Skin Glands and Structures

• An organ comprises different tissues performing specific functions.
• A system consists of organs working together to perform specific functions.
• Dermatology is the medical specialty focused on skin disorders.
• Skin, identified as an organ, is the largest in the body by surface area and weight.
• The integumentary system includes the skin and its accessory structures (hair, nails, oil, and sweat glands).
• The skin is also known as the cutaneous membrane or the integument.

Quick Overview of Skin

• The epidermis is the superficial layer of the skin, the dermis is the underlying layer, followed by the subcutaneous layer (hypodermis).
• The epidermis contains keratinized stratified squamous epithelium.
• The epidermis layers from superficial to deep are: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale/germinativum.
• The dermis consists of a papillary region (more superficial, areolar connective tissue) and a reticular region (deeper, dense irregular connective tissue).
• The subcutaneous layer (hypodermis) is not part of the skin and binds the skin to underlying tissues and organs; composed of areolar connective tissue and adipose tissue.

Functions of Skin

• The skin regulates body temperature through sweat evaporation and redirection of blood flow.
• The skin acts as a blood reservoir, holding 8-10% of the body’s total blood flow at rest.
• The skin protects against abrasion, UV light, heat, microbes, chemicals, and dehydration.
• Dendritic/Langerhans cells in the epidermis and macrophages in the dermis contribute to immune protection.
• The skin provides cutaneous sensation via receptors and nerve endings for touch, pressure, vibration, temperature, and pain.
• The skin excretes sweat (water, ions, organics, nitrogenous wastes) and absorbs lipid-soluble substances (vitamins A, D, E, K; CO2, O2, solvents).
• The skin synthesizes vitamin D through a multistep process involving the skin, liver, and kidneys, requiring UV light.
• Calcitriol, the most active form of vitamin D, facilitates calcium absorption in the gut.

Epidermal Cells

• The keratinized stratified squamous epithelium consists of keratinocytes, melanocytes, dendritic cells, and tactile epithelial cells; keratinocytes make up about 90% of the epidermis.
• Keratinocytes produce keratin and lamellar granules.
• Keratin protects against heat, microbes, and chemicals.
• Lamellar granules release a water repellent, preventing dehydration.
• Melanocytes, approximately 8% of the epidermis, produce melanin, which absorbs UV light to protect DNA.
• Melanin can be yellow-red or brown-black.
• Dendritic cells, about 5% of epidermal cells, originate from bone marrow and help immune cells recognize and destroy invaders.
• Tactile epithelial cells (Merkel cells) are the least numerous cells and are located in the stratum basale, contacting tactile/Merkel discs for tactile sensation.

Layers of Epidermis

• The epidermis is of ectodermal embryological origin and is avascular.
• The stratum corneum is the most superficial layer (25-30 layers of flat, dead keratinocytes filled with keratin) providing excellent protection and is constantly shed and replaced.
• The stratum lucidum is found only in thick skin (fingertips, palms, soles) and consists of 4-6 layers of clear, flat, dead keratinocytes filled with keratin.
• The stratum granulosum (transition layer) contains 3-5 layers of flattened cells undergoing apoptosis, with keratohyalin and lamellar granules.
• Lamellar granules release a water-proofing agent.
• The stratum spinosum contains 8-10 layers of mostly keratinocytes, melanocyte processes, and intraepidermal cells, joined by desmosomes; cells shrink during microscopy preparation, creating a spiny appearance.
• The stratum basale/germinativum is the deepest layer, containing a single layer of cuboidal/columnar keratinocytes, some stem cells, melanocytes, intraepidermal macrophages, and tactile epithelial cells connected by desmosomes and hemidesmosomes.

Keratinization and Growth of the Epidermis

• Keratinization is a form of apoptosis where keratinocytes fill with keratin, die, and slough off.
• Keratin intermediate filaments are reorganized into keratin by keratohyalin.
• Cells accumulate keratin as they move toward the skin's surface.
• Dead, keratinized cells create the superficial layers starting with the stratum granulosum and eventually slough off.
• The process takes 7-10 weeks in average epidermis (0.1mm) as epithelium lacks blood vessels (is avascular).

Dermis

• The dermis is the dried, treated animal skin; it has a superficial papillary region and reticular region.
• The papillary region is ~1/5 of the dermis and has areolar CT with collagen and elastic fibers.
• Dermal papillae increase surface area and have capillary loops, corpuscles of touch (Meissner's corpuscles), and free nerve endings.
• The reticular region is deeper and has dense irregular CT, consisting of thick collagen bundles with few coarse elastic fibers.
• Fibroblasts, macrophages, and adipocytes are near the subQ, and hair follicles and glands are embedded.

