A&P Ch 5 The Integumentary System
The Integumentary System
Overview of the Integumentary System
Definition: The integumentary system, or integument, is the largest system of the body.
It comprises approximately 16\% of total body weight.
It covers an average surface area of 1.5 to 2 square meters.
Components:
Cutaneous Membrane (Skin):
Epidermis: The superficial layer, composed of epithelium.
Dermis: The deep layer, composed of connective tissue.
Accessory Structures: Includes hair, hair follicles, nails, and exocrine glands.
Additional Components: The integument contains an extensive network of blood vessels and sensory receptors.
Subcutaneous Layer (Hypodermis):
A layer of loose connective tissue located deep to the cutaneous membrane.
It serves to separate the integument from the deep fascia surrounding underlying organs (e.g., muscles and bones).
The Epidermis
Structural Features:
The top, superficial layer of the skin.
Composed of stratified squamous epithelium.
Avascular: Lacks its own blood supply; nutrients and oxygen diffuse from capillaries located in the dermis.
Keratinocytes:
The primary cells of the epidermis.
Contain large quantities of keratin, a tough, fibrous protein responsible for the skin's protective properties.
Types of Skin:
Thin Skin:
Covers most of the body surface.
The epidermis has four distinct layers (strata).
Thick Skin:
Covers the palms of the hands and soles of the feet.
The epidermis has five distinct layers (strata).
Five Strata (Layers) of the Epidermis (from superficial to deep):
Stratum Basale (Stratum Germinativum):
The deepest epidermal layer, attached to the basement membrane by hemidesmosomes.
Contains basal cells (germinative cells), which are stem cells responsible for dividing and producing new keratinocytes.
Features epidermal ridges, which are projections that interlock with dermal papillae in the dermis.
This interlocking mechanism increases the surface area for attachment between the dermis and epidermis, enhancing structural integrity.
The contour of the skin surface aligns with the pattern of these epidermal ridges.
In thick skin, these ridges are visible as fingerprints, which increase surface area for improved grip and are genetically unique.
Specialized cells of the stratum basale:
Tactile (Merkel) cells: Found in hairless skin; these cells, along with sensory nerve endings, form tactile discs, acting as sensory receptors for touch.
Melanocytes: Cells that synthesize the pigment melanin. They are distributed throughout the stratum basale, and their processes extend into more superficial layers.
Stratum Spinosum:
Located superficial to the stratum basale.
Consists of 8 to 10 layers of keratinocytes, which are interconnected by desmosomes.
Keratinocytes in this layer originate from basal cell division in the stratum basale; some continue to divide within the stratum spinosum.
Contains dendritic (Langerhans) cells, which are immune cells that defend against microorganisms and play a role in preventing skin cancers.
Stratum Granulosum:
A thin layer superficial to the stratum spinosum.
Comprises 3 to 5 layers of keratinocytes.
Cells from the stratum spinosum are pushed into this layer as new cells are continually produced beneath them.
Stratum Lucidum:
A layer of dead keratinocytes found only in thick skin.
It is superficial to the stratum granulosum.
Stratum Corneum:
The outermost, top layer of the skin, directly exposed to the environment.
Consists of 15 to 30 layers of keratinized cells.
Keratinization: The process of forming protective layers of cells filled with keratin.
New cells generated in the stratum basale move upward, reaching the stratum corneum in approximately 7 to 10 days.
Exposed cells in the stratum corneum are shed after another 2 weeks.
Water Loss from Skin:
The stratum corneum provides water resistance but is not completely waterproof.
Insensible Perspiration: The continuous loss of water that diffuses across the stratum corneum and evaporates from the skin surface.
Approximately 500 ext{ mL} of water is lost per day through this process.
It is an unnoticeable and unpreventable form of fluid loss.
Fluid loss significantly increases if the stratum corneum is damaged (e.g., in cases of severe burns).
Sensible Perspiration: Water loss through the excretion of sweat from sweat glands.
This mechanism is crucial for cooling the body and regulating body temperature.
Epidermal Growth Factor (EGF):
A peptide growth factor that stimulates epithelial cell growth.
It is produced by the salivary glands and glands of the duodenum.
Laboratory Application: EGF is utilized in tissue culture to promote the growth of skin grafts.
Functions of EGF:
Promotes the division of basal cells.
Accelerates the production of keratin.
Stimulates the development and repair processes within the epidermis.
Stimulates secretions from various epithelial glands.
