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ANATOMY 6: FORMATION OF BODY CAVITIES AND DIAPHRAGM Review of Folding of Embryo: 4th week: transition from flat disc to hollow tubular structure faster proliferation of cells on upper surface of embryonic disk than lower surface bending/folding cranio-caudal: growth of nervous system lateral: growth of somites http://www.indiana.edu/~anat550/cvanim/index.html Review: Cranial Folding structures cranial to developing brain: oropharyngeal membrane, cardiogenic region, septum transversum folding causes cardiogenic region and septum transversum to be caudal to oropharyngeal membrane and located in the chest region septum transversum- most cranial; furthest from primitive streak diaphragm; connects cardiac field to amnion *think of envelop example* Day 27: formed primitive heart tube, starting to develop dorsal aorta, neural tube fusing Endoderm: epithelium internal organs and GI tract Ectoderm: epidermis, cutaneous glands, internal ear, lens, CNS, cells of PNS Review: Mesoderm Derivatives paraxial mesoderm: somites (next to notochord) intermediate mesoderm: urinary and reproductive tracts lateral mesoderm: o splits into 2 layers- upper continuous with amnion (parietal); and lower continuous with yolk sac (visceral) o parietal dermis of back, abdominal wall, outer linings of body cavities o visceral muscular layers and blood vessels of gut, circulatory system, external covering of internal organs Body Cavities pericardial cavity: around heart pleural cavities: one cavity around each lung peritoneal cavity: abdominal cavity; around abdominal viscera Formation of Intraembryonic Coelom (Early Body Cavity) formed from space created when lateral mesoderm splits into parietal and visceral layers forms as U shape around cranial embryonic disk continuous with chorionic cavity (extraembryonic coelom) embryo folds craniall- traps coelomic space between upper and lower layers embryo folds laterally- wraps around space to enclose space within body Divisions of Intraembryonic Coelom folds of tissue form and grow across coelomic space to separate regions pleuropericardial folds: between pericardial cavity and pericardioperitoneal canals pleuroperitoneal folds: separate the peritoneal cavity from the pericardioperitoneal canals Pleuropericardial folds form in intraembryonic coelom between developing heart and lungs grows medially separating 2 dorsolateral pleural cavities from single ventromedial pericardial cavity fuse with mediastinal meso around foregut to form pleuropericardial membrane Pleuroperitoneal folds form in dorsal aspec grow ventrally thin as they grow pleuroperitoneal membranes septum transversum: thick band of meso that translocates caudal to heart after folding pleuroperitoneal membranes- join septum transversum to separate pleural and peritoneal cavities Pericardial Cavity heart enlarges and pushes up into space of newly formed pericardial cavity fist into balloon analogy gives visceral and parietal pericardium Pericardium serous pericardium—from serosa (lining of early embryonic cavity – intraembryonic coelom) visceral pericardium- derived from visceral lateral mesoderm (floor of intraembryonic coelom) parietal pericardium- derived from parietal lateral mesoderm (roof of intraembryonic coelom) 1 ANATOMY 6: FORMATION OF BODY CAVITIES AND DIAPHRAGM parietal pericardium fuses with pleuropericardial folds (which become fibrous pericardium) separated by pericardial cavity Pleural Cavities dorsolateral to pericardial cavity lung buds form off developing trachea and push into pleural cavities as lungs grow- cavities expand lateral and ventral on either side of heart visceral pleura- derived from visceral lateral mesoderm parietal pleura- derived from parietal lateral mesoderm pleural cavity between Abdominal cavity parietal mesoderm lines elongating body wall visceral mesoderm adjacent to yok sac yolk sac shrinks visceral meso wraps around developing gut tube trapping b/n two visceral meso layers pleuroperitoneal folds form and join with septum transversum to separate pleural and peritoneal cavities 2 cavities fuse to form single cavity all GI structures develop in-between the 2 layers of visceral mesoderm membranes (intestines, spleen, pancreas, part of liver Peritoneum and Mesenteries parietal peritoneum: lines body wall visceral peritoneum: covers GI organs dorsal and ventral mesenteries- fused double layer of visceral peritoneum suspending and supporting developing gut tubes most of ventral mesentery breaks down single cavity gut suspended by dorsal mesentery coelom lined with epi derived from somatic mesoderm parietal pleura / pericardium / peritoneum organs in coelom covered with epi from splanchnic meso visceral pleura / pericardium / peritoneum where peritoneum reflects from body wall to gut tube bilayered mesenteries (important for innervation and vasculature) Formation of Diaphragm 4 embryonic components of diaphragm o septum transversum- forms central tendon grows dorsally to form pleuroperitoneal membranes (which are growing ventrally) o pleuroperitoneal membranes- form 2 small areas on either side of diaphragm o myoblasts- invade esophageal mesentery which elongates to form crura of diaphragm o muscle ingrowths- body wall musculature openings esophagus, aorta, vena cava pleuroperitoneal membranes- only small part of developed diaphragm greatest contribution: ingrowth of mm from body wall and central tendon innervation: when in cervical region, adjacent to 3-5th somites myoblasts and associated nerve fibers (3-5th spinal nn) from somites invade septum tranasversum phrenic n as diaphragm moves to thoracic position- brings cervical nn with it periphery innervated by thoracic nn because periphery is from ingrowth of muscles in body wall Malformation of Diaphragm congenital diaphragmatic hernia o pleuroperitoneal folds fail to form o pleuroperitoneal membrane fails to fuse with other components o mostly on L side o intestines, stomach, and liver may be found in thoracic cavity; respiratory distress Eventration- muscle of diaphragm not functional- intestine pushes upward but is not in thoracic cavity Major Developments Wk. 4: NT begins closing (rostral- day 26; caudal- day 28) Pharyngeal arches begin to form- primordia of many structures in face and neck Upper limb buds, otic pits, lens placodes, lower limb buds Heart pumps blood 2 ANATOMY 6: FORMATION OF BODY CAVITIES AND DIAPHRAGM 3