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True or false: Skin is the largest organ in the body composing 15-20% of body mass. As an organ, it is made up of a combination of tissues (epithelial, nerve, muscle, and CT).



What are the fxns of skin?

• Barrier (to physical, chemical and biologic agents)
• Thermoregulation (homeostasis)
• Conveys somatosensory information from environment to the nervous system
• Immunologic: antigen presentation/processing to effector cells 
• Endocrine: secretes hormones, cytokines, and growth factors; converts precursors into Vitamin D
• Excretion: secretion from sweat and sebaceous glands


What are the 3 layers of skin? What is the composition of each layer? What emybryonic tissue layer are they derived from?

epidermis, dermis, and hypodermis. epidermis and dermis are the 2 main layers

epidermis: derived from ectoderm. keratinized stratified squamous epithelium. continuous growth and turnover. is avascular. contains keratinoctyes, melanocytes, and Langerhan's cells

dermis: derived from mesoderm. dense irregular CT. imparts mechanical support

hypodermis: layer below dermis with variable amounts of adipose tissue separated by CT. insulates and serves as an energy source. 


What are somethings found associated with the epidermis (epidermal derivatives, skin appendages)?

hair and hair follicles

sebaceous glands

eccrine and apocrine glands (sweat glands)

mammary glands



note that eccrine glands go all the way to surface of skin while apocrine glands do not. apocrine glands dump their contents into hair follicles


What are the 2 types of skin? How are they classified/what are their differences? Where are the 2 types found in the body?

Thick and thin skin. Thick skin is classified by having a thicker epidermis (thick stratum corneum layer of epidermis). Also, thick skin does not have hair follicles. Thick skin is found on palmar and plantar surfaces while thin skin is everywhere else. Thick skin is adapted for a great amount of abrasion bc is subjected to mechanical stress. 


What are the layers of the epidermis? (just list)

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Stratum Corneum

Stratum Lucidum (will not have to ID histologically)

Stratum Granulosum

Stratum Spinosum

Stratum Basale


What is the stratum basale? What is its fxn/contents? What are the fxns of its cell types?

the stratum basale is also known as the germinativum. It is a single layer of mitotically active stem cells resting on a basal lamina that give rise to the main cell type in the epidermis-keratinocytes. The major fxn of keratinoctyes is to form the physical and chemical barrier of skin. They are specialzied for the production of cytokeratin proteins that organize as intermediate filaments (tonofilaments). The stem cells maintain extensive junctional attachments to each other via desmosomes and to underlying CT via hemidesmosomes. Melanocytes are also located in the stratum basale and synthesize melanin pigment from tyrosine. Melnocytes are neural crest cells that migrate to developing epidermis in the embryo. They are clear appearing in histo sections bc they immediately secree melanin for keratinocytes to absorb.


What is the stratum spinosum? What is its fxn/contents? 

multiple cell layers thick; spinous processes of keratinocytes attached to each other by multiple desmosomes


What is the stratum granulosum? What is its fxn/contents? 

Thin layer (1-3 cells thick) characterized by the appearance of intense basophilic granular staining in keratinocytes due to keratohyalin granules containing the precursor to filaggrin, which later bundles keratin filaments in the stratum corneum


What is the stratum corneum? What is its fxn/contents? 

most superficial layer of skin. Thickness varies depending on anatomic location and amount of skin abrasion. Are anucleate cells that lose organelles as they migrate toward the surface. Only contain keratin


Where is the water barrier located in skin and how is it formed?

The water barrier is located at the base of the stratum corneum and is formed by complex arrangement of proteins and lipids secreted by lamellar bodies of the stratum granulosum


What is the life cycle of a keratinocyte? Be sure to discuss difference in pH btwn layers.


