Human Reproduction

1. Human Reproduction Biology 122 – CHS (Chapter 39) Mr. Gallop

2. Objectives – Chapter 39 • Upon completion of this unit, you should be able to: • Differentiate between asexual and sexual reproduction. • Summarize the process of male and female sex cell development. • Analyze and describe in detail the structure and function of the male human reproductive system. • Analyze and describe in detail the structure and function of the female human reproductive system. • Explain the human reproductive cycles, identifying the main hormones involved. • Understand the process of negative feedback mechanisms as they pertain to the hormones associated with reproduction. • Discuss the process of fertilization and describe the events of the three trimesters of human pregnancy. • Consider the conditions necessary for the development of fraternal and maternal twins. • Describe and explain the onset of labour and birth, in general terms. • Research and explore reproductive technologies (such as in vitro fertilization) and consider the ethical implications of these technologies.

3. Asexual vs Sexual Reproduction • Reproduction occurs in animals in two distinct ways. • When one individual organism donates all of its DNA to the new offspring. This method is called asexual reproduction and is typically reserved for simpler lifeforms where diversity is less of a concern. • These creatures reproduce through mitosis and in essence make exact clones of themselves. These organisms are not able to adapt as quickly to changing environments since they are all genetically identical.

4. Sexual Reproduction • Sexual reproduction - meiosis and the creation of sex cells. Such unions lead to a genetic blending of the two parent organisms. • Genetic diversity follows such a union as the offspring are not simply identical clones of either parent. There tends to be a commonality in sexual reproduction in that female sex cells are large and immobile, while male sex cells are both small and mobile.

5. Human Reproduction • Humans invest a great deal of time and energy into rearing offspring. • A typical bacterium, with unlimited resources for living, would require only 36 hours to produce enough offspring to cover the entire earth's surface 30 cm deep in bacteria! • Elephants, allowed to mate at will and with unlimited resources, would require 750 years to produce a population of 19 million beasts.

6. Intro to Reproduction • Most primates - only one or two offspring in their lifetime; great deal of effort and time into each and every one of them. • In the human species, males and females produce sex cells inside sexual glands known as gonads. In males they are called testes, and in females they are the ovaries. Testes produce sperm, and ovaries produce eggs. Human males can produce 100 to 650 million sperm per day throughout their lifetime. Human males become fertile during puberty and remain fertile until death. So throughout their lives, they are capable of producing tremendous numbers of haploid sex cells.

7. Intro to Reproduction • Females are born with all of their sex cells already produced, all 400,000 or so. Of those 400,000, only 400 of them will actually reach maturity. Once the human female reaches puberty and is sexually mature, she will usually ovulate one egg per month that is capable of being fertilized. • Ovulation generally occurs for the first time during the early teen years. Ovulation stops after 400 months (about 33 years later). • When the two haploid sex cells, one from each parent, unite in the process of fertilization, a diploid zygote is produced. This zygote undergoes rapid mitosis and passes into an embryonic stage, followed by a fetal stage. Birth occurs approximately 266 days after fertilization.

8. Male Physiology Male and female sex organs are responsible not only for sexual reproduction but also for liquid waste excretion. In the male, the testes are carried externally on the body in a pouch of skin called the scrotum(descending into the scrotum during the seventh month of pregnancy). By carrying the testicles outside of the body, they are a few critical degrees cooler than the rest of the body. These slightly cooler temperatures allow the testicle to develop healthy, viable sperm.

9. Male Physiology • Sperm do not fully mature inside the testes but in a tube just outside of the testes called the epididymus. Mature sperm remain in this area until they are ejaculated. • During sexual activity, the Cowper's gland secretes a clear, alkaline substance designed to neutralize any urine in the urethra.

10. Male Physiology • During ejaculation, sperm are forced out of the epididymus and into the vas deferens, which connects to a duct emerging from the seminal vesicles. This joining of ducts forms a short yet wider ejaculatory duct. The seminal vesicles contribute seminal fluid to the sperm, creating semen. Seminal fluids make up about 60% of the volume of the total ejaculate. This fluid consists of: • Fructose: a sugar that feeds the sperm • Prostoglandins: believed to cause small contractions within the female to aid in fertilization • Alkaline fluids: to protect the sperm from the acidic environment found in the vagina

11. Male Physiology • The semen continues on through the prostate gland. This gland adds more nutrients to the semen, as well as an anticoagulant. From here the semen passes through the Cowper's gland (which has already lubricated and neutralized any acidity in the urethra). Once the ejaculate has passed through the Cowper's gland, it travels through the urethra and finally exits the body of the male.

