(Transcribed from Dr. Kalliecharan’s handout, 1 Mar 2000 by Brian Buschman)
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The skin is composed of three layers the first being the epidermis superficially which is of ectodermal origin and is stratified squamous epithelia. The middle layer is the dermis which is mesodermally derived and compose of dense irregular connective tissue. The intersection between the dermis and epidermis occurs at an interdigitating line called the papillae or dermal ridges. The deepest layer is the hypodermis (or subcutaneous layer) of loose connective tissue and fat which is the superficial fascia. The fat pads are called panniculus adiposus.
The skin has many functions including:
1) Protection from bacterial invasion.
2) Protection against water loss.
3) Protection against UV light.
4) Excretion as sweat.
5) Thermoregulation via sweating and capillary controlled conduction.
6) Sensory reception.
7) Synthesis of vitamin D.
Skin can be classified as either thick or thin.
Thick skin (palmer and planter):
1) Lacks hair follicles.
2) Lacks arrector pili muscles.
3) Lacks sebaceous glands.
4) Has sweat glands.
Thin skin has all of them but lacks a stratum lucidum.
The epidermis is avascular and made up of five layers and primarily four cell types. The layers are:
1) Stratum basale which is a single layer of columnar or cuboidal keratinocytes. Keratinocytes are the epithelial cells of the skin that are still alive and will eventually become the cells on the surface. They make keratin. This basal layer of cells is mitotically active but only at night. They posses intermediate filaments to bind desmosomes which bind cells on the upper and lateral surfaces to hold keratinocytes together. They also bind hemidesmosomes to anchor the keratinocytes to the basal lamina.
2) Stratum spinosum consists of polyhedral or flattened cells and is the thickest of the epidermal layers. The basally located keratinocytes are mitotically active. This region of mitotically active cells and the stratum basale together make up the Malpighian layer which is the only area in epidermis where mitosis takes place. The keratinocytes of the S. spinosum have a spiny appearance since they have more tonofilaments which reach the edges of the cells.
3) Stratum granulosum is made of three to five layers of flattened keratinocytes with non-membrane bound keratinohyalin granules which are made up of a precursor of keratin. They also contain membrane bound lamellar granules which fuse with the plasma membrane to release glycolipids that waterproof the skin.
4) Stratum lucidum is only seen in thick skin and has cells with many keratin filaments and it’s organelles have already been digested.
5) Stratum corneum has cells that are keratinized. They are held together by desmosomes and are sloughed off at the surface.
Langerhans’ cells are antigen presenting cells of the epidermis. They are found primarily in S. spinosum and have a star shaped appearance.
Merkel cells are in thick skin and have many dense granules. They have free nerve endings at their base and they are may be barocepeptors for cutaneous sensation but that is not know for sure.
Melanocytes are present in the S. basale, make melanin and melanin forming enzymes and are found in roughly equal numbers in all races. The difference in skin color comes from the content of melanin and carotene as well as the amount and color of blood vessels in the dermis. Melanocytes are derived from neural crest cells and have round cell bodies. They have extensions that reach between cells of the Malpighian layer which pass melanosomes to keratinocytes. Melanosomes are organelles that contain melanin and tyrosinase. Tyrosinase is an enzyme responsible for the conversion of tyrosine to melanin. They are bound to the basal lamina by hemidesmosomes but are not bound to adjacent keratinocytes. Melanocytes
The dermis is the vascular portion of the skin and has two layers.
1) The papillary layer is made of loose connective tissue, reticular fibers (collage types I and III) and elastic fibers. It contains macrophages, fibroblasts and mast cells. The papillary layer contains the capillary loops that nourish the epidermis and function in thermoregulation. It also has anchoring fibers that help hold down the epidermis.
2) The reticular layer is made of dense irregular connective tissue (collagen type I) and proteoglycans between the fibers. (The reticular layer hardly has any reticular fibers they are ironically in the papillary layer.)
The dermis contains hair follicles, sweat glands and sebaceous glands.
The arterial supply enters and branches into two layers:
1) Deep plexus which supplies the hypodermis and reticular layer.
2) Superficial plexus which supplies the reticular layer and papillary layer. The superficial layer as AV shunts involved in directing the blood to or from the skin for thermoregulation and blood pressure regulation.
There are four types of nerve endings in the skin. All are technically in the dermis but some project into the epidermis.
1) Free nerve endings lack myelin sheathes and end in the S. granulosum. They are concentrated in the thin skin near hair follicles and detect heat, cold and pain.
2) Meissner’s corpuscles are touch receptors in the dermal papillae of lips, nipples, fingers and such sensitive areas. They are covered by fibroblasts and collagen and have unmyelinated ends of myelinated axons.
3) Pacinian corpuscles are deep pressure receptors for mechanical or vibrating pressure. They have myelinated nerve endings in the deep dermis and hypodermis.
4) Raffini’s corpuscles are tension receptors that are sensitive to movement of the adjacent collagen fibers. They include a single myelinated fiber in a connective tissue sheath. They are in the dermis.
Hair comes from a hair follicle with a terminal dilation called the hair bulb. At the base of the bulb is the dermal papilla which contains the capillaries. Hair have an internal root sheath which surrounds the entire internal part of the hair and an external root sheath which is continuous with the epidermis. The glassy membrane separated the hair follicle from the dermis. The arrector pili muscles connect the follicle to the papillary layer of the dermis.
Fingernails grow from the nail root and the S. corneum folds over the nail to give the cuticle. The nail bed is a layer of epidermis below the nail. The nail matrix gives the nail and it’s rounded proximal edge is called the lunula.
Sebaceous glands are holocrine glands that secrete cells full of fat droplets into a duct system. They are concentrated in the face, scale and forehead.
Apocrine sweat glands are larger and secrete into hair follicles in the anal, axillary and areolar regions.
Exocrine sweat glands have simple coiled tubular ducts. There are two types of secretory units that secrete into the ducts, dark cells that line the luminal surface and secrete glycoproteins and clear cells that have no secretary granules but are involved in transporting salt and fluid.
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