provides mobility. ANDROGENS, notably TESTOS-
TERONE, and other hormones regulate spermato-
genesis. Spermatogenesis is a continuous process.
Fertilization The role of the sperm is to fertil-
ize the ovum (egg), the first step in establishing
PREGNANCY. SEXUAL INTERCOURSE, in which the
man’s erect PENISenters the woman’s VAGINA, is the
natural mechanism through which sperm gain
access to the woman’s reproductive tract. From 20
to 250 million sperm leave the testicles within the
semen, the fluid that nourishes and protects the
sperm, during each EJACULATION. The sperm swim
through the fluids in the vagina, enter the UTERUS
through the CERVIX, and continue to the entrance
of the FALLOPIAN TUBESat the top of the uterus.
Of the millions of sperm that begin this jour-
ney, most die before reaching the fallopian tube.
Surviving sperm continue through the fallopian
tube; fertilization takes place if there is an ovum
(egg) also in the fallopian tube and a sperm is able
to penetrate its surface membrane. Multiple fac-
tors influence this ability, including the shape of
the sperm head, the remaining motility of the
sperm tail to thrust the head through the ovum’s
membrane, and the environment within the fal-
lopian tube. Once a single sperm penetrates the
shell of the ovum, the ovum closes itself to further
penetration. Only the head of the sperm enters
the ovum; the tail of the sperm drops off outside
the ovum. Multiple pregnancies occur when two
or more sperm simultaneously penetrate the
ovum (identical multiples) or when two or more
OVAare present in the fallopian tubes (fraternal
multiples). Abnormalities of sperm structure or
motility may interfere with the sperm’s ability to
reach or penetrate the ovum.
For further discussion of sperm within the con-
text of the structures and functions of reproduc-
tion and sexuality, please see the overview section
“The Reproductive System.”
See also ASSISTED REPRODUCTIVE TECHNOLOGY(ART);
CELL STRUCTURE AND FUNCTION; FERTILITY; INFERTILITY;
SECONDARY SEXUAL CHARACTERISTICS; SEXUAL HEALTH.
spermatocele A cyst containing dead SPERMthat
forms in the epididymis. Doctors do not know
what causes spermatoceles, also called epididymal
cysts, to develop though suspect they result from
some sort of obstruction that blocks their flow
through the epididymis. A spermatocele is round,
firm, and clearly defined. A man may discover a
spermatocele during routine TESTICULAR SELF-EXAMI-
NATION(TSE) or the doctor may find it during ROU-
TINE MEDICAL EXAMINATION. A large spermatocele
may cause PAIN. ULTRASOUNDof the SCROTUMcon-
firms the diagnosis. No treatment is necessary for
a small spermatocele that does not cause symp-
toms. For large spermatoceles or spermatoceles
that cause pain, surgery to remove the spermato-
cele is the most effective treatment. However, sur-
gery may impair FERTILITY because it usually
involves removing a portion of the epididymis.
See also HYDROCELE; SURGERY BENEFIT AND RISK
ASSESSMENT; VARICOCELE.sperm donation The collection of a man’s SEMEN,
which contains SPERM, for use in FERTILITYtreat-
ments. The man obtains the semen for donation
through MASTURBATION to produce EJACULATION.
Sperm banks (facilities that collect and store
donated sperm) have varying policies for qualify-
ing sperm donors. In general a sperm donor must
be between the ages of 18 and 45, have no known
genetic or hereditary conditions, and have no
exposure to infectious diseases such as HIV/AIDS,
HEPATITIS, and SEXUALLY TRANSMITTED DISEASES(STDS).
Sperm donors have no legal rights or responsibili-
ties for children conceived with their sperm and
typically remain anonymous.
See also ASSISTED REPRODUCTIVE TECHNOLOGY(ART);
GESTATIONAL SURROGACY.stillbirth The death of the FETUSafter 24 weeks
of gestation. There are numerous causes for still-
birth; often the reason remains unknown. Some-
times the obstetrician cannot detect the fetal
heartbeat during a routine PRENATAL CARE visit;
more often the woman notices the fetus has
stopped moving. An ULTRASOUNDcan confirm the
death, after which the doctor induces labor to
deliver the baby. Death may also occur during
CHILDBIRTH. The loss of a PREGNANCYthrough still-
birth is emotionally traumatic for parents, family
members, and friends.
See also ABORTION; GRIEF; PREMATURE BIRTH; SUP-
PORT GROUPS.342 The Reproductive System