TBL1 - Skin

Question 1

What is the skin?

Answer 1

The skin, the body’s largest organ, consists of the epidermis, a superficial cellular layer, and the dermis, a deep connective tissue layer

Question 2

What is the epidermis?

Answer 2

1) The epidermis is a keratinized epithelium—that is, it has a tough, horny superficial layer that provides a protective outer surface overlying its regenerative and pigmented deep or basal layer
2) The epidermis has no blood vessels or lymphatics. The avascular epidermis is nourished by the underlying vascularized dermis
3) The skin is also supplied with afferent nerve endings that are sensitive to touch, irritation (pain), and temperature. Most nerve terminals are in the dermis, but a few penetrate the epidermis

Question 3

What is the dermis?

Answer 3

1) The dermis is a dense layer of interlacing collagen and elastic fibers. These fibers provide skin tone and account for the strength and toughness of skin
2) Although the bundles of collagen fibers in the dermis run in all directions to produce a tough felt-like tissue, in any specific location most fibers run in the same direction. The predominant pattern of collagen fibers determines the characteristic tension and wrinkle lines in the skin

Question 4

What are functions of the dermis?

Answer 4

1) The deep layer of the dermis contains hair follicles, with associated smooth arrector muscles and sebaceous glands
2) Contraction of the arrector muscles of hairs (L. musculi arrector pili) erects the hairs, causing goose bumps. Hair follicles are generally slanted to one side, and several sebaceous glands lie on the side the hair is directed toward (“points to”) as it emerges from the skin. Thus, contraction of the arrector muscles causes the hairs to stand up straighter, thereby compressing the sebaceous glands and helping them secrete their oily product onto the skin surface
3) The evaporation of the watery secretion (sweat) of the sweat glands from the skin provides a thermoregulatory mechanism for heat loss (cooling). Also involved in the loss or retention of body heat are the small arteries (arterioles) within the dermis. They dilate to fill superficial capillary beds to radiate heat (skin appears red) or constrict to minimize surface heat loss (skin, especially of the lips and fingertips, appears blue)

Question 5

What are other skin derived structures?

Answer 5

Other skin structures or derivatives include the nails (fingernails, toenails) the mammary glands, and the enamel of teeth

Question 6

What is the subcutaneous tissue composed of?

Answer 6

Located between the overlying skin (dermis) and underlying deep fascia, the subcutaneous tissue (superficial fascia) is composed mostly of loose connective tissue and stored fat and contains sweat glands, superficial blood vessels, lymphatic vessels, and cutaneous nerves. The neurovascular structures course in the subcutaneous tissue, distributing only their terminal branches to the skin

Question 7

What is the function of subcutaneous tissue?

Answer 7

1) The subcutaneous tissue provides for most of the body’s fat storage, so its thickness varies greatly, depending on the person’s nutritional state. In addition, the distribution of subcutaneous tissue varies considerably in different sites in the same individual. Compare, for example, the relative abundance of subcutaneous tissue evident by the thickness of the fold of skin that can be pinched at the waist or thighs with the anteromedial part of the leg (the shin, the anterior border of the tibia) or the back of the hand, the latter two being nearly devoid of subcutaneous tissue. Also consider the distribution of subcutaneous tissue and fat between the sexes: In mature females, it tends to accumulate in the breasts and thighs, whereas in males, subcutaneous fat accumulates in the lower abdominal wall
2) Subcutaneous tissue participates in thermoregulation, functioning as insulation, retaining heat in the body’s core. It also provides padding that protects the skin from compression by bony prominences, such as those in the buttocks

Question 8

What are dermal papillae?

Answer 8

In the human skin, the dermal papillae (DP) are small, nipple-like extensions (or interdigitations) of the dermis into the epidermis. At the surface of the skin in hands and feet, they appear as epidermal or papillary ridges (colloquially known as fingerprints)

Question 9

What is the function of dermal papillae?

