Textbook of Pediatric Gastroenterology, Hepatology and Nutrition Anupam Sibal, Sarath Gopalan, Akshay Kapoor, Vidyut Bhatia
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Gastroenterology1
  • Developmental Anatomy and Physiology of the Digestive Tract
  • Common Gastrointestinal Symptoms
  • Disorders of the Esophagus
  • Neurogastroenterology and Motility Disorders
  • Disorders of the Stomach and Small Intestine
  • Acute Diarrhea
  • Chronic and Persistent Diarrhea
  • Celiac Disease
  • Inflammatory Bowel Disease
  • Disorders of the Pancreas
  • Short Bowel Syndrome
  • Surgical Disorders of the Gastrointestinal Tract2

Developmental Anatomy and Physiology of the Digestive TractChapter 1

Yogesh Waikar
The digestive system comprises of the salivary gland, esophagus, stomach, duodenum, jejunum, ileum, colon with an accessory connecting hepatobiliary pancreatic system. It is important to keep in mind developmental, anatomical as well as physiological aspects of gastrointestinal tract, which forms the basis of clinical gastroenterology.
 
APPLIED DEVELOPMENTAL ANATOMY AND PHYSIOLOGY OF ESOPHAGUS
The human esophagus develops from foregut by four weeks of age. Incomplete division of trachea and esophagus between the fourth and fifth week of life can lead to developmental abnormalities like tracheo-esophageal fistula, duplication cyst and esophageal atresia. Lengthening of the esophagus is achieved primarily by ascent of the pharynx rather than descent of the stomach in the seventh week.1 Various genes and molecular pathways are involved in the development of the esophagus. Defects or mutation can lead to aberrations and congenital structural abnormalities. The esophagus starts at the lower border of cricoid, passing through the mediastinum thereby providing a convenient access for transesophageal biopsies of lymph nodes and mediastinal masses with the help of endoscopic ultrasound (EUS) and even transesophageal echocardiography.2 Narrowed portions of the esophagus that are easily negotiated using the endoscope are at the start at the oropharyngeal junction, aortic arch, the left main bronchus, and the diaphragm. A shorter intra-abdominal portion of the esophagus is one of the responsible factors for gastroesophageal reflux (GER) in infants.
The parasympathetic supply is from the vagal and the sympathetic supply to esophagus is from the cervical and thoracic sympathetic trunks and greater splanchnic nerves. The upper one-third of esophagus has predominantly striated muscle. The epithelium of the esophagus is non-keratinized squamous.
Various calculations are used to determine length of esophagus in children. The commonly used are Song et al.11 [Esophageal Length = 0.242 × (height) + 0.2078 cm], Strobel et al.12 [Esophageal Length = 0.226 × (height) + 6.7 cm] and Jolley et al.13 [length = 0.207 × (height) + 4.61] the esophageal length, defined as the length from the incisors to the gastroesophageal junction. Knowledge regarding esophageal length is important to calculate placement of esophageal probes for pH studies in children.
At 20 weeks of gestation, the fetus can swallow about 15 mL/day of secretions. The full-term neonate can swallow about 450 mL/day.14 At 34 weeks, 30-seconds of sucking at 2-minute intervals develop, coordinated with swallowing.15 The pressure difference between that at the fundus of the stomach and at the lower end of esophagus rises proportionately with postconceptional age.16 At 28 weeks of gestation, lower esophageal pressure is about 4 mm Hg and by term, it is 18 mm Hg.22
 
