Skin Structure: Epidermis and Dermis Layers

Epidermis Layers

• The epidermis has five layers: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basal.
• Stratum lucidum is exclusively found in thick skin.

Epidermis Cells

• The epidermis consists of keratinocytes (produce keratin), melanocytes (produce melanin), dendritic cells (macrophages activating the immune system), and tactile (Merkel) cells (sensory receptors).
• Melanocytes and Merkel cells are located in the basal layer.
• Dendritic cells, macrophages that activate the immune system, reside in the spinosum layer.
• Keratinization starts in the stratum granulosum.

Dermis Layers

• The papillary layer is areolar tissue, contains less collagen and elastic fibers, and includes Meissner's corpuscles.
• The reticular layer is dense irregular connective tissue, contains more collagen and elastic fibers, and includes arrector pili muscles, Pacinian corpuscles, eccrine glands, sebaceous glands, and apocrine glands.

Hair Follicle Structure

• Hair follicle layers include the internal root sheath, external root sheath, glassy membrane, and peripheral connective tissue sheath.
• The hair follicle contains the hair bulb, hair follicle receptor, hair follicle wall (the layers), hair matrix (responsible for hair growth), and hair papilla.
• Hair functions in protection and as sensory receptors.

Nail Components

• The nail consists of the nail plate (free edge, nail body with nail bed of basal cells, nail root), lunula (keratinocytes), and nail folds.

Sweat Glands

• Eccrine sweat glands release sweat directly onto the skin surface, particularly on the palms and forehead, for temperature control via exocytosis.
• Apocrine sweat glands secrete a milky substance into hair follicles in sexual areas via exocytosis.
• Sebaceous glands secrete oil for lubrication into hair follicles.

Thermoregulation and Skin Function

• Blood vessels regulate temperature through vasoconstriction (when cold) and vasodilation (when hot).
• The skin protects, regulates body temperature, provides sensation, performs metabolic functions, acts as a blood reservoir, and excretes waste.

Sensory Reception in the Skin

• Touch is detected by Meissner's corpuscles (fine touch), Pacinian corpuscles (deep touch), and Merkel cells.

Burn Classification

• First-degree burns affect only the epidermis.
• Second-degree burns affect the epidermis and part of the dermis.
• Third-degree burns affect the epidermis, dermis, and hypodermis.

Skin Cancer

• Basal cell carcinoma originates in the basal cell layer and is treatable if caught early.
• Squamous cell carcinoma occurs in the spinosum layer.
• Melanoma is cancer of the melanocytes; detection relies on the ABCD (Asymmetry, Border, Color, Diameter) rule.
• Moles are accumulations of melanocytes that can range from benign to melanoma.

Skeletal System Composition

• The human body has 206 bones.
• The skeletal system contains hyaline, elastic, and fibrocartilages.

Bone Functions

• Bones support, protect, facilitate movement, store minerals and growth factors (calcium and phosphorus in the matrix), form blood cells (red bone marrow), store triglycerides (fat), and produce hormones.

Axial Skeleton

• The axial skeleton consists of 80 bones: skull (8 cranial, 14 facial), auditory ossicles, hyoid, vertebral column (7 cervical, 12 thoracic, 5 lumbar, sacral, coccyx), and thoracic rib cage (7 true, 5 false, 2 floating).

Appendicular Skeleton

• The appendicular skeleton consists of 126 bones: upper and lower limbs, pectoral girdle, and pelvic girdle.

Bone Classification

• Bones are classified as long, short (carpals and tarsals), flat, irregular, and sesamoid.

Long Bone Structure

• A long bone consists of a shaft (diaphysis), proximal/distal epiphysis (ends), epiphyseal plate (growth plate/line), medullary cavity, metaphysis, and periosteum (covering).
• The distal/proximal epiphysis is made of hyaline cartilage, compact bone, and spongy bone.
• The term "dipole" refers to spongy bone between compact bone in flat bones.
• Yellow bone marrow is in the medullary cavity; red marrow is in the spongy bone and medullary cavity in children, and in the skull, vertebrae, ribs, and sternum in adults.

