Physiology%20of%20Reproduction

1
Physiology of Reproduction
2
(No Transcript)
3
(No Transcript)
4
Gonadal development

• Both the testes and the ovaries are derived from
  the same gonadal primordium.
• There are two sets of ducts, the Wolfian duct and
  the Mullarian duct.
• Development of the primary sexual characteristics
  depends directly on the endocrine environment
  during development.
• An individual can be forced into either a female
  development or a male development by application
  of the appropriate hormones, regardless of
  genetic makeup.
• In the absence of hormonal stimulation, the
  gonadal primordium will develop into ovaries and
  the Mullarian ducts will develop into the uterine
  ducts, uterus and vagina.

5

• Development
• The sex organs themselves, along with all their
  associated ducts and glands are referred to as
  the Primary sexual characters
• Secondary sexual characteristics are structures
  which will enhance reproduction, but are not
  necessarily required. For example, beard growth
  in men.

6

• Without hormonal stimulation, the Wolffian duct
  regresses.
• In males, the gonadal primordium begins to
  secrete testosterone and Mullarian Inhibiting
  Substance (MIS).
• Testosterone stimulates the development of the
  Wolffian ducts, which subsequently differentiate
  into the vas deferens, epididymis and seminal
  vesicles.

7

• MIS causes the Mullarian ducts to degenerate
• Estradiol can prevent MIS from stimulating
  Mullarian duct regression.
• Testosterone is converted into dihydrotestosterone
  (DHT) by the enzyme 5a-reductase.
• DHT influences the development of the external
  genitalia.
• The genital tubercle becomes the penis.
• The genital folds become the shaft of the penis.
• The genital swellings become the scrotum.

8

• Without DHT, the external genitalia are
  feminized.
• The genital tubercle becomes the clitorus.
• The genital folds become the labia minora.
• The genital swelling becomes the labia majora.
• Currently, the structure of MIS is not known, but
  it appears to be a glycoprotein.
• Circulating levels of androgens (and possibly
  estrogens) also trigger differential development
  in the brain. Animals exposed to androgens
  during a specific critical window will develop
  male reproductive behavior, regardless of the
  genotype or the physical phenotype.

9

• Development of secondary sexual characteristics
• This usually coincides with the final maturation
  of the gonads. In humans, this is referred to as
  puberty.
• Mechanism controlling onset is unclear, but
  appears to involve the loss of inhibition of
  gonadal development.
• One potential candidate (at least in males) is
  melatonin.
• During childhood, melatonin is produced in the
  pars intermedia of the pituitary gland.
• However, after childhood the pars intermedia
  stops producing melatonin. Melatonin synthesis
  and secretion are taken over by the pineal gland,
  but at a much reduced rate.

10

• This drastic drop in melatonin secretion (gt75)
  may trigger the secretion of sex steroids by the
  adrenal glands and/or the testes.
• In females, the situation may be different.
• There is good evidence that the hormone leptin is
  also involved.
• Leptin is a hormone released by adipose tissue.
• Circulating leptin levels may reflect total body
  fat storage by the body.
• In females, a certain minimum total-body fat
  content is required for puberty to progress and
  for maintenance of the menstrual cycle.

11
Male reproductive system
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
Spermatogenesis I

• The immature germ cell in the male is referred to
  as the spermatogonium.
• These cells are located just under the basement
  membrane of the seminiferous tubules, between
  adjoining sustentacular (Sertoli) cells.
• Since sperm production continues throughout adult
  life and at the peak, 100-200 million sperm can
  be produced daily, the spermatogonia are
  constantly renewed.
• The first step in spermatogenesis is a mitotic
  division of the spermatogonium. One of the
  daughter cells remains, to replace the original
  spermatogonium, while the other cell (now called
  a primary spermatocyte) undergoes meiosis.

19
Spermatid migration
20
Spermatogenesis II

• The first meiotic division yields two secondary
  spermatocytes. Usually, these secondary
  spermatocytes do not fully separate during cell
  division, leaving a direct cytoplasmic connection
  between the cells.
• Following the second meiotic division (again, an
  incomplete division), the cells are known as
  spermatids. As the germ cells are undergoing
  meiosis, they also migrate towards the lumen of
  the seminiferous tubule.
• As they approach the lumen, they shed much of
  their cytoplasm. They are attached to the
  Sustentacular cells, via specialized junctions,
  which provide nutrients.
• When the spermatids reach the lumen, they remain
  embedded within the sustentacular cells, where
  they undergo tail development, acrosome formation
  and nuclear condensation.
• Finally, the fully-formed spermatozoa are shed
  into the lumen of the seminiferous tubule, where
  they are carried to the epididymus. This whole
  process takes between 60 and 70 days.

