SEM_11_Digestive system and coelomic cavities_PARTE2.docx

Intestinal tract The intestinal tract consists of duodenum (with a U-shaped disposition starting next to the liver; it comprises the descending and ascending duodenum), jejunum (characterised by many loops), ileum (the portion ending into the caecum), caecum -also spelled cecum- (blind sac diverticu...

Intestinal tract The intestinal tract consists of duodenum (with a U-shaped disposition starting next to the liver; it comprises the descending and ascending duodenum), jejunum (characterised by many loops), ileum (the portion ending into the caecum), caecum -also spelled cecum- (blind sac diverticulum at the beginning of the colon), colon (U-shaped portion composed of ascending, transverse and descending colon), rectum, and anal canal. The caudal end of the foregut gives rise to the first part of the duodenum which is fixed to the liver via the bile duct. The midgut differentiates into the caudal part of the duodenum, jejunum, ileum, caecum, ascending colon and the first two-thirds of the transverse colon. The hindgut forms the caudal third of the transverse colon and the rectum. In a highly summarised description, besides the elongation of the tubular embryonic gut, the formation of the intestinal tract comprises the following main morphogenic events: The part of the midgut where the yolk sac is attached elongates quickly giving rise to the formation of a midgut loop which includes the mesentery artery at its centre. At this stage, the midgut undergoes a rapid phase of growth, so that the intestinal loop is displaced out of the abdominal cavity through the umbilical cord. This process is referred to as physiological umbilical herniation which is a normal occurrence during embryo development. Later, when the haematopoiesis moves to the bone marrow and the liver decreases in size, the intestines return to the abdominal cavity. Because the ventral mesentery is reabsorbed and quickly disappears, the whole gut including the rectum is only attached to the roof of the abdominal cavity by a dorsal mesentery. The main arterial supply to the intestine is the cranial mesenteric artery which runs longitudinally in the centre of the dorsal mesentery. As the embryo grows, the caecum develops as an outgrowth of the caudal limb of the midgut loop. The caecum establishes a morphological landmark to separate the small intestine from the large intestine. At a certain stage of development, the midgut loop rotates clockwise around the mesentery artery which can be considered as an imaginary dorso-ventral axis for intestinal rotation. In a first phase, the midgut loop rotates 180° so that the descending limb is displaced caudally, and the ascending limb is displaced cranially. The reduction of the umbilical hernia coincides with an additional rotation of the intestine around the mesentery artery so that the full intestinal rotation is about 360º. The intestinal rotation twists the dorsal mesentery around the mesenteric artery at the roof of the abdominal cavity forming the so-called root of the mesentery in adults. Although the shape and final position of the intestine is species-specific, there are some common features that can be drawn from this shared basic mechanism of intestinal rotation: The cranial part of the midgut loop gives rise to the U-shaped duodenum which is bent around the mesentery artery by the intestinal rotation. Because the cranial part of the duodenum is attached to the liver by the common bile duct, it begins in the right side of the abdominal cavity, near the liver; then, the descending duodenum stretches caudally along the right side; finally, the ascending duodenum turns cranially in a short final part. The jejunum is the more mobile part of the gut; it develops many coils and intestinal loops that occupy most portions of the ventral abdominal cavity. The ileum is a short portion located next to the caecum. The caecum becomes positioned in the right-hand side of the caudal abdomen, near the ilium bone. https://sway.office.com/TvZvoGMPxmhzCiDk#content=fBXlQXvm0UW55L - Formation of the midgut loop and rotation of the gut. The rapid lengthening of the midgut leads to the formation of the primitive loop that outgrows the available space in the abdominal cavity and escapes through the umbilical cord into the extra-embryonic coelom. This process is referred to as physiological umbilical herniation. The midgut loop rotates clockwise around a dorso-ventral axis formed by the cranial mesentery artery so that the developing gut returns to the abdominal cavity where they are accommodated. The last part of the midgut loop forms a U-shaped colon which is curved around the mesentery artery, like the duodenum but in the opposite direction. The initial part of the colon begins near the ilium bone as a continuation of the caecum, in the right-hand side of the caudal abdomen. From there, the ascending colon runs cranially; then, the transverse colon crosses from the right side to the left side of the abdomen; finally, the descending colon stretches caudally toward the rectum along the left side of the abdominal cavity. The evolutionary intestinal complexity among species is clearly associated with the wide array of feeding behaviours. For instance, carnivores have shorter and simpler gastrointestinal tracts compared to herbivores, which usually have longer and more compartmentalised digestive tracts. Among the latter, the most outstanding differences reside in the ascending colon and caecum. In horses, the ascending