Classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Order: Rodentia
Suborder: Sciurognathi
Family: Muridae
Subfamily: Murinae
Genus: Mus
Species: Mus musculus
ANATOMICAL AND PHYSIOLOGICAL CHARACTERISTICS
The head-body length of the mouse is6-10 cm, while the tail alone can be up to 10 cm long. The colour of the hair varies from white (albino) to dark grey, brownish or black and the body weight of an adult mouse ranges between 20 and 50 g.
The reduced body size implies a very high metabolism, with quite an accelerated heart rate (325-780/min) and respiratory rate (60-220/min) compared to other mammals; the basal temperature ranges between 36.5 and 38°C.
Sensory organs
In view of its poor eyesight and its predominantly nocturnal activity, Musmusculus has developed highly efficient sensory capabilities (hearing, taste and smell); its sense of hearing, in particular, enables it to perceive a wide range of ultrasounds and the sense of smell allows it to detect, also at a great distance, the pheromones commonly used for communication.
The ears are proportionately rather large and the eyes are black, except for in the albino strains in which the colour is red due to the genetic absence of iris pigmentation, as happens in other species.
Mouse skin
The skin is covered with hair whose colour varies from white (albino) to black, through different shades of brown and grey. The limbs and the tail seem hairless, but they are actually covered with an extremely thin hair, which is not always visible. Some immunodepressed strains are present, which, in addition to a functional deficiency of the thymus, are characterized by a genetic alteration of the hair bulbs and are thus nude; also nude are the so-called "hairless" mice, in which the lack of hair has nothing to do with alterations in the thymus and the immune system. Superficial glands are present in the skin of the mouse in the head and neck region; the glands appear as clusters of adipose tissue easily confused with the salivary glands and lymph nodes present in the same region.Five pairs of mammary glands are present, with their correspondingnipples, three pairs located in the thoracic region and two pairs in the abdominal region (Drawing 1). Mammary tissue is very abundant and extended (up to the cervicodorsal and the inguinal region), and is frequently affected by neoplastic diseases.
Skeleton and internal organs
The vertebral formula is C7, T13, L6, S4, Co 28 with some variations depending on the strain.The digits arefour in number plus one vestigial thumb in the front foot and five in the hind foot.
The dental formula is: I 1/1, M 3/3 (I = incisors, M = molar), for a total of 16 teeth and a very long diastema due to the lack of premolars and canines. The mouse is a rodent and, as such, it "crunches" very hard foods; it is an omnivorous animal, with continuously growing incisors (Fig. 1) whose growth is naturally contained through a constant consumption of the masticatory surfaces.
The stomach is divided into two functional parts: an inferior secretory glandular portion and a non-glandular portion located dorsally.
The heart is placed between the third and the fifth intercostal space.
The urinary system is not very different from that of other mammals except for the greater length of the loop of Henle, which allows the mouse to concentrate its urine three times as much as humans do; under physiological conditions, with urination, the mouse eliminates large amounts of proteins (up to 1 mg/ml of urine) and creatinine.
The mouse thermoregulates itself by dilating the veins of the tail and ears and by sweating from the sweat glands located in the plantar foot pads.
Genital apparatus (Drawing 2)
The two testes of the male are ovoid in shape and descend into the scrotum (separated scrotal sacs) around the second or third week of life. The spermatic cord is composed of the vas deferens,vessels and a voluminous adipose body.The annexed glandsare the prostate, the vesicular glands, coagulation glands (typical of rodents), bulbourethral and preputial glands. The penis is contained within a small scrotum and has a thick tunica albuginea which surrounds the corpora cavernosa; in the ventral part of the apex there is a cartilaginous process similar to the os penis of the dog. At restthe penis exhibits a flexure, caused by the contraction of the penis retractor muscles, which distends during erection. As in most rodents and lagomorphs, the inguinal canal remains patent also in the adult; for this reason, the testes may be alternatively present in the scrotum or held in the abdomen (Fig. 2). During the breeding season the testes increase in volume and are clearly visible.
