Ebook Human embryology and developmental biology (5/E): Part 2
➤ Gửi thông báo lỗi ⚠️ Báo cáo tài liệu vi phạmNội dung chi tiết: Ebook Human embryology and developmental biology (5/E): Part 2
Ebook Human embryology and developmental biology (5/E): Part 2
Chapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2and structures, including several types of nerves and glia, connective tissue, bones, and pigment cells. Its importance and prominence are such that the neural crest has often been called the fourth germ layer of the body. Not until adequate methods of marking neural crest cells became available— fi Ebook Human embryology and developmental biology (5/E): Part 2rst with isotopic labels and subsequently with stable biological markers, monoclonal antibodies, intracellular dyes, and genetic markers—did the neuraEbook Human embryology and developmental biology (5/E): Part 2
l crest become one of the most widely studied components of the vertebrate embryo. Most studies on the neural crest have been conducted on the avian eChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2e, especially for dissecting molecular controls, but it appears that most of the information on the biology of the neural crest derived from birds can be applied to mammalian embryos. Some important syndromes and malformations are based on abnormalities of the neural crest. Some of these syndromes a Ebook Human embryology and developmental biology (5/E): Part 2re presented in Clinical Correlation 12.1. at the end of the chapter.Developmental History of the Neural CrestThe neural crest originates from cells lEbook Human embryology and developmental biology (5/E): Part 2
ocated along the lateral margins of the neural plate. Tracing the history of the neural crest in any region involves consideration of the following: (Chapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2differentiation. Each of these phases in the development of the generic neural crest is covered before neural crest development in specific regions of the body.Origin, Induction, and SpecificationAccording to the most recent data, the earliest stages of neural crest induction may occur as early as g Ebook Human embryology and developmental biology (5/E): Part 2astrulation. but according to the classical model, the neural crest arises as the result of inductive actions by the adjacent non-neural ectoderm andEbook Human embryology and developmental biology (5/E): Part 2
possibly nearby mesoderm on the neural plate I Fig. 12.1). The ectodermal inductive signals are bone morphogenetic proteins I BMPs > and Wilts?FibroblChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2e of BMPs is complex and relates to a concentration gradient along the ectodermal layer as neurulation proceeds. The highest concentrations of BMP are seen in the lateral ectoderm, and cells exposed to these concentrations remain ectodermal. Cells within the neural plate are exposed to the lowest co Ebook Human embryology and developmental biology (5/E): Part 2ncentrations of BMP because of the local inhibitory actions of noggin and chordin (see Fig, 5.SD). and. by default, they remain neural. Cells at the bEbook Human embryology and developmental biology (5/E): Part 2
order of the neural plate are exposed to intermediate levels of BMP. and. in this environment. they are induced to form neural crest precursor cells.IChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2-1 and Msx-2,Dlx-5, Pax-3/Pax-7. and Gbx-2. These and other gene products turn on a network of genes that transform the epithelial neural crest precursor cells into mobile mesenchymal cells that break free from the neu-roepithehum of the neural tube.Epitheliomesenchymal Transformation and Emigration Ebook Human embryology and developmental biology (5/E): Part 2 from the Neural TubeWithin the neural tube, neural crest precursor cells are epithelial and are tightly adherent to other neuroepithelial cells throuEbook Human embryology and developmental biology (5/E): Part 2
gh a variety of intercellular connections. Prominent among them are the cadherius. Among the new transcription factors upregulated in induced neural cChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2e from the neural epithelium and then migrate away as mesenchymal cells.* Under the influence of snail-1 and snail-2, the profile of cadherins produced by the neural crest precursors changes from type I cadherius (e.g.. N-cadlieiiu and E-cadherin). which are strongly adhesive, to tspe II cadhe-I-Ins Ebook Human embryology and developmental biology (5/E): Part 2. which are less adhesiveNeural crest cells break free from the neural tube in the trunk at the level of the last-formed somite or rhe neural plate inEbook Human embryology and developmental biology (5/E): Part 2
