Transcript Chapter 28 Pregnancy and Development

Chapter 28
Pregnancy and
Human Development
G.R. Pitts, J.R. Schiller, and
James F. Thompson, Ph.D.
Pregnancy
 Events from
fertilization to birth
 Conceptus 
Embryo  Fetus :
the developing
offspring
 Gestation period:
the time during
which development
occurs
Fertilization
Capacitation: the process in the femal
reproductive tract whereby the ejaculated
sperm become capable of fertilizing the
egg
Acrosomal membrane must become fragile
Acrosomal reaction: release of the
digestive enzymes (acrosin, other
proteases) from the sperms’ acrosome
Hundreds of sperm must participate
Fertilization
 If timing is ideal,
sperm reach the
egg in the upper
third of the
uterine tube
 Sperm move by
flagellar action
but also receive
an assist from
uterine tube
peristalsis
Prevention of Polyspermy
Penetration of the egg membrane by the first
sperm causes the membrane to depolarize (Na+
influx) (fast block)
Cell membrane depolarization triggers release
of stored Ca++ from the endoplasmic reticulum
Ca++ causes cortical reaction resulting in
formation of fertilization membrane from cortical
vesicles (slow block)
Polyploid zygotes cannot survive
Similar to the events of an action potential at
the synaptic end bulb and muscle contraction
Early Events of
Fertilization
 Upon entry of sperm, the
secondary oocyte:
Completes meiosis II
Casts out the second polar
body
 The ovum nucleus swells,
and the two nuclei approach
each other
 When fully swollen, the two
nuclei are called pronuclei
 Pronuclei burst
 Fertilization – when the
pronuclei come together
(Blastomeres)
Preembryonic Development
 Zygote undergoes cleavage to morula and on to
blastocyst; should be completed in the uterine tube
Implantation
 Blastocyst “floats” in the uterus for 2-3 days
 Blastocyst implants 6-7 days after fertilization
 the trophoblasts then
proliferate and form two
distinct layers
Implantation
 Cytotrophoblast – cells of the
inner layer that retain their
cell boundaries
 Syncytiotrophoblast – cells in
the outer layer that lose their
plasma membranes and
invade the endometrium
 implantation completed by
14 days after ovulation
 hCG from the placental
chorion signals the
hypothalamus, pituitary,
and corpus luteum that
implantation has occurred
 steroid hormone levels are
maintained which prevents
uterine sloughing
(menses)
hCG
Implantation
• Endometrial epithelium
grows around
implanted blostocyst
• Chorion – develops
from trophoblasts after
implantation, continues
hCG stimulus
hCG
Placenta produces hCG,
estrogen, progesterone, etc.
 hCG maintains the
corpus luteum which
produces estrogen and
progesterone (positive
feedback)
 hCG informs the
hypothalamus and
pituitary that
implantation has
occurred
 eventually, the placenta
produces its own
estrogen &
progesterone to support
uterine proliferation
Placentation
 The chorion develops
fingerlike villi, which:
become vascularized
extend to the embryo
as umbilical arteries
and veins
lie immersed in
maternal blood
The Placenta
 Before becoming
three-layered, the
inner cell mass
subdivides into the
upper epiblast and
lower hypoblast
 These layers form
two of the four
embryonic
membranes
Placentation
 The embryo is supported by three external embryonic
membranes: chorion, allantois, and amnion.
