79. The Digestive System: General Introduction*

  1. Early development review
    1. 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
      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
      2. 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
        1. 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
    2. 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
  2. Major or principal stages of development
      1. 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
        1. The buccopharyngeal membrane is resorbed at the beginning of week 4, connecting the amniotic cavity with the digestive tube
        2. The derivatives of the cloacal membrane open at the end of week 9
      2. The development of the digestive system consists of
        1. A very complex anterior pharyngeal portion, the foregut
          1. The foregut extends from the buccopharyngeal membrane to the duodenum where the liver bud arises (the anterior intestinal portal)
        2. A very extensive growth in length of its middle or abdominal portion, the midgut
          1. The middle portion of the midgut remains connected to the yolk sac via the vitelline or omphalomesenteric duct
          2. 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)
        3. An intermingling and very close association with the urogenital system is found in relation to its terminal portion, the hindgut
          1. The hindgut extends from the posterior intestinal portal to the cloacal membrane
        4. The foregut, midgut, and hindgut are supplied by the celiac artery, the superior mesenteric artery, and the inferior mesenteric artery, respectively
    2. * Development of the oral cavity, the tongue, the salivary glands, and the teeth is described in other units of the text.

the digestive system:  general introduction*: image #1