The Skin

Question 1

The skin

Answer 1

AKA the integument
largest single organ in the body
has two components: the epidermis (superficial epithelial layer) and the dermis (the underlying CT)
- there is a loose CT hypodermis underneath the dermis, which is not part of the skin)

Question 2

functions of the skin

Answer 2

protection
sensation
thermoregulation
metabolism

Question 3

thick vs thin skin

Answer 3

thick skin has a thicker epidermis, and is typically found in areas that are subject to considerable abrasions, such as the palms of hands or the soles of feet
- thick skin has sweat glands, but lacks hair follicles and sebaceous glands
- thin skin has a thin epidermis, and contains hair follicles, sweat glands, and sebaceous glands

Question 4

epidermis layers

Answer 4

thin skin has 4 distinct layers, thick skin has 5
from deep to superficial:
1. strata basale
2. strata spinosum
3. strata granulosum
4. strata lucidum (thick only)
5. strata corneum

Question 5

cells of the epidermis

Answer 5

epidermis is stratified squamous epithelium, mostly composed of keratinocytes
- keratinocytes arise in the strata basale, and migrate until they reach the stratum corneum
- epidermis also contains melanocytes, merkel cells (detect sensation), and Langerhan cells (phagocytes)

Question 6

stratum basale composition

Answer 6

deepest layer, has a single layer of cuboidal to columnar cells that rest on the basement membrane of the epidermis, at the dermal-epidermal junction

Question 7

stratum basale characteristics

Answer 7

characterized by intense mitotic activity and contains progenitor cells for all the epidermal layers
- division of basal cells replace the more superficial keratinocytes that are lost at the epithelial surface

stratum basale contains melanocytes

Question 8

Merkel cells

Answer 8

found in basal layer of epidermis, and are most abundant in the fingertips and at the bases of hair follicles
- sensitive mechanoreceptors essential for light touch and sensation
- joined to keratinocytes by desmosomes

Question 9

basal lamina

Answer 9

cells of the stratum basale rest on the basal lamina, and are anchored there by hemidesmosomes, into which tonofilaments insert

desmosomes bind basal lamina together via lateral and upper surfaces

Question 10

divisions of the basal lamina

Answer 10

lamina lucida
lamina densa
both of these rest on the reticular lamina of the dermis

Question 11

stratum spinosum

Answer 11

consists of 3-10 layers of polygonal keratinocytes with euchromatic nuclei
- each time a basal cell divides in the stratum basale, a daughter cell is pushed to the stratum spinosum, where it begins to differentiate into a keratinocyte

Question 12

keratinocytes of the stratum spinosum

Answer 12

have a centrally located nucleus with nucleoli and cytoplasm that actively synthesizes keratins
- keratin filaments bundle to form tonofibrils

adjacent keratinocytes are attached by desmosomes, creating a spiny appearance

Question 13

stratum spinosum of thick skin

Answer 13

thick skin is subject to continuous friction and pressure, and as such have a thicker stratum spinosum with more abundant tonofibrils and desmosomes

Question 14

langerhan cells

Answer 14

APCs and macrophages
most commonly found in stratum spinosum, but present in all epidermis layers

Question 15

stratum granulosum

Answer 15

3-5 layers thick, composed of flattened keratinocytes filled with prominent keratohyalin granules
- keratohyalin and tonofibrils combine to form soft skin keratin
- simultaneously, the nuclei and other organelles in these cells degenerate, resulting in production of keratinized scales, which form the stratum lucidum and stratum corneum

Question 16

keratinocytes of the stratum granulosum

Answer 16

contain lamellar granules that release lipid contents by exocytosis into the intercellular space to form the epidermal water barrier

Question 17

stratum lucidum

Answer 17

only present in thick skin, thin translucent layer
- cells lack organelles and have flattened nuclei, densely packed, and filled with keratin filaments

Question 18

stratum corneum

Answer 18

most superficial layer of the epidermis, and consists of numerous layers of dead keratinized stratified squamous epithelium
- dry, dead layer helps prevent penetration of microbes and the dehydration of underlying tissues. it also provides protection against abrasion

Question 19

process of keratinization

Answer 19

1. keratinocytes in the stratum basale produce intermediate filaments (tonofilaments) composed of the protein keratin. filaments aggregate to form tonofibrils, which are prominent in the stratum spinosum
2. in the stratum granulosum, keratohyalin granules and tonofibrils condense to form keratin; simultaneously, organelles and nuclei are lost. keratin formed by this process is soft keratin of skin
3. the resulting flattened, keratin filled squamous forms stratum corneum, which are continuously shed at the cell surface

Question 20

the dermis

Answer 20

deep to the epidermis, and has 2 major components:
1. the papillary layer: superficial, consists of loose CT. has dermal papillae that project between epidermal ridges of the dermal-epidermal interdigitations

2. the reticular layer: composed of dense irregular CT and contains large blood vessels, nerves, and sometimes sweat glands

Question 21

sensory receptors

Answer 21

the skin contains different sensory receptors located in the epidermis, dermis, and hypodermis, which allow the skin to respond to stimuli like touch, pressure, and vibration

Question 22

Meissner’s corpuscles

Answer 22

encapsulated sensory receptors located at the apex of the sensory dermal papillae
- provide fine touch discrimination, and are most concentrated in fingertips, palms, soles, and lips

these corpuscles are located in the dermis and project into the epidermis

Question 23

Pacinian Corpuscles:

Answer 23

encapsulated sensory receptors located in the deep layer of the dermis and in the hypodermis
- respond to pressure and vibration

Question 24

associated skin structures

Answer 24

sweat glands, hair follicles, and nails

Question 25

sweat glands

Answer 25

develop of long epidermal invaginations embedded in the dermis
- two types: eccrine sweat glands and apocrine sweat glands

Question 26

eccrine sweat glands

Answer 26

simple coiled tubular glands widely distributed in the skin, most numerous on soles of feet
- secrete thin and watery secretions onto skin surface to cool and reduce body temperature
- secretory unit is pale staining, and simple cuboidal or columnar epithelium
- the duct is long and unbranched, travels through the dermis to epidermis, opens onto surface of skin. comprised of stratified cuboidal

Question 27

apocrine sweat glands

Answer 27

simple coiled tubular glands, large in size with a thick lumen
- ducts open into hair follicles, limited to the anal, areolar, and axillary regions
- develop after puberty, produce a thick, protein rich, and odorous secretion

Question 28

hair follicles

Answer 28

invaginations of the epidermal epithelium, extending through the dermis into the hypodermis
- hair is made of keratinized cells
- all skin has at least minimal hair except palms, soles, lips, penis, clitoris, and labia minor

Question 29

structures associated with hair

Answer 29

sebaceous gland: secretes sebum
follicle sheath: dense CT sheath that surrounds hair follicle, provides attachment for arrector pili muscle
hair shaft: extends beyond skin surface
arrector pili muscle: smooth muscle, contractions elevate the hair (goosebumps)
dermal papilla: contains capillary network to sustain hair