Flashcards in Establishment Of Body Plan Deck (57):
What is involved in the thickening of the neural plate?
-columnarization of midline Ectodermal cells overlying notochord (microtubules)
-change in cell adhesion molecules
What happens during the convergent extension period of the shaping of the neural plate?
-involves migration of ectodermal cells toward the midline
-ectodermal cells elongate along the anterior-posterior axis and become thinner, forming an elongated narrow plate
What occurs during the planar cell polarity stage of the shaping of the neural tube?
-causes cells to become oriented along an axis in a flat plane
-caused by asymmetric distribution of planar cell polarity proteins
-asymmetric distribution may be associated with Wnt gradient
What are the planar cell polarity proteins and where are the they concentrated?
-disheveled (concentrated posteriorly)
-prickle (concentrated anteriorly)
What does the planar cell polarity pathway form?
-convert elliptical gastrula into keyhole shaped neurula and are important in the folding of the neural plate into the neural tube
What signals are utilized in the PCP pathway?
What happens in the absence of the PCP pathway?
-neural folds remain far apart and neural tube closure is absent
What does the folding of the neural plate result in?
-formation of neural groove flanked by neural folds
What is the median hinge point during the folding of the neural plate?
-induced by notochord
-involves changes in shape of columnar cells to pyramidal shaped cells via apical constriction
-requires synthesis of actin filaments at apical ends of cells
What do the lateral hinge points require?
-involve apical construction by actin filaments
What elevates the neural folds?
-due to pushing inward by expanding non-neural epithelium
What mediates the formation of the neural tube?
-due to fusion of apical surfaces of neural folds
-mediated by glycoconjugates
When does closure of the neural tube begin?
-closure begins midway along neural tube around 21-22 days
-tube closure extends cranially and caudally
-anterior and posterior neuropores remain at day 23
Secondary neurulation occurs caudal to the posterior neuropore, what does this involve?
-involves formation of solid rod of mesenchymal cells (medullary cord)
-mesenchymal rod becomes hollow via cavitation
-not prominent in humans
What do prosomeres form?
What do rhombomeres form?
What are the regions of the tripartite and subsequently the pentapartite brain?
-Prosencephalon -> telencephalon, diencephalon
-rhombencephalon -> metencephalon and myelencephalon
What is the head organizing region? What factor organizes this process?
-consists of anterior visceral endoderm and prechordal plate
What needs to be expressed in the anterior notochord?
-Gbx-2 in hindbrain
Wnt-8 establishes what gradient?
-analogous to segment polarity gene (wingless) in drosophilia
What is the isthmic organizer? What does it release?
-signaling center established by boundary between Otx-2 and Gbx-2
-important in organizing the midbrain and the cerebellum
-releases Wnt-1 anteriorly ad FGF-8 posteriorly -> induce gradients of En-1(ant) and En-2(post)
What is the anterior neural ridge signaling center? What does it secrete?
-important in organizing telencephalon, parts of diencephalon, olfactory area, and pituitary gland
-secretes Shh and FGF-8
What is the zona limitans signaling center? What does it secrete?
-organizes border between dorsal and ventral thalamus
The human hindbrain is divided into 7 ___________.
What genes account for which rhombomeres?
-Krox20 -> r3 and r5
-Kreisler and Hoxa-1 -> r5
-RA gradient -> r4-r7
-Gbx-2 -> r1-r3
What gene determines cranial nerves and pharyngeal arch derivatives?
-Hox is stimulated by RA gradient which initiates expression of Hoxa-1 and Hoxb-1
What does the caudal part of the neural plate serve as?
-stem cell zone
-cells produced in this region continue to proliferate but do not differentiate as long as they are in this zone
Presomatic mesoderm flanks the neural tube. What does it secrete to maintain the stem cells in the neural tube to continue proliferating?
As the neural tube advances posteriorly, some cells fall out of the stem cell zone and begin to ____________.
The differentiation of the cells from the SCR is stimulated by what?
-RA produced by flanking somites -> RA blocks FGF-8 and Wnt-3a
What are the four 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
Mesoderm originates from __________, passes through primitive streak as ________________ spread laterally and establishes a continuous layer between ectoderm and endoderm.
What are the three zones of mesoderm?
-paraxial/segmental plate mesoderm
-lateral plate mesoderm
What is paraxial mesoderm?
-thick column of mesoderm closest to and parallel with the notochord
-becomes segmented into somites
What is intermediate mesoderm?
-narrow column of mesoderm lateral to the paraxial mesoderm
-gives rise to urogenital system
What is lateral plate mesoderm?
