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The facial region develops ventral to the rostral neural tube from parts of the notochord, and the pharynx, which is surrounded on both sides by a series of aortic arches. Between these structures and the ectoderm there are a large number of neural crest cells (neurectodermal origin) and mesenchyma cells, which stem from the mesoderm. They have a tendency for segmental ordering and form the various sections of the facial anlage and the pharyngeal arches. The face and the jaw stem from an unpaired forehead prominence (frontonasal process) and on both sides from the maxillary and mandibular processes, which stem from the first pharyngeal arch. Through differing growths of the individual processes the frontonasal process forms the upper half of the face (frontal process) and the nose while the middle part of the upper jaw and lip are formed by the medial and lateral nasal processes. The maxillary process fuses with the medial nasal process and forms the lateral part of the upper jaw and lip. The mandibular process forms the lower jaw, lip and the chin. Between the lateral nasal process and the maxillary process arises the nasolacrimal duct that connects the orbit with the nasal cavity.
The palate arises through the fusion of the unpaired median palatine process (primary palate) with the two lateral palatine processes (secondary palate).
The nose arises from the two nasal placodes in the frontonasal prominence. They subside and form two nasal sacs that are outwardly delimited on both sides by the lateral and medial nasal processes (the first forming later the nasal wings and the second the nasal septum). The nasal sacs (primary nasal cavity) open into the oral cavity via the posterior nasal orifices. Only somewhat later do these two nasal cavities widen towards the rear. They remain separated from each other in the middle by the nasal septum. The secondary palates separate them from the primary oral cavity.
The teeth form through the interaction between dental lamina (ectodermal ridge) in the upper and lower jaws and neural crest tissue. Initially they are cap-shaped and later bell-shaped. The ectodermal portion forms the enamel organ (ameloblasts) that forms the hard tooth enamel layer. In the interior the odontoblasts form from the neural crest tissue, which is responsible for dentin production. The salivary glands also stem from ectodermic sproutings.
The tongue arises from various anlagen in the pharyngeal floor region. From this the complicated innervation pattern of the tongue can be explained. The lingual musculature stems from the occipital somites that migrate into the tongue (lingual cord) with the glossopharyngeal nerve (cranial nerve XII).
In their interior all pharyngeal arches are delimited by the pharyngeal pouches and on the outside by the pharyngeal clefts. They form many of the various structures in the neck region.
From the 1st pharyngeal arch arise the upper and lower jaws. With mesenchyma of the 1st and 2nd pharyngeal arches that surrounds it, the 1st pharyngeal pouch and cleft forms the numerous small parts of the middle ear and the external acoustic meatus, respectively.
The 2nd, 3rd and 4th pharyngeal clefts obliterate and form the surface of the neck. Components of the 2nd to 4th pharyngeal arches form the skeleton, muscle and connective tissue portions of the neck.
Finally, from the 3rd and 4th pharyngeal pouches arise on both sides portions of the thymus, the parathyroid and the ultimopharyngeal body (only from the 4th pharyngeal pouch). The thyroid arises as an unpaired ventral sprouting from the upper pharynx (later base of the tongue) region (foramen cecum).
The intestines arise from a tube that forms from the endoderm. Cranially this tube ends at the oropharyngeal membrane and caudally at the cloacal membrane. It is subdivided into a foregut, midgut and hindgut. The midgut is connected with the umbilical vesicle. The differing development of the various intestinal sections is based on the local interactions with the surrounding mesenchyma (epithelio-mesenchymal interactions). Thus in the uppermost section, beside the thyroid, the primordia of the respiratory system, the pancreas, liver and gall bladder form as ventral sproutings.
The esophagus remains as a tube, obliterates, though, during the course of the embryonic development and only in the fetal period does it undergo a recanalization. It lengthens considerably, just like almost all intestinal sections, with the growth of the embryo. The stomach appears quite early as a spindle-shaped widening. It is hung on a dorsal and ventral mesogastrium and through rotation reaches its adult position. Due to its large increase in length the midgut forms transiently a loop into the umbilical coelom (physiologic umbilical hernia). With the straightening up of the embryo in the late embryonic period the intestinal loops can move back again into the abdominal cavity, but experience a rotation (270 degrees) about their own axis. This leads to the characteristic arrangement of the colon around the loops of the small intestine. Parts of the mesenterium also coalesce later with the posterior abdominal wall so that the ascending and descending colon lies secondarily retroperitoneal. Just as with the esophagus the lumen of the small intestine also obliterates for a certain time in the early fetal period in order to be recanalized again later.
The pancreas, liver and gall bladder form as sproutings from the intestine.
The primordium of the liver arises at the level of the transverse septum. Through the enormous increase in size it extends, though, into the abdominal cavity but remains still connected with the transverse septum (diaphragm) by the area nuda. The liver primordium, together with the omphalomesenteric vessel, form the complicated sinusoid system of the liver.
The pancreas arises from two components, the ventral and dorsal pancreases. Through a shifting of the ventral part around the duodenum the two join and form dorsally the definitive pancreas in the mesogastrium, which somewhat later, adheres to the posterior abdominal wall. Through the coalescence of the mesentery in this region the pancreas finally also comes to be positioned secondary retroperitoneal.
In the entire intestinal tract many abnormalities can arise. They range from stenoses, atresias, duplications, fistulas, diverticula to abnormal rotations. Genetic disorders can also be responsible for malformations in this region.
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