Answer 9

1) Blood vessels in the dermal papillae nourish all hair follicles and bring nutrients and oxygen to the lower layers of epidermal cells. The pattern of ridges they produce in hands and feet are partly genetically determined features that develop before birth. They remain substantially unaltered (except in size) throughout life, and therefore determine the patterns of fingerprints, making them useful in certain functions of personal identification.
2) The dermal papillae are part of the uppermost layer of the dermis, the papillary dermis, and the ridges they form greatly increase the surface area between the dermis and epidermis. Because the main function of the dermis is to support the epidermis, this greatly increases the exchange of oxygen, nutrients, and waste products between these two layers. Additionally, the increase in surface area prevents the dermal and epidermal layers from separating from each other by strengthening the junction between them. With age, the papillae tend to flatten and sometimes increase in number.
3) Dermal papillae also play a pivotal role in hair formation, growth and cycling

Question 10

Briefly describe the combined structure of the epidermis and dermis

Answer 10

The epidermis is composed of contiguous cells arranged in multiple layers, which are covered by a layer of keratin. Recognize the portion of the dermis adjacent to the epidermis consists of loose connective tissue, characterized by a variety of cells widely dispersed in clear interstitial fluid that also contains pink-stained collagen fibers and many small blood vessels

Question 11

What is the basal layer of skin?

Answer 11

1) The single layer of epithelial cells at the dermo-epidermal junction forms the basal layer of the epidermis
2) Mitosis of the basal cells generates the stratified (multi-layered), keratinized epithelium

Question 12

What are the different layers of keratinized epithelium?

Answer 12

1) The epidermis, a continually renewing epithelium, shows progressive differentiation and keratinization in a basal to superficial direction
2) Epithelial cells superficial to the basal layer are called keratinocytes because their cytoplasm accumulates the protein keratin as the cells move progressively toward the apical surface of the skin
3) The epidermis consists of cells that undergo mitosis, differentiation, maturation, and keratinization as they are displaced outward toward the skin surface to be shed. Four or five distinct layers, or strata, constitute the epidermis

Question 13

How are diseases such as psoriasis related to turnover times of basal cells to keratin-filled cells in the epidermis?

Answer 13

1) The normal time for turnover of keratinocytes from stratum basale to uppermost stratum corneum varies from 20 to 75 days. Turnover and transit times may be even more rapid in some diseases, such as psoriasis, in which transit time is about 8 days
2) Such a thin layer of epidermis allows you to see more directly the dermis, which is red

Question 14

What are desmosomes?

Answer 14

1) A desmosome, also known as macula adherens, is a cell structure specialized for cell-to-cell adhesion
2) A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes
3) Desmosomes help to resist shearing forces and are found in simple and stratified squamous epithelium (keratinocytes are connected by intercellular junctions called desmosomes). The intercellular space is very wide (about 30 nm)
4) Desmosomes are found in a central core region that bridges the gap between cells separates two identical electron-dense plaques

Question 15

What is the basement layer of epithelium?

Answer 15

1) Most epithelia rest on an amorphous extracellular layer—the basement membrane (basal lamina)—at the boundary between epithelium and underlying connective tissue. Where it surrounds other types of cells such as muscle cells, adipocytes, and Schwann cells, it is called an external lamina
2) A basement membrane supports and cushions epithelia, is a semipermeable sieve or selective filtration barrier, and controls epithelial cell differentiation in growth and tissue repair. These membranes vary in thickness

Question 16

What is the function of hemi-desmosomes?

Answer 16

Hemi-desmosomes link the basal surface of the stratified epithelium to the basement membrane and thereby counteract disruptive forces at the dermo-epidermal junction

Question 17

Compare carcinomas and adenocarcinomas (Note: tumor cells must breech the basement membrane to become malignant cells)

Answer 17

1) Epithelial cells have a high mitotic index and are exposed to the surface, which gives pathogens and carcinogens free access to them. The most common types of cancerous (or malignant) tumors (or neoplasms) in adults originate from epithelial cells; these tumors invade or metastasize to distant tissues and organs
2) Neoplasms that grow slowly are benign tumors and include papillomas, which arise from surface epithelium, and adenomas, which originate from glandular epithelium. Malignant neoplasms of surface epithelium are carcinomas, and those originating from glandular epithelium are adenocarcinomas

Question 18

How do desmosomes and hemi-desmosomes counteract mechanical forces that could cause blistering disorders?

