Cell Death by Holocrine Secretion: The Final Step of Epithelial Differentiation in Sebaceous Glands

Sebaceous glands consist of epithelial cells, known as sebocytes, that undergo differentiation to deliver the components of sebum into the sebaceous duct and eventually to the hair and skin surface. The final step of the terminal differentiation program is called holocrine secretion because the entire cell content is converted into sebum. Holocrine secretion is a mode of programmed cell death, which involves the degradation of the nucleus and other organelles and the rupture of the cell membrane. Here, we review the current knowledge of differentiation-associated death of sebocytes and discuss open questions regarding its mechanism and functions. In vivo studies have provided evidence for degradation of nuclear and mitochondrial DNA by lysosomal deoxribonuclease 2 (DNase 2), indicating a key role of lysosomes in holocrine secretion. We discuss the influence of tight junctions on the spatial localization of holocrine secretion within glands, the regulation of holocrine cell death by autophagy and potential mediators of membrane lysis. Further studies of holocrine secretion are needed to fully uncover its molecular control and to determine potential clinical implications.