Histoanatomy 3: Skin & Breast

Question 1

Skin

Answer 1

o Self-renewing organ covering all external surfaces (1-4mm thick)
o Continuous with mucous membranes at body openings
♣ Mucous membranes line ALL INTERNAL body surfaces that open to exterior (digestive, resp, urogenital tracts)

Question 2

LO: What are the functions of the skin?

Answer 2

• Barrier: physically protects from abrasion/ pathogens, permeability barrier limiting body fluid loss via evaporation, UV radiation barrier protecting cells from damage
• Immunological: contains APCs (detect antigen/ notify immune)
• Homeostatic: regulates body temp by controlling heat loss at surface
• Sensory: receptors detect pain, temp, touch, pressure, vibration
• Endocrine: vitD activation
• Excretory: sweat/ sebaceous/ apocrine gland secretions (limited) metabolite/ drug elimination
• Drug delivery: lipid-soluble substances (ie. nicotine, steroids, anti-nauseants) via patch

Question 3

LO: What are the tissue components of the skin?

Answer 3

• Skin consists of 2 layers:
  o Epidermis: stratified squamous keratinized epithelium
  ♣ Variety of cell types, laminated structure, thick/ thin skin
  o Dermis: underlying CT – consists of 2 layers: papillary/ reticular
  o (the subcutaneous layer aka hypodermis/ superficial fascia = separate from skin)
• Skin appendages/ accessory structures: hair, nails, multicellular exocrine glands (mammary)

Question 4

LO: What are the differences between thin/ thick skin and its distribution

Answer 4

• Thin/ thick refers to epidermis thickness only (NOT overall skin thickness)
• Thick skin: hairless, found on palms of hands/ soles of feet (most traction), contains merocrine sweat glands (ie. think sweaty palms!)
• Thin skin: usually with hair, everywhere else

Question 5

LO: What are the characteristics of epidermal layers?

Answer 5

• Epidermis layers: stratum corneum stratum granulosum stratum spinosum stratum basale ( BM dermis)

Question 6

S. basale (germinativum) keratinocytes

Answer 6

o Rapid basal cell cycling daughter cells that either stay behind as stem cells/ migrate superficially
o Keratins: proteins produced on free ribosomes accumulate in cytoplasm assemble into keratin intermediate filaments hemidesmosomes (connect basal cells to BM)/ desmosomes (connect to adjacent basal cells/ spinosum above) mechanical strength

Question 7

S. spinosum keratinocytes

Answer 7

o Limited mitosis
o Keratin intermediate filaments bundle together to strengthen intercellular adhesion via desmosomes
o Keratohyalin granules produced on free ribosomes (consist of intermediate filament-associated proteins)
o Membrane-bound lamellar granules form on RER (contain pro-barrier lipids)

Question 8

S. granulosum keratinocytes

Answer 8

o Undergo keratinization:
♣ Keratohyalin granules’ intermediate filament-associated proteins associate with keratin intermediate filaments protein masses that pack cytoplasm
♣ Lamellar granules release lipids via exocytosis intercellularly epidermal water barrier (ie. like mortar between bricks)
o Lose organelles/ die as move into s corneum

Question 9

• S. corneum keratinocytes

Answer 9

o Squamous, membrane-bound packages of protein soft keratin (vs hard keratin in hair)
o Desmosomes degrade as migrate towards surface desquamated/ shed

Question 10

LO: What cell types are found in the epidermis/ their functional roles?

Answer 10

keratinocytes, Merkel cells, Langerhans cells, melanocytes

Question 11

Keratinocytes

Answer 11

o Most predominant cell type, found throughout epidermis
o Arise through mitosis in s basale differentiate/ mature/ keratinize as migrate superficially through s spinosum s granulosum shed from s corneum (desquamation) 4-6 weeks later (rate production = loss, thus maintaining thickness)

Question 12

Merkel Cells

Answer 12

o Mechanosensitive touch receptors in s basale
o Vesicles containing neuroactive chemicals released with mechanical deformation from basal surface stimulate afferent sensory nerve endings (which crosses BM) report touch to SC
o Abundant in regions with acute touch sensation (fingertips)

Question 13

Langerhans Cells

Answer 13

Langerhans Cells
o In s spinosum (of epidermis in keratinized skin, as well as mucous membranes, superficial regional lymph nodes)
o APCs: phagocytose/ process antigens present epitopes on surface migrate to regional nodes/ present antigen to lymphocytes immune response
o Derived from monocyte-macrophage lineage (precursors enter epidermis from BVs differentiate into Langerhans)

Question 14

Melanocytes

Answer 14

o In s. basale (cell processes extend into spinosum)
o Melanin produced from tyrosine in membrane-bound melanosomes melanosomes transported to cell process tips transferred to adjacent keratinocytes = pigment donation (keratinocytes phagocytose melanocyte cell process tips/ melanosomes) melanin takes supranuclear position, shielding keratinocyte nuclei from UV pigment degraded over time by keratinocyte lysosomes
o Melanocyte to keratinocyte ratio in s basale varies depending on body region (more in regions exposed to sun)

Question 15

LO: What are the components of the dermis?