Bone Cells

• Osteogenic cells are stem cells that differentiate into osteoblasts.
• Osteoblasts are responsible for bone growth and matrix synthesis.
• Osteocytes maintain the bone matrix and bone cell lining.
• Osteoclasts break down and absorb the bone matrix.

Bone Matrix Composition

• The bone matrix includes organic compounds (osteoid with collagen and ground substance) and inorganic compounds (hydroxyapatite).
• The periosteum covers the outer surface of bones.
• The endosteum lines the internal surfaces of bones, including the medullary cavity.
• The dipole refers to spongy bone located between the inner and outer layers of the compact bone in flat bones.

Ossification

• Ossification is the process of bone formation during embryonic development, growth, and bone healing.
• Intramembranous ossification and endochondral ossification are the two ways ossification happens.
• Intramembranous ossification produces flat bones of the skull (frontal, temporal, parietal), maxilla, mandible, and the central thick part of the clavicle, originating from fibrous membranes.

Endochondral Ossification Steps

• Bone formation replaces hyaline cartilage through these steps
• Bone collar forms around diaphysis of the hyaline cartilage model.
• Cartilage calcifies in the center of the diaphysis and develops cavities.
• The periosteal bud invades the internal cavities, and spongy bone forms.
• The diaphysis elongates, and a medullary cavity forms; secondary ossification centers appear in the epiphysis.
• The epiphysis ossifies; hyaline cartilage remains only in the epiphyseal plates and articular cartilage upon completion.

Spongy vs Compact Bone Composition

• Compact bone has a central canal, concentric lamellae, osteons, circumferential lamellae, interstitial lamellae, Volkmann's canals, and canaliculi.
• Spongy bone contains trabeculae and canaliculi.

Bone Growth

• Interstitial growth is growth in length (stops after puberty).
• Appositional growth is growth in width.

Epiphyseal Plate Zones

• The epiphyseal plate consists of the resting zone, proliferation zone, hypertrophic zone, calcification zone, and ossification zone.
• Resting zone: Hyaline cells are inactive.
• Proliferation zone: Cartilage cells undergo mitosis (where cartilage grows).
• Hypertrophic zone: Older cartilage cells enlarge, grow in diameter, and lacuna walls become thin.
• Calcification zone: Matrix becomes calcified; cartilage cells die, matrix begins deteriorating.
• Ossification zone: New bone forms.

Bone Remodeling

• Bone remodeling replaces old bone with new bone through bone deposit (osteoblast formation) and bone resorption (osteoclast breakdown), influenced by hormones and external factors.

Hormones Affecting Growth

• Growth hormone acts on the epiphyseal plate.
• Thyroid hormone controls the metabolism of bone cells.
• Testosterone and estrogen (steroids) act on the proliferation and ossification zones.
• Glucocorticoids (from the adrenal cortex) and serotonin inhibit osteogenic cell differentiation into osteoblasts.

Blood Calcium Regulation

• Parathyroid hormone (PTH) is released from the parathyroid gland when stimulated by low calcium levels.
• The kidney synthesizes calcitriol, the active form of vitamin D, leading to higher calcium and phosphorus absorption from food.

Fracture Healing Stages

• Stages include hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Bone Diseases

• Osteomalacia is due to Vitamin D deficiency in adults.
• Rickets is due to Vitamin D deficiency in children.
• Osteoporosis is the loss of bone mass (primarily spongy bone); treatment includes bisphosphonates, selective estrogen receptor modulators, and denosumab.
• Paget's disease results in a "moth-eaten" appearance due to imbalanced remodeling.

Bone Counts

• There are 206 bones in the human body.
• The axial skeleton has 80 bones.
• The appendicular skeleton has 126 bones.

Facial Bones

• Facial bones include the nasal, lacrimal, palatine, inferior nasal conchae, vomer, maxillae, zygomatic, and mandible.

Cranial Bones

• Cranial bones include the frontal, parietal, temporal, occipital, sphenoid, and ethmoid.

