Histology of the Skin

The skin is the largest organ, typically accounting for 15-20% of total body weight in adults, presenting 1.5-2 m² of surface area to the external environment.
The skin is also known as the integument or cutaneous layer.
It consists of three main layers: the epidermis, dermis, and subcutaneous tissue (hypodermis).
The epidermis is a superficial layer made up of stratified squamous epithelium of ectodermal origin.
The dermis is a deeper layer made up of connective tissue of mesodermal origin. It contains collagen and elastic fibers, blood vessels, nerves, sweat glands, and hair follicles. This layer contributes to the skin's strength, elasticity, and sensory functions.
The subcutaneous tissue (hypodermis) is situated below the dermis and primarily composed of adipose tissue. This layer serves as insulation, energy storage, and a cushion against external trauma.
The epidermis has four layers in thin skin and five in thick skin: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.
The dermis has two sublayers: the papillary layer, and the reticular layer.
The papillary layer is thin and consists of loose connective tissue, fibroblasts, and mast cells. The reticular layer is thicker and makes up most of the dermis, containing dense irregular connective tissue and bundles of type I collagen.
The subcutaneous tissue binds the skin loosely to the underlying organs making it possible for the skin to slide over them.
The skin has a good capacity for repair, with healing stages.
The skin has various epidermal cells, including keratinocytes, melanocytes, Langerhans cells, and Merkel cells.

Skin Appendages

Skin appendages include hair follicles, sebaceous and sweat glands, and nails.
Sebaceous glands produce sebum, a mixture of fats and cellular debris, which helps retain skin flexibility, preserve moisture, protect from too much moisture, reduce damage from friction, facilitate cooling, and protect from UV damage.
Sebaceous glands are usually associated with hair follicles or directly connected to the surface of the skin.
Sweat glands include eccrine and apocrine glands.
- Eccrine glands regulate body temperature, producing sweat through a tightly regulated process of water and electrolyte excretion that cools the body.
- Apocrine glands are associated with hair follicles and become active during emotional stress. Their secretions are viscous and contain proteins; they are odourless, but after bacterial decomposition they give off body odours.
The hair plays a role in thermoregulation by trapping a layer of air close to the skin.
The hair also helps to wick away sweat.
Hair helps protect from sun's UV rays and keeps our bodies warm.
Nails are hard plates on the dorsal surface of each distal phalanx, primarily of keratin and consist of several different parts, including the visible nail plate, nail root, nail fold, lunula, hyponychium, and free edge. They act as hard protective barriers, provide counterforce, and prevent foreign objects like dirt and bacteria from entering the body.

Sensory Receptors

Sensory receptors are specialized structures in the body that respond to stimuli and transmit signals to the nervous system.
Types of receptors include mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, and nociceptors.
- Mechanoreceptors respond to mechanical stimuli (touch, pressure, vibration, stretch). Types include Meissner’s corpuscles, Pacinian corpuscles, and Ruffini endings.
- Photoreceptors respond to light. These are specialized receptors in the eyes (rods and cones) that convert light into electrical signals.
- Chemoreceptors respond to chemical stimuli (taste, smell). These are specialized structures, such as taste buds in the mouth and olfactory receptor cells in the nose.
- Thermoreceptors respond to temperature changes. They can act as free nerve endings in the skin and deeper tissues.
- Nociceptors respond to pain. They can be free nerve endings in various tissues and respond to stimuli that potentially cause tissues damage.

Nerve Endings

Nerve endings in skin sense different sensations.
Types include: Meissner’s corpuscles (light touch), Merkel discs (sustained pressure), Pacinian corpuscles (deep pressure and rapid changes in pressure), Ruffini endings (skin stretch), and free nerve endings (pain, temperature, and itch).

Skin Repair

Skin repair progresses through three main phases: inflammatory, proliferative, and remodeling.
The inflammatory phase involves blood clotting, release of growth factors, and infiltration of immune cells (like neutrophils and macrophages) to remove debris.
The proliferative phase focuses on cell migration and growth. Epithelial cells and fibroblasts migrate under the blood clot and proliferate to replace the damage. Macrophages and other cells release growth factors to aid in this phase. Blood vessels also regrow, and fibroblasts produce collagen.
The remodeling stage focusses on remodeling and normal vasculature reestablishment. Scar tissues form at the wound site, while the epidermis reestablishes continuity (but might have lost the ability to form new hair or glands).

Medical Applications

Skin cancers arise from cells of the basal or spinous layer of the epidermis. Basal cell carcinomas and squamous cell carcinomas are two types of skin cancer.
Albinism involves skin hypopigmentation due to a defect in melanin production
Vitiligo is a disorder characterized by localized skin depigmentation.
Merkel cell carcinomas are of clinical importance as they are aggressive skin cancers that are difficult to treat but less common than malignant melanoma.