SEM_11_Digestive system and coelomic cavities_PARTE1.docx

Digestive system and coelomic cavities Learning objectives Digestive system: Describe the development of the oesophagus. Understand the development of the single-chambered and multi-chambered stomach. Describe the rotational events that give rise to the definitive shape and position of the st...

Digestive system and coelomic cavities Learning objectives Digestive system: Describe the development of the oesophagus. Understand the development of the single-chambered and multi-chambered stomach. Describe the rotational events that give rise to the definitive shape and position of the stomach. Describe the development of the liver, gallbladder and pancreas. Describe the changes that take place in the midgut to form the adult small and large intestines. Understand the comparative development of the intestinal tract in domestic mammals. Understand the development of the cloaca and its derivatives. Consider briefly some congenital abnormalities in the development of the digestive system. Coelomic cavities: Describe the development of the serous body cavities and diaphragm. Digestive system The digestive system consists of the digestive tract and associated organs like salivary glands, the liver and pancreas. The gastrointestinal tract (GIT) consists of a hollow muscular tube starting from the oral cavity, where food enters the mouth, continuing through the pharynx, oesophagus, stomach and intestines to the rectum and anus, where the food is expelled. In general, the digestive system is considered to be a derivative of the embryonic gut: https://sway.office.com/TvZvoGMPxmhzCiDk#content=3Tx2doaxpG1C2g - Early derivatives of the gut The endoderm of the foregut becomes the epithelial lining of the pharynx, oesophagus, stomach, cranial duodenum, and the digestive glands (liver and pancreas). The blind end of the foregut is closed by the oropharyngeal membrane which is lined by endoderm on the inside and the ectoderm on the outside. At the location of the oropharyngeal membrane, the ectoderm forms a depression, the stomodeum, which is the origin of the oral cavity and associated organs (tongue and salivary glands). As development progresses, the oropharyngeal membrane breaks down and the oral cavity becomes continuous with the pharynx. - Foregut derivatives The endoderm of the midgut becomes the epithelial lining of the remaining small intestines, caecum, ascending colon, and part of the transverse colon. - Midgut derivatives The endoderm of the hindgut becomes the epithelial lining of the transverse and descending colon. The rectum develops from the caudal end of the hindgut which is known as the cloaca. The blind end of the embryonic cloaca is temporarily closed by a cloacal membrane which is formed by the endoderm on the inside and cover by the ectoderm on the outside. At the level of the cloacal membrane, the ectoderm forms a shallow depression called proctodeum, which will be the origin of the anus. Rupture of the cloacal membrane will allow the digestive tract to communicate with the exterior. - Hindgut derivatives Besides the inner epithelial lining, the wall of the gut includes a surrounding layer of visceral mesoderm which gives rise to the smooth muscles and connective tissues of the digestive system. Because the oral cavity will be considered with the development of the head, and because the pharynx has already been studied, this chapter begins with the study of the oesophagus. Oesophagus The oesophagus (commonly known as the food pipe or gullet) develops from the foregut caudal to the pharynx. The endoderm is the origin of the epithelial lining of the oesophagus. Along its length, the endoderm is surrounded by mesoderm which gives rise to the muscular layer and connective tissues of the oesophagus. In the cranial portion of the oesophagus, the mesoderm is contributed by last pharyngeal arches. This branchial mesoderm differentiates into striated muscle innervated by the vagus nerve. The caudal part of the oesophagus is coated by smooth muscle differentiated from visceral mesoderm. The extent to which the oesophagus is surrounded by striated or smooth musculature offers important species-specific variations. In most species of birds, the oesophagus develops the crop, an expandable saccular diverticulum used as a short-term food reservoir for later digestion. When it is present, its size can vary from a simple expansion to a prominent oesophageal pouch in the neck. - Development of the oesophagus. The oesophagus is a long tube from the pharynx to the stomach. It can be divided into the cervical, thoracic and abdominal parts. Stomach The early development of the stomach is marked by a spindle-shaped dilatation in the caudal part of the foregut. Initially, this primordial stomach is attached to the abdominal wall both dorsally and ventrally by the dorsal mesentery and the ventral mesentery, respectively. The portion of the dorsal mesentery that attaches the stomach to the dorsal abdominal wall, gives rise to the dorsal