1. Introduction to Tissue Level of Organization

Like all multicellular animals, the human body is composed of different types of cells. Groups of cells similar in structure and function are organized into tissues. Different tissues grouped together into a structural and functional unit are called organs. An organ system is a group of organs that function together to carry out the principal activities of the body.

The Four Basic Types of Animal Tissue

• Type
  Epithelial Tissue

  Covers body surfaces, lines hollow organs, body cavities, and ducts, and forms glands.

• Type
  Connective Tissue

  Protects, supports, and binds organs together, stores energy as fat, and provides immunity.

• Type
  Nervous Tissue

  Detects changes in conditions and responds by generating nerve impulses (action potentials).

• Type
  Muscular Tissue

  Generates the physical force needed to make body structures move and generates body heat.

Figure: Levels of Structural Organization. Cells combine into tissues, tissues combine into organs, and organs combine into organ systems that together make up the body. The four basic tissue types — epithelial, connective, nervous, and muscular — are the building blocks of every organ.

2. Epithelial Tissue

2.1 Characteristics & Cell Junctions

An epithelial tissue (or epithelium) consists of cells arranged in continuous sheets, in either single or multiple layers. Cells are closely packed with very little intercellular space between them.

Key Features

• Feature
  Avascular

  Lacks its own blood supply. Exchange of nutrients and wastes occurs via diffusion with adjacent, highly vascular connective tissue.

• Feature
  Nerve Supply

  Possesses its own nerve supply.

• Feature
  High Renewal Capacity

  Subjected to physical wear and tear, epithelial cells have a high capacity for cellular division and renewal.

• Feature
  Basement Membrane

  A thin extracellular layer that anchors the epithelium to underlying connective tissue.

Cell Junctions

• Junction
  Tight Junctions

  Form leak-proof seals between adjacent cell membranes.

• Junction
  Anchoring Junctions

  Adherens junctions & desmosomes fasten cells to one another or to extracellular material.

• Junction
  Gap Junctions

  Allow direct passage of ions and small molecules between adjacent cells for intercellular communication.

Figure: Epithelial Cell Junctions. Adjacent epithelial cells are joined near the apical surface by tight junctions (leak-proof seals), anchoring junctions such as desmosomes, and gap junctions (channels for direct cell-to-cell communication). The basement membrane anchors the epithelium to the underlying, vascular connective tissue.

2.2 Covering and Lining Epithelium

Classified based on layer arrangement and cell shape.

Layer Arrangement

• Layers
  Simple Epithelium

  Single layer of cells functioning in diffusion, osmosis, filtration, secretion, or absorption.

• Layers
  Pseudostratified Epithelium

  Appears to have multiple layers because nuclei lie at different levels, but all cells rest on the basement membrane (not all reach the apical surface).

• Layers
  Stratified Epithelium

  Two or more layers of cells protecting underlying tissues from wear and tear.

Cell Shapes

• Shape
  Squamous

  Thin, flattened cells arranged like floor tiles.

• Shape
  Cuboidal

  Cube-shaped cells, as tall as they are wide, often with central round nuclei.

• Shape
  Columnar

  Tall, column-like cells that protect underlying tissues and are specialized for secretion/absorption.

• Shape
  Transitional

  Variable shape that changes as organs stretch (e.g., in the urinary bladder).

2.3 Detailed Classification of Covering & Lining Epithelia

A. Simple Squamous Epithelium

Description: Single layer of flat, scale-like cells with a centrally located oval/flattened nucleus.
Locations: Lines heart, blood vessels, lymphatic vessels, air sacs (alveoli) of lungs, Bowman's capsule of kidneys, and inner surface of tympanic membrane; forms epithelial layer of serous membranes (peritoneum, pleura, pericardium).
Functions: Filtration, diffusion, osmosis, and secretion in serous membranes.

Figure: Simple Squamous Epithelium. Flat, tile-shaped cells form a single thin layer ideal for rapid filtration and diffusion.

B. Simple Cuboidal Epithelium

Description: Single layer of cube-shaped cells with a centrally located, round nucleus.
Locations: Covers surface of ovary, lines anterior surface of capsule of lens of eye, forms pigmented epithelium at posterior surface of eye, lines kidney tubules and smaller ducts of many glands, forms secreting portion of thyroid and pancreas.
Functions: Secretion and absorption.

Figure: Simple Cuboidal Epithelium. Cube-shaped cells with central round nuclei, well suited to secretion and absorption in ducts and glands.

C. Simple Columnar Epithelium (Nonciliated & Ciliated)

1. Nonciliated Simple Columnar Epithelium — Description: Single layer of nonciliated column-like cells with oval nuclei near the base; contains microvilli (fingerlike projections that increase plasma membrane surface area) and goblet cells (modified columnar cells secreting sticky mucus). Locations: Lines gastrointestinal tract from stomach to anus, ducts of many glands, and gallbladder. Functions: Secretion (mucus acts as lubricant/protective barrier) and absorption.

2. Ciliated Simple Columnar Epithelium — Description: Single layer of ciliated column-like cells with nuclei near the base; contains goblet cells interspersed. Locations: Lines a few portions of the upper respiratory tract, uterine (fallopian) tubes, uterus, some paranasal sinuses, central canal of spinal cord, and ventricles of the brain. Functions: Moves mucus and other substances by ciliary action (e.g., sweeps oocytes along the fallopian tubes).