Answer 18

1) Desmosomes are complex intercellular junctions that mediate and enhance cell adhesion by anchoring keratin filaments to keratinocyte plasma membranes. Cell-cell adhesion. Hemidesmosomes attach basal aspects of keratinocytes to the underlying basement membrane.
2) Some debilitating blistering disorders of skin result from disrupted epidermal adhesion and attachment

Question 19

Is basal cell carcinoma or squamous cell carcinoma more likely to breech the basement membrane? Which tumor is more prevalent?

Answer 19

1) Skin cancer is the most common malignant disease in North America. Two of three major types are basal cell carcinoma and squamous cell carcinoma (arise from keratinocytes)
2) Basal cell carcinoma accounts for more than 90% of all skin cancers; it grows slowly and seldom spreads to other parts of the body
3) Squamous cell carcinoma is associated with long-term exposure to sun and has a greater likelihood of metastasis

Question 20

What are melanocytes? What is melanin?

Answer 20

1) Melanocytes produce melanin pigment in cytoplasmic organelles called melanosomes, which are deposited in the cytoplasm of keratinocytes. Melanocytes determine color of skin and hair
2) The major determinant of color is not melanocyte number but activity (rate of melanosome production), which is affected by corticotropin from the pituitary
3) Melanosomes rearrange themselves within cells in response to external cues such as UV rays; they usually cluster near cell centers and can rapidly redistribute along microtubules to ends of dendritic processes
4) Darkly pigmented skin, hair, and eyes have melanosomes that contain more melanin

Question 21

How does tanning of the skin occur and why isn’t it a permanent change?

Answer 21

1) Tanning of the skin caused by UV exposure represents an increased eumelanin content of the epidermis. Its major purpose is enhanced protection against damaging effects of UV radiation on DNA
2) Recall that epithelial cells are constantly dividing, growing, and dying. Once tan skin cells die and are shaved off the epithelium, the original color of the skin returns

Question 22

What are langerhans cells?

Answer 22

1) Langerhans cells are dendritic cells (antigen-presenting immune cells) of the skin and mucosa
2) They are present in all layers of the epidermis
3) They also occur in the papillary dermis, particularly around blood vessels, as well as in the mucosa of the mouth, foreskin, and vagina
4) They can be found in other tissues, such as lymph nodes

Question 23

What is the function of langerhans cells?

Answer 23

1) In skin infections, the local Langerhans cells take up and process microbial antigens to become fully functional antigen-presenting cells
2) Generally, dendritic cells in tissue are active in the capture, uptake and processing of antigens. Once dendritic cells arrive in secondary lymphoid tissue, however, they lose these properties while gaining the capacity to interact with naive T-cells

Question 24

Describe the initial formation of hair

Answer 24

1) Hairs begin development as solid epidermal proliferations that penetrates the underlying dermis
2) At their terminal ends, hair buds invaginate. The invaginations, the hair papillae, are rapidly filled with mesoderm in which vessels and nerve endings develop
3) Soon, cells in the center of the hair buds become spindle-shaped and keratinized, forming the hair shaft, while peripheral cells become cuboidal, giving rise to the epithelial hair sheath

Question 25

Describe the elongation and appearance of hair

Answer 25

Continuous proliferation of epithelial cells at the base of the shaft pushes the hair upward, and by the end of the third month, the first hairs appear on the surface in the region of the eyebrow and upper lip

Question 26

Describe the formation of sebaceous glands around hair

Answer 26

The epithelial wall of the hair follicle usually shows a small bud penetrating the surrounding mesoderm. Cells from these buds form the sebaceous glands. Cells from the central region of the gland degenerate, forming a fat-like substance consisting of lipid-rich decomposed glandular cells (sebum) secreted into the hair follicle, and from there, it reaches the skin

Question 27

Describe the formation and use of arrector pili muscles

Answer 27

1) The dermal root sheath is formed by the surrounding mesenchyme. A small smooth muscle, also derived from mesenchyme, is usually attached to the dermal root sheath. The muscle is the arrector pili muscle
2) Arrector pili muscles attach to the follicular wall near the sebaceous glands
3) Involuntary contraction of the smooth muscle arrector pili compresses the glands to expedite sebum secretion into the follicle

Question 28

What is the function of sebum?

