Histological characteristics of oocyte differentiation in the captive longnose seahorse Hippocampus trimaculatus (Leach, 1814).

This study aimed to explore the reproductive histology and oocyte differentiation of the longnose seahorse Hippocampus trimaculatus (Leach, 1814) in captivity. Five mature healthy females were histologically observed. The reproductive systems of the five specimens exhibited similar morphological characteristics with a pair of saccular creamy white ovaries merging caudally into a single gonoduct. There were two germinal ridges lined with a layer of germinal epithelium (GE). The ovarian maturation of this species was considered asynchronous. The oogenic cells were classified into oogonia and oocytes at several developmental phases based on their size and characteristics. Oogonia were identified among the connective tissue in the middle area of the GE. The stromal compartment contained oocytes that were classified into four distinct phases: the primary growth (PG) phase having two steps (perinucleolar and oil droplets-cortical alveolar steps) and the secondary growth (SG) phase with three oocyte types, including early SG oocytes, late SG oocytes, and fully grown oocytes. The atretic oocytes (AO) were observed in all stages of oogenesis. Postovulatory follicles were also seen among the ovarian connective tissue. The occurrence of postovulatory follicles suggested that the specimens analysed in this study were in the spawning period. This research provides new insights into the identification of the reproductive cycles and morphological characteristics of the ovary of H. trimaculatus.

The male reproductive cycle of the brown basilisk Basiliscus vittatus (Squamata: Corytophanidae) from Tabasco, Southern Mexico.

We used histological and morphometric methods to study the testis and associated glands, including the epididymis, ductus deferens, and renal sexual segment (RSS), of specimens of Basiliscus vittatus sampled from Tabasco, Mexico (17.5926° N, 92.5816° W). Samples were collected throughout 1 year, which included the dry (February to May) and rainy (June to January) seasons. Spermatogenesis in B. vittatus is active throughout the year, but a significant increase in the testicular volume, diameters of seminiferous tubules, height of the germinal epithelium, spermiogenesis, and released spermatozoa occur in the dry season. During the rainy season, all aforementioned parameters decreased except the secretory activity of the epididymis and the RSS, which increased concomitant with an increase of the spermatozoa population within the ductus deferens. These data strongly suggest that B. vittatus reproduce year-round, but males exhibit a peak in spermatogenic activity during the dry season and a peak in insemination and/or copulation at the beginning of the rainy season. We highlight the importance of analyzing not only the testis but also accessory ducts and glands when determining the reproductive cycles of reptiles. The reproductive cycle of B. vittatus is discussed in relation to the environmental conditions of Southern Mexico and is compared to that of other squamates.

Morphological basis for maternal nutrient provision to embryos in the viviparous fish Ataeniobius toweri (Teleostei: Goodeidae).

In viviparous Mexican fishes of the family Goodeidae, embryos develop in the maternal ovarian lumen. They typically absorb maternal nutrients during gestation by means of "trophotaeniae," that is, specialized, elongated extensions of the hindgut that are exposed to the fluids, which occupy the ovarian lumen. The sole exception is Ataeniobius toweri, whose embryos lack trophotaeniae but are nevertheless matrotrophic. Thus, how its embryos obtain maternal nutrients is unclear. We studied a series of non-pregnant and pregnant ovaries of A. toweri using histology to identify the mechanism of maternal-embryo nutrient transfer. By early-gestation, embryos have depleted their yolk supplies. Yolks are released into the ovarian lumen and are ingested by the developing embryos, as shown by yolk material in their digestive tracts. The embryonic gut is lined by an epithelium consisting of columnar cells with apical microvilli, providing a means for nutrient absorption. Contrary to statements in the literature, embryos develop minuscule trophotaenial rudiments that extend slightly into the ovarian lumen. These structures are formed of an absorptive epithelium that overlies a vascular stroma, similar to the trophotaeniae of other goodeids. Through late gestation, vitellogenic follicles form and oocytes are discharged into the ovarian lumen, contributing to embryonic nutrition. Thus, histological evidence suggests that embryos chiefly obtain nutrients from ingestion of yolk and maternal secretions released into the ovarian lumen. This function possibly is supplemented by uptake via the small hindgut protrusions and other absorptive surfaces (e.g., the skin and the gill epithelium). Our observations are consistent with two evolutionary interpretations of the hindgut protrusions: (a) that they are rudimentary, evolutionary precursors of trophotaeniae formed by exteriorized hindgut; and (b) that they are vestigial remnants of trophotaeniae that were lost during a switch to a form of matrotrophy involving nutrient ingestion.

