pregnancy and development 2

hCG

– Hormone secreted by syncytiotrophoblast cells (which become part of chorion)

– Like LH, keeps corpus luteum secreting progesterone and estrogen

▪ Also helps prevent immune system from rejecting embryo

– Pregnancy tests use antibodies to detect hCG in blood or urine

▪ Usually detectable in blood two days after implantation (levels continue rising until end of second month; settle to low values by fourth month)

– Placenta takes over secretion of progesterone and estrogen (corpus luteum finally degenerates) between second and third month

Formation of the placenta

• By second month, placenta providing nutrients/oxygen to embryo and carrying away wastes

– Temporary organ that originates from both embryonic and maternal tissues

Formation of embryonic portion:

▪ Embryoblast forms layer of extraembryonic mesoderm that lines inner surface of trophoblast; together these structures form the chorion, which then Develops fingerlike projections called chorionic villi

Chronic villi

are then invaded by newly forming blood vessels, which extend to embryo as umbilical arteries and vein

▪ Erosion of endometrium produces large, blood-filled lacunae (intervillous spaces) in stratum functionalis

– Villi lie in spaces surrounded by maternal blood

Placentation 1

• Part of stratum functionalis between embryo and stratum basalis, called decidua basalis, forms maternal portion of placenta (decidua = “that which falls off”)

– Chorionic villi here increase in number and branches as fetus grows

• Part of stratum functionalis surrounding uterine cavity face of embryo, called decidua capsularis, expands to accommodate growing fetus

– Villi here degenerate as fetus grows

• Fully formed and functional by end of month 3, placenta consists of decidua basalis (maternal part) and associated chorionic villi (embryonic part)

– Provides nutritive, respiratory, excretory, and endocrine functions

• Substances exchanged (diffusion) between maternal and fetal blood across two barriers (so blood supplies don’t intermix):

– Membranes of chorionic villi

– Endothelium of embryonic capillaries

placentation 2

• Ability of syncytiotrophoblast cells (“hormone manufacturers”) to produce estrogens and progesterone matures more slowly than ability to secrete hCG

– If these hormones (particularly progesterone) are inadequate when hCG levels wane, endometrium degenerates and pregnancy spontaneously aborts

• During pregnancy, blood levels of estrogens and progesterone increase, promoting

– Growth and preparation of mammary glands for lactation

• Placenta secretes other hormones, including

– Human placental lactogen and relaxin

Embryonic events

Include Gastrula Formation and Tissue Differentiation, Which are Followed by Rapid Growth of the Fetus

• During process of implantation, blastocyst begins process of being converted into gastrula

– Three germ layers, as well as extraembryonic membranes, develop from gastrula

• Embryoblast divides into two layers: upper epiblast and lower hypoblast

– Now called the embryonic disc

Extraembryonic membranes

that form during first 2–3 weeks of development include:

1. amnion

2. yolk sac

3. allantois

4. chorion

Amnion

(“bag of waters”): epiblast cells form transparent membranous sac that surrounds amniotic cavity, which contains amniotic fluid

▪ Buoyancy protects embryo and helps maintain constant temperature

▪ Allows freedom of movement; prevents parts from fusing together

▪ Fluid initially derived from maternal blood, later fetal urine contributes

Yolk sac

primitive gut cells form sac that hangs from ventral surface of embryo

▪ Forms part of gut; source of earliest blood cells

▪ Location from which primordial germ cells migrate to gonads to form gametes

Allantois

small out-pocketing at caudal end of the yolk sac

▪ Structural base for umbilical cord; becomes part of urinary bladder

– Umbilical cord contains core of embryonic connective tissue, umbilical vein, and umbilical arteries; covered by amnion

Chorion

outermost membrane (helps form placenta); encloses embryonic body and all other membranes

Gastrulation 1

— germ layer formation

(during week 3): cellular rearrangements and migrations in which two-layered embryonic disc transforms into three-layered embryo with:

– Three primary germ layers: ectoderm, mesoderm, and endoderm

• Begins with appearance of primitive streak, a raised dorsal groove that establishes longitudinal axis of embryo

ectoderm

(“outer skin”) forms nervous system and epithelium of skin (epidermis)

• Cells on embryo’s dorsal surface now called ectoderm (formerly epiblasts

——consist mostly of cells that form epithelia

Endoderm

(“inner skin”) forms epithelial linings of digestive, respiratory, and urogenital systems, and associated glands

• Epiblasts from surface migrate medially to enter groove; first of these form endoderm (most inferior layer) as they displace hypoblasts of yolk sac

—consist mostly of cells that form epithelia

Mesoderm

(“middle skin”) forms virtually everything else

• Cells that follow push laterally, forming mesoderm (between endoderm and epiblasts)

– Notochord: rod of (aggregated) mesodermal cells that serves as first axial support of embryo

—a mesenchyme, an eembryonic tissue with star-shaped cells free to migrate widely within embryo

Organogenesis

Differentiation of the Germ Layers

• Gastrulation sets stage for organogenesis, formation of body organs and systems

• End of week 8, embryo ~ 30 mm (just over inch) long from head to buttocks

– All organ systems are recognizable

• Embryo begins as flat plate, then folds (as it grows) into cylindrical body resembling three stacked sheets of paper folding laterally into tube (at both ends)

Specialization of the endoderm

– Primitive gut formed from endodermal folding; becomes epithelial lining of G I tract

▪ Organs of G I tract become apparent; then oral and anal ends of gut open

– Mucosal lining of respiratory tract forms as outpocketing of foregut

▪ Glands (e.g., thyroid, parathyroid, thymus) arise as endodermal outpocketings further along tract

Specialization of the ectoderm

– First major event of organogenesis, neurulation, produces brain and spinal cord

▪ Induced by chemical signals from notochord

▪ Ectoderm over notochord thickens, forming neural plate

– Neural folds form by folding of neural plate, which then deepens, producing neural groove

– By day 22, neural folds fuse to form neural tube

▪ Anterior end of tube will form brain; rest of tube forms spinal cord

– Associated neural crest cells migrate widely to form:

• Cranial, spinal, and sympathetic ganglia and nerves; adrenal chromaffin cells

• Pigment cells of skin; and contributes to some connective tissues

– By end of month 1, forebrain, midbrain, and hindbrain appear

– Brain waves can be recorded by end of month 2

specialization of the mesoderm

– First evidence of mesodermal differentiation is appearance of notochord (eventually replaced by vertebral column)

– Three aggregates develop lateral to notochord:

1. Somite: 40 pairs, each has three functional parts

– Sclerotome cells produce vertebra (and rib at each associated level)

– Dermatome forms dermis of skin on dorsal part of body

– Myotome forms skeletal muscles of neck, trunk, limbs (via limb buds)

2. Intermediate mesoderm: forms gonads and kidneys

3. Lateral plate mesoderm: consists of two plates of cells (which cooperate to form the serosae of the coelom, or ventral body cavity)

Somatic mesoderm

forms dermis of skin in ventral region, parietal serosa of ventral body cavity, and most tissues of limbs

Splanchnic mesoderm

forms heart and blood vessels, most connective tissues of body, and wall of digestive and respiratory organs

Fetal circulation

• First blood cells arise in yolk sac

• End of week 3, embryo has system of paired vessels; those forming heart have fused

– Heart beats by day 22

Unique vascular modifications seen only during prenatal development and are occluded at birth include:

– Umbilical arteries and umbilical vein

– Three vascular shunts:

1. ductous venosus

2. foramen ovale

3. ductus arteriosus

Ductus venosus

bypasses liver; umbilical vein drains into ductus venosus which empties into inferior vena cava

Foramen ovale

bypasses pulmonary circulation via opening in interatrial septum

ductus arteriosus

bypasses pulmonary circulation; pulmonary trunk drains into ductus arteriosus, which drains into aorta