Stage 1. basal cells synthesize intermediate filaments (keratin) that bundle together as tonofilaments

Stage 2. keratinocytes in the stratum spinosum layer begin to make lamellar bodies (membrane bound probariier lipds and enzyme containing molecules)

Stage 3. keratinocytes in the granulosum cell layer produce keratohyalin granules and more lamellar bodies which are secreted by exocytosis in the ECM btwn the stratum granulosum and corneum (forming water barrier of epidermis)

Stage 4. stratum corneum cells are keratinized: granule contents released and keratin aggregates into tonofibrils. 

Stage 5: pH decreases from deep to superficial layer (pH activates proteases). Controls exfoliation of cells. Proteases cause molecular cascade leading to digestion of desmosomal proteins & detachment of most superficial layer of stratum corneum. 


What is the function of melanocytes? Where are they located? What product do they make and what is the fxn of their product? Where are they derived from embryologically?

Melanocytes are derived from neural crest cells that migrate to epidermis. Melanocytes are located in the stratum basale and secrete melanin that is immediately taken up by keratinoctyes via phagocytosis. Melanin abosrbs UV light to protect keratinocyte DNA. Aging is associated with a decrease in melanocytes-increased skin cancer risk


What is skin pigmentation determined by?

Skin pigmentation is primarly dependent on the quanity of melanin in keratinocytes and is regulated by how long melanin granules persis in the keratinocyte before degradation. Lighter skinned people are thought to have melanosomes that are degrated faster than darker skinned people. 


What are Langerhan's cells? Where are they located? What is their fxn? From where are they derived?

Langerhan's cells are derived from bone marrow and are a part of the mononuclear phagocytic system. They are antigen presenting cells in the epidermis. Upon uptake and processing of an antigen, the migrate and present antigen to T-cell in dermis or migrate to local lymph nodes to present antigens


Fingerlike extensions of the dermis, called ___ ___ project into the epidermis and similar ____ ____ project into the dermis. What are the differences btwn these 2 extensions?

Dermal papillae project into the epidermis and epidermal ridges (rete ridges) project into the dermis. Dermal papillare are longer and are more numerous on the hands and feet to accommodate mechanical stress/increased abrasion and form the patterned hand and foot prints. 


What are the 2 layers of the dermis and how do they differ?

superifical papillary layer at the level of dermal papillae. connective fibers smaller and tissue is less dense, contains threadlike elastic fibers

deeper reticular layer with large irregularly oriented collagen fibers and coarser elastic fibers


What are the components of the dermis? 

cellular components: fibroblasts that secrete ECM proteins. lymphocytes, mast cells, macrophates, granuloctyes, migrating Langerhan's cells

extracellular components: collagen fibers (predominantly type I), elastic fibers

other: blood vessels, smooth muscle, nerves, sensory receptors (nerves and sensory receptors are also located in hypodermis)


What are arrector pili muscles? Where are they located? What is their fxn and when do they come into play?

smooth muscle located in dermis in that elevates hair follicles. attached to base of hair follicle. part of thermoregulatory fxn of skin: elevate hairs when cold. also, when scared-innervated by sympathetic nervous system. when hair elevated, increases sensory awareness


The dermis and hypodermis have nerve and sensory receptors. What are free nerve endings and what type of information do they convey?

free nerve endings have no connective tissue structure surrounding nerve fibers. Afferent nerves carry pain, temperature, and also fxn as mechanorecptors via hair follicles. They are largely unmyelinated


What are Meissner's corpuscles? What are Pacinian corpuscles? Where are they located (in skin and in the body) and what type of info do they convey?

Meissner's corpuscles are type of touch receptor located in dermal papillae and convey fine discriminatory touch (2 point discrimination). Respond to low frequency stimuli. Most numerous in thick skin and lips.

Pacininan corpuscles: Located deep in dermis (as well as joints and internal organs). Convey vibration and deep pressure. Have multilayers CT capsule around a nerve fiber


What are the 2 types of sweat glands?

eccrine and apocrine


What is the classification of eccrine sweat glands? What are histological characteristics? Where are they located in the body? How are they innervated? How is their product released?

