12-2: Regulation of Human Reproduction

Question 1

two modes of reproduction in animals

Answer 1

-asexual and sexual

Question 2

sexual reproduction

Answer 2

-the fusion of haploid gametes forming a diploid zygote (embryo -> fetus)
• Female gamete = egg (aka oocyte) -> large, non-motile
• Male gamete = sperm -> small, motile
• Yields genetically-different organisms

Question 3

asexual reproduction

Answer 3

-new individuals generated without fusion of gametes
• Yields genetically-identical organisms (aka clones)
-Parthenogenesis in bees
= the egg develops without being fertilized, rare in vertebrates and occurs as a response to low population density

Question 4

Variation in patterns of sexual reproduction

Answer 4

-adaptation to a challenge in finding a mate in sessile, burrowing animals and some parasites = hermaphroditism

Question 5

Hermaphroditism

Answer 5

-each individual has both male and female
reproductive systems
• A result of that => any two individuals can mate

Question 6

bluehead wrasse

Answer 6

• Adaptation to a challenge of finding a mate in
bluehead wrasse
• Bluehead wrasses may reproduce in four
different ways throughout their lifetime:
• 1) as a female in a group spawning event;
• 2) as a female in a pair spawning event within the territory of a large male;
• 3) as a small male in a group spawning event; and
• 4) as a dominant, terminal male in a pair spawning event within its own territory.

Question 7

oysters

Answer 7

• Adaptation to maximize gamete
production in oysters
• The individuals reproduce as males
first, and then when their size is
greatest - as females (they produce
more gametes).
• A result of that => releasing more
gametes tends to result in more
offspring

Question 8

fertilization

Answer 8

• The union of sperm and egg: external or internal
• External: eggs released into environment -> male fertilizes them
-Requires: a moist habitat; proper timing; environmental cues; courtship behaviors
-Examples: many amphibians, some fishes, echinoderms
• Internal: sperm deposited in or near the female reproductive
tract -> fertilization within the female tract
-Requires: compatible reproductive systems; cooperative behavior
-Examples: mammals, reptiles, birds

Question 9

Functions of Sex Hormones

Answer 9

-Direct regulation of gametogenesis: promote gamete formation
-Additional functions (indirect regulation)
• The appearance of the primary sex characteristics (internal and external anatomy; e.g. development of structures directly involved in reproduction)
• Formation of the secondary sex characteristics (physical and behavioral characteristics, e.g. male vocalization, facial and pubic hair, aggressiveness, etc.)
-Upon reaching sexual maturity, human males carry out
spermatogenesis continuously, whereas human females produce gametes in cycles

Question 10

functions of reproductive cycle

Answer 10

• produce eggs
• allows successful fertilization
• maintain pregnancy

Question 11

human females’ reproductive tract

Answer 11

• eggs produced in ovary
• eggs fertilized in oviduct
• sperm comes thru the vagina
• fertilized egg (zygote) implants in uterine wall

Question 12

female reproductive cycles

Answer 12

• both controlled by hormones and synched:
  1) Follicular phase of the ovarian cycleoverlaps with the menstrual flow and proliferative phase of the uterine cycle.
  2) Luteal phase of the ovarian cycle overlaps with the secretory phase of `the uterine cycle

Question 13

The ovarian cycle: the follicular phase

Answer 13

-It lasts from day 1 to day 14 of a 28-day long cycle, and includes follicle growth and ovulation of a (usually) single egg.
-Ovarian follicle is a microscopic
structure in the ovary that
contains the developing oocyte
and secretes estrogen.
Previous figure in words
• This is what happens inside the ovaries:
1. FSH stimulates follicle growth in the ovaries. Several follicles
begin to grow in each cycle, but usually only one follicle from just one ovary matures, and the others disintegrate.
2. The cells of the growing follicle start to make estradiol (estradiol is the main type of estrogen in humans) themselves, as they continue to mature.
3. The maturing follicle containing a fluid-filled cavity, enlarges, forming a bulge at the surface of the ovary.
4. The follicle and adjacent wall of the ovary rupture, releasing the oocyte. This is termed ovulation.

Question 14

luteal phase

Answer 14

-It lasts from day 15 to day 28 of a 28-day long cycle
• This is what happens inside the ovaries:
1. LH stimulates the follicular tissue left behind in the ovary to
transform into a glandular tissue called corpus luteum.
2. The corpus luteum secretes progesterone and estradiol.
-The corpus luteum secretes progesterone, which is a steroid
hormone responsible for the development of the endometrium and maintenance if pregnancy occurred.
-If pregnancy didn’t occur, corpus luteum would have degenerate and become destroyed

Question 15

follicle growth

Answer 15

• This cycle includes follicle growth and ovulation of a usually single egg.
• The follicular phase of the ovarian cycle lasts from day 1 to day 14, and considers the following stages:
1) Primary follicle: this is in fact the primary oocyte with special cells surrounding it for nourishment. Only one follicle will actually mature each
month, and the others that begin the process soon degenerate.
2) Growing follicle: the cells of the growing follicle start to make estradiol (main estrogen in humans), and the egg matures.
3) Mature follicle: it contains an internal fluid-filled cavity, and it grows very large, forming a bulge near the surface of the ovary.
-The follicular phase ends in ovulation on day 14.

