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Chapter 18
Lecture Outline
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18.1 Fertilization
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A. Introduction
1.
2.
Fertilization – penetration of the ovum by
one sperm
The zygote is formed when the genetic
material of the ovum and sperm unite
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B. Sperm and ovum anatomy
1.
Sperm Anatomy
a. Head – contains the nucleus of 23
chromosomes and very little cytoplasm;
capped by the acrosome that contains
digestive enzymes
b. Middle piece – contains energyproducing mitochondria
c. Tail (flagellum) – for swimming
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2. Ovum anatomy
a.
b.
c.
Ovum contains cytoplasm, organelles ,
and 23 chromosomes
The plasma membrane is surrounded by
the zona pellucida
The corona radiata surrounds the zona
pellucida
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C. Steps of Fertilization
1.
Several sperm penetrate the corona
radiata
2. Several sperm attempt to penetrate the
zona pellucida
3. One sperm enters the ovum
a. Acrosome releases digestive enzymes
b. When a sperm binds to the ovum, their
plasma membranes fuse and this sperm
enters the ovum
c. Sperm and ovum nuclei fuse to produce
the zygote
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4. Prevention of polyspermy
a.
b.
c.
d.
Accidental entry of more than one sperm
will halt development of the zygote
When the one sperm fuses to the ovum
cell membrane, the membrane depolarizes
Release of cortical oocyte granules with
lysosomal enzymes
Zona pellucida becomes impermeable to
any other sperm
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Fertilization
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18.2 Development
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A. Processes of Development
1.
2.
3.
4.
Cleavage – rapid cell division (mitosis)
with no size increase
Growth – size increase
Morphogenesis – shaping of the embryo
and movement of cells
Differentiation – groups of cells take on
different structures and functions
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B. Extraembryonic membranes
1.
2.
Are not part of the embryo and fetus
Functions in humans:
a. Chorion – develops into the fetal half of
the placenta
b. Yolk sac – first site of blood cell
formation
c. Allantois – contains blood vessels that
become the umbilical blood vessels
d. Amnion – contains fluid to cushion and
protect the embryo
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Extraembryonic membranes
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C. Stages of Development
1.
2.
Human gestation of 280 days
Pre-embryonic development
a. Events of the first week
b. After fertilization, the zygote divides
repeatedly (cleavage)
c. A morula becomes a blastocyst
1) Inner cell mass surrounded by the
trophoblast; becomes the embryo
2) The trophoblast will become the
chorion
d. Each cell has the genetic capability of
becoming any tissue
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Pre-embryonic development
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3. Embryonic development
a. Second week
1) Embryo usually begins the process of
implantation
a) If implantation is successful, the
female is clinically pregnant
b) An ectopic pregnancy occurs if the
embryo implants in the uterine tube
2) The trophoblast begins to secrete HCG
a) Acts like LH
b) Stimulates corpus luteum to secrete
progesterone and the endometrium is
maintained
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Second week, cont
3) The inner cell mass separates from the
trophoblast
4) The yolk sac and amnion form
5) Gastrulation occurs
a) Inner cell mass becomes the
embryonic disk
b) Primary germ layers (ectoderm,
mesoderm, and endoderm) form
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Embryonic germ layers
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b. Third week
1) The nervous system becomes evident
a) Neural plate
b) Neural groove
c) Neural folds
d) Neural tube forms – later becomes the
brain and spinal cord
e) If the neural tube fails to close
completely, a neural tube defect will
result – spina bifida or anencephaly
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Third week, cont
2) Development of the heart begins
a) Begins with right and left heart tubes
b) Tubes fuse and begin to pump blood
c) Arteries and veins grow from the heart
tube
d) Tube twists so that the vessels are all
anterior
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Embryonic development
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c. Fourth and fifth weeks
1) Exchange of gases, nutrients, and wastes
can take place across the capillary
network within the chorionic villi
2) Blood vessels of the allantois become the
umbilical blood vessels
3) The umbilical cord completes formation
4) Limb buds appear
5) The head enlarges and the sense organs
become more prominent
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d. Sixth through eight weeks
1) Embryo is easily recognized as human
2) The head achieves its normal
relationship with the body with the
development of the neck
3) Nervous system has developed enough
to allow reflex actions
4) Embryo is about 1.5 inches long
5) All organ systems have formed
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Human embryo at 5 weeks
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4. Placenta
a. Usually fully functional by the end of the
embryonic period
b. Two portions:
1) Fetal portion composed of chorionic
tissue
2) Maternal portion composed of uterine
tissue
c. Placental membrane – epithelium of
embryonic capillary and epithelium of a
chorionic villus
d. Placenta supplies the fetus with its
nutritional and excretory needs – digestive
system, lungs, and kidneys do not function
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Placenta, cont
e.
