B8.031 Embryogenesis Flashcards
fertilization and implantation time frame
weeks 1-2
blastocyst normally implants in uterus
embryonic period
week 3-8
in first 8 weeks, embryos progress from a single cell to having established organ primordia and “recognizable” external body form
fetal period
week 9-birth (40)
extraembryonic membrane formation (big picture)
membranes protect and nurture the developing embryo; function in gas & waste exchange
shock absorber
placentation (big picture)
placenta formation nutrition and waste exchange with embryo
gastrulation (big picture)
forms 3 germ layers, which give rise to body structures via cell proliferation, migration, differentiation, and morphogenesis (big scale movement of tissues)
cardiovascular development and hematopoiesis (big picture)
oxygenation and nutrition (along with placenta)
blood cell formation
neurulation (big picture)
formation of the nervous system
when does implantation occur
5.5-6 days
what cavities are present in the developing embryo at the end of the 2nd month
amniotic
chorionic
yolk sac
uterine
blastocyst development at day 4.5 to 5
composed of:
1. outer cell mass = trophoblast cells
2. inner cell mass = embyoblast, localized to one pole
fluid penetration into intercellular spaces of ICM results in blastocele cavity
2 layers of embryoblast
epiblast (upper)
hypoblast (lower)
cavities present in blastocyst at day 7.5
blastocyst and amniotic cavities
structure of epiblast
high columnar cells adjacent to amniotic cavity
amnioblasts
subset of epiblast cells adjacent to cytotrophoblast
secrete amniotic fluid
structure of hypoblast
cuboidal cells adjacent to blastocyst cavity
what occurs around day 9
deeper embedment
transformed blastocyst cavity emerges
what is the exocoelomic cavity and its function
primitive yolk sac
plays early role in nutrition
later connects to umbilical cord by a yolk stalk
primitive yolk sac membrane
exocoelomic (Heusers) membrane
formed from hypoblast
what occurs around day 12
extraembryonic (chorionic) cavity forms; filled with 2 layers of extraembryonic mesoderm
origin: lining of the exocoelomic cavity
layers of extraembryonic mesoderm in the chorionic cavity
- extraembryonic splanchnopleuric mesoderm: covers yolk sac
2. extraembryoinc somatopleuric mesoderm: lines cytotrophoblast layer
what occurs on day 13
secondary yolk sac forms within and obliterates the exocoelomic cavity
exocoelomic cyst
remnants of exocoelomic cavity that are “pinched off”
what surrounds the secondary yolk sac
lined with extraembryoinc endoderm (from hypoblast)
surrounded by extraembryonic splanchnopleuric mesoderm
what lines the amniotic cavity
amnion (inner fetal membrane)
what lines the chorionic (extraembryonic) cavity
extraembryoinc somatopleuric mesoderm
purpose of gastrulation
goes from a bilaminar to a trilaminar germ disc
ectoderm, mesoderm, endoderm
beginning of gastrulation
primitive streak (furrow on upper surface) forms on epiblast around the tail of the embryo
process of invagination in gastrulation
invagination of a subset of epiblast cells
- subset of the invaginated cells displace hypoblast, creating the endoderm (ventral layer)
- subset of invaginating cells lie between epiblast and endoderm/hypoblast creating mesoderm
- epiblast cells remaining form ectoderm (dorsal)
how is gastrulation organized
primitive node
cells in epiblast that form elevated cluster at cranial margin of streak
when does gastrulation occur
starts week 3, continues through the end of the 4th week
how does the primitive node organize gastrulation
acts as a signaling center to pattern germ layers
movement of primitive streak
extends cranially, then regresses caudally
does NOT move up into head
describe the movement of cells in invagination
cells move lateral and cranial, influenced by signaling
- chemo attractive in perimeter
- chemo repulsive at midline
why is embryonic disc thickest at midline
this is where ingression is occuring
mesoderm only exists here before it has the chance to migrate out
eventual fate of the hypoblast
replaced by invaginating cells that form the endoderm
may contribute to lining of primitive yolk sac and extraembryonic endoderm and mesoderm
fate of epiblast cells
where epiblast cells migrate through primitive streak determines where cells will end up and what structures they will form
- notochord
- somites
- urogenital system
- body wall
- chorion
formation of the notochord overview
