Corticogenesis and folding process of the neopallium in the South American plains vizcacha, Lagostomus maximus.

The plains vizcacha, Lagostomus maximus, is a precocial hystricomorph rodent with a gyrencephalic brain. This work aimed to perform a time-lapse analysis of the embryonic brain cortical development in the plains vizcacha to establish a species-specific temporal window for corticogenesis and the gyrencephaly onset. Additionally, a comparative examination with evolutionarily related rodents was conducted. Embryos from 40 embryonic days (ED) until the end of pregnancy ( ∼ $\sim $ 154 ED) were evaluated. The neuroanatomical examination determined transverse sulci at 80 ED and rostral lateral and caudal intraparietal sulci around 95 ED. Histological examination of corticogenesis showed emergence of the subplate at 43 ED and expansion of the subventricular zone (SVZ) and its division into inner and outer SVZs around 54 ED. The neocortical layers formation followed an inside-to-outside spatiotemporal gradient beginning with the emergence of layers VI and V at 68 ED and establishing the final six neocortical layers around 100 ED. A progressive increment of gyrencephalization index (GI) from 1.005 ± 0.003 around 70 ED, which reflects a smooth cortex, up to 1.07 ± 0.009 at the end of gestation, reflecting a gyrencephalic neuroanatomy, was determined. Contrarily, the minimum cortical thickness (MCT) progressively decreased from 61 ED up to the end of gestation. These results show that the decrease in the cortical thickness, which enables the onset of neocortical invaginations, occurs together with the expansion and subdivision of the SVZ. The temporal comparison of corticogenesis in plains vizcacha with that in relative species reflects a prenatal long process compared with other rodents that may give an evolutionary advantage to L. maximus as a precocial species.

ERα and GnRH co-localize in the hypothalamic neurons of the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha).

Gonadotropin-releasing hormone (GnRH) is the key regulator of the hypothalamic-pituitary-gonadal axis. Estradiol (E2) affects GnRH synthesis and delivery. Hypothalamic estrogen receptors (ER) modulate GnRH expression acting as transcription factors. The South American plains vizcacha, Lagostomus maximus, is able to ovulate up to 800 oocytes per reproductive cycle, and shows continuous folliculogenesis with pre-ovulatory follicle formation and an ovulatory event at mid-gestation. The aim of this work was to analyze the hypothalamic expression of ER in the vizcacha at different gestational time-points, and its relationship with GnRH expression, serum luteinizing hormone (LH) and E2. The hormonal pattern of mid-gestating vizcachas was comparable to ovulating-females with significant increases in GnRH, LH and E2. Hypothalamic protein and mRNA expression of ERα varied during pregnancy with a significant increase at mid-gestation whereas ERß mRNA expression did not show significant variations. Hypothalamic immunolocalization of ERα was observed in neurons of the diagonal band of Brocca, medial preoptic area (mPOA), periventricular, suprachiasmatic, supraoptic (SON), ventromedial, and arcuate nuclei, and medial eminence, with a similar distribution throughout gestation. In addition, all GnRH neurons of the mPOA and SON showed ERα expression with no differences across the reproductive status. The correlation between GnRH and ERα at mid-gestation, and their co-localization in the hypothalamic neurons of the vizcacha, provides novel information compared with other mammals suggesting a direct action of estrogen as part of a differential reproductive strategy to assure GnRH synthesis during pregnancy.

Variation in progesterone receptors and GnRH expression in the hypothalamus of the pregnant South American plains vizcacha, Lagostomus maximus (Mammalia, Rodentia).

In mammals, elevated levels of progesterone (P4) throughout gestation maintain a negative feedback over the hypothalamic-hypophyseal-gonadal (H-H-G) axis, avoiding preovulatory follicular growth and preventing ovulation. Recent studies showed that in the South American plains vizcacha (Lagostomus maximus) folliculogenesis progresses to preovulatory stages during gestation, and an ovulatory process seems to occur at midgestation. The aim of this work was to analyze hypothalamic gonadotropin-releasing hormone (GnRH) and P4 receptors (PR) expression and luteinizing hormone (LH) secretion and correlate these with the functional state of the ovary in nonovulating and ovulating females and gestating females with special emphasis in the supposedly ovulating females at midgestation. We investigated P4 and LH serum levels as well as the distribution, localization, and expression of PR and GnRH in the hypothalamus of L. maximus at different time points during gestation and in nongestating, ovulating and nonovulating, females. A significant increment in GnRH, P4, and LH was detected in midpregnant vizcachas with respect to early-pregnant and to ovulating females. PR was also significantly increased in midpregnant animals. PR was detected in neurons of the preoptic and hypothalamic areas. Coexistence of both PR and GnRH in neurons of medial preoptic area and supraoptic nucleus was detected. Midpregnant animals showed increased number of PR immunoreactive cells at median eminence, localized adjacently to GnRH immunoreactive fibers. High expression of hypothalamic GnRH and PR, despite an increased level of P4, was correlated with the presence of antral, preovulatory follicles, and luteinized unruptured follicles at midgestation that suggest a possible role of the H-H-G axis in the modulation of ovulation during gestation in L. maximus.

Expression of androgen receptor, estrogen receptors alpha and beta and aromatase in the fetal, perinatal, prepubertal and adult testes of the South American plains Vizcacha, Lagostomus maximus (Mammalia, Rodentia).

Androgens and androgen receptor play a critical role in spermatogenesis and fertility in mammals, and estrogens and their receptors contribute to regulation of testicular function through initiation and maintenance of spermatogenesis and germ cell division and survival. However, results from different species are still far from establishing a clear understanding of these receptors in the different cell types from the testis. We analyzed the expression of androgen receptor, estrogen receptors α and ß and aromatase protein by immunohistochemistry and real-time PCR, in relation to proliferation followed by the expression of proliferation cell nuclear antigen (PCNA) and germinal identity by VASA protein, in fetal, perinatal, prepubertal and adult testes of Lagostomus maximus, a rodent with sustained germ cell proliferation and an increasing number of OCT-4-expressing gonocytes in the developing ovary. AR expression was restricted to Leydig cells and peritubular cells before sexual maturity, at which point it also became expressed in Sertoli cells. ERα and ERß were expressed in seminiferous tubules and the interstitium, respectively, in both fetal and prepubertal testes. In adult testes, both ERα and ERß co-localized in Leydig and peritubular cells. The aromatase enzyme, which converts androgenic precursors into estrogens, was detectable in all developmental stages analyzed and was restricted to Leydig cells. PCNA remained high until sexual maturity. ERα nuclear detection in germ cells and AR in Leydig cells in PCNA-positive cells suggest the possibility of a stimulatory effect of estrogens on spermatogonia proliferation. This effect might explain the increase found in VASA-expressing cells in the adult testis.