Basic Primary Tissues

Overview of Tissue Types

Histology is the study of tissues, which are groups of cells with similar structure and function. The human body contains four basic (primary) tissue types:

• Epithelial tissue: Covers body surfaces and lines cavities.

• Connective tissue: Supports, binds, and protects organs.

• Muscle tissue: Responsible for movement.

• Nervous tissue: Conducts electrical impulses and processes information.

Extracellular Matrix (ECM)

The extracellular matrix (ECM) is a network of non-cellular components present within tissues and organs, providing structural and biochemical support to surrounding cells.

• Functions: Provides support, segregates tissues, and regulates intercellular communication.

• Ground substance: The amorphous, gel-like component of ECM, composed of water, proteoglycans, and glycoproteins.

• Protein fibers: Includes collagen (strength), elastin (elasticity), and reticular fibers (support).

• Cell adhesion molecules (CAMs): Proteins that help cells stick to each other and to the ECM.

Tight Junction vs. Gap Junction

• Tight junctions: Seal adjacent epithelial cells, preventing passage of molecules between them.

• Gap junctions: Allow direct communication between cells via channels.

• Desmosome: A type of cell junction that provides strong adhesion between cells, especially in tissues subject to mechanical stress.

Epithelial Tissue

Epithelial tissue lines body surfaces, cavities, and forms glands. It serves as a barrier, protective layer, and is involved in absorption and secretion.

• Functions: Protection, absorption, filtration, secretion.

• Basement membrane: Thin layer anchoring epithelium to underlying connective tissue; produced by both tissues.

• Repair and renewal: Epithelial cells have high regenerative capacity; new cells arise from stem cells.

Exocrine vs. Endocrine Glands

• Exocrine glands: Secrete products into ducts (e.g., sweat, saliva).

• Endocrine glands: Release hormones directly into the bloodstream.

Modes of Secretion

• Merocrine: Exocytosis.

• Apocrine: Portion of cell is shed.

• Holocrine: Entire cell is released.

Connective Tissue

Connective tissue supports, binds, and protects other tissues and organs. It is characterized by cells embedded in an abundant extracellular matrix.

• General characteristics: Diverse cell types, abundant ECM, vascularity varies.

• Matrix components: Ground substance and protein fibers.

• Fibroblasts: Cells that secrete ECM components.

Phagocytes, Adipocytes, Mast Cells

• Phagocytes: (e.g., macrophages) engulf debris and pathogens.

• Adipocytes: Store fat.

• Mast cells: Mediate inflammation.

Protein Fibers

• Collagen, elastin, reticular fibers: Each provides different structural properties.

Ground Substance

• Determines tissue consistency: Fluid, gel, solid.

Physical Properties of Cartilage

• Determined by ECM composition.

Perichondrium

• Dense layer surrounding cartilage: Provides nutrients and support.

Muscle Tissue

Muscle tissue is specialized for contraction and movement. There are three types:

• Skeletal muscle: Voluntary movement, striated.

• Cardiac muscle: Heart contraction, striated, involuntary.

• Smooth muscle: Walls of hollow organs, non-striated, involuntary.

Myocytes are muscle cells; excitability refers to their ability to respond to stimuli.

Nervous Tissue

Nervous tissue consists of neurons and neuroglia. It is responsible for transmitting electrical signals and supporting neural function.

• Neurons: Conduct impulses; composed of cell body, axon, dendrites.

• Neuroglia: Support, protect, and nourish neurons.

• Axons vs. dendrites: Axons transmit impulses away; dendrites receive impulses.

Membranes

Types and Functions

Body membranes cover surfaces, line cavities, and form protective sheets.

• True membrane: Composed of epithelial and connective tissue.

• Mucosae: Line body cavities open to the exterior (e.g., digestive tract).

• Serous membranes: Line closed cavities; secrete lubricating fluid.

Tissue Repair

Regeneration, Fibrosis, and Scar Tissue

Tissue repair restores structure and function after injury.

