Biology 102
Fall 2001
R. Brundage

The Journey Begins

A. An incredible continuum of events gradually transforms the fertilized egg into an

embryo.

B. By the fifth week, the embryo is recognizable as human and will become more so in the

remaining months of gestation.

C. Overview

1. Male gonads (testes) produce sperm plus hormones for reproductive function and

secondary sexual characteristics.

2. Female gonads (ovaries) produce eggs plus reproductive hormones.

3. All gonads are similar in form until the seventh week of development; they do not

become functional until early adolescence.

I. Reproductive System of Human Males

A. Where Sperm Form

1. The testes reside in the scrotum, which is a few degrees cooler than body temperature

for proper sperm development.

2. Each testis is divided into about 300 lobes, each of which contains two or three

seminiferous tubules where sperm are continuously formed beginning at puberty.

B. Where Semen Forms

1. Sperm move from a testis ——> epididymis (for maturation and storage) ——> vas

deferens ——> ejaculatory ducts —— urethra (located inside the penis).

2. The sperm-bearing fluid—semen—is formed by secretions from the seminal vesicles

(fructose and prostaglandins) and the prostate (buffers against acidic vagina).

3. The bulbourethral glands secrete a mucus-rich fluid into the vagina during sexual

arousal.

C. Cancers of the Prostate and Testis

1. Prostate cancer kills about 40,000 older men annually.

a. There is no pain in the early stages and it may spread to other parts of the body without

notice.

b. Detection of this cancer is by physical examination and blood test for the prostate-

specific antigen.

2. There are about 5,000 cases of testicular cancer each year in the United States.

a. Once the cancer has metastasized, it kills as many as 50 percent of those stricken by it.

b. Monthly self-examination is very helpful for early detection.

II. Male Reproductive Function

A. Sperm Formation

1. Diploid spermatogonia undergo mitosis ——> primary spermatocytes, which undergo

meiosis I ——> haploid secondary spermatocytes, which undergo meiosis II ——>

haploid spermatids ——> mature sperm.

2. Sertoli cells in the tubule provide nourishment and chemical signals to the developing

sperm.

3. Each sperm has a head (nucleus and acrosome), midpiece (mitochondria), and tail

(microtubules).

B. Hormonal Controls

1. Testosterone, produced by Leydig cells located between the lobes in the testes,

stimulates spermatogenesis, the formation of reproductive organs and secondary

sex characteristics, and helps to develop and maintain normal (or abnormal?) sexual

behavior.

2. Luteinizing hormone (LH) is released from the anterior pituitary (under prodding by

GnRH from the hypothalamus) and stimulates testosterone production.

3. GnRH also causes the pituitary to release FSH, which stimulates the production of

sperm, beginning at puberty.

III. Reproductive System of Human Females

A. The Reproductive Organs

1. The egg is released from the ovary ——> oviduct ——> uterus (zygote will implant in its

lining, the endometrium).

2. The lower part of the uterus is the cervix, which extends into the vagina, which in turn

leads to the outer vulva (labia majora, labia minora, and clitoris).

B. Overview of the Menstrual Cycle

1. Most female mammals follow an estrous cycle; humans and other primates have a

menstrual cycle (there is no relationship between heat and fertility).

2. During each cycle an oocyte matures and escapes from the ovary and (if it is fertilized)

may implant in the endometrium; if there is no implantation, the uterine lining is

3. There are three major phases in the menstrual cycle:

a. In the follicular phase, there is menstrual flow, endometrial breakdown and rebuilding,

and maturation of the oocyte.

b. Ovulation is the rather quick release of the oocyte from the ovary.

c. During the luteal phase , the corpus luteum forms and the endometrium is primed for

possible pregnancy

4. FSH and LH stimulate the ovaries to secrete estrogens and progesterone, which in turn

promote changes in the endometrium (which may grow outside the uterus in a

condition known as endometriosis).

IV. Female Reproductive Function

A. Cyclic Changes in the Ovary

1. At birth about 2 million immature eggs (primary oocytes) are already present and

arrested in meiosis I.

2. Of the approximately 300,000 oocytes still present at age seven, only about 400- 500

will mature in a lifetime.

3. The follicle consists of a layer of cells (granulosa) surrounding the primary oocyte; the

4. During the menstrual cycle, one oocyte resumes meiosis I to form a secondary oocyte

and a polar body (both haploid).

5. At about midcycle, there is a surge of LH that causes ovulation—the release of the

secondary oocyte.

B. Cyclic Changes in the Uterus

1. During the first half of the cycle, the hypothalamus signals the anterior pituitary to

release LH and FSH, which in turn stimulate the ovary to secrete estrogen.

2. The corpus luteum persists for about twelve days, secreting progesterone that inhibits

further FSH and LH secretion.

3. If fertilization does not occur, the corpus luteum degenerates, progesterone and estrogen

levels fall, and FSH and LH are again secreted to begin another cycle.

V. Visual Summary of the Menstrual Cycle

[This section consists of an excellent diagram depicting the sequential events of the

menstrual cycle.]

VI. Pregnancy Happens

A. Sexual Intercourse

1. In male sexual arousal, the spongy tissue spaces inside the penis become filled with

blood to cause an erection.

2. During coitus, mechanical stimulation of the penis causes involuntary contractions that

force semen out and into the vagina.

