Morphology, composition, and deterioration of the embryonic rostral sheath of the smalltooth sawfish (Pristis pectinata)

Elongated rostra evolved in diverse animal groups as adaptations for feeding, defense, sensory perception, and reproduction. Sawfish rostra have tooth-like dermal denticles, referred to as rostral teeth, along their lateral margins. Embryos have a sheath, or covering, for the calcified rostral teeth during gestation, and it persists until after parturition. Little is known about the morphology and composition of the sheath. During 18 years of tagging juvenile smalltooth sawfish (Pristis pectinata), sheaths were documented for 36 neonates with stretch total lengths of 581–812 mm, and samples were collected from 6 specimens for laboratory evaluation. The multilayered, skin-like sheath, which cannot be easily removed manually, has a vascularized inner layer of connective tissue composed primarily of fibrous proteins (e.g., collagen, reticulin, and keratin) surrounded by an outer layer of columnar and spherical epithelial cells overlying a basement membrane. The columnar cells contain condensed chromatin and differentiate into the outermost spherical cells that contain carbohydrates. After birth, the sheath is shed evenly over 4 d, through sloughing and apoptosis, fully exposing the rostral teeth. The sheath is an ephemeral embryonic organ that protects the female and the embryos from injury during gestation and birth.