21
Mitosis vs. Meiosis
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
Male reproductive ducts

• The spermatozoa traverse the epididymus in 2 to 4
  weeks.
• During this time, they lose most of the remaining
  cytoplasm, as well as increase in mobility.
• The epithelial cells which line the epididymus
  secrete proteins which bind to the sperm cell
  membranes, to enhance their forward mobility and
  ability to fertilize an ovum.
• The sperm migrate into the ductus (or vas)
  deferens, where they can be stored for several
  months.
• The vas deferens runs up through the spermatic
  cord, conducting the sperm to the prostate
  gland.

26

• The end of each ductus deferens (two) enlarges
  to form ampullae, where sperm are stored until
  ejaculation.
• The prostate contains the first part of the
  urethra (prostatic urethra) which is where the
  ejaculatory ducts merge with the urethra.
• The urethra exits the prostate, penetrated the
  urogenital diaphragm and runs the length of the
  penis.

27
(No Transcript)
28
Male sexual response

• Erection
• The first phase of the male sexual response is
  erection of the penis, which allows it to
  penetrate the female vagina.
• This occurs when the erectile tissue of the penis
  becomes engorged with blood.
• When a male is not sexually aroused, the
  arterioles supplying the erectile tissues are
  constricted.

29

• During sexual excitement, a parasympathetic
  reflex is triggered that causes these arterioles
  to dilate (NO2).
• As a result, the vascular spaces of the penis
  fill with blood causing the penis to become
  enlarged and rigid.
• Expansion of the penis also compresses the veins
  retarding the outflow of blood and further
  contributing to the swelling of the penis.
• This reflex is initiated by a variety of stimuli
  ranging from thought to touch.

30
Ejaculation

• A spinal reflex is initiated, producing a
  sympathetic discharge to the genital organs.
• As a result, the reproductive ducts and accessory
  glands contract peristaltically discharging their
  contents into the urethra.
• The muscles of the penis undergo a rapid series
  of contractions propelling semen from the
  urethra.
• This is followed by muscular and psychological
  relaxation and vasoconstriction of the arterioles
  serving the penis, allowing blood to drain out of
  the erectile tissue, which subsequently causes
  the penis to become flaccid again.

31
(No Transcript)
32
Role of the Accessory Glands

• The seminal vesicles are paired glands that
  produce about 60 of the semen.
• Their secretions contain fructose sugar, ascorbic
  acid and prostaglandins.
• These are sac shaped glands, approximately 5
  centimeters long, which lie along side the
  ampullae of the ductus deferens.
• They each empty into a short duct, the
  ejaculatory duct, which merges with the terminal
  end of the ductus deferens.

33
(No Transcript)
34

• These, in turn, fuse with the prostatic urethra
  which runs from the bladder through the prostate
  gland.
• The alkalinity of the fluid serves to neutralize
  the normally acidic environment in the distal
  urethra and in the vagina.
• The fructose is supplied as an energy source for
  the sperm, and the prostaglandins serve to
  stimulate smooth muscle contractions in the
  vagina and cervix.
• This is thought to facilitate the uptake of sperm
  into the uterus.

35

• The bulbourethral glands are paired glands that
  secrete a small amount of thick clear mucus. This
  secretion is released prior to ejaculation and is
  believed to neutralize traces of acidic urine in
  the urethra.
• The prostate gland is a single gland, which
  secretes about one third of the semen volume. It
  secretes a milky, slightly acidic fluid
  containing citrate, acid phosphatase and several
  proteolytic enzymes. These enzymes are probably
  involved in breaking down the mucus plug in the
  cervix. They also appear to contribute to the
  motility and viability of the sperm

36
Semen Production

• Remember, Sperm seminal fluid semen.
• Semen provides a transport medium for the sperm.
  It also provides nutrients for the sperm and
  chemicals that protect them, activate them and
  facilitate their movement.
• The amount of semen released during ejaculation
  is relatively small, about 2-6 ml but it contains
  50-100 million sperm per ml.

37
Sperm capacitance

• Freshly ejaculated sperm are incapable of
  fertilizing an egg.
• As the sperm travel up the female reproductive
  tract, they lose cholesterol from their membranes
• When the sperm reach the fallopian tubes, the
  membranes around the acrosome are fragile enough
  to allow the release of the acrosomal enzymes.

38
Brain-testicular axis
39
Female reproductive system
40
(No Transcript)
41
(No Transcript)
42
OOGENESIS I

• This process is the equivalent of spermatogenesis
  in the male. However, the two processes are
  vastly different.
• In females, much of the process occurs during
  fetal development.
• The primitive germ cells undergo numerous rounds
  of mitosis, which produces millions of oogonia
  (2n).
• Most of these oogonia are resorbed (through a
  process called atresia).
• However, a few hundred thousand begin meiosis and
  enter prophase I. These are now referred to as
  primary oocytes.

43
OOGENESIS II

• There are no oogonia present in the adult female.
  
