Anatomy and Physiology 34B
Chapters 28: Pregnancy and Human Development
I. Overview
A. Fertilization
B. Preembryonic Development
C. Embryonic Development
D. Pregnancy and Childbirth
II. Fertilization (conception) - the penetration of an ovum by a sperm
and subsequent union of their genetic material usually occurs in the
distal half of the uterine tube. This includes the following events.
A. A woman usually ovulates one ovum per month
1. The ovum is surrounded by a “jelly coat” consisting of inner the
zona pellucida & outer corona radiata
2. These layers protect the ovum as it enters the uterine tube and is
carried toward the uterus by cilia lining the tube
B. During coitus, a male ejaculates bet. 100-500 million sperm into
the female’s vagina
1. Sperm must travel to the distal uterine tube within 5-10 minutes
of ejaculation, and in the process they undergo capacitation,
which enable the sperm to penetrate the ovum
2. Only about 100 sperm will live to encounter the ovum
3. The head of each sperm is capped with an acrosome, which
contains digestive enzymes that allow the sperm to digest the
outer jelly coat of the egg
4. When a sperm penetrates the zona pellucida, the ovum forms a
fertilization membrane that blocks other sperm
5. With the sperm’s entry, the ovum completes meiosis II, and the
haploid sperm and egg nuclei unite to form a diploid zygote
C. Prenatal (before birth) development can be divided into 3 periods:
1. Preembryonic - first 2 weeks; initiated by fertilization
2. Embryonic – weeks 3-8, in which the body organs are formed
2
3. Fetal - from 2 months to parturition (childbirth)
D. Gestation includes all of the above developmental periods
(conception to birth). In humans it’s about 280 days (40 weeks)
from the onset of the last menstrual period.
III. Preembryonic Development
A. Cleavage & Formation of the Blastocyst
1. Within 30 hrs after fertilization, the zygote undergoes mitotic
cell divisions without growth, called cleavage
2. When it is a solid ball of 16 cells, it is called a morula
3. The cells continue to divide and become a hollow, fluid-filled
ball of cells called a blastocyst
a. The outer layer of cells are the trophoblast, which will
become the chorion, and later the embryonic placenta
b. An inner layer of cells is the embryoblast, which will
become the embryo
B. Implantation in the uterine lining begins between the 5th & 7th day
after fertilization
1. The blastocyst embeds itself in the endometrium
2. The trophoblast secretes enzymes that digest a portion of the
endometrium
3. Endometrial cells cover the embedded blastocyst
4. Fingerlike projections extend from the trophoblast into the
endometrium
5. Human chorionic gonadotropin (HCG) secreted by the
trophoblast maintains the corpus luteum so it continues to
produce progesterone to maintain the endometrium
6. HCG production declines by the 10th week when the developing
placenta begins to produce estrogen & progesterone to maintain
the endometrium
3
C. Formation of the 3 Germ Layers begins during the 3rd week
1. An amniotic cavity forms between the inner cell mass &
trophoblast
2. The inner cell mass flattens into an embryonic disk and forms 3
germ layers:
a. The upper ectoderm, near the amniotic cavity
b. The lower endoderm, near the blastocyst cavity
c. Later, the mesoderm forms between ectoderm & endoderm
3. When all 3 germ layers are formed, the individual is an embryo
IV. Embryonic Development
A. During the embryonic period (3-8 wk), all of the body tissues &
organs form, as well as the placenta, umbilical cord, &
extraembryonic membranes
B. Germ layers differentiate into different tissues
1. Ectoderm (outer layer) eventually becomes:
a. Epidermis of the skin and its derivatives
b. Nervous tissue - brain, spinal cord, sense organs, pituitary
gland, & adrenal medulla
2. Mesoderm (middle layer) differentiates to form:
a. Muscle - smooth, cardiac, & skeletal
b. Connective tissue - cartilage, bone, ligaments, & blood
c. Dermis of the skin
d. Epithelial lining of blood vessels, body cavities, joint cavities
e. Notochord, which becomes the intervertebral discs
f. Gonads, kidneys, ureters, and adrenal cortex
3. Endoderm (inner layer) eventually forms:
a. Epithelial lining (mucous membranes) of digestive sys.,
respiratory sys., urinary bladder & urethra, and vagina
4
b. Liver & pancreas
C. Extraembryonic membranes outside the embryo include the
amnion, yolk sac, allantois, and chorion
1. Amnion - thin membrane derived from ectoderm and
mesoderm, it surrounds the embryo, forming an amniotic sac
filled with amniotic fluid.