prenotochordal cells that ingress through the primitive streak form the notochordal process (a hollow tube)
notochordal process flattens and some cells remain in the mesodermal layer; others intercalate in the hypoblast to form the notochordal plate
what 2 cell layers make up the notochord
mesodermal hypoblast (endoderm)
notochordal plate
cells proliferate and detach from the endoderm (as it replaces the hypoblast) and roll up to form a solid cord of cells
establishment of body axes
- left right
- dorsal ventral
- anterior posterior
occurs before and during gastrulation
results in embryonic asymmetry
abnormalities of axes formation results in developmental malformations
axis patterning genes in the anterior-posterior axis
AVE (anterior visceral endoderm)- acts with primitive node as a second signaling region in head
-expresses genes essential for head formation and other genes that inhibit formation of the primitive streak in the head
when is the cranial end of the embryo established
before gastrulation
what maintains the primitive streak (posterior embryo)
nodal
ventralization signaling
BMP4 (secreted throughout embryonic disc) ventralizes mesoderm; forms intermediate and lateral plate mesoderm
dorsalization signaling
chordin, noggin, and follistatin (secreted by node and prechordal mesoderm) antagonize BMP4 and thus dorsalize cranial mesoderm to form notocord from notochordal plate and paraxial mesoderm
dorsal-ventral axis formation at later stage
after notochord is formed - chordin, noggin, and follistatin are expressed by notochord and play a role in neural induction in CRANIAL region
brachyury (T) gene dorsalizes mesoderm in middle and CAUDAL embryo forming paraxial mesoderm
where is brachyury (T) gene expressed
node
notochordal precursor cells
notochord
goosecoid
dorsalizes cranial/head mesoderm
activates chordin
over/under expression of goosecoid
head malformations similar to conjoined twins phenotype
function of brachyury (T) gene
essential for cell migration through primitive streak (mesoderm formation)
AND dorsalizing mesoderm (paraxial mesoderm formation)
absence of brachyury (T) gene
decreased formation of mesoderm & decreased PA mesoderm (ie somites)
results in shortening of embryonic axis (caudal dysgenesis) that may result in fusion of the limb buds
axis patterning for the left-right axis
gene cascade initiated by FGF8 on the L side establishes expression of nodal on the L
SHH represses L sidedness genes on the R
how are cilia related to left-right axis formation
node contains ciliated cells; cilia rotate and set up directional fluid flow (signaling) patterns
situs inversus
transposition of viscera in thorax and abdomen
linked to altered fluid flow
when does cephalocaudal differentiation occur
mid 3rd week to mid 4th week
**gastrulation is still occurring caudally while differentiation is occuring cranially
when does the primitive streak stop supplying cells
regresses caudally until the end of the 4th week
why does the embryonic disc expand mainly in the cranial region
continuous migration of cells from the primitive streak in the cranial direction
association between teratogenesis and gastrulation
in 3rd week, when gastrulation begins, embryo is highly sensitive to teratogenic insult
holoprosencephaly
high doses of alcohol (consumed before mother knows she is pregnancy) lead to craniofacial malformations
when are holoprosencephaly defects initiated
days 19-21
when midline of the forebrain is established
features of holoprosencephaly
small forebrain
lateral brain ventricles fuse
fusion of lateral nasal prominences
failure of eye fields to separate properly
what is a sacrococcygeal teratoma
results from remnants of the primitive streak in the sacrococcygeal region
may contain tissues derived from all 3 germ layers
epidemiology of sacrococcygeal teratoma
more often observed in females
rare: 1 in 37,000
mature sacrococcygeal teratoma characteristics
cystic: enclosed in its own fluid containing sac
solid: made up of tissue, but not self enclosed
mixed: containing both solid and cystic parts
typically NOT malignant
cardiovascular system at the end of the 3rd week
villus capillaries contact vessels in the chorionic plate and connecting stalk, which in turn contact intraembryonic vessels
when is the villous capillary system ready for heartbeat
4th week of development
intermediate mesoderm derivatives
kidney
gonads
paraxial mesoderm derivatives
head somite -sclerotome -myotome -dermatome