• The primary oocytes are arrested in prophase I
  and become quiescent until puberty.
• Cyclical changes in LH and FSH will trigger three
  or four primary oocytes to finish meiosis each
  uterine cycle.
• During the two meiotic divisions, all the
  cytoplasm will stay with a single daughter cell,
  which is destined to become the ovum.
• The other three daughter cells simply develop as
  small polar bodies that are eventually degraded
  and resorbed.

44
(No Transcript)
45
(No Transcript)
46
(No Transcript)
47
(No Transcript)
48
(No Transcript)
49
(No Transcript)
50
Female Sexual Response

• As with Males, arousal is controlled by
  parasympathetic stimulation.
• Involves engorgement of the erectile tissues.
• Increased bloodflow to the external genitalia and
  vaginal walls.
• Stimulation of secretion of cervical mucous
  glands and greater vestibular glands.

51
Female Sexual Response (cont.)

• Rhythmic contact of the clitoris and vaginal
  walls, reinforced by touch sensations from the
  breasts and other stimuli, can lead to orgasm.
• As with male climax, female climax results in
  rhythmic peristaltic contractions of the uterus
  and vaginal walls and associated skeletal
  muscles.
• This is thought to enhance the migration of
  sperm up the female reproductive tract.
• Female climax is NOT required for fertilization.

52
(No Transcript)
53
(No Transcript)
54
(No Transcript)
55
(No Transcript)
56
(No Transcript)
57
Fertilization and pregnancy
58
(No Transcript)
59
Fertilization
60
Blocks to polyspermy

• If more than one sperm were to fertilize the egg,
  then the genetic complement would be 3n.
• In order to prevent multiple sperm penetrations,
  two responses have evolved in the egg.
• First, as soon as the first sperm head penetrates
  the egg, it triggers a massive influx of Na.
• This influx depolarizes the egg, making it
  positive inside. This repels the positively
  charged sperm, inhibiting penetration of more
  sperm.

61

• Second, the depolarization triggers an influx of
  Ca 2 . This Ca 2 facilitates the exocytosis of
  a number of secretory vesicles, known as cortical
  vesicles.
• The contents of these vesicles surrounds the egg,
  swells with water and gels, pushing other sperm
  away from the egg and blocking their entry.

62
(No Transcript)
63
(No Transcript)
64
(No Transcript)
65
Implantation
66
(No Transcript)
67
(No Transcript)
68
(No Transcript)
69
(No Transcript)
70
(No Transcript)
71
(No Transcript)
72
(No Transcript)
73
(No Transcript)
74
Chorionic villi
75
(No Transcript)
76
(No Transcript)
77
(No Transcript)
78
(No Transcript)
79
Placental hormones

• During early pregnancy, HCG is secreted by the
  syncitial trophoblasts.
• Later, the placenta secretes estradiol,
  progesterone, relaxin and somato-mammotropin.

80
Function of placental hormones

• HCG is similar to LH and maintains the corpus
  luteum in a functional state for 3-4 months.
• This keeps progesterone levels high and they
  maintain the functional endometrium.
• Relaxin increases flexibility in the pelvic
  joints, as well as suppressing release of
  oxytocin.
• Placental progesterone keeps the uterine wall
  intact.
• Somatomammotropin acts like prolactin and
  triggers the mammary glands to develop.
• Estrogen increases the sensitivity of the
  myometrium to mechanical irritation, as well as
  oxytocin stimulation.

81
Labour

• Towards the end of pregnancy, relaxin secretion
  falls off, thus, the uterus becomes more
  sensitive to oxytocin.
• Initially, the fetus secretes oxytocin into the
  maternal circulation.
• The oxytocin stimulates contractions, which push
  the head down against the cervix.

82

• This pressure on the cervix stimulates the
  release of oxytocin from the maternal pituitary
  gland.
• The maternal oxytocin causes more contractions of
  the uterus, forcing the head of the fetus against
  the cervix even harder.
• This is a positive feedback system.

83
Labour and delivery

• As the head of the fetus is pressed down against
  the cervix, it thins and then starts to dilate.
• This stage is known as the Dilation Stage and can
  last several hours, or days (usually around 8
  hours).
• Once the cervix has dilated, the fetus starts
  moving through the birth canal. Contractions are
  maximal and come about 2-3 minutes.
• This is known as the Expulsion Stage.
• If the vaginal wall has not stretched enough,
  tearing may occur.

84
Labour and delivery cont.

• There is also a chance that the fetus will get
  stuck in the birth canal (usually caused by
  insufficient molding of the head.
• In these cases, a cesarean section is performed.
• Finally, after expulsion of the fetus, the
  placenta detaches from the uterine wall and is
  delivered through the birth canal.
• This is known as the Placental Stage.

85
(No Transcript)
86
(No Transcript)
87
(No Transcript)
88
(No Transcript)
89
Nursing

• Two hormones are involved, PRL and oxytocin.
• PRL stimulates milk production, while oxytocin is
  required for the expression of milk from the
  breast.

90
Fertility issues