L3 The Skin and Epidermis

The Skin

• The skin is the largest organ in the human body
• The skin is responsible for protecting the body from the environment
• Skin is composed of three distinct layers: the epidermis, the dermis, and the hypodermis

Epidermis

• The epidermis is the outermost layer of skin
• The epidermis is made of stratified squamous keratinizing epithelium
• It is comprised of five layers:
  • Stratum basale: consists of a single layer of columnar epithelial cells; contains mitotically active cells; some cells contain melanin pigment
  • Stratum spinosum: formed of polygonal shaped cells; cells connected by desmosomes; contains membrane-coating granules
  • Stratum granulosum: consists of flattened elongated cells; contains keratohyalin granules
  • Stratum lucidum: present in thick skin but not in thin skin; consists of flattened non-nucleated cells
  • Stratum corneum: outermost layer of the epidermis; consists of 15-20 layers of dead, flattened, anucleate keratinocytes; protects against friction and water loss,

Cell Types in Epidermis

• Keratinocytes: the predominant cell type; originate in the basal layer; produce keratin; responsible for the epidermal water barrier
• Melanocytes: synthesize dark melanin pigment in melanosomes; transport melanosomes to adjacent keratinocytes
• Langerhans cells: located in stratum spinosum; branched star shaped cells; function as antigen presenting cells
• Merkel cells: located in the basal layer of the epidermis; contain small dense neurosecretory granules; function as mechanoreceptors for tactile sensation

Dermis

• The dermis is the layer of skin below the epidermis
• The dermis is composed of two layers:
  • Papillary layer: supports and nourishes epidermis; contains sensory nerves, lymphatics, and capillaries
  • Reticular layer: dense irregular connective tissue; collagen fibers limit stretch; elastic fibers provide flexibility

Structures in the Dermis

• Blood vessels
• Sensory (touch) nerve endings
• Sweat glands and their ducts
• Hair follicles
• Sebaceous glands
• Arrector pili muscles

Sebaceous Glands

• Made of secretory epithelial cells that secrete sebum (oily secretion)
• Secrete sebum via holocrine secretion (entire cell breaks up to form secretion)
• Sebaceous glands are absent in the palms and soles

Sweat Glands

• Widely distributed in the skin
• Each gland has a coiled body under the skin and a duct that opens onto the pores on the skin
• Most numerous on palms of hands, soles of feet, axilla, and groin
• Sweat glands are divided into two types:
  • Eccrine sweat glands: secrete sweat directly onto the surface of the skin
  • Apocrine sweat glands: secrete fluid into the sac of a hair follicle where it eventually comes out on the skin

Hair Follicles

• Down growths of epidermal cells into the dermis
• At the base of the follicle, a cluster of cells called the bulb is present
• Multiplication of cells in the bulb causes the hair to grow
• The part of the hair inside the skin is called the root
• The part of the hair above the skin is called the shaft

Arrector Pili Muscles

• Small bundles of smooth muscles that are attached to hair follicles
• Contraction causes the hair to stand erect and causes goose flesh

Nails

• Protect the tips of the fingers and toes
• Nails have a root embedded in the skin
• The exposed part is called the nail plate
• The nail grows from the epidermis from a germinal part called the nail bed

Cutaneous Nerve Endings

• Free nerve endings: perceive pain and temperature
• Merkel cells: tactile sensation
• Meissner's corpuscles: touch
• Pacinian corpuscles: vibration and pressure
• Ruffini endings: pressure

Sources of Skin Color

• Melanocytes: produce melanin, which provides UV protection; gives reddish-brown to brown-black color
• Carotene: contributes orange-yellow color; provided from diet (carrots and tomatoes)
• Hemoglobin: blood pigment

Functions of Skin

• Excretion: skin excretes some water and salts in sweat
• Protection: skin protects from bacteria, water loss, and excess heat and cold
• Temperature regulation: skin plays a major role in regulating body temperature
• Sensory function: skin is an organ of sense of touch, pain, temperature, and pressure
• Production of vitamin D

Clinical Notes About Skin

• Skin preparation for surgery: the skin harbors many bacteria; preparation involves shaving hair in the area, washing the sebum with soap, and cleaning the skin with antiseptic
• Burns: burns cause damage to the skin due to fire, electricity, or acids; structure of the skin is damaged, infection and water loss are common; burns are classified according to their severity (depth) as first, second, and third-degree burns
  • First-degree burn: only epidermis is damaged; heals and leaves no scar
  • Second-degree burn: damage to epidermis and upper region of the dermis; if carefully treated and infection prevented can heal without a scar
  • Third-degree burn: destroys the entire thickness of the skin; there is no pain as nerve endings are destroyed; regeneration is not possible; skin grafting must be done
• Determination of the burnt area: use the rule of nines to determine the area burnt