Microscopic organisation of the oviducal gland of the holocephalan elephant fish, Callorhynchus milii

Abstract

The study of chondrichthyan reproductive biology has a long history, but the structure and function of the holocephalan oviducal glands (OG) is poorly known; these organs are a vital component in the understanding of chondrichthyan life history. Histochemical techniques revealed that a fundamental zonation was evident in the OG of Callorhynchus milii, similar to most elasmobranchs. In sexually mature females, the following zones occurred (anterior to posterior): (1) club zone, periodic acid–Schiff positive (PAS+), indicating glycoprotein or any mucus substance containing neutral sugars, and Alcian blue positive, pH 2.5 (AB+), indicating the presence of sulfated and unsulfated acid glycosaminoglycans and sialoglycoproteins; (2) papillary zone (AB+); (3) baffle zone (PAS–, AB–); and (4) terminal zone (AB++). Using histological and histochemical techniques not used previously with the holocephalan group, we demonstrated that the structure and function of the OG zones were equivalent between oviparous elasmobranchs and C. milii, even though their final egg capsule morphologies differed. It was also evident that the club and papillary zones produce the egg jelly that surrounds the egg and the baffle zone formed the multilaminate egg capsule. Furthermore, the terminal zone had functions associated with sperm storage and the production of fine hairs that decorate the surface of the egg capsule.