mesogastrium, also referred to as the greater omentum which is inserted in the greater curvature of the stomach. The greater omentum enlarges and expands caudally into an ample apron-like fold that covers most of the intestines in the ventral abdominal wall. The portion of the ventral mesentery that attaches the stomach to the ventral abdominal wall, forms the ventral mesogastrium. By the subsequent growth of the liver, this ventral mesogastrium is divided into two parts: the lesser omentum between the lesser curvature of the stomach and the liver; ligaments of the liver (the falciform and coronary ligaments) between the liver and the abdominal wall and diaphragm. According to the feeding behaviour of the species, the embryonic stomach can remain as a single chamber (monogastric stomach) or gain in complexity to form a multi-chambered structure like in birds and ruminants. https://sway.office.com/TvZvoGMPxmhzCiDk#content=9lMPiaKB15cp02 - Formation of the stomach. The stomach comes from a dilation of the foregut between the oesophagus and the small intestine. The primitive dorsal mesentery keeps attached to the greater curvature of the stomach. This part is called the dorsal mesogastrium, also known as greater omentum. Back to the diaphragm, the ventral mesentery also remains attached to the lesser curvature of the stomach, forming the ventral mesogastrium also known as the lesser omentum. The ventral mesogastrium also connected to the liver which is interposed between the stomach and the ventral abdominal wall. Single-chambered stomach In monogastric animals, such as horses, pigs, carnivores or humans, the dorsal part of the fusiform stomach grows more rapidly than the ventral, leading to the formation of a flatten C-shaped sac with a convex greater curvature and a concave lesser curvature. The anatomical region into which the oesophagus opens is called the cardia. Further growth of the cranial aspect of the greater curvature gives rise to the fundus which lies above the level of the cardia. The large middle portion is termed the body and the most distal area is referred to as the pylorus. The endoderm is the origin of the epithelial lining of the stomach. Firstly, it is formed by simple columnar epithelium, later it exhibits species-specific regional differences. Simple columnar epithelium persists throughout the stomachs of carnivores, while in horses and pigs stratified squamous epithelium replaces columnar epithelium in defined gastric regions. In those regions of the stomach, where simple columnar epithelium persists, gastric glands develop from epithelial buds (glandular part). The regions covered with stratified squamous epithelium become non-glandular (non-glandular part). - Single-chambered stomach Image of the horse's stomach Two-chambered stomach: avian stomach Bird´s stomach consists of two chambers. The proventriculus or glandular stomach is the first chamber. It secretes a gastric acid for breaking down the food components. The second chamber is known as the ventriculus or gizzard. It is made of stratified squamous epithelium and heavy muscles for grinding the food. - Two-chambered stomach. Image of the digestive system in birds Four-chambered stomach: ruminant stomach The ruminant stomach consists of three compartments lined by stratified squamous epithelium (rumen, reticulum, and omasum) and one glandular compartment (abomasum). The early development of the ruminant stomach is the same as the simple stomach; subsequently, a series of dilations increase the final number of compartments. The rumen develops as an expansion of the fundus and greater curvature; the reticulum arises as a ventral pocket of the developing rumen; the omasum develops as a bulge along the lesser curvature; the abomasum develops from the pyloric portion. - Four-chambered stomach ruminant stomach Ruminant animals are characterized by the development of a complex stomach which includes various compartments. Commonly, they are referred to as polygastrics but this term could somehow be misleading since these compartments come from pouches and dilatations of a simple stomach. Stomach rotations Initially, the stomach is located in the midline, adjacent to the diaphragm in the cranial part of the abdomen, and dorsal to the developing liver which lies ventrally. In order to assume its final position in the abdominal cavity, the stomach undergoes two successive rotations. First rotation: The stomach rotates 90° around a craniocaudal axis to the left-hand side of the embryo while the liver is shifted to the right-hand side. After the first rotation, the greater curvature of the stomach is moved towards the left abdominal wall, while the liver lies adjacent to the right abdominal wall. - The first rotation of the stomach In order to assume its final position, the stomach undergoes two rotations that also affect other organs anchored to the mesogastrium, such as the liver, the spleen and the pancreas. From a caudal view, the first rotation is 90º counter-clockwise around a caudo-cranial axis. the