University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2021
Views:

294

Pages:

29
Author:

lankstoetp11

A tube on top of a tube !! During 3rd - 4th wks: q The top layer (ectoderm) of the trilaminar embryonic disc forms the neural plate that rolls up into a tube to form the brain and spinal cord. q Simultaneously, the ventral layer (endoderm) forms the gut tube. “Thus, the neural tube dorsally and the gut tube ventrally”. A tube on top of a tube !! The middle layer (mesoderm) holds the two tubes together and the lateral plate component of this mesoderm layer also splits into visceral (splanchnic) and parietal (somatic) layers. Ø The visceral layer rolls ventrally and connected to the gut tube. Ø The parietal layer, together with the overlying ectoderm, forms the lateral body wall folds, which meet ventrally in the midline to close the ventral body wall. Ø The space between visceral and parietal layers of lateral plate mesoderm is the primitive body cavity, which at this early stage is a continuous cavity, because it has not yet been subdivided into the pericardial, pleural, and abdominopelvic regions. Intraembryonic Coelom • The primordium of the embryonic body cavity. • Appears as a horseshoe-shaped cavity in the cardiogenic area and lateral mesoderm by the 4th week. • The bend in this cavity indicates the future pericardial cavity & the limbs indicate the future pleural and peritoneal cavities. • The distal part of each limb opens laterally into extra-embryonic coelom (EEC). • During cranial folding of embryo, the pericardial cavity becomes ventral to the foregut. • The pericardioperitoneal canals: • Pass on each side of the foregut (future esophagus) • Lie dorsal to septum transversum • Open into the peritoneal cavity • During horizontal folding, the limbs of the coelom are brought together on the ventral aspect of the embryo. • The coelom is lined by mesothelium derived from the somatic mesoderm (parietal layer) and the splanchnic mesoderm (visceral layer). • The peritoneal cavity loses its connection with the extraembryonic coelom during the 10th week. Parietal layer Visceral layer Division of Embryonic Body Cavity • Par??ons form in each pericardioperitoneal canal, separa?ng the pericardial cavity from the pleural cavi?es, and the pleural cavi?es from peritoneal cavity. • As the bronchial buds grow into the pericardioperitoneal canals, a pair of membranous ridges is produced in the lateral wall of each canal: § Pleuropericardial folds superior to the developing lungs. § Pleuroperitoneal folds inferior to the developing lungs. Pleuropericardial Membranes • As the pleuropericardial folds enlarge, they form partitions that separate the pericardial cavity from the pleural cavities. • The bronchial buds grow laterally from the caudal end of the trachea into the pericardioperitoneal canals (future pleural cavities). • As the pleural cavities expand ventrally, they grow into the body wall in the angle between the body wall and a ridge raised by the common cardinal vein and the phrenic nerve. • The expansion of the pleural cavities results in splitting the mesenchyme into: § An outer layer that forms the thoracic wall. § An inner layer that forms the pleuropericardial membrane (becomes the fibrous pericardium). The pleuropericardial membranes project into the cranial end of the pericardioperitoneal canals. • With the growth & descent of the heart and expansion of the pleural cavi?es, the pleuropericardial membranes expand & move medially. 7th week 8th week By 7th week, the membranes fuse with the mesenchyme ventral to the esophagus forming the primordial mediastinum, thus, closing the pleuropericardial openings. Phrenic nerve The fused pleuropericardial membranes form the fibrous pericardium. (Note the position of phrenic nerve in the fibrous pericardium). Pleuroperitoneal Membranes • Develop from the pleuroperitoneal folds that are attached dorsolaterally to the abdominal wall and their free edges project into the caudal end of the pericardioperitoneal canals. • As the developing lung enlarges cranially and liver expands caudally, these folds become more prominent and gradually become membranous. • Are soon invaded by the myoblasts (primitive muscle cells). • During 6th week, the pleuroperitoneal membranes extend ventromedially and fuse with the dorsal mesentery of the esophagus and the septum transversum. This results in closure of the pericardioperitoneal openings. The right opening closes slightly earlier than the left. Mesenteries Ventral mesentery After embryonic folding • The caudal part of the foregut is connected to the anterior and posterior abdominal walls by the ventral & dorsal mesentery respectively. Ventral mesentery • The midgut and the hindgut are suspended in the peritoneal cavity from the posterior abdominal wall by the dorsal mesentery. • The ventral mesentery degenerates in the region of the future peritoneal cavity, extending from the heart to the pelvic region. What is a mesentery? • Double layer of peritoneum enclosing a mass of mesoderm. • Connects the organ to the body wall • Carries vessels, nerves & lymphatics for the organ. • Is the site where the visceral (Splanchnic) peritoneum continues as parietal (Somatic) peritoneum Development of Diaphragm q The diaphragm develops from four embryonic components: 1. 2. 3. 4. Septum transversum Pleuroperitoneal membranes Dorsal mesentery of esophagus Muscular ingrowth from lateral body walls 1 3 2 4 Transverse section of a 12-week embryo Septum Transversum • A mesodermal tissue • Located: § Between the pericardial cavity and the yolk sac. § Ventral to the foregut and the pleuroperitoneal membrane • Grows dorsally from the ventrolateral body wall. yolk sac • Forms an incomplete partition between the thoracic cavity and the abdominal cavity. • Expands and fuses with the pleuroperitoneal membranes and the mesenchyme ventral to the esophagus. Septum transversum is the primordium of the central tendon of the diaphragm. • During 6th week, the three basic components: 1. Septum transversum 2. Pleuroperitoneal membranes 3. Mesentery of esophagus 1 3 Fuse with each other and form a complete partition between the thoracic and abdominal cavities 2 4 • During 9th – 12th weeks the lungs and pleural cavities enlarge, burrowing into the body wall, splitting it into: § External layer that becomes part of the body wall. § Internal layer that contributes muscles to peripheral portions of diaphragm, external to the parts derived from the pleuroperitoneal membranes. Septum transversum: Central tendon Pleuroperitoneal membranes: form large portion of fetal diaphragm but represent a smaller portion in infants. Dorsal mesentery of esophagus: Crura Body wall: peripheral muscular part Positional Changes & Innervation of Diaphragm • During the 4th week, the septum transversum lies opposite the 3rd – 5th cervical somites. • During 5th week, myoblasts from these somites move to the developing diaphragm bringing their nerve fibers with them (phrenic nerve). • Thus, because the the diaphragm development originated in the cervical region, it is innervated by phrenic nerve C3-5. • Rapid growth of the body of embryo results in further descent of diaphragm • By the 6th week, the diaphragm lies at the level of the thoracic somites. • By the end of 8th week the dorsal end of diaphragm lies at the level of first lumbar vertebra. • When the 4 parts of the diaphragm fuse, the mesenchymal cells from the septum transversum extend into the other three parts, change into myoblasts, and give rise to the muscles of the diaphragm. Thus, phrenic nerve supplies all the muscles of diaphragm The phrenic nerve also supplies sensory fibers to diaphragm except in the peripheral region which is derived from the body wall and brings its nerve supply (lower intercostal nerves) with it. Congenital Anomalies Congenital hiatal hernia: because of large esophageal hiatus Congenital diaphragmatic hernia: Commonly through a posterolateral defect in diaphragm. Mostly on left side. Left lung shows hypoplasia Eventration of diaphragm: because of defective musculature

Body Cavities, Mesenteries & Diaphragm

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