Chapter 28 Pregnancy and Human Development G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.
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Transcript of Chapter 28 Pregnancy and Human Development G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.
Chapter 28Pregnancy and
Human Development
G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.
PregnancyEvents 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 eggAcrosomal membrane must become fragile
Acrosomal reaction: release of the digestive enzymes (acrosin, other proteases) from the sperms’ acrosomeHundreds 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 PolyspermyPenetration 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 surviveSimilar to the events of an action potential at
the synaptic end bulb and muscle contraction
Early Events of FertilizationUpon entry of sperm, the
secondary oocyte:Completes meiosis II Casts out the second polar
bodyThe ovum nucleus swells,
and the two nuclei approach each other
When fully swollen, the two nuclei are called pronuclei
Pronuclei burstFertilization – when the
pronuclei come together (Blastomeres)
Preembryonic DevelopmentZygote 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
Implantation the trophoblasts then proliferate and form two distinct layers
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
PlacentationThe 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 embryoamnion – 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 appearsthis 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 foldsnext, head and tail folds appearan 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 PatternsUmbilical vein routes oxygenated nutrient-laden blood
first to the liver, then to the general circulation Ductus venosus – the venous shunt which
bypasses the liver
Fetal Circulation Patterns3 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
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 occurDyspnea (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
ParturitionDilation 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
Expulsionplacental 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 Lifeonce 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
Lactationthe production of milk by the mammary glands
estrogens, progesterone, and lactogen stimulate the hypothalamus to release a prolactin-releasing factor
the anterior pituitary responds by releasing prolactin
Colostruma yellowish solution rich in vitamin A, protein, minerals,
and IgA antibodies is released the first 2–3 days is followed by true milk production
LactationAdvantages 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
LactationOther 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 nutritionshould begin between 6
and 12 months after birthsometimes a difficult
transition for both mother and child
Parental Care and SocializationMother 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