Morphology of yolk and pericardial sacs in lecithotrophic and matrotrophic nutrition in poeciliid fishes.

We used histological techniques to describe the morphology of the yolk and pericardial sacs in developing embryos of the lecithotrophic species Girardinus creolus, Gambusia puncticulata, Limia vittata, and Quintana atrizona, in comparison with the extreme matrotrophic Heterandria formosa. In lecithotrophic species, the yolk sac was enlarged and lasted until the final stages of development, while in H. formosa it was completely absorbed soon after fertilization. Lecithotrophic poeciliids showed a pericardial sac with a single layer of blood vessels covering the dorsal surface of the cephalic region only, while H. formosa showed a more complex largely vascularized pericardial sac covering the entire dorsal surface, except the caudal region. In advanced gestation of G. creolus, a vascular plexus of the yolk sac reaches the pharyngeal region, behind the gills, suggesting that the pharynx may play a role in embryonic nutrition in lecithotrophic species. These morphological evidences suggest that matrotrophy derives from lecithotrophy.

Morphological patterns of cell death in ovarian follicles of primary and secondary growth and postovulatory follicle complex of the annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae).

The dynamics of cellular development and homeostasis of the ovary depend on the balance between proliferation and cell death throughout the reproductive cycle. Millerichthys robustus is an annual fish whose ovarian follicles develop asynchronously, allowing daily reproduction from sexual maturity until death. The objective of this research is to describe, histologically, the processes of follicular atresia and regression of postovulatory follicular complexes (POC) throughout a reproductive cycle of M. robustus. Patterns of cell death were documented by apoptosis in atretic follicles and POC, and necrosis in the POC after ovulation with an associated inflammatory response. Atretic follicles were seen from the onset of sexual maturity, during week three post-hatching (PH), both in primary growth (from the Cortical alveoli step, with folliculogenesis completed) and secondary growth Stages, with a higher prevalence in the latter. POCs were observed in different stages of regression from week four PH until the death of the fish. The apoptotic characteristics found were: (i) fragmentation of the nuclear membrane and zona pellucida, and liquefaction of the cortical alveoli and yolk; (ii) follicular cells becoming phagocytic, increasing their size, and migrating within the oocyte; and (iii) formation of an intrafollicular lumen, a product of phagocytosis of the oocyte constituents and dispersed pigments that remain after the digestion of yolk and cortical alveoli. The morphological changes of the follicular cells of the POC, from a squamous morphology after ovulation to columnar during its regression with PAS+ contents, was documented, suggesting a secretory activity.

Structure of the testis and spermatogenesis of the viviparous teleost Poecilia mexicana (Poeciliidae) from an active sulfur spring cave in Southern Mexico.

We describe the histological characteristics of the testis and spermatogenesis of the cave molly Poecilia mexicana, a viviparous teleost inhabiting a sulfur spring cave, Cueva del Azufre, in Tabasco, Southern Mexico. P. mexicana has elongate spermatogonial restricted testes with spermatogonia arranged in the testicular periphery. Germ cell development occurs within spermatocysts. As spermatogenesis proceeds, the spermatocysts move longitudinally from the periphery of the testis to the efferent duct system, where mature spermatozoa are released. The efferent duct system consists of short efferent duct branches connected to a main efferent duct, opened into the genital pore. Spermatogenesis consisted of the following stages: spermatogonia (A and B), spermatocytes (primary and secondary), spermatids, and spermatozoa. The spermatozoa are situated within spermatocysts, with their heads oriented toward the periphery and flagella toward the center. Once in the efferent duct system, mature spermatozoa are packaged as unencapsulated sperm bundles, that is, spermatozeugmata. We suggest that the histological characteristics of the testis and spermatogenesis of P. mexicana from the Cueva del Azufre, and the viviparous condition where the spermatozoa enter in the female without been in the water, have allowed them to invade sulfurous and/or subterranean environments in Southern Mexico, without requiring complex morphofunctional changes in the testis or the spermatogenetic process.

