Scanning electron microscopy (SEM) provides three dimensional details of external surface structures and supplements ultrastructural information provided by transmission electron microscopy (TEM). Animals composed of watery jellylike tissues such as hydras and other coelenterates have not been considered suitable for SEM studies because of the difficulty in preserving such organisms in a normal state. This study demonstrates 1) the successful use of SEM on such tissue, and 2) the unique arrangement of batteries of nematocysts within large epitheliomuscular cells on tentacles of Hydra littoralis.Whole specimens of Hydra were prepared for SEM (Figs. 1 and 2) by the fix, freeze-dry, coat technique of Small and Màrszalek. The specimens were fixed in osmium tetroxide and mercuric chloride, freeze-dried in vacuo on a prechilled 1 Kg brass block, and coated with gold-palladium. Tissues for TEM (Figs. 3 and 4) were fixed in glutaraldehyde followed by osmium tetroxide. Scanning micrographs were taken on a Cambridge Stereoscan Mark II A microscope at 10 KV and transmission micrographs were taken on an RCA EMU 3G microscope (Fig. 3) or on a Hitachi HU 11B microscope (Fig. 4).
Using scanning and transmission electron microscopy (SEM and TEM), we observed batteries of 20-30 nematocytes, cnidoblasts, sensory cells, and other neurons invaginated within epitheliomuscular cells of Hydra. In an attempt to determine how the cells were structurally interrelated, we isolated cells of Hydra littoralis for further study by SEM and TEM.To isolate cells, 30 Hydra littoralis were placed for 30 minutes in a P19 culture medium modified by omitting calcium and magnesium ions, adding 20% sucrose and stabilizing at pH 7.2 with sodium phosphate, then pipetted gently to free the epithelial cells from the mesoglea. Glutaraldehyde was added to make a 5% solution and the cells were fixed 1 hour at 0-4°C, centrifuged 10 minutes at 1000 X g, resuspended in cacodylate buffer twice, and placed 2 hours in 4% osmium tetroxide at 0-4°C.
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