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The old saying goes, "Without the skin, where can the hair attach?" As an appendage of human skin, hair's growth and development are closely linked to the skin. To understand the physiology and pathology of human hair, we must start with the skin.
What Does Skin Look Like and How Is It Structured?
Skin, located on the body's surface, acts as the first line of defense and has crucial functions. In terms of weight and area, it's the largest organ, constituting about 16% of body weight, covering 1.5 to 2 square meters in adults, and about 0.21 square meters in newborns.
To the naked eye, skin appears as a network of fine wrinkles and grooves of varying depths. Deep grooves divide the skin into different shapes, like triangles or polygons, especially pronounced in flexible areas like palms, wrists, and necks.
Under a magnifying glass, the skin surface reveals numerous tiny ridges ("papillae") and grooves. On these ridges are small, indented pores - sweat glands. Apart from areas like palms, soles, lips, glans penis, labia minora, most skin has hair follicles.
Under a microscope, skin comprises three layers: the epidermis, dermis, and subcutaneous tissue (see Figure 1). The outermost, the epidermis, consists of tightly packed cells, without nerves or blood vessels. Beneath it lies the dermis, distinguished by a wavy boundary with the epidermis, containing various fibers, blood vessels, lymphatics, nerves, and skin appendages. The subcutaneous tissue below the dermis, or fat layer, varies in thickness based on nutrition, gender, age, and location. Body fat largely depends on this layer's thickness.
What Does Hair Look Like and How Is It Structured?
Hair, a skin appendage, is a cylindrical keratin structure. The part above the skin is the "hair shaft"; below the skin is the "hair root," ending in an onion-shaped "hair bulb" (see Figure 1).
- Epidermis: The outermost shield of our skin, composed of five distinct layers: the stratum corneum, lucidum, granulosum, spinosum, and basale. Think of it as the skin's frontline defense against the external world.
- Dermis: Beneath the epidermis lies the living, bustling neighborhood of the dermis. It's packed with nerves, blood vessels, and other critical structures, giving our skin its resilience and flexibility.
- Subcutaneous Tissue: The innermost layer of the skin, a cozy blanket of fat and connective tissue. It's not just for warmth and cushioning; it's a fortress for immune defense and a hub for hormone activity.
- Stratum Corneum: The outermost barrier of the epidermis, made up of deceased skin cells. It's the unsung hero, offering both protection and waterproofing.
- Stratum Lucidum: A thin, clear layer found only in the thick skin of palms and soles. It regulates moisture, preventing excessive loss or absorption.
- Granular Layer: Acting like a vigilant guard, this layer filters out invaders and shields us from harmful UV rays.
- Spinosum: A storage unit for the skin cells, where they bide their time, preparing for their eventual journey to the surface.
- Basal Layer: Nestled at the epidermis' deepest reaches, this layer houses melanocytes, the artists behind our unique skin tones.
- Blood Plexus: A network of blood vessels in the upper dermis, akin to a bustling highway, providing nutrients and oxygen.
- Sweat Ducts: These tubular structures are like little highways, transporting sweat from its source to the outer world of our skin.
- Collagen Fibers: The dermis's structural proteins, conferring strength and elasticity. They are the skin's backbone, quite literally.
- Deep Blood Plexus: Situated in the lower dermis, these vessels are vital lifelines for sweat glands, hair follicles, and more.
- Hair Bulb: The birthplace of hair, nestled at the base of the hair follicle.
- Hair Papilla: A tiny, nipple-like structure at the bottom of the hair bulb, supplying nutrients and oxygen to fuel hair growth.
- Sweat Glands: Little factories producing sweat, integral for temperature regulation and waste excretion.
- Sweat Pores: The skin's mini exits, where sweat makes its grand debut on the skin's surface.
- Meissner's Corpuscles: Delicate structures that sense the lightest of touches, adding nuance to our tactile world.
- Hair Shaft: The visible part of hair, the flagpole emerging from the skin.
- Dermal Papillae: These small, nipple-like projections in the dermis offer nourishment, support, and protection, like little guardians.
- Sebaceous Glands: Tiny glands secreting sebum, nature's moisturizer for skin and hair.
- Arrector Pili Muscle: When these muscles contract, they cause hairs to stand on end, creating the familiar goosebumps.
- Hair Root: The anchor of the hair, embedded in the hair follicle, and a lifeline for the cells in the hair bulb.
- Hair Follicle: A mini-organ supporting hair growth, like a nurturing cradle for each strand.
- Pacinian Corpuscles: Structures attuned to pressure, translating the world's pushes and prods into sensory experiences.
- Adipose Tissue: A community of fat cells, acting as energy reservoirs, warmth providers, and shock absorbers.
Hair is widespread, covering almost the entire body, except for palms, soles, and certain other areas. Generally, hair can be divided into terminal (coarser, darker) and vellus (finer, lighter) types. Under a microscope, hair is seen to consist of a medulla, cortex, and cuticle. The medulla forms the hair's core; the cortex, its main part, contains pigment in colored hair. The cuticle, a layer of overlapping cells, covers the hair's surface.
What Is the Structure of a Hair Follicle?
Each hair originates from a depression in the epidermis, the hair follicle (see Figure 1). The follicle, a skin extension, surrounds the hair root and is attached to the arrector pili muscle and sebaceous gland, with sweat glands also opening into it.
The hair follicle comprises the epidermal hair root sheath and a connective tissue sheath. The epidermal sheath originates from the epidermis, extending into the dermis and subcutaneous fat layer, wrapping around the hair. The connective tissue sheath, formed from the dermis, envelops the outer hair root sheath without a clear boundary with the dermis' connective tissue.
A fine smooth muscle, the arrector pili, attaches at one end to the dermis and at the other to the follicle's connective tissue sheath. This muscle, controlled by the sympathetic nervous system, causes the hair to stand up straighter, creating "goosebumps" when contracted.