7.2.2: Shaft

There are three layers of the hair shaft – the cortex, cuticle, and medulla - which make up the visible hair shaft seen in terminal hair follicles.
The central medulla contains cells with a honeycomb-like appearance and may be absent or fragmented in the terminal hairs. In fine, vellus hair fibers the medulla is always absent.
The cortex, which looks like coils of rope, form the central layer of the hair shaft. They are longitudinally-shaped fibrous cells which are positioned around the medulla. These cells are packed with keratin filaments. The cortex contains color determining melanosomes. The two main hair pigments are pheomelanin, a yellow/red pigment that gives red, blonde, or auburn hair; and eumelanin, a pigment that produces black or brown hair.
The outermost layer of the hair fiber, the cuticle, which looks like slates on a roof, is a thin and translucent layer, thereby allowing light to penetrate to the cortex pigments.

Shaft Structure

The structure of a hair shaft is organized into three "alpha-helixes" which are twisted together to form a protofibril.
These protofibrils are then bundled together to form a ‘cable’ known as a microfibril.
The microfibrils are embedded in a protein matrix of high sulfur content (which aid in the strength of the hair shaft), and are cemented together into an irregular fibrous cortical bundle called a macrofibril .
Finally, these macrofibrils are grouped together to form the rope-like structure of the cortex of the hair fiber.