Ultrastructural observations of previtellogenic ovarian follicles of the caecilians Ichthyophis tricolor and Gegeneophis ramaswamii

Abstract

The ultrastructural organization of the previtellogenic follicles of the caecilians Ichthyophis tricolor and Gegeneophis ramaswamii, of the Western Ghats of India, were observed. Both species follow a similar seasonal reproductive pattern. The ovaries contain primordial follicles throughout the year with previtellogenic, vitellogenic, or postvitellogenic follicles, depending upon the reproductive status. The just-recruited primordial follicle includes an oocyte surrounded by a single layer of follicle and thecal cells. The differentiation of the theca into externa and interna layers, the follicle cells into dark and light variants, and the appearance of primordial yolk platelets and mitochondrial clouds in the ooplasm mark the transition of the primordial follicle into a previtellogenic follicle. During further development of the previtellogenic follicle the following changes occur: i) the theca loses distinction as externa and interna; ii) all the follicle cells become the dark variant and increase in the complexity of ultrastructural organization; iii) the nucleus of the oocyte transforms into the germinal vesicle and there is amplification of the nucleoli; iv) the primordial yolk platelets of the cortical cytoplasm of the oocyte increase in abundance; v) the mitochondrial clouds fragment and the mitochondria move away from the clouds, leaving behind the cementing matrix, which contains membrane-bound vesicles of various sizes, either empty or filled with a lipid material; vi) the perivitelline space appears first as troughs at the junctional points between the follicle cells and oocyte, which subsequently spread all around to become continuous; vii) macrovilli and microvilli develop from the follicle cells and oocyte, respectively; and viii) the precursor material of the vitelline envelop arrives at the perivitelline space. The sequential changes in the previtellogenic follicles of two species of caecilians are described.

Stages in follicle cell/oocyte interface during vitellogenesis in caecilians Ichthyophis tricolor and Gegeneophis ramaswamii: a transmission electron-microscopic study

“…The sequential cytological changes in the two caecilians from these perspectives are believed to be comparable with those known for the anurans X. laevis (Dumont 1972;Wallace and Bergink 1974;Wallace 1985;Wallace and Selman 1990), Rana esculenta (Wartenberg and Gusek 1960), Rana pipiens (Ward 1962) and Rana tigrina (Sretarugsa et al 2001) and the urodeles Notophthalmus viridescens (Hope et al 1963) and Necturus maculosus (Kessel and Ganion 1980). The formation of the vitelline envelope in X. laevis (Dumont 1972) and R. tigrina (Sretarugsa et al 2001) is first detectable in stage II oocytes as is the case in the caecilians (Beyo et al 2007a(Beyo et al , 2007b. However, a study in X. laevis (Yamaguchi et al 1989) has indicated that the cytoplasm of oocytes as early as stage I contains components reacting to anti-vitelline coat antibodies, suggesting that the oocyte plays a role in synthesizing its own vitelline envelope.…”

“…The two caecilians, which overlap in their distribution in the Western Ghats of Southern India, differ little in their reproductive cyclicity, structural changes in the ovarian follicles and dependence on environmental factors (Beyo et al 2007a(Beyo et al , 2007b. Hence, we have traced the developmental sequence of the follicle cell/oocyte interface for these two species together.…”

“…c Dense granules (dg), oocyte cortex (oc) and extension of the follicle cell (fc) cytoplasm into the tunnel (tl) of the vitelline envelope. Bar 0.31 μm the first arrival of the material of the vitelline envelope in the perivitelline space (Beyo et al 2007a(Beyo et al , 2007b.…”

“…Among the urodeles the salamander Notophthalmus viridescens (Hope et al 1963) and the newt Necturus maculosus (Kessel and Ganion 1980) are two species that have been studied in adequate detail. No reports of TEM analysis of caecilian ovarian follicles exist, except for our description of the previtellogenic follicles of Ichthyophis tricolor and Gegeneophis ramaswamii of the Western Ghats of India (Beyo et al 2007a(Beyo et al , 2007b. However, the light-microscopic morphology of the ovarian changes in relation to follicular dynamics is known for Ichthyophis beddomei Nadkarni 1993a, 1993b) and Typhlonectes compressicauda (Exbrayat 2006).…”

“…The TEM analysis of the follicle cell/oocyte interface of I. tricolor and G. ramaswamii involved the same basic methodology as described earlier (Beyo et al 2007a(Beyo et al , 2007b. I. tricolor (Ichthyophiidae) and G. ramaswamii (Caeciliidae) were collected from terraced plantations of mixed coconut and rubber from Thekkada (08°37′N, 76°57′E) in the Trivandrum district of Kerala and Maramalai (08°26′N, 77°24′E) in the Kanyakumari district of Tamil Nadu, Southern India, from June 2004 to June 2005.…”