APPLIED DEVELOPMENTAL ANATOMY AND PHYSIOLOGY OF STOMACH
In the fifth week of gestation, the stomach appears fusiform in shape and dilated in the median plane. The rotation and growth of stomach in abdominal cavity is responsible for its innervation pattern.17 Gastric volvulus results due to abnormal rotation of one part of the stomach around another part which may be organoaxial, mesenteroaxial or combined.
The columnar epithelium of stomach is established by seventeenth week of gestation. The mucus-producing pit cells can be visualized proximally towards the gastric lumen and acid-secreting parietal cells are visualized more distally towards the middle and lower regions of the gastric gland. Chief cells secrete pepsinogen and pre­dominate at the base of glands. Neuroendocrine cells, including enterochromaffin cells, enterochromaffin-like (ECL) cells, and D cells (somatostatin), are seen at the base of the gastric gland. The proximal stomach is responsible for the storage 4of food and receptive relaxation.18 The distal stomach, the antrum and pylorus, is responsible for grinding and emptying solid food. The gastric emptying rate is dependent on many factors like osmolality, consistency, temperature, pH, size of food particles, antral distention, concen­tration of lipid, protein, and acid in the duodenum, and colonic distention.19 Gastric motor activity appears between 14 and 24 weeks, starting at the gastric pacemaker (on greater curvature of the stomach).20 The normal frequency of gastric slow waves is 3 cycles per minute (CPM). Gastric slow-wave activity can be measured noninvasively using the electrogastrography.21 Migrating motor complexes (MMC), a propagated sequence of contractions that migrates from the stomach into the intestine and toward the ileum every 90 to 120 min consists of 3 phases. Phase 1 is a pattern of quiescence that always follows phase 3. Phase 2 is a period of irregular contractions. Phase 3 is a distinctive pattern of regular high-amplitude contractions repeating at a maximal rate for 3 to 10/min. MMC begins in the esophagus or stomach. Motilin is responsible for initiat­ing phase 3 contractions that begin in the stomach and are observed by 32 weeks of gestation.
Water, electrolytes, hydrochloric acid, and glycoproteins, mucin, intrinsic factor, and enzymes are secreted by stomach (Table 1.1). Central cephalic, local enteric exocrine epithelial cells and endocrine-like neural regulatory cells regulate gastric secretions. In newborns, gastric pH ranges from 6.0 to 8.0. On day 2 to 3 pH reduces to 1 to 3 slowing stabilizing by 2 years.22
Table 1.1   Common secretions in stomach and their corresponding cells
Gastrin → Antral G cell
Histamine → Ecl cell
Adolescents somatostatin → D cell
Gastric lipase → Chief cell
Hcl and intrinsic factor → Parietal cell
Gherlin → Gr cells in the basal part of oxyntic gastric gland
 
