Skin Biology

Epidermis

Epidermis is composed of stratified squamous epithelium. These cells arise from the basal layer and become progressively more keratinized as their cytoplasm is replaced by keratin. Cells become progressively more superficial.

The epidermis is composed of keratinocytes, melanocytes, Langerhans cells (immune cells), and Merkel cells (mechanoreceptors).

Epidermis has five layers:

Stratum corneum - dead keratinized cells make up the outermost layer, protecting the other layers of skin. Sebum produced here is bactericidal to Staphylococcus and Streptococcus.

Stratum lucidum is a clear layer of skin present only in the thick skin of the palms and feet.

Stratum granulosum is composed of mature keratinocytes containing granules of keratohyalin. This layer is the predominant site of protein synthesis.

Stratum spinosum contains large kerainocytes joined to each other by tonofibrils.

Stratum germinativum, or basal layer, is composed of skin stem cells hadherent to the basal lamina. This layer contains melanocytes.

Dermis

The dermis accounts for 95% of skin thickness. The papillary dermis is superficial. It is predominantly composed of cells and fine collagen. The reticular dermis is deeper, containing fewer cells and thicker collagen fibres.

ECM components of the dermis include:

• collagen
• elastin
• proteoglycans

The vascular plexus lies between the papillary and reticular dermal layers.

Hair and Glands

Hair follicles are compused of medulla, cortex, and outer cuticle.

Hair follicles are composed of inner root sheath from epidermis and outer root sheath from dermis.

Sebaceous glands drain into each follicle.

Three-quarters of hairs are in the anogen (growth) phase, while one-quarter are in the telogen (resting) phase.

Eccrine glands...

Apocrine glands...

Sebaceous glands..

Blood Supply

Deep vessels arise from the aorta and give rise to the main arterial supply of the skin via interconnecting vessels:

• Fasciocutaneous vessels provide most of the blood supply to the limbs by travelling along fascial septae.
• Musculocutaneous vessels arise directly from deep vessels, divide multiple times as they enter muscle bellies, and travel to the skin. They provide the main blood supply to the skin of the torso

Vascular plexuses exist at multiple layers within the skin:

• subfascial plexus
• prefascial plexus
• subcutaneous plexus
• subdermal plexus: the main source of bleeding following injury
• dermal plexus: predominantly arterioles; important in thermal regulation
• subepidermal plexus: capillaries important in nutrition

Angiosomes are tissue regions supplied by a specific artery.

Vessels tend to follow nerves and have fixed desintations though varied origins.

Control of Blood Flow

Arteriovenous anastamoses connect arterioles with efferent veins. Control is mediated by a number of factors:

Sympathetic innervation of arterioles, precapillary sphincters, and anastamoses controls rate of blood flow.

Epinephrine causes vasoconstriction

histamine and bradykinin cause vasodilation, as do low oxygen saturation, high CO2, and acidosis.