Penile spines are prevalent throughout the primate world, however they are absent in humans. Penile spines are keratinized, androgen dependent structures that protrude from the glans of the penis. The lack of penile spines has been linked to a longer time of intromission during copulation and is hypothesized to be part of an adaptive suite linked to pair-bonding in humanity’s early hominid ancestors. Other appendages of the skin, such as hair, nails, mammary glands, and teeth all show similar developmental origins through specific interactions between the epithelium and mesenchymal tissue. The purpose of this study is to determine the efficacy of Milligan’s Trichrome stain for distinguishing between muscle, collagen, and keratin during skin appendage development. Once this was established, the histology of penile spine, hair, and nail development was compared during postnatal mouse development. Milligan’s Trichrome stain proves to be effective for distinguishing keratinized structures and may even be useful in distinguishing between hard and soft keratins. In agreement with previous studies on skin appendage development, penile spine formation is shown to be largely similar to that of hair and nails. However, penile spines develop substantially later than these other structures, with their entire development occurring postnatally. Placodes appear at P7, buds appear by P15, and the keratinized spines appear at P20. By P63 they are fully formed keratinized spines that are exposed to the exterior.
