Atlas of Drosophila Development by Volker Hartenstein

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Intestinal Tract pages 28-29 | 30-31 | 32-33 | 34 | 35

The epithelium of the larval alimentary tract originates from two germ layers, endoderm and ectoderm. The endoderm, also called anterior and posterior midgut rudiment (amg, pmg), gives rise to most of the midgut (Poulson 1950; see Skaer; Costa et al.; both this volume). In the blastoderm fate map [stage 5], the anlagen of the midgut rudiments occupy a position at the anterior and posterior embryonic poles. The ectoderm surrounding the anterior and posterior midgut rudiments forms the foregut and hindgut, respectively. Foregut and hindgut are divided up into structurally and functionally different parts. The spatial arrangement of the anlagen of these structures are indicated in the diagram of the blastoderm fate map (ph, pharynx; es, esophagus; pv, proventriculus). During gastrulation, the anterior and posterior midgut rudiments invaginate. The anterior midgut rudiment loses its epithelial characteristics a short time after gastrulation and becomes a solid cluster of rounded, apolar cells flanked by the anterior mesoderm. In contrast, the posterior midgut remains epithelial until late stage 10.

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The early development of the primordia of the foregut and the hindgut is also quite different. The foregut primordium does not start invaginating before stage 10; after that, invagination is a slow process that ends only in late embryogenesis [stages 15-17]. On the other hand, the hindgut primordium invaginates right after the posterior midgut rudiment [stages 7-9]. In the stage 7 embryo depicted, both the posterior midgut rudiment (pmg) and the already invaginated part of the hindgut primordium (hg) form a pouch called the amnioproctodeal invagination (or proctodeum, pr).

During stage 9, the remaining parts of the hindgut primordium enter into the embryo, leading to an increase in length of the amnioproctodeal invagination. At the same time, this structure is pushed anteriorly by the elongating germ band. Proliferation of the gut primordia takes place during stages 8-10. All cells of the primordium of the foregut and the midgut primordia divide three times in a parasynchronous way (Hartenstein and Campos-Ortega 1985). The hindgut primordium also performs three complete divisions; after that, some parts of it (e.g., what will become the Malpighian tubules) divide for a fourth time. (as) Amnioserosa; (ms) mesoderm.