Morphology of the Hatching Glands in Betta splendens (Teleostei: Perciformes)

Abstract

The hatching of many teleost embryos from their eggs begins with the release of hatching enzymes from unicellular hatching glands. The enzymes weaken the chorionic membrane, which is then easily broken by the moving larva. Although there have been numerous ultrastructural and biochemical studies on the hatching glands of fishes, with few exceptions (Yamamoto et al., 1979), none have described the morphology of the hatching glands using scanning electron microscopy (SEM). Here we describe for the first time the presence of hatching glands in the Siamese fighting fish, Betta splendens. These were identified using a combination of immunostaining, SEM, TEM, and light microscopy. Using SEM, we describe their morphology and distribution. With TEM and light microscopy, we describe their ultrastructure. Hatching glands are unicellular and are distributed over the head, yolk sac, body, mouth, and gills of embryonic fishes in a species-specific pattern (Ishida, 1985). The size of hatching glands varies among species from 5-30 mm in diameter and 7-20 mm in height. The ultrastructure of hatching glands has been studied in representatives of many teleosts including clupeiforms (Rosenthal and Iwai, 1979), cyprinids (Ouji, 1955, 1966; Willemse and Denuce, 1973), characids (Willemse and Denuce, 1973), esocids (Schoots et al., 1983) salmonids (Yokoya and Ebina, 1976), an osmerid (Yanai et al., 1956), atherinomorphs (Yamamoto, 1963), and perciforms (Helvik et al., 1991a, 1991b). From these ultrastructural studies, the process of hatching granule formation has been inferred. Although they are generally considered to be unicellular holocrine glands (Ishida, 1944a, 1944b; Yamamoto, 1963), Schoots et al. (1983) report a combination of holocrine and merocrine characteristics for the hatching gland cells of pike embryos (Esox lucius). They showed that the hatching enzyme is released by an exocytotic secretory process, characteristic of merocrine glands, but in a single massive release, which is characteristic of holocrine glands. Moreover, the hatching gland cells degenerate by apoptosis, unlike true holocrine cells. The hatching enzyme has been characterized from Oryzias latipes (Yasumasu et al., 1989a, 1989b) Coregonus peled (Luberda and Strzezek, 1991; Luberda et al., 1992), Salmo gairdneri (Hagenmaier, 1974), and Esox lucius (Schoots et al., 1982), and the timing of hatching enzyme gene expression has been examined in 0. latipes (Inohaya et al., 1995).