Comparison of Ovarian Morphology and Follicular Disturbances between Two Inbred Strains of Cotton Rats (Sigmodon hispidus)

Most mammalian ovarian follicles contain only a single oocyte having a single nucleus. However, two or more oocytes and nuclei are observed within one follicle and one oocyte, respectively, in several species, including cotton rat (CR, Sigmodon hispidus). The present study compared ovarian histology, focusing on folliculogenesis, between two inbred CR strains, HIS/Hiph and HIS/Mz. At 4 weeks of age, ovarian sections from both the strains were analyzed histologically. Multi-oocyte follicles (MOFs) and double-nucleated oocytes (DNOs) were observed in all stages of developing follicles in HIS/Hiph, whereas HIS/Mz had MOFs up to secondary stages and lacked DNOs. The estimated total follicles in HIS/Mz were almost half that of HIS/Hiph, but interstitial cells were well developed in HIS/Mz. Furthermore, immunostaining revealed no clear strain differences in the appearance of oocytes positive for Ki67, PCNA, and p63 in MOF or DNOs; no cell death was observed in these oocytes. Ultrastructural analysis revealed more abundant mitochondrial clouds in oocytes of HIS/Hiph than HIS/Mz. Thus, we clarified the strain differences in the CR ovary. These findings indicate that early events during folliculogenesis affect the unique ovarian phenotypes found in CRs, including MOFs or DNOs, and their strain differences.

Small mammals killed in discarded bottles along roadsides in central Nebraska

Littered debris along roadways traps and kills small vertebrates. In Nebraska, at least 20 species of small mammals are small enough to enter openings of discarded bottles and cans. We surveyed roadsides for littered bottles and cans containing vertebrate remains in central parts of the state. We observed 459 bottles and 278 aluminum cans along 17.6 km of roadsides in Nebraska. Littered bottles contained 41 vertebrate remains representing nine taxa of small mammals. Glass bottles contained the majority of individuals, plastic bottles had a few individuals, and no vertebrate remains were detected in aluminum cans. Harvest mice (Reithrodontomys spp.) were the most frequently observed taxa trapped in bottles, followed by short-tailed shrews (Blarina spp.). Remains of a juvenile Hispid Cotton Rat (Sigmodon hispidus) in a glass bottle suggest that more species are at risk when individuals are young. Bottles with openings higher than their base contained more vertebrates (21.1%) than those lying flat (3.8%) or those with openings facing downward (1.2%). Overall, about 5.7% of bottles had vertebrate remains in Nebraska, which is slightly greater than studies in the eastern United States. Densities of littered bottles were much lower along roads in Nebraska than roadways in the eastern United States, resulting in fewer mortalities per km. Human population density appears associated with littered debris, thus numbers of vertebrates killed in these roadside hazards likely is greater in eastern Nebraska and near population centers where most of the state’s population reside. In Nebraska, six species of small mammals are species of conservation need, and thus, at risk from littered debris. Reduction and removal of litter along roadsides not only is aesthetically pleasing, but it can also reduce mortality of small mammals and other animals, such as small insects.

Unique histological features of the tail skin of cotton rat (Sigmodon hispidus) related to caudal autotomy

ABSTRACTCaudal autotomy in rodents is an evolutionarily acquired phenomenon enabling escape from predators, by discarding the tail skin after traumatic injuries. The histological mechanisms underlying caudal autotomy seem to differ among species. Cotton rats (Sigmodon hispidus), which are important laboratory rodents for human infectious diseases, possess a fragile tail. In this study, we compared the tail histology of cotton rats with that of laboratory rats (Rattus norvegicus), which have no fragility on their tail, to elucidate the process of rodent caudal autotomy. First, the cotton rats developed a false autotomy characterized by loss of the tail sheath with the caudal vertebrae remaining without tail regeneration. Second, we found the fracture plane was continuous from the interscale of the tail epidermis to the dermis, which was lined with an alignment of E-cadherin+ cells. Third, we found an obvious cleavage plane between the dermis and subjacent tissues of the cotton-rat tail, where the subcutis was composed of looser, finer, and fragmented collagen fibers compared with those of the rat. Additionally, the cotton-rat tail was easily torn, with minimum bleeding. The median coccygeal artery of the cotton rat had a thick smooth muscle layer, and its lumen was filled with the peeled intima with fibrin coagulation, which might be associated with reduced bleeding following caudal autotomy. Taken together, we reveal the unique histological features of the tail relating to the caudal autotomy process in the cotton rat, and provide novel insights to help clarify the rodent caudal autotomy mechanism.

Developmental Changes of the Ovary in Neonatal Cotton Rat (Sigmodon hispidus)

The reproductive characteristics and ovarian development in cotton rats (Sigmodon hispidus, CRs) are unclear, although CRs are commonly used as animal models in biomedical research. We previously reported that young (6–8 weeks) CRs showed multi-oocyte follicles (MOFs) and double nucleated oocytes (DNOs) in different stages of follicles. The developmental changes in neonatal CR ovaries were investigated in the present study and were compared with our findings in previous studies of unique phenotypes, particularly in oocytes. CR ovaries at postnatal days (PND) 0, 4, and 7 were obtained from the Hokkaido Institute of Public Health. Samples were analyzed by light and transmission electron microscopy. The general histology and folliculogenesis in CR ovaries were similar to those in other experimental rodents. However, DNOs were observed in all age categories and were frequently observed in primordial follicles, whereas MOFs started to develop from PND4 with greater frequency in primary follicles. Almost all developing follicles expressed DEAD (Asp-Glu-Ala-Asp) box polypeptide 4 and forkhead box L2, which are representative markers of oocytes and follicular epithelial cells, respectively. Ki-67 staining demonstrated the proliferative activity of granulosa cells, but not of oocytes, in follicles. Moreover, rapid folliculogenesis of CR due to a small number of apoptotic oocytes was suggested, based on results of the terminal deoxynucleotidyl transferase dUTP nick end labeling assay, confirming the formation of DNOs or MOFs. These findings clarify the development of unique phenotypes of neonatal CR ovaries and support it as a useful model to better understand folliculogenesis and oocytogenesis as well as their abnormalities in humans and other animals.

Cause and Treatment of Exophthalmos in Aged Cotton Rats (Sigmodon hispidus)

Aged cotton rats (Sigmodon hispidus) from an established breeding colony displayed signs of spontaneous exophthalmos. Of a total of 118 colony animals that were older than 6 mo of age, 37 (31%) displayed signs of exophthalmos. These rats were clinically healthy and had no other signs of disease. Ophthalmic exams, molecular and microbiologic testing, and histopa- thology were performed to determine the cause of the exophthalmos and to provide appropriate treatment. Environmental monitoring records were also reviewed for vivarium rooms in which the cotton rats were housed. Histopathology findings supported that the exophthalmos in these cotton rats was secondary to retro-orbital thrombosis associated with cardiomyopathy. The exophthalmic eyes were treated by either removal of the affected eye (enucleation) or surgical closure of the eyelids (temporary tarsorraphy). Enucleation of the exophthalmic eye was the best intervention for these aged cotton rats. These findings demonstrate the potential for a high incidence of ocular problems occurring secondary to cardiomyopathy in aged cotton rats. Enucleation as a therapeutic intervention for exophthalmic eyes in aged cotton rats prolongs the morbidity-free time span during which these aged animals can be used experimentally.