Superfetation in the viviparous fish Heterandria formosa (Poeciliidae).

Heterandria formosa is a viviparous poeciliid native of the southeastern of United States of America. H. formosa exhibits unique reproductive features as: (a) production of extremely small eggs with scarce quantity of yolk (microlecithal eggs), (b) consequently, a high level of matrotrophy and development of a complex follicular placenta, (c) ovarian sperm storage that allows the continuous fertilization of oocytes and production of offspring and (d) development of high degree of superfetation. The degree of superfetation refers to the number of broods in different simultaneous stages of gestation. Morphological evidence of the degree of superfetation in H. formosa has not been documented. Therefore, and because of the general interest in the complex process of superfetation, the goal of this study is to morphologically define the degree of superfetation of H. formosa through two procedures: (a) histological analysis of entire ovaries in gestation and (b) dissection of visible embryos and the histological analysis of the remaining ovarian tissue. Results indicate that H. formosa can gestate up to seven broods at the same time.

Ovarian structure, folliculogenesis and oogenesis of the annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae).

Cellular aspects of oocyte development of the Mexican rivulus Millerichthys robustus were morphologically described in order to analyze ovarian function and the cellular recruitment dynamics associating it with life history strategies of annual killifishes. Millerichthys is an iteroparous batch spawner with continuous oocyte recruitment and indeterminate fecundity with asynchronous development of the follicles. It has two ovaries of cystovarian type, with a central lumen, which communicates with the outside through the caudal region of the ovary, that is, the gonoduct. From the walls of the ovary, irregular lamellae composed of germinal epithelium and vascularized stroma project. Oogenesis starts with oogonial proliferation, found alone or in nests within the germinal epithelium. The oogonia come into meiosis becoming oocytes and advancing to the chromatin nucleolus stage and to early primary growth stage. Folliculogenesis is completed in the primary growth stage and cortical alveoli step. Follicles moves toward the stroma, but they continue to be attached to the germinal epithelium through the basement membrane until ovulation. The inclusion of fluid yolk in the follicles during the secondary growth stage was observed. During ovulation, the follicle collapsed, the oocyte was released into the lumen, and the constitutive elements of the post-ovulatory follicle complex remained in the stroma.

Testicular structure and spermatogenesis of the oviparous goodeids Crenichthys baileyi (Gilbert, 1893) and Empetrichthys latos Miller, 1948 (Teleostei, Cyprinodontiformes).

The cyprinodontiform family Goodeidae comprises some 51 species, including subspecies, of freshwater fishes all of which are at risk or are extinct in the wild. It is classified in two allopatric subfamilies: the Goodeinae, endemic to the Mexican Plateau, and the Empetrichthyinae, known only from relict taxa in Nevada and southern California. The 41 species of goodeins are all viviparous and share a set of well-documented reproductive characters. In contrast, the recent species or subspecies of empetrichthyins are all oviparous and relatively poorly known, yet of critical interest in understanding the evolution of livebearing in the family. We previously described ovarian structure and oogenesis in empetrichthyins using archival museum specimens of females and here extend that study to males. Testicular characters of two species of empetrichthyins, Crenichthys baileyi, and Empetrichthys latos, are studied and compared directly with those of one species of viviparous goodeid, Ataeniobius toweri. The testis is a restricted spermatogonial type in both the Empetrichthyinae and the Goodeinae: spermatogonia are found solely at the distal termini of lobules, a diagnostic character of atherinomorph fishes. Morphology of the differentiation of germinal cells during spermatogenesis is similar in both subfamilies. In the oviparous C. baileyi and E. latos spermatozoa are free in the deferent ducts. In contrast, the spermatozoa of viviparous goodeids are organized into numerous bundles called spermatozeugmata, a characteristic of most fishes that practice internal fertilization. Differences between the goodeid subfamilies are interpreted relative to the oviparous versus viviparous modes of reproduction. Archival museum specimens are a reliable source of data on reproductive morphology, including histology, and may be the only specimens available of rare or extinct taxa.

Change of lecithotrophic to matrotrophic nutrition during gestation in the viviparous teleost Xenotoca eiseni (Goodeidae).

