sm68n3343 10/9/04 12:42 Página 343 SCI. MAR., 68 (3): 343-353 SCIENTIA MARINA 2004 Ultrastructural studies of oogenesis in Bolinus brandaris (Gastropoda: Muricidae)* MARÍA JOSÉ AMOR1, MONTSERRAT RAMÓN2 and MERCÈ DURFORT1 1 Departamento de Biología Celular, Facultat de Biologia, Diagonal 645, 08028 Barcelona, Spain. E-mail: [email protected] 2 Institut de Cièncias del Mar (CSIC), Passeig Marítim de la Barceloneta 37-49, 08003, Barcelona, Spain. SUMMARY: Ultrastructural studies of oogenesis in Bolinus brandaris are described. Although the initial phase of oogen- esis is common to most animal species, vitellogenesis can be considered a species-specific characteristic. In the vitellogen- esis of B. brandaris, mitochondria and endoplasmic reticula play a relevant role in the formation of myelinised membranous systems. Nuclear envelope, Golgi body and the oocyte plasma membrane invaginations are three possible origins for annu- late lamellae. The latter can be considered membranous reservoirs. There are two sources for the vitellum: exogenous (from follicular cells) and endogenous (from the endoplasmic reticulum of the same oocyte). Key words: vitellogenesis, mitochondria, endoplasmic reticulum, annulate lamellae, imposex. RESUMEN: ESTUDIO ULTRAESTRUCTURAL DE LA OOGENESIS DE BOLINUS BRANDARIS (GASTROPODA: MURICIDAE). – Aunque las primeras fases de la oogénesis son comunes en la mayoría de las especies animales, la vitelogénesis sin embargo, puede con- siderarse específica para cada una de ellas. En la vitelogénesis de Bolinus brandaris, cabe destacar el papel de las mitocon- drias y del retículo endoplasmático en la formación de sistemas membranosos que asemejan figuras mielínicas. La envoltu- ra nuclear, el complejo de Golgi y las invaginaciones de la membrana plasmática oocitaria podrían constituir los posibles orígenes de las láminas anilladas, que pueden considerarse un reservorio membranoso. El vitelo a su vez, puede tener dos orígenes: extrínseco (procedente de las células foliculares) e intrínseco (sintetizado por el retículo endoplasmático del pro- pio oocito). Palabras clave: vitelogénesis, mitocondria, retículo endoplasmático, láminas anilladas, imposex. INTRODUCTION affected by the imposex phenomenon (i.e. penis and spermduct are superimposed onto the female gono- Muricidae comprise more than 2000 Neogastro- choristic ducts; Smith, 1971), in response to trib- pod species distributed worldwide. Some of these utyltin (TBT) pollution in sea water, mainly caused species are commercially exploited in, for instance, by anti-fouling paints. This phenomenon is caused by Thailand (Nugranad et al. 1994) and Chile (Gutiérrez the alteration of the steroid metabolism (Oehlmann et and Gallardo, 1999). Bolinus brandaris (Linnaeus, al. 1993) and has been shown to cause alterations to 1758) is common in the Mediterranean and consti- the genital tract (Oehlmann et al., 1991). tutes a locally important resource in Spain, Italy and One of the first reports about prosobranch repro- Turkey (Martín et al. 1995). On the other hand, B. duction is by Schitz (1920), who examined gameto- brandaris, like many other prosobranchia species, is genesis with light microscopy in Hexaplex trunculus. Further details on the ultrastructure of members of the *Received June 16, 2003. Accepted November 20, 2003. family Muricidae are provided by Bottke (1972) in OOGENESIS IN BOLINUS BRANDARIS 343 sm68n3343 10/9/04 12:42 Página 344 Viviparus contectus; Durfort (1973a) in Murex ele- meiosis are characterised by the presence of a round nensis; Griffon and Gomot (1979) in Viviparus vivip- nucleus 6 µm in diameter, in which drops of hete- arus; Buckland-Nicks (1973) and Buckland-Nicks rochromatin are spread throughout the nucleoplasm. and Chia (1976) in Littorina sitkana; Buckland-Nicks The cytoplasm is nearly empty, except for some et al. (1982 and 1983) in Fusitriton oregonensis; and mitochondria and dictyosomes (Fig. 1A). The most Romanova (1978) in Littorina saxatilis From 1987 to developed apparatus is the centriole, shown as a 1990 we studied the spermatogenesis of B. brandaris microtubule organising centre (Fig. 2A). (Amor, 1987; Amor and Durfort, 1990a,b). In the pre- During meiosis, the most outstanding phase is sent study we document the ultrastructural character- prophase I, in which the nucleus enlarges and chro- istics of the gametogenesis of female B. brandaris mosomes and synaptonemal complexes appear. As affected by imposex. is usual at this stage, the cells are undifferentiated and the nucleus/cytoplasm ratio is high. Vitellogenesis in B. brandaris can be divided MATERIALS AND METHODS into three main stages: In the first stage, the cell is 30 µm in diameter Thirty female individuals of Bolinus brandaris and the nucleus (22 µm) is round or