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.

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.

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.