Histology Exam 2: Integument

Front 1

What tissue type is the dermis made of?

Back 1

Dense irregular connective tissue (mostly type I collagen) and some elastic tissue

Front 2

Describe the two layers of the dermis.

Back 2

Superficial (papillary) region: loosely organized collagen fibers; contains capillary network, nerve endings, immune system cells.

Deeper (reticular) layer: densely packed collagen bundles with more elastic fibers.

Front 3

Name and describe the deepest layer of the skin.

Back 3

Subcutis or hypodermis:

- mainly adipose tissue, but also contains larger blood vessels that drain/supply the dermal vasculature

- anchors skin to adjacent structures (e.g. skeletal muscle)

- forms superficial fascia and outer fascia of adjacent skeletal muscle and bone

Front 4

Describe the interdigitating structures between the epidermis and the dermis. What is their function?

Back 4

Rete ridges (aka rete pegs) are downward growths from the epidermis that penetrate the upper dermis. Dermal papillae are projections from the superficial dermis.

They provide resistance to frictional shearing force and increase surface area of contact between the two layers.

Front 5

Name and describe the deepest layer of the epidermis.

Back 5

Stratus basale (stratum germinativum):

- single layer of small cuboidal to low columnar cells
- less cytoplasm
- closely spaced
- slightly basophilic
- may contain melanin
- cells connected to each other and to keratinocytes by desmosomes
- connected to underlying basal lamina by hemidesmosomes
- are the mitotically active stem cells from which new cells (keratinocytes) arise

Front 6

Name and describe the second-deepest layer of the epidermis.

Back 6

Stratum spinosum (prickle cell layer):

- several cells thick
- larger cells than stratum basale
- desmosomes between cells (spinous connections)
- produce keratins (cytokeratins) that assemble into tonofilaments
- in mammals also contain lamellar bodies (aka membrane-coating granules)
- as cells mature and move towards surface, they increase in size and flatten

Front 7

What are tonofilaments?

Back 7

- intermediate filament formed by aggregation of keratins (cytokeratins)

- extend into cytoplasm and attach to dense plaque of desmosome

Front 8

Name and describe the most superficial layer of the nonkeratinized portion of the epidermis.

Back 8

Stratum granulosum (granular cell layer):
- 1-3 cells thick
- tight junctions between keratinocytes (permeability barrier)
- increased number of lamellar bodies that then release their contents into intercellular spaces
- contain keratohyaline granules

- oldest cells in this layer undergo modified apoptosis: degeneration of nucleus and metabolic apparatuses by lysosomal enzymes

Front 9

Describe keratohyaline granules.

Back 9

- basophilic
- contain filaggrin and trichohyalin, which promote aggregation of keratin into tonofibrils (soft keratin)

Front 10

What are lamellar bodies?

Back 10

- aka membrane-coating granules

- oval, membrane-bound organelles that contain a mixture of lipids, proteins, hydrolytic enzymes

- when their contents are released into intercellular space (stratum granulosum), glycolipids coat cell surface in stratum lucidum to form watertight seal

- only seen with EM

Front 11

Name and describe the most superficial layer of the epidermis.

Back 11

Stratum corneum (cornified cell layer):

- flattened, desiccated, no nucleus/organelles

- almost entirely filled with keratin filaments (so very eosinophilic)

- thickened plasma membranes coated by lipid-rich matrix (water barrier)

- cells are desquamated (exfoliated) through proteolytic process that degrades desmosomes; active process, not passive

Front 12

What are the major biochemical components of cornification?

Back 12

Menthos in rubber cement:

- cornified envelope (hard shell of menthos): proteins joined by transglutaminase

- keratin filaments (chewy inside)

- intercellular lipids (rubber cement)

Front 13

Which layer of the skin does most of the metabolism of compounds?

Back 13

Spinous cell layer

Front 14

What are the components of the immunologic barrier of the skin?

Back 14

In epidermis: Langerhans cells, keratinocytes

In dermis: dermal dendrocytes, lymphocytes, macrophages

Front 15

How does the skin act as a structural barrier against bacteria?

Back 15

- too dry for bacteria to live

- pH too high

- outer layer constantly shedding

Front 16

How does the skin act as an innate (non-specific) immunologic barrier?

Back 16

- keratinocytes can release antimicrobial peptides

- keratinocytes and to a lesser extent, Langerhans cells, release cytokines that attract neutrophils and macrophages

Front 17

How does the skin act as an adaptive (specific) immunologic barrier?