12. Male Sex Cell Development • Inside the testes there are small tightly-coiled tubes known as seminiferous tubules. (1/2 km of these) • Inside the tubules, sperm are created—a process known as spermatogenesis. Cells in these tubules are constantly undergoing mitosis, creating vast numbers of sperm. • Final preparations occur in the epididymus, a small holding area consisting of tubes lining an area outside each testis. The process of creating sperm takes 65 to 75 days (from spermatogonia to sperm).

13. Male Hormone System At puberty, males begin secreting a hormone called gonadotropin releasing hormone, or GNRH. This hormone is released by the hypothalamus in the brain. The GNRH trickles down a small tube into the anterior pituitary. The anterior pituitary responds by releasing two more hormones: leutenizing hormone (LH) and follicle stimulating hormone (FSH). Both hormones are destined for the testes.

14. Leutenizing hormone • LH affects the interstitial cells to begin to produce testosterone. Testosterone has many functions—increase sex drive, promote hair growth in pubic regions, as well as promote maturation in the penis and testes. • High levels of testosterone in the blood will cause the hypothalamus to slow down its release of GNRH, setting up an internal negative feedback loop that ultimately controls how much more testosterone is released.

15. Follicle Stimulating Hormone • FSH causes an increase in sperm production. Sperm production is also increased by the presence of testosterone.

16. Female Physiology • Throughout a female's fertile years, the ovaries produce a single egg each month. • Eggs are released during ovulation. Ovulated eggs travel down the fallopian tube, also known as the oviduct. This is where fertilization usually occurs. • After fertilization, the zygote continues down the fallopian tube until it reaches the uterus. At the time of ovulation, there is an abundance of blood lining the uterus. The inner lining of the uterus is engorged with blood in anticipation of the arrival of a fertilized egg. • The swollen inner area of the uterus is called the endometrium. If the egg is fertile, it will become embedded in the endometrium and implant itself. If an unfertilized egg arrives, the endometrium will be shed within a matter of days. This is the "bleeding" of menstruation. The outer lining of the uterus, the myometrium, is very muscular and will contract, causing cramping. This results in the inner lining breaking free and being shed. The contraction of the myometrium is also instrumental during the process of childbirth.

17. Female Physiology • At the lower end of the uterus is a small opening known as the cervical canal, which is surrounded by the muscular cervix. This muscular ring separates the uterus from the vagina and holds the fetus inside the womb until birth. When the baby's head begins to pass the cervix, the cervix is said to be dilated and can be measured to determine how soon the baby is likely to be born. The urethra is located between the vagina and the clitoris, and is responsible for urinary waste excretion. The clitoris is located at the apex of the labium minor and its primary function is for sexual stimulation. The labium major are folds of skin which enclose the labium minor and the vestibule. The labium minor are smaller folds of skin which also enclose the vestibule. The vestibule in turn encloses the vaginal opening and the urethra.

18. Female Sex Cell Development • The creation of an egg is referred to as oogenesis (remember that the creation of sperm is spermatogenesis). • In oogenesis, cell division is uneven. Spermatogenesis aims to produce millions of sperm, while oogenesis aims to produce just one egg.

19. The Menstrual Cycle • Less than 30% of all women have a menstrual cycle that runs the so-called classic 28 days. (most between 20 and 40 days.) This cycle has four phases: the flow phase, the follicular phase, ovulation and the luteal phase. • The flow phase marks the beginning of the menstrual cycle. This phase is the only outwardly visible sign of menstruation. During the flow phase the egg is flushed out of the body due to the muscular contractions of the outer wall of the uterus. This phase lasts from four to six days, and the intensity of the flow can vary greatly from one month to the next.

20. The Menstrual Cycle • As soon as one egg has been flushed from the uterus, preparations immediately begin for next month's egg. This phase is known as the follicular phase. Within the ovary, several follicles begin to develop. • While several follicles may begin to develop, only one will fully mature and ovulate. As the primary follicle matures, the support cells produce estrogen. This causes blood to gather in the uterus (the creation of another endometrium). The endometrium continues to grow for a week or so.

21. The Menstrual Cycle • The follicle continues to develop and moves toward the wall of the ovary. The follicle becomes very large and eventually ruptures through the wall of the ovary. This eruption releases the oocyte into the fallopian tube. This is ovulation. Ovulation generally occurs on the day 14, midway through the menstrual cycle.

22. The Menstrual Cycle • The remaining cells develop into a new structure called the corpus luteum. This development marks the beginning of the luteal phase. • The corpus luteum manufactures two hormones—estrogen and progesterone. The estrogen causes even more blood and nutrients to gather in preparation for receiving the recently ovulated egg. The second hormone, progesterone, causes several changes in the woman's body. Initially it prohibits further ovulation so that no other eggs can be released from the ovaries. • The corpus luteum maintains itself for about 10 days. After that time, uterine contractions do begin—the endometrium, which is now quite substantial in size, is flushed from the body. This marks the end of the current menstrual cycle and the beginning of the next one.