Comprehensive Study Notes on the Integumentary System

Components and Primary Functions of the Integumentary System

• System Components: The integumentary system is composed of the skin, hair, nails, and various glands (such as sweat and sebaceous glands). It is the largest organ in the human body, accounting for approximately 16\,\text{%} of total physical weight. If laid out flat, the skin would cover nearly $1.7\,\text{m}^2$ of ground.

• General Protective Functions: The system serves as a barrier against the external environment, protecting the body from wind, water, and ultraviolet (UV) radiation.

• Pathogen Defense: The skin acts as the first line of defense against microbes and pathogens. Sweat contains a chemical called Dermcidin, which possesses antibiotic properties. The surface of the skin is home to millions, billions, or even trillions of bacterial cells (the normal bacterial flora), which help prevent pathogens like Staphylococcus aureus from over-colonizing and causing infection.

• Physical Barrier: The skin prevents allergens, viruses, and bacteria from penetrating into deeper tissues like muscle and bone. The stratum corneum, the most superficial layer of the skin, contains heavy concentrations of keratin and lipids to form this waterproof and protective seal. Without the skin, the body would be a mass of tissue and fluids fatally exposed to the elements.

• Water Management: A major function of the skin is preventing water loss from the human body. In burn victims, the loss of skin leads to a critical loss of water and an increased vulnerability to infections.

• Thickness Ranges: The skin varies in thickness depending on its location, ranging from $0.5\,\text{mm}$ at its thinnest points to $4\,\text{mm}$ at its thickest.

Thermoregulation and the Nervous System

• Neural Control: The integumentary system regulates body temperature through a tight association with the sympathetic nervous system (the "fight or flight" division), which continuously monitors body temperature and initiates motor responses.

• The Hypothalamus: This brain structure acts as the body's thermostat, typically setting the internal temperature at $98.6^{\circ}\text{F}$ ($37^{\circ}\text{C}$).

• Response to Heat (Hyperthermia):

  • Vasodilation: Blood vessels increase in diameter. Superficial blood vessels allow heat carried by the blood to dissipate into the external environment. This results in the skin appearing flushed or reddish.

  • Sweating: Ecrine glands secrete water and salts. Evaporation of this sweat from the skin surface removes heat from the body. Even without noticeable sweating, the body secretes approximately $500\,\text{mL}$ of "insensible perspiration" daily. During vigorous activity or high heat, this can increase to $0.7$ to $1.5\,\text{L/hour}$.

  • Muscle Relaxation: Muscles involved in temperature regulation relax to maximize heat loss.

• Response to Cold (Hypothermia):

  • Vasoconstriction: Blood vessels decrease in diameter, shunting blood toward the core of the body to conserve heat. This makes the skin appear pale or even blue.

  • Shivering: Skeletal muscles undergo rapid contraction/friction to generate heat.

  • Arrector Pili Action: Small smooth muscles called arrector pili, attached to hair follicles, contract. This pulls the hair follicle upright, creating "goosebumps."

  • Insulation Mechanism: These bumps create a textured surface that makes it harder for air to brush against the skin and remove heat through convection. The upright hairs trap an insulating layer of air close to the skin.

Nutrient Synthesis and Bone Health

• Vitamin D Synthesis Pathway:

  1. The skin is exposed to UV radiation from the sun.

  2. This exposure triggers the release of Cholecalciferol (Vitamin $D_3$).

  3. The liver converts Cholecalciferol into Calcidiol.

  4. The kidneys convert Calcidiol into Calcitriol.

• Active Vitamin D: Calcitriol is the active form of Vitamin D. It is essential for the absorption of calcium ($Ca$) and phosphorus ($P$) from the intestines. Without Vitamin D, these minerals remain in the intestinal tract (effectively "outside" the body) and cannot be utilized.

• Fortification: Common drinks like milk and orange juice are fortified with Vitamin D to assist in mineral absorption.

• Clinical Implications:

  • Rickets: Occurs in children with Vitamin D deficiency. It results in decreased bone density and "bowed" legs, making it difficult for the skeleton to support the body's weight.

  • Osteomalacia: A softening of the bones in adults and the elderly due to Vitamin D deficiency.

  • Osteoporosis: A condition involving decreased bone density. Females are more prone due to a drop in estrogen during menopause. Estrogen normally stimulates osteoblasts (bone-building cells). When estrogen is low, osteoclasts (bone-breaking cells) become relatively more active. Males experience a similar but slower process called andropause related to decreasing testosterone level. Weight-bearing exercises help prevent this by putting stress on the bone to increase strength.

Sensory Reception and Structures

• Receptor Types:

  • Mechanoreceptors: Detect physical touch and movement. Each hair follicle is wrapped in sensory nerves to detect movement (e.g., an insect landing on the skin).

  • Thermoreceptors: Detect changes in temperature.

  • Free Nerve Endings (Nociceptors): Detect pain and tissue damage. In deep burns, these endings are destroyed, which is why severe burns can become painless.

• Specific Tactile Receptors:

  • Meissner's Corpuscles: Located superficially in the dermis; they detect light touch and changes in texture or shape. These are crucial for the ability to read Braille via fingertips.

  • Merkel's Disks: Found in the deepest layer of the epidermis (stratum basale). They provide information on pressure, position, shapes, and edges. A mnemonic for these is "Steve Urkel" from the show Family Matters; just as Urkel's large glasses identify shapes and edges, these disks detect those tactile details. There are approximately $750$ Merkel cells per $\text{cm}^2$ in the fingertips.

