Osteology of the musk shrew (Suncus murinus)

In the present study using 29 adult musk shrew of both sexes, most findings were identified to be similar to those by Sharma (1958) on the osteology , and there was no remarkable difference between skeletal structure of wild musk shrew in India and that of breeding species in Japan except the number of the vertebra lumbalis and existance of the dens caninus . However, it was elucidated that the os articulare coccygis like H-letter in shape is characteristic for this animal, and shows 17 components as many between interspace ventral to the adjacent os coccygis , and that there exists the proc. spinosus appendicularis nominated newly over the vertebra thoracica II, which plays significant role to head and neck as origin or insertion for strong m. serratus dorsalis, m. splenius, and m. semispinalis .     

Scent marking in the musk shrew (Suncus murinus)

The hormonal regulation of scent marking was examined in male and female musk shrews. In males castration depressed flank, perineal, and throat marking frequencies, and a physiological dose of testosterone restored perineal and throat marking to intact levels. Females exhibited high levels of marking whether tested in a non-pregnant state, late in gestation, or when post-partum. Moreover, ovariectomy had no influence on marking behavior. Adrenalectomy, in addition to ovariectomy, caused a significant decline in two of the three marking behaviors examined, flank and perineal marking. These results demonstrate that these sexually monomorphic scent marking behaviors are under gonadal control in male musk shrews, but not in females. Furthermore, two of the three different types of scent marking examined in female musk shrews appear to require adrenal hormones. Throat marking in the female musk shrew apparently is not regulated by steroidal hormones.     

Potential energetic implications of emesis in the house musk shrew (Suncus murinus)

During the course of studies investigating novel anti-emetic therapies we serendipitously observed a previously unreported behaviour related to emesis in the house musk shrew. This behaviour consisted of spontaneous ingestion of vomit in about half of the animals (males and females) in which emesis was induced by either nicotine (4 mg kg-1 sc.) or horizontal motion (1 Hz, 4 cm, 10 min). Analysis of vomit samples and gastric contents revealed that in a "typical" individual the gastric contents would be voided by as few as 3 vomits. Energetic calculations of the metabolisable energy of food, gastric contents, vomit and field metabolic rate (FMR) predict that a male weighing 60 g would lose 17.3% of its hourly energy requirement for FMR if it vomited once. A 40 g female, however, would experience an hourly energy loss of approximately 22.8%. The possible energetic consequences and resulting ecological implications of this unusual behaviour are discussed.     

Structure of the palate and pharynx in the house musk shrew (Suncus murinus)

Anatomical features of the palates and pharynges in Suncus murinus were examined through a dissecting microscope. The palatopharyngei (PP) on both sides join at the caudal, median line of the palatine velum and converge in an area immediately caudal to a mucosal notch, the pharyngeal torus before radiating into the dorsal wall of the esophagus. Some muscle bundles of the PP are attached to the veli palatini cartilage. The nasopharyngeal cartilage is situated rostral to curved mucosal plicae of the palatine velum. The pharyngeal torus interlocks with the rostrally lying corniculate tubercles. The structural relationships of the corniculate tubercles, pharyngeal torus and PP suggest that muscular tension of the PP sustains the epiglottis and retains it in the nasopharynx, i.e. the PP acts as a circumscribed sphincter around the epiglottis and establishes a short, secure air passageway to the larynx. The nasopharyngeal and veli palatini cartilages could keep the passageway in shape.     