a. Functions include:
1) Ensures symmetrical growth & development
2) Cushions & protects the embryo/fetus
3) Helps maintain a constant pressure & temperature
4) Permits freedom of fetal movement
b. Amniotic fluid is composed of fluids from maternal blood
plasma, fetal urine, and cells sloughed off by the fetus
c. In amniocentesis, amniotic fluid is withdrawn by syringe (at
14-15 wks), then the cells are tested for genetic abnormalities
2. Yolk sac - established from cells on the underside of the
embryonic disc during the end of the 2nd week
a. Does not contain nutritive yolk
b. Produces blood cells until the liver forms the 6th week
c. Primordial germ cells form from the yolk sac wall, then
migrate to the gonads to form spermatogonia or oogonia
d. Folds with the embryonic disc to become the primitive
digestive tract and part of the umbilical cord
3. Allantois - forms during the 3rd week as a small outpocketing of
the posterior end of the yolk sac
a. Gives rise to the umbilical cord
b. Becomes part of the urinary bladder
4. Chorion - outermost extraembryonic membrane
a. Becomes the fetal half of the placenta as fingerlike villi
extend into the endometrium
5
b. The villous chorion forms embryonic blood vessels; as the
embryonic heart forms, the blood is circulated close to the
uterine wall
c. In chorionic villus sampling, a sample of chorionic villus is
obtained (at 10-12 wks), then is tested for genetic
abnormalities
D. Placenta - vascular structure formed from both maternal &
embryonic tissues that attach the fetus to the mother’s uterus
1. The placenta forms from day 11 through week 12, with most
development during the embryonic period
2. As chorionic villi digest their way through uterine blood vessels,
eventually forming a blood filled cavity called the placental
sinus
3. The chorionic villi grow into vascular branched structures
surrounded by maternal blood in the sinus
4. Blood does not flow directly between mother & fetus, rather
substances diffuse across the shared placental membrane
a. Oxygen and nutrients diffuse across the placenta from mother
to fetus via blood in the umbilical vein
b. CO2 and wastes diffuse from fetus to mother via blood
carried by the umbilical arteries
c. The placenta’s blood barrier prevents some large pathogens
from diffusing but allows smaller pathogens, such as drugs,
viruses, some bacteria, and maternal antibodies
5. The placenta also functions as an endocrine gland, secreting
both glycoproteins and steroid hormones to maintain the
endometrium.
E. Umbilical cord - connects the fetus to the placenta and contains 2
umbilical arteries & 1 umbilical vein
6
F. Major structural changes in the embryo by week
1. Third week – gastrulation & neurulation occur
a. A linear band called the primitive streak occurs along the
dorsal midline of the embryonic disk and cells invaginate
b. This gives rise to the primitive node at the cranial end,
which gives rise to the mesoderm of the head and notochord
c. The notochord and somites form along the midline axis to
begin formation of the embryonic skeleton and muscles
d. Along the dorsal midline ectoderm above the notochord a
neural plate forms, which folds into a neural groove, then a
neural tube, from which the brain and spinal cord develop
2. Fourth week – embryo folds into a tadpole shape, germ layers
differentiate into specialized tissues
a. A connecting stalk that will become the umbilical cord
forms from the embryo to the placenta
b. The heart begins to beat
c. Head & jaws become apparent
d. Primordial eyes, brain, spinal cord, lungs, & digestive organs
develop
e. Superior & inferior limb buds are seen as small swellings
3. Fifth week
a. The head enlarges and the developing eyes, ears, and nasal
pits become obvious
b. Appendages form from the limb buds, with paddle shapedhand & foot plates
4. Sixth week
a. The head is larger than the trunk
b. The brain is undergoing extensive differentiation
c. Limbs lengthen and are flexed, with notches appearing
between the digital rays in the hand & foot plates
5. Seventh & eighth weeks
a. Body organs are formed
7
b.
c.
d.
e.
f.
g.
The nervous system is starting to coordinate body activity
Neck is apparent
Eyes are well developed, but lids are stuck together
Nostrils are developed but plugged with mucus
External genitalia are formed, but still undifferentiated
All body systems are developed (but still immature) by the
end of the 8th week
h. From this time on, the embryo is called a fetus
G. The developing embryo/fetus is particularly susceptible to
teratogens (birth defect causing substances) during the first
trimester (first 3 months of development).
