Integumentary System

The integumentary system is the first line of defense in the human body and includes skin, hair, nails, and glands.

Unit 5 Synopsis

Affixes and Roots

Albino-: white

Melan(o)-: black

Erythro-: red

Cyan-: blue

-graft: moving tissue from one area to another

Cautero-: burn

Cryo-: cold

Icthyo-: dry, scaly

Cut-; Cutaneo-; Derm(a)-; Dermato-: Skin

Sud-: sweat

Seb-: oil

Lip(o)-: fat

Trich-: hair

Epi-: on top or upon

Strati-: layer

Functions

The primary function of the integumentary system is protection, it is the first line of defense by inhibiting pathogens from entering the body.

Other functions include the synthesis of vitamin D, thermoregulation, osmoregulation, sensation through the processing of environmental stimuli, excretion.

Structures

There are three main layers of skin: epidermis, dermis, and hypodermis.

It has a surface area of 1.2-2.2 sq m

Epidermis

The epidermis is the most superficial layer of skin that is made of epithelial tissue. The epidermis is avascular, therefore dependent on deeper layers (dermis) for nutrients.

Epidermal cells:

1. Melanocytes: produce melanin. Melanin is a pigment that keratinocyte nuclei from radiation damage.

2. Keratinocytes: produce keratin, the protective, waterproofing protein.

3. Dendritic cells: also known as Langerhans cells, ingest foreign invaders and activates the immune system.

4, Tactile epithelial cells: also known as Merkel cells. Located where the dermis and epidermis attach, combine with nerve endings that allow for touch sensory reception.

The epidermis is further divided into 5 layers:

The most superficial layer is the stratum corneum.

Stratum lucidum

Stratum granulosum

Stratum spinosum

Most deep layer is the stratum basalis

Dermis

The dermis is deep to the epidermis and superficial to the hypodermis. It is further divided into two layers. Most of the dermis is made of dense connective tissue, contains blood vessels, glands, hair follicles, and nerves.

Layers of the Dermis

1. Papillary Dermis

Made of areolar connective tissue, interwoven loosely with collagen, elastic fibers, and blood vessels

Dermal papillae: projections from its surface into the epidermis that contain capillary loops, nerve endings, and touch receptors.

Lie on top of dermal ridges that form the ridges in the epidermis, which form our fingerprints.

2. Reticular Dermis

Made of dense, irregular connective tissue and houses a network of blood vessel and thick bundles of collagen fibers that give the dermis strength and resiliency

Makes up 80% of the dermis

The collagen fibers in the reticular dermis run in various planes, but mainly parallel to the skin’s surface. Separations between these bundles of fibers form cleavage (tension) lines in the skin that are invisible externally, but are used by surgeons when they make incisions (if they cut parallel to these lines, the skin will gape less and thus heal more readily!)

Hypodermis

Aka subcutaneous tissue
Technically not part of the skin
- because it is superficial to the fascia (tough connective tissue) that wraps the skeletal muscles
Mainly made of adipose tissue (fat)
Functions:
- Insulation against heat loss
- Energy storage
- Shock absorber
- Anchors skin to underlying structures (mostly muscles)

Accessory Structures

In addition to the skin, there are several accessory structures that form the integumentary system, including hair, nails, and glands.

All of these “skin appendages” or accessory structures (hair, nails, sweat glands, and oil glands) are derived from epithelial cells in the epidermis and extend into the dermis.

Hair

Hair on our arms helps us to sense if an insect is on us before it stings/bites.
Hair on our heads helps to prevent heat loss and guards against sunlight and physical trauma.
Nose hairs help filter large particles when we inhale.
Eyelashes help shield our eyes.
Hair = long filaments made of dead keratinocytes
- Made of 3 layers:
  - Medulla = central core made of soft keratin
  - Cortex = bulky layer
  - Cuticle = single layer of overlapping cells, like shingles on a roof
    - When the cuticle wears away at the tip of the hair shaft it causes split ends
- The hair root is under the skin and the shaft is above the skin.
- The medulla is absent in fine hair.
  - Hair conditioner smooths out the “shingles” on the cuticle of the hair.

Hair follicles = tubular invaginations in the epidermis that hair grows out of
- Deep end of root = hair bulb
- Arrector pili = smooth muscle cell bundle that contracts to raise hair upright
  - The arrector pili muscle also causes goose bumps in response to cold temperatures.
- Root hair plexus (hair follicle receptor) nerve fiber endings that perceive touch and wrap around each bulb
  - Hair grows because cells in the hair bulb of the follicle divide rapidly, making new hair cells that push the older part of the hair upward and outward.
  - Each follicle goes through a growth cycle that has an active growth phase and a resting phase. The length of these phases varies in different parts of the body!
- The shape of the hair follicle plays a major role in the texture of your hair
  - The flatter the follicle, the curlier the hair will be

Nails

Made of heavily keratinized cells
3 parts = root (embedded in skin), plate/body, and free edge (what you trim)
Rests on part of epidermis called the nail bed
- Our nails appear pink under the nail bed because of all the capillaries in the underlying dermis!
Nail matrix = where nail grows

Glands

Our skin has sweat glands, that mainly work to keep us from overheating, and sebaceous (oil) glands, that mainly work to prevent water loss.
- We have over 3 million sweat glands distributed throughout the body – everywhere except the nipples and parts of external genitalia.
- Sweat is 99% water + some salts, traces of metabolic nitrogenous wastes, and dermcidin (a peptide that can kill microbes!). It is usually acidic (pH = somewhere between 4 and 6)
Two types of sweat glands:
- Eccrine sweat glands: most abundant; extra in palms, forehead, and soles of feet
- Apocrine sweat glands: lie deeper in dermis and are larger; come in during puberty and empty into the hair follicles around the armpits and groin
  - 2 types of modified apocrine glands = ceruminous glands in the ear canal that make ear wax, and mammary glands that secrete milk
  - Both of these types of sweat glands are associated with specialized secretory cells that contract when stimulated by the nervous system to push sweat through the duct and to the surface.

Functions

Protection 🡪 creates the following barriers:
- Physical: from its many layers of cells and its continuity
- Chemical: skin secretes natural antibiotics (defensins) and most secretions have a low pH, both for killing bacteria
  - Melanin = a chemical pigment shield to protect against UV damage; made by melanocytes
- Biological: immune cells
  - Dendritic cells in the epidermis patrol beneath the surface to capture invaders 🡪 immune response
  - Macrophages in the dermis act as a 2nd line of defense 🡪 an immune response if invaders make it through the epidermis
Body Temperature Regulation
- Loses heat by causing vasodilation of dermal blood vessels and releasing sweat to cool the body down as it evaporates
- Retains/conserves heat by causing vasoconstriction of dermal blood vessels
Blood Storage:
- Holds up to 5% of the body’s blood volume at a time
Synthesis of Vitamin D:
- Allows the body to absorb calcium
Excretion:
- Some nitrogenous wastes get eliminated via sweat (but most is released in urine)
Sensations and Sensory Receptors:
- Tactile (Meissner’s) corpuscles and tactile epithelial (Merkel) cells: texture changes and slow vibrations; allow us to feel clothing on our skin
- Lamellar (Pacinian) corpuscles: deeper pressure and fast vibrations
- Hair follicle receptors (root hair plexus): allow us to feel when the wind blows our hair
- Free nerve endings: sense pain (nociception)
- Ruffini endings: stretch