colon enlarges and bends over itself giving rise to a U- shaped loop which is the origin of the four distinct parts in the equine ascending colon. In pigs and ruminants, additional loops of the ascending colon give rise to a spiral colon that adopts a coiled arrangement. - Development of the large intestine in carnivores In carnivores, the intestinal tracts remain short and simple - Development of the large intestine in horses In horses, the ascending colon expands to form a long narrow U-shaped loop in which the two limbs are kept tightly apposed by the mesocolon. - Development of the large intestine in ruminants In ruminants, the ascending colon enlarges and coils to form the spiral colon Intestinal congenital anomalies Failure in the vitelline duct involution can result in a) Umbilical fistula: a hollow connection between the jejunum and umbilicus as a result of a persistent vitelline duct; b) Meckel´s diverticulum: a blind-ended diverticulum in the jejunum; c) Vitelline cyst: a closed sac next to the intestines or to the umbilicus; The intestinal tract and oesophagus normally undergo temporary atresia (no lumen) during development because of epithelial proliferation. Re-canalisation occurs by formation of vacuoles that coalesce to form the ultimate lumen. Congenital intestinal atresia (failure to re-canalize) or stenosis (narrow lumen) is a congenital anomaly that can occur at localized sites anywhere along the oesophagus or intestines. Congenital umbilical hernia. A congenital umbilical hernia is a congenital malformation of the navel (umbilicus). An umbilical hernia occurs when part of the intestine protrudes through the umbilical opening in the abdominal muscles. Omphalocele. It is a rare abdominal wall defect in which the intestines, liver, and occasionally other organs remain outside of the abdomen in a sac because of a defect in the development of the muscles of the abdominal wall. - Congenital umbilical fistula and Meckel´s diverticulum - Umbilical hernia - Omphalocele in a newborn dog Cloacal derivatives The cloaca is the enlarged end of the hindgut which receives the ending parts of the urogenital system (nephric ducts and the Müllerian ducts) and digestive system (colon). At the caudal end of the cloaca, the endoderm apposes to the surface ectoderm to form the cloacal membrane which eventually breaks down. In birds, reptiles, and amphibians, the cloaca persists throughout adult life as a common chamber for urogenital and digestive systems. In mammals, the cloaca is divided at an early stage of development into two chambers: the rectum and urogenital sinus. The partition of the cloaca is carried out by an urorectal septum that splits the cloaca into a dorsal gastrointestinal component (rectum and anal canal) and ventral renal/genital component (urogenital sinus). The urorectal septum eventually becomes part of the perineal body: the muscular and fibrous tissues between the rectum and the urogenital organs. https://sway.office.com/TvZvoGMPxmhzCiDk#content=Q8pQHyxuOj9PyE - Development of the cloaca The initial cloaca is the common early endoderm lined space of the hindgut that will later become partitioned by a septum into a dorsal gastrointestinal component (rectum) and ventral renal/genital component (urogenital sinus). The dorsal chamber, which is continuous with the hindgut, becomes the rectum and anal canal, including the anal membrane. Eventually, the anal membrane degenerates but if the tissue closing the anus persists after birth it will result in a congenital defect called anal atresia or imperforate anus. In such a case, communication may develop between the blind anus and the urethra (rectourethral fistula) or the vagina (rectovaginal fistula), or, rarely, the bladder. The ventral chamber is called the urogenital sinus and it is continuous with the allantois. The urogenital sinus is the origin of the urinary bladder and urethra, which will be considered in the next chapter. - 3a) Normal development of the anus. 3b y 3c) Anal atresia: the tissue closing the anus may be a thin membrane of skin (type I) or several centimetres thick (type II). 3d) Anal atresia type III. The rectum ends in a blind pouch before entering the pelvis. A fistula often extends from the rectal pouch to the perineum or the urethra in males and to the vagina in females (3e). Serous body cavities The primary lateral mesoderm quickly split into two sheets, the lateral parietal (somatic) mesoderm and medial visceral (splanchnic) mesoderm. The intervening space or cavity bounded by the two sheets of the lateral mesoderm is referred to as the coelom. The coelomic epithelium gives rise to the serous epithelium, or simply serous membranes, which limits and covers the surface of the coelomic; subsequently, the coelom is divided to give rise to the body cavities: peritoneal cavity, pleural cavity and pericardial cavity. https://sway.office.com/TvZvoGMPxmhzCiDk#content=2PczWm0ZUKDdtb - Coelomic cavities In the early embryo, the intra-embryonic coelom forms as a single cavity appearing in the lateral plate mesoderm. This single cavity (coelom) is limited by the somatic and splanchnic mesoderm and will later be portioned into the three main body cavities: pericardial, pleural and peritoneal cavities. Peritoneal cavity Before the embryonic body begins to fold, the coelom is arranged as two right and left coelomic cavities that extend from thoracic levels to the pelvis. When the wall of the body closes ventrally, the left and right medial sheets meet in the midline and fuse dorsal and