Differentiation Ectoderm

1 differentiation

1.1 initial appearance
1.2 development

1.2.1 gastrulation


1.3 later development

1.3.1 neurulation
1.3.2 organogenesis

differentiation
initial appearance

the ectoderm can first observed in amphibians , fish during later stages of process called gastrulation. @ start of process, developing embryo has divided many cells separating embryo, hollow sphere of cells called blastula, 2 parts, animal hemisphere , vegetal hemisphere. animal hemisphere of blastula become ectoderm.

early development

like other 2 germ layers, mesoderm , endoderm, ectoderm forms shortly after egg fertilized, , rapid cell division initiates. epidermis of skin originates less dorsal ectoderm surrounds neuroectoderm @ gastrula stage of embryonic development. position of ectoderm relative other germ layers of embryo governed selective affinity , meaning inner surface of ectoderm has strong (positive) affinity mesoderm, , weak (negative) affinity endoderm layer. selective affinity changes during different stages of development. strength of attraction between 2 surfaces of 2 germ layers determined amount , type of cadherin molecules present on cells surface. example, expression of n-cadherin crucial maintaining separation of precursor neural cells precursor epithelial cells. ectoderm instructed become nervous system notochord, typically positioned above it.

gastrulation

during process of gastrulation, special type of cells called bottle cells invaginates hole on surface of blastula called dorsal lip of blastopore. once lip has been established, bottle cells extend inward , migrate along inner wall of blastula known roof of blastocoel. once superficial cells of animal pole destined become cells of middle germ layer called mesoderm. through process of radial extension, cells of animal pole once several layers thick divide thin layer. @ same time, when thin layer of dividing cells reaches dorsal lip of blastopore, process occurs termed convergent extension. during convergent extension, cells approach lip intercalate mediolaterally, in such way cells pulled on lip , inside embryo. these 2 processes allow prospective mesoderm cells placed between ectoderm , endoderm. once convergent extension , radial intercalation underway, rest of vegetal pole, become endoderm cells, engulfed prospective ectoderm, these top cells undergo epiboly, ectoderm cells divide in way form 1 layer. creates uniform embryo composed of 3 germ layers in respective positions.

later development

once there embryo 3 established germ layers, differentiation among these 3 layers proceeds. next event take place within ectoderm process of neurulation, results in formation of neural tube, neural crest cells , epidermis. these 3 components of ectoderm each give rise particular set of cells. neural tube cells become central nervous system, neural crest cells become peripheral , enteric nervous system, along melanocytes, facial cartilage , dentin of teeth, , epidermal cell region give rise epidermis, hair, nails, sebaceous glands, olfactory , mouth epithelium, eyes.

neurulation

neurulation proceeds primary , secondary neurulation, both positioning neural crest cells between superficial epidermal layer , deep neural tube. during primary neurulation, notochord cells of mesoderm signal adjacent, superficial ectoderm cells reposition in columnar pattern form cells of ectodermal neural plate. cells continue elongate, group of cells above notochord change shape, forming wedge in ectodermal region. these special cells called medial hinge cells (mhp). now, ectoderm continues elongate, ectodermal cells of neural plate fold inward. inward folding of ectoderm virtue of cell division continues until group of cells form within neural plate. these cells termed dorsolateral hinge cells (dlhp), , once formed, inward folding of ectoderm stops. dlhp cells function in similar fashion mhp cells regarding wedge shape, however, dlhp cells result in ectoderm converging. convergence led ectodermal cells above dlhp cells known neural crest. neural crest cells pull adjacent ectodermal cells together, leaves neural crest cells between prospective epidermis , hollow, neural tube.

organogenesis

ectodermal specification

all of organs rise ectoderm such nervous system, teeth, hair , many exocrine glands, originate 2 adjacent tissue layers: epithelium , mesenchyme.

several signals mediate organogenesis of ectoderm such as: fgf, tgfβ, wnt, , regulators hedgehog family. specific timing , manner ectodermal organs form dependent on invagination of epithelial cells. fgf-9 important factor during initiation of tooth germ development. rate of epithelial invagination in increased action of fgf-9, expressed in epithelium, , not in mesenchyme. fgf-10 helps stimulate epithelial cell proliferation, in order make larger tooth germs. mammalian teeth develop ectoderm derived mesenchyme: oral ectoderm , neural crest. epithelial components of stem cells continuously growing teeth form tissue layers called stellate reticulum , suprabasal layer of surface ectoderm.