Histology of the Integumentary System PDF

Histology of the Integumentary System Describe the composition and function of the integumentary system. Overview (composition and function):  The skin and its derivatives (hairs, sebaceous glands, sweat glands, and nails) are known as the integumentary...

Histology of the Integumentary System Describe the composition and function of the integumentary system. Overview (composition and function):  The skin and its derivatives (hairs, sebaceous glands, sweat glands, and nails) are known as the integumentary system.  Skin is composed of an epidermis and a dermis which lie on the hypodermis (superficial fascia).  Skin covers the entire external surface of the body and is classified as either thick (hairless) or thin (hairy).  Thick skin is found on the palmar surface of the hands and plantar surface of the feet, thin skin covers the rest of the body.  Skin is a protective barrier against UV light, mechanical insults, chemical insults, thermal insults, dehydration, and invasion by microorganisms.  Skin is the largest sensory organ in the body and contains receptors for pain, temperature, discriminative touch, crude touch, and vibration.  Control of body temperature through the regulation of heat loss is facilitated by the evaporation of sweat and vasodilation of vessels in the dermis.  Synthesis of vitamin D takes place in the skin.  Histology of the Integumentary System Describe the Epidermis. The Epidermis is a self‐renewing keratinized, stratified squamous epithelium with the following layers: Stratum basale (germinativum) Stratum spinosum Stratum granulosum Stratum lucidum Stratum corneum The majority of cells in the epidermis are keratinocytes, but there are some non‐keratinocytes: Melanocytes (produce melanin) Merkel cells (mechanoreceptors) Langerhans cells (Antigen Presenting Cells). The epidermis does not contain any blood vessels so nutrients and wastes pass through by diffusion.  Histology of the Integumentary System Describe the structure of the epidermis: stratum basale, stratum spinosum. Kierszenbaum A. L. Histology and Cell Biology © Elsevier Stratum basale (the deepest layer): Single layer of cuboidal cells which rests on a basement membrane on the dermis and is anchored to it by hemidesmosomes. This is the mitotic layer where keratinocytes start the cycle of proliferation and maturation which ultimately results in their death. Melanocytes and Merkel cells are present in this layer. Stratum spinosum: Several layers of interdigitated cells layers (also called prickle cells) connected to each other by desmosomes. Cells have many cytoplasmic processes or spines which become apparent when skin is processed for light microscopy. These connections provide tensile strength to this layer. Cells have prominent nucleoli and are basophilic due to the synthesis of keratin proteins (filaments). Upper cell layers have more filaments and are beginning to synthesize keratohyalin granules (aggregations of filaments). Langerhans cells and the occasional lymphocyte are the only non‐keratinocytes present.  Histology of the Integumentary System Describe the structure of the epidermis: stratum granulosum, stratum lucidum, stratum corneum. Stratum granulosum: 2 to 5 cells thick and is the most superficial layer of nonkeratinized cells. Cells characterized by numerous basophilic keratohyalin granules. Keratohyalin granules overfill cells destroying contents and rupturing cells. These cells release hydrophobic glycophospholipids which render the epidermis relatively waterproof. Stratum lucidum: Few layers of dead and dying cells that appears translucent due to acidophilic staining. Only present in thick skin. Stratum corneum: Stratum corneum is shed continuously and new cells are formed in the stratum basale and pushed upwards (turnover time is about 25‐50 days). Flattened dead cells without nuclei that are filled with soft keratin (squames), as opposed to hard keratin in hair and nails. Desquamation occurs in the outer layers as the desmosomes break down.  Histology of the Integumentary System Describe the structure and function of melanocytes. Gartner, L.P., Color Textbook of Histology © Elsevier Melanocytes:  Melanin pigment forming cells located in the stratum basale that have long cytoplasmic processes to contact cells in the stratum basale and stratum spinosum.  In H&E stains melanocytes have a clear area around the nucleus.  Melanin is transferred to other cells in melanosomes via the cytoplasmic processes. Melanocytes are attached to the basement membrane by hemidesmosomes but are not attached to surrounding basal cells by desmosomes.  Skin color is determined by the size and number of melanosomes.  