What is the primary substance secreted by sebaceous glands?

Sebaceous glands primarily secrete an oily substance known as sebum. This secretion serves several important functions for the skin and hair. Sebum helps to lubricate the skin and hair, providing moisture and preventing dryness. Additionally, it plays a role in protecting the skin by forming a barrier that can prevent the growth of certain bacteria and fungi. The presence of sebum also contributes to the overall health of the skin's microbiome, promoting a balanced environment.

The other options represent different biological substances or functions unrelated to the primary role of sebaceous glands. Sweat is produced by sweat glands and is primarily involved in temperature regulation. Keratin is a structural protein found in hair, nails, and the outer layer of skin, and is not secreted by sebaceous glands. Collagen fibers, which provide structural support to skin and connective tissues, are produced by fibroblasts and do not originate from sebaceous glands. Thus, the focus on sebum accurately captures the primary function of these glands in the skin.