Answer 28

1) Sebum waterproofs and lubricates the skin and hair of mammals
2) Sebaceous secretions in conjunction with apocrine glands also play an important thermoregulatory role. In hot conditions, the secretions emulsify the sweat produced by the eccrine glands and this produces a sheet of sweat that is not readily lost in drops of sweat. This is of importance in delaying dehydration. In colder conditions, the nature of sebum is seen to become more lipid and in coating the hair and skin, rain is effectively repelled

Question 29

Differentiate between thin and thick skin

Answer 29

1) On the basis of the structural complexity and thickness of the epidermis, skin is classified into thick or thin. Thick skin, which is glabrous (lacks hair follicles and shafts), is found on palms of the hands and soles of the feet; thin skin covers most of the remaining body surface
2) Whereas the multilayered epidermis of thick skin is 0.8-1.5 mm thick, the epidermis of thin skin is 0.07-0.15 mm thick, with fewer cellular layers

Question 30

What is the difference between sweat and sebum?

Answer 30

1) Sweat is secreted by sweat glands (skin surface) while sebum is secreted by sebaceous (oil) glands
2) Sweat is fluid in nature while sebum is oily in nature
3) Sweat is composed of mainly sodium chloride & water while sebum is composed of oil or wax like secretion
4) The function of sweat is for temperature control & excretion, while the function of sebum is for lubrication

Question 31

How is a sweat gland formed?

Answer 31

1) Long invaginations of the epidermis into the dermis and superficial fascia form the sweat glands
2) In the dermis and superficial fascia, terminal ends of the invaginations form secretory units (acini), which are connected to the apical surface of the skin by tortuous ducts derived from the remaining portions of the invaginations
3) An acinus refers to any cluster of cells that resembles a many-lobed berry, such as a raspberry (acinus is Latin for “berry”). The berry-shaped termination of an exocrine gland, where the secretion is produced, is acinar in form

Question 32

Differentiate between ducts and secretory units of sweat glands under light microscopy

Answer 32

LM eccrine sweat gland in the dermis:

1) Eccrine sweat glands are simple, coiled tubular glands consisting of secretory and narrower excretory duct portions
2) In the transverse and oblique sections of the coiled secretory portion (Se) of the gland, secretory cells have a relatively pale cytoplasm and border a prominent central lumen
3) Several smaller, more darkly stained profiles of the duct (Du) are seen with their characteristic double cuboidal epithelium
4) Surrounding dermis contains abundant capillaries (Cap)

Question 33

Cite locations of the odoriferous sweat glands and explain the musky odor of their secretions

Answer 33

1) Apocrine sweat glands, also known as odoriferous sweat glands, are large, branched glands found in axillae, scrotum, prepuce, labia minora, nipples, and perianal regions
2) Their yellow, viscous, oily secretion has an acrid or musky odor in response to bacterial decomposition

Question 34

Describe the dermo-epidermal junction

Answer 34

1) The dermis (De) is less cellular than the epidermis (Ep)
2) The papillary dermis is loose connective tissue with collagen fibers (Co) interspersed with mononuclear cells
3) Contains many small blood vessels (capillaries)

Question 35

Describe the connective tissue of the dermis

Answer 35

1) Fibroblasts produce the collagen and elastic fibers and macromolecules (e.g., proteoglycans and glycosaminoglycans) normally present in the dermis
2) Multiple types of collagen are created by variations in amino acid compositions, and Type I collagen forms the thick fibers of the dermis
3) Collagen fibers reside in the connective tissue and their cable-like structure provides tensile strength to the dermis
4) Distensible elastic fibers also reside in the dermis thus enabling the skin to passively recoil after stretching or distension
5) Aside from fibroblasts, other connective tissue cells in the dermis include macrophages, mast cells, adipocytes, plasma cells, and lymphocytes (contribute to immune response)

Question 36

Describe collagen fibers in the deep dermis

Answer 36

Type I collagen fibers are more densely packed in the deep dermis, positioned between the superficial dermis and superficial fascia. Thus, the deep dermis is characterized by fewer cells and less interstitial fluid than the loose connective tissue of the superficial dermis

Question 37

What type of collagen do basement membranes contain and why?

Answer 37

Basement membranes (ignore their three layers) contain Type IV collagen organized into fine fibrous meshworks that are essential for selective filtration functions of the basement membrane

Question 38

What is adipose tissue?

