02-05: Body Plan Development I Flashcards
What are the stages of neural tube development?
1- Formation of thickened neural plate
2- Shaping of neural plate
3- Folding of neural plate
4- Formation of neural tube
What happens during the formation of thickened neural plate stage of neural tube development?
- columnarization of midline ectodermal cells overlying notochord
- CUBOIDAL»_space; COLUMNAR
- involves microtubules
What happens during the shaping of the neural plate stage of neural tube development?
- process of convergent extension
- rectangle»_space; space
What role does planar polarity play in the shaping of neural plate stage of neural tube development?
-
What are the 2 hinge points of the neural tube?
- Median hinge point
- Lateral hinge point
What is the median hinge point during the folding stage of the neural plate?
- induced by notochord to cause folding
- COLUMNAR»_space; PYRAMIDAL
What is the lateral hinge point during the folding stage of the neural plate?
completes the folding process
What elevates the neural folds during the folding stage of the neural plate?
pushing inward by expanding surrounding non-neural epithelium
What causes the formation of the neural tube?
fusion of apical surfaces of neural folds
What is the formation of the neural tube mediated by?
glycoconjugates
Where does the formation of neural tube begin?
MIDWAY along the neural tube (21-22 days)
Where does the neural tube extend?
cranially»_space; caudally
When do the Anterior and Posterior neuropores of the neural tube form?
day 23
What causes the columnar cells to transform into wedge cells?
actin & myosin
**contractile mechanism at one end of the cell and not the other end of the cell
What induces neural tube segmentation?
- notochord
- anterior visceral endoderm
- chordal plate
** +Wnt8 gradient
Where does segmentation occur?
in an existing tubular structure
What are the 3 organizing (signaling) centers in neural tube segmentation?
- isthmic organizer
- anterior neural ridge
- zona limitans
What is the isthmic organizer important for during neural tube segmentation?
important for organizing the midbrain and cerebellum
What does the isthmic organizer secrete?
Wnt1
FGF8
What is the anterior neural ridge important for during neural tube segmentation?
important for organizing (ventral aspect of forebrain):
- telencephalon
- parts of diencephalon
- olfactory area
- pituitary gland
What does the anterior neural ridge organizer secrete?
Shh
FGF8
What is the zona limitans important for during neural tube segmentation?
important for organizing the border between the dorsal and ventral thalamus
What does the zona limitans secrete?
Shh
What do the rhombencephalon segmentation genes do?
set up basic pattern of segmentation in the hindbrain
What does Krox 20 demarcate?
r3
r5
What do Kreisler and Hoxa1 demarcate?
r5
What does the Retinoic acid gradient demarcate?
r4-r7
What does Gbx2 demarcate?
r1-r3
What does the pattern of Hox gene expression determine?
cranial nerves and pharyngeal arch derivatives
What stimulates the expression of Hoc genes?
Retinoic acid gradient»_space; initiates expression of Hoxa1 and Hoxb1
What does Retinoic acid do?
sets up a gradient
What part of the neural plate possesses the properties of a stem cell zone?
the caudal most part
What happens if Retinoic acid inhibits FGF8?
mitosis will not occur»_space; differentiation
Why is neural crest tissue sometimes considered to be a 4th germ layer?
forms many structures
What are the sensory placodes?
thickenings in overlying ectoderm (sensory ganglia)
What is the origin of the sensory (ectodermal) placodes?
from ectoderm or neural crest
What are the 3 zones of mesoderm?
