Integumentary System
Integumentary System: Overview and Key Roles
Primary protective function: provides protection to the body; keratin strengthens tissues; epidermis forms a water-resistant barrier to prevent dehydration and slow water loss.
UV protection: melanin in the epidermis is a brown pigment that blocks UV radiation, which can damage DNA and is a major cause of skin cancer; melanin also protects nutrients in the blood from sunlight damage.
Temperature regulation: skin regulates body temperature via two mechanisms:
Evaporative cooling: sweat (a watery secretion) is produced and evaporates from the skin surface, taking heat with it.
Vasodilation: increasing blood flow through the dermis carries heat away from the body.
Sensation: the skin contains somatosensory receptors that allow us to sense the external environment; hair follicle receptors detect hair movement; other receptors provide touch, temperature, pain, and itching sensations.
Vitamin D synthesis: photochemical synthesis requires UVB radiation; epidermal cells convert cholesterol to cholecalciferol (vitamin D). Vitamin D is fat-soluble and often supplemented because dietary intake and sun exposure alone may be insufficient.
Tissue organization and layers: skin is organized into three layers – epidermis (surface), dermis (deep to epidermis), and hypodermis (deepest).
Epidermis: keratinized stratified squamous epithelial tissue.
Dermis: superficial layer of loose areolar connective tissue and a deeper layer of dense irregular connective tissue.
Hypodermis (subcutaneous layer): adipose tissue.
Dermal papillae: upward projections of the dermis containing capillaries that supply nutrients to epidermal cells and remove waste.
Blood supply: capillaries in the dermal papillae transport nutrients to epidermal cells and remove waste.
Function summary: protection, temperature regulation, sensation, vitamin D synthesis, and aesthetic/biochemical signaling roles (e.g., pigmentation, hair growth).
Skin Structure: Epidermis, Dermis, and Hypodermis
Epidermis: keratinized stratified squamous epithelium.
Dermis: two layers – papillary layer (loose areolar connective tissue) and reticular layer (dense irregular connective tissue).
Hypodermis: adipose tissue.
Dermal papillae: projections that increase surface area for capillaries and nutrient exchange.
Epidermis: Layers, Cells, and Histology
Thin skin generally has four layers; thick skin has five layers (addition of stratum lucidum).
Stratum basale (also called stratum germinativum):
Contains stem cells that divide to produce keratinocytes.
Melanocytes are present; tactile discs (Merkel cells) are also found here.
Basal cells give rise to newly formed keratinocytes.
Stratum spinosum: contains metabolically active keratinocytes; spiny appearance due to desmosome attachments and dehydration during histology prep.
Keratinocytes in this layer accumulate melanin granules from melanocytes.
Langerhans cells (dendritic leukocytes) are distributed throughout stratum spinosum and sometimes in nearby dermis; they defend against infection.
Stratum granulosum: keratinocytes begin to keratinize; cytoplasm fills with keratin; lipid granules are excreted to help create a water-resistant barrier; cells begin to flatten and prepare for apoptosis.
Stratum lucidum: clear layer found only in thick skin (palms and soles).
Stratum corneum: most superficial; dead keratinocytes filled with keratin; surrounded by glycolipids to maintain water resistance; continually shed and replaced by cells from deeper layers; turnover about two weeks.
Histology visuals:
Thin skin shows a thinner stratum corneum and no stratum lucidum.
Thick skin shows exaggerated epidermal ridges and a prominent stratum lucidum between stratum granulosum and stratum corneum.
Epidermal Cells and Their Roles
Keratinocytes: primary cell type in epidermis; synthesize keratin, an intermediate filament protein, to provide epidermal strength.
Melanocytes: located in the deepest epidermal layer; produce melanin; extend long processes to transfer melanin to keratinocytes; melanin contributes to skin color and UV protection.
Merkel cells (tactile discs): specialized somatosensory receptors in stratum basale; detect changes in skin shape and contribute to light touch.
Langerhans cells: specialized leukocytes (dendritic cells) in stratum spinosum and some in dermis; defend against infection by recognizing pathogens; monitor for abnormal cells and can trigger removal of cancerous or malfunctioning cells.
Pigments and Skin Color
Melanocytes produce melanin; melanin is stored in secretory vesicles and transferred to keratinocytes; the number of melanocytes does not determine skin color; activity of melanocytes (melanin production) and its distribution determine darker skin.
Melanosomes in keratinocytes form a thicker melanin layer with darker skin; tanning is an adaptive increase in melanin production after UV exposure.
Local pigmentation variants:
Freckles and moles: local increases in melanin.
Heme: red pigment in erythrocytes; blood flow affects skin color. Oxygenated hemoglobin is bright red; deoxygenated hemoglobin appears darker red; visible blue tint can occur in veins through skin (cyanosis).
Carotene: orange pigment from diet (beta-carotene is a vitamin A precursor) contributing to orange skin color; excessive carotene can lead to keratosis (orange tint) as seen in carrots overconsumption.
