Anatomy & Physiology Study Notes: Tissues

Chapter: Tissues

Introduction to Tissues

In human anatomy and physiology, tissues are groups of cells with similar structure and function. All cells in the body can be classified into one of four primary tissue types, each with specialized roles essential for the body's structure and function.

• Epithelial tissue: Covers exposed surfaces and lines internal passageways and chambers.

• Connective tissue: Fills internal spaces, provides structural support, and stores energy.

• Muscular tissue: Contracts to produce movement and includes skeletal, cardiac, and smooth muscle.

• Neural tissue: Conducts electrical impulses and carries information throughout the body.

Epithelial Tissue

Characteristics of Epithelial Tissue

Epithelial tissue consists of closely packed cells with minimal extracellular space, forming continuous sheets or layers. It is avascular (lacks blood vessels) and highly regenerative.

• Cell arrangement: Cells are bound close together, with no intercellular space.

• Surfaces: Has apical (exposed) and basal (attached to basal lamina) surfaces.

• Avascularity: Nutrients are obtained via diffusion or absorption.

• Regeneration: High capacity for renewal via stem cell division near the basal lamina.

Functions of Epithelial Tissue

• Physical protection: Shields underlying tissues from abrasion, pathogens, and chemicals.

• Control of permeability: Regulates movement of substances in and out of the body.

• Absorption: Uptake of nutrients and other substances.

• Secretion: Produces and releases substances such as mucus, hormones, and enzymes.

Specializations of Epithelial Cells

• Microvilli: Increase surface area for absorption.

• Cilia: Move substances across the epithelial surface.

Maintaining Epithelial Integrity

• Intercellular connections: Cell junctions, cell adhesion molecules (CAMs), and intercellular cement provide strength and stability.

• Attachment to basal lamina: The cell membrane attaches to the basement membrane for support.

• Self-renewal: Epithelial cells are continually replaced by stem cell division.

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells.

• Simple epithelium: One layer of cells.

• Stratified epithelium: Two or more layers of cells.

Cell shapes:

• Squamous: Flat cells.

• Cuboidal: Cube-shaped cells.

• Columnar: Taller than wide, column-shaped cells.

Types of Epithelial Tissue

Type	Location	Function
Simple Squamous	Lining body cavities (mesothelium), heart, blood vessels (endothelium)	Reduces friction, absorption, secretion
Stratified Squamous	Surface of skin, mouth, esophagus, rectum, vagina	Protection against abrasion, pathogens, chemicals
Simple Cuboidal	Thyroid gland, ducts, kidney tubules	Secretion, absorption, limited protection
Stratified Cuboidal	Large exocrine ducts (rare)	Structural support, some secretion and absorption
Simple Columnar	Stomach, intestines, gallbladder, uterine tubes, kidney collecting ducts	Secretion, absorption, protection
Stratified Columnar	Pharynx, epiglottis, anus, mammary glands, salivary glands, urethra	Protection
Pseudostratified Ciliated Columnar	Nasal cavity, trachea, bronchi, male reproductive tract	Protection, secretion
Transitional	Urinary bladder, renal pelvis, ureters	Ability to stretch extensively

Glandular Epithelia

• Exocrine glands: Secrete via ducts (serous, mucous, mixed glands).

• Endocrine glands: Secrete hormones directly into the bloodstream (no ducts).

Gland structure:

• Simple glands: Tubular or alveolar (acinar).

• Compound glands: Tubuloalveolar (combines tubular and alveolar).

Methods of secretion:

• Merocrine (Eccrine): Secretion by exocytosis (e.g., salivary glands).

• Apocrine: Shedding of the apical portion of the cell (e.g., mammary glands).

• Holocrine: Cell bursts apart, releasing contents (e.g., sebaceous glands).

Connective Tissue

Characteristics of Connective Tissue

Connective tissue is the most abundant tissue type in the body. It consists of specialized cells, extracellular protein fibers, and ground substance, which together form the matrix. Most connective tissues have more matrix than cells, except adipose tissue.