In the female, the ovaries are ventral to the last two lumbar vertebrae and are often surrounded by abundant adipose tissue, which makes their visualisation difficult. The fallopian tubes are thin and very convoluted and the uterus is simple and bicornuate with a single cervix, unlike in the rat. The urethra does not open into the vestibule of the vagina, opening independently at the base of the clitoris.
REPRODUCTION AND PERIPARTUM
In the female the nipples are clearly visible and the sex differentiation marker is represented by the anogenital distance (distance between the anus and the genitalia), which in the male is about 1.5-2 times greater compared to the female (Figures 3 and 4).
The duration of the reproductive life of the female is extremely variable, ranging between 7-18 months, for a total of 6-10 litters, or even more. The mouse is a continuous polyoestral species and is therefore capable of breeding throughout the year; in the wild, the highest fertility rate occurs during the seasons when food availability is greater (Fig. 5).
The first oestrus occurs approximately at 28-40 days of life, however it is better not to force very young females to breed, at least until 50-60 days of life. A premature mating would find an animal not yet properly developed, thus compromising its reproductive career, its proper growth and in addition with the risk of not completing the pregnancy successfully. The same can be said for the male. Instead, a late mating (over 10 weeks) increases the risk of dystocia, due to the fusion of the symphysis pubis, the presence of fat deposits within the birth canal, agalactia caused by an improper development of mammary glands as well as ovarian cysts.
After mating a vaginal plug (secretion of the male accessory sex glands) is formed, which persists for approximately 16-24 hours; for this reason, the female can only mate once per day. Ejaculation is certain only when the male falls onto its side. The duration of the oestrous cycle is of 4-5 days, with the oestrus itself lasting 12 hours. Except for the postpartum oestrus (14-28 hours after parturition), during lactation no other heat cycles are present; if the postpartum oestrus is not used for mating the cycle will start again 2-5 days after weaning.
The first macroscopic alterations of gestation may be seen after approximately 13 days, with an evident increase in the body weight of the female; fetuses, at this time, become palpable (Figures 6 and 7).
The placenta is of the haemochorionic type and allows a very efficient exchange between mother and fetuses, also in terms of antibodies. The gestation period is 19-21 days, after which inept baby mice are born, weighing between 0.75 and 2.0 grams (Figures 8 and 9). The optimal production of baby mice (10-12) is between the second and the eighth pregnancy. Hair starts to grow already the day after birth, and after the 10-12th day the entire body is covered. The baby mice are weaned at 21 days, unless the litter is too large or the mice are poorly developed; in this case, the duration of nursing may continue up to 28 days of life (Fig. 10). Should the female have been mated exploiting the postpartum oestrus, the gestation would in this case be prolonged by approximately 3-5 days thanks to a delay in embryo implantation in order to allow the gravid mother to nurse and wean the previous litter before delivering the new one.
Parental care is frequent and attentive, unlike what happens with the rabbit. The mother proceeds to nurse her offspring several times a day, even dividing the litter into two smaller groups should it be too numerous. The female will dig a little nest where possible, in sawdust or occupying an already present nest. Mice kept for breeding can be bred ina colony or monogamously. In a colony, one male and 2-6 females are housed together and baby mice are removed after weaning. Monogamy consists in keeping a male and a female constantly together and always removing baby mice after weaning. Females which are bred together cooperate in looking after the litters (Fig. 11). These systems allow the exploitation of post partum oestrus because the female heat period is detected directly by the male. Alternatively, females bred in monogamy or in a colony can be separated from the male shortly before delivery; in this case, however, in order to take advantage of the first useful heat females should be taken back to the male within 14-28 hours after parturition and kept under observation in order to verify that the mating takes place correctly, with the female’s acceptance of the male; afterwards, females will be returned to the litter. Females kept in groups without a male will tend not to go into heat; after the introduction of the male heat should occur after approximately 72 hours (Whitten effect). A pseudopregnancy may prolong the period between two subsequent oestrous cycles.