Week 4 – Integumentary System 1
Skin (cutaneous membrane) – external surface of body; weighs about 7% of body weight
Composed of two parts:
EPIDERMIS – superficial, thinner, epithelial tissue portion
DERMIS – deeper, thicker, connective tissue portion
o Deep in the dermis is SUBCUTANEOUS LAYER (aka hypodermis), but is not part of the skin.
Fibers extend from the dermis and anchor the skin to the subcutaneous layer, which connects to muscles. The
subcutaneous layer stores fat, large blood vessels that supply the skin, and nerve endings sensitive to pressure
Epidermis – composed of keratinized stratified squamous epithelium; contains four cell types:
90% of epidermal cells and produces
o Keratin – a tough fibrous protein that helps protect skin and underlying tissues
o Lamellar granules – contains a waterproof sealant
8% of epidermal cells
Produces melanin – yellow-red/brown-black pigment that contributes to skin color and absorbs UV light
Helps other cells of the immune system recognize an invading microbe and destroy it
M ERKEL CELLS
Least numerous of epidermal cells and located in the deepest layer of the epidermis
Helps with the sensation of touch
Thin (hairy) skin – most regions of the body have an epidermis with four layers (no stratum lucidum)
Thick (hairless) skin – where exposure to friction is greatest (i.e., palms and soles), five layers
Deepest layer of epidermis
Single row of cuboidal/columnar keratinocytes with some stem cells that continually produce new
Cytoskeleton of keratinocytes contains filaments that form keratin in its more superficial epidermal layers
Consists of numerous keratinocytes in 8-10 layers
Keratinocytes in this layer produce coarser bundles of keratin
When prepared for microscopic examination, they appear to be covered with thornlike spikes
Provides strength and flexibility Week 4 – Integumentary System 2
STRATUM G RANULOSUM
~middle of epidermis
3-5 layers of flattened keratinocytes that are undergoing apoptosis (programmed cell death)
Darkly stained granules of a protein called keratohyalin
Lamellar granules release a lipid-rich secretion to help propel water
Present only in thick skin
4-6 layers of flattened, clear, dead keratinocytes
Provides additionally “toughness” for the skin
Numerous layers of flat, dead keratinocytes
Cells are continuously shed and replaced by cells from the deeper layers
The multiple layers of dead cells help protect deeper layers from injury and foreign invasion
Keratinization and Growth of the Epidermis
1. Newly formed cells in the stratum basale are slowly pushed to the surface
2. As cells move up, they accumulate more and more keratin = keratinization
3. Then they undergo apoptosis
4. The keratinized cells are eventually sloughed off and are replaced by underlying cells that go through the
same process that takes about 4-6 weeks
Dermis – composed of dense irregular connective tissue containing collagen and elastic fibers
Great ability to resist pulling or stretching
Has the ability to stretch and recoil easily
Thickness varies among the body but it is always thicker than the epidermis
Cells present in the dermis are mostly fibroblasts, macrophages, and a few adipocytes near the bottom
Blood vessels, nerves, glands, and hair follicles are embedded in dermis
Divided into two regions:
1. Papillary region ⅕ of dermis
o Contains thin collagen and fine elastic fibers
o Surface area greatly increased by dermal papillae – small nipple-shaped structures that project
into the undersurface of the epidermis; contains capillary loops (blood vessels)
Some contain nerve endings sensitive to touch and free nerve endings
2. Reticular region
a. Attached to subcutaneous layer
b. Consists of thick collagen fibers, scattered fibroblasts, some adipose, and other cells
c. Blood vessels, nerves, hair follicles, sebaceous (oil) glands, and sudoriferous (sweat) glands
occupy space between fibers
d. Provides dermis with strength, the ability to stretch, and the ability to return to original shape
after stretching Week 4 – Integumentary System 3
Epidermal ridges – “fingerprints”
Creates strong bond between epidermis and dermis
Increases surface area of the epidermis increases grip of the hand or foot by increasing friction
Increases tactile sensitivity
Everyone has fairly different fingerprints, even identical twins.
Structural Basis of Skin Color – Pigments of Skin
The amount of melanin causes the skin’s color to vary from pale yellow to yellowish-brown to black.
Melanocytes mainly in epidermis of penis, nipples/areola, face, and limbs. Also in mucous membranes.
The NUMBER of melanocytes is about the same in all people. It is the AMOUNT OF PIGMENT they
produce and transfer to keratinocytes that causes various skin colors.
Melanin can accumulate in patches called freckles.
Age spots are also accumulations of melanin due to age and exposure to sun.
Moles are benign localized overgrowth of melanocytes.
Melanin is synthesized from the amino acid tyrosine in the presence of an enzyme called tyrosinase.
Synthesis occurs in melanosomes. Exposure to UV light increases activity which in turn causes a “tan”.
Tans help protect the body against further UV radiation.
Melanin absorbs UV radiation, prevents damage to DNA
A small amount of UV light exposure is necessary for vitamin D synthesis.
Light skinned individuals have little melanin in the epidermis so it appears translucent
o Pink to red color depending on O content of the blood moving through capillaries in the dermis
o When skin is “red” it has the most hemoglobin, the oxygen-carrying pigment in RBC’s.
Yellow-orange pigment that gives egg yolks and carrots their color, precursor of vitamin A
Synthesizes pigments needed for vision
Stored in the stratum corneum and fatty areas of the subcutaneous layer after excessive dietary intake.
------------------------5.2 – Accessory Structures of the Skin ------------------------------------------------
Present on most skin except the palms and soles
Hair on the head guards scalp from injury and the sun’s rays, and decreases heat loss
Eyebrows and eyelashes protect eyes from foreign particles
Function in sensing light touch (hair root plexus touch receptors around hair follicles)
Hair is composed of columns of dead, keratinized epidermal cells bonded by extracellular proteins Week 4 – Integumentary System 4
Shaft – superficial, projects above skin surface
Root – deep, penetrates into dermis and
sometimes into the subcutaneous layer
Made of three cylindrical layer