1. The part of the interior of the earth, or another planet, located below the crust and above the core, extending to a depth of about 3,500 kilometers below the surface.

The hot plastic asthenosphere, part upper mantle and lower crust about 186 miles (300 kilometers) thick, separates the more brittle crust-mantle lithosphere above from the mesosphere below.

This is thought to be responsible for the movement of the lithospheric plates (crustal plates) which slowly "carry" the continents around the planet.

The more solid mesosphere, which is located below he asthenosphere, includes part of the upper and all of the lower mantle.

2. The nuclear zone of the developing neural tube between the marginal layer and the ependymal layer (covering of internal and external surfaces of the body, including the lining of vessels and other small cavities); which forms the gray matter of the central nervous system.