the head by changing their shape and properties from those of typical neuroepithelial cells to those of mesenchymal cells. Important to this process Chapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2 N-CAM.'Snail-2 IS iso expressed ducnf guttviatioa by cdb of the epiblatt after they htne entered the walk of the primitive atreak and are about to leave aa tneieo:hyEbook Human embryology and developmental biology (5/E): Part 2
st growth factor-8, N-C AM, neural cell éứnesion moleculeInductionGbx-2BMP, WritiZ~*Msx-1.-2FOF-8EmigfatfonLoss of N-CAM.E- and N-CadhemE-cadherin. anChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2iated into certain structures (e.g.. spinal ganglia). CAMS are often expressed again.In the head. where closure of the neural plate has not yet occurred, neural crest cells must penetrate the basal lamina underlying the neural plate. This IS accomplished by the production of enzymes that degrade com Ebook Human embryology and developmental biology (5/E): Part 2ponents of the basal lamina and by sending out processes that penetrate the basal lamina. In the trunk, neural crest cells do not leave the neuroepithEbook Human embryology and developmental biology (5/E): Part 2
elium until after the neural tube has formed. They do not. however, have to contend with penetrating a basal lamina because the dorsal part of the neuChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2al crest cells first encounter a relatively cell-free environment rich in extracellular matrix molecules (Fig. 12.2). In this environment, the cells undergo extensive migrations along several well-defined pathways. These migrations are determined by intrinsic properties of the neural crest cells and Ebook Human embryology and developmental biology (5/E): Part 2 features of the external environment encountered by the migrating cells.Neural crest migration is influenced by a variety of molecules residing in thEbook Human embryology and developmental biology (5/E): Part 2
e extracellular matrix. Although the presence of a basal lamina can inhibit their emigration from the neural rube, neural crest cells often prefer to Chapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2ar matrix permissive for migration include molecules found in basal laminae, such as fibronectin, laminin, and type IV collagen (Fig. 12.3). Attachment to and migration over these substrate molecules are mediated by the family of attachment proteins called iiiteglius Other molecules, such as chondro Ebook Human embryology and developmental biology (5/E): Part 2itin sulfate pioteoglvvans. are not good substrates for neural crest cells and inhibit their migration.Neural crest cells emigrate from the neural tubEbook Human embryology and developmental biology (5/E): Part 2
e or neural folds in streams, with each cell in contact with neighbors through filopodial contacts. During their migratory phase, neural crest cells aChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2ptor pairs Robo Slit Neinopiliii Semaphoi'ln and Ephiin Eph (see Table 11.1). Much less is known about attractive influences on neural crest cell migration. During migration, neural crest cells extend protrusions that both test the environment and are part of the propulsive mechanism. If an inhibito Ebook Human embryology and developmental biology (5/E): Part 2ry influence IS encountered, the protrusions collapse through signalsRg. 12.2 Scanning eleciron micrograph of a chide embryo, showng the early migratiEbook Human embryology and developmental biology (5/E): Part 2
on of neural crest cells (arrow) out of the neural tube (NT) The subectodermal pathway of neural crest migration (asterisk) is relatively cell free, bChapter 12Neural CrestThe neural crest, whose existence has been recognized for more than a century, forms an exceptionally wide range of cell types a Ebook Human embryology and developmental biology (5/E): Part 2arity pathway (see p. 87). This mechanism acts as a brake when the cells encounter an inhibitory environment, but it is also involved in their forward propulsion In a migrating stream of neural crest cells, contact with the cells behind also results in the pulling of protrusions at the trailing edge Ebook Human embryology and developmental biology (5/E): Part 2 of the cells, thus resulting in a net forward motion of the leading cells. Specific examples of the environmental control of neural crest cell migratEbook Human embryology and developmental biology (5/E): Part 2
ions are given later in this chapter. Much remains to be learned about what causes neural crest cells to stop migrating, but often they stop migratingGọi ngay
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