Survey of oviducal gland structure and function in elasmobranchs

Abstract

We report the results of a comparative survey of the structure and function of oviducal glands (OG) of selected elasmobranch fish with differing modes of reproduction using light and scanning electron microscopy, e.g., Scyliorhinus canicula, S. stellaris, Raja erinacea, R. eglanteria, R. clavata, Squalus acanthias, Mustelus canis, and Urolophus jamaicensis. Oviducal glands consistently display four fundamental zones regardless of the type of reproduction of the particular species. The zones correspond to lamellae that extend full width across the gland lumen. Formerly, zones of OG were designated as albumen secreting and shell secreting. This oversimplified terminology does not accurately apply to all species. We have adopted the terminology recently introduced (Hamlett et al. [1998] Cybium, in press) that refers to the four basic zones on a morphological basis rather than on a purported function that may not be applicable across species lines. This allows comparisons to be accurately made between species. Oviducal glands have a proximal club zone, papillary zone, baffle zone, and terminal zone. Variations in the makeup of each zone may show species variability, but the fundamental organization is maintained. The club and papillary zones replace the former designation of the albumen zone. The club zone is so named because of its shape when viewed in transverse section with the light microscope. Similarly, the papillary zone is characterized by a papillary or conical profile when viewed in section. The club and papillary zones are responsible for producing the various jelly coats that surround the egg. The baffle zone produces the various types of egg investments seen in elasmobranchs. In oviparous species, such as the sharks S. Canicula and S. Stellaris and the skates R. Erinacea, R. Eglanteria,and R. Clavata, tubular glands produce secretory components that pass to secretory ducts. Secretory ducts are confluent with a spinneret that has paired baffle plates that manipulate the secretory material as it emerges from the secretory duct. Secretory material from adjacent secretory ducts blends in transverse grooves that extend across the full width of the gland, thus one transverse groove is responsible for one secreted layer. The yellow spotted stingray, U. Jamaicensis, is unusual in that it does not produce an egg investiture other than jelly coats and therefore lacks baffle plates. Despite variations among species, the capsule-producing OG we studied seems to use the same basic assembly process to produce tough, flexible, and selectively permeable egg capsules, candles, or egg envelopes. This involves the extrusion of a capsule material through dies, each of which opens between two baffle plates and extrudes a flattened ribbon containing precisely and complexly orientated molecules. The dies discharge into transverse grooves in the main lumen of the gland, each groove thus secreting a single lamella of the egg covering. J. Exp. Zool. 282:399–420, 1998. © 1998 Wiley-Liss, Inc.