Week 3 Flashcards
what things occur in the 3rd and 4th weeks of development?
neurulation, early organogensis, derivates of the mesodermal germ layer, somite formation & differentiation, early cardiogenesis and embryonic folding
what is neurulation? describe the process
clinical correlations?
neural plate forms the neural tube
- the mesoderm and endoderm secrete bone morphogenic protein-BMP-4 which induces epidermal tissue
- but notochord releases signaling proteins noggin and chordin to block the activity of BMP-4. by blocking BMP4 it can become neural tissue
- ectoderm now thickens axially and forms the neural plate
- neural folds come from the folding of lateral edges—forms neural groove
- and these approach the midline and fuse. fusion begins in CERVICAL region and proceeds CRANIALLY AND CAUDALLY.
- need fusion of neural tube so that it closes and cranial and caudal ends communicate with the amniotic cavity
- can get spina bifida (posterior/caudal neuropore doesn’t close) or
- anencephaly: anterior neuropore/cranial doesn’t fuse
importance of the neural crest
neural crest cells are neuroectoderm cells that undergo an EPITHELIAL-MESENCHYMAL transition before they migrate into the underlying mesoderm.
-they are located along the lateral border of the neural plate and fuse to form the neural tube
-importnace:
precursors for numerous tissue types including the PNS, adrenal medulla, melanocytes, skeletal and muscular structures of the head, and portions of the cardiac outflow tract
describe early organogensis
pretty much form ear and eye first.
- ear: ectodermal thickening on either side of HINDBRAIN—otic disc—forms internal ear
eye: groove on each side of FOREBRAIN becomes the optic sulcus —-that forms the globe of the eye
explain the derivatives of the mesodermal germ layer
intraembryonic mesoderm is divided into 3 regions
1) paraxial mesoderm
- loose mesenchyme on either side of the NOTOCHORD
- gives rise to vertebrae, some of skull, skeletal muscles, fibrous CT, and dermis of skin on the dorsal body wall
think: “thick things”
2 ) lateral to the paraxial is the intermediate mesoderm:
-this gives rise to the kidney and ureters, gonads, reproductive ducts and the adrenal cortex
will form a solid cord of mesenchyme called the urogenetial ridge
3) lateral plate mesoderm:
lateral to the intermediate. also mesenchym. it gives rise to the dermis on front of body wall, wall of GI and respiratory, appendicular skeleton, and visceral layers.
splanchnic vs somatic mesoderm?
splanchnic mesoderm is associated with the endoderm
somatic mesoderm is associated with the ectoderm
come from lateral plate
describe somite formation, dispersal, and differentiation
somites come from the paraxial mesoderm and they are transient structures that develop first in the CRANIAL direction, then in the cranial-caudal direction
-about 42 to 44 pairs form
begins day 20
- localized mesenchyme aggregates form somitomeres (presomites)
- these undergo a MESENCHY TO EPITHELIAL transition and further condense to somites
*first 7 somitomeres in cranial region do not form somites
differentiation:
somites lose epithelial characteristic and become mesenchymal again.
-they further differentiate into
-cells in the ventral and medial walls of the somite loose epithelial characteristic and shift to surround the neural tube and notochord —these are SCLEROTOME—gives rise to the vertebral column, cartilage, and bone
and dorsolateral area cells are :
DERMOMYOTOME : dorsolateral cells that form. the cells on the medial surface are myotome and give rise to body wall and limb muscles
dermatome give rise to dermis
what is epaxial and hypaxial
myotome subdivisions
epiaxial= muscles located dorsally and innervated by dorsal rami
hypaxial are muscles located ventrolaterally and innervated by ventral rami
what are dorsalizing factors of somite differentiation?
what are ventralizing factors?
SHH and noggin are dorsalizing factors released from the notochord/neural tube
N=notochord, noggin,
Wnt and BMP4 are ventralizing factors released from the lateral plate
what is the first organ to become functional in the embryo
-how do blood vessels arise?
heart
blood vessels arise via 2 processes:
first the mesodermal cells are induced to form angioblasts—precursors for blood vessel and blood cell formation
- vasculogensis: blood vessels arise from blood islands
- angiogenesis: blood vessels sprout from existing vessels
when do angioblasts form? and where?
which are the first forming vessesl
form in yolk sac at day 16
-the first vessels are yolk sac, heart, and aorta and these come from blood islands or vasculogenses and the rest come from these primary ones via angiogenesis
how is the heart formed? what region?
heart formed from splanchnic mesoderm (remember this is from the lateral mesoderm and refers to influence from the endoderm) in the CEPHALIC region of the embryo by day 30 (so first month)
describe embryonic folding . when does this happen?
what planes?
this occurs weeks 3 and 4
usually occurs bc of differential growth rates and it transforms the embryo to a 3D CYLINDER
-2 distinct planes : median and transverse
1) cranio-caudal folding in the median plane is caused by the growth of the CNS and the amnion.
- neural tube and the amnion elongate but yolk doesn’t grow this constrains the embryo and pulls the cranial and caudal end to a C shape
2) lateral folding: in transverse plane:
occurs simultaneous with crania caudal folding. driven by growth of amnion.
what are the results of embryonic folding?
form body (going from flat to tubal)
- relocate structures from dorsal to ventral wall
- close ventral body wall
- elevation of embryo into amniotic cavity
by week 4 what occurs?
- heart is now beating
- embryonic circulation established btwn yolk sac and placenta
- neural tube closed with EXCPTION of anterior and posterior neuropores
- cranio-caudal folding and lateral folding
- somite formation still occurring
