The integumentary system consists of skin, the largest organ of the body that serves as a barrier from the outside world, and its adnexal structures, such as hair, nails, sebaceous, apocrine, and mammary glands (Sundberg et al. "Skin and Adnexa", pp. 511-542).
Epidermis is the outermost skin layer, facing the outside environment, and consists of stratified squamous epithelial cells called keratinocytes (Sundberg et al. "Skin and Adnexa", pp. 511-542).
Mammary glands are modified apocrine (sweat) glands that secrete milk, the primary food source of newborn mammals (Sundberg et al. "Skin and Adnexa", pp. 511-542).
Sundberg, JP, Booth, CJ, Nanney, LB, Fleckman, P, King Jr., LE
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