Integumentary System: Skin and Appendages

• Integuments are the skin's natural outer covering.
• Protective, sensory, temperature regulation, excretory, storage, and identification functions are served.
• Protection is provided as a physical barrier against tissue damage.
• Tactile, painful, and thermal stimuli affect receptors and free nerve endings.
• The amount of blood flow in cutaneous vessels regulates temperature, especially concerning heat loss.
• Excretion occurs through the secretion of water and salt in sweat.
• Vitamin D precursors are stored; in sunlight, precursors of Vitamin D (7-dehydrocholesterol) convert to cholecalciferol, or active Vitamin D.
• Fingerprints are unique to each individual.
• The integumentary system comprises skin and skin appendages.
• Skin is the human body's largest organ.
• Sweat glands, sebaceous glands, hairs, and nails are considered skin appendages.
• The epidermis is avascular.
• It consists of stratified squamous keratinized epithelium of variable thickness.
• All but the deepest layers are composed of dead cells.
• Depending on location, the epidermis may have four to five layers.
• Palms and soles have five layers because they are subject to the most friction.

Layers of the Epidermis

• Stratum basale is a single layer in contact with the dermis, and contains four cell types.
  • Keratinocytes: produce keratin, which toughens and waterproofs the skin.
  • Melanocytes: synthesize the pigment melanin, which protects against UV radiation and light.
  • Tactile cells (Merkel cells): aid in tactile reception.
  • Non-pigmented granular dendrocytes (Langerhans cells): protective macrophage cells that ingest bacteria and debris.
• Stratum spinosum consists of several stratified layers.
  • Keratinocytes have a spiny appearance due to shape changes.
  • Limited mitosis occurs.
• Stratum granulosum consists of only 3-4 flattened rows of cells.
  • Cells appear granular due to keratohyaline granules.
• Stratum lucidum's nuclei, organelles, and cell membranes are no longer visible, making it appear clear.
  • Exists only in lips and thickened skin on soles and palms.
  • Contains a translucent substance called eleidin.
  • Glans penis and clitoris are examples.
• Stratum corneum consists of 25-30 layers of flattened, scale-like anucleated cells that are continuously shed.
  • It is the cornified surface and the skin's protective layer.
  • Cornification occurs through keratinization and hardening as cells die and are pushed to the surface.
• Skin color results from a combination of three pigments: melanin, carotene, and hemoglobin.

Melanin

– brown-black pigment produced by melanocytes in the stratum basale.

• It guards the skin against UV rays.
• Gradual sun exposure increases melanin production, leading to tanning.
• Skin defects include:
  • Albino: X chromosome defect with normal melanocytes but lacks tyrosinase.
  • Freckles: Aggregated patches of melanin.
  • Vitiligo: Lack of melanocytes in specific areas, causing white spots.

Carotene

• The source is from carrots.
• It imparts a yellowish pigment found in epidermal cells and the dermis' fatty parts.
• Abundant in Asians.
• Combination with melanin contributes to yellowish-tan skin tone.

Hemoglobin

• Not a pigment of the skin.
• It is an oxygen-binding pigment in red blood cells.
• Oxygenated blood flowing through the dermis results in pinkish skin tones.

Surface Patterns

• Congenital Patterns: inborn or acquired.
  • Includes fingerprints and friction ridges present on palms and soles.
  • Formed by the pull of elastic fibers within the dermis, preventing slippage when grasping objects.
• Acquired Lines:
  • Deep flexion creases are found on the palms.
  • Shallow flexion lines appear on knuckles and other joints.
• Furrows of the forehead and face (wrinkles) are acquired from continual facial muscle contractions.
• Langer Lines consist of collagen fibers that present lines of tension oriented in a non-random pattern.
  • Surgical incisions made parallel to these lines promote better wound healing.