Chorion (outer membrane) forms
from the embryonic trophoblast
 chorion forms the bulk
of the placenta
 chorionic villi develop
and become
vascularized
 villi are surrounded by
maternal blood vessels
in the uterine lacunae
 nutrients, gases, and
wastes are exchanged
by diffusion between
the maternal and fetal
circulations
Allantois
 Allantois – a small outpocketing
at the caudal end of the yolk sac
 structural base for the umbilical
cord
 becomes part of the urinary
bladder
 Yolk sac – hypoblast cells that
form a sac on the ventral surface
of the embryo
 forms part of the digestive tube
 produces earliest blood cells and
vessels
 is the source of primordial germ
cells
Umbilical cord with fetal blood
vessels develops from allantois
Amnion (inner membrane)
envelops and protects embryo
 amnion – epiblast cells form a
transparent membrane filled
with amniotic fluid – a
maternal plasma filtrate
 amniotic fluid comes from
maternal blood, and, later,
fetal urine adds to it
 amniotic fluid acts as a liquid
shock absorber to protect the
fetus
 helps maintain uterine
internal homeostatis
 amniotic fluid may be
sampled to determine certain
aspects of fetal health
Gastrulation
During the 3rd week, a
primitive streak
appears
this raised dorsal groove
establishes the
longitudinal axis of the
embryo
The first cells that
enter the groove form
the endoderm
Gastrulation
 the cells that follow push
laterally between the cells
forming the mesoderm
 the cells that remain on
the embryo’s dorsal
surface form the
ectoderm
 the two-layered
embryonic disc becomes
a three-layered embryo
 the primary germ layers
form: ectoderm,
mesoderm, and endoderm
The Primary Germ Layers
 form populations of stem cells from which
all body tissues and organs are derived
 Ectoderm – forms structures of the
nervous system and skin epidermis
 Endoderm – forms epithelial linings of
the digestive, respiratory, and urogenital
systems
 Endoderm and ectoderm give rise to the
epithelial tissues
 Mesoderm – forms all other tissues (all
connective tissues, bone, all types of
muscle, blood and blood vessels, the
gonads and the adrenal cortex)
Body Plan and Tissues Develop,
Then Organ Systems Develop
 Head, Trunk, and Limb
Buds Develop
 The Notochord and
Neural Tube organize
on the dorsal surface
 The Peritoneal Cavity
(coelom) and Primitive
Gut (archenteron)
organize beneath the
ventral surface
Organogenesis
 Neurulation – the first
event of organogenesis
gives rise to the brain and
spinal cord
 induced by the notochord
 Ectoderm over the
notochord thickens,
forming the neural plate
 the neural plate folds
inward as a neural groove
with prominent neural
folds
Organogenesis
 by the 22nd day, the neural
folds fuse into a neural
tube, which pinches off into
the body
 the anterior end becomes
the brain; the rest becomes
the spinal cord
 associated neural crest
cells give rise to the
cranial, spinal, and
sympathetic ganglia of the
PNS
Endoderm Specializations
 embryonic folding begins with lateral folds
 next, head and tail folds appear
 an endoderm tube forms the epithelial lining of the GI
tract
Endoderm Specializations
 organs of the GI tract
become apparent,
and oral and anal
openings perforate
 endoderm forms the
epithelium linings of
the hollow organs of
the digestive and
respiratory tracts
Mesoderm Specializations
 three mesoderm aggregates
appear lateral to the notochord
 the somites produce the
vertebrae, ribs, dermis of the skin,
and skeletal muscles of the neck,
trunk, and limbs
 intermediate mesoderm forms
the gonads and the kidneys
 lateral mesoderm
 somatic mesoderm forms dermis of
the skin in the ventral region parietal
serosa of the ventral body cavity,
bones, ligaments, and dermis of the
limbs
 splanchnic mesoderm forms the
heart, blood vessels and most
connective tissues of the body
Fetal Circulation Patterns
 Umbilical vein routes oxygenated nutrient-laden blood
first to the liver, then to the general circulation
 Ductus venosus – the venous shunt which
bypasses the liver
 3 shunts transfer
oxygenated blood from
the right to the left side
of the heart to bypass
the pulmonary
circulation
 Foramen ovale –
opening in the interatrial
septum
 Interventricular foramen
– opening in the
interventricular septum
 Ductus arteriosus –
anastamosis transfers
blood from the pulmonary
trunk to the aorta
Fetal Circulation
Patterns
Maternal Changes During Pregnancy