lateral mesoderm derivatives
splanchnic (visceral/organs)
somatic (body)
extra-embryonic
derivates of the somite
vertebrae and ribs (bones) - sclerotome
dermis of the skin and back - dermatome
skeletal muscles of the back, body wall, and limbs - myotome
overview of the urogenital system formation
3 overlapping kidney systems are formed cranial to caudal
- pronephric
- mesonephric
- metanephric
cloaca
common embryological cavity
excretory ducts of both the urinary and genital systems enter here
heart formation in weeks 3-4
lateral folding results in cardiac tube formation
vasculogenesis
de novo formation of endothelial tubes
angiogenesis
sprouting of tubes off existing vessels
what are hemangioblasts
common mesodermal precursors for blood cells (hematopoietic cells) and endothelial cells (angioblasts); located in blood islands
what cell types are needed for vasculogenesis
both blood cells and endothelial cells
what are blood islands
endothelial and blood cell precursors present in the wall of the yolk sac
intraembryonic vascular formation
from splanchnic LPM
occurs slightly after extraembryonic vessel formation (from somatic and splanchnic extraembryonic mesoderm)
site changing through the process of hematopoiesis
- *blood cells initially form from splanchnic mesoderm**
1. extraembryonic: blood islands of yolk sac; transitory (week 3)
2. aorta-gonad-mesonephros region
3. liver: colonized by cells from the AGM (2-7 months)
4. bone marrow: colonized by cells from the liver; definitive (7th month of gestation)
capillary hemangiomas
occur in 10% of births
may occur anywhere; but often in craniofacial regions
can be focal or diffuse
can disappear without treatment
ectodermal derivatives
neural tube
neural crest (epithelial to mesenchymal transformation)
epidermis
sweat glands and hair follicles
derivatives of the endodermal germ layer
epithelial lining of: GI tact, respiratory tract, urinary bladder, auditory tube, liver, pancreas, cloaca
gut formation from endoderm and cephalocaudal folding
embryonic disc begins to bulge in amniotic cavity as result of brain vesicle growth
folding is most pronounced in head and tail
cephalocaudal folding promotes endoderm lined cavity incorporated into embryo body
formation of umbilical cord
portion of the allantois (evagination of hindgut) is surrounded by mesoderm of connecting stalk
together form umbilical cord
blood vessels form from mesoderm
foregut
bounded by buccopharyngeal membrane until its rupture in 4th week
midgut
temporarily communicates with yolk sac
hindgut
terminates temporarily at cloacal membrane until its rupture in 7th week
function of lateral folding in formation of the gut tube and ventral body wall
lateral folding results from rapid growth of somites
embryo assumes rounded appearance
gut forms tube and ventral body wall is established
midgut connection with yolk sac becomes long and narrow (vitelline duct)
relationship of gut to umbilical cord
at 10 weeks; intestinal loops are herniated into the yolk sac DUCT and are in the umbilical cord due to size constraints of the abdominal cavity
defects of the ventral body wall
failure to close after lateral folding ectopia cordis (ectopic heart) bladder exstrophy gastroschisis cloacal exstrophy
bladder exstrophy
eversion; failure of pelvic region to close
gastroschisis
herniation of intestines through abdominal wall
cloacal exstrophy
failure of pelvic region to close
diagnosis of ventral body wall defects
- presence of high maternal serum AFP levels (similar to spina bifida)
- US imaging
respiratory development overview
25 days
epithelial lining of larynx, trachea, bronchi, and lungs is endodermal in origin
respiratory diverticulum/lung bud is outgrowth from ventral wall of foregut (4 weeks)
tracheoesophageal septum later divides foregut into: dorsal esophagus, ventral trachea, lung buds
what are the pharyngeal arches
5 paired structures surrounding the pharynx
form MSK structures in head and neck
resembles gill formation in fishes and amphibians
derivation of pharyngeal arches
derived from paraxial mesoderm (somitomeres and occipital somites) and neural crest cells
covered with ectoderm, lined by endoderm
what do the pharyngeal arches contribute to
formation of face, neck, mouth, larynx, pharynx
muscles, arteries, connective tissue, cartilage, parts of skeleton
when does external body form development take place
7-8th week development
limbs have 3 segments
most major organs and organ systems formed