Dermis

• Deeper and thicker than the epidermis.
• A strong, stretchy envelope aiding to hold the body together.
• Blood vessels nourish the epidermis.
• Contains numerous collagenous, elastic, and reticular fibers that support the skin.
• Highly vascular and glandular with many nerve endings and hair follicles.
• Papillary Layer which makes your fingerprints:
  • It is in contact with the epidermis.
  • Accounts for approximately 1/5 of the dermis.
  • Dermal papillae are numerous projections extending into the epidermis.
  • Dermal papillae contain capillary loops, which supply nutrients to the epidermis.
  • Some papillae house pain and touch receptors (Meissner's Corpuscles).
• Reticular Layer:
  • The deepest skin layer.
  • Contains blood vessels, sweat, and oil lands and some deep pressure receptors (Paccinian corpuscles).
  • Many phagocytes engulf bacteria penetrating the epidermis.

Cutaneous Glands

• Exocrine glands that release secretions via ducts to the skin's surface.
  • Sebaceous glands exist
  • Sweat glands exist
• Sebaceous Glands
  • All over the skin, excluding palms and soles.
  • Ducts typically empty into hair follicles or directly onto the skin.
  • Sebum: a secretion consisting of oily substances.
  • Fragmented cells keep skin soft, moist, and prevent brittleness.
  • Sebum contains antibacterial chemicals.
• Sudoriferous Glands
  • Distributed widely, especially on palms, soles, axillary, and pubic areas.
  • Secretions evaporate to cool the body.
  • Primary function is reflex regulation of body temperature through surface water evaporation.
  • Merocrine (Eccrine):
  • Found all over, mostly on the body, forehead, back, palms, and soles. -Secretions reach the skin via ducts that open directly onto the surface through sweat pores.
  • Apocrine;
  • Larger glands in axillary and pubic regions that empty into hair follicles.
  • Only functional after puberty.
  • A thick secretion of organic substances broken down by bacteria, which causes body odour.
  • Odoriferous secretion, a subconscious sexual attractant.

Mammary Glands

• Located within the breasts.
• They are specialized sudoriferous (sweat) glands that secrete milk during lactation.
• Under the stimulus of the pituitary glands.

Hair

• Characteristic of all mammals, but differs in distribution, function, density, and texture.
• Humans are relatively hairless except for the scalp, face, pubis, and axilla.
• Men have more obvious hair due to male hormones.
• Certain areas are hairless: palms, soles, lips, nipples, penis, and labia minora.
• Eyelashes live for 3-4 months; scalp hair lives for 3-4 years.
• Protection is the primary function.
• Example where the scalp, hair, and eyebrows protect from sunlight.
• Parts of the Hair
  • Shaft: The visible and dead portion projecting above the skin.
  • Root: Enclosed in the follicle.
  • Hair Bulb Matrix: The growth zone containing melanocytes that color the hair.
• Layers of Hair in Cross-Section:
  • Medulla: Inner part.
  • Cortex: Thick middle part. -Cuticle: Covers the cortex, forming a toughened outer portion.
• Three Distinct Kinds of Hair in Humans:
  • Lanugo: Fine, silky fetal hair appearing in the last trimester of development, usually only evident in premature newborns.
  • Angora: Grows continuously, such as scalp hair, and hair on the face of males.
  • Definitive: Grows to a certain length and then ceases, which includes eyebrows, eyelashes, pubic and axillary hair.
• Sebaceous glands and arrector pili muscles attach to hair follicles.
• Muscle contraction due to thermal or psychological stimuli pulls the hair follicle upright, causing goosebumps.

Nails

• Found on the distal dorsum of fingers and toes.
• Formed from a hardened, transparent stratum corneum of the epidermis.
• Components:
  • Nail Body: Visible attached portion.
  • Nail Root: Covered by skin.
  • Free Edge: Distal exposed border attached to the undersurface of the hyponychium.
  • Hyponychium: Beneath the free edge.
  • Eponychium (Cuticle): Covers the nail root.
  • Nail Matrix: Proximal part of the nailbed where growth occurs.
  • Lunula: Whitish crescent-shaped area at the proximal aspect of the nail.
• Made of transparent and nearly colourless, appear pink due to rich supply of blood in the dermis below.