Sutures of the Cranium

• The sutures are coronal (frontal), sagittal (between the two parietal bones), lambdoid (back), and squamous (sides)

Sinuses

• The largest sinuses are the maxillary sinuses, located under the eyes.
• The sphenoid sinuses are near the sella turcica.

The Hyoid Bone

• The hyoid bone provides attachments for muscles that act on the tongue, larynx, and pharynx.

Vertebrae

• The five types are cervical, thoracic, lumbar, sacral and coccygeal.

Primary Curvatures of the Spine

• The thoracic and sacrococcygeal curvatures are primary.

Secondary Curvatures of the Spine

• The cervical and lumbar are secondary.

Spinal Abnormalities

• Spinal abnormalities include scoliosis (lateral curve), kyphosis (hunchback), and lordosis (arch).

Types of RIbs

• Ribs include true (1-7), false (8-12), and floating (11-12) ribs; true ribs connect directly to the sternum, while false ribs connect indirectly.

Girdles

• The pectoral girdle includes the scapula and clavicle.
• The pelvic girdle includes the hip.
• The surgical neck is more commonly fractured than the anatomical neck.

Bones of the Upper Limb

• Pronation of the forearm involves the radius crossing over the ulna, with palms facing downward.

Carpal Bones

• The carpal bones are scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, and hamate.

The Coaxe

• The coaxe consists of the ilium, ischium, and pubis.

Tarsal Bones

• The tarsal bones are talus, calcaneus, navicular, cuboid, medial cuneiform, intermediate cuneiform, and lateral cuneiform.

Foot Arches

• The foot has medial longitudinal, lateral longitudinal, and transverse arches.
• Medial Longitudinal Arch: Runs along the inner side of the foot; absorbs shock.
• Lateral Longitudinal Arch: Runs along the outer side of the foot; provides support and balance.
• Transverse Arch: Runs across the width of the foot; helps distribute weight evenly.

Joints

• A joint is a connection between two or more bones, allowing movement and flexibility.

Structural Classification of Joints

• The three structural classifications of joints are fibrous, cartilaginous, and synovial.

Fibrous Joints

• Bones are joined by dense regular connective tissue, with no joint cavity, and are either synarthrosis or amphiarthrosis.

Cartilaginous Joints

• Joined by hyaline cartilage or fibrocartilage, with no joint cavity, and are either synarthrosis or amphiarthrosis.

Synovial Joints

• Freely moveable with a joint cavity and are classified as diarthrosis.

Mobility vs Stability

• A joint with more mobility is less stable.

Functional Classes of Joints

• Synarthroses include sutures, gomphoses (tooth), and synchondroses (ribs plate).
• Amphiarthroses include syndesmosis (between radius and ulna), and symphyses (vertebrae disk, knee).
• Synovial joints are everything else.

Synovial Joint Structures

• Synovial joints have articular cartilage, a joint (synovial) cavity, an external fibrous layer, an inner synovial membrane, synovial fluid, ligaments, and nerves and blood vessels.

Factors Affecting Joint Mobility

• Factors that impact the mobility of muscle are: muscle tone, shape of articular surface, and ligament number and locomotion.

Joint Types

• Gliding joint example: wrist and ankle.
• Pivot joint example: neck.
• Hinge joint example: knee.
• Saddle joint example: thumb.

Angular Motion

• Angular motion is a change in the angle between articulating bones.

Anatomical Movements

• Abduction: away from midline
• Adduction: towards midline
• Pronation: palm down
• Supination: palm up

Joint Types

• Shoulder and hip: ball and socket
• Elbow and knee: hinge

Connective Tissues

• Ligament: connects bone to bone, keeps joints stable.
• Tendon: connects muscle to bone, allowing muscles to move the bones.
• Glenoid Labrum: A piece of cartilage in the shoulder that helps keep the arm bone in place in the shoulder socket.
• Bursa: A small fluid-filled cushion that reduces friction and helps things move smoothly, like between bones and muscles.