1. Alcohol causes more birth defects than any other drug, and can
cause fetal alcohol syndrome, which is characterized by a small
head and mental retardation
2. Cigarette smoking contributes to fetal and infant mortality.
Women who smoke while pregnant have twice as many
miscarriages as those who do not smoke
3. All drugs should be avoided during pregnancy, unless prescribed
by a physician who knows the woman is pregnant
V. Pregnancy
A. Hormones of pregnancy include estrogens, progesterone, HCG
1. The corpus luteum secretes progesterone and estrogens to
maintain the endometrium in the first 7-12 weeks of
development
2. The blastocyst secretes human chorionic gonadotropin (HCG),
which stimulates the corpus luteum to continue to secrete its
hormones
3. At about 12 weeks, the placenta begins to secrete the majority of
estrogens and progesterone. Functions are
a. Estrogens stimulate tissue growth in the fetus and mother
8
b. Progesterone and estrogen suppress pituitary secretion of
FSH and LH, thereby preventing the development of more
follicles, as well as promoting mammary gland development
c. Progesterone suppresses uterine contractions and prevents
menstruation
B. Other hormones, such as thyroid & parathyroid hormones,
glucocorticoids, adosterone, and relaxin, also contribute to the
developments of pregnancy
C. Adjustments to Pregnancy occur in many body systems
1. Digestive system – exhibits morning sickness, constipation, and
heartburn because steroid hormones inhibit intestinal motility
and the growing uterus compresses digestive organs
2. Circulatory system – blood volume and cardiac output
increases; pressure from the uterus may cause hemorrhoids and
varicose veins
3. Respiratory system – breathing becomes more rapid as O2
demand and CO2 sensitivity increase, yet the lungs are
compressed by the uterus, causing shallow breathing
4. Urinary system – glomerular filtration and urine output increase
to dispose of maternal and fetal wastes, but bladder capacity is
reduced by pressure from the uterus
5. Integumentary system- skin of the breasts and abdomen grow
and may cause striae (stretch marks)
D. Childbirth
1. In the 7th month of gestation, the fetus usually turns into a headdown vertex position
2. Late in pregnancy, the uterus may exhibit Braxton-Hicks
contractions weeks before the true labor contractions occur
3. True labor contractions are stimulated by uterine stretching and
oxytocin
a. In the positive feedback theory of labor, stretching of the
cervix triggers reflex contraction of the uterine body, which
pushes the fetus downward and stretches the cervix even more
9
b. Cervical stretching also activates a neuroendocrine reflex that
results in oxytocin secretion, which stimulates more intense
uterine contractions
4. Stages of labor include dilation, expulsion, and the placental
stage
a. Dilation involves widening of the cervical canal to a diameter
of 10 cm and thinning of the cervical tissue. Fetal
membranes rupture and discharge amniotic fluid
b. Expulsion of the fetus begins when the baby’s head enters
the vagina and lasts until the baby is entirely discharged. At
the end of this stage the umbilical cord is clamped and cut
c. During the placental stage, the placenta, amnion, and other
components of the afterbirth are discharged. The afterbirth
is examined to ensure that there are no abnormalities
5. Puerperium is the first 6 weeks postpartum (after birth), during
which the mother’s anatomy and physiology return nearly to
their original condition
VI. Lactation is the synthesis and ejection of milk from the mammary
glands
A. During pregnancy, estrogen, GH, insulin, glucocorticoids,
prolactin, and progesterone stimulate the development of the
mammary glands
B. For 1-3 days postpartum, the mammary glands secrete a fluid
called colostrum, which is higher in protein than milk, but lower
in fat and lactose. It also contains immunoglobulins that provide
some immunity to infection
C. Prolactin is secreted during pregnancy but cannot stimulate milk
synthesis until after the placenta is shed, due to the inhibitory
effects of steroidal hormones
D. The nursing infant stimulates neuroendocrine reflexes in which the
pituitary gland secretes oxytocin and prolactin
10
1. Oxytocin triggers contraction of myoepithelial cells of the acini,
making milk flow down the lactiferous ducts to the nipple
2. Prolactin stimulates synthesis of the milk that will be used for
the next feeding