stomach is dragged toward the lefthand side while the liver is pushed toward the righthand side. Second rotation: The stomach turns about 45º around a dorso-ventral axis, positioning the stomach transversally in the abdominal cavity. The caudal portion of the stomach, which is the pylorus and proximal duodenum, moves from its left position to the right-hand side of the abdomen, next to the liver. This rotation establishes the transverse orientation of the stomach in adults, with its cranial portions to the left and the caudal portions to the right. - The second rotation of the stomach From a caudal view, the second rotation is 45º clockwise around a dorso-ventral axis. The second rotation drags the pylorus toward the right side close to the liver. Liver The liver develops from the last portion of the foregut that will form the beginning of the intestinal tract (the descending duodenum). An outgrowth of the endoderm gives rise an evagination known as the hepatic diverticulum. Near the duodenum, the hepatic diverticulum splits into two parts. The hepatic part (pars hepatica) grows and branches inside of the former haematopoietic sinus; after hollowing its initial solid core, these branches give rise to the intrahepatic bile ducts, while the cells of their walls differentiate into the liver cells or hepatocytes. The cystic part (pars cystica) grows adjacent to the liver, without penetrating inside, and turns into the gall bladder that keeps connected to the common bile duct by a cystic duct. Some animals (rats, horses, camels, elephants, rhinoceros and etc.) and birds (pigeons, turtle-doves, ostriches and etc.) do not develop the cystic part and therefore, the gall bladder is not present in those animals. The hepatic diverticulum will remain attached to the duodenum forming the common bile duct, which opens into the major duodenal papilla joined with the ventral pancreatic duct. https://sway.office.com/TvZvoGMPxmhzCiDk#content=yAYoSCeBiM5aWy - Development of the liver The liver develops from an endodermal diverticulum that arises in the ventral aspect of the caudal region of the foregut. The diverticulum divides into the hepatic bud, which branches into the developing hepatic cords, and the cystic part, which turns into the gallbladder. Pancreas The pancreas originates from two separate endodermal evaginations in the caudal end of the foregut (duodenum) known as pancreatic buds or pancreatic diverticula. The ventral pancreatic bud develops as an outgrowth of the hepatic diverticulum, that grows attached to the ventral mesogastrium, near the liver and descending duodenum. The pancreatic duct originated from this ventral pancreatic portion is referred to as the pancreatic duct, which joins the common bile duct before opening into the major duodenal papilla. The dorsal pancreatic bud, from which much of the parts of the pancreas are originated, arises as an outpouching of the dorsal surface of the future duodenum at the caudal end of the foregut. It grows between the layers of the dorsal mesogastrium near the stomach. The duct of the dorsal portion of the pancreas, which is called the accessory pancreatic duct, opens into the minor duodenal papilla. As a consequence of the rotation of the stomach and liver, the dorsal and ventral pancreatic buds overlap and fuse to form a single anatomical structure, the pancreas, consisting of a pancreatic body (the common fused part) and two lobes: the right pancreatic lobe (derived from the ventral bud), and the left pancreatic lobe (derived from the dorsal bud). Also, the two pancreatic ducts anastomose to form a common drainage system. This allows one pancreatic duct to become smaller than the other or even disappear. Which pancreatic duct is destined to become smaller or absent depends on the species. In humans, horses and dogs, both ducts persist in their entirety, but in the latter, the accessory pancreatic duct is the larger one. The pancreatic duct is not found in cattle and pigs while the accesory pancreatic duct is absent in small ruminants and cats. The endodermal cells of the pancreatic buds proliferate in a glandular-fashion way giving rise to the pancreatic ducts and associated secretory acini of the pancreas. These structures constitute the exocrine part of the pancreas. Some endodermal cells which lose their connections with the duct system, develop into the endocrine cells of the pancreas. These clusters of endodermal cells form the pancreatic islets (islets of Langerhans) which gradually differentiate into glucagon, insulin and somatostatin producing cell populations. https://sway.office.com/TvZvoGMPxmhzCiDk#content=CIhSS55mT0pCRD - Development of the pancreas The pancreas develops from dorsal and ventral endodermal outgrows of the caudal region of the foregut. The ventral pancreatic bud arises in a common outgrowth with the hepatic diverticulum. The rotation of the stomach affects the pancreatic buds so the dorsal and ventral pancreatic buds overlap and fuse in a single organ consisting of a body and left and right lobes.