14 Braz. J. Morphol. Sci., 2009, vol. 26, no. 1, p. 14-18 P. axelrodi are still unknown (HARRIS and PETRY, 2001). Such information that can aid culturing techniques of this spe- cies on a larger scale is extremely necessary, thereby reducing the fishing pressures on native stocks. Thus, the present study contributes to a better understanding of the reproductive bi- ology of P. axelrodi through histological and histochemical investigations of the oocytes of mature females. 2 Material and methods Paracheirodon axelrodi specimens acquired from an aquar- ium store were maintained in a communal 100 L aquarium during nine months. Artificial lighting was controlled to achieve a 12:12 hours L:D photoperiod. The system was heated to maintain a temperature of 26 °C. During this time, the fishes were fed twice a day with regular commercial fish food (47% protein) and Artemia salina nauplii given 3 times a week as a food complement. Females (n = 8) were anesthe- tized and then sacrificed. Ovaries were removed and fixed in Bouin’s fluid for 10 hours, embedded in paraffin inclusion, cut in 5 µm sections and stained by hematoxilin and eosin (HE). Histological sections derived from both ovaries were analyzed. The oocyte development stages were established based on the morphological alterations of the nucleus, cytoplasm and coats (BAZZOLI, 2003). The presence of carbohydrates, proteins and lipids in the oocytes was determined using clas- sical histochemical techniques according to Pearse (1985): Periodic acid-Schiff (PAS), salivary amylase + PAS, Alcian blue pH 2, 5, Alcian blue pH 0, 5, ninhidrina-Schiff and Sudan black B. 1 Introduction Capture of small-sized ornamental fishes captured in the Negro River and tributaries present a great value in the na- tional and international trade markets. Approximately 70% of the fish exported from the Amazon State comes from the Negro River (ANJOS, SIQUEIRA and AMORIM, 2007) with the municipalities of Barcelos and Santa Isabel do Rio Negro being the main sale locations (CHAO, 2001; FREITAS and RIVAS, 2006). Although dozens of species are regularly exported from the Amazon as ornamental fish, the demand is centered on a small number of species. The main species is the cardinal tetra Paracheirodon axelrodi (SCHULTZ, 1956), representing al- most 3/4 of the export trade in fish (ANJOS, SIQUEIRA and AMORIM, 2007). Its metallic blue color makes it one of the most popular species among Amazonian ornamental fishes trad- ed. They are small-sized fish, reaching up to 5.1 cm in standard length (GEISLER and ANNIBAL, 1986). In the wild, most P. axelrodi species have longevity of little more than one year (GEISLER and ANNIBAL, 1986), but in captivity they are reported to live for more than five years (CHAO, 2001). Recently, P. axelrodi has been the subject of studies in its natural environment (WALKER, 2004; MARSHALL, FORSBERG and THOMÉ-SOUZA, 2007). Recent experi- mental work provided a better understanding of its biology because this species is easy to maintain in captivity. Laboratory experiments conducted by Anjos and Anjos (2006) demon- strated batch spawning of the species and identified the fe- cundity to be between 154 and 562 adhesive eggs. Like other ornamental species of economic importance and interest on the international market, aspects of the biology of Oogenesis of the cardinal tetra Paracheirodon axelrodi Schultz (1956): a histological and histochemical study Brito, MFG. 1 * and Bazzoli, N. 2 1 Departamento de Biologia, Universidade Federal de Sergipe – UFS, Cidade Universitária Prof. José Aloísio de Campos, Av. Marechal Rondon s/n, Jardim Rosa Elze, CEP 49100-000, São Cristóvão, SE, Brasil 2 Programa de Pós-Graduação em Zoologia, Pontifícia Universidade Católica de Minas Gerais – PUC Minas, Av. Dom José Gaspar 500, Coração Eucarístico, Prédio 41, CEP 30535-610, Belo Horizonte, MG, Brasil *E-mail: marcelictio@hotmail.com Abstract A histological and histochemical study of Paracheirodon axelrodi oogenesis was conducted. Four types of oocytes were determined, presenting a thin zona pellucida and squamous follicle cells in all developmental stages. The yolk globules in vitellogenic oocytes are spherical and the micropyle possessed an ample vestibule and short micropylar canal. Atresic follicles were frequent, since ovulation did not occur. The histochemical reactions demonstrated the presence of neutral glycoproteins in the zona pellucida and follicle cells; glycopro- teins and lipids in the yolk globules and carboxylated acid glycoconjugates in the cortical alveoli. The knowl- edge of the reproductive parameters becomes an important tool in captivity breeding programs, reducing the fishing effort on the native stock. Keywords: Paracheirodon axelrodi, reproduction, oocyte, follicle atresia.