tip of the PENIS. In women the contractions of
orgasm occur as rhythmic waves along the walls
of the VAGINA. Aside from being a source of intense
pleasure, orgasm appears to serve as a mechanism
to facilitate the movement of SPERMthrough the
vagina. This is important from a reproductive per-
spective as the vagina presents a fairly hostile
environment for sperm, which are not able to sur-
vive longer than an hour or two within it.
Though a man can have an orgasm without
ejaculating, he cannot ejaculate without orgasm.
After orgasm a man enters a refractory period dur-
ing which his body recovers from the experience.
During this time the mechanism of ERECTIONdoes
not respond to sexual stimulation and many men
feel the overwhelming desire to fall asleep. The
length of the refractory period varies with age and
among men, ranging from 10 to 20 minutes for a
man in his 20s to an hour or longer for a man 50
or older. A woman does not have a refractory
period and may continue or revive sexual arousal
indefinitely. The consistent inability to reach
orgasm is a form of SEXUAL DYSFUNCTIONthat may
have physiologic or emotional foundations.
See also ERECTILE DYSFUNCTION; MASTURBATION;
RETROGRADE EJACULATION; SEXUAL INTERCOURSE.
ova The female cells of reproduction, also called
eggs or gametes. An ovum, also called an oocyte
(single egg cell), is a haploid cell; it contains one
half of the genetic material necessary for human
life. At birth the OVARIEScontain about 400,000
follicles, each of which holds a single immature
ovum. At PUBERTYthe follicles begin to ripen, with
usually one ovum coming to maturity with each
MENSTRUAL CYCLE. Over the course of a woman’s
reproductive years her ovaries produce 400 to 600
ripened ova. About 10 to 20 times as many ova
begin but do not complete the maturation process.
The ovaries eventually absorb ova that fail to
reach maturity.
Ovulation The sequence of hormonal and
physiologic changes that bring an ovum to matu-
rity is OVULATION, which takes place during the
start of the menstrual cycle’s luteal phase around
day 14 of the menstrual cycle (day 1 being the
first day of MENSTRUATION). The PITUITARY GLAND
releases first a surge of FOLLICLE-STIMULATING HOR-
MONE(FSH), which activates an ovarian follicle. The
follicle secretes ESTROGENS, which begin the matu-
ration process for the ovum the follicle contains.
The pituitary gland then secretes LUTEINIZING HOR-
MONE(LH), which induces the ovarian follicle to
produce PROGESTERONE. The progesterone brings
the ovum to full maturity and the follicle rup-
tures, releasing the ovum for capture into the fal-
lopian tube.
Fertilization, implantation, and conception
The smooth MUSCLE walls of the fallopian tube
contract in a gentle, wavelike pattern that draws
the ovum through the tube toward the UTERUS.
When SPERMare also present in the fallopian tube,
fertilization takes place. Typically, though many
sperm attempt to penetrate the outer membrane
of the ovum only one succeeds. The chemical
composition of the ovum’s membrane alters once
the sperm is within the ovum, preventing other
sperm from following. The nuclei of the gametes
(ovum and sperm) fuse to form a single diploid
cell, called a ZYGOTE. As the zygote moves along
the fallopian tube toward the uterus it continues
to grow and divide. By the time the zygote
reaches the uterus it has become a two-layered
mass of cells called a blastocyst. The outer layer of
the blastocyst attaches to the endometrium; as
pregnancy continues this layer becomes the PLA-
CENTA and the inner layer develops into the
EMBRYO. The completion of fertilization and
implantation is CONCEPTION.
For further discussion of the ova within the
context of the structures and functions of repro-
duction and sexuality, please see the overview sec-
tion “The Reproductive System.”
See also ASSISTED REPRODUCTIVE TECHNOLOGY(ART);
CELL STRUCTURE AND FUNCTION; FALLOPIAN TUBES; FER-
TILITY; GAMETE; INFERTILITY; PREGNANCY; SECONDARY
SEXUAL CHARACTERISTICS; SEXUAL HEALTH.ovarian cancer A malignant (cancerous) tumor
that develops in the tissues of the ovary. Ovarian
cancer may arise from any of the ovary’s three
types of cells—germ, stromal, and epithelial—
though about 90 percent of ovarian cancers arise
from the ovarian epithelium (the membranous
covering of the ovary). Ovarian epithelial cancer
occurs most commonly in women over age 60
(after MENOPAUSE). Though tumors are typically
noncancerous or cancerous, ovarian epithelialovarian cancer 309