Unique structure of the esophago-gastric junction of the house musk shrew (Suncus murinus)

The house musk shrew (Suncus murinus) belongs to the Order Insectivora, and has been used for the research in comparative anatomy as one of the most primitive placental mammals. Another feature of this shrew is its ability to easily vomit which mimics the human emesis or motion sickness response. The house musk shrew has thus been utilized as a rare small experimental animal for studies on the neurophysiological mechanism of vomiting. However, there is no report investigating the morphological background of vomiting in this species. The purpose of this study is to provide detailed morphological and histological features of the house musk shrew stomach as they possibly correlate to vomiting. The stomachs of ten female house musk shrews were used. Six of them were the wild type (Jic: SUN), two were the high-emesis strain (Jic: SUN-Her) and the rest of them were the low-emesis strain (Jic: Sun-Ler). In addition to the macroscopic anatomy, the region of esophago-gastric (EG) junction and the gastric groove were observed using the light and transmission electron microscopy. Although evident differences in structure of stomach were not found among the three strains, some interesting findings in comparative anatomy were noted. The circular valve-like thick fold was seen at the cardiac portion, which protruded into the esophageal lumen forming a deep groove between its frilled edge and the esophagus. The second frilled ridge was often found as inner ridge of this valve-like thick fold. The esophago-gastric junction between the stratified squamous and the simple columnar epithelium was found at the edge of the second frilled ridge. The lamina propria of the frilled edge was occupied by loose connective tissue and many large lumens of lymphatic vessels. The lamina muscularis mucosae, which developed in the esophageal region, was not in the main frilled edge. A well-developed inner muscle layer was found around the base of the fold, which seemed to correspond to the human lower esophageal sphincter. Cardiac glands occupied most of the thick cardiac wall, forming complicated crypts lined by simple columnar epithelium, and ducts of cardiac gland opened to these crypts. Since the house musk shrew has no esophageal gland, these cardiac glands may actively protect the lower part of the esophagus. In the cardiac wall, the inner circular and outer longitudinal muscle layer largely crossed each other obliquely as same as other reports. The transition area from the striated to the smooth muscle was observed in the sphincter surrounding the distal end of the cardiac wall. The gastric groove, lined by simple columnar epithelium in the lesser curvature, which has been reported to play a role as a shortcut from the cardia to the pylorus in other species including rodents, was also confirmed in the house musk shrew. The mucosal fold in the boundary between the esophageal and the gastric epithelium of house musk shrew may correspond to the structure called the limiting ridge (in mouse, rat and hamster), the teeth-like fimbria or Grenzfalten (in vole), and the gastric teeth (in crustacean and mollusk). The valve-like mucosal fold protruding into the esophageal cavity, the well developed huge cardiac glands, and the cardiac sphincter localized distally to the cardiac gland appear to facilitate the regurgitation of the stomach content, that is, vomiting. These findings suggest that this structure might have developed to support the feeding habit of house musk shrew, and that the differences of strains in vomiting may be determined by neurophysiological mechanisms.     

Retinofugal projections in the house musk shrew, Suncus murinus.

Retinofugal projections in the house musk shrew (Suncus murinus) were studied with the WGA-HRP method. After WGA-HRP injection into the vitreous cavity of one eye, terminal labeling was seen in the suprachiasmatic nucleus, dorsal and ventral lateral geniculate nuclei, pretectum and superficial layer of the superior colliculus. The terminal labeling in the suprachiasmatic nucleus was more marked on the side ipsilateral to the injection than on the contralateral side, whereas that in other regions was seen mainly on the contralateral side. A retino-intergeniculate leaflet projection was observed. No unequivocal terminal labeling was found in the lateroposterior thalamic nucleus.     

Cortisol facilitates induction of sexual behavior in the female musk shrew (Suncus murinus)

The role of cortisol in sexual behavior in the female musk shrew (Suncus murinus) was examined. High levels of cortisol were associated with sexual receptivity, as indicated by species-typical tail-wagging behavior, during brief (15-min) mating tests. When cortisol production was blocked by metyrapone, an 11-beta-hydroxylase inhibitor, females exhibited reduced sexual behavior relative to controls, an effect that was reversed with acute cortisol replacement. These results indicate that cortisol facilitates, rather than inhibits, sexual behavior in this species and expands the comparative understanding of hypothalamic-pituitary-adrenal (HPA) effects on reproduction.     

