Germ layer differntiation

1、1 Chapter 7 Late embryonic development 2Germ layersorgansEctodermEpidermis and its derivatives (hair, scale), nervous system, neural crest（神经嵴）（神经嵴）MesodermSkeleton, muscle, kidney, heart, bloodEndodermEndodermis（内皮）（内皮） of gut, liver, lungs31. The nervous system and the epidermis Primary neurulatio

2、n (初级初级神经胚形成神经胚形成)(1) Formation of the neural tube（神经管）（神经管）Two major ways of neurulation:The process by which the three ectoderm regions are made physically and functionally distinct from one another is called neurulation（神经胚形成）（神经胚形成）.An embryo undergoing this process is called neurula（神经胚）（神经胚）.4

3、416556nCells of the neural plate (NP) can be distinguished as elongated cells in the dorsal region of the ectoderm7nThe cells surrounding the NP direct the NP cells to proliferate, invaginate（内（内陷）陷）, and pinch off from the surface（离开表面）（离开表面）to form a hollow（中空的）（中空的）tube.nIn general, the anterior

4、portion of the NT is made by primary neurulation. 8In amphibians and amniotes:羊膜动物羊膜动物9nThe cells of the NP are characterized by expressing the Sox family of transcription factors (Sox1, 2, 3) that:nActivate the genes that specify cells to be NP;nInhibit the formation of epidermis and neural crest b

5、y blocking the transcription and signalling of BMPs. (the important principle of development)10Four stages of primary neurulation:nFormation of the NP（cells elongated).nThe edges of NP thickens and move upward to form the NF（neural fold, 神经褶神经褶）.nBending of the NP to form the neural groove （神经沟）（神经沟

6、）.nThe NFs migrate toward the midline, eventually fusing to form the NT.11Neurulation in the human embryo12nCertain genes, such as: Pax3, sonic hedgehog, and openbrain, are essential for the formation of the mammalian neural tube.nDietary(饮食的饮食的) factors, such as cholesterol and folic acid（叶酸）（叶酸）,

7、also appear to be critical.Human neural closure requires:13无脑畸形无脑畸形脊柱裂脊柱裂14Neural tube defects:nFailure to close site 2 in NT results in a lethal condition, anencephaly(无脑畸形).nFailure to close the posterior neuropore（神经孔） around day 27 results in spina bifida.nThe failure of the entire NT to close i

8、s called craniorachischisis（ 颅脊柱裂）. 15The separation of NT from the surface ectoderm is thought to be mediated by the expression of different cell adhesion molecules（细胞粘连分子）（细胞粘连分子）E-cadherin（钙粘连蛋白）（钙粘连蛋白）N-cadherin16 Secondary neurulation（次级神经（次级神经胚形成）胚形成）17nThe neural tube arises from the coalesce

9、nce (结合处结合处) of mesenchyme cells to form a solid cord that subsequently create a hollow (中空的）中空的） tube.nThe posterior portion of the NT is made by secondary neurulation.18(2) Differentiation of the NTnAnatomical（解剖的）（解剖的）level: the NT and its lumen bulge（凸出）（凸出）and constrict（收缩）（收缩） to form the cham

10、bers of the brain and the spinal cord（脊髓）（脊髓）.nTissue level: the cell populations within the tube wall form the different functional regions of the brain and the spinal cord.nCellular level: the neural epithelial cells differentiate into the numerous types of nerve cells （neurons, 神经元）神经元）and suppor

11、tive cells （glia, 神经胶质神经胶质).Three levels:19原始脑泡原始脑泡次生脑泡次生脑泡脑腔脑腔端脑端脑间脑间脑中脑中脑后脑后脑髓脑髓脑2021Chordin,Noggin,FollistatinCerberus,Frzb,DickkopfHead inducers: ILGFs视视黄黄酸酸 The a-p axis222210-8 M RA10-6 M RAControl23nRetinoic acid（RA: 视黄酸）视黄酸）causes anterior to posterior transformation of neural fate.nLow RA l

12、evel permit anterior Hox genes to be expressed. nHigh RA level repress anterior Hox genes and activate posterior Hox genes to be expressed.24A-P polarity in all vertebrates becomes specified by the expression of Hox genes颈颈胸胸腰腰后头部后头部骶骨骶骨尾部尾部 2526nHox genes pattern the A-P axis and help to specify po

13、sitions along this axis.nThis patterning mechanism is extremely ancient.27 The dorsal-ventral axis28nThe newly formed NT is influenced by : BMP4 and BMP7 from the roof epidermis; Shh protein from the notochord.nThen the signals are : BMP4 and Shh within the tube. nBMP4 establishes a cascade of TGF-

14、factors, spreading ventrally. Shh diffuses dorsally as a gradient.nThe neurons of the spinal cord are given their identities by their exposure to these gradients of the factors.2930Neurogenic transcription factorsExpressing areaFunctionOver-expression of either factor increase the proportion of neur

15、ons NeurogeninWhere form the first primary neuronsCause activation of NeuroDNeuroDIn primary neurons from open neural plate stagesFormation of neurons(3) Neurogenesis 31Lateral inhibition(旁侧抑制旁侧抑制)mediated by the Delta-Notch system amplifies an initially minor difference between cellsNeurogeninDelta