Health-related skin color indicators:
Cyanosis: blue color indicating hypoxia (low blood oxygen). Assessed in nail beds, lips, and eyes; varies with skin tone.
Jaundice: yellow coloration from elevated bilirubin (heme breakdown product) due to liver dysfunction or accelerated heme breakdown; common in neonatal jaundice due to immature liver.
Contusion: bruising; red/blue/purple/black discoloration due to internal bleeding.
Petechiae and purpura: small and numerous bruising-like lesions from internal bleeding; associated with various diseases.
Sun exposure and health implications:
Sun tanning increases melanin to protect against UV damage; higher melanin blocks more UV light, reducing DNA damage and skin cancer risk but may reduce vitamin D synthesis; darker skin can have higher risk of vitamin D deficiency at higher latitudes due to reduced UVB exposure.
Vitamin D and health considerations:
Vitamin D is essential; deficiency can occur more readily in individuals with darker skin at higher latitudes; supplementation may be needed.
Racism and health disparities context (from the study):
Racism contributes to health disparities; a study comparing siblings found higher hypertension rates associated with darker skin color, after controlling for confounders; illustrates broader links between skin color, social determinants, and health outcomes.
Accessory Structures of the Integumentary System
Hairs (pili):
Hair is primarily keratin and consists of dead keratinized cells; originates from hair follicles (epidermal tissue folded into the dermis).
Hair shaft: exposed portion that extends beyond the skin; hair root: portion within the follicle; hair bulb: deep enlargement containing actively dividing basal cells (hair matrix) that produce keratinized cells.
Hair papilla: connective tissue core with blood vessels and nerves that nourish growing hair within the bulb.
Hair matrix: mitotically active epithelial cells that divide and differentiate to form keratinized hair.
Hair functions: protection (e.g., scalp protects skull; nose/ear/eyelash/eyebrow hair reduces particle ingress; eyebrows prevent sweat from entering eyes), sensory input (hair follicle receptors detect movement), thermoregulation, and communication (facial hair growth signals maturity in males after puberty; thicker beard conveys masculine signals).
Erector pili: smooth muscle attached to hair follicle; when activated, hair stands up (goosebumps) and can increase insulation by trapping a thicker layer of warm air around the body.
Glands associated with hair:
Sebaceous glands: oil-secreting glands connected to hair follicles; stratified cuboidal epithelium; produce sebum to lubricate hair/skin, condition hair, and provide antibacterial protection.
Eccrine sweat glands: most common sweat glands; coiled tubular structure; stratified cuboidal epithelium; secrete watery sweat through a duct to the skin surface to cool the body.
Apocrine sweat glands: located in axillary, pubic regions, face, and nipples; stimulated by hormones during puberty; produce oily, lipid- and protein-rich sweat that bacteria can feed on, contributing to body odor; ducts connect to hair follicles.
Nails:
Produced from modified epidermal tissue; nail matrix contains stem cells that divide to form hard keratin of the nail.
Nail body is composed of dead keratinocytes; cuticle/eponychium covers the proximal nail root; pink appearance due to underlying dermal blood vessels.
Nails provide protection, extra hardness at digit tips, aid in scratching/tearing, and assist grip.
Receptors and Neural Structures in the Skin
Hair follicle receptor: detects hair movement; contributes to touch.
Other skin nerves: pre-nerve endings extend through the dermis and epidermis; provide pain, temperature, and itching sensations.
Mechanoreceptors:
Lamellar corpuscles (Pacinian corpuscles): deep in reticular dermis and subcutaneous layer; detect deep pressure and high-frequency vibrations.
Tactile corpuscles (Meissner’s corpuscles): nerve endings encapsulated in connective tissue; highly sensitive to light touch; concentrated in fingertips within the dermal papillae of the papillary layer.
Tactile discs (Merkel cells): neuroepithelial cells in the deepest epidermal layer (stratum basale) that detect bending and light touch.
Glands and Their Histology
Sebaceous glands: simple branched alveolar exocrine glands; stratified cuboidal epithelium; secrete sebum to condition and lubricate hair/skin; antibacterial.
Eccrine glands: most widespread; secret watery sweat; coiled tubular structure; duct opens to skin surface via pore.
Apocrine glands: localized to axillary, pubic, facial regions; duct connects to hair follicle; secretion contains lipids and proteins; stimulated by puberty hormones; odor produced by bacterial metabolism.
Nails: Anatomy and Function (Detailed)
Nail matrix: site of nail formation; contains stem cells that produce keratinocytes.
Nail root: portion under proximal skin fold.
Body (plate) of nail: visible, pink due to dermal blood vessels.
Cuticle/eponychium: proximal fold covering the nail root.
Growth and replacement: nail cells are keratinized and become part of the hard keratin structure; turnover and replacement maintain nail integrity.
Skin and Health: Summary of Pigment-Related Conditions and Risks
Sun exposure and skin cancer risk: UV damage linked to basal cell carcinoma, squamous cell carcinoma, and melanoma; protection via melanin reduces risk but melanin can also reduce vitamin D synthesis.
Vitamin D considerations: deficiency risk higher with darker skin at higher latitudes; supplementation sometimes necessary.