Skin exposure to UVB light stimulates chemical changes in melanin.  Melanin granules protect cell nucleus from UV radiation.  Histology of the Integumentary System Describe the location and function of Langerhans cells and Merkel cells. Langerhans cells (left): Spider‐like cells scattered within the stratum spinosum. Lack desmosomes (i.e. they are able to move). Are an APC cell because they uptake, process, and present antigens to T‐lymphocytes. Merkel cells (single, right): Located in the stratum basale and connected to keratinocytes by desmosomes. Most numerous in the fingertips. Are mechanoreceptor cells associated with the terminal unmyelinated portion of a myelinated nerve fiber. Respond to directional deformation of the epidermis and the direction of hair movement (discriminative touch). Histology of the Integumentary System Bullous Pemphigoid – An Autoimmune Blistering Disease. Which layer is this?  Histology of the Integumentary System Describe the structure of the dermis: papillary layer, reticular layer. Dermis: A dense irregular connective tissue matrix of collagen and elastic fibers which consists of papillary and reticular layers Provides a tough flexible base for the epidermis. Contains the vascular supply for the epidermis and dermis. Contains a variety of sensory nerve endings, macrophages and fibroblasts. Papillary layer (epidermal‐dermal interface): The superficial layer that lies just beneath the basement membrane of the epidermis. Contains dermal papillae (aka rete ridges) which interdigitate with epidermal pegs (rete pegs) Dermal papillae are the basis for finger prints. Reticular layer: The deep layer, merges with the deep aspect of the papillary layer.  Histology of the Integumentary System Describe the structure of the dermis: papillary layer, reticular layer. Reticular layer (continued): The predominant orientation of collagen fibers determines the lines of skin tension (cleavage or Langer’s lines) which are important for surgical incisions. Histology of the Integumentary System Why are Tattoos Permanent? Tattoo needle Into which layer is the tattoo needle depositing the ink? What layers must the needle pass through?  Histology of the Integumentary System Why are Tattoos Permanent? What happens to the ink once it is deposited so that it does not seep out?  Histology of the Integumentary System Describe the structure of the hypodermis (superficial fascia or subcutis). Hypodermis:  Layer of adipose tissue and loose connective tissue between dermis and deep fascia covering muscle.  Varies in thickness depending on the region of the body.  Insulates, allows for movement of the skin, contains the lower portions of sweat glands and hair follicles, and adipose cells which store triglycerides.  Dense in the scalp, palms and soles where it binds skin to underlying structures, part of the deep fascia and known as aponeuroses.  Histology of the Integumentary System Describe the structure of thin skin. Thin skin (also known as non‐acral skin) is actually a misnomer because it only refers to the thickness of the epidermis. The skin on the upper back region is actually the thickest on the body but it is a classified as thin skin with a very thick dermis. Epidermis: Much thinner, but all layers reduced, stratum lucidum is absent. Still a keratinized stratum corneum. Dermis: Structure of the dermis is the same in thin skin as it is in thick skin, but in some areas it is thicker than in thick skin. Most thin skin contains hair follicles (F) with arrector pili muscles (A), sebaceous glands (S), and scattered sweat glands (G). There are fewer and shallower dermal papillae. S = sebaceous gland, F = hair follicle, A = arrector pilli muscle, G = sweat gland  Histology of the Integumentary System Describe the innervation of skin. FUNDAMENTAL NEUROSCIENCE FOR BASIC AND CLINICAL APPLICATIONS. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. Innervation of Skin: Cutaneous branches of spinal nerves give off fine branches (twigs) that enter a dermal nerve plexus located at the base of the dermis. Within the plexus, individual nerve fibers divide further and overlap extensively with others before terminating at more superficial levels of the skin. Somatosensory nerve fibers (GSA): o These nerve fibers carry general body sensations (pain, temperature, discriminative touch, nondiscriminative touch or crude touch, pressure, vibration, and conscious proprioception) from the skin to the spinal cord and then to the brain. o If the terminal portion of the nerve fiber is surrounded by some type of connective capsule it is called an encapsulated nerve ending, and if there is no connective tissue capsule, it is called a free nerve ending (nonencapsulated). Autonomic nerve fibers (GVE): o These nerve fibers carry motor information to smooth muscle structures in the skin. The autonomic fibers to skin are ONLY postganglionic sympathetic fibers which innervate blood vessels, arrector pili muscles, and sweat glands. Nonencapsulated nerve endings:  Merkel cells: Are mechanoreceptor cells and associated with the terminal unmyelinated portion of a myelinated nerve fiber. Respond to directional deformation of the epidermis and the direction of hair movement (discriminative touch).  