What part of the hair extends from the hair papilla to the body surface

Hair shaft

Explanation:

Each hair is made up of a base called hair root and hair shaft which protrudes outwards. Growth of the hair follicle is due to continuous addition of new dead cells at the base of the hair. At the base of the hair is a dermal or hair papilla from which the hair root develops.

Erector pili muscles attached to the follicle alter the angle between the hair shaft and skin and therefore vary the amount of air trapped between the hair and skin.

cell regulation is conducted by special proteins.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.