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Embryonic Folding Notes

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This is an extract of our Embryonic Folding document, which we sell as part of our Organisation of the Body Notes collection written by the top tier of Oxford students.

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Embryonic folding

Differential growth and active reshaping causes the embryo to fold
-End of third week- embryo is flat/ovoid trilaminar disc that is formed during gastrulation
-the ectoderm gives rise to the skin and the nervous system
-the mesoderm gives rise to muscle, connective tissue, blood, bone
-endoderm gives rise to the gut and associated organs
-notochord, neural tube, somites stiffens the dorsal axis so folding occurs on the cranial, caudal, lateral margins of the disc- this gives the ventral surfaces of the body Rostral/Cephalic folding- moves rostral structures venral
-rostral and caudal folding is due to expansion of the neural tube
-cephalic portion of the embryonic axis overgrows the yolk sac, cephalic neural plate flexes sharply
-flexure of the cephalic neural plate causes thin cranial rim of the disc to fold under forming the ventral surface of the face, neck and chest
-cephalic folding causes the buccopharangeal membrane (Cranial to the neural plate) to the region of the future mouth
-the cardiogenic region (cranial to the buccopharangeal membrane) translocated towards the future chest.
-cephalic folding also creates the septum transversum- thick bar of mesoderm between the cardiogenic region and the cranial margin, this is translocated until it is wedges between the cardiogenic region and the edge of the yolk sac
-septum transversum forms the initial partition that seperates the thoracic cavities from the abdominal cavities- it gives rise to part of diagphragm and ventral mesentry of the stomach and duodenum Caudal folding- caused by the growth of the spinal cord
-rapidly lengthening neural tube and somites overgrow the caudal region of the yolk sac

-stifness of the dorsal axis leads to caudal rim of the germ disc-containing the cloacal membrane to fold and become the ventral surface of the embryo
-connecting stalk connects the caudal end of the germ disc to the developing placenta-folding causes the connecting stalk to move cranialy and merges with the neck of the yolk sac
-the root of the connecting stalk has the allantois Lateral folding- edges of the germ disc fuse at the midline
-4th week- grows rapidly in length and differential growth of various structures- embryonic disc and amnion grow vigorously but yolk sac hardly grows - lateral folding of the embryo - amnion surrounding the embryo- left and right sides of the amnion into close proximity and fusing at the ventral midline
-left and right sides of the embryo flex sharply - narrows the neck of the yolk sac
-at the head and tail ends of the embryo these lateral edges make contact with each other, ectodermal, mesodermal, endodermal layers
-ectoderm covers the entire surface of the 3D embryo except the surface of the umbilical region where the yolk sac and connecting stalk emerge Fusion of the lateral edges of the endoderm creates the Gut tube
-the endoderm of the trilaminar germ disc gives rise to the Lining of the small intestine
-cranio-caudal folding and lateral folding of the embryo creates the fore and hind duct at the superior and inferior end of the endoderm
-cranial end of the foregut is capped by the buccopharangeal membrane which under programmed cell death forms the opening of the mouth. The caudal end of the hind gut is capped by the cloacal membrane which by programmed cell death forms the anus and urogenital system
-the midgut- initially open to the yolk sac but lateral folding thins the neck of the yolk sac and reduces the communication of the midgut with the yolk sac
-end of 6th week gut tube is fully formed and the neck of the yolk sac has been reduced to a slim stalk- vitelline duct

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