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WVSOM - Embryology-4

Early Morphogenesis (weeks 3 - 8)

QuestionAnswer
Formation of the neural tube Neurulation (progenitor of numerous tissues, particularly CNS)
What structures does the ectoderm generate? CNS; PNS; sensory epithelia of ears, nose, eyes; epidermis including hair and nails; subcutaneous and mammary glands; pituitary gland; teeth enamel
What occurs at the beginning of week 3? Notochord in anterior 3/4 of germ disc secretes GF that induce overlaying ectoderm to differentiate into neural plate
Neuroectoderm Refers to thickened, columnar cells that compose neural plate
Neural folds Lateral edges of neural plate that elevate (end of week 3)
Neural groove Medium of neural plate depresses to form groove
Neural tube Inversion and fusion of neural tube in anterior and posterior directions; begins in area that will develop into neck (4th somite)
Openings that persist before final closure Cranial and caudal neurospores; open to the amniotic cavity
When does the cranial neurospore close? ~day 25 (18-20 somite stage)
When does the caudal neurospore close? Day 27 (25 somite stage)
What does the closure of the cranial and caudal neurospores indicate? Completion of neurulation
What forms the spinal cord? The narrow posterior region of the neural tube
What forms the brain vesicles? The broad anterior portion of the neural tube
Neural crests Lateral borders of the neural folds; after closure, neural crests dissociate & migrate thru mesoderm as mesenchymal cells (ex. of how mesenchyme not always mesodermal)
Neural crest cells Ectodermal mesenchyme; from numerous structures (i.e. ganglia & cranial nerves, schwann cells, melanocytes, adrenal medulla, craniofacial bones, connective tissues, conotruncal heart cushions)
Mesenchyme Loosely woven, embryonic stem cell derived from any germ layer
Mesoderm development Produces muscle, connective tissue, cartilage, bone, blood vessels, kidneys, spleen, suprarenal cortex
Mesoderm differentiates into 3 regions Paraxial, intermediate, lateral mesoderm
Lateral mesoderm differentiation Coelom develops in lateral mesoderm by coalescence of intercellular spaces; lateral mesoderm lining dorsal surface of coelom = somatic mesoderm layer; ventral surface lining coelom = splanchnic mesoderm layer
Somatic (parietal) mesoderm layer + overlaying ectoderm = somatopleura; will develop into lateral & ventral walls of body cavity; connects w/ extraembryonic mesoderm (covers amnion)
Splanchnic (visceral) mesoderm layer + underlining endoderm = splanchnopleura; produces wall of gut; continuous w/ extraembryonic mesoderm covering yolk sac
Intermediate mesoderm Located in cervical and thoracic regions; produces segmental clusters = nephrotomes (posteriorly produce unsegmented nephrogenic cord); both regions generate components of urinary system
Paraxial mesoderm Segmented into somitomeres; each segment consist of concentric whorls of cells on each side of embryo
Neuromere Somitomere in anterior region that forms beside neural plate segment; contribute to head mesenchyme
Somites Somitomeres organized in medial and posterior regions; first develop in occipital region on day 20; give rise to segmental pattern on body
How many pairs of somites develop? 42-44 pairs (4 occipital, 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 8-10 coccygeal); first occipital and last 5-7 coccygeal somites later disappear; others develop into axial skeleton
Sclerotomes Somite ventral and medial walls that lose cohesion, producing mesenchymal cells that surround notochord
Dermomyotome Remaining dorsal wall of each somite; laminates into 2 layers (myotome, dermatome)
What do sclerotomes produce? Vertebra (cartilage to bone)
What do myotomes differentiate into? Muscles, attached to each vertebra and limbs
What do dermatomes produce? Underlying dermis of the skin
Endoderm development Produces digestive tract, as well as epithelia and stromata for numerous other organs
Gut Digestive tub that begins as a tube; formation = passive process (produced by inversiion of entire embryo due to disproportionate growth of ectoderm); rapid expansion of dorsal ectoderm elongates embryo; pulls ectoderm in anterior & posterior directions
Ectodermal elongation causes what to happen? Embryo to buckle ventrally, constricting connectiion w/ yolk sac; at same time, somite growth inverts embryo laterally (helps fold endoderm into tubular gut); segregates majority of gut from yolk sac & coelom from chorionic cavity
