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Development of the foregut |
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| The foregut anlage can be traced
back to blastoderm stage It forms a ring behind the anlage of the anterior
midgut at the anterior tip of the embryo. The cells of the foregut start
to invaginate during stage 10, when the stomodeum forms. Internalization
of the foregut structures continues until stage 16, partially by means of
mitotic divisions and partially by incorporating additional cells from the
neighbouring ectoderm. The invaginated cells form a monolayered epithelium. Only cells at the floor of the stomodeum, which establish contact with the endodermal anterior midgut primordium, lose their epithelial organization. During stages 12 and 13, the subdivisions of the foregut become morphologically distinct. Cells in the roof of the part of the stomodeum that will become the oesophagus form three invaginations, which represent the primordia of the stomatogastric nervous system. From stage 13 onwards, the hypopharyngeal lobes ventrally and the tip of the clypeolabrum dorsally form the stomodeal opening. By this stage, four morphologically distinct subdivisions of the stomodeum can be recognized. These comprise (from anterior to posterior) the primordia of the pharynx, oesophagus, inner layer of the proventriculus, and recurrent layer of the proventriculus. With the progression of head involution at stage 14-16, the anteriormost part of the foregut, represented by the atrium, invaginates. At the border between atrium and pharynx, the common duct of the slaivary glands meets the foregut. |
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