Ultrastructural Characteristics of Keratinization and Secretion in the Epidermis of Some Antarctic Fishes in Relation to Their Cold Adaptation

ABSTRACT Ultrastructural characteristics on keratinization and secretion in the epidermis of some Antarctic fishes in relation to their cold adaptation. Acta Zoologica (Stockholm). Antarctic fishes evolved specific adaptations in the skin, the first organ contacting freezing water. We examined the epidermis of two red‐blooded species ( Trematomus bernacchii and T. newnesi ; family Nototheniidae) and a white‐blooded species ( Chionodraco hamatus ; family Channichthyidae) to provide morphological evidence of cold resistance. The thick epidermis comprises basal, intermediate and apical regions. Cubic‐columnar basal keratinocytes are joined to the thick lamina densa of the basement membrane along their plasma membrane. In the intermediate and apical regions, keratinocytes flatten and store keratins, forming an electron‐dense cortical region. Numerous desmosomes tightly connect keratinocytes, forming a mechanical barrier that may prevent crystal ice entry. During keratinocyte differentiation, the central cytoplasm remains electron‐pale and is enriched in tubular/vesicular mitochondria, SER cisternae and smooth vesicles. Lipids might contribute to thermal barrier formation across the epidermis. Moving toward the skin surface, flattening keratinocytes form microridges, while their organelles degenerate. Lipid degradation may release anti‐freeze glycerol that adds to glycoprotein/glycolipid secretions. Likely, more mechanisms are working together to protect the skin from freezing. The thick basement membrane and numerous desmosomes help prevent ice entry, while lipids provide thermal insulation. These two adaptations would support the protection exerted by the presence of antifreeze glycoproteins and high tissue osmolarity.