Teleosts possess unique features of the female reproductive system compared with the rest of vertebrates, features that define the characteristics of their viviparity. Viviparity involves new maternal-embryonic relationships detailing the most diverse structures during gestation that include embryonic nutrition. In order to analyze the morphological features of the complex nutrition in viviparous teleosts during intraovarian gestation, this study utilizes the goodeid Xenotoca eiseni as a model. Ovarian gestation in X. eiseni, as in all goodeids, is intraluminal; the early embryo moves from the follicle to the ovarian lumen where gestation continues. The scarce yolk in the oocytes implies that the initial lecithotrophy is replaced by matrotrophy, with nutrients provided via maternal tissues. The nutrients are absorbed by the embryo mainly by trophotaenia, extensions of the embryonic intestine into the ovarian lumen. This histological study analyses the structures involved in these two types of nutrition and when they occur during gestation in X. eiseni. The morphology displayed in this study demonstrated the extended simultaneity of lecithotrophy and matrotrophy during gestation with the progressive reduction of lecithotrophy and increase of matrotrophy. Similarly, it describes the development of complex embryonic structures for metabolic exchange with the maternal tissues associated with matrotrophy; specifically the branchial placenta and mainly the trophotaenia.

Insemination, intrafollicular fertilization and development of the fertilization plug during gestation in Heterandria formosa (Poeciliidae).

The viviparous teleost Heterandria formosa is a remarkable species for its reproductive characters including: (a) the smallest oocyte in viviparous fish species; (b) a high level of matrotrophy with a complex placenta; and (c) the highest level of superfetation. Superfetation involves (d) the continuous development of oocytes and fertilization at the same time with embryos in gestation. The sequential fertilization of oocytes requires (e) storage of spermatozoa in the ovary. Among these characteristics, fertilization is of fundamental interest, specifically the intrafollicular fertilization of poeciliids, species that do not present micropyle, and the consequent formation of the fertilization plug, a structure developed at the periphery of the follicle where the entrance of spermatozoa occurs. Both processes intrafollicular fertilization and formation of the fertilization plug have been rarely described. There is only one study illustrating, the fertilization plug of H. formosa with a drawing. In the context of reproductive aspects of H. formosa, the goal of this study is to describe the morphology of the ovary during insemination, intrafollicular fertilization and development of the fertilization plug. After insemination, spermatozoa enter the ovary and occupy folds of the lamella near follicles of all stages of oogenesis, the delle, where the germinal epithelium establishes contact with the follicular epithelium. The results of the present study provide evidence that both epithelia open at the distal end of the delle, this morphological change allow that the spermatozoa to make contact with the zona pellucida of the oocyte. After fertilization, the delle becomes blocked by proliferation of cells of the germinal epithelium, to form the fertilization plug that persists throughout gestation. Abundant reticular fibers and blood vessels are seen around the fertilization plug. Persistence of the fertilization plug suggests that it could be the site where the juvenile will gain entrance to the ovarian lumen during birth.

Ovarian structure and oogenesis of the extremophile viviparous teleost Poecilia mexicana (Poeciliidae) from an active sulfur spring cave in Southern Mexico.

The structure of the ovary and oogenesis of Poecilia mexicana from an active sulfur spring cave is documented. Poecilia mexicana is the only poeciliid adapted to a subterranean environment with high hydrogen sulfide levels and extreme hypoxic conditions. Twenty females were captured throughout one year at Cueva del Azufre, located in the State of Tabasco in Southern Mexico. Ovaries were processed with histological techniques. P. mexicana has a single, ovoid ovary with ovigerous lamella that project to the ovarian lumen. The ovarian wall presents abundant loose connective tissue, numerous melanomacrophage centers and large blood vessels, possibly associated with hypoxic conditions. The germinal epithelium bordering the ovarian lumen contains somatic and germ cells forming cell nests projecting into the stroma. P. mexicana stores sperm in ovarian folds associated with follicles at different developmental phases. Oogenesis in P. mexicana consisted of the following stages: (i) oogonial proliferation, (ii) chromatin nucleolus, (iii) primary growth, subdivided into: (a) one nucleolus, (b) multiple nucleoli, (c) droplet oils-cortical alveoli steps; (iv) secondary growth, subdivided in: (a) early secondary growth, (b) late secondary growth, and (c) full grown. Follicular atresia was present in all stages of follicular development; it was characterized by oocyte degeneration, where follicle cells hypertrophy and differentiate in phagocytes. The ovary and oogenesis are similar to these seen in other poeciliids, but we found frequent atretic follicles, melanomacrophage centers, reduced fecundity and increased of offspring size.