16、NeuroDNeurogeninDeltaNeuroDHigh Delta Neurogenin NeuroDLow Notch activityLow Delta Neurogenin NeuroDHigh Notch activityNeuronal differntiationDeltaNotchNon-neuronal differntiationDeltaNotch32nNotch signaling represses neurogenin with consequent non-neuronal differentiation.33SG1G2Mitotic cellsThe ea

17、rly neuroepithelium is only one cell thick. The nuclei are present at many different levels3435Vertical cleavage(垂直分裂垂直分裂)Horizontal cleavage （水平分列）（水平分列）All mitotic cellsOne mitotic cell;One new neuron36 nIt is associated with the localization of Numb. nAfter horizontal division, internalized neuro

18、n has none Numb protein.nThe neuroblasts（成神经细胞）（成神经细胞） divide to form neuronal precursors called ganglion（神经节）（神经节） mother cells.37 The horizontal divisions means that the neural tube ceases to be one cell thick. 38Neuronal migration on glial cell process（神经胶质细胞突起）（神经胶质细胞突起）(边缘边缘区区)腔面区腔面区mantle zone

19、外膜区外膜区39ventricular zone脑室区gray matterwhite matterThree layers of the later NT:neuronal cell bodiesaxonsmantle zone套膜区套膜区marginal zone边缘区40nThe mitotic luminal (内腔的内腔的) layer becomes the ventricular zone（脑室（脑室区）区）.nCells moving in from the ventricular layer form a mantle zone（套膜区）（套膜区）.nAs the neuro

20、ns produce their axons(轴轴突突), they build up to form a cell-poor (with axons) marginal zone （边缘区）（边缘区）.41（4）The specification of neural crest（NC,神经嵴）神经嵴）Although derived from the ectoderm, the neural crest has sometimes been called the fourth germ layer because of its importance.42神经外胚层神经外胚层神经褶神经褶43神

21、经嵴细胞神经嵴细胞44nThe NC cells are present in the dorsal part of the NT on closure but soon migrate into the surrounding tissues.45荧光葡聚糖荧光葡聚糖黑色素细胞黑色素细胞背根神经节背根神经节交感神经节交感神经节肾上腺髓质肾上腺髓质NC cells are pluripotent（多能的）（多能的） before migration46 Migrates ventrally to give rise to pigment cells. Migrates dorsal-later

22、ally to produce dorsal root ganglia(背背根神经节根神经节), etc. 47Multipotential NC cellsWnt BMP2 GGF Endothelin+Wntglucocorticoid hormones糖皮质激素Paracrine factors encountered in the environment help specify the NC-derived lineages in the trunk sensory neuronsAutonomic neuronsSchwann cells; gliaMelanocytesAdren

23、omedullary cells48The NC cells migrate extensively to generate numerous of cell types:nSympathetic and parasympathetic ganglia（交感和副交感神经节）（交感和副交感神经节）.nAdrenal medulla cells (肾上腺髓质细胞肾上腺髓质细胞).nThe pigment cells.nCartilage (软骨软骨), bone, cranial neurons, glia, and connective tissues （结缔组织）（结缔组织）of the he

24、ad.49502. Mesoderm and organogenesis51A Belgian Blue bull showing the double muscling phenotype. A Piedmontese cattle showing the double muscling phenotype in Italy5253(1) Chordamesoderm: forms the notochod, a transient organ that functions mainly in inducing the formation of the neural tube and est

25、ablishing a-p axis.(2) Paraxial mesoderm: will form somites-will produce many of the connective tissues of the back (bone, muscle, cartilage, and dermis).12 3454(3) Intermediate mesoderm: forms the urogenital system, consisting of the kidneys, the gonads, and their associated ducts, the cortical por

26、tion of the adrenal gland.(4) Lateral plate system: -heart, blood vessels, blood cells of the circulatory system, as well as the lining of the body cavities. 55脏壁中胚层脏壁中胚层体腔体腔体壁中胚层体壁中胚层体节体节中间中胚层中间中胚层562.1 Somitogenesis（体节发生）（体节发生）57体节体节Somite58The formation of somitesnThe somitogenesis（体节发生）（体节发生）are

27、 periodicity.nStarting: paraxial mesoderm cells become somitomeres（体节球）（体节球）.nThe somitomeres become compacted（致（致密的）密的）and bound together and eventually separate to form individual somites. 59nSomites formation depends on a “clock and wave” mechanism: Notch and Wnt pathways provide an oscillating s

28、ignal (clock) ; a rostal-to-caudal gradient provides a moving “wave” of an FGF that sets the somite boundaries.60nMutations affecting Notch pathway Notch, its ligands Delta-like 1and 3 (Dll1 and Dll3) are responsible for aberrant vertebra formation in mice and humans.61nDelta-like3 mutation gives ri

29、se to aberrant vertebra formation in mice 62nOne of the genes regulated by Notch is hairy1 .nSomite formation correlates with the wavelike expression of hairy1 gene in the chick.63Sclerotome (生骨节生骨节) cells can be distinguished from the rest of the somite, becoming mesenchymal cells again.Sclerotome

