Anatomy and Physiology of Hair and Nails

The Hair Follicle and Arrector Pili

The hair is housed within a structure called the hair follicle. This structure consists of an epithelial root sheath (an infolding of the epidermis) and a dermal root sheath.

Attached to the hair root is a small band of smooth muscle called the arrector pili. When these muscles contract, they cause the hair to stand upright, a phenomenon commonly known as "goosebumps".

Microscopic Layers of the Hair Shaft

If you were to view a transverse section (a cross-cut) of a hair shaft, you would see three distinct layers:

• Medulla: The soft, central core of the hair.
• Cortex: A highly organized layer of keratinocytes that provides the hair with its structural strength.
• Cuticle: The outermost layer, consisting of a single layer of overlapping keratinocytes.

Hair Types, Texture, and Color

Not all hair is created equal. The type of hair present on the body changes throughout life and varies by location.

Hair Types

• Lanugo: Fine, nonpigmented hair that covers nearly the entire body of a fetus; it typically falls out around birth.
• Vellus: Thin, nonpigmented hair that covers most of the body in adults.
• Terminal: Thick, coarse, and pigmented hair found on the scalp and surrounding the eyes. After puberty, terminal hair often replaces much of the vellus hair.

Texture and Color

The texture of your hair is determined by the shape of the hair follicle's cross-section:

• Round follicles produce straight hair.
• Oval follicles produce wavy hair.
• Flattened follicles produce curly hair.

Color is dictated by pigment granules located within the cells of the cortex:

• Eumelanin: Provides brown and black colors.
• Pheomelanin: High concentrations produce red hair, while intermediate amounts produce blond hair.
• Gray/White hair: This occurs due to a scarcity or total absence of melanin in the cortex, often accompanied by the presence of air in the medulla.

Exam Tip: If a question asks about the relationship between follicle shape and hair type, remember: Round = Straight, Oval = Wavy, and Flat = Curly.

Anatomy of the Nails

Nails are hard accessory structures located at the ends of the digits (fingers and toes). They are composed of keratinocytes and serve to protect the tips of the digits.

The most visible part of the nail is the nail plate. The portion of the nail plate that is visible is referred to as the nail body. Much like the hair root, nail growth occurs at the matrix, which is the area containing actively dividing cells.