Free nerve endings: Extend into the lower layers of the epidermis. For pain, temperature, itching, and non‐discriminative touch.  Hair follicle receptors (peritrichial nerve endings): Associated with the lower portion of a hair follicle for discriminative touch and vibration (movement of hair).  Histology of the Integumentary System Describe the innervation of skin. FUNDAMENTAL NEUROSCIENCE FOR BASIC AND CLINICAL APPLICATIONS. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. Encapsulated nerve endings (mechanoreceptors):  Meissner’s corpuscles: Located in the dermal papillae for discriminative touch (e.g. reading Braille text).  Pacinian corpuscles: Located in the hypodermis (approx. 300 in the hand) near sides of fingers and palms, especially sensitive to vibration.  Ruffini’s endings: Located deep in the dermis and respond to shearing stress.  Histology of the Integumentary System Describe the innervation of skin. Meissner’s corpuscle Pacinian corpuscles  Histology of the Integumentary System Describe the vascularization of skin. Kierszenbaum A. L. Histology and Cell Biology © Elsevier Vascular supply: Amounts to approximately 5% of cardiac output. Increased blood flow (as much as 20 times) facilitates heat loss, decreased blood flow prevents heat loss. Arteries in the subcutaneous plexus in the hypodermis give rise to two plexuses: Subpapillary plexus (upper horizontal): Vascularizes the upper portions of the dermis and the epidermis (also any skin appendages in these areas). Gives rise to a capillary loop in each dermal papillae. Cutaneous plexus (lower or deep horizontal): Vascularizes the hypodermis and deeper portions of the dermis (also any skin appendages in these areas). Venous drainage parallels the arterial supply.  Histology of the Integumentary System Describe the vascularization of skin. Lymphatic vessels are blind endothelial cell–lined spaces located below the papillary layer of the dermis, collecting interstitial fluid for return to the blood circulation. They also transport Langerhans cells, lymphocytes and macrophages to regional lymph nodes.  Histology of the Integumentary System Describe the structure of the skin appendages: hair. Hairs: Keratinized structures produced by hair follicles. The portion of the hair shaft above the epidermis is often just called hair. Hairs consist of a central medulla surrounded by a cortex (hard keratin) and a cuticle (see next slide). *All skin appendages are derived from the epidermis which is derived from ectoderm. Hair follicle (= shaft, bulb, and connective tissue sheaths):  Tubular invagination of epidermis which produces the hair.  Lowest portion of the hair follicle is the hair bulb (or hair root) which is filled with connective tissue called the dermal hair papilla.  Growth occurs in the matrix of the hair bulb (found in the lower part of the hair bulb).  The portion of the hair follicle surrounding the hair shaft is divided into an inner root sheath and an outer root sheath derived from the epidermis, and then a connective tissue sheath derived from the dermis.  Histology of the Integumentary System Male Pattern Baldness. Hair types: Lanugo hairs: First hair of the fetus, thin and unpigmented. Shed before birth and replaced with vellus hair. Vellus hair: Soft, short, colorless, and very fine hairs that are replaced by terminal hairs except on areas such as the forehead. Terminal hairs: Hard, long, large, coarse, and dark hairs found throughout the body (scalp, eyebrows, arm pits, perineum). Hair Growth, 3 phases: Anagen – growth phase, lasts 2‐7 years and 90% of hair are in this phase Catagen – regression, interruption of blood supply, follicle shrinks to a fraction of original size, lasts 2‐3 weeks Telogen – resting phase, shedding, this phase lasts for 10‐12 weeks and affects 5‐15% of follicles *Follicles genetically sensitive to DHT are found in common patterns – temples and crown of head, hence male pattern baldness  Histology of the Integumentary System Describe the structure of the skin appendages: arrector pili muscles, sebaceous glands. SG AP Arrector pili muscles: Composed of smooth muscle. Attach to connective tissue sheath at an angle. Contraction causes hair to stand up and goose bumps in the skin. Innervated by postganglionic sympathetic fibers (activated by cold or fear). Sebaceous glands: Usually found in association with hair follicles but are also found independently (lips, eyelids, glans of penis, labia majora, nipples). Develop as an outgrowth of the external root sheath. Secrete sebum onto the hair follicle (picture on right) for waterproofing and moisturizing the hair and the surface of the skin. Contraction of arrector pili muscles aids in the expulsion of sebum.  