Chapter 45 Human Reproduction and Development

I. Human Reproductive System
A. 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.
B. Male Reproductive Organs
1 Where Sperm Form
a. The testes reside in the scrotum, which is a few degrees cooler than body temperature for
proper sperm development.
b. 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.
1) Diploid spermatogonia undergo mitosis Æ primary spermatocytes, which undergo meiosis I Æ
haploid secondary spermatocytes, which undergo meiosis II Æ haploid spermatids Æ
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).
2 How Semen Forms
a. Sperm move from a testis Æ epididymis (for maturation and storage) Æ vas deferens Æ
ejaculatory ducts Æ urethra.
b. The sperm-bearing fluid—semen—is formed by secretions from the seminal vesicles (fructose
and prostaglandins) and the prostate (buffers against acidic vagina).
c. The bulbourethral glands secrete a mucus-rich fluid into the vagina during sexual arousal.
C. Hormonal Control of Male Reproductive Functions
1 Testosterone, produced by interstitial cells 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 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.
D. Female Reproductive Organs
1 Two ovaries in the lower abdominal cavity release eggs (monthly) and secrete estrogen and
progesterone.
a. At birth about 2 million immature eggs (oocytes) are already present and arrested in meiosis I.
b. Of the 30,000–40,000 oocytes still present at puberty, only about 400 will mature in a
lifetime.
c. Meiosis II will not be completed in each egg unless fertilization occurs.
2 The egg is released from the ovary Æ oviduct Æ uterus (zygote will implant in its lining, the
endometrium); 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
E. Menstrual Cycle
1 Most female mammals come into “heat” or estrus; humans and other primates have a menstrual
cycle (there is no relationship between heat and fertility), and the uterine lining is sloughed at
the end of each cycle of (approximately) 28 days.
2 Ovarian Function
a. The follicle consists of a layer of cells (granulosa) surrounding the primary oocyte; the
granulosa cells gradually deposit a layer of material (zona pellucida) around the follicle.
b. During the menstrual cycle, one oocyte resumes meiosis I to form a secondary oocyte and a
polar body (both haploid).
c. Ovulation is the release of the secondary oocyte; the follicle remains behind to develop into the
corpus luteum.
d. 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.
e. At about midcycle, there is a surge of LH that causes ovulation.
f. The corpus luteum persists for about twelve days, secreting progesterone that inhibits further
FSH and LH secretion.
g. 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.
3 Uterine Function
a. Estrogen and progesterone cause the endometrium to develop and prepare for pregnancy.
b. When fertilization does not occur, progesterone and estrogen levels fall and the endometrium
begins to disintegrate and is sloughed during the menstrual flow, which usually lasts for
three to six days.
c. Sometimes endometrial tissue is stimulated by estrogen to grow outside the uterus; this serious
condition is called endometriosis.
F. 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.

II. From Fertilization to Birth
A. 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.
B. 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. Membranes Around the Embryo
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 yolk sac gives rise to the digestive tube.
b. The allantois does not function in waste storage (as it does in birds) but is active in oxygen
transport.
c. The amnion is a fluid-filled sac that keeps the embryo from drying out and acts as a shock
absorber.
d. The chorion, a protective membrane around the embryo, forms a portion of the placenta and
secretes a hormone (chorionic gonadotropin) that maintains the uterine lining after
implantation.
e. The umbilical cord connects the growing human embryo to parts of the yolk sac, allantois, and
chorion.
D. The Placenta
1 The placenta is a combination of endometrial tissue and embryonic chorion.
2 Materials are exchanged from blood capillaries of mother to fetus, and vice versa, by diffusion; the
maternal blood and fetal blood do not mix!
E. Embryonic and Fetal Development
1 The first trimester is a time of rapid development.
a. Gastrulation results in the formation of three germ layers.
b. Organ development proceeds rather slowly after the fourth week: nerve cord, heart,
respiratory organs, extremities and digits, and a tail (even in humans).
2 The second trimester encompasses months 4, 5, and 6; the individual is now called a fetus; the heart
is beating; fuzzy hair (lanugo) covers the body.
3 The third trimester extends from month 7 until birth; the earliest delivery in which survival on its
own is possible is the middle of this trimester.
F. Birth and Lactation
1 Birth begins with contractions of the uterine muscles; the cervical canal dilates, and the amniotic
sac ruptures.
2 The fetus is expelled accompanied by fluid and blood; the umbilical cord is severed; finally the
placenta is expelled.
3 The mammary glands first produce a special fluid for the newborn; then, under the influence of
prolactin, they produce milk.

III. Postnatal Development, Aging, and Death
A. The stages of postnatal development are: newborn (first two weeks) Æ infant (2 weeks to 15 months) Æ
child (to 12 years) Æ pubescent (individual at puberty) Æ adolescent (from puberty to 3–4 years
later) Æ adult.
B. During the aging process, cell structure and function deteriorate.

IV. 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 Spermicidal foam and jelly are toxic to sperm but not reliable unless used in combination with a
barrier device.
6 A diaphragm fits over the cervix and prevents entry of sperm into the uterus.
7 Condoms prevent sperm deposition in the vagina but must be used with care.
8 The Pill contains synthetic female hormones and prevents ovulation when taken faithfully according
to directions.
9 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.
10 Abortion by several methods (including the drug RU-486) can terminate a pregnancy, but for many
this is not a preferred method of birth control.
C. In Vitro Fertilization
1 About 15 percent of American couples are infertile.
2 Externally conceived embryos implant successfully in only about 20 percent of the attempts.