• Regeneration: Replacement of damaged cells with the same type.

• Fibrosis: Replacement with scar tissue (collagen).

• Scar tissue: Non-functional, dense connective tissue.

Integumentary System

General Functions and Structure

The integumentary system includes the skin and its accessory structures. It protects the body, regulates temperature, and provides sensory information.

• General functions: Protection, sensation, thermoregulation, excretion, vitamin D synthesis.

• Accessory structures: Hair, nails, glands.

• Cutaneous membrane: The skin; composed of epidermis and dermis.

Cell Types

• Keratinocytes: Most abundant.

• Melanocytes: Pigment.

• Merkel cells: Touch.

• Langerhans cells: Immune.

Thin vs. Thick Skin

• Thick skin: Thick skin on palms/soles; thin skin elsewhere.

Layers of Epidermis

• Stratum basale, spinosum, granulosum, lucidum (thick skin only), corneum.

Melanin

• Pigment produced by melanocytes; determines skin color.

Wrinkles

• Caused by aging, loss of collagen and elastin.

Callus

• Thickened skin due to friction; protective function.

Dermis Layers

• Papillary (superficial), reticular (deep); provide strength and elasticity.

Nerve Fibers

• Sensory and motor functions in skin.

Vellus vs. Terminal Hairs

• Vellus: Fine, unpigmented; terminal: coarse, pigmented.

Skin Regeneration

• High regenerative capacity due to stem cells.

Bones and Bone Tissue

Structure and Function

Bones provide support, protection, movement, mineral storage, and blood cell formation.

• Primary functions: Support, protection, movement, mineral storage, hematopoiesis.

• Red vs. yellow marrow: Red: hematopoietic; yellow: fat storage.

• Compact vs. spongy bone: Compact: dense, strong; spongy: porous, lightweight.

Bone Cells

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-resorbing cells; involved in osteolysis.

• Osteocytes: Mature bone cells.

Lamellae Types

• Concentric, interstitial, circumferential.

Trabeculae

• Lattice-like structures in spongy bone.

Periosteum

• Outer membrane of bone; contains osteogenic cells.

Endosteum

• Lines internal bone surfaces.

Bone Formation

• Intramembranous ossification: Bone develops from mesenchymal tissue (e.g., skull).

• Endochondral ossification: Bone forms from cartilage model (e.g., long bones).

Articular Cartilage

• Covers joint surfaces; reduces friction.

Growth

• Longitudinal growth: Lengthening of bones at epiphyseal plates.

• Appositional growth: Increase in bone diameter.

Bone Mass

• Influenced by activity, hormones, nutrition.

Hormones

• Parathyroid hormone: Increases blood calcium.

• Calcitonin: Decreases blood calcium.

Bone Fracture Repair

• Involves hematoma formation, fibrocartilaginous callus, bony callus, and remodeling.

Joints (Articulations)

Classification and Structure

Joints are sites where two or more bones meet, allowing for movement and flexibility.

• Functional classification: Synarthrosis (immovable), amphiarthrosis (slightly movable), diarthrosis (freely movable).

• Structural classification: Fibrous, cartilaginous, synovial joints.

• Fibrous joints: Connected by dense connective tissue; e.g., sutures, syndesmoses.

• Cartilaginous joints: Connected by cartilage; e.g., synchondroses, symphyses.

• Synovial joints: Most movable; contain joint cavity, synovial fluid.

Support Structures

• Ligaments: Bone to bone.

• Tendons: Muscle to bone.

• Bursae: Fluid-filled sacs.

• Tendon sheaths: Protective coverings for tendons.

Classes of Synovial Joints

• Plane, hinge, pivot, condyloid, saddle, ball-and-socket.

Example Table: Types of Joints

Key Equations

• Bone growth rate:

• Calcium homeostasis:

Additional info: Academic context and definitions have been expanded for clarity and completeness.