Essentials of Anatomy and Physiology

Human Development—growth of a fertilized
egg into a human individual

Fertilization—the union of the nuclei of egg
and sperm; usually takes place in the fallo-
pian tube

1. Sperm undergo final maturation (capacitation)
   within the female reproductive tract; the acrosome
   contains enzymes to digest the membrane of the
   ovum.


2. The 23 chromosomes of the sperm join with the 23
   chromosomes of the egg to restore the diploid
   number of 46 in the zygote.


3. A zygote has 22 pairs of autosomes and one pair of
   sex chromosomes; XX in females, XY in males (see
   Fig. 21–1).

Implantation (see Fig. 21–2)—5 to 8 days
after fertilization

1. Within the fallopian tube, the zygote begins
   mitotic divisions called cleavage to form two-cell,
   four-cell, eight-cell stages, and so on.


2. A morula is a solid sphere of cells that divides fur-
   ther to form a hollow sphere called a blastocyst.


3. A blastocyst consists of an outer layer of cells called
   the trophoblast and an inner cell mass that contains
   the potential embryo; the cells are stem cells, not
   yet specialized. The trophoblast secretes enzymes
   to form a crater in the endometrium into which the
   blastocyst sinks.

Embryo—weeks 1 through 8 of gestation
(see Fig. 21–3)

1. In the embryonic disc, three primary germ layers
   develop: ectoderm, mesoderm, and endoderm (see
   Table 21–1).


2. By the eighth week of gestation (end of 2 months),
   all organ systems are formed (see Table 21–2).

Embryonic Membranes (see Fig. 21–3)

1. The yolk sac forms the first blood cells and the cells
   that become spermatogonia or oogonia.


2. The amnion surrounds the fetus and contains amni-
   otic fluid; this fluid absorbs shock around the fetus.


3. The chorion develops chorionic villi that will con-
   tain blood vessels that form the fetal portion of the
   placenta.

Fetus—weeks 9 through 40 of gestation (see

Table 21–2)

1. The organ systems grow and mature.


2. The growing fetus brings about structural and
   functional changes in the mother (see Table 21–3).

Placenta and Umbilical Cord

1. The placenta is formed by the chorion of the
   embryo and the endometrium of the uterus;
   the umbilical cord connects the fetus to the pla-
   centa.


2. Fetal blood does not mix with maternal blood; fetal
   capillaries are within maternal blood sinuses (see
   Fig. 21–5); this is the site of exchanges between
   maternal and fetal blood.


3. Two umbilical arteries carry blood from the fetus
   to the placenta; fetal CO 2 and waste products dif-
   fuse into maternal blood; oxygen and nutrients
   enter fetal blood.


4. Umbilical vein returns blood from placenta to fetus.


5. The placenta is delivered after the baby and is
   called the afterbirth.

Placental Hormones

1. hCG—secreted by the chorion; maintains the
   corpus luteum so that it secretes estrogen and
   progesterone during the first few months of gesta-
   tion. The corpus luteum is too small to secrete
   sufficient hormones to maintain a full-term preg-
   nancy.


2. Estrogen and progesterone secretion begins within
   4 to 6 weeks and continues until birth in amounts
   great enough to sustain pregnancy.


3. Estrogen and progesterone inhibit FSH and LH
   secretion during pregnancy and prepare the mam-
   mary glands for lactation.


4. Progesterone inhibits contractions of the
   myometrium until just before birth, when proges-
   terone secretion begins to decrease.


5. Relaxin inhibits contractions of the myometrium
   and permits stretching of the pubic symphysis.

Parturition and Labor

1. Gestation period ranges from 37 to 42 weeks; the
   average is 40 weeks.


2. Labor: first stage—dilation of the cervix; uterine
   contractions force the amniotic sac into the cervix;
   amniotic sac ruptures and fluid escapes.

Human Development and Genetics 493

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