
3. Spermiogenesis
4. Spermiation
(Spermatids → spermatozoa) (Release of sperm into
duct)
Spaces between tubules (interstitium) contain
interstitial cells (of Leydig)
•  Secrete androgens
6!
3
Spermatogenic Cells
One spermatogonium (primordial germ cell)
Mitosis produces
•  One new spermatogonium and…
•  One primary spermatocyte
Meiosis I produces
•  Two secondary spermatocytes
Meiosis II produces
•  Four spermatids
Spermiogenesis
•  Four spermatozoa
7!
Seminiferous Tubules
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Martini 2004!
8!
4
Sustentacular (Sertoli or Nurse) Cells - 1!
Extend from basement membrane to lumen
Functions:
1.  Form blood-testis barrier
Sertoli cells are joined by tight junctions
A. Prevents immune response against sperm
•  Sperm-specific antigens
B. Produce tubular fluid
C. Separate tubular fluid from interstitial fluid
•  (Tubular fluid high in androgens)
9!
Sustentacular (Sertoli) Cells - 2!
2.  Support mitosis and meiosis
•  Influenced by FSH
•  Control movement and release of
developing sperm
3.  Support spermiogenesis
•  Provide nutrients, chemical stimuli
4.  Secrete inhibin
•  Adjusts rate at which sperm develop
•  Inhibits FSH
•  ↑ Sperm production → ↑ inhibin secretion
→ ↓ FSH secretion
10!
5
Sustentacular (Sertoli) Cells - 3!
5.  Secrete androgen-binding protein
•  Keeps androgens in tubule fluid
•  Required for sperm production
6.  Phagocytize degenerate sperm
7.  Secrete estrogen
•  Function unknown, but apparently
necessary
FSH stimulates spermatogenesis
LH stimulates testosterone production
11!
Meiosis Terms - 1!
Spermatogenesis and oogenesis involve both
mitosis and meiosis
Chromosome = colored body
•  Composed of DNA and proteins
•  Visible as a separate unit under the ‘scope
Chromatid
•  One of a pair of identical, connected DNA
strands
•  Each is a copy of the original chromosome
12!
6
Meiosis Terms - 2!
Diploid number (of chromosomes)
•  Two complete sets of chromosomes
•  Diploid cell has two copies of each gene
(maternal and paternal chromosomes)
•  2n = 46 in humans
Haploid number (of chromosomes)
•  One-half the normal number (n = 23)
•  Haploid cell has 1 copy of each gene
(maternal or paternal chromosome**)
Gamete = sperm or ovum has n chromosomes
13!
Meiosis Terms - 3!
Zygote
•  Cell resulting from the fusion of sperm and
egg
Gene
•  A discrete unit of heredity
•  Composed of DNA
•  Located on a chromosome
•  Codes for a particular protein or RNA
Allele
•  An alternate form of a gene
14!
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Meiosis Terms - 4!
Homologous chromosomes
•  Two chromosomes belonging to a pair
(One from mom, one from dad)
•  Contain the same genes, but potentially
different alleles
•  22 of 23 pairs are homologous = autosomes
•  1 pair = sex chromosomes (XX or XY)
15!
Purpose of Meiosis!
1.  Take a cell that is diploid (2n) and make 4
gametes that are haploid (n) and…
2.  Take a cell that has one “dose” of DNA and
make 4 gametes that each have one-half the
normal “dose” of DNA
Then:
Sperm (n) + Ovum (n) = zygote (2n) → new
person
- and Sperm (1/2 DNA) + Ovum (1/2) DNA = zygote (1DNA) →
new person
16!
8
2n = 4!
1”dose” of
DNA!
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Figure 19.8!
Meiosis II
Spermatids/
spermatozoa
Secondary
spermatocytes
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Meiosis I
Primary
spermatocyte
DNA replication
Spermatogonium
Meiosis Overview !
2n = 4!
2 “doses”!
of DNA!
n = 2!
1 “dose”!
of DNA!
n = 2!
1/2 “dose”!
of DNA! 17!
Spermatogenesis!
(1D)
DNA replication!
(2D)
(1D)
(1/2D)
Life: The Science of Biology!
Pruves et al. 2001!
18!
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Spermatogenesis and Oogenesis
Spermatogenesis
Figure 19.11!
Oogenesis
19!
Crossing-over Occurs During Meiosis I!
•  Duplicated chromosomes
(bivalents) line up in
homologous pairs
•  Form tetrads
•  Homologous pairs line up on
equatorial plate
•  Orientation on equatorial plate
(maternal/paternal) is random
•  Non-sister chromatids (on
different chromosomes)
exchange DNA
•  2 or 3 crossovers per pair of
chromatids produces new
combinations of genes
Individual
chromosomes
DNA Replication
Duplicated
chromosomes
Bivalents =
2 sister chromatids
each
Tetrads
Crossing over
20!
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Number of Genetic Combinations - 1!
1.  Random orientation of chromosomes during
Meiosis I
•  Alignment of maternal and paternal
homologous chromosomes on equatorial
plate is random
•  Possible combinations = 2n
•  n = haploid number of chromosomes
•  223 = 8 million combinations per sperm or
egg
2.  Crossing-over
•  About 2 or 3 per chromosome pair
21!
Number of Genetic Combinations - 2!
3.  Random fertilization
Ignoring crossing-over…
•  8 million combinations for each female
gamete
•  8 million combinations for each male
gamete
•  64 trillion chromosome combinations for
each male-female pair
22!
