Biology of primordial germ cells in vertebrates with emphasis in urodeles amphibians.

Primordial germ cells (PGCs) are highly specialized cells that play a relevant role in the maintenance and evolution of the species, since they create new combinations of genetic information between the organisms. Amphibians are a class of amniote vertebrates that are divided into three subclasses, the anurans (frogs and toads), the urodeles (salamanders and newts), and the gymnophiones (caecilians). The study of PGCs in amphibians has been addressed in more detail in anurans while little is known about the biology of this cell lineage in urodeles. Studies in some urodeles species have suggested that PGCs are of mesodermal origin, specifying in the lateral plate mesoderm at the late gastrula stage. With classical experiments it shown that, there is an induction of mesoderm, therefore most likely urodeles PGCs develop from unspecialized mesodermal tissue that responds to extracellular signals. However, some fundamental biological processes of PGCs such as the analysis of their specification, arrival, and colonization to the gonads, and their maintenance and differentiation into mature and fertile gametes remain to be elucidated. Therefore, knowledge about the biology of PGCs is of great importance to ensure the perpetuation of urodeles amphibians, as some species are in danger of becoming extinct.

Placentation in the Mexican scincid lizard Plestiodon brevirostris (Squamata: Scincidae).

Viviparity is the reproductive pattern in which females gestate eggs within their reproductive tract to complete their development and give birth to live offspring. Within extant sauropsids, only the Squamata (e.g., snakes, lizards, and amphisbaenians) evolved viviparity, representing 20% of the existing species. The genus Plestiodon is represented by 43 species and is one of the most widely distributed genera of the Scincidae in Mexico. The goal of this research has been to determine the placental morphology and ontogeny during gestation in the lizard Plestiodon brevirostris. Specimens were dissected to obtain the embryonic chambers and the embryos were categorized to carry out the correlation between the development stage and the placenta development. The embryonic chambers were processed using the conventional histological technique for light microscopy. The identified embryonic stages were 4, 29, 34, 36, and 39. A thin eggshell surrounds the egg in early developmental stages; however, this structure is already absent in the embryonic hemisphere during the developmental stage 29. The results revealed that P. brevirostris is a lecithotrophic species, but a close maternal-fetal relationship is established by tissue apposition. Ontogenically, the placental types that form in the embryonic hemisphere are the chorioplacenta, choriovitelline placenta, and chorioallantoic placenta; whereas the omphaloplacenta is formed in the abembryonic hemisphere. The structure of the chorioallantoic placenta in P. brevirostris suggests that it may play a role during gas exchange between the mother and the embryo, due to the characteristics of the epithelia that comprise it. The structure of embryonic and maternal epithelia of the omphaloplacenta suggests a role in the absorption of the eggshell during gestation and possibly in the transport or diffusion of some nutrients. In general, it is evident that ontogeny and placental characteristics of P. brevirostris match those of other species of viviparous lecithotrophic scincids.

The ultrastructure of spermatid development during spermiogenesis within the rosebelly lizard, Sceloporus variabilis (Reptilia, Squamata, Phrynosomatidae).

Several recent studies have mapped out the characters of spermiogenesis within several species of squamates. Many of these data have shown both conserved and possibly apomorphic morphological traits that could be important in future phylogenetic analysis within Reptilia. There, however, has not been a recent study that compares spermiogenesis and its similarities or differences between two species of reptile that reside in the same genus. Thus, the present analysis details the changes to spermiogenesis in Sceloporus variabilis and then compares spermatid morphologies to that of Sceloporus bicanthalis. Many of the morphological changes that the spermatids undergo in these two species are similar or conserved, which is similar to what has been reported in other squamates. There are six main character differences that can be observed during the development of the spermatids between these two sceloporid lizards. They include the presence (S. variabilis) or absence (S. bicanthalis) of a mitochondrial/endoplasmic reticulum complex near the Golgi apparatus during acrosome development, a shallow (S. variabilis) or deep (S. bicanthalis) nuclear indentation that accommodates the acrosomal vesicle, filamentous (S. variabilis) or granular (S. bicanthalis) chromatin condensation, no spiraling (S. variabilis) or spiraling (S. bicanthalis) of chromatin during condensation, absence (S. variabilis) or presence (S. bicanthalis) of the longitudinal manchette microtubules, and the lack of (S. variabilis) or presence (S. bicanthalis) of nuclear lacunae. This is the first study that compares spermiogenic ultrastructural characters between species within the same genus. The significance of the six character differences between two distantly related species within Sceloporus is still unknown, but these data do suggest that spermiogenesis might be a good model to study the hypothesis that spermatid ontogeny is species specific.

Seasonal spermatogenesis in the Mexican endemic oviparous lizard, Sceloporus aeneus (Squamata: Phrynosomatidae).

Oviparous species of Sceloporus exhibit either seasonal or continuous spermatogenesis and populations from high-elevation show a seasonal pattern known as spring reproductive activity. We studied the spermatogenic cycle of a high-elevation (2700 m) population of endemic oviparous lizard, Sceloporus aeneus, that resided south of México, D.F. Histological analyses were performed on the testes and reproductive ducts from individual lizards collected monthly. This population of S. aeneus showed a seasonal pattern of spermatogenesis, with 4 successive phases common in other lizards. These include: 1) Quiescence in August, which contained solely spermatogonia and Sertoli cells; 2) Testicular recrudescence (September-January) when testes became active with mitotic spermatogonia, spermatocytes beginning meiosis, and the early stages of spermiogenesis with spermatids; 3) Maximum testicular activity occurred from March to May and is when the largest spermiation events ensued within the germinal epithelia, which were also dominated by spermatids and spermiogenic cells; 4) Testicular regression in June was marked with the number of all germs cells decreasing rapidly and spermatogonia dominated the seminiferous epithelium. February was a transitional month between recrudescence and maximum activity. The highest sperm abundance in the lumina of epididymides was during maximum testicular activity (March-May). Thus, before and after these months fewer spermatozoa were detected within the excurrent ducts as the testis transitions from recrudescence to maximum activity in February and from maximum activity to quiescence in June. Maximum spermatogenic activity corresponds with warmest temperatures at this study site. This pattern known as spring reproductive activity with a fall recrudescence was similar to other oviparous species of genus Sceloporus.