Mammalian Reproduction

The Male Pattern or Hormones Involved in Spermatogenesis

•Testosterone is the major androgen or steroid hormone involved in sperm development

•Gonadotropin Releasing Hormone (GnRH) is released from the hypothalamus and travels to the anterior pituitary

•GnRH stimulates the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH)

•FSH: increases sperm development

•LH: stimulates the Leydig cells of testes to produce testosterone which promotes sperm production

Figure 46.14 Hormonal control of the testes

Figure 46.8 Reproductive anatomy of the human male

Figure 46.11x Spermatogenesis: Seminiferous tubules (left), sperm in semen (right)

Figure 46.12 Structure of a human sperm cell

Major Players in Semen Production

1. Seminiferous tubules: where sperm form

2. Leydig Cells: are placed all around the seminiferous tubules and produce testosterone

3. From S. tubules to the epididymis

4. Vas Deferens: carries sperm from epididymis to ejaculatory duct

5. E. duct passes semen into urethra

6. Glands adding secretions to semen:

• Seminal vesicles: alkaline; contains fructose; a coagulating enzyme, vitamin C and prostaglandins

•Prostate gland: anticoagulant enzymes, citrate (sperm nutrient)

•Bulbourethral glands: secretes fluid that neutralizes acidic urine in the urethra; generally released before orgasm; main contain some sperm.

7. Prostaglandins stimulate uterine contractions

8. Anticoagulants will liquefy the sperm for sperm movement to fallopian tubes.

9. Sperm are viable for about 2-4 days

Figure 46.11 Spermatogenesis

Major Components of Making Sperm

1. Occurs in the seminiferous tubules

2. Spermatogonia are the cells in the s. tubules that produce sperm cells

3. Sperm cells start at the periphery of the s. tubule and move towards the lumen as they undergo meiosis and mature

4. So:

•Spermatogonia develop into primary spermatocyte (2n)

•At the end of meiosis I they are 2o spermatocytes (1n)

•Go through meiosis II and are called spermatids

•Differentiate into sperm cells (spermatozoa)

•All spermatids develop into sperm cells

•Sperm cells are made continuously after puberty.

•No “resting periods”: uninterrupted sequence of events.

Figure 46.8 Reproductive anatomy of the human male (continued)

Female Pattern or Hormonal Involvement in Ovulation

1. Again, males produce sperm continuously, females do not, releasing only one egg (maybe a few) during each cycle

2. Menstrual vs. estrous cycle

Menstrual Cycle and the Power of Hormones

Menstrual Flow Phase

•Day 1: first day of menstrual flow

the endometrium

loss of functional part of endometrial lining

•Day 2: loss of endometrium continues

Proliferative Phase

•Days 7-21: endometrium regenerates the lost tissue and prepares for next ovulation

•Blood vessel growth re-establishes itself.

Secretory Phase

•About 2 weeks in length

•More blood vessel growth (vascularization)

•If implantation does not occur, then you are back to Day 1 and menstruation occurs.

Figure 46.13b Oogenesis

Figure 46.9x Ovary (left) and follicle (right)

Figure 46.15 The reproductive cycle of the human female

The Ovarian Cycle and Hormonal Influences

Follicular Phase

•Several follicles enlarge within the egg and each begin to develop. One of these follicles will continue with egg maturation while the others disintegrate.

•Ovulation occurs with one egg being released from the ovary.

•What stimulates the development of the follicles?

•FSH: released from the anterior pituitary

•Ovary has receptors on cells for FSH

•FSH production is controlled by GnRH from hypothalamus

•LH is also released by has no effect due to lack of receptors on the ovary.

•Upon FSH stimulation, follicle cells release estrogens

•These estrogens begin stimulation of uterine lining for fertilized egg implantation. So before ovulation, the uterus is preparing its lining by thickening and increasing blood vessel growth.

•Estrogen levels rise steeply which causes a stimulation of hypothalamus to release GnRH.

•This causes a spike in FSH and LH

•At this time receptors for LH are present on the follicle

•This causes a completion of follicle development / maturation

•The LH surge causes ovulation: egg is viable for about 1-2 days.

Figure 46.10 Ovulation

Luteal Phase

•LH causes the follicle to form the corpus luteum.

•C. luteum is a gland that secretes estrogens and progesterone.

•As estrogen and progesterone levels increase, they neg. feedback on hypothalamus to decrease GnRH output which then decreases FSH and LH.