-thin plate of mesoderm lateral to intermediate mesoderm
-splits to form lining of body cavities and mesoderm of most internal organs as well as limbs
What are somitomeres?
-initial pairs of segments of mesenchyme (barely discernible as segments) that begin to develop along and on either side of the neural plate in the paraxial mesoderm
-new somitomeres are added caudally as the primitive streak regresses
When do the first somites appear?
-after 20 pairs of somitomeres have formed (day 20), the first pair of somites forms caudal to the 7th pair of somitomeres at their expense of the 8th pair of somitomeres
What are somites?
-more dense blocks of mesoderm that form along the notochord
Describe how somitomeres are replaced by somites.
-caudally, a pair of somitomeres is transformed into a pair of somites every few hours
-as pairs of somites are formed, pairs of somitomeres are also formed at the anterior end of the primitive node
-for each pair of somites formed from somitomeres, a new pair of somitomeres is formed at the caudal end of the paraxial mesoderm
-11 pairs of somitomeres are kept constant at the caudal end of the paraxial mesoderm
Where do cells travel to form the different segments of the vertebrae?
-cells migrating through primitive streak will form cervical, thoracic, and lumbar vertebrae
-cells migrating from the tail bud will form the sacrum and coccyx
What are the 4 stages of the formation of a somite?
1. Epithelial stage
2. Epitheliomesenchymal transformation stage
3. Separation of the myotomes
4. Breakup of epithelial dermatome into dermal fibroblasts
What is the basic mechanism of the wavefront mechanism?
-increase in [FGF-8] -> mitosis in mesenchymal cells in posterior primitive streak (proliferating cells)
-increase in [RA] more anteriorly opposes action of FGF-8 (segmenting cells)
-balance between FGF-8 and RA that results in cellular determination toward somitogenesis
-Mesp-2 balances the two
What is the segmentation clock and what are the important signaling pathways?
-used to determine size of somites segments
-at each critical location that will serve to divide adjacent somites, lunatic fringe will become concentrated at the future anterior border of the somite and c-hairy will be concentrated at future posterior
-cells at anterior border express Eph A (receptor) and cells at posterior will express Eph B (ligand), resulting in fissure of adjacent somites
What are the steps is somite differentiation?
1. Ectoderm dorsal to somites -> Wnt-6 -> paraxis in somite
2. Paraxis + decrease Snail -> mesenchymal cell to epithelial cell conversion
3. Formation of somitocoel
4. Notochord -> Shh + noggin -> Pax1 + Pax9 in ventral part of somite = sclerotome
5. Wnt genes in dorsal neural tube -> transformation of dorsal half of somite into dermomyotome
6. Dermomyotome -> expression of Pax3, Pax7, paraxis -> separation off dermomyotome into dorsal dermatome and ventral myotomes
What is the dermomyotome?
-dorsi-lateral part of the somite
What two signals create a balance that causes the myotomes portion to myogenic lineage? Where is it signaled from?
-signaled from dorsal neural tube
-Shh and Wnt
-Noggin inhibits BMP-4 which inhibits myogenesis
What does BMP-4 do besides inhibit myogenesis?
-stimulates cells from ventrolateral dermomyotome to migrate from the somite and into the limb bud
FGF from the myotomes signals the sclerotome to do what?
-produce scleraxis, which causes the anterior and posterior borders of each somite to form the syndetome which is the precursor of tendons
What is sclerotomal breakup?
-sclerotomal portion of each somite breaks up into anterior and posterior portion
-posterior portion of one somite joins the anterior portion of the adjacent somite
-this creates a gap that allows nerves to grow out from the neural tube into the epaxial musculature
What two signals expressed creates intermediate mesoderm? What does intermediate mesoderm express?
-responds to BMP (lateral ectoderm) and activin (par axial mesoderm)
-cranial and caudal portions of mesoderm is dependent on expression of Hox4 and Hox11
Where do the early heart forming cells arise? Where do they migrate from and to? What do they form?
-arise in epiblast
-migrate through primitive streak
-cells migrating through anterior primitive streak form the outflow tract
-cells migrating through middle form ventricles
-cells entering posteriorly form atria
What are the cardiac tubes? How many layers do they have? What forms between the two layers?
-bilaterally paired tubes that form the cardiogenic mesoderm -> fuse beneath foregut to form single tube
-consists of inner and outer layer (outer = myocardium and inner = endocardium)
-cardiac jelly forms between the two layers -> specialized extracellular matrix
What is the source of pericardium and myocardial fibroblasts?
What are the genes involved in early heart formation?
What are the two layers of the lateral mesoderm?