Histology of the Integumentary System Describe the structure of the skin appendages: sweat glands. Eccrine sweat glands: Major type of sweat gland found throughout the body except in the axilla and perineum. Simple coiled tubular glands that secrete a watery fluid onto the skin surface. Coiled secretory portion is located deep in the dermis or in the hypodermis (diagram on left), dermal duct (straight duct) located in the dermis. Spiral duct located in the epidermis, opens up on a sweat pore. Merocrine secretion process (only product released). Innervated by postganglionic sympathetic fibers. Apocrine sweat glands: Apocrine (decapitation) secretion, milky and odorless, but bacterial breakdown produces a distinctive odor. Larger than eccrine sweat glands and found in the axillary, areola, and perineal regions. Become functional during puberty under hormonal influence. Innervated by postganglionic sympathetic fibers. *Body heat loss requires vasodilation in the skin and sweating. Sweating is also stimulated by emotional stimuli.  Histology of the Integumentary System Describe the structure of the skin appendages: nails. The nail unit (or apparatus) consists of the nail plate, nail bed, nail matrix, nail folds, nail grooves, eponychium, and hyponychium. Nail plate: consists of a free edge, body, lunula, and root.  Translucent avascular plates of compacted dead keratinized (cornified) cells containing hard keratin which has a high sulfur content.  Forms a protective covering over the distal phalanges of the fingers and toes.  The nail root is the most proximal portion of the nail plate. It is surrounded by the nail matrix and covered by the eponychium.  The free edge is the most distal portion of the nail plate.  The lunula is a half moon area which represents the distal extent of the nail matrix.  The nail plate corresponds to the stratum corneum and stratum granulosum of the surrounding epidermis of the thin skin and is held tightly to the underlying nail bed by desmosomes.  Histology of the Integumentary System Describe the structure of the skin appendages: nails. Gartner, L.P., Color Textbook of Histology © Elsevier Nail bed: Continuous with the stratum basale and stratum spinosum, highly vascularized, has grooves for attachment to the nail plate, and contributes about 20% to the formation of the nail plate. Nail matrix: Located at the proximal aspect of the nail bed and surrounds the nail root. Continuous with the stratum basale and stratum spinosum and forms about 80% of the nail plate. Nail folds: Folds of skin on the proximal and lateral borders of the nail plate. Nail grooves: Furrows between the nail folds and the nail bed. Eponychium or cuticle: The edge of the skin fold that covers the nail root. Continuous with the surrounding stratum corneum. Composed of hard keratin and does not desquamate. Protects the matrix from irritants, bacteria, etc. Hyponychium: Thickened epidermal layer beneath the free edge of the nail plate. Continuous with the surrounding stratum corneum.  Histology of the Integumentary System Clinical correlations of the integument. Acne: Commonly involves the sebaceous glands and hair follicles. Impacted sebum and keratinous debris in hair follicles reduces their patency (obstruction). Anaerobic bacteria near obstruction may be a contributing factor. Aging: The epidermis decreases in thickness and the number of layers reduces, dermal papillae become shallower, and decreased vascularity. Albinism: An inherited recessive absence or defect in tyrosinase. Baldness: The conversion of terminal hair follicles into vellus hair follicles. Callus: A hyperplasia of the stratum corneum due to pressure or friction. Squamous cell carcinoma: Arises in the keratinocytes of the epidermis, is locally invasive and may metastasize, lesions are usually on the head and neck regions. Characterized by a hyperkeratotic scaly plaque or nodule that often bleeds or ulcerates. Verruca (plantar wart): Benign epidermal growths caused by papilloma viruses.  