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First published online May 23, 2006
doi: 10.1242/10.1242/dev.02406

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Mechanisms of elongation in embryogenesis

Department of Biology, University of Virginia, Charlottesville, VA 22901, USA.

View larger version (25K): [in a new window]   Fig. 1. Straightening and elongation of the notochord. (A,B) The amphibian (Xenopus) notochord straightens and elongates from the early (A) to late (B) tailbud stage (anterior is to the left, dorsal at the top). (C,D) The early tailbud notochord consists of transversely stacked flattened cells that are the shape of pizza slices (C), which vacuolate and swell during notochord elongation (D). (E) The notochord is encased in a sheath of extracellular matrix with an average fiber angle of 54° (fibers shown in green). Isolated intact notochords straighten and lengthen in solutions of low osmotic strength (gray), and bend and shorten in solutions of high osmotic strength (black). (F) The cells swell but notochord morphogenesis is lost when the sheath is enzymatically digested. (G) The mechanical behavior of fiber-wound hydraulic skeletons has been studied experimentally by varying the water pressure (P) inside latex tubes, which were embedded with taffeta fiber windings at varying angles and suspended in a water bath. (H) Geometric parameters were measured, including fiber angle, diameter, length, curvature (a/b) and shortening or elongation (2b). (I,J) Mechanical properties were measured, including flexural stiffness [the force (F) necessary to produce bending between two supports; I] and isometric force production in pushing and pulling (J). (A,B,E-J) Reproduced, with permission, from Koehl et al. (Koehl et al., 2000).

View larger version (35K): [in a new window]   Fig. 2. Elongation of the nematode embryo. (A-C) The dorsal surface of the nematode embryo shows intercalation of dorsal hypodermal cells (green), which elongates the dorsal aspect of the embryo, giving it a comma shape (lateral view, G). B is an enlargement of A: the basal surfaces of the intercalating dorsal hypodermal cells show medially directed protrusions. (D-F) Ventral views and (G-J) lateral views of ventral closure, during which the ventral hypodermal cells (red) move across the underlying neuroblast cells (purple) and meet in the midline. (K-M) Hypodermal-mediated elongation beyond the comma stage involves a circumferential actin microfilament cytoskeleton (yellow) in the dorsal (green) and ventral (red) hypodermal cells, and a circumferential contraction of the lateral or seam hypodermal cells (blue). In regions overlying muscles (K and enlargement), trans-epithelial attachments (TEAs) develop and connect the underlying matrix and muscle (purple) to the overlying cuticle (embryonic sheath). TEAs consist of fibrous organelles (FOs) - electron-dense plaques similar to hemidesmosomes on the ECM/muscle side and on the embryonic sheath side of the hypodermis; FOs are connected by intermediate filaments that span the hypodermal cells. Anterior is to the right in A-J. (A-J) Adapted, with permission, from Simske and Hardin (Simske and Hardin, 2001); (K-M) adapted, with permission, from Ding et al. (Ding et al., 2004).

View larger version (31K): [in a new window]   Fig. 3. Convergent extension of the Drosophila hindgut. (A, parts a,b) Sagittal views show invagination of the posterior midgut (pmg, purple), proctodeal ring (pr, green), and visceral mesoderm (vm, orange) during germ band extension. (A, parts c,d; B) During germ band retraction, the hindgut elongates (yellow, small intestine; green, large intestine). (C) This elongation occurs by circumferential cell intercalation. (D) Elongation is regulated by the JAK/STAT pathway, including the ligand Unpaired (Upd), the receptor Dome (Dome, Domeless or Master of Marelle), JAK (Hop, Hopscotch) and Stat (Stat92E or Marelle). (E) A model for regulation of hindgut convergent extension is shown, in which unpaired (upd) mRNA is expressed in the small intestine (gray), and the encoded protein diffuses posteriorly, establishing a gradient (lavender). Upd activates Stat and positively regulates Stat protein levels, resulting in a Stat protein and activity gradient (blue) that, in turn, regulates circumferential intercalation and the resulting convergent extension. Anterior is to the left in A-C,E. Adapted, with permission, from Lengyel and Iwaki (Lengyel and Iwaki, 2002) and Johansen et al. (Johansen et al., 2003).