ventrally to the intestine that is trapped in the middle. In this way, the intestine is held between the right and left coelomic cavities by a dorsal sheet, or dorsal mesentery, which extends to the ceiling of the abdomen, and by a ventral sheet, or ventral mesentery, which extends to the bottom of the abdomen. At a certain stage of development, most of the ventral mesentery disappeared in the middle intestine region. Therefore, the left and right celomic cavities become openly connected in the ventral part of the abdomen, resulting in a single definitive cavity that is called the peritoneal cavity. The coelomic epithelium is transformed into the abdominal serous membrane that is called the peritoneum, which lines the abdominal serous cavity or peritoneal cavity. The descent of the testicles from the abdomen into the scrotum is accompanied by a pouch of the peritoneum called the vaginal cavity that is lined by expansions of the peritoneum into the inguinal rings (vaginal tunic, "tunica vaginallis" in Latin). https://sway.office.com/TvZvoGMPxmhzCiDk#content=Z5uzPsECdhB1vV - Peritoneal cavity. In the abdominal region, the gut is trapped between the left and right intra-embryonic coeloms, which surround the developing gut and form the mesenteries and other visceral ligaments. Atrophy of the ventral mesentery results in the coalescence of the left and right coelomic cavities forming a single peritoneal cavity lined by the peritoneum. Pericardial and pleural cavities In the embryonic thorax, the ventral part of the coelomic cavity constitutes the pericardial cavity which is dorsally connected with the bilateral pleural cavities. Initially, pleural cavities are small spaces into which lung buds project. As the lungs grow, pleural cavities enlarge and carve into the body wall. The coelomic wall that surrounds the lung is transformed into the pleura while the coelomic part that envelops the heart forms the pericardium. Eventually, the ventral pericardial cavity and the dorsal pleural cavities are separated by the growth of two bilateral pleuropericardial folds which appear as small membranes or ridges of mesenchyme projecting into the primitive undivided thoracic cavity. These membranes fuse in the midline with the mesoderm that surrounds the oesophagus, nerves and vessels that run along the thorax between both pleural cavities. All these structures that remain in the middle are not lined by any serous membrane and constitute what is called the mediastinum. https://sway.office.com/TvZvoGMPxmhzCiDk#content=tBtWobkVYcq0Sy - Separation of the pleural and pericardial cavities. Dorsally, the developing lungs protrude into the pleural cavities and thus become surrounded by the pleura. Ventrally, the heart is initially trapped in the middle between the right and left pericardial cavities but the atrophy of the mesocardium results in the formation of a single pericardial cavity. Formation of the diaphragm The coelomic cavities are transversally divided by the formation of the diaphragm, a domed musculotendinous structure with a critical role in respiration. The formation of the diaphragm establishes a transversal partition between the abdominal cavity, which accommodate the peritoneal cavity, and the thoracic cavity, which hold the pericardial and pleural cavities. The diaphragm is a complex structure, which components arise from different parts of the embryo: In the ventral part of the embryo, the septum transversum forms an initial barrier separating the heart from the liver. This septum grows from the ventral wall of the embryo as a thin, mesodermal partition between the thorax and the abdomen. In the dorsal part of the embryo, the abdominal cavity (peritoneal cavity) remains temporarily connected to the thoracic cavities (pleural cavities) through bilateral pleuroperitoneal canals. Later, a pair of pleuroperitoneal folds grow from the dorsal wall to meet the septum transversum. The median portion of the diaphragm develops from the dorsal oesophageal mesentery (mesoesophagus). The crura of the diaphragm develop from muscle fibres that grow into the oesophageal mesentery. The peripheral parts of the developing diaphragm is formed by muscular fibres migrated from cervical somites. Accordingly, the phrenic nerves stretch all along the thoracic cavity, from their cervical origin in the spinal cord (C3-C5) to innervate the diaphragm’s musculature. The peripheral parts of the diaphragm are also formed from body wall mesenchymal tissue that splits off medially and fuses with the other components. Congenital diaphragmatic hernia. If the diaphragm is absent or partially formed results in an abnormal opening (hernia) that allows abdominal organs (usually stomach and intestines) to move into the chest cavity compressing the heart and lungs. The extent of the defect varies from a small deficiency in the closure of the pleuroperitoneal canal to a complete absence of the diaphragm (diaphragm agenesis). This condition can lead to underdevelopment of the lungs (pulmonary hypoplasia), potentially resulting in life-threatening breathing difficulties that are apparent from birth. https://sway.office.com/TvZvoGMPxmhzCiDk#content=gxOnsbdyW51ciW - Separation of the pleural and pericardial cavities. Dorsally, the developing lungs protrude into the pleural cavities and thus become surrounded by the pleura. Ventrally, the heart is initially trapped in the middle between the right and left pericardial cavities but the atrophy of the mesocardium results in the formation of a single pericardial cavity.