Embryology L7 PDF

Embryology L7 The Gut Tube and the Body Cavities  A TUBE ON TOP OF A TUBE During the third and fourth weeks, 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 by the process called neurulation....

Embryology L7 The Gut Tube and the Body Cavities  A TUBE ON TOP OF A TUBE During the third and fourth weeks, 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 by the process called neurulation. Almost simultaneously, the ventral layer (endoderm) rolls down to form the gut tube, such that the embryo consists Of a tube on top of a tube: the neural tube dorsally and the gut tube ventrally. The middle layer (mesoderm) hold the two tubes together and the lateral plate component of this mesoderm layer also splits into visceral (splanchnic) and parietal (somatic) layers. The visceral layer rolls ventrally and is intimately connected to the gut tube; the parietal layer, together with the overlying ectoderm, forms the lateral body wall folds (one on each side of the embryo), which move ventrally and meet in the midline to close the ventral body wall. 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 abdomino-pelvic regions.  FORMATION OF THE BODY CAVITY At the end of the third week, intraembryonic mesoderm differentiates into paraxial mesoderm, which forms somitomeres and somites that play a major role in forming the skull and vertebrae; intermediate mesoderm, which contributes to the urogenital system; and lateral plate mesoderm, which is involved in forming the body cavity. Soon after it forms as a solid mesodermal layer, clefts appear in the lateral plate mesoderm that coalesce to split the solid layer into two: (1) the parietal (somatic) layer adjacent to the surface ectoderm and continuous with the extra-embryonic parietal mesoderm layer over the amnion. Together, the parietal (somatic) layer of lateral plate mesoderm and overlying ectoderm are called the somatopleure; (2) the Visceral (splanchnic) layer adjacent to endoderm forming the gut tube and continuous with the visceral layer of extraembryonic mesoderm covering the yolk sac. Together, the visceral (splanchnic) layer of lateral plate mesoderm and underlying endoderm are called the splanchnopleure. The space created between the two layers of lateral plate mesoderm constitutes the primitive body cavity. During the fourth week, the sides of the embryo begin to grow ventrally forming two lateral body wall folds.  FORMATION OF THE BODY CAVITY These folds consist of the parietal layer of lateral plate mesoderm, overlying ectoderm, and cells from adjacent somites that migrate into the mesoderm layer across the lateral somitic frontier. As these folds progress, the endoderm layer also folds ventrally and closes to form the gut tube. By the end of the fourth week, the lateral body wall folds meet in the midline and fuse to close the ventral body wall. This closure is aided by growth of the head and tail regions (folds) that cause the embryo to curve into the fetal position. Closure of the ventral body wall is complete except in the region of the connecting stalk (future umbilical cord). Similarly, closure of the gut tube is complete except for a connection from the midgut region to the yolk sac called the Vitelline (yolk sac) duct. This duct is incorporated into the umbilical cord, becomes very narrow, and degenerates with the yolk sac between the second and third months of gestation. (Note that throughout the process of body cavity and gut tube development, the parietal and visceral layers of lateral plate mesoderm are continuous with each other at the junction of the gut tube with the posterior body wall)  SEROUS MEMBRANES Some cells of the parietal layer of lateral plate mesoderm lining the body wall of the primitive embryonic cavity become mesothelial and form the parietal layer of the serous membranes lining the outside of the peritoneal, pleural, and pericardial cavities. In a similar manner, some cells of the visceral layer of lateral plate mesoderm form the Visceral layer of the serous membranes covering the abdominal organs, lungs, and heart. Visceral and parietal layers are continuous with each other as the dorsal mesentery, which suspends the gut tube from the posterior body wall into the peritoneal cavity. Dorsal mesentery extends continuously from the caudal limit of the foregut to the end of the hindgut. Ventral mesentery exists only from the caudal foregut to the upper portion of the duodenum and results from thinning of mesoderm of the septum transversum, a block of mesoderm that forms connective tissue in the liver and forms a scaffold for forma- I tion of the diaphragm. these mesenteries are double layers of peritoneum that provide a pathway for blood vessels, nerves, and lymphatics to the organs. Clinical Correlates: Ventral Body Wall Defects  DIAPHRAGM AND THORACIC CAVITY the septum transversum is a thick plate of mesodermal tissue occupying the space between the thoracic cavity and the stalk of the yolk sac. The septum is derived from visceral (splanchnic) mesoderm surrounding the heart and assumes its position between the primitive thoracic and abdominal cavities when the cranial end of the embryo grows and curve into the fetal position. This septum does not separate the thoracic and abdominal cavities completely but leaves large openings, the pericardioperitoneal canals, on each side of the foregut. When lung buds begin to grow, they expand caudolaterally within the pericardioperitoneal canals. As a result of the rapid growth of the lungs, the pericardioperitoneal canals become too small, and the lungs begin to expand into the mesenchyme of the body wall dorsally, laterally, and ventrally. Ventral and lateral expansion is posterior to the pleuropericardial folds.  DIAPHRAGM AND THORACIC CAVITY At first, these folds appear as small ridges projecting into the primitive undivided thoracic cavity. With expansion of the lungs, mesoderm of the body wall forms two components: (1) the definitive wall of the thorax and (2) the pleuropericardial membranes, which are extensions of the pleuropericardial folds that contain the common cardinal veins and phrenic nerves. Subsequently, descent of the heart and positional changes of the sinus venosus shift the common cardinal veins toward the midline, and the pleuropericardial membranes are drawn out in mesentery-like fashion. Finally, they fuse with each other and with the root of the lungs, and the thoracic cavity is divided into the definitive pericardial cavity and two pleural cavities. In the adult, the pleuropericardial membranes form the fibrous pericardium.  FORMATION OF THE DIAPHRAGM Although the pleural cavities are separate from the pericardial cavity, they remain in open communication with the abdominal (peritoneal) cavity by way of the pericardioperitoneal canals. During further development, the opening between the prospective pleural and peritoneal cavities is partially closed by crescent-shaped folds, the pleuroperitoneal folds, which project into the caudal end of the pericardioperitoneal canals. By the seventh week, the folds become thinner to form the pleuroperitoneal membranes that grow medially to fuse with the mesentery of the esophagus that forms the crura of the diaphragm around the aorta. The pleuroperitoneal membranes then continue to grow until they fuse with each other and completely cover the septum transversum. These muscle cells originate from cervical segments three to five (C3—C5) to form the musculature of the diaphragm.  FORMATION OF THE DIAPHRAGM Innervation of the diaphragm is via the phrenic nerves that are derived from ventral primary rami of spinal nerves carrying motor and sensory fibers that also originate from C3 to C5. The reason that muscle cells and nerve fibers from cervical segments populate and innervate the diaphragm is because the cervical region is where diaphragm development originated in the fourth week. Later, in the sixth week, differential growth, whereby growth of the dorsal part of the embryo (brain and vertebral column) is greater than the ventral part, causes the diaphragm to move ventrally and caudally to its final location, resulting in the phrenic nerves taking a long course to reach their destination. In summary, the diaphragm is derived from the following structures: 1. The two pleuroperitoneal membranes form the central tendon of the diaphragm 2. Muscular components from somites at cervical segments three to five 3. The mesentery of the esophagus, in which the crura of the diaphragm develop SUMMARY THANK YOU！