30、cells migrate toward the NT生骨节生骨节生骨节生骨节64The medial cells form a primaxial myotome (近轴肌节近轴肌节) beneath the dermamytome (生皮生皮肌节肌节), while the lateral cells form an abaxial myotome (远远轴肌节轴肌节)生骨节细胞生骨节细胞远轴肌节远轴肌节生皮肌节生皮肌节近轴肌节近轴肌节中央生皮肌中央生皮肌节区节区远轴体节芽远轴体节芽生骨节生骨节近轴生皮肌近轴生皮肌节唇节唇成肌细胞成肌细胞A layer of myotome forms b

31、eneath the epithelial dermamyotome65生生骨骨节节远轴肌远轴肌近轴肌近轴肌66High concentrations of Shh induceexpression in those cells fated to become thesclerotome.Sclerotome (生骨节生骨节) 远轴肌远轴肌近轴肌近轴肌Certain concentrations of neurotrophin-3 (NT3) from the dorsal NT specify the dermotome.67Wnt proteins (probably Wnt1 and W

32、nt3a), in combination with low concentrations of Shh from the notochord and floor plate induce the primaxial myotome.远轴肌远轴肌近轴肌近轴肌Wnt proteins from the epidermis, in conjunction with BMP4 and Fgf5 from the lateral plate mesoderm induce the abaxial myotome.68nHigh concentrations of Shh induceexpressio

33、n in those cells fated to become thesclerotome.nWnt proteins (probably Wnt1 and Wnt3a), in combination with low concentrations of Shh from the notochord and floor plate induce the primaxial myotome.nCertain concentrations of neurotrophin-3 (NT3) from the dorsal NT specify the dermotome.nWnt proteins

34、 from the epidermis, in conjunction with BMP4 and Fgf5 from the lateral plate mesoderm induce the abaxial myotome.69Somites form:nThe cartilage of the vertebrae and ribs (椎骨椎骨和肋骨的软骨和肋骨的软骨).nThe muscles of the rib cage（胸腔）（胸腔）, limbs, abdominal wall（腹部壁）（腹部壁）, back, and tongue.nThe dermis（真皮）（真皮）of t

35、he dorsal skin.nThe tendons (肌腱肌腱) that connect the muscles to the bones.nVascular（血管的）（血管的）cells that contribute to the formation of the aorta (主动脉主动脉) and the intervertebral（椎间）（椎间） blood vessels.702.2 Myogenesis (肌肉发生)nMuscle cells come from two cell lineages in the somite: the primaxial and the

36、abaxial myotome.nIn both lineages, paracrine factors instruct cells to become muscles by inducing them to synthesize the MyoD protein.71nThe myotome cells producing the myogenic bHLH (basic helix-loop-helix) proteins (MyoD, Myf5) are the myoblasts: committed muscle cell precursors.72nWhen the growth

37、 factors are used up, the myoblasts cease dividing, align, and fuse into myotubes.nThe myotubes become organized into muscle fibers that spontaneously contract.旁旁分分泌泌因因子子Conversion of myoblasts into muscles in culture73nMyostatin (MSTN) is a member of the TGF-family that functions as a negative regu

38、lator of skeletal muscle development and growth in mammals.nThe Double muscle breeds of cattle that have significantly more muscle mass than standard breeds were found to carry natural mutations in the myostatin gene, Such as Belgian blue and Piedmontese74A Belgian Blue bull showing the double muscl

39、ing phenotype. A Piedmontese cattle showing the double muscling phenotype in Italy75Overexpression of the MSTN prodomain in skeletal muscle of transgenic zebrafish have no effect on the expression of myogenic regulatory genes. However, the adult transgenic fish showed hyperplasia (增生增生) in their ske

40、letal muscles compared with non-transgenic control. Furthermore, Myostatin gene silenced by RNAi show a zebrafish giant phenotype (Acosta et al., 2005).762.3 Osteogenesis: The development of bonesnThree distinct lineages generate the skeleton:n Somites generate the vertebral skeletonn Lateral plate

41、mesoderm produces the limb skeletonn Cranial NC generates the branchial arch and craniofacial bones and cartilage.77Two modes:(1) The direct conversion of mesenchymal tissue into bone is called Intramembranous ossification (膜内成骨膜内成骨)78成骨细胞成骨细胞 骨细胞骨细胞Targeting of the Cbfa1 in mice prevents the bone f

42、ormation CBFA1 may activate the genes expressing osteocalcin （成骨（成骨素素 ）、）、 osteopontin（骨桥蛋白）（骨桥蛋白）and other extracellular matrix that help the bone formation.79nIn the skull, NC-derived mesenchymal cells proliferate and condense into compact nodules. nSome of these cells change their shape to become osteoblasts (成骨细胞).nThe osteoblasts secrete osteoid matrix that is able to bind calcium.nOsteoblasts that become embeded in the calcified matrix become osteocytes (骨细胞).80（2）Endochondral ossification (软骨内成骨软骨内成骨): The mesenchymal cells differ