any structural anomaly of the highly specialized, sebum-producing epithelial cells of the sebaceous glands that release their content by rupture of the cell membrane and cellular degradation (holocrine secretion); these cells are most commonly found in the skin in association with hair follicles (forming the pilosebaceous unit), where they arise from hair follicle keratinocytes, but there are also sebaceous glands not associated with a hair follicle; the latter have special functions as secretion of pheromones or corneal protection