The Reproductive System
conversion of the ruptured follicle to a corpus luteum and
stimulates its secretory activity. (3) Rising levels of progesterone
and estrogen inhibit the hypothalamic-pituitary-gonadal (HPG)
axis, the corpus luteum deteriorates, ovarian hormones drop to
their lowest levels, and the cycle begins anew.
The Uterine (Menstrual) Cycle
Varying levels of ovarian hormones in the blood trigger events of
the uterine cycle.
During the menstrual phase of the uterine cycle (days 1–5),
the functional layer sloughs oﬀ in menses. During the
proliferative phase (days 6–14), rising estrogen levels stimulate
its regeneration, making the uterus receptive to implantation
about one week aFer ovulation. During the secretory phase (days
15–28), the uterine glands secrete nutrients, and endometrial
vascularity increases further.
±alling levels of ovarian hormones during the last few days of the
ovarian cycle cause the spiral arteries to become spastic and cut
oﬀ the blood supply of the functional layer, and the uterine cycle
begins again with menstruation.
Effects of Estrogens and Progesterone
Estrogen promotes oogenesis. At puberty, it stimulates the growth
of the reproductive organs and the growth spurt and promotes
the appearance of the secondary sex characteristics.
Progesterone cooperates with estrogen in breast maturation and
regulation of the uterine cycle.
Female Sexual Response
Te female sexual response is similar to that of males. Orgasm in
females is not accompanied by ejaculation.
Sexually Transmitted Infections
Sexually transmitted infections (S²Is) are infectious diseases
spread via sexual contact. Gonorrhea, syphilis, and chlamydia are
bacterial diseases. Syphilis has broader consequences than most
other sexually transmitted bacterial diseases since it can infect
organs throughout the body. ²richomoniasis is a parasitic infection.
Genital herpes and genital warts are viral infections; genital warts,
caused by the HPV virus, are implicated in cervical cancer.
Developmental Aspects of the Reproductive
Embryological and Fetal Events
Genetic sex is determined by the sex chromosomes: an X from
the mother, an X or a Y from the father. If the fertilized egg
contains XX, it is a female and develops ovaries; if it contains XY,
it is a male and develops testes.
Gonads of both sexes arise from the mesodermal gonadal ridges.
Te mesonephric ducts produce the male accessory ducts and
glands. Te paramesonephric ducts produce the female duct system.
Te external genitalia arise from the genital tubercle and associated
structures. Te development of male accessory structures and external
genitalia depends on the presence of testosterone produced by the
embryonic testes. In its absence, female structures develop.
Te testes descend into the scrotum from the abdominal cavity.
Puberty is the interval when reproductive organs mature and
become functional. It begins in males with penile and scrotal
growth and in females with breast development.
Te uterine wall is composed of the outer perimetrium, the
myometrium, and the inner endometrium. Te endometrium
consists of a functional layer (stratum functionalis), which sloughs
oﬀ periodically unless an embryo has implanted, and an underlying
basal layer (stratum basalis), which rebuilds the functional layer.
Te vagina extends from the uterus to the exterior. It is the
copulatory organ and allows passage of the menstrual ﬂow or a baby.
The External Genitalia and Female Perineum
Te female external genitalia (vulva) include the mons pubis,
labia majora and minora, clitoris, and the urethral and vaginal
oriﬁces. Te labia majora house the mucus-secreting greater
The Mammary Glands
Te mammary glands lie over the pectoral muscles of the chest
and are surrounded by adipose and ﬁbrous connective tissue.
Each mammary gland consists of many lobules, which contain
Physiology of the Female Reproductive System
Oogenesis, the production of eggs, begins in the fetus. Oogonia,
the diploid stem cells of female gametes, are converted to primary
oocytes before birth. Te infant female’s ovaries contain about
1 million primary oocytes arrested in prophase of meiosis I.
At puberty, meiosis resumes. Each month, one primary oocyte
completes meiosis I, producing a large secondary oocyte and a
tiny ﬁrst polar body. Meiosis II of the secondary oocyte produces
a functional ovum and a second polar body, but does not occur in
humans unless a sperm penetrates the secondary oocyte.
Te ovum contains most of the primary oocyte’s cytoplasm. Te
polar bodies are nonfunctional and degenerate.
The Ovarian Cycle
During the follicular phase (days 1–14), several primary follicles
begin to mature. Generally, only one follicle per month completes
the maturation process, becoming the dominant follicle. Late in
this phase, the oocyte in the dominant follicle completes meiosis
I. Ovulation occurs about day 14, releasing the secondary oocyte
into the peritoneal cavity. Other developing follicles deteriorate.
In the luteal phase (days 15–28), the ruptured follicle is converted
to a corpus luteum, which produces progesterone and estrogen
for the remainder of the cycle. If fertilization does not occur, the
corpus luteum degenerates aFer about 10 days.
Hormonal Regulation of the Ovarian Cycle
Beginning at puberty, the hormones of the hypothalamus, anterior
pituitary, and ovaries interact to establish and regulate the ovarian
cycle. Establishment of the mature cyclic pattern, indicated by
menarche, takes about four years. Leptin serves a permissive role in
puberty’s onset, stimulating the hypothalamus when adipose tissue is
suﬃcient for the energy requirements of reproduction.
Te hormonal events of each ovarian cycle are as follows: (1)
GnRH stimulates the anterior pituitary to release ±SH and LH,
which stimulate follicle maturation and estrogen production.
(2) When blood estrogen reaches a certain level, positive
feedback exerted on the hypothalamic-pituitary-gonadal axis
causes a sudden release of LH that stimulates the primary oocyte
to continue meiosis and triggers ovulation. LH then causes