Cross sections (3 m) were stained with Ferric Hematoxylin/Eosin (HE); Toluidine Blue (TB); Schiff Periodic Acid + Ferric Hematoxylin + Metanil Yellow (MY) [37]; and with the Reticulin Method that enhances basement membranes. males arise from the sex reversal of females [8,9,10,11,12]. They are widely distributed across Central and South America [13]. In this species, sex change is correlated to the length of the individual, where reversion takes place in those between 25.0 and 60.0 cm total length [9,12,14]. The sex reversal process in begins in reproductive females and is characterized by a disorganization of the gonadal architecture; intense proliferation of myoid cells; appearance of new germline cysts located at the edges of the ovarian lamellae; massive degeneration of the female germ cells; intense phagocytic activity; increase of vascularization; and the presence of melanomacrophage centers [10,11,15]. During the gonadal development of teleosts, a constant remodeling of the interstitial tissue is required, according to the changes undergone by the germinal epithelium, either during reproductive IFNGR1 cycles [16] or gonadal differentiation [17]. This remodeling of the connective tissue involves events of degradation and new synthesis of extracellular matrix components [18,19]. The degradation of the matrix components is effected by several proteolytic enzymes, with the matrix metalloproteinases (MMPs) being the main ones involved in this process [20]. Metalloproteinases (MMPs) are a group of structurally-related proteins (endopeptidases), calcium and zinc dependent, and active at physiological pH [21,22]. These enzymes act on the degradation of many components of the extracellular matrix during tissue remodeling and may be involved in the regulation of cell-cell and cell-matrix signaling [22,23]. In mammals, studies show that some of the major MMPs are involved in remodeling processes of the male germinal epithelium [24] and Indole-3-carboxylic acid in the rupture of the ovarian follicle [25]. A recent study also shows the expression of proteinase genes and proteolytic enzymes in gonad development of the mouse [26]. However, in aspects regarding the reproductive biology in teleosts, there are few studies that relate gonadal remodeling to the action of the MMPs and none regarding their role during sex reversal. Another important aspect during sex reversion is the role of the steroid hormones. In teleosts, sexual steroids are considered the main factor of gonadal sex development and reproduction and are involved on sex reversal in hermaphroditic species [1,3,12,15,27]. These hormones are produced by gonadal somatic cells: follicle and theca cells, in females; Indole-3-carboxylic acid Sertoli and, especially, Leydig cells in males. In most teleost fishes, as in other vertebrates, Leydig cells are usually located singly or in small clusters in the interstitial compartment of the testis [28]. Leydig cells have features of steroid-producing cells, such as steroid dehydrogenase enzymes (essential for the biosynthesis of most steroid hormones) and receptors for hormone/polypeptide growth factors [29,30]. In fact, different approaches and techniques have been used in teleost Leydig cells and have confirmed that these cells are the main source of testicular steroids [17,31,32,33,34,35]. Leydig cells, also in the swamp eel specimens, from 25 to 54.9 cm total length, were collected in the Tiet river, in the Penpolis region, S?o Paulo State, Southeastern of Brazil (2117 S; 4947 W). Fish were transferred to the laboratory and anesthetized by immersion in a solution of 0.1% benzocaine. Handling of animals was performed in compliance with international standards on animal welfare (Canadian Council on Animal Care, 2005Ottawa, Canada), as well as being in compliance with the local Ethical Committee from Instituto de Biocincias de BotucatuBotucatu, SPBrazil (n. 580-IBBUNESP). 2.2. Sample Preparation for Light Microscopy Animals were anesthetized and immediately following, were sacrificed by decapitation and the gonads were quickly removed. Samples were then fixed in glutaraldehyde 2% and paraformaldehyde 4% solution in Sorensen buffer (0.1 M at 7.2 pH) Indole-3-carboxylic acid for at least 24 h at room temperature. After fixation, the samples were dehydrated in a crescent ethanol series and embedded in Historesin (Leica HistoResin?, Buffalo Grove, IL, USA). Cross sections (3 m) were stained with Ferric Hematoxylin/Eosin (HE); Toluidine Blue (TB); Schiff Periodic Acid + Ferric Hematoxylin + Metanil Yellow (MY) [37]; and with the Reticulin Method that enhances basement membranes. The Reticulin stain [38] uses an oxidizing agent, potassium permanganate, to oxidize aldehyde groups. Subsequently, the oxidized aldehyde groups are detected by the deposition of positive silver ions followed by their reduction.