Hair Anatomy Dallas
Anatomy of hair explained by Dr Sam Lam;
Hair follicles cover the entire skin surface except the palms, soles, glans penis, and labia minora. There are approximately 5 million hair follicles on the body with 100,000 on the scalp. The scalp has the greatest density of hair follicles with roughly 300 to 500 hairs per square centimeter.
The hair follicle may be divided anatomically into four parts:
- The bulb consisting of the dermal papilla and matrix
- The suprabulbar area from the matrix to the insertion of the arrector pili muscle
- The isthmus that extends from the insertion of the arrector pili muscle to the sebaceous gland
- The infundibulum that extends from the sebaceous gland to the follicular orifice.
The lower portion of the hair follicle consists of five major portions:
- The dermal papilla
- The matrix
- The hair shaft, consisting from inward to outward the medulla, cortex, and cuticle
- The inner root sheath (IRS) consisting of the inner root sheath cuticle, Huxley’s layer, and Henle’s layer
- The outer root sheath (ORS).
The base of the follicle is invaginated by the dermal papilla, which has a capillary loop that passes through the papilla. Signal transduction and communication between the dermal papilla and the matrix cells influence how long and how thick the hair shaft will grow. The melanocytes within the matrix also produce the pigment in the hair shaft.
The hair medulla is only partially keratinized and therefore appears amorphous and may not always be present. The hair cortex cells lose their nuclei during their upward growth and do not contain any keratohyaline or trichohyaline granules unlike the ORS and IRS, respectively. The keratin of the cortex is hard in contrast to the IRS or epidermis, which are soft. The cuticle is firmly anchored to the IRS cuticle.
The cuticle of the IRS consists of a single layer of flattened overlapping cells that point downward and interlock tightly with the upward angled cells of the hair shaft cuticle. Huxley’s layer is composed of two cell layers, whereas the outer Henle’s layer is only one cell thick. Just before the isthmus, the IRS becomes fully keratinized but disintegrates at the level of the isthmus. Although the IRS is not present in the emerging hair shaft, the IRS serves as a strong scaffold in the lower portion of the hair follicle.
The ORS extends from the matrix all the way to the sebaceous gland, where it becomes the surface epidermis. The ORS is thinnest in the hair bulb and progressively becomes thicker with the thickest portion at the level of the isthmus.
The portion where the arrector pili muscle inserts is known as the bulge area and is thought to be the location for the stem cells that can help regenerate the hair follicle.