- paraxial (segmental plate) mesoderm
- intermediate mesoderm
- lateral plate mesoderm
Which mesoderm zone is closest to the notochord?
paraxial/segmental plate mesoderm
What is a coelom?
cavity lined with mesoderm
- thoracic
- pericardium
- abdominal
What are somitomeres?
initial pairs of segments of mesenchyme that begin to develop along and on either side of the neural plate in the paraxial mesoderm
**barely discernable as segments
When are new somitomeres added?
added caudally as the primitive streak regresses
What happens after 20 pairs of somitomeres have formed (day 20)?
the first pair of somites form behind (caudal to) the 7th pair of somitomeres
What are somites?
more dense blocks of mesoderm that form along the notochord
What happens to a pair of somitomeres every few hours?
a pair of somitomeres is transformed in the caudal direction into a pair of somites
What happens as a pair of somites are formed?
pairs of somitomeres are also formed at the ANTERIOR end of the primitive node
What occurs because of the transformation and formation of somites?
11 pairs of somitomeres are kept constant at the causal end of the paraxial mesoderm
What are the 2 mechanisms involved in somitogenesis?
- Wavefront
- Segmentation clock
What results in a cessation of somitogenesis?
the balance between the opposing gradients of:
- Retinoic acid
- FGF8
What do oscillating molecules in the Notch pathway lead to?
a defining of the Anterior and Posterior borders of each somite
Stimulate the expression of:
-Lunatic fringe = at Anterior border of a future somite
-C-Hair = at Posterior border of a future somite
Where is the Retinoic acid gradient strongest?
cranial
Where is the FGF gradient strongest?
caudal
What do ephrins do?
maintain the intersomitic spaces
What role does the down regulation of Snail play in somitogenesis?
-
What are the 4 features of the history of a somite?
- epithelial stage
- epitheliomesenchymal transformation stage
- separation of the myotome
- breakup of epithelial dermatome into dermal fibroblasts
What are the sclerotome ventral derivatives?
- ventral bodies
- their intervertebral disks
What are the sclerotome lateral derivatives?
- distal ribs
- some tendons
What are the sclerotome dorsal derivatives?
- dorsal part of neural arch
- spinous process
What are the sclerotome central derivatives?
- pedicles and ventral parts of neural arches
- proximal ribs
- transverse processes of vertebrae
What are the sclerotome medial (meningotome) derivatives?
- meninges
- BV of meninges
What are the arthrotome derivatives?
- intervertebral disks
- vertebral joint sufaces
- proximal ribs
What are the dermatome derivatives?
- dermis
- blade of scapula
What are the dorsomedial myotome derivatives?
intrinsic back muscles (epaxial)
What are the ventrolateral myotome derivatives?
limb muscles or muscles of ventrolateral body wall (hypaxial)
What are the neurotome derivatives?
endoneurial and perineruial cells
What are the sydentome derivatives?
tendons of epaxial musculature
What is a dermomyotome?
the dorso-lateral part of a somite
What creates a balance that commits the myotome portion of the somite to commit to the myogenic lineage?
- Shh signaling from the notochord
- Wnt signaling from the dorsal neural tube
What does Noggin do?
- inhibits BMP4 which would normally inhibit myogenesis
- allows myogenesis to occur
What produces BMP4?
the lateral plate
What does BMP4 do?
- suppresses myogenesis in the ventrolateral dermomyotome
- stimulates cells from this area to migrate from the somite into the limb bud
What produces FGF?
the myotome
What does FGF do?
signals the sclerotome to produce scleraxis
What does scleraxis do?
causes Anterior and Posterior borders of each somite to form the syndetome
What is the syndetome?
the precursor of tendons
What does the sclerotomal portion of each somite break up into?
an Anterior and a Posterior portion
What creates gaps that allow nerves to grow out from the neural tube into the epaxial segmental musculature?
the joining of the posterior portion of one somite and the anterior portion of an adjacent somite
What does the future intermediate mesoderm respond to, to become intermediate mesoderm?
- BMP [from lateral ectoderm]
- activin [from paraxial mesoderm]
What does the intermediate mesoderm form?
- pronephros
- mesonephros (later)
Which way do the heart-forming cells move through the primitive streak?
caudal»_space; cranial
Where is the cardiac crescent located?
on the cranial end
Where is the first blood cell from?
yolk sac
