Histology and ultrastructure of the coenenchyme of the octocoral Swiftia exserta, a model organism for innate immunity/graft rejection

Abstract

The octocoral Swiftia exserta has been utilized extensively in our laboratory to study innate immune reactions in Cnidaria such as wound healing, auto- and allo-graft reactions, and for some classical "foreign body" phagocytosis experiments. All of these reactions occur in the coenenchyme of the animal, the colonial tissue surrounding the axial skeleton in which the polyps are embedded, and do not rely on nematocysts or directly involve the polyps. In order to better understand some of the cellular reactions occurring in the coenenchyme, the present study employed several cytochemical methods (periodic acid-Schiff reaction, Mallory's aniline blue collagen stain, and Gomori's trichrome stain) and correlated the observed structures with electron microscopy (both scanning and transmission). Eight types of cells were apparent in the coenenchyme of S. exserta, exclusive of gastrodermal tissue: (i) epithelial ectoderm cells, (ii) oblong granular cells, (iii) granular amoebocytes, (iv) morula-like cells, (v) mesogleal cells, (vi) sclerocytes, (vii) axial epithelial cells, and (viii) cnidocytes with mostly atrichous isorhiza nematocysts. Several novel organizational features are now apparent from transmission electron micrographs: the ectoderm consists of a single layer of flat epithelial cells, the cell types of the mesoglea extend from beneath the thin ectoderm throughout the mesogleal cell cords, the organization of the solenia gastroderm consists of a single layer of cells, and two nematocyst types have been found. A new interpretation of the cellular architecture of S. exserta, and more broadly, octocoral biology is now possible.

Keywords: Cnidaria; Coenenchyme; Gorgonians; Nematocysts; Octocoral biology.

Fig. 1

(a) A complete small colony (approximately 25 cm tall) of Swiftia exserta in a holding aquarium. (b) Branchlet removed from a Swiftia exserta colony in artificial seawater. Scale in millimeters.

Fig. 2

Schematic representation of an octocoral. Diagram redrawn from Bayer et al. (1983) by Ellen Bigger Streeter.

Fig. 3

Swiftia exserta coenenchyme and ectoderm. (a) Longitudinal paraffin section stained with Mallory’s aniline blue (connective tissue stain) showing the hollow axial skeleton (ax skel) at the bottom, several circular solenia (sol) in the mesoglea between the skeleton and the outer rind of cells, and the blue mesoglea. (b) A cross-section of the coenenchyme by TEM shows light areas within the coenenchyme that hold remnants of sclerites (sc) which do not stain with electron-dense dyes. Ectoderm (EcD) is found at the bottom of the image and gastroderm (GaD) at the top. Cell cords (cc) can be seen permeating the fibrous mesoglea (M). (c and d) High magnification TEM shows the microvilli in section and the thin, flattened ectoderm cells filled with electron-lucent granules (panel c) while panel (d) gives a survey of the stratified rind of the coenenchyme, with thin ectoderm cells (EcD) at the top covering oblong granular cells with electron-dense granules and amorphously shaped granular amoebocytes. At center-right is a type I nematocyst (nem I). Oblong granular cells (og), granular amoebocytes (ga), fibrous mesoglea (M), and sclerites (sc) are indicated in the panels.

Fig. 4

Cell cords in the coenenchyme of Swiftia exserta with embedded sclerites (sc). (a) TEM overview showing the cellular architecture of the coenenchyme from the ectoderm (EcD) into the fibrous mesoglea (M). (b) A closer view of the dense packing and the irregular shape of the cell cords filled with oblong granular cells (og, with electron-dense granules) and granular amoebocytes (ga, of indeterminate shape and with granules staining less intensely and of different sizes) shows the continuity of cells from beneath the thin ectodermal cells into the cell cords. (c) Paraffin section stained with Mallory’s aniline blue showing the acellular nature of the mesogleal matrix. The orange nucleus of a sclerocyte is visible towards the bottom right of the sclerite space (small arrowhead). Blue staining of the fibrous mesoglea matrix indicates a collagen-like stain affinity. An isolated, single mesogleal cell (mc) is indicated by a long arrow. The basal lamina (bl) is indicated by a short arrow. (d) An area of the mesoglea with an isolated cell cord embedded within the fibrous mesogleal matrix shows a single amoebocytic mesogleal cell (arrowhead to mc) and the basal lamina (short arrow to bl) that separates the gastroderm cells (GaD) from the mesoglea (M). The insert at the top right shows a morula-like cell (Mor).

Fig. 5

Solenia-type endo-/gastroderm in Swiftia exserta. (a) Solenia-type gastroderm (GaD) lining is visible at the upper part in a paraffin section stained with Mallory’s stain. The staining indicates that one of the digestive cell types is acidophilic (bright orange-red granules in cytoplasm). A basal lamina (bl) structure isolates the gastroderm from the mesoglea. (b–d) Transmission electron micrographs of solenia show different aspects. A solenium near the axial skeleton (ax skel) and axial epithelia (ae) is seen. This solenium is separated from the fibrous mesoglea (M) by a basal lamina-like layer (short arrow to bl). This basal lamina layer is faintly visible in panels (b) and (c), and clearly visible in panel (a) (arrow and bl). The arrowhead indicates a zymogen-like cell. Several secretory cells (sc) and a secretory granule (sg) are indicated. (c) Low magnification TEM of a section through a solenia (sol) showing several types of digestive cells. Long arrows indicate phagocytic vesicles. Short arrow indicates the basal lamina-like structure. A mucus-secreting cell’s granules are indicated (muc), as are several secretory cells (sc). Panels (b) and (c) show secretory cells (sc) packed with granules (sg) and phagocytic cells with membrane ruffles. To move liquid through the solenia (sol lumen), a flagellated cell with flagellum, seen in panel (d) with a section perpendicular to the flagellum (arrowhead) and base, is used.

Fig. 6

Swiftia exserta desmocytes and axial epithelia. (a) Desmocyte plaques (des) and axial epithelia (ae) with H&E staining from paraffin sections. (b–d) TEM images demonstrate the elevation of the axial epithelial cells (ae) above the plane of the desmocytes (panel b) as well as an active axial epithelial cell capable of secreting gorgonin. Panels (c) and (d) show the fine structure of the desmocyte anchors into the mesoglea and the many secretory vesicles and golgi bodies found in the axial epithelial cells (panel c). Insert in (d) shows the septae found in these desmocyte-to-axial epithelium junctions. Junctions between the desmocyte and axial epithelium cell are indicated by arrowheads in panels (b–d). The axial skeleton (ax skel) and mesoglea (M) are indicated in panels (b–d).