Functions of Connective Tissue

• Structural framework for the body

• Transport of fluids and dissolved materials

• Protection of organs

• Support, surround, and connect other tissues

• Energy storage

• Defense against microorganisms

Categories of Connective Tissue

• Connective Tissue Proper: Matrix of fibers (loose and dense)

• Fluid Connective Tissue: Matrix of liquid (blood and lymph)

• Supporting Connective Tissue: Matrix of fibers and sometimes insoluble calcium salts (cartilage and bone)

Connective Tissue Proper: Cells and Fibers

• Fixed cells: Maintenance, repair, energy storage

• Wandering cells: Defense and repair

Cell Type	Function
Fibroblasts	Produce connective tissue fibers
Fibrocytes	Maintain connective tissue fibers and matrix
Macrophages	Phagocytize pathogens and damaged cells
Adipocytes	Store lipid reserves
Mesenchymal cells	Stem cells that can differentiate into other cell types
Melanocytes	Synthesize melanin

Types of fibers:

• Collagen fibers: Long, unbranched, provide tensile strength

• Reticular fibers: Highly branched, resist multidirectional forces

• Elastic fibers: Branched, wavy, contain elastin for flexibility

Types of Connective Tissue Proper

• Loose connective tissue: Areolar, adipose, reticular

• Dense connective tissue: Dense regular, dense irregular, elastic

Areolar Tissue

• Contains every cell and fiber type

• Flexible and resilient; underlies epithelial tissue

• Connects skin to muscle; allows independent movement

Adipose Tissue

• Dominated by adipocytes

• Located in hypodermis, buttocks, surrounds organs

• Functions: Cushioning, thermal insulation, energy storage

Reticular Tissue

• Contains reticular fibers

• Forms supporting framework of organs (liver, spleen, kidney, lymph nodes, tonsils, appendix, bone marrow)

Dense Regular Connective Tissue

• Collagen fibers arranged in one direction

• Locations: Tendons (muscle to bone), aponeuroses (muscle to muscle), ligaments (bone to bone)

• Elastic ligaments allow modest stretching

Dense Irregular Connective Tissue

• Collagen fibers arranged in multiple directions

• Locations: Nerve and muscle sheaths, dermis

• Resists tension from multiple directions

Membranes

Membranes are formed by the combination of epithelial and connective tissues. They serve as protective barriers and line various body cavities.

• Mucous membranes: Wet membranes connected to the exterior (e.g., digestive, respiratory tracts)

• Serous membranes: Line body cavities (e.g., pleura, pericardium, peritoneum)

• Cutaneous membrane: The skin; thick, dry, water-resistant

• Synovial membrane: Lines joint capsules

Fluid Connective Tissues

• Blood: Plasma matrix with water and proteins; contains erythrocytes (RBCs), leukocytes (WBCs), and platelets

• Lymph: Plasma that has left blood vessels; contains lymphocytes (a type of WBC); referred to as interstitial fluid

Supporting Connective Tissue

• Cartilage: Made of chondrocytes, gel matrix of chondroitin sulfate, avascular, supports framework

• Bone: Solid matrix of calcium phosphate and collagen fibers, maintained by osteoblasts and osteoclasts, organized into osteons

Type	Location	Function
Hyaline Cartilage	Connection between ribs and sternum, tracheal rings	Support, flexibility
Elastic Cartilage	Auricle of ear, epiglottis, auditory tube	Flexible and resilient
Fibrous Cartilage	Knee joints, intervertebral discs	Resists compression, tough
Bone	Skeletal system	Support, protection, movement

Muscle Tissue

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

• Skeletal muscle: Striated, voluntary, multinucleated; responsible for body movement

• Cardiac muscle: Striated, involuntary; found only in the heart

• Smooth muscle: Non-striated, involuntary; found in digestive, respiratory, and vascular systems

Nervous Tissue

Nervous tissue is specialized to conduct electrical signals throughout the body. It consists of two main types of cells:

• Neurons: Transmit electrical impulses; composed of soma (cell body), axon (nerve fiber), and dendrites

• Neuroglia: Supporting cells that protect and support neurons

Example: The nervous tissue in the brain and spinal cord coordinates body functions by transmitting signals between different body parts.

Additional info: The study of tissues is called histology. Understanding tissue structure and function is essential for diagnosing diseases and understanding organ systems.