Human hair

Embryonically, hairs develop as a blunt tipped cone of inward growth of epithelial cells pushing down into the dermis.

The progressing tip of this inward movement becomes flattened and then develops a central concavity where the dermal papilla of the hair root forms.

The dermal papilla contains the connective tissues of the dermis and a blood supply of a few tiny capillaries.

The column of epithelial cells that develops extends deep into the dermis and constitutes the follicle of the hair.

The area of epithelial cells above the dermal papilla will grow to form the mitotically active hair matrix. Some cells in hair matrix are pushed upward to become part of the growing hair.

The hair itself becomes a filament of dead keratinocytes.

Typically the filament of hair has three layers of keratinized cells from the center to the outside: A central medulla, a thick middle layer of cortex cells, and a thin outer cuticle of one or two layers of dead flattened cells.

The wall of the follicle surrounding the hair shaft consists of an inner root sheath and an outer root sheath. The inner root sheath grows from the hair matrix. The outer root sheath is continuous with the stratum basale and stratum spinosum of the surface of the skin.

The inner root sheath has three layers: An inner root sheath cuticle of flattened cells which lies next to the hair’s cuticle layer, Huxley’ layer of somewhat flattened cells as a middle layer, and Henle’s layer is a single row of cuboidal cells lying next to the outer root sheath.

Hair follicles appear, mature, and then eventually cease functioning and disappear on a regular basis.