Functional morphology of the gonoduct of the viviparous teleost Poeciliopsis gracilis (Heckel, 1848) (Poeciliidae).

Female teleosts do not develop Müllerian ducts; consequently, the ovary of teleosts contains two zones: germinal and gonoduct. The gonoduct lacks germinal cells, but has relevant functions in the reproductive process. We describe the functional morphology of the gonoduct in the viviparous teleost Poeciliopsis gracilis during nongestation and gestation stages. This study tests the hypothesis that the gonoduct functions as a barrier between the germinal zone and the exterior. By providing information about morphology and function of the gonoduct we show that this part of the ovary has an essential role in the reproduction of teleosts. The ovaries were processed by histological technique and stained with hematoxylin-eosin (H-E), Masson's trichrome, toluidine blue and periodic acid-Schiff (PAS). The gonoduct is divided into three regions: cephalic, middle, and caudal. In the cephalic and middle regions there are mucosal folds that extend into the gonoductal lumen, forming structures similar to a cervix. The caudal region has two portions: the anterior contains a dorsal invagination and exocrine glands among columnar cells; the posterior has a ventral flexion and stratified epithelium with apical secretory cells. The morphology of this epithelium indicates two functions: (a) secretory by the apical columnar cells, and (b) protection through the stratification. Another peculiarity of the caudal region is that both ducts, reproductive and digestive, converge in a common cavity at their caudal ends, forming a cloacal region. The histology of the gonoduct indicates relevant functions including: (1) the control of the luminal diameter by the muscle and the presence of mucosal folds, like a cervix; (2) the relationship with the spermatozoa during insemination and storing them in mucosal folds; (3) the support of immunological processes; (4) secretory activities; (5) forming the duct during birth; and (6) possibly, acts as a barrier against parasite infestations.

Oogenesis: From Oogonia to Ovulation in the Flagfish, Jordanella floridae Goode and Bean, 1879 (Teleostei: Cyprinodontidae).

We provide histological details of the development of oocytes in the cyprinodontid flagfish, Jordanella floridae. There are six stages of oogenesis: Oogonial proliferation, chromatin nucleolus, primary growth (previtellogenesis [PG]), secondary growth (vitellogenesis), oocyte maturation and ovulation. The ovarian lamellae are lined by a germinal epithelium composed of epithelial cells and scattered oogonia. During primary growth, the development of cortical alveoli and oil droplets, are initiated simultaneously. During secondary growth, yolk globules coalesce into a fluid mass. The full-grown oocyte contains a large globule of fluid yolk. The germinal vesicle is at the animal pole, and the cortical alveoli and oil droplets are located at the periphery. The disposition of oil droplets at the vegetal pole of the germinal vesicle during late secondary growth stage is a unique characteristic. The follicular cell layer is composed initially of a single layer of squamous cells during early PG which become columnar during early vitellogenesis. During primary and secondary growth stages, filaments develop among the follicular cells and also around the micropyle. The filaments are seen extending from the zona pellucida after ovulation. During ovulation, a space is evident between the oocyte and the zona pellucida. Asynchronous spawning activity is confirmed by the observation that, after ovulation, the ovarian lamellae contain follicles in both primary and secondary growth stages; in contrast, when the seasonal activity of oogenesis and spawning ends, after ovulation, the ovarian lamellae contain only follicles in the primary growth stage. J. Morphol. 277:1339-1354, 2016. © 2016 Wiley Periodicals, Inc.

Conserved form and function of the germinal epithelium through 500 million years of vertebrate evolution.