Eccrine sweat glands are simple coiled tubular glands distributed widely in the body. Conta a ligher stianed secretory segment and a darker stained duct segment. Note that you can tell it is a duct also by the fact that ducts have stratified cuboidal epithelia. Reasborption of ions along the gland towards the surface forms hypotonic sweat. 

Eccrine sweat glands are regulated by autonomic cholinergic innervation for temperature regulated sweat and by adrenergic innervation for emotional related secretion. 

Contain myoepithelial cells (specialized contractile epithelial cell) that surround the base of the secretory segment which when contracted, causes rapid excretion of sweat from the gland. 


What is the classification of apocrine sweat glands? What are histological characteristics? Where are they located in the body? What is their product and how/where is it released? When in development do they become active? Discuss MHC.

Apocrine sweat glands are simpled coiled tubular and branched tubular glands with a duct reaching the skin through a hair follicle. They are located it eh axilla, inginal, and perianal areas. Modified version in outer ear produces ear wax. Have a large diameter lumen that stores secretory product, a mix of protein, lipid, and carbs that is released via myoepithelial cells. Become active during puberty. Composition and scent of secretions (causing body odor and pheromone release) related to major histocompatibility complex (MHC) and mate seleciton. Male pheromones directly affect female menstrual cycle and female pheromones influence male hormone levels and their perception of females.


What is the classification of sebaceous sweat glands? What are histological characteristics? Where are they located in the body? What is their product and how/where is it released? When in development are the activated?

Sebaceous glands are outgrowths of the epithelium that give rise to hair and their connection to hair is maintained. They are simple branched acinus exocrine glands. Acini accumulate oily lipid called sebum and are secreted via holocrine secretion-cell bursts releasing contents and also causes its apoptosis. Activated during puberty via sex hormones.


Discuss temperature regulation in skin. 

Temperature regulation in skin involves both eccrine sweat glands and blood vessels in the dermis. Increased body and skin temp causes arterial vasodilation in the dermis which increases blood flow and heat dissipation as well as increased sweat gland activity which cools the body via evaporation of water. Decreased body and skin temp causes arterial vasoconstriction and heat conservation. Arterial-venous connections that bypass capillary beds (called arteriovenous anastomoses) and precapillary spincters combined also alter blood flow through the skin. 


Generally, what are epidermal cancers caused by? What are 3 types of epidermal cancers and what are their characteristic/prognosis/treatment?

long term exposure to UV radiation

basal cell carcinoma: most common. microscopically resembles cells in stratum basale. slow growing tumor that usually does not metastasize and recommended tx is surgical removal

squamous cell carcinoma: highly atypical cells in all layers of epidermis. usually have small painless patch or nodule surrounded by inflammation. variable differentation patterns. can metastasize to lymph nodes if basement membrane is disrupted. tx depends on size, location, histologi (includes surgical excision, cryotherapy, electrodissection, chemo, radiation)

malingant melanoma: most serious. originate form melanocytes. contain large nuclei with irregular contours. grows in all directions and can extend to dermis. have irregular borders. tx is surgery if localized along with chemo if advanced

see pg 403 of first aid


What is primary union of skin repair? secondary union?

Primary union: following surgical incisions/suture application reduces extent of repair area needed, minimizing scar formation
Secondary Union: occurs in traumatic wounds with separated edges, more extensive cell/tissue 
loss. Wound healing involves production of granulation tissue (specialized tissue formed during repair). Requires stimulated growth of dermis and epidermis


What does repair of the dermis and epidermis involve in secondary union?

Dermal repair involves:

-Clot formation

-Removal of damaged collagen fibers via macrophages
-Granulation tissue formation
-Re-epithelialization of exposed surface
-Proliferation of fibroblasts and migration of myofibroblasts involved in wound contraction

Epidermal repair
-Proliferation of basal keratinocytes in stratum basale in undamaged tissue surrounding wound
-Wound site covered by a scab which is a dehydrated blood clot with dead neutrophils

-Proliferating basal cells migrate underneath the scab and across the wound surface
-Eventual restoration of multilayered epidermis
-Overlying scab freed with desquamating cells