The Dermis
Structures and Functions:
The deeper layer of skin, situated beneath the epidermis.
It firmly anchors epidermal accessory structures, such as hair follicles and sweat glands.
Contains an intricate network of blood vessels and nerves.
Layers of the Dermis:
Papillary Layer:
The superficial layer of the dermis, located directly beneath the epidermis.
Composed of areolar tissue.
Contains capillaries, lymphatic vessels, and sensory neurons.
Named for the dermal papillae, which are projections that interlock with the epidermal ridges.
Dermatitis: An inflammation of the dermis.
Can be caused by infections, radiation, mechanical irritation, or exposure to chemicals (e.g., poison ivy).
May manifest as itching or pain.
Reticular Layer:
The deeper layer of the dermis, situated above the subcutaneous layer.
Consists of dense irregular connective tissue, which is rich in collagen and elastic fibers.
Its fibers extend and connect the reticular layer to both the overlying papillary layer and the underlying subcutaneous layer, providing strong structural linkage.
Dermal Strength and Elasticity:
Collagen Fibers:
Possess high tensile strength; they can bend easily but do not stretch significantly.
They contribute strength and limit the flexibility of the skin, helping to prevent tissue damage.
Elastic Fibers:
Provide the skin with its ability to stretch and recoil to its original shape.
Crucial for skin flexibility.
Skin Turgor: Refers to the strength and flexibility of the skin, which is largely dependent on its water content.
Tension Lines (Cleavage Lines):
Patterns formed by parallel bundles of collagen and elastic fibers within the dermis.
These lines allow the skin to effectively resist applied forces.
Clinical Significance:
A surgical incision made parallel to tension lines typically heals well with minimal scarring.
A cut made perpendicular to tension lines tends to pull open, leading to more pronounced scarring.
Dermal Blood Supply:
Cutaneous Plexus: A deep network of blood vessels located along the border between the reticular layer of the dermis and the subcutaneous layer.
Subpapillary Plexus: A network of smaller vessels situated within the papillary layer of the dermis.
Contusion (Bruise): Results from damage to the blood vessels within the dermis, causing blood to leak into surrounding tissues.
The Subcutaneous Layer (Hypodermis)
Description:
Also known as the hypodermis.
It stabilizes the position of the skin and connects the dermis to underlying tissues (e.g., muscles).
Important Note: The subcutaneous layer is not considered a part of the skin (integumentary system) itself.
Composition: Primarily composed of adipose tissue.
Subcutaneous Fat: Serves multiple functions including energy storage, thermal insulation, and physical padding.
Vascularity: Contains large arteries and veins, acting as a significant reservoir for blood.
Clinical Relevance: This layer is a common site for subcutaneous injections, administered using hypodermic needles.
Skin Color
Factors Accounting for Individual Differences in Skin Color:
Role of Epidermal Pigmentation:
Carotene:
An orange-yellow pigment.
Often found in orange-colored vegetables.
Accumulates in epidermal cells, the dermis, and the subcutaneous layer.
Can be converted into vitamin A, which is essential for the maintenance of epithelia and the synthesis of photoreceptor pigments in the eye.
Melanin:
A group of pigments that can be red-yellow (pheomelanin) or brown-black (eumelanin).
Synthesized by melanocytes.
Stored in intracellular vesicles called melanosomes, which are then transferred to keratinocytes.
Individuals with darker skin tones possess larger and more numerous melanosomes.
The number of melanocytes and the rate of melanin production can vary across different regions of the body.
Protective Function: Melanin provides crucial protection to the skin from harmful ultraviolet (UV) radiation, which can damage DNA and lead to skin cancer.
Sunlight Response: Melanin synthesis accelerates significantly upon exposure to sunlight, leading to tanning.
Albinism: A genetic condition characterized by a deficiency in melanin production, resulting in a lack of pigment in skin, hair, and eyes.
Role of Dermal Circulation: Blood flow and the level of oxygenation within the dermis significantly influence skin color.
Hemoglobin: The protein in red blood cells that carries oxygen.
It appears bright red when bound to oxygen.
When blood vessels in the dermis dilate (widen), increased blood flow causes the skin to redden.
Conversely, when dermal blood vessels constrict (narrow), reduced blood flow makes the skin appear pale.
Hemoglobin turns dark red when it is not carrying oxygen.
Cyanosis: A bluish discoloration of the skin resulting from a significant reduction in local blood supply and oxygen levels.
Can be caused by conditions such as extreme cold, heart failure, or severe asthma.
