T15- Development of the back Flashcards
What is somitogenesis
formation of the somites
When does somitogenesis occur
between days 20 and 30
Somitogenesis
What does somitogenesis begin with
regression of the primative streak which leaves behind the notochord
Somitogenesis
What happens as the primative streak leaves behind the notochord
formation of somitomere pairs occur on either side of the notochord in the cranial reigon
Somitogenesis
After the _____ pair of somitomeres are formed in the _______ reigon, no more somitomeres are formed; instead the ________________ will differentiate into ___________
7th; cranial; paraxial mesoderm; somites
Somitogenesis
Where does the first pair of somites appear?
in the occipital reigon
Somitogenesis
When does the first pair of somites appear
around day 20
Somitogenesis
At what rate do somites form?
a rate of about 3 somite pairs/day
Somitogenesis
In total, how many somite pairs will develop?
42-44 pairs of somites
Somitogenesis
How many pairs of somites in each reigon
Occipital reigon: 5 Cervical reigon: 8 Thoracic reigon: 12 Lumbar reigon: 5 Sacral region: 5 Cocxygeal reigon: 8-10
Epithelization of somite
What happens when the somite undergoes epithelization
it forms around a somitocoel
Epithelization of somite
What happens in the ventral reigon of somite
epithelial cells transform back into mesenchyme and are pushed centrally; forms the sclerotome
Epithelization of somite
What do sclerotome cells surround
the neural tube and the notochord
Epithelization of somite
What is the sclerotome divided into
Dorsal, ventral, lateral and central sclerotome
Epithelization of somite
Where does dorsal sclerotome end up
between the neural tube and surface ectoderm
Epithelization of somite
Where does ventral sclerotome end up
circling around the notochord
Epithelization of somite
Where does lateral sclerotome end up
migrating laterally and ventrally
Epithelization of somite
Where does central sclerotome end up
it stays in close connection with the dermatome
Epithelization of somite
What happens to the dorsal reigon of the somite
it becomes the dermomyotome
Epithelization of somite
How is the myotome formed?
cells from the VLL and DML transform back into mesenchyme and migrate beneath the dermatome
Epithelization of somite
Where is the VLL and DML originally located
on either side of the dermatome
Epithelization of somite
Where is the dermatome originally located?
between the VLL and the DML
Somite polarization: sclerotome polarization
Which protiens induce the ventral part of the somite to form into sclerotome
SHH from the floor of the neural tube and NOGGIN from the notochord
Somite polarization: sclerotome polarization
Which protien does the sclerotome express
PAX-1
Somite polarization: sclerotome polarization
What does PAX-1 control
vertebrae formation and chondrogenesis
Somite polarization: Polarization of Myotome (DML)
Which protiens is the DML influenced by
WNT protiens
Somite polarization: Polarization of Myotome (DML)
Where do the WNT protiens come from
the dorsal aspect of the neural tube
Somite polarization: Polarization of Myotome (DML)
What does WNT instruct the cells of the DML to do
To differentiate into muscle precursor cells and to express the muscle gene MYF5
Somite polarization: Polarization of Myotome (DML)
What do the cells of the DML become
muscles of the ventrolateral wall and the flexors of limbs
Somite polarization: Polarization of Myotome (VLL)
Which protiens are the cells of the VLL influenced by
WNT protiens which come from the overlying ectoderm and BMP4 from lateral plate mesoderm
Somite polarization: Polarization of Myotome (VLL)
BMP4 has what type of influence on cells of VLL
inhibiting influence
Somite polarization: Polarization of Myotome (VLL)
What do the cells of the VLL become due to protien influence
muscle precursor cells
Somite polarization: Polarization of Myotome (VLL)
What protien do the muscle precursor cells express
MYOD
Somite polarization: Polarization of Myotome (VLL)
What do muscle precursor cells go on to become
intrinsic back muscles and extensors of the limb
Somite polarization: Polarization of Dermatome
Which protien influences the dermatome cells
NT-3 from the dorsal neuraltube
Somite polarization: Polarization of Dermatome
What protien does the dermatome cells express as a result of protien influence
PAX3
Formation of vertebrae: Resegmentation of sclerotome
How does the sclerotome resegment?
The sclerotome forms clusters so that individual bones can be formed, and each sclerotome cluster divides into a cranial portion and a caudal portion
Formation of vertebrae: Resegmentation of sclerotome
What does the cranial and caudal portions of each sclerotome consist of
cranial portion: loosely packed cells
caudal portion: densely packed cells
Formation of vertebrae: Resegmentation of sclerotome
The vertebral body is formed from what parts of the sclerotome
The caudal half of the cranial sclerotome and the cranial half of the caudal sclerotome
Chondrification of vertebrae
What does the costal process become
The transverse process
Chondrification of vertebrae
What does the centrum become
The vertebral body
Chondrification of vertebrae
What is the chondrification center
the site where mesenchyme turns into cartilage
Chondrification of vertebrae:
When does formation of the chondrification center occur
about 6 weeks into development
Ossification of vertebrae
When do primary ossification centers appear
at about 7 weeks
Ossification of vertebrae
When can ossification centers in the neural arch be seen
in the 8th week
Ossification of vertebrae
When does full ossification of the cartilagenous process complete
at about age 25
Normally, _________ chondrification centers fuse at the end of the _________ to form the ____________
two; embryonic period; cartilagenous centrum
__________ failure of ________ chondrification center results in________
unilateral; one; hemivertebra