 Chadwick’s sign – the vagina develops a purplish hue
 Breasts enlarge and their areolae darken
 The uterus expands, occupying most of the abdominal cavity
 Lordosis is common due to the change of the body’s center of gravity
Maternal Changes During Pregnancy
 Relaxin causes pelvic ligaments and the pubic symphysis to
relax
 Typical weight gain is about 29 pounds
 GI tract – morning sickness occurs due to elevated levels of
estrogen and progesterone
 Urinary system – urine production increases to handle the
additional fetal wastes
 Respiratory system – edematous and nasal congestion may
occur
 Dyspnea (difficult breathing) may develop late in pregnancy
 Cardiovascular system – blood volume increases
25-40%
 Venous pressure from lower limbs is impaired, resulting in
varicose veins
Regulation of Parturition (birth)
 Labor and Delivery are
regulated cooperatively
by hormones and the
ANS
 Relaxin is secreted by
the corpus luteum; it
helps to soften the cervix
and relax the pelvic
ligaments in preparation
for childbirth
Regulation of Parturition (birth)
 Cortisol from fetus increases estrogen
 Estrogen peaks during the last weeks
of pregnancy increasing oxytocin
receptors and antagonzing P4 causing
myometrial weakness and irritability
 Weak Braxton Hicks contractions may
take place
 As birth nears, the fetus produces
oxytocin and the placenta produces
prostaglandins causing uterine
contractions
 Emotional and physical stress:
 activates the hypothalamus
 sets up a positive feedback mechanism,
releasing more oxytocin
Parturition
 Dilation  Expulsion of
Neonate  Expulsion of
Placenta
 Eventually conscious
motor commands add the
“push” for delivery
Dilation
 from the onset of labor until the
cervix is fully dilated (10 cm)
 initial contractions are 15–30
minutes apart and 10–30 seconds
in duration
 the cervix thins (effaces) and
dilates
 the amnion ruptures, releasing
amniotic fluid (breaking of the
water)
 engagement occurs as the infant’s
head enters the true pelvis
 the head rotates face down
 Longest part of parturition (6-12 h)
Expulsion
 from full dilation to delivery of
the infant
 strong contractions occur
every 2–3 minutes and last
about 1 minute
 the urge to push increases in
labor without local anesthesia
 crowning occurs when the
largest dimension of the head
is distending the vulva
Expulsion
 placental delivery is
accomplished within 30
minutes of birth
 afterbirth – the placenta
and its attached fetal
membranes
 all placenta fragments
must be removed to
prevent postpartum
bleeding
Extrauterine Life
 once carbon dioxide is no longer eliminated by the
placenta, central acidosis occurs
 this excites the respiratory centers to trigger the first
inspiration
 this requires tremendous effort – airways are tiny
and the lungs are collapsed
 once the lungs inflate, surfactant in alveolar fluid
helps reduce surface tension
 umbilical arteries and vein constrict and soon
become fibrosed
Lactation
 the production of milk by the mammary glands
 estrogens, progesterone, and lactogen
stimulate the hypothalamus to release a prolactinreleasing factor
 the anterior pituitary responds by releasing
prolactin
 Colostrum
a yellowish solution rich in vitamin A, protein, minerals,
and IgA antibodies
is released the first 2–3 days
is followed by true milk production
Lactation
 Advantages of breast milk for the infant
fats and iron are better absorbed
its amino acids are metabolized more efficiently than those
of cow’s milk
beneficial chemicals are present – IgA, other
immunoglobulins, complement, lysozyme, interferon, and
lactoperoxidase
interleukins and prostaglandins are present, which prevent
overzealous inflammatory responses
its natural laxatives help cleanse the bowels of meconium
Lactation
 Other advantages of breast feeding for the infant
improved maternal-child bond
improved neurological development
appropriate jaw, teeth and overall facial development as
well as speech development
reduced risks for breast cancer and ovarian cancer
Weaning
 The transition from milk
to other forms of
nutrition
should begin between 6
and 12 months after birth
sometimes a difficult
transition for both mother
and child
Parental Care and Socialization
 Mother provides milk
 Father and siblings and,
perhaps, other relatives may
provide additional food,
care, support and protection
 Maturity and socialization
develop slowly over a period
of years, even decades!
End Chapter 28