Nutritional deficiency and placenta calcification underlie constitutive, selective embryo loss in pregnant South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

Neurovascular description in the South American plains vizcacha, Lagostomus maximus (Chinchilloidea, Caviomorpha). A study involving evolutionarily related species of Caviomorpha and Muroidea

Oxygenated blood is required for the adequate metabolic activity of the brain. This is supplied by the circle of Willis (CoW) and the vertebrobasilar and carotid systems. The CoW ensures blood flow in case of arterial stenosis or occlusion. Different animal models have been explored for the CoW morphological and functional study. This work aims to characterize the vascular architecture of the CoW of the plains vizcacha, Lagostomus maximus (Suborder: Hystricomorpha), and to compare it with evolutionarily related species of Caviomorpha and Muroidea. The blood supply in adult plains vizcachas was studied using latex cerebrovascular casts and angiography. A caudo-rostral flow direction was determined, beginning in the spinal and vertebral arteries and converging in the basilar artery which bifurcates in the carotid-basilar communication in the caudal communicating arteries. In the first third of its course, the caudal cerebral arteries project laterally, and the middle and rostral cerebral arteries bifurcate from their rostral terminal segment, supplying the temporo-parietal and frontal cortex. The CoW architecture is mainly conserved between rodent species. Likewise, the small neurovascular variations observed could be considered phylogenetic morphological variations more than evolutionary adaptations. The absence of the rostral communicating artery that generates the rostral open architecture of the CoW in the vizcacha as in the other analyzed species, supports the need for a revision of the CoW classical function as a security system. Finally, this work supports the importance of expanding our understanding of brain anatomy among species, which may contribute to a better understanding of functional neuroanatomy.

Hypothalamic GnRH expression and pulsatility depends on a balance of prolactin receptors in the plains vizcacha, Lagostomus maximus

In mammals, gestation is considered a physiological hyperprolactinemia status. Prolactin (PRL) is one of the modulators of gonadotropin-releasing hormone (GnRH) neurons function. The South American plains vizcacha (Lagostomus maximus) is a unique model to study the regulation of hypothalamic GnRH neurons by direct and indirect steroid-dependent pathways. The aim was to characterize the hypothalamic expression of endocrine markers in vizcacha during gestation as well as their response to experimental induced hyperprolactinemia. The possible involvement of PRL regulatory pathways on GnRH in the context of hypothalamic and pituitary reactivation in mid-gestating vizcachas was discussed. Using two in vivo approaches, we determined changes in the hypothalamic expression and distribution of prolactin receptor (PRLR), tyrosine hydroxylase (TH), and dopamine type 2 receptor. A significant increment in the number of tuberoinfundibular dopaminergic (TIDA) neurons was determined in the arcuate nucleus from early to term pregnancy. On the other hand, at preoptic area, the number of both TH+PRLR+ and GnRH+PRLR+ double-labeled neurons significantly decreased at mid-pregnancy probably allowing the recovery of GnRH expression indicating that both types of neurons may represent the key points of PRL indirect and direct pathways modulating GnRH. Moreover, in a model of induced hyperprolactinemic vizcachas, the inhibitory effect of PRL on GnRH at both expression and delivery levels were confirmed. These results suggest the concomitant participation of both PRL regulatory pathways on GnRH modulation and pinpoint the key role of PRL on GnRH expression enabling the recovery of the hypothalamic activity during the gestation in this species.

Embryonic-placental relationship in Lagostomus maximus as compared to other hystricognath rodents and eutherian mammals

Reproductive specializations in caviomorphs (infraorder Hystricognathi), are very peculiar within the Order Rodentia. These include long gestations, the birth of offspring with an extreme degree of precociality, and short lactation periods. This study describes the embryo-placental relationship of viable implantation sites (IS) of the plains viscacha, Lagostomus maximus, after 46 post-coital days. The observations recorded in this study are comparatively discussed with those of other hystricognaths and eutherians. At this stage, the embryo resembles that of other eutherians. At this time of embryo development, the placenta exhibits a size, shape, and organization similar to that it will have in its mature state. Besides, the subplacenta is already highly folded. These characteristics are adequate to sustain the development of future precocial offspring. The mesoplacenta, a structure present in other hystricognaths and related to uterine regeneration is described for the first time in this species. This detailed description of the placental and embryonic structure contributes to the knowledge of the reproductive and developmental biology of the viscacha, as well as that of hystricognaths. These characteristics will allow testing other hypotheses related to the morphology and physiology of the placenta and subplacenta, and their relationship with the growth and development of precocial offspring in Hystricognathi.

Placental glycotype of the caviomorph rodent Lagostomus maximus and its evolution within Eutheria

The main evolutionary milestone in the oviparity-viviparity transition is placentation. The placenta is an organ with great morphological diversity among eutherians. The expression of different glycosidic residues (Gr) in the near-term placenta constitutes its glycotype. In this study, the expression of different Gr was determined by lectin histochemistry in early, midterm, and near-term placentas of the plains viscacha (Lagostomus maximus), a caviomorph rodent with the highest poliovulatory rate and embryonic resorption rate among eutherians. Besides, a matrix with the expression of each Gr in the exchange trophoblast of viscacha and other eutherians was constructed to map and infer phylogenetic and evolutionary relationships. Between early, midterm, and near-term placentas, variations in the pattern expression of Gr were observed. The glycotype of the near-term placenta is composed of a high diversity of Gr. Reconstruction of the ancestral state for each Gr present in the near-term placenta showed a diverse scenario: some sugars were common to the species of Placentalia included in this study. In the analyzed species with synepitheliochorial and epitheliochorial placentas, no differential glycosylation patterns between them were observed. In species with invasive placentas, such as the endotheliochorial placentas of Carnivora, some common Gr were detected among them, while others were species-specific. In species with hemochorial placenta, the same Gr are shared. Particularly, in the viscacha greater differences with species of the Hominidae and even Muridae families were observed. Nevertheless, greater similarities with other caviomorph rodents were detected. Placental glycotype of each species constitutes an excellent tool to achieve phylogenetic and evolutionary inferences among eutherians.