The umbilical cord transports fetal blood
to and from the placenta
1) Two umbilical arteries carry fetal blood
to the placenta for gas and waste
removal
2) One umbilical veins carries oxygen and
nutrients from the placenta to the fetus
f.
The placenta begins to produce
progesterone and estrogen to maintain
the pregnancy
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The placenta
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5. Fetal development
a. Third and fourth months
1) Head growth slows and the rest of the
body increases in length
2) Epidermal refinements appear –
fingernails, nipples, eyelashes,
eyebrows, and hair
3) Cartilage begins to be replaced by bone
4) Possible to distinguish males from
females
5) During the fourth month, the fetal
heartbeat can be auscultated
6) About 6 inches long and weighs 6 oz.
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Fetal development, cont
b.
Fifth through seventh months
1) Fetal movement can be felt by the mother
2) Lanugo and vernix caseosa
3) Eyelids are fully open
4) About 12 inches long and weighs 3 lbs.
c.
Eighth through ninth months
1) Fetus usually rotates so that the head is
pointed towards the cervix
2) Weight gain due to an accumulation of fat
3) About 20½ inches long and weighs 7½ lbs.
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Human Development Summary
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Five to seven month fetus
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D. Development of male and female sex
organs
1. Gender is determined at the moment of fertilization
2. At six weeks, both males and females have
Wolffian and Müllerian ducts
3. Gonads start developing in the seventh week
4. Genes on the Y chromosome cause the production
of testosterone
a. Causes testes to develop from indifferent
tissue
b. Stimulates the Wolffian ducts to become male
genital ducts and enter the urethra
c. Testes secrete anti-Müllerian hormone that
causes the Müllerian ducts to regress
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Development of male and female sex
organs, cont.
5.
If there is no Y chromosome present
a. Ovaries develop instead of testes
b. Müllerian ducts develop into the uterus
and uterine tubes and Wolffian ducts
regress
6. At 14 weeks, both ovaries and testes are
in the abdominal cavity
a. During the last trimester, ovaries and
testes descend into the pelvic cavity
b. Testes then descend into the scrotal
sacs via the inguinal canal
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7. External genitalia
a.
At six weeks – a small bud of indifferent
tissue is seen between the legs
b. At nine weeks – urogenital groove appears
c. By fourteen weeks
1) The groove has disappeared in males
and the scrotum has formed
2) In females the groove becomes the
vaginal opening and the labia majora
and labia minora form
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Male & female reproductive organs
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18.3 Birth
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A. Introduction
1.
Labor is marked by regular, long-lasting
contractions
2. A positive feedback mechanism causes the
onset and continuation of labor
a. Stretching of cervix stimulates oxytocin
release
b. Oxytocin stimulates the uterine muscles
c. Uterine contractions push the fetus
forward causing the cervix to stretch
more
3. Expulsion of the mucous plug is usually
the first sign that birth is imminent
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B. Stages of Birth (parturition)
1.
Stage 1
a. Effacement
b. Amniotic membrane will probably
ruptured if it has not already
c. Ends when the cervix has dilated
completely (about 10 cm)
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Stages of Birth, cont
2.
Stage 2
a. Uterine contractions occur every 1-2
minutes and last about 1 minute each
b. Baby’s head descends into the vagina
c. The baby is delivered
3. Stage 3
a. The placenta is delivered
b. About 15 minutes after delivery of the
baby, the placenta is dislodged from the
uterus and expelled into the vagina
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Stages of parturition
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C. Effects of Pregnancy on the Mother
1.
Early pregnancy
a. Nausea and vomiting
b. Loss of appetite and fatigue
2. Then weight gain due to:
a. Breast and uterine enlargement
b. Weight of the fetus
c. Amount of amniotic fluid
d. Size of the placenta
e. Increase in her own body fluid
f. Storage of proteins, fats, and minerals
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3. Physiological changes due to placental
hormones
a.
Progesterone
1) Relax smooth muscle
2) Reduced uterine motility
3) Reduced maternal immune response to
fetus
b. Estrogen
1) Increased uterine blood flow
2) Increased renin-angiotensin-aldosterone
activity
3) Increased protein biosynthesis by liver
c. Peptide hormone – increased insulin
resistance
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4. Other maternal changes
a.
b.
c.
d.
e.
Increase in pulmonary values
Stress incontinence
Edema and varicose veins
Possibility of pregnancy-induced
diabetes
Striae gravidarum (stretch marks)
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