•This seems appropriate because there is no sense stimulating follicle development yet until we see if the ovulated egg gets fertilized.

•Estrogen and progesterone levels maintain endometrium.

•C. luteum lasts for 8-10 days, then disintegrates causing estrogen and progesterone levels to decrease.

•This causes blood vessel disintegration of the endometrium and hence the loss of the endometrium

•Now hypothalamus can produce / release FSH which causes a new follicle to develop. So, yes, at the end of a menstrual cycle there is also the development of the next follicle /egg.

Figure 46.9 Reproductive anatomy of the human female

Figure 46.9 Reproductive anatomy of the human female (continued)

Figure 46.13a Oogenesis

Oogenesis or Ovum (egg) Development

1. As an embryo, cells develop in the ovaries that are called oogonium (stem cells that will eventually produce eggs).

2. By birth, the oogonium have matured into primary oocytes and this is a females lifetime supply of eggs. These primary oocytes are 2n and are “waiting” at prophase of meiosis I.

3. At puberty, age 9 –12, each primary oocyte develops into a secondary oocyte (1n) and a polar body (1n). The secondary oocyte is now in metaphase of meiosis II and “waits” there.

4. This secondary oocyte is ovulated.

5. If fertilization occurs, the secondary oocyte finished meiosis II and another polar body is produced.

Differences Between Spermatogenesis and Oogenesis

1. Cytokinesis is unequal in oogenesis. First polar body is smaller than secondary oocyte. The secondary oocyte will eventually become the egg. Polar bodies degenerate.

• In spermatogenesis, all cells made develop into sperm cells.

2. At birth, the ovary has all the primary oocytes it will ever have.

• In males, the cells from which sperm develop divide throughout his life.

3. Sperm are made constantly; eggs are made cyclically with rest periods in between.

Figure 46.16 Formation of the zygote and early postfertilization events

After about 24 hours

Ball of cells reaches uterus in 3-4 days.

By about 1 week, ball of cells has flattened into a blastocyst

Figure 46.17 Placental circulation

Figure 46.18 Human fetal development

Figure 46.19 Hormonal induction of labor

Figure 46.20 The three stages of labor

Birth Control Methods

1. Rhythm Method: no intercourse for a few days before and after ovulation to a few days after. Ovulated egg is viable for 1-2 days and sperm are viable for 2-4 days.

2. Male and Female Condoms

3. Diaphragm

4. Cervical Cap

5. IUDs: prevent implantation in uterus

• Plastic or metal

• Some problems with vaginal bleeding, tearing of uterus, tubal pregnancies.

6. BC Pills

• Combined Oral Contraceptives have both E and P

• Differing levels of estrogen and progesterone to prevent ovulation; but for one week you may take a placebo, allowing your body to cycle and menstruate.

• With doctor’s supervision you may be able to avoid the placebo, continue with hormones and skip your period totally

• Prevents ovulation (most of time)

• Since modern day BC pills have less estrogen than “old school pills”, an egg can be released (unexpectedly) 2-10% of time

• Endometrial lining is so thin that even if fertilization occurs no implantation is likely.

•Side Effects: headaches, acne, weight gain, vaginal infections, depression. So if you are already susceptible to headaches, this may, in my opinion, cause more since you have a tendency towards them.

•Health Risks: especially among smokers you may encounter abnormal blood clotting and heart attacks, cancer, gallbladder disease.

7. Minipill or Progestin (progesterone) only; considered safer than combined oral conceptive because it lacks estrogen which produces some cardiovascular problems.

• Can be in pill form or implants under the skin

• Pill form: suppresses ovulation

• Implant: Contains only progestin (progesterone) and this forms a mucus cap at the cervix and also thins the lining of the uterus to prevent implantation.

• Norplant: capsule injected under skin lasting 5 years

• DepoProvera: injected every 3 months

• Some references say there is no ovulation; others say may ovulate.

8. Fallopian Tubes can be tied / vas deferens can be cut (vasectomy)

9. RU 486

• progesterone receptor blocker

• Can be used within first 7 weeks

• Doctors supervision

• Hospitalized during treatment

• Uterine lining is shed so implanted embryo is lost

• Prostaglandins injected to induce contractions

10. Morning-after Pill or Levonelle

• Contains a female-type hormone “levonorestrel.

Figure 46.21 Mechanisms of some contraceptive methods