Dashing down a track, sprinters rely on the special properties of the fast-twitch muscle fibers interspersed throughout their skeletal muscles. Meanwhile, long-distance runners are more concerned with performance of the slow-twitch muscle fibers in those same muscles. In humans, these fibers differ in the number of mitochondria they contain, but both types of muscle are formed from a cellular syncytium: many muscle cells fuse together into a multinucleate mass.

The hormone leptin is critical for maintaining metabolic homeostasis. Secreted by adipose tissue after a meal, leptin binds to specific leptin receptors in certain brain regions, particularly the arcuate nucleus (Arc) region of the hypothalamus.

Dendritic cells (DCs) are a type of immune cell that patrol tissues, on the lookout for microbial invaders. When DCs encounter a pathogen, they chop it up into tiny pieces and then carry samples of it to local lymph nodes. There, they display their finds to another kind of immune cell, the T cell, which then mounts a full-fledged immune response against the invader.

The hormone leptin is critical for maintaining metabolic homeostasis. Secreted by adipose tissue after a meal, leptin binds to specific leptin receptors in certain brain regions, particularly the arcuate nucleus (Arc) region of the hypothalamus.

Dashing down a track, sprinters rely on the special properties of the fast-twitch muscle fibers interspersed throughout their skeletal muscles. Meanwhile, long-distance runners are more concerned with performance of the slow-twitch muscle fibers in those same muscles. In humans, these fibers differ in the number of mitochondria they contain, but both types of muscle are formed from a cellular syncytium: many muscle cells fuse together into a multinucleate mass.

Dendritic cells (DCs) are a type of immune cell that patrol tissues, on the lookout for microbial invaders. When DCs encounter a pathogen, they chop it up into tiny pieces and then carry samples of it to local lymph nodes. There, they display their finds to another kind of immune cell, the T cell, which then mounts a full-fledged immune response against the invader.