The germinal epithelium, i.e., the site of germ cell production in males and females, has maintained a constant form and function throughout 500 million years of vertebrate evolution. The distinguishing characteristic of germinal epithelia among all vertebrates, males, and females, is the presence of germ cells among somatic epithelial cells. The somatic epithelial cells, Sertoli cells in males or follicle (granulosa) cells in females, encompass and isolate germ cells. Morphology of all vertebrate germinal epithelia conforms to the standard definition of an epithelium: epithelial cells are interconnected, border a body surface or lumen, are avascular and are supported by a basement membrane. Variation in morphology of gonads, which develop from the germinal epithelium, is correlated with the evolution of reproductive modes. In hagfishes, lampreys, and elasmobranchs, the germinal epithelia of males produce spermatocysts. A major rearrangement of testis morphology diagnoses osteichthyans: the spermatocysts are arranged in tubules or lobules. In protogynous (female to male) sex reversal in teleost fishes, female germinal epithelial cells (prefollicle cells) and oogonia transform into the first male somatic cells (Sertoli cells) and spermatogonia in the developing testis lobules. This common origin of cell types from the germinal epithelium in fishes with protogynous sex reversal supports the homology of Sertoli cells and follicle cells. Spermatogenesis in amphibians develops within spermatocysts in testis lobules. In amniotes vertebrates, the testis is composed of seminiferous tubules wherein spermatogenesis occurs radially. Emerging research indicates that some mammals do not have lifetime determinate fecundity. The fact emerged that germinal epithelia occur in the gonads of all vertebrates examined herein of both sexes and has the same form and function across all vertebrate taxa. Continued study of the form and function of the germinal epithelium in vertebrates will increasingly clarify our understanding of vertebrate reproduction. J. Morphol. 277:1014-1044, 2016. © 2016 Wiley Periodicals, Inc.

Morphological development of the structures related to annualism in the ovarian follicle of the killifish Millerichthys robustus (Costa,1995) (Teleostei: Cyprinodontiformes).

Ovaries of five females of the annual fish teleost species Millerichthys robustus were processed, and the development of the cortical alveoli, zona pellucida and secondary envelope during oogenesis were described. We also documented the origin of the cortical alveoli in time and space similar to the Balbiani body; the synthesis of three generations of cortical alveoli and an active zona pellucida prior to vitellogenesis, which is implicated in the entry of oils to the interior of the oocyte. We found that in this species, the diameter of the alveoli is greater than in the other teleost fish species reported in the literature, except for Fundulus heteroclitus, in which the diameter is similar. The thickness of the zona pellucida recorded in M. robustus is the greatest reported to date. Likewise, two periods of secondary envelope deposition were documented: filaments during pre-vitellogenesis and, subsequently, trapeze-shaped projections during the maturation of the oocytes. We report about development of structures that are considered key for the survival of embryos in annual fish during the long periods of diapause in their extreme habitats. The development of peripheral structures described here probably reflects the changes in the physiology of the oocytes in M. robustus. J. Morphol. 277:1219-1230, 2016. © 2016 Wiley Periodicals, Inc.

The occurrence of spermatozoa in the ovary of the gynogenetic viviparous teleost Poecilia formosa (POECILIIDAE).

The reproductive mode of the female viviparous teleost Poecilia formosa (Poeciliidae) represents the phenomenon known as gynogenesis; that is, parthenogenetic development is initiated by spermatozoa which are needed for physiological activation of the egg and the initiation of gestation, but spermatozoa are prevented from contributing to the genome of the embryo. For the reason that no previous histological analyses of the ovary of this species during the reproductive cycle has been published the present study has been conducted. This study examined the histology of the ovary of P. formosa during nongestation and gestation phases and identified the presence of spermatozoa inside the ovary. Spermatozoa were observed in folds of the ovarian epithelium of P. formosa during both the nongestation and gestation phases. Sperm storage as documented in this study is a very important trait for the gynogenetic viviparous fish P. formosa contributing to the understanding of this species reproduction.

Permanent germinal epithelium and reproductive cycle of Atractosteus tropicus (Lepisosteiformes: Lepisosteidae) males, Tabasco, Mexico.