Answer 15

• Papillary layer
  o Loose CT, rich in collagen type I/ delicate collagen type III (reticular fibres), fine elastic fibres
  o Richly vascularized (nourishes epidermis)
  o Numerous cell types
• Reticular layer
  o Thicker, less cellular than papillary
  o Dense irregular CT with coarser bundles of collagen type I (tensile strength), elastic fibres (resiliency/ stretch)
  o Fibre bundles aligned along regular tension lines in skin = Langer’s lines
  ♣ Fibres may break with rapid stretch/ not recoil stretch marks
  ♣ Skin incisions parallel to Langer’s lines heal with least scarring

Question 16

Dermis blood supply

Answer 16

o Subcutaneous plexus in hypodermis = site of hypodermic/ subQ injections; feeds into
o Cutaneous plexus at dermal-subcutaneous junction supplies reticular layer; feeds into
♣ AV anastomoses in cutaneous plexus controls blood flow into subpapillary plexus, thus heat loss across body surface
o Subpapillary plexus at papillary-reticular junction; feeds into
o Capillary loops in dermal papillae that serve dermal-epidermal interface

Question 17

Hair

Answer 17

o Keratinized filaments produced by follicles: include both epithelial/ CT components; associated with sebaceous glands/ arrector pili (smooth muscle)
o Everywhere except thick skin (palms/ soles), lips, urogenital openings
o Sebaceous glands: produce sebum (lubricates hair/skin); usually open into hair follicle
o Arrector pili contraction:
♣ Thermoregulation: causes hair to stand on end in cold trap warm air closer to body
♣ Compresses sebaceous gland sebum secretion
o Hair bulb: expanded follicle base containing hair matrix (germinative cells), which gives rise to hair/ its inner covering
o Dermal papilla: high vascularized peg of loose CT nourishing avascular matrix
o Melanocytes within matrix supply keratinocytes with melanin (type/ density hair colour)
o Follicular bulge: niche of epidermal stem cells at arrector pili attachment sustaining hair growth/ sebaceous glands

Question 18

Types of hair

Answer 18

♣ Vellus: fine, pale hair distributed over most of body
♣ Terminal: coarse, dark hair
• Androgen-independent terminal hair follicles on heads/ eyelashes/ eyebrows
♣ Follicles proportional in size to hairs they produce
♣ Vellus hair follicles can be converted into terminal/ vice versa in androgen-dependent manner
• Increase androgen at puberty terminal in axillae/ pubic areas of both sexes (and faces of males)
• Androgenetic alopecia (male-patterned baldness): genetic predisposition for conversion of terminal follicles on head vellus

Question 19

Hair growth

Answer 19

♣ 3 phases:
• Anagen: active growth (rate = 0.33mm/ day)
• Catagen: involution/ regression of both hair/ follicle towards skin surface
• Telogen: inactive, resting shed
♣ Duration of phases varies in different body regions – length of anagen determines uncut hair length (longest in scalp = 3 years)

Question 20

Sebaceous glands

Answer 20

o Over most of body except thick skin (palms/ hands)
o Duct opens into upper hair follicle OR directly at skin surface in hairless regions (glans penis, clitoris, eylids, nippes)
o Hair follicle + sebaceous gland + arrector pili = pilosebaceous unit
o Hair follicles/ sebaceous glands dense in face/ scalp

Question 21

Sweat glands

Answer 21

o 2 types (apocrine/ eccrine) with distinct distributions, secretions, functions
o Each has secretory/ duct portions, secrete by exocytosis (apocrine = misnomer), secretory portions associated with myoepithelial cells (located between secretory cells/ BM, surrounding secretory acini – contract with sympathetic input/ hormones glandular secretion)

Question 22

Apocrine sweat glands

Answer 22

♣ In axillae, areolae, groin
♣ Become functional at puberty due to elevated sex hormones
♣ Ducts open into hair follicles (superficial to sebaceous glands)
♣ Secretion stimulated by emotions (fear, sexual excitement) – initially odourless odour develops with bacterial metabolism
♣ Produce pheromones in other mammals (? vestigial in humans)

Question 23

Eccrine sweat glands

Answer 23

♣ Numerous, widely distributed, including thick skin
♣ Ducts open onto skin surface (not associated with hair follicle)
♣ Functional from birth thermoregulation/ limited excretion – sweat evaporation carries away heat
♣ Sweating: interstitial fluid secreted from dermis acinus lumen Na+ reabsorbed as transferred along duct skin surface

