Introduction to Human Physiology - 7 Reproductive Systems

Source: My personal notes from Introduction to Human Physiology | Coursera

Fertility Monitors

Commercial Clearblue product detects levels of LH and broken down estrogen (E3G) in urine to analyze timing of female cycle and ovulation.

Male Reproductive System

• Hypothalamus Pituitary (H-P) Gonad Axis

• View through the testes (the male gonad) - pronounced like testis

• The circled area is a cross section of a tubule

• Lumen of tubule (inside tube) shows nuclei (black flecks) of forming sperm.

Sexual Differentiation

How do you become a male or female?

There are several steps, which could go wrong

• Chromosomes: XY = male, XX = female

• Phenotypic sex = sex you appear to be; appearances from gonad hormones.

Things that could go wrong:

• Gonadal sex doesn’t match chromosomal sex due to mutation of SRY gene on Y chromosome. In this case, ovaries form instead of testes.

H-P-Gonad Axis, Common Axis

Called Common Axis since males and females have similar axes

• Release of GnRH is pulsatile.

• FSH = follicle stimulating hormone

• Sertoli promote formation of sperm. It secrete inhibin to feedback to FSH production

• Leydig cell secrete testosterone for sperm and feedback to pituitary

Maturation of the H-P-Gonad Axis

• Kisspeptin are the neurons that receive feedback from testosterone and control GnRH hormone secretion.

• Children exhibit low amounts of testosterone and estrogen; however, it is understood children have sensitive kisspeptin neurons which keep sex hormones low. During puberty, kisspeptin neurons sensitivity will alter though it is not well understood.

Key Concepts

Male Sperm and Hormone Production

Testis Structure

• Inside testis are tubules, formation of sperm occurs in tubules after puberty

• Males can be fertile their whole lives.

Sperm Production

Cross section of a tubule.

• Pink cells are sertoli cells. Sperm need testosterone and estrogen.

• Leydig cells create DHT will is a more powerful androgen than testosterone.

• It takes many sperm to fertilize an egg due to inefficient process. For example, a male with < 20 million sperm/ml is considered infertile.

• Sperm mature in 90 days.

• In other words, male sperm production is efficient, though fertilization of female egg is inefficient

Androgens

• DHT are important for sexual characteristics (e.g. hair growth).

• Note the anabolic actions (building of muscle mass).

• Prostate is an androgen receptive tissue, so more testosterone increases its size if there is too much. Excess testosterone could be used as a form of birth control, though remember sperm take 90 days for maturation so birth control will be in effect several months later.

Androgen Dependent Tissues

Estrogen and the Male Skeleton

Estrogen is important for males in these areas:

• Epiphyseal plate gets fused.

• Pathology: males with estrogen receptor/aromatase inactivation causes continual skeleton growth. Testosterone grows skeleton, but estrogen is the one to signal stopping of growth.

Key Concepts

Female Reproductive System

• Hypothalamus Pituitary (H-P) Gonad Axis

• Image shows 3 cells forming in follicle.

Common Axis

Very similar to male H-P-G axis

• Pulsatile release of Gonadotropin-releasing hormone (GnRH) resulting in Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

• FSH acts on granulosa cells in ovaries. Granulosa cells produce some estrogen and progesterone.

• LH act on theca cells that are outside of granulosa cells.

• One exception in similarities to male axis. Once each cycle, there is an increases in estrogen which causes increase in GnRH

Roles of the Ovary

Females have an extra organ compared to males - the uterus. The ovary controls the uterus

Oocyte Number

• Female germ cell (oocytes) cycle.

• The germ cells are generated in embryogenesis (they are born with the cells).

• In contrast to males, females are born with 10-20 million germ cells, though in the female’s lifetime only 400-500 are ovulated in her life (usually 1 egg per month). Not as efficient for germ cell production and maturation.

Oocyte Maturation

• Oocyte is surrounded by granulosa cells. Granulosa cells are surrounded by theca cells.

• FSH acts on cells in follicle (granulosa).

• LH and FSH matures follicles - how is it that only one follicle is matured at a time? It is a race in maturation.

  • LH and FSH makes follicle produce more estrogen, LH and FSH receptors increase in follicle (a positive feedback loop).

  • Eventually mature follicle signals a decrease in LH and FSH (particularly FSH). This results the death of less mature follicles, meanwhile the mature follicle with its many receptors is able to survive - the “chosen follicle”. It continues to mature.

Ovulation

• LH surge (increase) causes ovulation.

• Egg leaves from follicle. The old follicle is a corpus luteum and continues producing estrogen and makes progesterone. Corpus luteum is the reason for the naming of LH (luteinizing). LH forms corpus luteum.

Role of kisspeptin in LH Surge

• Females have kisspeptin neurons like males, they also have a second set of kisspeptin neurons that respond to the increase in estrogen close to ovulation that causes the LH surge.

Phases of the Ovary

Uterus and the ovaduct.

#3 shows follicle maturation and chosen follicle

#6 after about 2 weeks, if there is no pregnancy, corpus luteum degenerates. Estrogen and progesterone production decreases.

Key Concepts

Menstrual Cycle

Understanding the uterus.

• Estradiol = estrogen

• At beginning small increase in FSH and LH versus end of last cycle

The Menstrual Cycle with the Uterus

• In beginning of cycle (first 7 days), menstruation starts. Lining of uterus sloughs off.

• Ovary is controlling the phase of the uterus though hormones.

  • First phase - estrogen, proliferative phase. Lining getting taller.

  • Second phase (luteal phase) - estrogen and progesterone, secretary phase. Prepares for implantation of embryo. Lining of uterus is getting taller.

Pregnancy

• Embryo makes hCG (human chorionic gonadotropin). It is like LH

• The hCG maintains the corpus luteum and continues elevated estrogen and progesterone levels.

  • Luteal phase is maintained.

  • hCG will be spilled out into urine and can be measured in at home urine test. It is also in blood.

Human Chorionic Gonadotropin (hCG) and Sex Hormone Levels during Pregnancy

• Notice hCG is at 0 (first day of menstruation).

• During implantation stage, hCG dramatically increase. It eventually decreases rapidly at 1st trimester. BUT estrogen and progesterone continue to rise despite reduce hCG

  • After 1st trimester, corpus luteum no longer produces estro. and prog. And placenta makes the sex hormones.

Regulation of Estrogen and Progesterone Receptors

• Estrogen alone makes the uterus be in proliferative phase.

• Estrogen increases the number of progesterone <— this process is called priming. It is getting the tissue ready for future prog. secretions.

• Progesterone eventually stops continual positive feedback of estrogen. This explains how giving estrogen hormones could cause cancer of the lining of the uterus since there is a continual feedback cycle.

Oral Contraceptives

Prevent pregnancy

• 1960s: mostly progesterone and a little estrogen, to inhibit GnRH.

• 1980s: synthetics with longer half lifes

• Actions:

  • prevent LH surge and ovulation.

  • Cervix mucous is thickened, preventing sperm entry