Development of the digestive system.

The primitive gut forms during the fourth week of the development as a result of cephalocaudal and lateral folding of the embryo. This endoderm lined cavity is incorporated into the embryo, while the yolk sac and the allantois remain temporarily by outside the embryo.

The endoderm of the primitive gut gives rise to the epithelium and glands of the digestive tract. The muscular and fibrous elements of the digestive tract are derived from the splanchnic mesoderm. The epithelium at the cranial and caudal extremities of the digestive tract is derived from the ectoderm of the stomodeum and the proctodeum (anal pit).

The primitive gut is divided into four parts: a) the pharyngeal gut which extends from the buccopharyngeal (oropharyngeal) membrane to the respiratory (tracheobronchial) diverticulum; b) the foregut, extending from the tracheobronchial diverticulum to the liver outgrowth; c) the midgut, extending from the liver outgrowth to the junction of the right two thirds and left one third of the transverse colon in the adult (posterior intestinal porta); d) the hindgut, extending from the posterior intestinal porta to the cloacal membrane. The liver, biliary apparatus, pancreas and the respiratory system arise as diverticula from the foregut.

Along the entire length the intestinal tube is suspended from the dorsal body wall by a dorsal mesentery. Along the segment of its length, it is attached to the ventral body wall by a ventral mesentery.

Development of the teeth.

Each tooth has a mesodermal and an ectodermal component, the latter forming only the enamel. At five to six weeks, the oral ectoderm develops horseshoe-shaped linear thickening in the upper and lower jaws. These labiodental laminae are at first solid and bifid, extending deeply into underlying mesoderm.The outer labial limb soon splits to form the groove between the lip and the alveolar process of the jaw (the future vestibule) while the inner limb, the dental lamina, develops a series of bud-like thickenings, or tooth germs, numbering five in each half jaw, one for each deciduous tooth. Later, starting at ten to twelve weeks, a second series of tooth germs develops on the lingual side, these numbering eight in each half jaw (five to replace deciduous teeth plus three to form the molars, which are not preceded by a deciduous tooth).

Each epithelial tooth germ, still attached above to the dental lamina by a cord of the cells, is at first “cap”-shaped, but becomes invaginated from below by a mesenchymal papilla to become the “bell stage” or enamel organ, the whole embedded in connective tissue (the dental sac). Soon, this completely invests the developing tooth when the connection to the dental lamina breaks down and disappears. The peripheral part of the enamel organ is formed by a single layer of epithelial cells that are low cuboidal on the convexity of the bell (the outer enamel or dental epithelium) and more columnar in the concavity (the inner enamel or dental epithelium). The two meet at the rim of the bell, the cervical area or neck, this marking the future cementoenamel junction. Internally in the bell, the cells are stellate, separated by intercellular spaces (the stellate reticulum), but form a more regular layer called the stratum intermedium adjacent to the inner enamel epithelium that become the enamel-forming ameloblasts.

By the time the ameloblasts are differentiated, the peripheral mesodermal cells of the dental papilla adjacent to them became arranged in a regular manner, one cell thick, at the odontoblasts, the two separated only by basal lamina material that later breaks down. By about twenty weeks of gestation, hard tissues of the both begin to form. Uncalcified predentin is formed first and increases in thickness by apposition on the internal surface. The predentin extends down towards the neck, and it increases in thickness, cytoplasmic processes of the odontoblasts are formed as the dentinal fibres. The odontoblasts form collagen and matrix, mainly glycosaminoglycans, and calcification then follows in the first-formed matrix, transforming predentin to dentin. Because mineralization occurs after the formation of fibres and ground substance, there always is a thin layer of predentin adjacent to the odontoblasts. Once dentin formation has been initiated, ameloblasts commence to form enamel on the dentin surface. The first enamel formed is seventy per cent minerals and thirty per cent organic matrix, whereas mature enamel is ninety nine per cent minerals. Thus, the matrix is necessary for crystallite deposition and/or orientation, and later it is lost almost completely. With an increase in thickness of the enamel, the ameloblasts recede from the dentin.

At the periphery of the enamel organ in the future neck region, i.e., at the edge of the bell, where inner and outer enamel epithelia come together, a fold of epithelial cells develops and grows downward toward the root. This is the epithelial root sheath (of Hertwig). Root development occurs shortly before tooth eruption and gradually processes as the crown emerges through the gingiva. Odontoblasts develop in relation to the epithelial sheath of Herwig and form dentin. Cementum develops from mesenchyme of the periodontal membrane. The epithelial sheath of Herwig disappears only when the root is formed completely.

During eruption of a permanent tooth, the deciduous tooth superficial to it gradually is resorbed by growth pressure, osteoclasts being prominent during the process. The deciduous tooth when finally shed consists only of the upper portion of the crown, the remainder having been resorbed.

Development of the pancreas.

The pancreas arises from two diverticula, ventral and dorsal, from the junction of foregut and the midgut. The diverticula fuse, the epithelial lining branching to form acini connected to the primary outgrowths by a duct system. Some epithelial buds lose their connection to the duct system and differentiate into islet tissue. However, it has been suggested that some of the islet cell types may arise from cells of the neural crest that migrate into the pancreas early in development. During the development, the duct systems of the two diverticula become interconnected so that although the dorsal diverticulum forms the bulk of the pancreas, its secretion passes to the ventral outgrowth, the duct of which becomes the main pancreatic duct. The proximal part of the duct of the dorsal diverticulum remains as the accessory pancreatic duct, which opens into the duodenum at a higher level than the main duct. The latter has a common opening into the duodenum with the common bile duct from the liver.

Development of the liver.

The liver develops as a ventral diverticulum (endoderm) of the foregut and midgut junction and extends anteriorly into the mesenchyme of the septum transversum. Proliferation of the edodermal cells gives rise to the cords and plates of hepatic cells, which at first are tubular in arrangement with cells arranged around a central lumen. The sinusoids develop from vascular tissue associated with the vitelline veins, which themselves form the portal vein. Mesenchyme associated with the portal vein and that of the septum transversum develops into the connective tissue and capsule of the organ.

The original diverticulum of the gut and its main branches remain tubular as the bile and hepatic ducts. The gallbladder and cystic duct develop as a diverticulum from the main duct.

The liver is one of the main blood-forming organs in the fetus and remains this potentiality in the adult.

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