Question 24

Mammary glands

Answer 24

o Modified apocrine sweat glands – develop along milk lines (in mammals, transient embryonic structures that extend from axillae inguinal region – mammary glands develop in pairs along lines)
♣ Supranumary nipples may persist along lines
o Parenchyma (functional cells of organ) derived from epithelia/ supported by surrounding CT stroma (support tissue of organ)
o Each consists of 15-25 lobes, served by lactiferous duct system that forms expanded lactiferous sinuses before opening independently at nipple
♣ Nipple, areola, lactiferous sinuses, rudimentary duct systems present in both sexes until puberty

Question 25

Female breasts at puberty

Answer 25

♣ Nipples/ areolae enlarge, areolar sebaceous follicles enlarge Montgomery’s tubercles
♣ Adipose accumulates in pectoral region, lactiferous duct systems expand (surrounded by loose CT), dense CT septa (Cooper’s ligaments) develop between duct systems/ extend from dermis to dense fascia overlying pectoralis major (support breast weight)
♣ Each lobe consists of separate lobules, connected to lactiferous duct system by extra lobular terminal duct
• Lobule + lobular terminal duct = terminal duct lobular unit (TDLU)
• Within lobule, intralobular terminal ducts open dead-ended terminal ductules (rudimentary in nonpregnant/ non-lactating female – develop into functional secretory acini during pregnancy/ lactation)
♣ Loose CT lobular stroma becomes edematous in premenstrual period slight breast enlargement
♣ Myoepithelial cells/ smooth muscle fibres encircle lactiferous ducts/ sinuses – contract to expel milk in lactating female

Question 26

Psoraisis

Answer 26

o Accelerated keratinocyte cell cycling 1 week epidermal turnover (vs 4-6) epidermal layer thickening and accelerated keratinization/ desquamation chronic inflammation (red, itchy, scaly, defective skin barrier)

Question 27

Warts

Answer 27

o Benign epidermal growths caused by papilloma viral infection of keratinocytes

Question 28

Albinism

Answer 28

o Results from defect in melanin-production pathway no melanin produced in skin/ hair

Question 29

Basal cell carcinomas

Answer 29

o In s basale
o Majority of skin cancers due to UV radiation damage
o Grow slowly, rarely metastasize

Question 30

Squamous cell carcinomas

Answer 30

o Arises from s spinosum cells due to UV radiation/ x-radiation/ chemical carcinogens
o 2nd most common skin CA, more likely to metastasize than basal

Question 31

Malignant melanoma

Answer 31

o In melanocytes of s basale
o Rarer, but aggressive skin CA
o Invades dermis spreads by lymph/ blood

Question 32

Factors affecting skin colour

Answer 32

o Melanocyte activity
♣ Ratio of melanocytes – keratinocytes consistent between races vs melanocyte activity is not
♣ More direct sunlight at equator adaptive changes evolution of darker-skinned people with greater UV protection
♣ Less direct sunlight in north adaptive changes fairer skinned people to sustain vitD3 synthesis with less UV
o Adaptive changes
♣ 2 genetically-determined forms of melanin: red-yellow pheomelanin, brown-black eumelanin
• Obvious implication in hair colour; also influences skin
♣ Melanin production rate higher in darker-skinned coarser melanin granules in keratinocytes
♣ Melanin lysosomal degradation rate lower in darker-skinned more persistent melanin granules found in more superficial epidermal layers
o Skin colour and aging
♣ Melanocytes decrease with age paler skin/ grey hair decreased UV protection (need increased sunscreen)
o Tanning: UV radiation increases melanin production tan/ keratinocytes protected BUT by time melanin increased, damage may have already occurred
o Vitiligo: due to autoimmune destruction of melanocytes in symmetrical patches (often in hands/ face – need UV protection!)

Question 33

Dermal-epidermal interface

Answer 33

o Epidermal ridges/ dermal papillae SA between epidermis/ dermis
♣ More pronounced in body regions experiencing greater mechanical stress
♣ The greater the SA, the greater the:
• # hemidesmosomes, thus adhesion between epidermis/ BM
• Gas/ nutrient/ metabolite exchange between avascular epidermis/ vascularized dermis
• # dividing cells in germinal layer, thus # keratinocytes produced/ moving superficially
• (all important in thick skin, which has greatest dermal-epidermal SA)
o Dermal ridges: more gross folds of dermal-epidermal interface that create “friction ridges” on hands/ feet (only present in thick skin) = basis of dermatoglyphs/ fingerprints

Question 34

Acne vulgaris

Answer 34

o Chronic inflammation of pilosebaceous unit
o Commonly in adolescence due to puberty hormone changes ccelerated keratinzation in follicle + sebaceous gland hypersecretion blocked follicle sebum accumulation anaerobic bacteria flourish inflammation
o Tx: systemic Abx, topical steroids

Question 35

Breast carcinoma

Answer 35

o Various epithelial components of mammary glands may undergo cancerous change
o Lobar carcinomas arise from terminal ductules
o Ductal carcinomas arise from terminal ducts (intralobular/ extralobular)
o Since men also have rudimentary mammary glands, can also develop breast CA (incidence lower)