Average litter size: 8

Characteristics:

The main characteristics of the NMRI nude mouse are the absence of the thymus and absence of hair due to impaired keratinization resulting in breakage of the hairs within the follicles. There is a general agreement that the hairless condition and the failure of thymus development are due to some basic defect in the embryonic ectoderm, and the first detectable abnormalities in the development of the nude thymus are in the ectodermal component occurring during the formation of a cervical vesicle on day 11 of embryonic life.

The immune system is characterised by: a small population of T cells; the antibody response is confined to IgM class; response to T cell dependent antigens is low; a compensatory increase in the level of natural killer cells resulting in a slightly increase in the level of NK cells in comparison to normal NMRI mice.

Husbandry and Breeding Data:

• Breeding: Although many nude mice on inbred background might be difficult to breed and rear due to the increased susceptibility towards infections, the NMRI-nu mouse seems to be more resistant to infections and may in some instances be kept under conventional conditions for a shorter period. The breeding is carried out by mating homozygous males with heterozygous females. Approximately 50% of the offspring is heterozygous and 50% is homozygous.
  The gnotobiotic NMRI-nu mice are bred in a production isolator. Large scale production takes place in laminar air flow barrier breeding units reserved for this purpose alone. The animals are kept under constant temperature 22 + 2°C, relative humidity (55 + 5%) and at a day-night cycles of 12 hours light (6 am - 6 pm) and 12 hours dark.
• Skin and Hair: It is more appropriate to characterise the condition of the nude as a recurrent development loss of hair due to imperfect keratinization in the hair shaft. Although studies using conventional histology have shown no particular abnormal appearance of the epidermis of nude mice (Flanagan, 1966), studies using electron microscope have revealed that the keratinization processes are obviously deeply impaired (Köpf-Maier et al., 1990). The vibrissae are present in all fetuses at 16^th day of gestation, but at birth the nude mice can be differentiated from heterozygotic littermates by the appearance of the vibrissae. In the nude mice the number of whiskers are less than in the normal littermates, and those which are present are bent and coiled.
• Immunology: Immune deficient athymic nude mice are highly susceptible to infections with facultatively pathogenic viruses and bacteria. It is therefore indispensable during breeding and experimentation of nude mice to protect them from possible infections. Mouse hepatitis virus and Sendai virus infection cause chronic wasting disease and shorten lifespan in nude mice (Fujiwara, 1982).
  Colitis, colonic hyperplasia, diarrhoe, and rectus prolaps may be caused by Citrobacter freundii and possible other enterobacteria (Bieniek & Tobermeyer 1976, Barthold et al. 1977). Abscesses in nude mice may be caused by Staphylococcus aureus, Proteus spp, Pasteurella pneumotropica, or Group G Stretococcus (Rygaard 1973, Dagnæs & Bisgaard 1990, Dagnæs et al. 1991).

Application in BioMedical Research:

• Human Tumor Xenografts: Soon after the discovery of the nude mutation, it was recognized that one of the most promising research areas with the nude mouse was the study of heterotransplantation of human tumors (Povlsen 1977). Due to the high reproductive performance and high vigor, the Bom:NMRI-nu strain has gained a widespread use as host for transplanted human tumors and for therapeutic studies on human tumors, transplanted to nude mice (Povlsen 1978, Fogh & Giovanelli 1978, Fogh & Giovanelli 1982, Spang-Thomsen & Visfeldt 1977, Spang-Thomsen 1985, Engelholm 1987, Bowen & Winnograd 1991). Many cultured human tumor cell lines have been characterized by their ability to induce tumors in nude mice (Fogh et al 1977).
  
  Compared to other nude mice or immune deficient mice, the NMRI-nu mice has a relative low take rate for a human breast cancer (Dagnæs-Hansen et al 1992). Maruo et al (1982) found that the tumor growth rate of human gastric cancers was high in outbred nude mice (NIH) compared to nude mice on a inbred background (BALB/c, C57BL/6J).
• Immunotoxicology and Teratotoxicology: Recently it has been suggested that immune deficient animals should be included in toxicological studies of xenobiotics and in safety testing of immunoreactive pharmaceuticals (Svendsen et al. 1987, Juul et al. 1992). Fetal rat limbs buds grafted in athymic mice might also be used for the testing of teratogenicity of drugs (Shiota et al 1990).