Answer 38

1) Adipose tissue is a specialized loose connective tissue that contains large numbers of adipocytes. It functions in insulation and padding and provides a ready source of fuel for metabolic processes
2) Dense microcirculation of white fat insures lipid droplets are a ready source of metabolic fuel
3) Except in emaciated or obese states, adipose tissue normally constitutes 10%-15% of body weight

Question 39

What is the structure of white fat?

Answer 39

White fat of the superficial fascia consists of adipocytes with large fat droplets filling their cytoplasm

Question 40

What is the structure of brown fat?

Answer 40

1) Brown adipose tissue is recognized by a characteristic color, which is due to rich vascularity and to lipochromes in numerous mitochondria within adipocytes
2) Adipocytes of brown fat are filled with many small lipid droplets interspersed by large numbers of mitochondria
3) Understand brown fat constitutes 2% of body weight in newborns but is more limited in adults

Question 41

What is the function of brown fat?

Answer 41

1) In humans, its main function is heat generation, or nonshivering thermogenesis
2) Mitochondria sit between the lipid droplets and play a role in mediating heat production by oxidation of fatty acids

Question 42

What is the function of white fat?

Answer 42

White adipose tissue is used as a store of energy

Question 43

What are types of arteries, veins, and capillaries that contribute to microcirculation?

Answer 43

Arterioles, venules & capillaries.

1) Arterioles: Arterioles have relatively narrow lumina and thick muscular walls
2) Venules: Venules are the smallest veins. Venules drain capillary beds and join similar vessels to form small veins
3) Capillaries: simple endothelial tubes connecting the arterial and venous sides of the circulation that allow the exchange of materials with the interstitial or extracellular fluid (ECF)

Question 44

Where does the microcirculation of skin reside?

Answer 44

1) Smaller arteries, veins, and capillaries constitute the main vasculature in the dermis
2) Networks of these small vessels form deep plexuses in the reticular dermis and superficial plexuses in the papillary dermis, which are connected by communicating vessels

Question 45

What do the superficial and deep plexus of the dermis vacularize?

Answer 45

1) The superficial plexus supplies the superficial dermis and adjacent, avascular epidermis (all epithelia lack a microcirculation)
2) The deep plexus supplies the deep dermis and superficial fascia

Question 46

How do subcutaneous arteries contribute to microcirculation?

Answer 46

1) Subcutaneous arteries serially feed the deep and superficial plexuses
2) Many arteriovenous anastomoses (shunts) in deeper layers of the dermis, especially in the dermis of fingers, lips, and toes, are direct connections between arterioles and venules and lack an intervening capillary network

Question 47

Why are dermal arteriovenous shunts open in response to cold ambient temperatures, and closed in response to hot ambient temperatures?

Answer 47

Blood flow through the dermal plexus allows a greater loss of heat, whereas opening of ateriovenous shunts in the subdermal plexus retains blood deeper in the tissue to reduce heat loss

Question 48

How do bluish, reddish, and yellowish skin colorizations provide clues for diagnosing certain clinical conditions?

Answer 48

1) When the blood is not carrying enough oxygen from the lungs, such as in a person who has stopped breathing or in a person having a defective circulation that is sending an inadequate amount of blood through the lungs, the skin can appear bluish (cyanotic)
2) Skin injury, exposure to excess heat, infection, inflammation, or allergic reactions may cause the superficial capillary beds to become engorged, making the skin look abnormally red, a sign called erythema
3) In certain liver disorders, a yellow pigment called bilirubin builds up in the blood, giving a yellow appearance to the whites of the eyes and skin, a condition called jaundice. Skin color changes are most readily observed in people with light colored skin and may be difficult to discern in people with dark skin

Question 49

How are capillaries formed?

Answer 49

Capillaries are formed by endothelium, a single layer of squamous epithelial cells residing on a basement membrane

Question 50

What are tight capillaries?

