third to eighth weeks Flashcards
what is the difference between mesoderm and mesenchyme
Mesoderm refers to cells derived from the epiblast and extraembryonic tissues.
Mesenchyme refers to loosely organized embryonic connective tissue regardless of origin
how does the neural crest form
As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbors, this population is the neural crest
what happens to the neural crest as it leaves the neuroectoderm
will undergo an epithelial-to-mesenchymal transition by active migration and displacement to enter the underlying mesoderm
Crest cells from the trunk region leave the neuroectoderm after closure of the neural tube and migrate along one of two pathways:
1) a dorsal pathway through the dermis, where they will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles, and
(2) a ventral pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons, Schwann’s cells, and cells of the adrenal medulla.
what cells contribute to the craniofacial skeleton
neural crest cells migrating from the cranial neural folds, leaving the neural tube before closure
other than craniofacial skeleton what do neural crest cells contribute to
neurons for cranial ganglia, glial cells, melanocytes
what are the neural crest cells sometimes referred to as
fourth germ layer
what do high and low levels of BMP cause
High BMP induce epidermis formation, intermediate levels, at the border of the neural plate and surface ectoderm, induce the neural crest; and very low concentrations cause formation of neural ectoderm
intermediate concentrations of BMPs, together with _________________, induce _____ and other transcription factors that “specify” the___________
FGF and WNT proteins
PAX3
neural plate border
what do the transcription factors from intermediate BMP levels induce
a second wave of transcription factors, including SNAIL and FOXD3, and SLUG
what is the function of
a) SNAIL and FOXD3
b) SLUG
a) specify cells as neural crest
b) promotes crest cell migration from the neuroectoderm
what organs and structures does ectodermal layer give rise to
- CNS
- PNS
- The epithelium of external auditory canal
- The sensory epithelium of the ear, nose, and eye
- The epidermis, including the hair and nails
what else does the ectodermal layer give rise to
Subcutaneous glands,
The mammary glands,
The pituitary gland,
And enamel of the teeth
what causes anencephaly
failing of the neural tube to close in the cranial region
what happens if the closure fails in the neural tube anywhere from the cervical region caudally
a defect called spina bfida
what is the most common site for spina bfida to occur
lumbosacral region
what reduces the incidence of spina bfida
folic acid (folate)
what happens to the cells of the mesodermal layer by approximately the 17th day
cells close to the midline proliferate and form a thickened plate of tissue known as paraxial mesoderm
what happens to the mesodermal layer more laterally
remains thin and is known as the lateral plate
what connects the paraxial and the lateral plate mesoderm
Intermediate mesoderm
what happens with the apperance and coalescence of intercellular cavities in the lateral plate and what forms as a result
this tissue is divided into two layers
• A layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer
• A layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer
these layers line a newly formed cavity, the intraembryonic cavity
what segments does the paraxial mesoderm form
somitomers which first appear in the cephalic region (cephalocaudally)
where do somites first appear
in the occipital region of the embryo
new somites appear in__________ sequence at a rate of approximately ________________, at the end of the fifth week
craniocaudally
three pairs per day
At the end, embryo will have 34-36 pairs 3 ______, 8 ______, 12 _______, 5 ______, 5 ______ and 1 to 5 _________ pairs
occipital, cervical, thoracic, lumbar, sacral coccygeal pairs
what does Formation of segmented somites from unsegmented presomitic (paraxial) mesoderm depend on
NOTCH and Wnt
what does increase in Notch protein do
activates other segment-patterning genes that establish the somite
Boundaries for each somite are regulated by what
retinoic acid (RA) and a combination of FGF8 and WNT3a
RA is expressed at____________ cranially and __________________ caudally, whereas the combination of FGF8 and WNT3a proteins is expressed at _____________ caudally and ______ ones cranially
high concentrations
decreases in concentration
higher concentrations
lower
what do the somites first form
presomitic mesoderm, as a ball of mesoderm
what happens to the somites by the beginning of the fourth week
cells in the ventromedial walls of the somite lose their epithelial characteristics become mesenchymal again and shift their position to surround the neural tube and notochord. these cells form the sclerotome
what does the sclerotome differentiate into
vertebrae and ribs
what do Cells at the dorsomedial and ventrolateral edges of the upper region of the somite form and what do the cells between them form
precursors for muscle cells, while cells between these
two groups form the dermatome
what happens to cells from muscle precursor groups
become mesenchymal again and migrate beneath the dermatome to create the dermomyotome
what do cells from the ventrolateral edge of the somite form
migrate into the parietal (somatic) layer of lateral plate mesoderm to form most of the musculature for the body
wall and most of the limb muscles
what do cells in the dermomyotome ultimately form
dermis for the skin of the back and muscles for the back, body wall (intercostal muscles) ,and some limb muscles
