The Hair Follicle

Macroscopic Anatomy

Hair follicles are downward extensions of the epidermis that project into the underlying dermis. Each follicle gives rise to a strand of hair – which grows from its base and extends towards the skin surface.

The visible portion of the hair that emerges above the epidermis is the shaft, while the portion embedded within the skin is known as the root. The root expands distally into the hair bulb, which surrounds a vascular structure called the dermal papilla (provides the hair bulb with oxygen and nutrients). Just above the bulb, the follicle narrows to form the isthmus.

The hair shaft itself is composed of three concentric layers of keratinised cells (dead epidermal cells that have replaced their internal structures with the protein keratin). From innermost to outermost, these layers are:

• Medulla – a central core of loosely arranged cells, often absent in fine hair.
• Cortex – the thickest layer, rich in keratin filaments and pigment granules that determine hair strength and colour.
• Cuticle – a thin, protective outer layer of overlapping keratin scales that interlock with the inner root sheath.

By OpenStax College [CC BY 3.0] via Wikimedia Commons

Fig 1
Structure of the hair follicle and hair shaft.

Microscopic Structure

Histologically, the hair follicle is arranged into several concentric layers that support and protect the developing hair shaft.

From innermost to outermost, these include:

• Inner root sheath – composed of three layers: Henle’s layer, Huxley’s layer and the outer cuticle. It closely surrounds the hair shaft, guiding it upwards through the follicle before disintegrating near the isthmus.
• Outer root sheath – a downward continuation of the epidermis made up of living keratinocytes. It provides structural support and houses stem cells capable of regenerating after injury.
• Glassy membrane – a thickened basement membrane separating the follicular epithelium (hair matrix and inner and outer root sheaths) from surrounding connective tissue.
• Connective-tissue sheath – a fibrous outer layer, rich in collagen and elastic fibres that anchors the follicle to the dermis.

At the base of the follicle lies the hair matrix, containing rapidly dividing matrix cells and pigment-producing melanocytes. Matrix cells differentiate into the three layers of the hair shaft and the three layers of the inner root sheath. Melanocytes transfer melanin granules to the developing hair cells to provide colour.

Adapted from work by Cuevas-Diaz Duran, R.; Martinez-Ledesma, E.; Garcia-Garcia, M.; Bajo Gauzin, D.; Sarro-Ramírez, A.; Gonzalez-Carrillo, C.; Rodríguez-Sardin, D.; Fuentes, A.; Cardenas-Lopez, A., [CC BY 4.0], via Wikimedia Commons

Fig 2
Microscopic structure of the hair follicle

Associated Structures of the Hair Follicle

The function of the hair follicle depends on several surrounding supporting structures that contribute to movement, lubrication, sensation and nourishment:

• Arrector pili muscle – a small band of smooth muscle that attaches to the follicle at an oblique angle. When it contracts, the hair shaft becomes more erect, producing goosebumps as part of a thermoregulatory response. Contraction of this muscle also assists in expelling sebum from the follicle.
• Sebaceous gland – a lobulated gland that releases a lipid-rich substance called sebum into the follicular canal. Sebum drains into the canal just above the point where the arrector pili muscle inserts, lubricating both the hair and surrounding skin. Together, the hair follicle and sebaceous gland form the pilosebaceous unit.
• Innervation – dense networks of sensory nerve endings encircle the follicle, forming part of the tactile apparatus of the skin. Sympathetic nerve fibres supply the arrector pili muscles and sebaceous glands, allowing autonomic control of their activity.
• Blood supply – the dermal papilla at the follicle base receives a fine capillary network from the deep dermal plexus. It delivers oxygen and nutrients essential for the active growth phase of the hair cycle.

Hair Growth Cycle

Hair follicles progress through repeated, asynchronous cycles of growth and rest. Each phase involves distinct cellular and structural changes within the follicle:

Around 85–90% of scalp follicles are in anagen at any one time, while non-scalp follicles have shorter cycles.

Adobe Stock, Licensed to TeachMeSeries Ltd

Fig 3
The hair growth cycle.