neural development 1

trophectoderm and inner cell mass

the developing of the the amion through cavitation of inner cell mass with the trophectoderm and extra-embryonic endoderm

9 - extension of the extra-embryonic endoderm around the blastocoel cavity 10.5 - characteristic mesh of extra-embryonic endoderm with the blastocoel cavity 11.5 - extra-embryonic mesoderm 13 - rhesus macaque embryo - externsion of the primitive streak from caudal epiblast, subsequent to extensive development of the extra-embryonic mesoderm, has a secondary yolk sac

a group of cells which are stem cells

an epiblast sheet, as the sheet develops so does the neural form

primitive streak is in the future spinal chord top part is the organiser which sends out signals to change external surroundings

with neural plate induction movement of the organiser reorganises tissue the organiser is a source of tissues that can direct the neural plate

when BMP is expressed throughout the xenopus blastula, the neural plate form from the prospective ectoderm when BMP signalling is inhibited by antagonists from the organiser

BMP antagonising activity from the organiser has lead to the formulation of the default model for neural induction from ectoderm cells will form neural tissue unless they receive signals that induce epidermal differentiation - so the default state is the neural state so neural induction requires signals that inhibit the epidermal inducer BMP$ BMP antagonists in the organiser include Noggin and Chordin the organiser secretes the BMP antagonists

they interfere with BMP signal transduction FGF-MAPK signalling contributes to neural induction MAPK inhibition of smad1 activity, an intermediate in the BMP signalling pathway that would otherwise translocate to the nucleus to activate BMP target genes FDF, IGF and HGF can also stimulate this

anrerior mesoderm induces forebrain posterior mesoderm induces spinal chord if neurula anterior mesoderm is grafted into early gastrula and induces and extra head with eyes and forebrain if neurula posterior mesoderm is grafted into early gastrula it gives a trunk and tail

the neural plate roles up called neurulation neural crest ells form the dorsal neural tube and give rise to the PNS including glangia in head and gut future motor neurones and associated interneurons are locarted ventrally and form the ventral root sin the spinal chord commissural neurons are secondary and their associated inter-neurones differentiate primarily in the dorsal region spinal chord extends their axons on the side the originated

the neural crest cells located laterally and dorsally in the neural tube

a group of non neural cells in the ventral midline of the neural tube the roof plate is non neural cells in the dorsal midline the floor and roof plates produce signals that pattern the neural tube along its dorso-verntal axis the floor plate is induced by signals secreted by the mesodermal notochord immediately below the neural tube

patterning can be seen, it induces a second floor plate

sonic hedge hog, this then forms a gradient of activity from ventral to dorsal in the neural tube and acts as the ventral patterning signal

Shh from the floor plate by retinoic acid secreted by the adjacent mesoderm

the isthmus, separating the midbrain and hindbrain regionalisation of the CNS is progressive and cell to cell signalling plays an important role in this local organising centres within the brain such as the isthmus is located at the midbrain to hind brain border to help subdivide neural tissue into regions and functionally distinct cell populations

grafting isthmus converts anterior midbrain to posterior midbrain grafting the isthmus to the posterior hind brain converts this to anterior hindbrain patterning mid and hind brain - FGF signalling induces r1 gene, engrailed 2 in move posterior hindbrain isthmus patterns tissues via FGF/Wnt signalling hind brain is divided into sections called rhombomeres which have distinct molecular identities hox genes provide a combinatorial code that distinguishes rhombomeres

transcription factors conserved across species and are an ancient means to convey personal identity the order they are in on the chromosome is the same order they are expressed in they code for specific identities

it converts rhombomere 4 to rhombomere 2 altering expression of hox genes least to homeotic transformations distinct hox gene combinations define different antero-posterior regions of the body axis including the spinal chord retinioc acid provided by somites promotes anterior hox genes

retinoic acid is derived from vitamin a lack of and excess of retinoic acid leads to developmental defects in chicken eggs and mammalian embyro retinol is supplied by maternal circulation retinol binds to retinol binding proteins in extracellular space and is transported into cells by rhe transport receptor retinol binds to CRBP and produces retinoic acid in a 2 step reaction retinol is first metabolised by retinaldehyde by retinol dehydrogenases or alcohol dehydrogenases retinaldehyde is then metabolised to retinoic acid by retinaldehyde dehydrogenases

• induce anterior neural gene= inhibition of BMP signalling • experience posteriorising factors = retinioc acid, FGF and Wnt signalling the neural tube is induced with anterior character and is trnasformer or posteriorised further subdivisions of nervous systems take place as development proceeds

from a stem zone in the tail bud cells in the stemzone imclude neuro-mesodermal projenisotrs which contribute to both spinal chord and paraxial mesoderm as the body axis elongates nmps are induced and maintained by fgf and wnt signalling which also promotes expression on caudal (5’0 how genes