In the anatomy of the brain, the prosencephalon, also known as the forebrain, is a large set of structures consisting of the diencephalon and telencephalon. Diencephalon structures include the thalamus, hypothalamus, and other nuclei of grey matter, or neurons, near the third ventricle, a fluid-filled cushioning area. The telencephalon refers to the cerebral cortex, the largest area of the brain, along with its white matter projections and support cells, and the basal ganglia.
During development, the brain initially forms as three regions, and later develops into five. These developmental regions consist of the prosencephalon, the mesencephalon or midbrain, and the rhombencephalon or hindbrain. Even during this period, the prosencephalon manages key functions such as emotional display, feeding, sleeping, and body temperature. Only later does this region separate into two distinct structures in higher mammals.
In humans and other higher mammals, there are many functions regulated by the prosencephalon. The cerebrum is involved in processing most types of sensory information, as well as coordinating and executing motor functions. Just as importantly for humans, it plays a role in higher-order processes like decision-making, memory, speech, and comprehension.
Several other critical processes are mediated in the diencephalon. Neurons in the thalamus receive incoming sensory signals, and route them to appropriate regions in the cerebral cortex. The nearby hypothalamus releases hormones that regulate sexual behavior, and regulates automatic processes like hunger, thirst, and circadian rhythms influencing sleep. Glands like the pituitary and pineal are also located here, where they assist in growth and homeostasis functions.
Proper development of the prosencephalon requires the presence of chemical signals released at appropriate times. At first, neural and stem cells begin to develop in a small area called the neuraxis. Cells on the anterior side of the neuraxis secrete proteins that stimulate differentiation between anterior cells and those at the posterior end of the neuraxis. Anterior cells develop into the prosencephalon, and posterior cells eventually become parts of the midbrain and hindbrain.
Following this initial division between anterior and posterior, there is another activation of signaling proteins. Cells at the dividing line between neural and non-neural tissue on the anterior side release proteins, such as the network of proteins called Wnt. These signals induce the growth and differentiation of distinct prosencephalon tissue. Telencephalon and diencephalon tissue form shortly afterward, but they still comprise a single structure. Other proteins help this brain region to develop properly along dorsal to ventral lines, and to differentiate into discrete structures.