Spontaneous embryonic death in plains viscacha (Lagostomus maximus - Rodentia), a species with unique reproductive characteristics

Spontaneous embryonic death is a conserved reproductive event in Eutherians. The macro and microscopic characteristics of this type of death are similar between the different taxa. However, in the hystricomorphic rodent plains viscacha (Lagostomus maximus) is exceptional in terms of massiveness (80% embryonic resorption). In this species, of the 10-12 implantation sites (IS) (half in each uterine horn), only the caudal embryos will survive, resorbing the cranial and intermediate IS. We hypothesize that uterine structural variations in L. maximus restrict growth and promote embryo death, with the consequent loss of placental homeostasis in the cranial and middle IS. In this study, different studies (ultrasonography, macroscopy and microscopy) were carried out to analyze different aspects of the intermediate gestation of L. maximus (46 days postcoitus). Ultrasonographic studies revealed that the cranial and middle IS (IS-1, IS-2, and IS-3) had no recognizable embryonic and placental structures as compared to the caudal implantation sites (IS-4). Macroscopically, the areas corresponding to the embryos in the cranial and middle IS were occupied by a necrotic black semi-fluid mass. Moreover, the placenta in these IS was undifferentiated. However, in the caudal IS both the embryo and its placenta were distinguishable. Using histological and immunohistochemical techniques, it was observed that the placentas of IS-1, IS-2 and IS-3 were disorganized and showed hemorrhage, inflammatory infiltration containing neutrophils, macrophages, mast cells and foreign body giant cells, apoptotic trophoblast, and a layer of collagen fibers and fibroblasts that circumscribed each of these IS. In contrast, the placenta of the caudal IS showed an organized maternal-embryonic interface. The characteristics observed in IS in resorption of viscachas in intermediate gestation show that, regardless of gestation time, embryonic death has a similar macro and microscopic morphological pattern among eutherians with invasive placentation. However, the massiveness and sectorization of embryonic death in the plains viscacha make the species a unique model for the study of this reproductive event.

Placentation and embryo death in the plains viscacha (Lagostomus maximus)

Caviomorpha are an exceptional group among rodents due to their extended gestational period and the delivery of precocial offspring. Among them, Lagostomus maximus is characterized by its polyovulation, polyembryony, and the highest embryonic death known in mammals. Its chorioallantoic placenta is hemomonochorial, an ancestral character among rodents. It resembles more the human placenta than the murine models. As in all caviomophs, the chorioallantoic placenta is divided in a main placenta and a subplacenta. The former is organized in labyrinth lobes surrounded by trophospongium, as in most caviomorphs. The giant cells (more numerous than in other caviomorphs) near the decidua could be related to invasiveness. During placentation of L. maximus, uterine natural killer cells are found. These cells have been related to invasiveness and remodeling of blood vessels in Mus musculus and Homo sapiens, although in other caviomorphs are not frequently found. In L. maximus, the placenta develops in all conceptuses (5-6 per uterine horn). Necrosis was observed in each implantation site at day 70 post-coitum, except in that closest to the vagina in each horn. This process of embryo death followed by resorption begins at day 26-30 post-coitum. Recently, we found variations in the percentage of blood vessel and uterine gland areas that could explain the regional differences in embryo survival. The characteristics of the placenta and implantation of L. maximus are important to stablish a unique model for studying placentation as well as early embryonic death, of interest for human and veterinary medicine.

Prenatal development in Lagostomus maximus (Rodentia, Chinchillidae): A unique case among eutherian mammals of physiological embryonic death

Embryonic death followed by resorption is a conserved process in mammals. Among the polyovular species, Lagostomus maximus (plains viscacha) constitutes a model of early and physiological embryonic death, since out of a total of 10-12 implants, 8-10 are resorbed during early/intermediate gestation, surviving are only the most caudal implantations of each uterine horn. This regular reproductive event is unique to this species, but many characteristics of the implantations during the early gestation of L. maximus, when embryonic death processes begin are unknown. The aim of the present work was to analyze the implantation sites of this species using morphological, morphometric, histochemical, lectinhistochemical, and immunohistochemical techniques to infer the possible causes of this event. Macroscopically, the length and width of the implantation sites significantly increased in a craniocaudal direction. Histochemically, the implantation sites did not differ in the expression of glycoconjugates and glycosidic residues. Furthermore, no variations were observed in cell renewal, hormone receptor expression, and decidualization. Both the glandular and vascular areas of the implantation sites significantly increased in the craniocaudal axis. Some necrotic cells and an inflammatory response with a predominance of lymphocytes and fibrin were observed in the cranial and middle but not in the caudal implantation sites. We conclude that signs of embryonic death and resorption are already observed in the early gestation of L. maximus. Our results reaffirm the hypothesis that postulates the key potential role of uterine glands and blood vessels in the gestation of the species, with emphasis on embryonic death. This pattern of embryonic death in L. maximus makes this species an unconventional mammalian model, which adds to the peculiarities of polyovulation (200-800 oocytes/estrus) and hemochorial placentation.