The repeated epilation (Er) is an autosomal dominant mutation mapped to Chromosome 4 that was first reported in 1960. It was probably radiation induced. Several congenic lines were created from the original outbred stock.
Heterozygous mice are indistinguishable from wildtype siblings until around 2 weeks of age, when they begin to lose hair in patches. These mice grow more slowly and some die. The embryonic hair cycle normally ends around two weeks of age but it is extended about one week in Er/+ mice. Hair is lost from head to tail. Unlike their littermate controls, Er/+ mice develop papillomas and squamous cell carcinomas beyond the age of 6 months. Homozygous (Er/Er) embryos can be recognized in utero at 1.35 days post conception (dpc) when they develop abnormalities of the skin, tail, and limbs. Stumpy legs and tails are evident at 18.5 dpc. Facial dysmorphism is also observed. Homozygous mice born die soon after because they can not breathe due to a closed oral cavity.
Foot plates are shortened with no digital separation. Nails are absent or poorly developed. The tail is bent and short. Homozygous mice develop local thickenings in the skin as early as 13.5 dpc. By 14.5 dpc the flank epidermis is about twice normal thickness while it remains thin on the dorsal head. Beginning around 17.5 dpc the skin wrinkles in +/+ or Er/+ mice but remains smooth in Er/Er mice. By 18.5 dpc the epidermis forms rete ridge like structures, which are not usually found in wildtype mice. The stratum corneum becomes markedly thickened. Hair follicles are scarce.
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