!Objectives: 1) Describe the tube within a tube body plan 2) Describe derivatives of the mesodermal germ layer 3) Describe how folding creates foregut, midgut, and hindgut 1) Tube within a tube body plan - outer ectodermal tube and inner endodermal tube!!3) Folding-->foregut, midgut, and hindgut - At end of the 3rd week the embryo was a flat, ovoid, trilaminar disc!- During the 4th week, the embryo folds which generates the verterbrate body forrm!- Main force for this= differential growth of various structures: embryonic disk and the amnio grow a lot, but the yolk sac doesn't (it's like mushroom top growing around)!- Cranial to the oropharyngeal membrane we get a U shaped cardiogeneic area--> will give rise to the heart. Curve= future pericardial cavity and sides= future pleural and peritoneal cavities!- the septum transversum forms from the cranial to cardiogenic area-->it separates the coelom into thoracic and abdominal cavities-->give rise to part of diaphragm!- Embryo undergoes cranio-caudal and lateral folding, begins on day 22/23!- Dorsal axis of the embryo is stiffened by the notochord, the neural tube, and the somites!-->most of the folding is in the thin and flexible outer rim of the embryonic disc!- Cranial-caudal folding - forward growth of the neural plate causing the disc to fold under!--> forms the ventral surface of the future face, neck, and chest!- heart going into the chest!- the oropharyngeal membrane moves to the regio of the future mouth and brings the cardiogenic area (U shaped part at head) and the septum transversum toward future chest - part of the def. yolk sac is incorporated into the embryo as the foregut!- Day 23: caudal rim of embry. disc (with cloacal membrane) folds under-->becomes part of ventral surface of embryo!- during caudal folding, part of the yolk sac is also incorp. and becomes the hindgut - the connecting stalk is moved cranially until it merges w/the neck of the yolk sacc (-->to form the umbilicis!!- Lateral Folding - so the R and L sides of the embryonic disc flex vertrally very sharply-->constricts and narrows the neck of the yolk sac!- at the head and tail ends, the lateral edges contact each other and zip up towards the site of the future umbilicis - Main idea= the cranial, caudal, and lateral margins of the disc fold completely under the dorsal axial structures and give rise to the ventral surface of the body !- Clinical correlation: Anterior body wall defects!- fusion doesn't go as expected, failure of the anterior body wall to form properly!- portion of the GI system herniates beyond the anterior wall!- omphalocele= bowel is membrane covered!- gastroschisis= bowel protrudes through the body wall!- in thoracic wall, ectopia cordis= heart exposed on the surface!!2) Germ layers and derivatives!- ectoderm= epidermis, CNS and PNS, eyes, internal ears, many connective tissues in head - endoderm= epithelial linings of resp. and digestive tracts - mesoderm= all connective tissues in the trunk, skeletal muscles, blood cells and vessels, all visceral smooth muscle and serosal linings of all body cavities, reproductive and excretory organs, most of cardivascular system!- around the notochord there are developing tissues of the mesoderm!- paraxial mesoderm= immediately lateral to notochord - intermediate mesoderm= lateral to paraxial mesoderm!- forms only in the trunk!- produces the urinary system and parts of genital system - lateral plate mesoderm= lateral to intermediate mesoderm!- in trunk only: stays unsegmented and forms a flattened sheet!- splits into two layers:!1) splanchnic mesoderm: ventral layer assoc. w/the mesoderm, -->viscera (inner organs) coverings and part of visceral wall!- intraembryonic splanchic mesoderm + assoc. endoderm= splanchnopleure 2) somatic mesoderm: dorsal layer assoc. w/ectoderm, -->inner lining of body wall and toparts of limbs!- intramembryonic somatic mesoderm + assoc. ectoderm= somatopleure!!1) Tube w/in a tube cont. Formation of Gut Tube - space between somatic and splanchnic mesoderm is originally open to the chorionic cavity!- when the folds of the embryo fuse, the space is enclosed and becomes the intraembryonic coelem!- the serous membranes lining the cavity form from the 2 layers of the lateral plate mesoderm: inside of the body wall is lined w/somatic mesoderm and visceral organs covered on inside by splanchhic mesoderm - the yolk sac is getting incorporated as this folding occurs!- when the cranial, caudal, and lateral edges of the embryo all meet and fuse, the cranial and caudal portions are converted into blind-ending tubes--future foregut and hindgut!- gut tube= fully formed by end of 6th week!- oropharyngeal membrane ruptures at end of 4th week!- cloacal membrane ruptures during 7th week!- as a result of all the folding-->the tube w/in a tube body plan is est.!- emrbyo= 2 main tubes: an outer ectodermal tube forming the skin and the inner endodermal tube form the gut!- space between the 2 tubes is filled w/mesoderm!- during 4th week= subdivide coelom into:!- pericardial cavity!- peitoneal cavity!- pleural cavity!!4th week of development outline - Day 21: embryo is a trilaminar germ disc!- Day 22: embryonic folding begins!- Day 28: folding is
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