APPLIED DEVELOPMENTAL ANATOMY AND PHYSIOLOGY OF INTESTINES
Small intestine starts from the pylorus to the ileocecal valve, occupying the central and lower parts of the abdominal cavity consisting of duodenum, jejunum, and ileum. The average length of the small intestine is 250 to 300 cm in the newborn.26 The caliber of the small intestine gradually diminishes from its origin to its termination. The duodenum constitutes approximately the first 25 cm of the small intestine in adults; the remaining length is arbitrarily divided into the proximal two-fifths, designated as the jejunum, and the distal three-fifths, designated as the ileum.
The first portion of the duodenum begins at the pylorus and ends at the neck of the gallbladder, the second portion, descends from the neck of the gallbladder along the right side of the vertebral column to the level of the third lumbar vertebra. The third portion, courses over to the third lumbar vertebra, passing from right to left, with inclination upward, lying inferior to the origin of the superior mesenteric artery in front of the aorta. Fourth portion ascends immediately to the left of the aorta, up to the level of the second lumbar vertebra, where it makes a ventral turn to unite with jejunum.
The jejunum is thicker and more vascular than the ileum, diminishing in size with distal progression. The intestinal luminal diameter is also greatest in the jejunum, shrinking in diameter as it progresses distally. The plicae circulare are crescentic luminal protrusions of submucosa covered by mucosa, running circumferentially along the inside diameter of the intestinal wall, are most prominent in the distal duodenum and proximal jejunum, decreasing in number and size towards the ileum and do not smooth out when the intestine is distended.27 The Peyer’s patches are more prominent during childhood and regress in size and number with advancing age are predominantly seen in ileum.
The mesentery begins as an anterior reflection of the posterior peritoneum, attached to the posterior abdominal wall along a line extending from the left side of the body of the lumbar vertebra to the right sacroiliac joint.
The ileocecal valve opens when a peristaltic wave overcomes the resting resistance at the terminal ileum.
The colon is approximately 60 cm long in the newborn, increasing to approximately 150 cm in the adult. The caliber of the large intestine is greatest at the cecum and gradually diminishes as it approaches the rectum. The colonic wall remains fairly constant in thickness throughout its entire length and lacks the villi. Small intestinal villi are formed by 16 weeks.26 Colonic villi presist in fetal life disappears by 29 weeks. Colon is larger in caliber as compared to small intestine. Large intestine is mostly retroperitoneal and fixed. Its outer longitudinal muscular layer is in form of three distinct longitudinal bands, teniae coli, extending from the cecum to the rectum. Colon has a characteristic sacculated haustra. The luminal surface of colon is interrupted by plicae semilunares. Appendices epiploicae are fatty projections found scattered over the free surface of the entire large intestine, with the exception of the cecum, vermiform appendix, and rectum. The anal canal is about 2 cm long in the infant, increasing to about 4.5 cm in the adult.27
5Appendix measures between 2 and 20 cm long, with length longest in childhood.28 It generally shrinks during further development in adult life.
Intestinal motility is well established by 32 weeks of gestation.26
Anal canal are the vertical projections bounded below by anal valves at the level of pectinate line. The depression between anal canals is called as anal fold. The pectinate line represents the junction between endodermal (columnar epithelium) and ectodermal (squamous) portions of anal canal.27 The contraction of puborectalis muscle pulls the rectum forward to retain stool, and the relaxation straightens the anal canal, allow­ing defecation. This is important to understand in normal defecography. The anal canal is surrounded by complex muscle fibers under voluntary control called as external anal sphincter and under involuntary control called as internal anal sphincter.
Interstitial cells of Cajal are present within the myenteric plexus between the circular muscle and the submu­cosa and are responsible for intestinal contrac­tile activity and regulation of intestinal tone.
Brunner’s gland secret mucus in the duodenum. They contribute to a protective alkaline pH and promote gallbladder contractility. They also help in promoting pancreatic secretions. Crypts of intestinal villi are lined by undifferentiated columnar epithe­lial cells, goblet cells, Paneth cells, tuft cells, cup-like cells, and enteroendocrine cells. The villous also contain similar columnar cells. Cells overlying Peyer’s patches in ileum are M cells which act as antigen presenting cells. The apical surfaces of the intestinal epithelial cells carry multiple brush-border transporters important for absorption of nutrients. Similar to the enterocytes, surface epithelial cells of colon are called as colonocytes. Paneth cells secrete a wide spectrum of antimicrobial peptides (AMPs) against gram-negative and gram-positive bacteria, fungi, protozoa, and viruses.26 The junctions between enterocytes allow for physiologic passage of fluids, electrolytes, and small macromolecules comprising up to 11 amino acids.28 Pathologic insult to enterocytes may loosen up these junctions thereby increasing permeability of particles.
 
APPLIED DEVELOPMENTAL ANATOMY OF PANCREAS
The pancreas forms as a result of fusion of dorsal and ventral pancreatic buds of the foregut. The ventral pancreas rotates clockwise around the duodenal axis. The dorsal pancreatic bud forms the anterior part of the head of the pancreas, the body, and the tail of the pancreas. The ventral pancreatic bud forms the posterior part of the head of the pancreas and the posterior part of the uncinate process. Abnormalities in development or in rotation of these buds lead to development of structural congenital abnormalities of pancreas.
The dorsal and ventral duct systems fuse resulting in the longer dorsal duct draining into the proximal part of the ventral duct to form the main pancreatic duct of Wirsung. The proximal portion of the dorsal duct forms an accessory of Santorini.29
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