The tropical gar A. tropicus plays an important ecological role as it regulates other fish stocks in different water bodies in Southeastern México. Nevertheless, wild populations are declining, and one conservation alternative is the aquaculture production and basic knowledge of reproductive biology; for males, this requires the study of germ and somatic structures of testes, to characterize the reproductive cycle, and to provide basic knowledge for exploitation and conservation models and strategies. With this aim, a total of 24 males with an average Lp = 47.2 cm were collected from wild populations from the Laguna Pomposú, municipality of Jalpa de Mendez (18°19' - 93°01'12" W), Tabasco, Mexico. Fish were collected with a trawl net and were transported live to the Tropical Aquaculture Laboratory, División Académica de Ciencias Biológicas (DACBiol), Universidad Juárez Autónoma de Tabasco (UJAT). Males were slaughtered by prolonged immersion in MS222. Testes samples were collected from each specimen and were processed using the standard histological procedures, that consisted of dehydration in an ascending ethanol series, xylol, embedding in paraffin, sectioning at 7 µm, and staining with hematoxylin-eosin (HE). The diameter of 20 seminiferous tubules (Tse), height of germinal epithelium (Egl), gonadosomatic index (IGS) and gonad volume (gV) were determined monthly. Based on morphometric and morpho-physiological characteristics, the testes consisted of a network of anastomosed tubules with non-restricted cystic spermatogenesis, and a permanent germinal epithelium. This is the first report of a permanent germinal epithelium in A. tropicus. Five reproductive classes were histologically identified: Class I Regressed; Class II Early Maturation; Class III Mid Maturation; Class IV Late Maturation; Class V Regression. Monthly GSI, gV and Tse values were lower in January and February, the testis showed spermatozoa remains and a regenerating discontinuous germinal epithelium. In March spermiogenesis increased and proliferation of spermatogonia decreased. Male tropical gar followed a seasonal reproductive cycle, indicated by the monthly variation of the reproductive classes and the reproductive season processes observed, and for which temperature and rainfall seem to stimulate reproductive activity and spermiation.

Branchial placenta in the viviparous teleost Ilyodon whitei (Goodeidae).

Intraluminal gestation, as it occurs in viviparous goodeids, allows a wide diversity of embryo-maternal metabolic exchanges. The branchial placenta occurs in embryos developing in intraluminal gestation when ovarian folds enter through the operculum, into the branchial chamber. The maternal ovarian folds may extend to the embryonic pharyngeal cavity. A branchial placenta has been observed in few viviparous teleosts, and there are not previous histological analyses. This study analysis the histological structure in the goodeid Ilyodon whitei. The moterno ovarian folds extend through the embryonic operculum and reach near the gills, occupying part of the branchial chamber. These folds extend also into the pharyngeal cavity. In some regions, the epithelia of the ovarian folds and embryo were in apposition, developing a placental structure in which, maternal and embryonic capillaries lie in close proximity. The maternal epithelium has desquamated cells which may enter through the branchial chamber to the pharyngeal cavity and the alimentary tract. The complex processes that occur in the ovaries of viviparous teleosts, and its diverse adaptations for viviparity, as the presence of branchial placenta, are relevant in the study of the evolution of vertebrate viviparity.

Proliferation of oogonia and folliculogenesis in the viviparous teleost Ilyodon whitei (Goodeidae).

Oogonial proliferation in fishes is an essential reproductive strategy to generate new ovarian follicles and is the basis for unlimited oogenesis. The reproductive cycle in viviparous teleosts, besides oogenesis, involves development of embryos inside the ovary, that is, intraovarian gestation. Oogonia are located in the germinal epithelium of the ovary. The germinal epithelium is the surface of ovarian lamellae and, therefore, borders the ovarian lumen. However, activity and seasonality of the germinal epithelium have not been described in any viviparous teleost species regarding oogonial proliferation and folliculogenesis. The goal of this study is to identify the histological features of oogonial proliferation and folliculogenesis during the reproductive cycle of the viviparous goodeid Ilyodon whitei. Ovaries during nongestation and early and late gestation were analyzed. Oogonial proliferation and folliculogenesis in I. whitei, where intraovarian gestation follows the maturation and fertilization of oocytes, do not correspond to the late oogenesis, as was observed in oviparous species, but correspond to late gestation. This observation offers an example of ovarian physiology correlated with viviparous reproduction and provides elements for understanding the regulation of the initiation of processes that ultimately result in the origin of the next generation. These processes include oogonia proliferation and development of the next batch of germ cells into the complex process of intraovarian gestation.