Embryo+folding



Folding of Embryo, Basic Body Formation, Coelom Formation

1) Describe results of lateral and longitudinal folding processes that occur in embryo a. Folding in median plane i. Produces head and tail folds ii. Head fold 1. Beginning of 4th wk, neural folds in cranial region thicken to form primordium of brain 2. Initially, developing brain projects dorsally into amniotic cavity 3. Later, developing forebrain grows cranilally beyond oropharyngeal membrane and overhangs developing heart 4. Septum transversum, primordial heart, pericardial coelom and oropharyngeal membrane move onto ventral surface of embryo 5. During longitudinal folding, part of endoderm of yolk sac is incorporated into embryo as foregut a. Foregut lies between brain and heart 6. Before folding, embryonic coelom consists of a flattened, horseshoe-shaped cavity 7. After folding, pericardial coelom lies ventral to heart and cranial to septum transversum 8. After folding, septum transversum lies caudal to heart where subsequently develops into central tendon of diaphragm iii. Tail fold 1. Folding of caudal end results primarily from growth of distal part of neural tube 2. Caudal eminence (tail region) projects over cloacal membrane (anus) 3. Before folding, primitive streak lies cranial to cloacal membrane 4. After folding primitive streak lies caudal to cloacal membrane 5. During folding, part of endodermal germ layer incorporated into embryo as hindgut (primordium of descending colon) 6. Terminal part of hindgut dilates slightly to form cloaca (primordium of urinary bladder and rectum) 7. Connecting stalk attached to ventral surface of embryo and allantois (diverticulum of yolk sac) partially incorporated into embryo b. Folding in horizontal plane i. Folding of sides produces right and left lateral folds ii. Produced by rapidly growing spinal cord and somites iii. Primordia of ventral wall fold toward median plane, edges roll ventrally and form roughly cylindrical embryo iv. As abdominal wall forms, part of endoderm incorporated as midgut (primordium of small intestine) v. After folding, wide connection between midgut and yolk sac reduced to a yolk stalk vi. Region of attachment reduced to narrow umbilical region 1. Umbilical cord forms from connecting stalk 2) Define a. Coelom i. Cavity b. Mesentery i. Double layer of peritoneum c. Septum Transversum i. Thick plate of mesodermal tissue that occupies space between thoracic cavity and yolk stalk ii. Primordium of central tendon of diaphragm 3) Define and describe adult structures derived from primitive intraembryonic coelom a. Divided into the three body cavities i. Pericardial cavity ii. Pleural cavity iii. Peritoneal cavity b. Cavities have parietal wall lined by mesothelium i. Future parietal layer of peritoneum ii. Derived from somatic mesoderm c. Cavities have visceral wall covered by mesothelium i. Future visceral layer of peritoneum ii. Derived from splanchnic mesoderm 4) Describe/define origin of cells lining intraembryonic coelom and its definitive derivatives a. mesoderm b. gives rise to the three body cavities i. pericardial ii. two pericardioperitoneal canals iii. peritoneal cavity 5) Describe configuration of intraembryonic coelom following development of head fold a. Before head fold the intraembryonic coelom is a flattened horseshoe shaped cavity b. after folding the pericardial coelom lies ventral to the heart and cranial to the septum transversum. (see page 79 of textbook) 6) Describe/define a. Pleuropericardial membrane i. Partitions that separate pericardial cavity from pleural cavity ii. Contains common cardinal veins that drain primordial venous system into sinus venosus of primordial heart iii. Project into cranial ends pericardioperitoneal canals iv. Become mesentery-like folds extending from lateral b. Pleuroperitoneal membrane i. Pleuroperitoneal folds closes off boundary between pericardioperitoneal canals as it grows to meet central tendon. 7) Describe the relative position of the diaphragm during its early development. a. As diaphragm migrates downwardà costodiaphragmatic recesses formed. Notice heart as in contact with septum transversum, thus still have connection. 8) List the four major tissue sources contributing to the definitive diaphragm. Correlate each with the related area of the definitive diaphragm. a. Septum transversum i. Central tendon of the diaphragm b. Pleuroperitoneal membranes i. Primordial diaphragm c. Dorsal mesentery of esophagus i. Crura of the diaphram d. Muscular ingrowth from lateral body walls i. Costodiaphragmatic recesses 9) Describe the probable developmental errors that produce: a. congenital posterolateral diaphragmatic hernia i. caused by defective formation and/or fusion of pleuroperitoneal membrane with the rest of the diaphragm. Results in pulmonary hypoplasia and excess amniotic fluid. b. congenital hiatal hernia i. part of the stomach passing through an excessively large esophageal hiatus - the opening in the diaphragm where the esophagus and vagus nerves pass through c. congenital retrosternal hernia. i. commonly associated with body wall defects in umbilical region. Results in intestine pushing into pericardial sac or heart pushing into peritoneal sac.