Hagfish Skin and Slime Glands

Abstract

The scaleless hagfish employs two modes of secretion, holocrine by theslime glands and to a lesser extent merocrine by the epidermis forrelease of its voluminous viscous exudate. In both cases, largeorganized entities of intermediate filaments (IFs) are requisiteparticipants. In the epidermis, each small mucous cell contains anIF-rich, basket-like structure ('capsule') which serves tocompartmentalize the mucin-rich secretory vesicles within the apicalregion for subsequent release. Each epidermal thread cell contains atleast one large IF-biopolymer ('thread') which may affect both thephysical properties of the epidermis and the epidermal exudate. Bycontrast, the single IF-rich thread biopolymer precisely localized ineach gland thread cell, interacts synergistically with mucins from thegland mucous cells to loosely organize water into a viscous mass ofslime (stage 1), which, after physical perturbation, forms even moremassive IF-aggregates ('cables') accompanied by the release of water(stage 2). The massive gland thread exhibits a linearly alignedIF/microtubule motif and is destined for extracellular export tofunction in an aqueous environment. Comparisons of the hagfish threadIF-polypeptide sequences (gamma and alpha, with high threonine-contents)reported herein show no preferred identity to any other type ofsequenced IF including those from higher vertebrates, a cephalochordateand invertebrates. Inasmuch as several keratin traits exist, we havecharacterized gamma and alpha as homologues of type I and II epidermalkeratins, respectively.