oval. It shows were collected in April 1999 from a coastal Mediter- one or two nucleoli (1.6 µm), in which we can often ranean site (Sant Carles de la Ràpita, Spain) at distinguish granular and fibrillate phases (Figs. depths of 15 to 25 m using an artisanal dragged gear 1B(a) and 2B). The nuclear envelope shows abun- (Martín et al. 1995). Imposex in this site was moni- dant well-developed pores (1 pore per µm2 of about tored in a previous study (Ramón and Amor, 2001) 90 nm in diameter). Slight invaginations increase and reached 99.7% of the females examined the envelope area and favour the passage of nuclear (N=301). The shell was cracked with a vice and the precursors for vitellogenesis (Fig. 1B(a)). The cyto- gonads of imposex females were carefully removed. plasm increases in volume and its organelles Thin sections were fixed in 10% formalin and increase in both number and in volume. Thus, we stained with hematoxylin-eosin and the PAS tech- can distinguish vacuoles of various sizes, PAS posi- nique (i.e. cytochemical stain with periodic acid and tive ß-glycogen granules and vesicles (4-5 µm) full Schiff reagent) for study by light microscopy. Thin- of material of varying electrondensity. Mitochondria ner sections were processed for transmission elec- are well-developed (1.5 µm long) and can be rod- tron microscopy following routine double fixation, shaped, curved or elongate. The cristae and matrix i.e. glutaraldehyde and 2.5% OsO4, both buffered are well formed. Both are near the nucleus and their using Sörensen’s phosphate buffer. Samples were number increases progressively, leading to the for- embedded in Spurr’s resin after progressive dehy- mation of mitochondria clusters. Mitochondria dration. About 1 mm-thick sections were obtained divide by bipartition or gemmation (Fig. 2C). At the and stained with methylene-blue borax to select the end of this stage, several mitochondria loose their areas most suitable for the ultrathin sections. These cristae and become vesicles, and some are invaded were about 30 nm thick and were cut using a by pre-vitelline material (Figs. 1B(a) and 2C). Reichert-Omu ultramicrotome with a diamond In the plasma membrane, intercellular bridges knife. Sections were picked up on copper grids and and desmosomes can be seen among young oocytes, stained with uranyl acetate and lead citrate. Thiery’s as well as microvilli (Fig. 1B). Desmosomes and technique (Thiery 1967) was sometimes used, and intercellular spaces showing accumulations of pre- then the sections were picked up on gold grids. We vitellogenic material are also shown among oocytes used a 301 Philips transmission electron microscope and follicle cells caused by electrondense vesicles at the Serveis Científico-Tècnics of the University possibly from follicle cells origin (Figs. 3A(a,b) and of Barcelona. 3B). Sometimes, the plasma membrane invaginates to form small vacuoles, which aggregate to create a large, round reticule of annulate lamellae (Figs. RESULTS 3A,B). The Golgi body is well developed and is formed by several dyctiosomes, with abundant cis- Oogenesis of Bolinus brandaris followed four ternae. These produce vesicles of varying electron- main stages: premeiosis, meiosis, vitellogenesis and density, multivesicular bodies and also annulate the formation of the mature oocyte. Premeiosis and lamellae (Fig. 3C(b)). Clusters of annulate lamellae 344 M.J. AMOR et al. sm68n3343 10/9/04 12:42 Página 345 FIG. 1. – A: Ultrastructural panoramic view of the ovary. Young oocytes can be seen (star). The nucleus (n) showing the chromatin dispersed in small drops, the nucleolus (arrowhead) and the mitochondria (m) are apparent in the cytoplasm. Note the non-germinal cells, apparently connective cells (asterisk), too. B: Detail of an oocyte in early stages of oogenesis. (a): The nucleus (black N) shows a compact and large nucleolus (white N). The chromatin is granulated, although some condensed droplets appear in the caryoplasm (arrowheads). The nucleus envelope shows ‘undulations’ and nuclear pores (black asterisk). The cytoplasm is limited by the plasma membrane (pm) showing microvil- li (white arrowhead). In the cytoplasm, note the clusters of mitochondria (mt). Vesicles of variable electrondensity (star) are detected, togeth- er with lysosomes (L). The white asterisk shows electrondense material embedding degenerate mitochondria, which still remain cristae (black arrow). Other electrondense vesicles with mitochondria morphology are also observed (black arrows). Outside the oocyte, a follicular cell(F) with glycogen granules (g) are shown and below, connective tissue (C) is detected; (b): microvilli (asterisk), present between two oocytes (O1 and O2), are shown. OOGENESIS IN BOLINUS BRANDARIS 345 sm68n3343 10/9/04 12:42 Página 346