Back 17

Langerhans cells phagocytose foreign antigens, break it down within their lysosomes, migrate to lymph node, and present it to a T-lymphocyte to initiate immune response.

Front 18

What are Langerhans cells?

Back 18

- one type of antigen-presenting cell

- dendritic (processes extend between keratinocytes)

- precursors in bone marrow migrate to epidermis and differentiate there

- lobulated nuclei

- best seen with immunohistochemical staining

Front 19

How does the skin act as a photoprotective barrier (against ionizing ultraviolet radiation)?

Back 19

Hair, melanocytes, keratinocytes

Front 20

What are melanocytes?

Back 20

- scattered around basal cells of stratum basale [also found in external root sheath and hair matrix of follicles, in ducts of sweat glands, in sebaceous glands]

- round cell body with long, cytoplasmic processes that extend into stratum spinosum

- transfer melanin granules into cytoplasm of neighboring keratinocytes via cytocrine method called pigment donation

Front 21

What are the two types of melanin?

Back 21

Eumelanin: most common type, only photoprotective one; confers dark brown to black color

Pheomelanin: yellow to red

Front 22

What are Merkel's cells?

Back 22

- modified keratinocytes with epithelial and neural characteristics

- oval to elongate cells with processes parallel to epidermal surface

- present as aggregates in tylotrich pads

- contain cytoplasmic granules, keratin filaments, melanosomes

- bound to adjoining keratinocytes by desmosomes

- can function as mechanoreceptors as Merkel's corpuscle (Merkel's cell in close contact with afferent, unmyelinated nerve ending; neuron penetrates basal lamina and expands into a disc near the base of the Merkel's cell)

Front 23

Describe the dermal-epidermal junction.

Back 23

(Basement membrane zone)

Thin membrane synthesized by keratinocytes and fibroblasts. Minimal barrier function. Must be intact for re-epithelialization to occur.

Front 24

What are the two anatomic regions of the dermis?

Back 24

Adventitial dermis: near/surrounds epithelial structures

Reticular dermis: away from epithelial structures

Front 25

Describe components of the dermis.

Back 25

- collagen: provides tensile strength

- elastic fibers: mainly elastin, fibrillin; provide elasticity and recoil after stretching

- connective tissue mucins (glycosaminoglycans, proteoglycans): bind water (otherwise skin would be leather)

Front 26

Name the regions of the hair follicle.

Back 26

Hair shaft:
- infundibular region: from opening of follicle down to insertion of sebaceous duct

- isthmus region: sebaceous duct to arrector pili muscle

- inferior region: arrect pili muscle down, including hair bulb

Front 27

Describe the organization of canine hair follicles.

Back 27

Primary hairs (large) are cranial, and get progressively smaller caudally.

Front 28

Name the four stages of the hair cycle.

Back 28

1) Anagen
2) Catagen
3) Telogen
4) Exogen

Front 29

Describe anagen.

Back 29

- active growth of hair follicle

- inner root sheath: thin, smooth, pink layer next to hair shaft

- hair shaft not attached to inner root sheath, so there may be visible space

- sometimes the hair shaft will drop out, leaving empty space in middle of follicle

- prominent hair bulb

Front 30

Describe catagen.

Back 30

- end of active growth of hair

- hair converts to club hair: hair shaft attaches to inner root sheath, which cuts off the hair from its blood supply and the cells that produce new hair

- bulb replaced by keratin

Front 31

Describe telogen.

Back 31

- dormant phase of growth

- inner root sheath is lost when hair shaft undergoes tricholemmal cornification (bright red, serrated border, unlike pink and smooth inner root sheath)

- hair shaft is tightly annealed to root sheath

Front 32

Describe exogen.

Back 32

- shedding phase of cycle, when hair exits the follicle

Front 33

Describe sinus hairs.

Back 33

- aka whiskers, vibrissae, tactile hairs

- connective tissue adjacent to the follicle is different: vascular sinus that amplifies vibrations picked up by the hair

- cavernous sinus (has fibroeslastic trabeculae); dogs and cats also have annular sinus (no trabeculae)

- skeletal muscle fibers attach to outer layer of dermal root sheath (allows some voluntary control of movement)

- nerves penetrate external dermal sheath, ramify in trabeculae and inner dermal sheath

Front 34

External root sheath vs. internal (inner) root sheath

Back 34

External root sheath: derived from (and continuous with) epidermis-- not from hair bulb.

Internal root sheath: generated by matrix cells in hair bulb; 3 layers of soft keratin.

Front 35

Describe the hair bulb.