Warning signs for skin cancer (ABCDE):
A: asymmetry – one half of the lesion differs from the other half.
B: border – irregular, scalloped, poorly defined borders.
C: color – variation in color within the lesion (tan, brown, black, white, red, or blue).
D: diameter – usually larger than 6 ext{ mm} (pencil eraser size).
E: evolving – the lesion changes rapidly in size, shape, or color.
Major Accessory Structures: Summary of Roles and Clinical Relevance
Hair: provides protection, sensory input, thermoregulation, and social signaling via facial hair growth; goosebumps contribute to insulation.
Glands: sebaceous, eccrine, and apocrine glands regulate moisture, temperature, lubrication, and odor; apocrine secretions contribute to puberty-associated odor.
Nails: protect fingertip tissue and aid in manipulation and grip; indicators of overall health can be influenced by systemic conditions.
Oncogenesis and Skin Cancer: Mechanisms and Classifications
Cancer basics: oncogenesis is the transformation of normal cells into cancerous cells due to genetic mutations that disrupt tissue growth control and apoptosis.
Cell cycle regulation: tumor suppressor genes slow down the cell cycle, repair DNA, or initiate apoptosis; mutations decrease function (e.g., p53 tumor suppressor gene).
Oncogenes: mutated genes that speed up the cell cycle; proto-oncogenes encode normal growth-signaling proteins; mutations lead to constitutive signaling.
Examples in skin cancer: RAF mutations are common and can drive the MAPK cascade (RAF → MEK → ERK) promoting uncontrolled cell division; epidermal growth factor receptor (EGFR) pathway involvement.
Benign tumors: do not spread; examples include acrochordon (skin tags), lipomas, and melanocytic nevi (moles) which are usually symmetrical with smooth borders and uniform color, often smaller than 6 ext{ mm}.
Malignant tumors: invasive and potentially metastatic; include basal cell carcinoma, squamous cell carcinoma, and melanoma.
Basal cell carcinoma: most common, least likely to metastasize; arises from mitotically active stem cells in stratum basale; commonly sun-exposed areas; treatment includes surgery, cryosurgery, or topical chemotherapy.
Squamous cell carcinoma: arises from uncontrolled growth of keratinocytes in stratum spinosum; can invade dermis and spread; treatment may involve surgery, chemotherapy, radiation, or immunotherapy.
Melanoma: uncontrolled growth of melanocytes; highly metastatic and aggressive; often arises from a pre-existing melanocytic nevus; appearance: asymmetrical, irregular border, color variation, diameter often > 6 ext{ mm}, evolving rapidly; treatment includes surgical removal, chemotherapy or modern targeted therapies and immunotherapies; prognosis depends on stage and early detection.
Diagnostic visuals: histology examples show malignant cells spreading in dermis for basal cell carcinoma and squamous cell carcinoma; melanoma shows deep invasion into dermis and surface spread of mutated melanocytes.
Treatments overview: surgical excision is common; cryosurgery; topical chemotherapy; immunotherapy; targeted therapies; newer approaches are reducing the use of traditional chemotherapy for melanoma.
ABCDE mnemonic reminder: a set of criteria to assess potential melanoma risk in nevi or lesions.
Skeletal System: Overview, Functions, and Homeostasis
Primary functions: support, movement, protection, mineral storage, and blood cell formation.
Support: bones provide a framework to support soft tissues and maintain posture against gravity.
Movement: joints (articulations) enable movement; muscles attached to bones via tendons contract to create motion.
Protection: skull protects the brain; ribs and sternum protect lungs and heart; vertebrae protect the spinal cord.
Mineral storage: calcium stored in bones; can be mobilized to maintain blood calcium levels; bone breakdown is regulated by the endocrine system.
Blood calcium homeostasis: set point for blood calcium is 10 ext{ mg/dL}; PTH stimulates osteoclasts to break down bone and release calcium; PTH also stimulates kidney calcium reabsorption and activates vitamin D in the kidneys; vitamin D then promotes calcium absorption in the small intestine.
Calcitonin role: if blood calcium is too high, the thyroid gland releases calcitonin; it inhibits osteoclast activity and promotes calcium storage by osteoblasts; reduces renal calcium reabsorption, increasing calcium loss in urine.
Hemopoiesis: production of blood cells (hemopoiesis/hematopoiesis) occurs in red bone marrow; hematopoietic stem cells differentiate into red blood cells, white blood cells, and platelets.
Red vs yellow bone marrow:
Red bone marrow: active in hematopoiesis; located deeply in bones (e.g., proximal femur and humerus; ribs, sternum, iliac crest, vertebral bodies in adults).
Yellow bone marrow: mainly adipose tissue; replaces red marrow with age but some red marrow remains in adults.
Bone anatomy and counts: the human skeleton comprises 206 bones.
Axial vs appendicular skeleton: the skeleton is classified into two major divisions – axial (skull, vertebral column, ribs, sternum) and appendicular (limb bones and girdles); distribution and function differ in protection and movement roles.