Answer 50

1) Tight capillaries, the most common type, are found in all muscle tissues and in areas with a blood-tissue barrier, such as the bloodbrain barrier (central nervous system), blood-air barrier (lungs), and blood-thymus, blood-ocular, and blood-testis barriers
2) Tight capillaries have uninterrupted endothelium. Many tight junctions, desmosomes, and gap junctions link endothelial cells in these capillaries
3) Their reduced permeability restricts indiscriminate passage of material from capillary lumen to surrounding tissues
4) This exchange is essential for diffusion of nutrients from the blood into the interstitial fluid and diffusion of metabolic wastes from the interstitial fluid into the blood

Question 51

Describe mast cell and vascular response to injury/laceration

Answer 51

1) The inflammatory response, the basic reaction of the body to injury, occurs in vascularized connective tissue and involves all of its structural elements, including mast cells
2) Venule (20-30 μm) shows transient vasoconstriction in response to injury
3) Histamine release from mast cells increases permeability of capillaries and venules (temporarily opens the capillary tight junctions) and results in local edema and emigration of leukocytes (plasma proteins and monocytes (white blood cells) leak into the interstitial fluid surrounding the site of injury) and monocytes from circulation. They stimulate local cell proliferation, which leads to production of connective tissue elements involved in repair of damaged tissues

Question 52

What occurs as a response towards histamine release from mast cells at the site of injury?

Answer 52

1) The resultant increase of interstitial fluid volume (aka edema) in the dermis causes localized swelling of the skin
2) In the edematous fluid, monocytes become macrophages that engulf invading pathogens (e.g., bacteria)
3) Degradation of the pathogens occurs via phagocytosis whereby lysosomal enzymes cleave antigens into small peptide fragments, and some of the fragments are ferried to the macrophage cell membrane for selective recognition by the dermal lymphocytes
4) Epidermal Langerhans cells adjacent to the injury site can similarly degrade antigens; thus immune responses typically accompany the inflammatory response
5) After 10-15 minutes, histamine levels are down-regulated and deduce fibroblasts restore the injured dermis
6) Angiogenesis, proliferation of endothelial cells from surrounding capillaries, restores the damaged microcirculation

Question 53

How do basal cells and the basement membrane contribute to remodeling of the epidermis after skin lacerations?

Answer 53

1) After injury by trauma or infection to skin and other soft tissues, the capacity of epithelial cells to undergo mitosis and regenerate is clinically important
2) Complex reparative events known as wound healing include an inflammatory phase followed by proliferative and remodeling stages
3) Epithelial cells from nearby areas replicate, change shape, and migrate across the defect to cover the wound
4) Cells secrete various growth factors and activators that enhance repair
5) Angiogenesis, whereby new capillaries grow from endothelial cells, also occurs
6) The basement membrane is critical for rapid recovery; if it is destroyed, healing is relatively slow

Question 54

What is the largest sensory organ in the body?

Answer 54

Skin is the largest sensory organ of the body; thus, it contains several sensory nerve terminals that perceive different sensations

Question 55

What is a Meissner’s corpuscle? (How to determine on a VM which papillae have meissners corpuscle)

Answer 55

1) Meissner’s corpuscles (or tactile corpuscles) are a type of mechanoreceptor. They are a type of nerve ending in the superficial dermis of the skin that is responsible for sensitivity to light touch
2) Any physical deformation in the corpuscle will cause an action potential in the nerve. Since they are rapidly adapting or phasic, the action potentials generated quickly decrease and eventually cease (this is the reason one stops “feeling” one’s clothes).
3) If the stimulus is removed, the corpuscle regains its shape and while doing so (i.e.: while physically reforming) causes another volley of action potentials to be generated

Question 56

What is a Pacinian corpuscle?

Answer 56

1) Lamellar corpuscles, Pacinian corpuscles, are one of the four major types of mechanoreceptor. They are nerve endings in the skin responsible for sensitivity to vibration and pressure. The vibrational role may be used to detect surface texture, e.g., rough vs. smooth
2) Lamellar corpuscles are rapidly adapting (phasic) receptors that detect gross pressure changes and vibrations in the skin

Question 57

Where are Meissner’s and Pacinian corpuscles typically located? (located anywhere else?)

Answer 57

Meissner’s and Pacinian corpuscles are typically present in thick skin of the finger-tips, palms of the hand, and soles of the feet

Question 58

How do eccrine sweat glands help maintain body temperature?

Answer 58

Eccrine sweat glands are simple, coiled tubular glands consisting of secretory and narrower excretory duct portions. With cholinergic innervation, they mainly serve a thermoregulatory role and maintain body temperature by evaporative heat loss