  • Pacinian Corpuscles: Located deep within the dermis. They require gross pressure to activate and are sensitive to deep pressure and vibrations.

• Neural Processing: The hands and fingertips occupy a disproportionately large area of the somatosensory cortex in the brain compared to other body parts, allowing for extreme tactile sensitivity.

Layers of the Skin

• Hypodermis (Subcutaneous Layer): The layer beneath the skin. While sometimes referred to as the third layer of the skin, the name "subcutaneous" literally means "below the skin."

• Dermis: The vascular layer of the skin containing connective tissue, blood vessels, and nerves. It has two sub-layers:

  • Papillary Layer: The thinner, superficial layer containing dermal papillae (projections that dip into the epidermis).

  • Reticular Layer: The thicker, deeper layer of the dermis.

• Epidermis: The most superficial, avascular layer composed of stratified squamous epithelial tissue. It relies on the diffusion of oxygen and nutrients from the vascular dermis below.

Microscopic Anatomy of the Epidermis

• Cell Types:

  • Keratinocytes: The most abundant cells in the epidermis. They produce the protein keratin. As they move superficially, they flatten, fill with keratin (cornification), and eventually die.

  • Basal Stem Cells: Cuboidal stem cells located in the stratum basale. They undergo continuous mitosis to produce new keratinocytes.

  • Melanocytes: Located in the stratum basale. They produce melanin (black/brown or reddish pigment) and package it into melanosomes, which are distributed to keratinocytes. Melanin hovers over the cell nucleus like a shield to protect DNA from UV-induced mutations.

  • Langerhans Cells (Dendritic Cells): Specialized immune cells that act as macrophages. They are Antigen-Presenting Cells (APCs); after phagocytizing a pathogen, they present pieces of it (antigens) to T cells to recruit an immune response.

• The Five Layers (Deep to Superficial): Use the acronym "Boys Say Girls Look Cute."

  1. Stratum Basale: A single layer of basal cells, melanocytes, and Merkel cells. Separated from the dermis by the basement membrane.

  2. Stratum Spinosum: Eight to ten layers thick; appears "spiny" due to desmosomes (cell junctions) that interlock to strengthen the bond between cells.

  3. Stratum Granulosum: Three to five layers of flatter cells. It has a grainy appearance as cells generate large amounts of keratin.

  4. Stratum Lucidum: A smooth, translucent layer of dead, flattened keratinocytes. It is only found in thick skin (palms of hands, soles of feet, and digits).

  5. Stratum Corneum: The most superficial layer exposed to the environment. It consists of dry, dead, keratinized cells that protect against abrasion and dehydration.

• Epidermal Ridges: These are portions of the epidermis that dip down into the dermis. They increase surface area and friction, forming the basis of fingerprints and enhancing grip.

Glandular Secretions

• Eccrine Sweat Glands: Distributed all over the body. Their ducts lead directly to the skin surface. They produce watery sweat primarily for thermoregulation.

• Apocrine Sweat Glands: Located in specific areas (axillary/armpits and pubic regions). Their ducts empty into hair follicles. They produce a thicker sweat that bacteria break down, resulting in body odor (BO).

• Mammary Glands: Modified apocrine glands that secrete milk. They contain both merocrine and apocrine components, with the apocrine portion providing the fat content of the milk.

• Ceruminous Glands: Modified apocrine glands that produce earwax.

• Sebaceous Glands: Located in the dermis; they secrete sebum, an oily substance that lubricates the skin and hair.

Pigmentation and Pathophysiology

• Pigment Types: Eumelanin (black/brown) and Pheomelanin (reddish color of freckles and red hair).

• Tanning: UV radiation stimulates keratinocytes to release chemicals that trigger melanocytes to produce more melanin, darkening the skin as a defense mechanism.

• Albinism: A genetic disorder where a DNA mutation prevents the production of melanin, even though melanocytes are present. Characteristics include:

  • Strabismus: Crossed eyes or eyes that do not move together.

  • Nystagmus: Bouncing or involuntary movement of the eyes.

  • Photophobia: Extreme sensitivity to light because the retina lacks melanin to absorb excess light, causing light to bounce around inside the eye.

  • Astigmatism: Blurred vision caused by multiple focal points on the retina.

  • Cancer Risk: Higher susceptibility to skin cancer because the lack of melanin allows UV radiation to damage DNA, specifically affecting tumor suppressor genes and proto-oncogenes.

  • Ageing: UV damage can destroy fibroblasts in the dermis. Since fibroblasts produce collagen and elastic fibers, their destruction leads to a loss of support for the epidermis, resulting in wrinkles.

Questions & Discussion

• Question: In an individual with osteoporosis who is low in Vitamin D and has no thyroid, why would they be prescribed Calcitriol (Calcitron)? How does it work to keep bones strong?

• Response: Calcitriol functions as the active form of Vitamin D. Its mechanism involves signaling the cells of the body to produce specific proteins that bind to calcium. This binding allows the cells in the intestines to absorb the calcium from the diet/medication and transport it into the body. Essentially, it facilitates the uptake of the mineral necessary for bone building.

• Question: Which layer is someone touching when they touch their own skin?

• Response: That is the stratum corneum, which is the most superficial layer of the epidermis (stratified squamous epithelial tissue).

• Question: Where is the border between the dermis and epidermis on a histology slide?

• Response: The border is the line where the tightly packed epithelial cells of the epidermis meet the more randomly arranged connective tissue and extracellular matrix (collagen and elastic fibers) of the dermis. The cells right at this border are typically the basal stem cells.