3. Ejaculation in the male, and similar contractions in the female, are termed orgasm.

B. Fertilization

1. Of the 150 million to 350 million sperm deposited in the vagina during coitus, only a

few hundred ever reach the upper region of the oviduct where fertilization occurs.

2. Only one sperm will successfully enter the cytoplasm of the secondary oocyte after

digesting its way through the zona pellucida.

a. The arrival of that sperm stimulates the completion of meiosis II, which yields a mature

ovum.

b. The sperm nucleus fuses with the egg nucleus to restore the diploid chromosome

number.

VII. Formation of the Early Embryo

A. Pregnancy lasts an average of 38 weeks.

1. Embryo formation takes about two weeks.

2. The embryonic period lasts from the third to the end of the eighth week.

3. The fetal period extends from the eighth week until birth.

B. Cleavage and Implantation

1. During the first few days after fertilization, the zygote undergoes repeated cleavages as

it travels down the oviduct.

2. By the time it reaches the uterus, it is a solid ball of cells (morula), which is transformed

into a blastocyst.

a. Before the first week ends, the blastocyst contacts and adheres to the uterine lining.

b. The inner cell mass of the blastocyst is transformed into an embryonic disk that will

develop into the embryo proper within the next week.

C. Extraembryonic Membranes

1. The extraembryonic membranes inside a shelled egg (such as birds) are also formed in

human development.

2. The membranes and their functions are:

a. The amnion is a fluid-filled sac that keeps the embryo from drying out and acts as a

shock absorber.

b. The yolk sac becomes a site for blood cell formation.

c. The chorion, a protective membrane around the embryo, forms a portion of the placenta

and secretes a hormone (human chorionic gonadotropin) that maintains the uterine

d. The allantois does not function in waste storage (as it does in birds) but is active in

blood formation and formation of the urinary bladder.

VIII. Emergence of the Vertebrate Body Plan

A. By the third week of development, a two-layered disk consisting of ectoderm and

endoderm has formed.

1. The "primitive streak", a forerunner of the neural tube from which the brain and spinal

cord will form, has appeared.

2. Some cells also form the notochord, from which the vertebrae will form.

B. Toward the end of the third week, mesoderm has developed and is giving rise to

somites—segments of bones and skeletal muscles.

IX. Why Is the Placenta So Important?

A. The placenta is a combination of endometrial tissue and embryonic chorion.

B. Materials are exchanged from blood capillaries of mother to fetus, and vice versa, by

diffusion; the maternal blood and fetal bloods do not mix!

X. Emergence of Distinctly Human Features

A. By the end of the fourth week the embryo has embarked on an intricate program of cell

differentiation and morphogenesis, including development of limbs, circulation,

and umbilical cord.

B. The second trimester encompasses months four, five, and six; the individual is now

called a fetus; the heart is beating; fuzzy hair (lanugo) covers the body.

C. The third trimester extends from month seven until birth; the earliest delivery in which

survival on its own is possible is the middle of this trimester.

XI. Focus on Health: Mother as Provider, Protector, Potential Threat

XII. From Birth Onward

A. Giving Birth

1. Birth begins with contractions of the uterine muscles; the cervical canal dilates, and the

amniotic sac ruptures.

2. The hormone oxytocin induces powerful uterine contractions.

3. The fetus is expelled accompanied by fluid and blood; the umbilical cord is severed;

finally the placenta is expelled.

B. Nourishing the Newborn

1. The mammary glands first produce a special fluid for the newborn; then, under the

influence of prolactin, they produce milk.

2. Oxytocin is released in response to suckling and further increases the milk supply.

C. Regarding Breast Cancer

1. In the USA, over 100,000 women develop breast cancer.

2. Obesity, high cholesterol, and high estrogen levels contribute to the cancer but with

early detection and treatment, chances for cure are excellent.

D. Postnatal Development, Aging, and Death

1. The stages of postnatal development are: newborn (first two weeks) ——>

infant (2 weeks to 15 months) ——> child (to 12 years) ——> pubescent

(individual at puberty) ——> adolescent (from puberty to 3–4 years later) ——> adult.

2. During the aging process, cell structure and function deteriorate.

XIII. Control of Human Fertility

A. Some Ethical Considerations

1. When does development begin? When does life begin?

2. What about overpopulation compared to available resources?

3. What about the consequences of unwanted pregnancies?

B. Birth Control Options

1. Abstention is most effective but unrealistic.

2. In the rhythm method, there is no intercourse during the days when an egg is capable of

being fertilized.

3. Withdrawal before ejaculation would seem to be effective but is not.

4. Douching is similarly ineffective due to the speed with which sperm enter the uterus.

5. Surgery to cut and tie the oviducts (tubal ligation) or vas deferens (vasectomy) is

effective and generally considered an irreversible method to prevent sperm and egg

union.

6. Spermicidal foam and jelly are toxic to sperm but not reliable unless used in

combination with a barrier device.

7. A diaphragm fits over the cervix and prevents entry of sperm into the uterus.

8. Condoms prevent sperm deposition in the vagina but must be used with care.

9. The birth control pill contains synthetic female hormones and prevents ovulation when

taken faithfully according to directions.

10. Progestin injections (Depo-Provera) or implants (Norplant) inhibit ovulation over

several months.

11. RU-486, the "morning-after pill" intercepts pregnancy by blocking fertilization or

preventing implantation.