Richard D. Campbell

Origin and Continuity of Desmosomes

The problems of desmosome formation and maintenance are different from those of the other cell components discussed in this volume. Desmosomes, and other cell attachment devices, are not organelles but rather are modifications of the cell periphery. No specific chemical or ultrastructural components have been identified in desmosomes which are not common to the rest of the peripheral cytoplasm and membrane of the cell. The problem of desmosome origin, therefore, seems to be one of localization and organization of ubiquitous components, a process less well understood than the reproduction of true organelles.

Terminology for Morphology and Cell Types

Hydra exist as polyps, i.e., as sedentary forms of coelenterates. A single animal has the form of a tube, about 5–20 mm long and 0.3–1.0 mm wide, bearing a whorl of hollow tentacles near one end. Animals undergoing asexual reproduction produce buds, which arise as evaginations of the body wall. The body wall consists of two concentric epithelia, the ectoderm and endoderm, separated by a common acellular basement membrane, the mesolamella or mesoglea. This trilaminar structure extends throughout the entire body column tentacles and buds. The ectoderm* (or epidermis) faces the environment, whereas the endoderm (or gastrodermis) lines the hollow cavity called the gastric cavity (coelentron, gut). In describing the morphology of hydra, the adjectives apical (or distal) and basal (or proximal) refer to the directions toward the tentacles and base, respectively.

Identifying Hydra Species

Hydra species differ in morphology, development, physiology, and ecology. Repeatability of experiments therefore will depend upon correct identification. I outline a procedure for distinguishing between the types of hydra used most frequently in research. Three major criteria are used: (1) presence or absence of symbiotic algae, (2) arrangement of tentacles on buds, and (3) shape of the holotrichous isorhiza nematocyst.

Preparing Histological Sections for Light Microscopy

To section hydra that have been embedded in paraffin for histological analysis with the light microscope.

Eliminating All Nonepithelial Cells Using Colchicine

To eliminate the interstitial cell population of hydra using colchicine treatments (Campbell, 1976; Marcum and Campbell, 1978). The resulting viable animals consist only of two layers of epithelial cells plus the noncellular mesoglea. These animals are termed epithelial hydra.

Whole Mounts for Light Microscopy

To prepare whole mounts, thereby enabling one to study large areas of hydra in a single microscopic field in which spatial relationships are preserved between cells, and also to preserve experimental animals for future microscopic analyses.

Culturing Hydra of the Same Species but of Different Sizes

Size and morphology are controlled by patterning agents such as morphogenetic and positional information fields. In order to study the nature of these fields it is useful to be able to modify them. Controlling hydra size by varying the amounts of food given to them provides an experimental method for manipulating the sizes and properties of patterning fields, and also for obtaining hydra of particular sizes for experiments.

Marking Epithelial Cells in Living Hydra with India Ink

To vitally mark epitheliomuscular cells with carbon particles so that those cells can be used to follow the displacement of small groups of cells or to demarcate the boundary of a tissue graft.