Final answer:
The central core of a hair fiber is called the medulla. It is one of the three zones of the hair shaft, which also includes the cortex and the cuticle, and it is encased by keratinized cells.
Step-by-step explanation:
The central core of a hair fiber is called the medulla. The structure of hair can be understood by examining a hair follicle, which has several components. The hair shaft, which is what we typically see above the skin, consists of three layers: the cuticle, the cortex, and the innermost zone known as the medulla. The cuticle is the outermost layer of the hair made of very hard, keratinized cells. The middle layer is the cortex, comprising compressed, keratinized cells that influence the hair's texture, such as straight or curly. The medulla forms the core of the hair, although not all hair types have a medullary layer. The external hair that is visible and extends from the skin is entirely composed of dead, keratin-filled keratinocytes, which means it has no sensation, and cutting or shaving it does not impact the structure.