The Reproductive System
Pregnancy and Embryonic Development.
Development
Once the egg cell or ovum ruptures from the ovary in
ovulation, it must be fertilized within 12 to 24 hours. Once
ejaculated, the sperm cell remains viable within the female
reproductive tract for 12 to 48 hours. Some sperm can
remain viable for up to 72 hours. For fertilization to occur,
sexual intercourse must occur no more than 72 hours before
ovulation or no later than 24 hours after ovulation. It takes
the ovum 24 hours to go approximately one-third down the
uterine or fallopian tube. If fertilization is going to take
place, it will occur in the upper two-thirds of the uterine
tube.
The sperm cells swim by means of their beating fla-
gella up the vagina and uterus and into the uterine tube.
They are attracted to the egg cell by chemicals secreted by
the ovum. Although hundreds of sperm cells are sur-
rounding the ovum and rupturing their acrosomes to re-
lease enzymes to penetrate the egg cell, only one sperm
will penetrate the egg and join its genetic material with the
genetic material of the egg to produce a fertilized egg or
zygote (ZYE-gote).
As the zygote moves down the uterine tube, it un-
dergoes a series of rapid mitotic divisions, resulting in a
hollow ball-like mass of cells called a blastula (BLASS-
tyoo-lah) or blastocyst. By the time it reaches the uter-ine
cavity, it consists of about 100 cells. At this stage, part of
the blastocyst develops into the chorionic vesicle
(KOH-ree-on-ik), and it secretes human chorionic
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gonadotropin, a hormone that causes the corpus lu-teum of
the ovary to continue producing its hormones to maintain
the lining of the uterus.
By the 7th day following ovulation, the developing
embryo has embedded itself in the endometrial lining of the
uterus. Meanwhile, the three primary germ layers are being
formed by mitotic divisions. The ectoderm will form skin
and the nervous system, the endoderm will form the
linings of internal organs and glands, and the mesoderm
will form muscles, bone, and the rest of the body tissues.
The blastocyst’s inner cell mass forms these primary germ
layers and its trophoblast, the large fluid-filled sphere, now
begins to form projections called chorionic villi, which
will interact with the uterine tissues to form the placenta
(plah-SEN-tah) (Figure 19-12).Once the placenta is formed, the embryo, which looks
like a three-layered plate of cells, becomes surrounded by a
fluid-filled sac called the amnion (AM-nee-on). The
embryo is attached by a connecting stalk of tissue called the
umbilical cord. The placenta delivers nutrients and
oxygen to and removes wastes and carbon dioxide from the
embryonic blood. All exchanges with the mother are made
through the placenta. By the 9th week of development, the
embryo is now called a fetus.By the 9th week of development the embryo looks
definitely human, the placenta has become an endocrine
organ secreting estrogen and progesterone to maintain
pregnancy, and the corpus luteum is now inactive. Laterhealth AlertPrevention of Sexually (^)
Transmitted Diseases (^)
The one absolute way to prevent the trans-
mission of sexually transmitted diseases
(referred to as STDs), like syphilis and gon-
orrhea, is through abstinence. HIV, which causes
AIDS, can also be transmitted sexually.
However, sexually active individuals can help
prevent the occurrence of STDs by practicing
what is called “safe sex.” Safe sex practices
include: always using a latex condom (HIV can
pass through other types of condoms, such as
animal skin condoms); avoiding the
exchange of bodily fluids like blood, semen, or
vaginal secretions; and avoiding contact with
genital or oral sores (which may con-tain the
genital herpes virus). Good personal hygiene
and yearly medical checkups, along with self-
awareness and examination of the genital
areas, will help prevent the spread of disease-
causing microorganisms. Any ab-normalities,
like sores or discharges from the reproductive
tract, should be immediately brought to the
attention of a physician.