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This study describes the microscopic anatomy of the reproductive systems of the …

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- Microscopic Anatomy of the Reproductive System in Two Sympatric Species of Fissurella BruguiƩre, 1789 (Mollusca: Vetigastropoda)

F. latimarginata and F. cumingi are gonochoric species. The reproductive system in both these species is simple, and copulatory organs are absent. The reproductive system is intimately associated to the excretory systems. The right kidney is much more developed than the left kidney in both species and there is a single gonad which is connected only with the right kidney. Upon careful examinations of the dissections, the reproductive systems oiF. latimarginata and F. cumingi were indistinguishable in microscopic organization, and so a single description is presented here for both species. The reproductive system is composed of a) a single gonad, b) a renopericardial duct that transport the gametes to the distal portion of the right kidney, it has a gland attached to its inner wall and is connected by the nefrostome to the pericardium; and c) the distal portion of the right kidney which receives the gametes from the duct. The gametes leave the kidney through its aperture toward the mantle cavity, and from there reaching the exterior (Fig.1). The renopericardial duct which carries the gametes from the gonad to the right kidney is referred to from now on as the "genital duct".

Gonad: This organ is a multilobed sac with variable morphology and size depending on the stage of maturity of the individual. The color in males varies from white-cream to yellow and in females is dark green; in both sexes it is well separated from other organs of the visceral mass. The gonad occupies a ventral position in the body cavity, below the digestive gland and stomach, and is dorsal to the foot. In mature individuals it is swollen, and extends from the posterior portion of the visceral cavity toward its anterior end, occupying all the available space. The gonad is covered with a cubical to low columnar epithelium, under which lies a muscle/ connective tissue layer, from which projects septa of connective tissue toward a common gonadic cavity. The germinal cell line develops around the septa, interacting with the somatic cells in a centrifugal organization towards the gonadic cavity (Figs. 2A, B).

Genital duct and kidney opening: Seen in a transversal section genital duct arises in the upper right mid-zone of the gonad and projects upward in the visceral cavity parallel to the muscular wall of the limpet (Figs. 2C; 3A, C), to which it is anchored by a thin mesentery. A gland is proximally attached to the internal wall of the genital duct lumen (Figs. 2C, G, H; 3A, C). Soon after the gland appears the nephrostome (Figs. 2D, I), which is an aperture bearing a ciliated epithelium joining the duct with the pericardial cavity at the level of the right auricle. Then the duct continues its course below the muscular floor of the pericardium toward the mid-region of the animal to open into the distal part of the right kidney, near the base of the right kidney opening (Figs. 2F, K; 3B, D). Distally, the duct near the kidney shows a widening of the lumen and finger-like epithelial folds that fail to fill the lumen, with apparent difference in length between both sexes (Figs. 2E, J; 3B). The transversal section shows two types of simple epithelia all along the genital duct, one above and one below, supported by a muscular/connective tissue layer. The upper epithelium is observed as a continuation of the epithelium which covers the gland associated with the wall of the genital duct, while the lower epithelium is a continuation of the gonadic wall isolating the gonad from the digestive gland. The upper epithelium is formed of cylindrical cells having a vesicular appearance, generally with basal nuclei, among which are intercalated goblet cells. The epithelial cells release large quantities of cytoplasmic vesicles and secretions into the lumen of the genital duct. Prior to their detachment the vesicles remain connected to the cells by cytoplasmic peduncles. Once the vesicles are released they appear to break open in the lumen and release their contents. The lower epithelium is also secretory, and is formed of flatter cells which are more densely stained. The cells are smaller near the distal portion of the duct and their borders are poorly defined. The histology of the mid-portion of the genital duct in males of F. latimarginata and F. cumingi is shown in Figs. 3G, H.

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