Induction of emetic response to staphylococcal enterotoxins in the house musk shrew (Suncus murinus)

The emetic responses induced by staphylococcal enterotoxin A (SEA), SEB, SEC2, SED, SEE, SEG, SEH, and SEI in the house musk shrew (Suncus murinus) were investigated. SEA, SEE, and SEI showed higher emetic activity in the house musk shrew than the other SEs. SEB, SEC2, SED, SEG, and SEH also induced emetic responses in this animal model but relatively high doses were required. The house musk shrew appears to be a valuable model for studying the mechanisms of emetic reactions caused by SEs.     

Fine structure of spermatozoa of the Asiatic musk shrew,Suncus murinus

The fine structure of spermatozoa of the Asian musk shrew, Suncus murinus, has been investigated using air-dried and critical-point-dried replicas in addition to freeze-fractured specimens. Asymmetry in the structure of the head, notably the acrosome, can be recognized in critical-point-dried replicas as a slight dorsoventral convexity not apparent in air-dried specimens. Freezeetching reveals a dense accumulation of intramembranous particles just anterior to the posterior ring, but relatively few other characteristic membrane conformations associated with the head. Gelatin smears visualized with the light microscope show a vigorous lytic response, but preliminary efforts to remove the acrosome by physiological or enzymatic incubation have thus far not been successful.     

Gross anatomical study of the sympathetic cardiac nerves in the house musk shrew (Suncus murinus)

The sympathetic cardiac nerves originating from the cervical and upper thoracic sympathetic ganglia in the house musk shrew (Suncus murinus) were examined using macroscopic and whole‐mount immunohistochemical methods. Based on the results, the nerves were macroscopically classified into the following three groups: nerves innervating the cervical sympathetic ganglia mainly to the arterial porta of the heart; nerves supplying the stellate and thoracic sympathetic ganglia at the level of T2–T5 or T6 for both the arterial and venous portae of the heart; and nerves innervating the thoracic sympathetic ganglia at the level of T4–T9 to the esophagus and lung and then the heart via the blood vessels within the mediastinal pleura. These findings in the house musk shrew suggest a possible primitive morphological pattern of the cervical and thoracic sympathetic nervous system that may be related to those in other mammals, including humans. Anat Rec, 2007. © 2007 Wiley‐Liss, Inc.     

Immunohistochemical study of adenohypophysial gonadotropes in the musk shrew, Suncus murinus L. (Insectivora)

Most gonadotropes in the pituitary of male Suncus murinus L. are immunostained with both LH beta and FSH antisera, whereas only a few gonadotropes are positive only for LH beta. The former gonadotropes consist of polygonal or polyhedral cells of medium size, and can be identified by the presence of two types of secretory granules of low density: smaller ones (about 150-250 nm in diameter) which are spherical in shape, and larger ones of irregular form with a maximum diameter of about 1,500 nm. The endoplasmic reticulum is not prominent, while the Golgi apparatus is distinct. Mitochondria are found throughout the cytoplasm but are often concentrated in and around the region of the Golgi apparatus. These fine structural features are somewhat similar to those of the classical Barnes "FSH cells" and Kurosumi-Oota "FSH cells".     

Tropical photoperiods affect reproductive development in the musk shrew, Suncus murinus.

The purpose of the present studies was to determine whether reproductive development in male musk shrews (original stock from Guam Island, 13 degrees N latitude) could be altered by small, ecologically relevant changes in photoperiod. In the first experiment, effects of changes in photoperiod equivalent to those seen between the 2 solstices on Guam Island (90 min) on reproductive maturation were investigated. The results showed that a 90-min difference in photoperiod had a significant effect on weights of various androgen-sensitive target tissues. Furthermore, there was little evidence that the preweaning photoperiod had an effect on the response to the postweaning photoperiod. In the second experiment, effects of changes in photoperiod equivalent to those seen between the equinoxes and solstices on Guam Island (45 min) on reproductive maturation were investigated. The results showed that both a decrease and an increase in photoperiod by 45 min had a significant effect on weights of various androgen-sensitive target tissues. Overall, these results suggest that animals living close to the equator can potentially use small changes in day length to alter or time reproductive function.     