Back 35

- invagination of loose connective tissue at base of hair bulb is called dermal papilla

- dark blue matrix cells (cuboidal) overlie dermal papilla

Front 36

Hard keratin vs. soft keratin

Back 36

Hard keratin (nails, hair):
- formed without keratohyaline
- high in sulfur; immersed in little lipid
- harder, more resistant to mechanical stress

Soft keratin (skin):
- aggregates of keratin filaments (by filaggrin and trichohyalin in keratohyaline granules)

Front 37

What is the arrector pili muscle?

Back 37

Smooth muscle located caudal to the hair; originates in superficial layer of dermis and inserts on connective tissue sheath that surrounds the external root sheath.

When it contracts, it pulls the base of the follicle caudally so the hair stands upright.

Front 38

Describe sebaceous glands.

Back 38

- holocrine secretion of sebum (lipids, proteins) into a duct called pilosebaceous canal, which connects to hair follicle

- lubricates hair shaft as it exits infundibulum, adds sheen to coat, may have antibacterial activity

- newly formed cells (reserve cells) have round nuclei, eosinophilic cytoplasm (smooth ER)

- differentiated, sebum-producing cells are foamy, filled with lipid droplets; look like brown fat but have central nuclei and no connection to basal lamina

Front 39

Describe circumanal glands.

Back 39

- modified sebaceous glands found only in canines

- described as "hepatoid glands" because the cells resemble liver cells: round, grainy pink cytoplasm; round vesicular nuclei with euchromatin

Front 40

Describe apocrine glands.

Back 40

- merocrine secretion of pheromones, body odor, perspiration (in horses only)

- coiled, tubular glands; secretory portion in deep dermis; simple cuboidal, eosinophilic cells around a wide lumen

- secretory duct: bilayered epithelium, cuboidal cells

Front 41

Describe eccrine glands.

Back 41

- independent of hair follicles

- simple, blind-ended, coiled, tubular structure made of secretory segment and duct segment (acrosyringium)

- produce watery secretion (sweat)

- worthless in haired skin, so only found on footpads of dogs/cats

Front 42

Names and functions of subcutaneous fat.

Back 42

Aka subcutis or panniculus

Insulates, protects, acts as storage

Front 43

What is the laminar corium?

Back 43

Another name for dermis in the claw and hoof.

Front 44

Name the layers of the equine hoof.

Back 44

- stratum externum (or tectorium): outside wall of hoof

- stratum medium (aka coronary epidermis)

- stratum internum

- laminar corium (dermis)

Front 45

Describe the stratum externum of the hoof.

Back 45

- thin outermost layer of epithelium

- extension of perioplic epidermis (periople is outer covering of hoof)

- cornified epithelial cells; appear soft, white, shiny

Front 46

Describe the stratum medium of the hoof.

Back 46

- bulk of the wall of the hoof

- has prominent tubular horn (hard keratinized tubules/papillae) and intertubular horn (surrounds tubules)

- originates from coronary epidermis, grows downwards towards the ground

Front 47

Describe the stratum internum of the hoof.

Back 47

- innermost epithelial layer of hoof wall

- epidermal lamellae interdigitate with laminar corium

- primary epidermal lamellae: keratinized, fused with inner portion of stratum medium,

- secondary epidermal lamellae: oriented at ~right angles to primary lamellae; core of stratum spinosum cells with adjacent stratum basale cells

- insensitive (lack nerve endings)

Front 48

Describe the laminar corium of the hoof.

Back 48

- arises from dermis that covers the distal phalanges

- sensitive (contain nerve endings)

Front 49

What are Pacinian corpuscles?

Back 49

- found in upper dermis and hypodermis [also joints, periosteum, internal organs]

- myelinated nerve ending surrounded by capsule structure (like cross-sectioned onion: series of concentric lamellae with thin fluid between each layer)

- respond to deep pressure and vibration through displacing of capsule lamellae, which causes depolarization of the axon

Front 50

What are Meissner's corpuscles?

Back 50

- touch receptors that respond to low-frequency stimuli

- in papillary region of dermis (just beneath lamina basale), in hairless skin

- look like loose twisted skein of wool

- tapered cylinder perpendicular to surface, with 1-2 unmyelinated nerve endings (of myelinated nerves) spiraling inside, and flattened Schwann cells around

Front 51

Describe mammary glands.

Back 51

- modified apocrine sweat glands that develop under the influence of sex hormones

- branched tubuloacinar morphology

- during active lactation, basal part of acinar epithelial cells has well-developed ER, central spherical nucleus; lumen of gland filled with secretion

- both merocrine (milk proteins) and apocrine (lipids) secretion