Question 16

luteal phase 2

Answer 16

• From the ovulation onward, which are days 15 to 28, the luteal
phase takes place, through the following stages:
• Empty follicle: once the follicle releases the egg, it becomes empty.
• Corpus luteum: follicle cells transform into a glandular structure called
corpus luteum (corpus=body, luteum=yellow). Corpus luteum secretes
progesterone and estradiol.
• Corpus luteum regression: if fertilization does not occur the corpus luteum
disintegrates. If pregnancy occurs, it continues to secrete progesterone,
which maintains the thick uterus lining for zygote to implant.
• If no pregnancy, the cycle resumes again.

Question 17

menstrual cycle

Answer 17

• menstrual flow (day 0-5): lining of uterus (endometrium) is shed
• proliferative phase (day 6-14): endometrium builds up
• secretory phase (day 15-28): endometrium continues to thicken, if no pregnancy, it sheds

Question 18

hypothalamus hormones

Answer 18

-GnRH stimulates anterior pituitary gland

Question 19

anterior pituitary hormones

Answer 19

-stimulated by GnRH produces LH and FSH also referred as gonadotropins, target the Hormonal regulation in human females

Question 20

ovaries hormones

Answer 20

-estrogen, progesterone

Question 21

Hormonal regulation in human females

Answer 21

• Negative feedback => lower concentrations of estradiol and
progesterone inhibit GnRH and LH and FSH
• Positive feedback => higher concentrations of estradiol stimulate hypothalamus to increase its output of GnRH
• This positive feedback results in final maturation of the follicle.

Question 22

hormone changes

Answer 22

Hypothalamus: continuously releases gonadotropin-releasing hormone (GnRH), 24/7, unless actively prevented from doing so by other hormones (most notably progesterone).
• FSH and LH are released into blood due to influence of GnRH from the hypothalamus. FSH and LH stimulate the follicle to grow.
• Estradiol is released due to positive influence of FSH and LH. Estradiol stimulates
the growth of the endometrium of the uterus. Throughout most of the cycle, estradiol inhibits the release of gonadotropin by the hypothalamus and LH/FSH
by the anterior pituitary.
• Day 12: Things change!
• On about this day, estradiol levels get so high that its influence radically changes.
This is unexpected, but it happens. On day 12, estradiol switches roles and positively stimulates the hypothalamus (and consequently anterior pituitary) resulting in a surge of LH and FSH at this time

Question 23

day 0-7

Answer 23

• Uterus is shedding its
lining;
• A follicle is beginning to
develop. It is producing
estradiol at this time;
• Estradiol level is still low
=> it suppresses LH and
FSH secretion via negative
feedback inhibition

Question 24

day 8-14

Answer 24

• The follicle is larger. It
produces more estradiol. It
also produces progesterone;
• Large amount of estradiol
=> positive feedback on LH
and FSH secretion. This
causes a spike in LH
secretion;
• The LH surge triggers
ovulation.

Question 25

day 15-21

Answer 25

• The corpus luteum
develops from the
remaining follicle. It
secrets lots of
progesterone and
some estradiol.
• Rising progesterone =>
lowers LH and FSH
production, causing
thickening of the
endometrium.

Question 26

day 22-28

Answer 26

No fertilization => the corpus
luteum degenerates =>
progesterone drops =>
degeneration of the
endometrial lining
• GnRH, LH, and FSH are not
inhibited any more;
• LH and FSH rise;
• A new menstrual cycle begins.

Question 27

menopause

Answer 27

-after ~500 cycles
-btwn ages 46-54
• During these years the ovaries lose their responsiveness to FSH and LH, resulting in a decline of estradiol production by the ovaries.
-Possible consequences are: hot flushes, vaginal and bladder discomfort,
even heart disease and osteoporosis.
• Osteoporosis is losing of the bone density and deterioration of bone tissue.
One in four women over the age of 50 has osteoporosis. (Men also suffer
from it, but with 50% less occurrence than women!)

Question 28

hormone regulation in males

Answer 28

Hypothalamus and its gonadotropin releasing hormone play the same
role as in females - affecting anterior pituitary to produce same hormones, LH and FSH. However, the latter two have different target
organs than in females.
• FSH affects the Sertoli cells, which nourish sperm cells. In turn, Sertoli cells produce inhibin which shuts of FSH secretion.
• Note that ovaries also produce inhibin, even though we did not point that before.
• LH affects Leydig cells which produce testosterone. Testosterone affects spermatogenesis and exerts negative feedback onto pituitary
and hypothalamus.