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