Histology of the Integumentary System Clinical correlations of the integument. Normal nail growth and structure of nails is important in the clinical assessment of nails. Finger nails grow about.1 mm per day and toenails grow about.03 mm per day. – Onychocryptosis (ingrown nail) ‐ is when one edge of a nail is overgrown by a lateral fold. – Onychomycosis ‐ is a fungal infection (thickening, rough, splitting) of a nail. Skin repair from an incision or laceration requires: – Dermal Repair: Damaged collagen fibers removed by macrophages. New collagen fibers and ground substance produced by fibroblasts. Approximating the edges of the wound or incision with sutures, steri‐strips, or tissue glue reduces the amount of repair and thus reduces scaring. – Epidermal Repair: Keratinocytes in the stratum basale proliferate and cover the wound site with a scab. Proliferating cells migrate under the scab and as they desquamate they free the overlying scab. Skin grafts: – A full thickness skin graft is the epidermis and dermis which leaves the deepest portions of the hair follicles and glands. – A split thickness skin graft is the epidermis and some of the dermis.  Histology of the Integumentary System Summary Skin consists mainly of a superficial stratified squamous epithelium, the epidermis, and a thicker layer of connective tissue, the dermis, which overlies a subcutaneous hypodermis. Epidermis The epidermis consists of keratinocytes that undergo a terminal differentiation process called keratinization in a series of steps that form distinct epidermal strata or layers. The stratum basale is one layer of mitotically active cuboidal cells attached by hemidesmosomes to the basement membrane and to each other by desmosomes. The stratum spinosum has several layers of polyhedral cells attached to each other by desmosomes at the tips of short projections containing bundled keratin, or tonofibrils. The stratum granulosum is a thinner layer of keratinocytes, now flattened and filled densely with keratohyaline granules containing filaggrin and other proteins binding the tonofibrils. The superficial stratum corneum protects against water loss, friction, and microbial invasion, and consists of flattened, terminally differentiated cells, or squames, which are slowly lost.  Histology of the Integumentary System Summary The epidermis-dermis interface is enlarged and strengthened by interdigitating epidermal ridges or pegs and dermal papillae in which microvasculature also supplies nutrients and O2 for the epidermis. Melanocytes in the basal epidermis synthesize dark melanin pigment in melanosomes and transport these to adjacent keratinocytes, which accumulate them to protect nuclear DNA from UV damage. Antigen-presenting cells (APCs) called Langerhans cells form a network through the epidermis, intercepting and sampling microbial invaders before moving to lymph nodes in an adaptive immune response. Dermis The dermis has two major layers: a superficial papillary layer or loose connective tissue with a microvascular plexus, and a thicker dense irregular reticular layer containing larger blood vessels. Cutaneous Sensory Receptors Sensory receptors in the epidermis include free nerve endings, which detect pain and temperature extremes, and basal Merkel cells, light-touch (tactile) receptors associated with sensory fibers. Other cutaneous sensory structures include Meissner corpuscles, encapsulated elliptical mechanoreceptors that surround sensory axons and also detect light touch. Deeper in the dermis and subcutaneous layer are Pacinian corpuscles, which are ovoid and much larger than Meissner corpuscles, for detection of pressure or firm touch.  Histology of the Integumentary System Summary Epidermal Appendages Hairs form in hair follicles, in which keratinocytes comprising the matrix of the deep hair bulb proliferate rapidly and undergo keratinization to form the medulla, cortex, and cuticle of a hair root. A large dermal hair papilla penetrates the base of the hair bulb, and its vasculature supplies nutrients and O2 for proliferating and differentiating cells. The growing hair root is surrounded by internal and external root sheaths continuous with the epidermis, a glassy membrane formed in part by the basal lamina, and a connective tissue sheath. Nails are formed in a manner similar to hairs: keratinocytes proliferate in the matrix of the nail root and differentiate with the formation of hard keratin as a growing nail plate with edges covered by skin folds. Sebaceous glands produce sebum, secreting this oily substance onto hair in the follicles or pilosebaceous units. Eccrine sweat glands in the dermis produce sweat that is mostly water onto the skin surface, where its evaporation provides an important mechanism for cooling the body. Apocrine sweat glands are restricted to skin of the axillae and perineum, develop after puberty, and secrete protein-rich sweat onto the hair of hair follicles.