The extracellular matrix of hydra is a porous sheet and contains type IV collagen

Abstract

Hydra, as an early diploblastic metazoan, has a well-defined extracellular matrix (ECM) called mesoglea. It is organized in a tri-laminar pattern with one centrally located interstitial matrix that contains type I collagen and two sub-epithelial zones that resemble a basal lamina containing laminin and possibly type IV collagen. This study used monoclonal antibodies to the three hydra mesoglea components (type I, type IV collagens and laminin) and immunofluorescent staining to visualize hydra mesoglea structure and the relationship between these mesoglea components. In addition, hydra mesoglea was isolated free of cells and studied with immunofluorescence and scanning electron microscopy (SEM). Our results show that type IV collagen co-localizes with laminin in the basal lamina whereas type I collagen forms a grid pattern of fibers in the interstitial matrix. The isolated mesoglea can maintain its structural stability without epithelial cell attachment. Hydra mesoglea is porous with multiple trans-mesoglea pores ranging from 0.5 to 1 microm in diameter and about six pores per 100 microm(2) in density. We think these trans-mesoglea pores provide a structural base for epithelial cells on both sides to form multiple trans-mesoglea cell-cell contacts. Based on these findings, we propose a new model of hydra mesoglea structure.

Fig. 1

Western blot analysis of rat monoclonal antibody JK2. This antibody reacts with the NC1 domains of type IV collagen from different sources including hydra.

Fig. 2

Immunofluorescent labeling of different hydra mesoglea components. Mid-body region mesoglea is viewed through the ectoderm. The oral-aboral body axis runs vertically in all panels as indicated by the double headed arrow in panel A. (A) Type IV collagen labeling. (B) Type IV collagen signal viewed at a higher magnification. (C) Laminin labeling. Arrowheads indicate perpendicular lines (see Results). (D) Laminin distribution at a higher magnification. (E) Type I collagen shows grid-like arrangement. (F) Type I collagen at a higher magnification. Arrowheads indicate perpendicular fibers (see Results). Bar in A = 20 µm for A, C, E; bar in B = 10 µm for B and D; bar in F = 10 µm.

Fig. 3

Isolated mesoglea stained with monoclonal antibodies to hydra laminin (A) and type I collagen (B). (A) Isolated mesoglea retains the shape of a polyp with tentacles and body column. (B) An optical cross-section through isolated mesoglea reveals different surface properties on ectodermal (ect) and endodermal (end) side. Bar in A = 200 µm; bar in B = 20 µm.

Fig. 4

Immunofluorescent double labeling of different mesoglea components. (A–F) Double labeling of type I collagen and laminin. (G–L) Double labeling of type I collagen and type IV collagen. (M–R) Double labeling of laminin and type IV collagen. Mesoglea is viewed either through the ectoderm (A–C, G–I, M–O) or by optical cross sections (D–F, J–L, P–R). The oral-aboral axis runs vertically in all panels as indicated by the double-headed arrow in panel A. Arrowheads in D, E, and K indicate trans-mesoglea pores (see Results). Bar in A = 10 µm for A–C, G–I, M–O; bar in D = 5 µm for D–F; bar in J = 5 µm for J–L; bar in P = 5 µm for P–R.

Fig. 5

Scanning electron micrographs of isolated hydra mesoglea; (A–C) ectodermal side, (D–E) endodermal side. The double-headed arrows in (A) and (D) indicate the oral-aboral axis. The arrowheads in (A) point to the thin and fibrous materials that may represent basal lamina. The arrowheads in (B) indicate trans-mesoglea pore openings. (C) is the higher magnification of an area in (B). Bar in D = 10 µm for A, B, and D. Bar in E = 2 µm for C and E.

Fig. 6

A schematic model drawing of hydra mesoglea structure and its relationship with epithelial cells. Mesoglea is a porous structure with multiple trans-mesoglea pores that allow cell processes to protrude through and contact each other. Mesoglea is composed of two sub-epithelial basal lamina zones that contain laminin and type IV collagen. These basal lamina layers extend into the trans-mesoglea pores along with cell processes. The interstitial matrix is located in the center and contains type I collagen.