Foregut Anterior part of gut
Midgut Medial part of gut
Hindgut Posterior part of gut
Vitelline duct (yolk sac stalk) Connection of midgut with yolk sac; by week 5, will merge w/ connecting stalk to form umbilical cord; later, midgut will separate from vitelline to become free in coelom
Oropharyngeal membrane Anterior end of foregut anchored by prechordal plate; after week 3, membrane ruptures & opens gut to amniotic cavity; opening will develop into mouth
Cloacal membrane Anchor for posterior of hindgut; in conjunction w/ portion of allantois, will form cloaca (develops into anus)
Embryonic circulatory system Mid-week 3: blood vessels begin to develop thru-out embryonic & extraembryonic mesoderm; mesenchymal cells = clusters of angioblasts; center of angioblasts = blood cells; periphery of angioblasts = endothelial cells to produce blood vessels
What happens as the vessels proliferate? They interconnect to establish embryonic circulatory system (runs thru-out mesoderm; includes heart tube, splanchnic extraembryonic mesoderm, somatic extraembryonic mesoderm, connecting stalk)
Chorionic villi Composed of cytotrophoblast cells which become organized into columns that extend thru syncytiotrophoblast; major component of placenta
Primary villi Consist of cytotrophoblast cells, surrounded by synctiotrophoblast
Secondary (placental) villi Transformation of primary villi when extraembryonic mesoderm penetrates, producing mesodermal cores
Tertiary villi Secondary villi with embryonic blood vessels in core
Outer cytotrophoblast shell Cytotrophoblast cells that spread out between syncytiotrophoblast and endometrium to form epithelium
Stem (anchoring) villi Villi that extend completely through syncytiotrophoblast
Terminal (free) villi Villi that branch off of stem villi; purpose = nutrient exchange in placenta
Two components of the placenta Embryonic (develops from chorion) and maternal component (derived from surrounding endometrium)
Maternal component of placenta Known as decidua
Intervillous spaces Lacunar network of decidua that expands and coalesces; forms between villi
Maternal blood flow Through intervillous spaces
Embryonic blood flow Through vessels w/i villi
Does maternal and fetal blood mix? NO; discourages isoimmunization (immunorejection of genetically foreign fetal tissue)
How is nutrients and oxygen passed from mother to fetus? Diffused across walls of villi
Progress of chorionic villi Initially completely surround blastocyst; as development proceeds, they regress from abembryonic pole and become concentrated at embryonic pole, producing placenta
Fetal expansionAs the embryo grows into a fetus, where does it expand into? Embryo into fetus -> amniotic cavity -> chorionic cavity; amniotic cavity supplants chorionic cavity
What is the membrane that surrounds the fetus composed of? Amnion and chorion
"Water break" Expulsion of amniotic fluid
Rh factors Erythrocyte (RBC) surface antigens
Isoimmunization Ex: mother = Rh-; feuts = Rh+; fetal blood in maternal circulatory system = mother's immune system produces antibodies against foreign Rh antigens
Erythroblastosis fetalis Anti-Rh antibodies that cross placenta, resulting in fetal RBC hemolysis; results from isoimmunization; syndrome characterized by anemia, elevated bilirubin (accumulates in brain = brain damage), hydrops, pallor (blue baby), death
How is a severe case of erythroblastosis fetalis diagnosed? treated? Diagnosed with spectrophotometric analysis of amniotic fluid; treated with intrauterine transfusion of RBC packed blood
How would fetal blood get into mother's circulatory system? Placenta abruption; placenta ruptures/separates from uterine wall resulting in intrauterine bleeding; most common mechanism = fetal blood released into uterus when placenta extruded after birth
What is a problem with placenta abruption? It can immunize mother against Rh+ factors, causing her body to attack subsequent pregnancies
What treatment is available for isoimmunization? RhoGam (anti-Rh0D antibodies) -> D is most common isoform; treat Rh- mother with product to clear any fetal Rh+ RBCs from mother's circulatory system before they can elicit immune response
RhoGam sensitization Reduces Rh- mothers from 10-20% to < 1%
Created by: JaneO