Epidermal Ridges

• The skin of the palms, fingers, and toes has epidermal ridges that complement dermal papilla.
• Epidermal ridges strengthen bond between dermis and epidermis and increase friction and grip.
• They also increase the number of touch receptors and increase sensitivity.

Thick vs. Thin Skin

• Thin skin is everywhere except the palms, palmar surface of digits, and soles.
• Thick skin is on the palms, palmar surface of digits, and soles.
• Thickness of thin skin: 0.1-0.15 mm.
• Thickness of thick skin: 0.6-4.5 mm.
• Thin skin has no stratum lucidum; the stratum spinosum and corneum are thinner.
• Thick skin has a stratum lucidum and a thicker stratum spinosum and corneum.
• Epidermal ridges are less organized in thin skin.
• Epidermal ridges are present and well-developed in thick skin.
• Hair follicles and arrector pili are present in thin skin and absent in thick skin.
• Sebaceous glands are present in thin skin and absent in thick skin.
• Sudoriferous glands are fewer in thin skin and more numerous in thick skin.
• Sensory receptors are fewer in thin skin and more dense in thick skin.

Structural Basis of Skin Color

• Melanin is yellow-red or brown-black and is made by melanocytes from tyrosine, and transferred to keratinocytes; freckles and age spots are accumulation of melanin.
• Carotene is yellow-orange, a precursor to vitamin A, and accumulates in the stratum corneum, the fat of the dermis, and subcutaneous layer.
• Hemoglobin is the red-burgundy pigment in red blood cells, and iron in heme groups binds O2.

Skin Color as a Diagnostic Clue

• Albinism: white-pink skin due to a lack of melanin.
• Vitiligo: irregular white spots of skin due to immune system malfunction.
• Cyanotic: bluish tint in nail beds and skin due to lack of O2.
• Jaundice: yellow in whites of eyes and skin due to increased bilirubin from bile breakdown.
• Erythema: redness of skin due to engorgement of dermal blood vessels from injury or inflammation.
• Pallor: paleness of skin due to shock or anemia.

Accessory Structures of the Skin

• Hair, glands, and nails are epidermal derivatives.

Skin Glands

• The four main types of exocrine glands are sebaceous, sudoriferous (eccrine and apocrine), ceruminous, and mammary.
• Sebaceous (oil) glands are simple branched acinar glands that secrete into hair follicles; they are absent in palms/soles and activated during puberty.
• Sebum prevents hair from becoming brittle, softening skin, decreases water loss, and inhibits the growth of some bacteria.
• Sudoriferous (sweat) glands have two types: Eccrine and Apocrine; both are simple, coiled tubular, and merocrine glands.
• Eccrine sweat glands function throughout life, stimulated by temperature and stress, that secrete watery solutions, and are more abundant than apocrine glands.
• Apocrine sweat glands function after puberty, stimulated by sex hormones and stress, that secrete viscous solutions, and are located in the axilla and groin areas.
• Ceruminous glands are modified sweat glands in the external ear that secrete cerumen/wax, which traps foreign material and microbes.

Hair and Nails

• Hair is a column of dead keratinized cells that has associated sebaceous/oil and arrector pili muscles.
• Nails are packed, hard, keratinized epidermal cells that grow at the root.
• Arrector pili are smooth muscles (from ectoderm) that cause goosebumps.

Skin Wound Healing

• Epidermal wound healing occurs in response to superficial damage (abrasion, minor burn).
• Basal cells migrate to the injury site stimulated by epidermal growth factor and stop migrating when they contact other epidermal cells (contact inhibition).
• Deep wound healing occurs in injuries that extend into the dermis, even subQ, and has 4 phases.
• Inflammatory phase: blood clot forms, inflammation occurs to eliminate microbes, and mesenchymal cells develop into fibroblasts.
• Migratory phase: the clot forms a scab closing off the wound, epithelial cells migrate beneath the scab, and fibroblasts create collagen.
• Proliferative phase: growth of epithelium below the scab, with continued collagen production.
• Maturation phase: loss of scab, collagen fibers organize, blood vessels return to normal.
• Scar tissue is densely arranged collagen with less elasticity and fewer blood vessels.

Systemic Effects of Burns

• Systemic effects of burns are loss of water and blood, leading to shock.
• Other effects are bacterial infection, reduced blood circulation, decreased urine production, and a diminished immune response.

Explore eccrine vs. apocrine glands, sebaceous secretions, and stress-related gland stimulation. Discover sebum's role in skin homeostasis, epidermal ridge functions, and thick vs. thin skin differences. Examine hair follicle and gland embedding in the dermis and composition of the reticular region.