Chapter 13 neural tube

BMP signals on ectoderm

Surface ectoderm: high levels of BMP: forms epidermis

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Neural crest:moderate levels of BMP: Forms nervous system, adrenal medulla, melanocytes , facial cartilage, teeth

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Neural plate/neural tube: low BMP levels(SOX TF expressed): brain spinal chord, motor neurons, retina

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SOX TF affects on neural plate

(1)activate genes that specify neural plate

(2)inhibit formation of epidermis and neral crest by blocking and transcription/signaling of BP

\-by blocking BMP SOX establishes neural plate that acts as the Precoursor to forming all cell types in the CNS

Two principle modes of neurulation

1. Primary neurulation: cells surrounding the neural plate cells direct the neural plate cells to proliferate, invaginate and seperate from the surface forming an underlying hollow tube

2. Secondary neurulation: neural tube arises from aggregate mesynchyme into a solid cord the makes cavities that eventually connect into a hollow tube

In many vert. There is a combination of the two that happen at different portions of the embryo where primary forms the anterior portion and posterior is formed by secondary neurulation.

Neural tube complete when two tubes join together at the transition zone where a combination of both mechanisms

Primary neurulation steps

1. Elongation and folding of neural plate (ap direction)

2. Bending of the neural plate by forming bottle cells to then make hinges(hinge points) where the medial cells are bound to the notochord creating the nerual groove

3. Convergence of the two lateral hinge points this happens at the dorsal points of the embryo and extends upward creating a driving force that furthers convergence

4. Closure of the neural tube: once the folds are brought into contact at the dorsal midline the folds adhere to one another. The surface ectoderm fuse with their counterparts and during this cells at the apex delaminate to form neural crest cells

Regulation of hinge points

1. Epithelial cells adopt a wedge shape via locallized contraction of actinomyosin complexes at the border of the cells creating apical constriction making bottle cell esc shapes

2. Cells also divide much faster in these regions creating denser areas adding force which is hypothesized to increase buckling

3. There is a morphogenetic pathway at work. SHH is released from the notochord which inhibits hogging thats expressed at the dorsal neural folds. Noggin inhibits BMP and BMP inhibits MHP which is activated by SHH

4. SHH is used for floor plate expression

Secondary neurulation

1. Takes place in the most posterior region of the embryo during tailbud elongation

2. Production of mesenchyme cells from the prospective ectoderm and mesoderm f

3. Then condensation of cells into meddulary cord below the surface ectoderm.

4. The mesenchyme goes through a mesenchyme epithelial transition and then creates follow spaces that come together into a hollow cavity.

5. As the embryo trunk elongates ectodermal cells posterior nerual tube cells express Sox2 and teh mesodermal cells express Tbx6 TF that forms somites

Anterior posterior axis

1. Early mammal neural tube is straight but changes drastically.

2. Anterior region becomes the three primary vesicles: the fore brain, midbrain, and hind brain.

3. By the time the posterior end of neural tube closes the secondary vesicle has already formed.

Opposing morphogens

dorsal-ventral polarity of the neural tube is induced by morphogenetic signals coming from its immediate environment

the ventral pattern is imposed by the notochord, whereas the dorsal pattern is induced by the overlying epidermis

Specification of the axis is initiated by two major paracrine factors: Sonic hedgehog protein (Shh) originating from the notochord, and TGF-β proteins originating from the dorsal ectoderm

Sonic hedgehog secreted from the notochord induces the medial hinge point cells to become the floor plate

the highest concentrations of Shh develop into the progenitor cells for motor neurons and a class of interneurons called V3 neurons, whereas moderate and lower levels of Shh induce increasingly more dorsal progenitor populations

BMP4 and BMP7 are found in the epidermis. Just as the notochord establishes a secondary signaling center—the floor plate cells—on the ventral side of the neural tube, the epidermis establishes a secondary signaling center by inducing BMP4 expression in the roof plate cells of the neural tube.

The importance of TGF-β superfamily factors in patterning the dorsal portion of the neural tube

All Axes Come Together

dorsal-ventral patterning by TGF-β and Shh morphogens pertains to cell fates throughout the CNS along the rostral-caudal axis

Progenitor cells in the anterior regions of the neural tube (which become the brain and most of the spinal cord) adopt a proneural fate directly from the epiblast

Cells in the posterior begin as bipotential neuromesodermal progenitors (NMPs) that undergo a transition to become neural or somitic cell types, with the neural cells forming the caudal end of the neural tube

NMPs are born in the caudal lateral epiblast during tailbud elongation and are positively maintained by Fgf8 and Wnt signals

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