Disease-Related Changes in Skin Color:
Jaundice: A yellowing of the skin and the whites of the eyes, occurring when the liver is unable to excrete bile, leading to a buildup of yellow pigment in the body.
Pituitary Tumor: Some tumors of the pituitary gland can result in an excess production of melanocyte-stimulating hormone (MSH), causing the skin to darken.
Addison's Disease: A disorder that causes the pituitary gland to release excess adrenocorticotropic hormone (ACTH), which has a structure and effect similar to MSH, leading to skin darkening.
Vitiligo: An autoimmune condition that causes the progressive loss of melanocytes, resulting in characteristic white patches on the skin.
Clinical Note: Skin Cancer
Basal Cell Carcinoma:
Originates in the stratum basale layer of the epidermis.
The most prevalent type of skin cancer, primarily caused by prolonged exposure to UV radiation.
Typically does not metastasize (spread to other parts of the body).
Squamous Cell Carcinoma:
Affects the squamous cells within the epidermis of the skin.
Caused exclusively by exposure to UV radiation.
Similar to basal cell carcinoma, it generally does not metastasize.
Malignant Melanoma:
An aggressive form of cancer that originates from melanocytes.
Metastasis (spread) is common and can be rapid.
Signs to look for (ABCDE rule): Asymmetry (A), irregular Border (B), mottled or varied Color (C), Diameter Larger than 6 ext{ mm} (D), and Evolution or changing size, shape, and color (E).
Vitamin D\text{3}
Interaction between Sunlight and Vitamin D\text{3} Production:
Production: Epidermal cells produce Vitamin D\text{3} (cholecalciferol) when exposed to ultraviolet (UV) radiation from sunlight.
Conversion to Calcitriol: The liver and kidneys collectively convert Vitamin D\text{3} into calcitriol, its active form.
Role of Calcitriol: Calcitriol is absolutely essential for the absorption of calcium and phosphate ions by the small intestine.
Rickets:
A disease caused by insufficient Vitamin D\text{3} levels.
Results in abnormally weak and flexible bones that may bend under the weight of the body.
Also leads to other structural changes in the skeleton due to impaired bone development.
Hair
Description:
A nonliving, long, cylindrical accessory structure.
Produced by a hair follicle.
Originates deep within the dermis and projects through the epidermis to the skin surface.
Covers almost the entire body, with notable exceptions including the lips, palms of the hands and sides of fingers, soles of the feet and sides of toes, and portions of the external genitalia.
Functions of Hair:
Protection, Cushioning, and Insulation: Helps protect the scalp from UV radiation, cushions light impacts, and provides thermal insulation.
Guard Openings: Prevents foreign particles and insects from entering body openings (e.g., nose, ears, eyes via eyelashes).
Sensory Receptors: Serves as sensory receptors due to the presence of nerve endings around the follicle.
Hair Follicles:
Structures that originate deep in the dermis.
Each follicle is encased in a dense connective tissue sheath.
The base of the follicle is surrounded by sensory nerves, forming a root hair plexus, which allows for sensation when the hair is moved.
Arrector Pili Muscle: An involuntary smooth muscle attached to the hair follicle.
Its contraction causes hairs to stand upright (piloerection).
This action results in the formation of "goose bumps."
Regions of the Hair:
Hair Root: The portion of the hair that anchors it within the skin.
It begins at the base of the hair (hair bulb) and extends approximately halfway to the skin surface.
Hair Shaft: The portion of the hair that extends from the end of the hair root and forms the protruding, visible part of the hair.
Hair Structure (Layers from central to surface):
Medulla: The central core of the hair.
Cortex: The intermediate layer, surrounding the medulla.
Cuticle: The outermost, surface layer of the hair.
Hair Production:
Begins at the hair bulb, located at the base of the hair follicle.
The hair bulb surrounds the hair papilla, which is a connective tissue containing capillaries and nerves essential for hair growth.
At the base of the hair bulb, epithelial cells form a layer known as the hair matrix.
Basal cells in the hair matrix continually divide to produce new hair cells.
As new cells are produced, existing hair cells are pushed upward and out of the skin.
During this upward movement, the cells keratinize (fill with keratin) and subsequently die.
Hair Growth Cycle: The cyclical process of hair growth and shedding.
Cells of the hair root absorb nutrients and incorporate them into the growing hair structure.
During the growth phase, the hair root remains firmly attached to the matrix of the follicle.
At the conclusion of the growth cycle