Surface secretions of the skin of Blennius (Lipophrys) pholis L.

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Abstract
Mucus is secreted to the surface of the body and fin webs of Blennius pholis by superficial epithelial cells and by goblet cells. Some goblet cells secrete sulphated acid glycoproteins, others produce a mucus which is neutral or mixed in its reactions. The superficial epithelial cells of these areas secrete sulphated acid glycoproteins, seen by electron microscopy as electron‐lucent or moderately lucent vesicles; this secretion is not normally visible external to the skin in transmission electron microscope (TEM) sections. These cells do not react to the bromphenol blue test for proteins. Over part of the surface of the pelvic fins and the distal parts of the rays of the pectoral fins, the skin contains no goblet cells and bears a thick external secretion, or cuticle, containing protein and glycoprotein which is mainly neutral in reaction, although some cells at the edges of the region secrete weakly sulphated or non‐sulphated acidic glycoprotein. The protein content of the columnar superficial epithelial cells of these regions correlates with the fibrous nature of the secreted cuticular layer as seen by TEM; the columnar cells are characterized by extensive ribosomal endoplasmic reticulum and vesicles which stain darkly with phosphotungstic acid, less so with uranyl acetate. The distal part of the cell, containing these vesicles, reacts positively to the PAS stain. In some places the borders of the zones with fibrous cuticle are characterized by cuboidal superficial epithelial cells which give a strong positive reaction to alcian blue at pH 1.0, indicating the presence of sulphated acid glycoproteins, but also react positively to the bromphenol blue test for proteins.