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Epithelial interactions in Hydra: apoptosis in interspecies grafts is induced by detachment from the extracellular matrix

Sergey G. Kuznetsov, Friederike Anton-Erxleben and Thomas C. G. Bosch

Zoologisches Institut, Christian-Albrechts Universität zu Kiel, 24116 Kiel, Germany

View larger version (66K): [in a new window]   Fig. 1. Apoptosis is induced at the graft border in heterografts. Control (24 h old) homografts of H. oligactis/H. oligactis (A) and H. vulgaris/H. vulgaris (E), and heterografts H. oligactis/H. vulgaris (B) and H. vulgaris/H. oligactis (C) vitally stained with Acridine Orange. (D) H. vulgaris/H. oligactis heterograft stained by the TUNEL method. (F—H) Graft sites indicated by the frames in B—D, respectively, shown at higher magnification. O, H. oligactis tissue; V, H. vulgaris tissue. Broken lines indicate the graft border. Arrows show Acridine Orange- or TUNEL-positive cells undergoing apoptosis. The cells in focus are localized in the ectodermal epithelium. Note that most of them are apparently restricted to the H. vulgaris tissue adjacent to the graft border.

View larger version (13K): [in a new window]   Fig. 2. Stability of H. vulgaris/H. oligactis heterografts after grafting. Values are means ± S.D. (N=5). The solid line is the polynomial trendline through the data points.

View larger version (70K): [in a new window]   Fig. 3. Displacement of H. vulgaris endoderm in heterografts. (A) 1-day-old heterograft with the H. vulgaris half placed distally and the H. oligactis half proximally. Inset, grafting site at higher magnification. (B) Longitudinal section through a heterograft similar to that shown in A. (C) Schematic representation of the section shown in B. Vul, tissues of H. vulgaris; Oli, tissues of H. oligactis. Note the clump of H. vulgaris endodermal tissue hanging into the gastric cavity. (D,E) H. vulgaris/H. oligactis heterografts 2 and 24 h after grafting, respectively. H. oligactis endoderm is labelled with fluorescent latex beads. (F) Brightfield view of the animal shown in E. Arrowheads indicate heterologous contact in the ectoderm.

View larger version (66K): [in a new window]   Fig. 4. Species-specific antibodies recognize epithelial cell surface antigens. (A) H. vulgaris ectodermal epithelial cells are stained with species-specific antiserum visualized with FITC-labelled secondary antibody. Note the strong staining of the apical surface (glycocalix). (B) Uncontrasted ultrathin section through the H. vulgaris (vul)/H. oligactis (oli) graft site. Antibodies coupled with 10 nm gold particles mainly stain glycocalyx of H. vulgaris. (C) Contrasted ultrathin section through the H. vulgaris/H. oligactis graft site (same preparation as in Fig. 5) stained with gold-coupled antibodies. Arrows indicate the heterotypic cellular contact. Scale bars, 0.5 µm.

View larger version (108K): [in a new window]   Fig. 5. Ultrastructure of homotypic and heterotypic septate junctions in the ectoderm of a heterograft (24 h old). (A) The left and the right white arrowheads show homotypic junctions between two cells of H. oligactis (o), enlarged in (B), and two cells of H. vulgaris (v), enlarged in (D), respectively. Note the regular septae and spaces between cell membranes. (C) Enlargement of the heterotypic junction indicated by the black arrowhead in A. White arrows in A point to the heterotypic contact with a few irregular septae and spaces between cell membranes shown in C. g, glycocalyx. Note the species-specific differences in thickness of glycocalyx. Scale bars, 0.5 µm.

View larger version (176K): [in a new window]   Fig. 6. Ultrastructure of the mesoglea in the chimeric region of a heterograft (24h old). (A) Overview of the graft site and (B) its schematic representation. Oli-ecto, H. oligactis ectoderm; Vul-ecto, H. vulgaris ectoderm; Oli-endo, H. oligactis endoderm; Vul-endo, H. vulgaris endoderm. Frames labelled C—F are enlarged in (C—F), respectively. Note the abnormal morphology of mesoglea (asterisk) in the chimeric region (D—F). (G) Mesoglea in control H. vulgaris polyps. Scale bars, 10 µm (A); 5 µm (C—G).

View larger version (69K): [in a new window]   Fig. 7. Heteroaggregates slowly develop to ecto-/endodermal chimeric polyps. (A—E) H. vulgaris/H. oligactis heteroaggregates 10h, 24h, 7 days, 21 days and 28 days after reaggregation, respectively. H. vulgaris cells are stained with species-specific antiserum visualized with FITC-labelled secondary antibody (green). Unlabelled cells of H. oligactis, couterstained with Toluidine Blue, appear red. (F) Histogram indicating the number of head structures per aggregate (numbers in columns) on day 7 after reaggregation. o/o and v/v, control homoaggregates from H. oligactis or H. vulgaris alone, respectively. v/o, heteroaggregates. (G—I) Interstitial cell differentiation products such as nematocytes are not eliminated in heterografts. Shown is the portion of a tentacle of a heterograft (3 days old) stained with an antiserum specific for H. vulgaris cells. One H. oligactis nematocyte (arrowhead) is embedded next to several H. vulgaris nematocytes (arrows).