原腸胚形成

原腸胚形成。原腸胚形成發生於卵裂之後，原腸胚形成完成後，胚胎進入原腸胚時期，開始過程。新形成的三個胚層的細胞會組合並發育為器官。每一個胚層的細胞都能發育為特定的器官和組織。外胚層會發育為表皮、神經嵴，以及之後會發育為神經系統的組織。中胚層細胞位於外胚層細胞和內胚層細胞之間，能發育為體節，生成肌肉，以及屬於肋骨、椎骨的軟骨。另外，中胚層還能發育為真皮、脊髓、血管與血液、骨，以及結締組織。內胚層細胞則會發育為消化系統和呼吸系統的上皮，比如肝和胰腺。原腸胚形成過程結束後，機體的細胞都會組織為被上皮等締合的細胞包圍的群體，或者像間充質那樣分散在各處。原腸胚形成的分子機制以及所需時間在不同的生物中是不同的。不過，不同生物之間的原腸胚形成仍然有一些共同點，如：一，胚胎的拓撲結構會發生變化，從單連通的表面。「原腸胚」，發育生物學的先驅之一，曾這樣說：「出生、結婚、死亡都不是你一生中最重要的時刻，原腸胚形成才是。」。體內.總覽.在羊膜上出現的一個開口，胚孔的出現有密切關係。注意胚孔並不是囊胚階段上囊胚腔上的開口，而是一個新的開口。胚孔能推動囊胚表面的細胞聚合在一起。在羊膜上，原腸胚形成可以分為以下幾個大步驟：一，胚胎變為不對稱；二，原條形成；三，原條表皮細胞發生。同時，原條區域細胞發生內移，形成胚層。原口動物和「口」。對稱性的失去.在原腸胚形成的準備階段，胚胎會形成遠近軸，打破前後對稱性。上述過程受Nodal訊號通路的調控。原條的產生.原條在原腸胚形成的初始階段產生。原條位於後側胚胎外組織以及表皮之間的連結處以及內移發生的區域。原條的形成與科氏鐮區域的細胞中的NODAL訊號通路以及由胚胎外組織啟用的BMP4訊號通路有很大關係。Cer1和Lefty1透過拮抗Nodal訊號通路，能將原條形成限制在特定區域。原條區域會繼續往遠端生長。已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。During the early stages of development, the primitive streak is the structure that will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the first embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the antero-posterior body axis, becoming the first symmetry-breaking event in the embryo, and marks the beginning of gastrulation. This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position, where they will differentiate into the three germ layers. The localization of the cell adhesion and signaling molecule beta-catenin is critical to the proper formation of the organizer region that is responsible for initiating gastrulation.上皮-間充質轉換與內移.In order for the cells to move from the epithelium of the epiblast through the primitive streak to form a new layer, the cells must undergo an epithelial to mesenchymal transition (EMT) to lose their epithelial characteristics, such as cell-cell adhesion. FGF signaling is necessary for proper EMT. FGFR1 is needed for the up regulation of SNAI1, which down regulates E-cadherin, causing a loss of cell adhesion. Following the EMT, the cells ingress through the primitive streak and spread out to form a new layer of cells or join existing layers. FGF8 is implicated in the process of this dispersal from the primitive streak.體外原腸胚形成.許多科學家都在嘗試透過體外實驗的方法與胚胎實驗進行對比對照，以更加深入的研究原腸胚形成的過程。通常，研究人員會使用2D。不過，體外實驗多少會和體內實驗存在差異，因而與體內的胚胎發育進行對比是必要的。已隱藏部分未翻譯內容，歡迎參與[ 翻譯]。To illustrate this, the guided differentiation of mouse ESCs has resulted in generating primitive streak-like cells that display many of the characteristics of epiblast cells that traverse through the primitive streak (e.g. transient brachyury up regulation and the cellular changes associated with an epithelial to mesenchymal transition), and human ESCs cultured on micro patterns, treated with BMP4, can generate spatial differentiation pattern similar to the arrangement of the germ layers in the human embryo. Finally, using 3D embryoid body- and organoid-based techniques, small aggregates of mouse ESCs (Embryonic Organoids, or Gastruloids) are able to show a number of processes of early mammalian embryo development such as symmetry-breaking, polarisation of gene expression, gastrulation-like movements, axial elongation and the generation of all three embryonic axes (anteroposterior, dorsoventral and left-right axes).