Pakistan J. Zool., vol. 46(2), pp. 389-397, 2016. Intrauterine Development of Female Genital Organs in Cavia porcellus (Rodentia: Caviidae) Amilton Cesar dos Santos, 1* Phelipe Oliveira Favaron, 1 Diego Carvalho Viana, 1 Amanda Olivotti Ferreira, 1 Fernanda Menezes Oliveira e Silva, 1 Dayane Alcântara, 1 Bruno Gomes Vasconcelos, 2 Rose Eli Grassi Rici, 1 Antônio Chaves de Assis-Neto 1 and Maria Angélica Miglino 1 1 Department of Surgery, Sector of Anatomy of Domestic and Wild Animals, School of Veterinary Medicine and Animal Science, University of Sao Paulo, Sao Paulo SP, Brazil 2 Institute of Health Science, Federal University of Mato Grosso, Sinop MT, Brazil A B S T R A C T To contribute to the knowledge of the embryology of an important experimental model for studies related to the reproduction, a study was conducted to describe the morphological events of sexual differentiation of female genital organs during the intrauterine life of Cavia porcellus from 20 th to 68 th gestational day. Eighteen embryos and fetuses at days 20, 25, 30, 45, and 60 of pregnancy and near term (n = 3 animals for each gestational period) were utilized. Embryos and fetuses were dissected, and the genital organs processed for light microscopy. Sexual differentiation on females of Cavia porcellus was observed only after the 25th day of gestation with the development of the ovary. However external genitalia acquired female characteristics only at 45 days of gestation. At the end of gestation, the external genitalia had labia, vaginal external ostium occluded by the vaginal closure membrane and penile clitoris with the external urethral ostium opening at the top of the organ. Microscopically, the tubular genital organs (uterine tubes and horns, uterus and vagina) were lined with internal epithelium and loose connective tissue at 30 days of pregnancy. At the end of the pregnancy tissue layers characteristics of tubular genital organs are slightly differentiated. INTRODUCTION The guinea pig (Cavia porcellus, Caviidae, Linnaeus, 1758) is a hystricomorph rodent, widely used as animal model for research development, especially that related to placentation (Miglino et al., 2004; Mess, 2007) and development of embryonic membranes (Oliveira et al., 2012; Vasconcelos et al., 2013), which have been studied by members of our research group. Other studies as female reproductive cycle (Stockard and Papanicolaou, 1917, 1919; Selle, 1922), immunological studies against vaginal diseases, which in this species closely resemble those found in women (Lilley et al., 1997) and toxicological tests (Leite et al., 2002; Savransky et al., 2013) also were conducted. In addition, the classic study performed by Phoenix et al. (1959) using Cavia porcellus as experimental model, and embryological and hormonal descriptions of Jost (1965) gave rise the model for sexual differentiation, widely used in books of embryology and developmental biology (Wolpert, 1998; Sinowatz, 2010). In adults Cavia porcellus, Cooper and Schiller (1975) describe in general terms the anatomy of this __________________________________ * Corresponding author: amiltonsantoss@bol.com.br 0030-9923/2016/0002-0389 $ 8.00/0 Copyright 2016 Zoological Society of Pakistan species, although it has been noticed the absence of more detailed data concerning the urogenital tract. Regarding the embryology of this species Evans and Sack (1973) describes some events of embryonic development. In relation to the genital organs however, only the presence of the external genitalia in a given period is described, but development of internal and external genitalia events were not described by these authors. Mammals initiate embryonic development without phenotypic sexual differentiation, and an ovary is not required for the female phenotype, but a testicle and the production of androgenic hormones are essential for the development of a male phenotype, since both, Wolff and the Müllerian ducts are found in sexually undifferentiated embryos. In females, Müllerian ducts develop into a duct- gonadal system and Wolff ducts atrophy. In the male the opposite occurs, stimulated by anti-Müllerian hormone and androgens produced by the testicles (Wolpert, 1998; Arnold, 2009; Nakamura, 2010; Sinowatz, 2010). In order to contribute to the knowledge of the embryology of this important experimental model, the aims of this study was to describe the morphological events related to the sexual differentiation of the female genital organs in Cavia porcellus with ages ranging from the 20 th to the 68 th day of pregnancy. Article Information Received 25 March 2015 Revised 22 April 2015 Accepted 13 June 2015 Available online 1 March 2016 Authors’ Contributions MAM and ACAN designed and coordinated the study. DCV and AOF performed macroscopic analysis, FMOS and DA performed dissections. ACDS and POF performed light microscopy of samples. BGV and AOF performed vaginal cytology analysis. ACDS wrote the article. POF and REGR helped in writing the article. Key words Cavia porcellus, guinea pig, penile clitoris, sexual differentiation, reproductive tract.