Glands, unicellular

Although secretions from the (so-called) penetration glands undoubtedly play a role in penetration, their nature and function are largely unknown. The glands have been particularly studied in H. diminuta and H. nana (205, 888, 889). In most species, glands have been described as syncytial (Fig. 8.16) or multicellular, but in H. nana they appear to be unicellular (205). One or two pairs of ducts may be present and these are supported by microtubules. Secretion has been described as being holocrine in some species (as in trematode miracidia and cercariae) but apocrine and merocrine secretory mechanisms have also been proposed for others (205, 242, 443). 

However, fibroin mRNA survives for several days, during which each mRNA molecule is translated repeatedly to yield 10 fibroin molecules. Thus, the whole unicellular silk gland makes 10 molecules or 300 mg of fibroin. 

Columnar cells are longer than they are wide the small intestine is a tubular organ lined with a simple columnar epithelium composed of a single layer of cells. Unicellular glands are scattered throughout this type of tissue and secrete mucus. 

The usual source of TDTase is the calf thymus gland. It contains 5-10 mg of TDTase per kg of fresh tissue. The enzyme is also found at high levels (100—200 U/10 cells) in certain leukemic lymphocytes (e.g., human leukemia Molt-4 and mouse thymoma PI 798) and at a detectable level in bone marrow. TDTase activity is not found in prokaryotes nor in unicellular eukaryotes. 

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