Animals - Reproduction

December 5, 2002

1. To date we have discussed a number of organ systems whose structure and function are finely tuned to the physiological needs of the organism - e.g. acquisition of H₂O, O₂, and nutrients; elimination of metabolic waste products; sensory and motor interactions with the environment; defense against infection. But successfully accomplishing these goals will have no effect on the future of the species unless the individual can reproduce, i.e. generate offspring carrying many of its own genetic alleles.

2. Some animals are capable of asexual reproduction (e.g. sea anemones; flatworms), but the vast majority rely on sexual reproduction as their primary means of generating offspring.

• One essential step in sexual reproduction is the production of haploid sex cells or gametes (female = egg; male = sperm). Most animals are diploid, and produce gametes by meiosis.
• Fertilization is the fusion of egg and sperm. It generates a new individual endowed with a mixture of both maternal and paternal genetic alleles.
• The behaviors associated with fertilization vary greatly between animal species.
  1. Many aquatic animals rely on external fertilization - i.e. gametes are released into the water, and the egg and sperm encounter one another outside the parents' bodies.
  2. Most terrestrial animals - and some aquatic animals - rely on internal fertilization in which the female retains the egg(s) within her body and receives sperm from the male. Internal fertilization increases the probability that sperm and egg will meet successfully.
  3. Animals are only capable of sexual reproduction at certain times in their life. For instance, in humans sexual reproduction is not possible until after puberty, and females can no longer support fertilization or pregnancy after menopause (ca. 46-54 years of age).

3. The mammals evolved a unique mode of reproduction and development, and we will here focus on the reproduction of humans.

• In most animals, the fertilized egg develops as an embryo without any direct connection to its mother's body. But in humans and other 'placental' mammals, the fertilized egg divides to form a blastocyst (composed of about 100 cells) that implants into the wall of the mother's uterus.
  1. Following implantation, the embryo gains access to the mother's circulatory system and relies on her respiratory, digestive, excretory, and immune systems for its needs.
  2. The gestation period (fertilization to birth) in humans is 9 months. After the first 8 weeks, the implanted embryo is referred to as a fetus.
• To prepare itself for fertilization and implantation, the reproductive tract of a human female undergoes a sequence of physiological changes called the menstrual cycle. [Note: non-primate mammals experience a similar reproductive cycle called 'estrus'.]
  1. On average, the human menstrual cycle is 28 days in length [see Campbell, Fig. 46.15].
  2. During the cycle, the walls of the uterus - called the endometrium - thicken and become heavily invested with blood vessels (vascularization).
  3. On day 14 of the cycle, one of the ovaries releases a mature, fertilizable egg into the female reproductive tract, an event called ovulation.
• If fertilization leads to implantation, the an outer layer of the implanted embryo grows together with the endometrium to form an organ called the placenta.
  1. In the placenta, the maternal capillaries expand to form large chambers called blood pools.
  2. Fetal blood vessels grow new capillary beds that are pressed up against these maternal blood pools, forming protuberances called chorionic villi [see Campbell, Fig. 46.17]. In these villi the maternal and fetal blood are only separated by two thin endothelial cells, allowing for ready exchange of O₂/CO₂, nutrients, and metabolic waste products.
• If implantation does not occur, the uterus will shed and expel the extra layers of endometrium ('menstruation') at the end of the cycle. [In mammals that have estrus, the thickened endometrium is resorbed rather than being expelled.]
• Although not directly relevant to today's lecture, one of the other unique reproductive adaptations of mammals is the production of mother's milk, which the offspring will feed upon after birth.

4. The menstrual cycle is coordinated by an interaction between various parts of the endocrine system.

• Two key control elements are the gonadotropins - follicle-stimulating hormone (FSH) and luteinizing hormone (LH) - which are secreted by the anterior pituitary gland.
• At the beginning of the menstrual cycle, cells in the ovary have receptors for FSH but not LH. Each haploid egg is contained in a sphere of diploid cells - together, they are called a follicle - and in response to FSH a few of these follicles enter the latter stages of maturation.
  1. The maturing follicles grow in size, and they increase their normally low-level secretion of the steroid sex hormone estrogen. [Note: this makes FSH a 'tropic' hormone.]
  2. This increase in estrogen concentration stimulates the connective tissue of the endometrium to thicken and vascularize as the cycle proceeds.
  3. Once stimulated by FSH, the maturing follicle also begins to express the receptor for LH.
• When the blood concentration of estrogen reaches a certain threshold, it stimulates secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus. In turn, the GnRH stimulates increased FSH and LH secretion by the pituitary, resulting in a sharp upswing in their blood concentrations [see Campbell, Fig. 46.15].
  1. The sudden surge in LH concentration on days 11-13 of the cycle - combined with the recent appearance of LH receptor - stimulates one of the maturing follicles to burst open and release its egg into the reproductive tract (= ovulation).
  2. After ovulation, LH causes the remainder of the follicle to transform into a glandular structure called the corpus luteum.
• The corpus luteum continues to secrete estrogen [see above], but also begins to synthesize and secrete another, chemically related steroid called progesterone [see Campbell, Fig. 45.13] that was not present at earlier stages of the menstrual cycle.
  1. Progesterone works together with estrogen to maintain the thickened endometrium.
  2. In addition, progesterone acts on the hypothalamus (and, indirectly, the pituitary) to inhibit any further FSH and LH secretion. The latter two hormones rapidly return to their original levels [see Campbell, Fig. 46.15].
• If the ovulated egg does not give rise to an implanted embryo, the loss of FSH and LH will cause the corpus luteum to degenerate, which brings on the end of the menstrual cycle.
  1. Degeneration of the corpus luteum induces menstruation. It deprives the endometrium of the progesterone - and much of the estrogen - needed to maintain its extra layers, causing those layers to degenerate and be lost.
  2. The absence of progesterone also releases the hypothalamus and pituitary from inhibition [see above]. The pituitary beings to secrete FSH and LH, beginning a new menstrual cycle.

5. If the ovulated egg is fertilized and implants, the resulting pregnancy halts the menstrual cycle.

• Following implantation, the embryonic portion of the placenta (called the 'chorion') begins to secrete a peptide hormone named human chorionic gonadotropin (HCG).
  1. HCG replaces FSH and LH in keeping the corpus luteum alive. This maintains progesterone secretion, which prevents the shedding of the endometrium and the onset of another menstrual cycle.
• The chorion of the embryo is the only human tissue that synthesizes HCG protein. Thus, many pregnancy tests rely on the presence or absence of HCG in a women's blood or urine.

Learning Goals

1. What are the differences between external fertilization and internal fertilization?

2. Learn the sequence of the events that occur during the human menstrual cycle at the level described in these notes. This means learning the 5 different hormones (GnRH; FSH; LH; estrogen; progesterone), which organs secrete them, and what effect they have on the other tissues.

3. What is implantation, and why does it interrupt the menstrual cycle?

4. What is the placenta? What is the anatomical basis of chemical exchange between the maternal and fetal blood supplies in the placenta?