View larger version (38K): [in a new window]   Fig. 4. Drosophila Malpighian tubule elongation. (A) Malpighian tubules form as evaginations from the hindgut-midgut junction (blue, parts a,b), and first elongate by cell division (parts b,c), and later by circumferential cell intercalation (parts c-e). Anterior is to the left. (B) Cell intercalation reduces the circumference from about eight cells to two cells, while elongating the tubule commensurately. (C) Mesenchymal cells from the visceral mesoderm (orange) intercalate into the primary cells (pc) to form the stellate cells (sc). (A) Adapted, with permission, from Lengyel and Iwaki (Lengyel and Iwaki, 2002); (C) Adapted, with permission, from Jung et al. and Denholm et al. (Jung et al., 2005; Denholm et al., 2003).

View larger version (52K): [in a new window]   Fig. 5. The development of the Drosophila ovary. (A) At puparium formation, the ovary is an anteroposteriorly stratified structure, consisting of apical cells (light gray), presumptive terminal filament (TF) cells (green), germ cells (dark gray), somatic cells (magenta) and presumptive basal stalk cells (blue); anterior is to the top and lateral to the left in all diagrams. About 20 ovarioles are formed as the TF cells intercalate laterally into stacks of elongated cells in register anteroposteriorly, which then separate into stacks of discs (A, parts a-d, B). As they do so, the apical cells form an epithelial sheet and move posteriorly between the stacks of discs, separating them into TFs (A, parts a-d). (C) The posteriorward (arrows) invasion of the apical cell epithelium then separates the germ cells and associated somatic cells and, finally, basal stalk cells into ovarioles (D,E). (F) The apical cell epithelium with an underlying basal lamina (red) separates the basal stalk cells into arrays of several cells in diameter (part a). These cells intercalate transversely to form a longer, narrower array, thus forming the elongated basal stalks and completing the separation of the ovarioles (parts b,c). A similar process occurs among the somatic cells associated with the germ cells, to elongate the interfollicular stalk, which separates the newly formed follicles from the germarium. Anterior is to the top and posterior to the bottom in all figures. Adapted, with permission, from Godt and Laski (Godt and Laski, 1995).

View larger version (28K): [in a new window]   Fig. 6. Elongation of the leech body plan from teleoblasts. (A) The teleoblasts of the leech (annelid) embryo produce elongating germinal bandlets through the sequential production of blast cells by highly unequal divisions. On each side, four ectodermal bandlets (N, O/P and Q) come into lateral apposition, while the mesodermal (M) bandlet comes into apposition from beneath. The five bandlets form the germinal band, which extends across the embryo (arrow, A) as bandlet cells are added posteriorly by highly unequal cell divisions of the parent teloblasts. The germinal bands anneal with their contralateral partner in a rostrocaudal sequence at the midline to form the germinal plate. (B,C) As the germinal bandlets extend by cell division and anneal, they, and the micromere cap overlying them, expand over the surface of the embryo in epiboly (B, curved arrows, early stage 8, animal view; C, late stage 8, rostral view). (A) Reproduced, with permission, from Nelson and Weisblat (Nelson and Weisblat, 1991); (B,C) reproduced, with permission, from Smith et al. (Smith et al., 1996). Asterisks indicate apposition of germinal bands.

View larger version (82K): [in a new window]   Fig. 7. Pronephric duct formation and posterior elongation in the urodele amphibian. (A) Dye marks (red) placed across the duct (dark gray) and adjacent tissues at the early tailbud stage show posterior extension of the duct by the late tailbud stage, rather than development in situ from underlying mesoderm. (B,C) Scanning electron micrographs (SEMs) show the posterior end of the pronephric duct at mid-tailbud (B), and the overlapping of anterior cells on more posterior ones (C, arrows). (D) SEM showing protrusions of pronephric duct cells attached to underlying intermediate mesodermal cells (arrows). Posterior is to the left in A and to the lower left in B. (A) Adapted, with permission, from Poole and Steinberg (Poole and Steinberg, 1981); (B-D) adapted, with permission, from Poole and Steinberg (Poole and Steinberg, 1977).

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