The electrocardiogram (ECG) and impulse conducting system (ICS) of the Asian musk shrew,Suncus murinus

Summary The impulse conducting system (ICS) in the heart of the Asian musk shrew,Suncus murinus, was investigated by means of electrocardiography and light microscopy.In contrast to recent results denying an ICS in this species there exists a physiologically and morphologically well defined conductile tissue not differing from that in other mammals. Recognizable and typical EGGs were obtained with a P wave, PQ interval, QRS complex, and T wave. In accordance with the electrocardiographical pattern, anatomical examination revealed an ICS consisting of a SA node, AV node, AV bundle and ventricular Purkinje system (limbs and terminal branches). Histologically the specific tissue differs from the working myocardium by having fewer myofibrils, mitochondria, and blood vessels. Moreover the AV system is partially surrounded by a connective tissue sheath.     

Scanning electron microscopy of the ovary in the house musk shrew, Suncus murinus

The structure of the house musk shrew (Suncus murinus) ovary, prepared by the Osmium-DMSO-Osmium method, was examined with a scanning electron microscope. Many ovarian follicles in various stages of development can be seen in the musk shrew ovary, but fully matured follicles (Graafian follicles) are never observed. In the inner layer of zona granulosa of the preantral follicles, the follicular cells containing a large amount of glycogen deposition in the cytoplasm are observed. Folded and collapsed zona pellucida surrounding the remnant of the degenerated oocyte, which is thought to be a residual form of atretic follicle, is observed in a small, crypt-like depression on the surface of the ovary where it has been split by cracking.     

Peroxidase activity in the submandibular gland of the house musk shrew, Suncus murinus (Soricidae, Insectivora)

Endogenous peroxidase activity in the submandibular gland of the house musk shrew, Suncus murinus was cytochemically investigated by light and electron microscopy using 3,3'-diaminobenzidine-tetrahydrochloride salt (DAB). The submandibular glands of male Suncus murinus at 8-month-olds were excised and diced into small pieces. In general, salivary glands are structurally divided into a terminal portion comprising a secretory portion and duct system. The submandibular gland of the Suncus murinus, the terminal portions consisted of proximal and distal acinar cells. On the other hand, a granular duct cell of the duct system contained a number of characteristic myelin-like bodies. In the present study, the peroxidase reaction products were localized in the secretory granules of the proximal acinar cells and in the endoplasmic reticulum, Golgi apparatus and myelin-like bodies of the granular duct cells. These reaction products were reduced when 5 mM 3-amino-1,2,4-triazole was added to the reaction medium. Additionally, release of peroxidase into the lumen was observed. In conclusion, the proximal acinar and granular duct cells formed peroxidase and may have performed excretory secretions. Moreover, the peroxidase positive myelin-like body consisted of lamellated membrane and its outer surface membrane continued to the endoplasmic reticulum.     

Retinal cysts in the eye of the Asian musk shrew, Suncus murinus, (Mammalia, Insectivora)

The development of retinal cysts in the eye primordia of Suncus murinus embryos was studied. Retinal infoldings were first identified in the 17.5-day-old embryo (crown rump length--CRL = 6.3 mm). At this time, the apex of the developing retinal fold could be identified attached to the lens. The pigmented epithelium did not take part in the formation of the cyst. The retinal fold closed, to form a cyst, by day 23 of embryonic development (CRL = 14.0 mm). The newly formed retinal cyst remained continuous with the retina and extended toward the lens where it attached to the lens epithelium. Retinal cysts had an oval, bilaterally symmetrical shape and a narrow lumen, which, lined by the internal limiting membrane, occasionally contained cellular debris. The dimensions of the cysts were usually 100 to 150 microns (diameter) by 200 to 250 microns (length). Retinal cysts appeared in 45 per cent of the embryos examined.