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Body cavities Formation of body cavities • At ^ end of ^ 3rd week: ^ lateral plate mesoderm which is involved in ^ formation of body cavity, it will differentiate into 2 layers ( a) ^ parietal (somatic) layer adjacent to ^ surface ectoderm & continuous with extra-embryonic parietal mesoderm layer covering ^ amnion. (b) ^ visceral (splanchnic) layer adjacent to ^ endoderm & continuous with^ visceral layer of extra-embryonic mesoderm covering ^ yolk sac. • During ^ 4th week, ^ sides of ^ embryo begin to grow ventrally forming 2 lateral body wall folds. These folds consist of ^ parietal layer of lateral plate mesoderm. ^ endoderm also folds ventrally & closes to form ^ gut tube. • By ^ end of 4th week, ^ lateral body wall folds meet in ^ midline & fuse to close ^ ventral body wall. This closure is aided by head & tail folds that cause ^ embryo to curve into ^ fetal position. • Closure of ^ ventral body wall is completed except in ^ connecting stalk, similarly closure of ^ gut tube is completed except for vitelline duct. Serous membranes • Cells of ^ parietal layer of lateral plate mesoderm lining ^ intra-embryonic cavity become mesothelial & form the parietal layer of ^ serous membranes lining ^ outside of ^ peritoneal, pleural, & pericardial cavities. • Similarly, cells of ^ visceral layer of lateral plate mesoderm form the visceral layer of ^ serous membranes covering ^ abdominal organs, lungs, & heart. • Visceral & parietal layers are continuous with each other at the dorsal mesentery . Which suspends ^ gut tube rom ^ posterior body wall into ^ peritoneal cavity. • Dorsal mesentery extends from ^ caudal limit of ^ foregut to ^ end of hindgut. • Ventral mesentery exists only from ^ caudal foregut to ^ upper portion of ^ duodenum & results from thinning of mesoderm of septum transversum. • These mesenteries are double layers of peritoneum that provide a pathway for blood vessels, nerves,& lymphatics to ^ organs. • ^ septum transversum is a thick plate of mesoderm occupying ^ space between ^ thoracic cavity & ^ stalk of yolk sac. Diaphragm & thoracic cavity • Diaphragm divides ^ body cavity into ^ thoracic & peritoneal cavities. • It develops from 4 components:(a) septum transversum (central tendon) (b) pleuroperitoneal membranes. (c) dorsal mesentery of ^ esophagus, & (d) muscular components from somites at cervical level s three to five (C3-5) of ^ body wall. • Since ^ septum transversum is located initially opposite cervical segments three-five & since muscle cells for ^ diaphragm originate from somites at these segments, ^ phrenic nerve also arises from these segments of ^ ^ spinal cord (C3,4,&5) keep ^ diaphragm alive. • Congenital diaphragmatic hernias involving a defect of ^ pleuroperitoneal membrane on ^ left side occur frequently. • ^ thoracic cavity is divided into ^ pericardial cavity & two pleural cavities for ^ lungs by ^ pleural cavities for ^ lungs by ^ pleuropericardial membranes. • Initially, ^ gut tube from ^ caudal end of ^ foregut to ^ end of ^ hindgut is suspended from ^ dorsal body wall by dorsal mesentery.