EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
79. The Digestive System: General Introduction*
Review of MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D.
- Early development review
- INTRODUCTION: the entoderm appears about day 8 and rapidly forms the yolk sac and the epithelial lining of the digestive tract and its associated glands. The process of body cylinder formation divides the yolk sac into 2 parts toward the end of month 1
- Part 1 is extraembryonic and is the yolk sac itself, which regresses early and disappears at about 3 months. The head, tail, and lateral folds incorporate the dorsal part of the yolk sac and the allantois into the embryo
- Part 2 is intraembryonic and is the primitive gut which is the early origin of the epithelium of the digestive tube and its accessory glands, the liver, the pancreas, and the biliary apparatus
- The epithelium at the cranial and caudal ends of the tract is derived from ectoderm of the stomodeum (primitive mouth) and the proctodeum (anal pit), respectively
- THE SPLANCHNIC MESODERM, which is formed about day 15 during gastrulation, surrounds the entoderm and provides the digestive tract with its connective tissue, muscle, and serous (peritoneal) coverings
- Major or principal stages of development
- GENERAL INFORMATION
- The digestive tube is initially closed cephalically by the buccopharyngeal or oropharyngeal membrane (a bilaminar membrane: ectoderm externally and entoderm internally) and caudally by the cloacal membrane
- The buccopharyngeal membrane is resorbed at the beginning of week 4, connecting the amniotic cavity with the digestive tube
- The derivatives of the cloacal membrane open at the end of week 9
- The development of the digestive system consists of
- A very complex anterior pharyngeal portion, the foregut
- The foregut extends from the buccopharyngeal membrane to the duodenum where the liver bud arises (the anterior intestinal portal)
- A very extensive growth in length of its middle or abdominal portion, the midgut
- The middle portion of the midgut remains connected to the yolk sac via the vitelline or omphalomesenteric duct
- The midgut extends from just caudal to the liver bud (the anterior intestinal portal) to a point where, in the adult, the right two-thirds and left one-third of the transverse colon are found (the posterior intestinal portal in the embryo)
- An intermingling and very close association with the urogenital system is found in relation to its terminal portion, the hindgut
- The hindgut extends from the posterior intestinal portal to the cloacal membrane
- The foregut, midgut, and hindgut are supplied by the celiac artery, the superior mesenteric artery, and the inferior mesenteric artery, respectively
- * Development of the oral cavity, the tongue, the salivary glands, and the teeth is described in other units of the text.
