Which gland's secretions are mainly composed of lipids?

The sebaceous gland's secretions are primarily composed of lipids, specifically sebum, which is an oily substance that helps to lubricate and protect the skin and hair. Sebum consists of a mixture of triglycerides, wax esters, squalene, and free fatty acids, making it crucial for maintaining skin hydration and providing a barrier against external elements.

In contrast, the apocrine gland secretes a more viscous fluid that contains proteins and lipids, but it is not predominantly lipid-based. The eccrine gland primarily produces a watery secretion that helps in thermoregulation and does not contain significant amounts of lipids. The hypothalamus, being a part of the brain, does not exhibit glandular activity related to the secretion of lipids; its functions are centered around regulating bodily homeostasis and endocrine functions through hormonal control. Hence, the sebaceous gland is the clear choice for a gland whose secretions are mainly lipid-based.