11
Spermiogenesis!
Maturation of spermatids into spermatozoa
•  Takes about 5 weeks
•  Spermatids attached to each other by
cytoplasmic bridges
•  Surrounded by Sertoli cells
•  Spermiation = release of sperm from Sertoli
cells
•  About 300 million sperm produced per day
•  Live about 48 hours in female tract
23!
Spermatozoon Structure !
Head contains
DNA
Acrosome
•  Hyaluronidase
•  Proteases
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(Martini 2004)
Middle piece
•  Many mitochondria
Flagellum
24!
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The Epididymis
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Martini 2004!
25!
Epididymis (Plural = Epididymides)!
6 - 7 meters of coiled tubes
•  Pseudostratified columnar epithelium with
stereocilia (are actually microvilli)
•  Sperm expelled to ductus deferens or
absorbed
1.  Monitors and adjusts fluid composition
•  Stereocilia absorb fluid, concentrate sperm
2.  “Recycles” damaged, deformed sperm
3.  Stores sperm (about 2 wks), facilitates
maturation
•  Sperm become motile
26!
•  Still require capacitation to fertilize egg
13
Ductus (Vas) Deferens!
About 18 inches (45 cm)
long
Stores sperm (for several
months)
Propels sperm during
ejaculation
Has 3 smooth muscle layers
Runs within spermatic cord
Passes through inguinal
canal
Pseudostratified epithelium
Ductus
deferens
Ureters
Seminal
vesicle
Prostate
gland
Bulbourethral
gland in
pelvic
diaphragm
27!
Urethra!
Passageway for sperm (semen) and urine
Three portions:
•  Prostatic urethra in prostate gland
•  Membranous urethra in urogenital diaphragm
•  Penile (spongy) urethra in CCU
Terminates as external urethral orifice
28!
14
Seminal Vesicles are Paired Structures!
Secretions = about 60% of ejaculate
1. Alkaline pH
•  Neutralize male urethra and female tract
2. Fructose and citric acid
•  Substrates for ATP production
3. Prostaglandins
•  Increase sperm motility and viability
•  May stimulate contractions of female tract
4. Clotting factors
•  Form seminal clot in female within 5 min
(Significance unknown)
29!
Prostate Gland - Not Prostrate Gland!!
Secretes about 25-30% of ejaculate
1. Slightly acidic (pH ≈ 6.5)
2. Citric acid = ATP synthesis substrate
3. Proteolytic enzymes
•  E.g. Prostate specific antigen (PSA)
Breaks down seminal clot
4. Prostaglandins
•  ↑ motility, ↑ female contractions(?)
5. Seminalplasmin
•  Decrease infection in female(?)
6. Acid phosphatase - unknown function
30!
15
Bulbourethral (Cowper’s) Glands!
Paired
Found in pelvic diaphragm
Open into penile urethra
•  Secretion:
1. Alkaline - protects sperm from
acidic female tract
2. Mucus
•  Lubricates urethra and
glans penis
•  Protects sperm from
abrasion
31!
Seminal Fluid (Semen)!
Sperm + secretions from all accessory glands
Spermatozoa are a small part of semen
Typically 2 - 5 ml/ejaculation
•  Contains 20 - 100 million sperm
Slightly alkaline (pH 7.2 - 7.7)
1.  Neutralizes acidic female tract
2.  Transport medium for sperm
3.  Provides nutrients for sperm (fructose, citrate)
4.  Enzymes activate sperm (after ejaculation)
5.  Contains seminalplasmin (antibiotic)
32!
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Penis!
1. Pathway for voiding urine
2. Introduces sperm into vagina
Three masses of erectile tissue
•  Two corpora cavernosa
penis (CCP)
•  One corpus cavernosum
urethra (CCU)
(a.k.a. corpus spongiosum)
Glans penis
•  Distal end
•  Erectile, sexual sensations
33!
Testosterone - the Major Androgen!
Steroid derived from cholesterol
Testosterone production - negative feedback
•  ↓ blood [testosterone] → ↑ GnRH release →
↑ FSH and LH release
•  LH stimulates interstitial cells of Leydig to
produce testosterone
34!
17
Effects of Testosterone - 1!
Binds receptors in nucleus, regulates gene
expression
1.  In embryo
•  Development of reproductive system
•  Descent of testes
•  Development of external genitalia
2.  At puberty
•  Development, enlargement of sex organs
•  Development of secondary sexual
characteristics
•  Facial hair, deepening of voice, etc.
35!
Effects of Testosterone - 2!
3.  Other effects
•  Male sex drive (libido)
•  Stimulates spermatogenesis
•  Stimulates protein synthesis (anabolic
steroid)
•  High levels cause epiphyseal plate closure
(with estrogen)
•  Refuses to ask for directions J
Inhibin (from Sertoli cells)
•  Inhibits FSH and therefore sperm production
36!
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37!
Erection!
A parasympathetic reflex
1. Flaccid (non-erect) state:
•  Arterial supply to erectile tissue restricted
2. Erect state:
•  Parasympathetic nerve fibers release nitric
oxide (NO)
•  Arterial branches dilate → ↑ blood flow
in
•  Cavernous tissues swell → erection
38!
19
Ejaculation!
A sympathetic reflex
Smooth muscle sphincter at base of bladder
constricts
Contractions of:
•  Ampulla, ductus deferens
•  Seminal vesicles
•  Ejaculatory ducts
•  Prostate gland
•  Skeletal muscles in pelvic floor, base of penis
39!
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