Figure: Simple Columnar Epithelium. Left: nonciliated columnar cells with apical microvilli, interspersed with mucus-secreting goblet cells (G). Right: ciliated columnar cells whose beating cilia move mucus and other substances across the epithelial surface.

D. Pseudostratified Columnar Epithelium

Description: Appears to have multiple layers because cell nuclei are disposed at different levels; all cells are attached to the basement membrane, but not all extend to the apical surface. Can be ciliated or nonciliated.
Locations: Ciliated variety lines the airways of the upper respiratory tract.
Functions: Secretion and movement of mucus by ciliary action.

Figure: Pseudostratified Columnar Epithelium. Every cell touches the basement membrane, but only the tall, ciliated columnar cells extend all the way to the apical surface; the shorter basal cells stop short of it. Because nuclei sit at whatever height each cell's mid-body reaches, they appear at multiple staggered levels, giving a false ("pseudo") impression of multiple layers. A goblet cell (G) is shown among the ciliated cells.

E. Stratified Squamous Epithelium

Description: Several layers of cells; the apical layer consists of squamous cells, whereas deep layers range from cuboidal to columnar. Basal cells continuously divide to replace surface cells.
Keratinized Variety: Apical layers contain keratin (a tough protective protein); forms the superficial layer of skin.
Nonkeratinized Variety: Does not contain keratin; remains moist.
Locations: Nonkeratinized lines wet surfaces such as the mouth, esophagus, part of epiglottis, part of pharynx, vagina, and covers the tongue. Keratinized forms the epidermis of skin.
Functions: Protection against abrasion, water loss, ultraviolet radiation, and foreign invasion.

Figure: Stratified Squamous Epithelium. Dividing basal cells push upward, flattening into a protective squamous surface layer — well suited to areas subject to abrasion, such as skin and the lining of the mouth and esophagus.

F. Stratified Cuboidal Epithelium

Description: Two or more layers of cells in which cells in the apical layer are cube-shaped.
Locations: Ducts of adult sweat glands and pancreatic ducts.
Functions: Protection, limited secretion and absorption.

Figure: Stratified Cuboidal Epithelium. Two layers of cube-shaped cells line the ducts of sweat glands and the pancreas, providing extra protection while retaining secretory/absorptive ability.

G. Stratified Columnar Epithelium

Description: Several layers of irregularly shaped cells; only the apical layer consists of columnar cells.
Locations: Lines part of the urethra, large excretory ducts of some glands (e.g., esophageal glands), small areas in the anal mucous membrane.
Functions: Protection and secretion.

Figure: Stratified Columnar Epithelium. Only the apical layer is columnar; the basal layers are irregularly shaped, together providing protection while secreting mucus in large excretory ducts.

H. Transitional Epithelium

Description: Appearance is variable ("transitional"). In the relaxed state, it looks like stratified cuboidal epithelium; as the tissue stretches, cells flatten and take on a squamous appearance.
Locations: Lines the urinary bladder, portions of the ureters, and urethra.
Functions: Permits distension (stretching) without tearing.

Figure: Transitional Epithelium. In its relaxed state (left), transitional epithelium is several layers thick, with large, rounded "umbrella" cells domed over the surface. As the organ (e.g., bladder) distends (right), those same cells flatten and the tissue thins to fewer layers, allowing it to stretch without tearing.

2.4 Glandular Epithelium

A gland consists of epithelium that secretes substances into ducts, onto a surface, or into the blood.

1. Endocrine Glands

Description: Ductless glands. Secretions (hormones) enter interstitial fluid and diffuse directly into the bloodstream.
Locations: Pituitary, pineal, thyroid, parathyroid, adrenal glands, pancreas, ovaries, testes, thymus.
Function: Produce hormones that regulate metabolic and physiological activities.

2. Exocrine Glands

Description: Secrete products into ducts that empty onto the surface of a covering and lining epithelium (e.g., skin or the lumen of a hollow organ).
Locations: Sweat, oil, and earwax glands of skin; digestive glands such as salivary glands and the pancreas.
Function: Produce substances like sweat, oil, earwax, saliva, and digestive enzymes.

Classification of Exocrine Glands by Mechanism of Secretion:

• Type
  Merocrine Glands

  Secretions are synthesized on ribosomes attached to rough ER, processed/packaged by the Golgi complex, and released via exocytosis in secretory vesicles without loss of cell material (e.g., salivary glands, pancreas).

• Type
  Apocrine Glands

  Accumulate secretory product at the apical surface of the secreting cell, then pinch off that portion from the rest of the cell to release the secretion; the remaining cell repairs itself (e.g., mammary glands).

• Type
  Holocrine Glands

  Accumulate secretory product in the cytosol; as the cell matures, it ruptures completely, releasing the product; the sloughed cell is replaced by a new cell (e.g., sebaceous/oil glands of skin).

Figure: Mechanisms of Exocrine Secretion. Merocrine glands release product by exocytosis with no cell loss. Apocrine glands pinch off the apical portion of the cell along with the secretion. Holocrine glands accumulate product until the entire cell ruptures and is replaced.

3. Connective Tissue

3.1 General Components & Classification

Connective tissue is one of the most abundant and widely distributed tissues in the body. Unlike epithelial tissue, it consists of two basic elements:

• Element
  Cells

  Derived from embryonic mesenchymal cells (e.g., fibroblasts, macrophages, plasma cells, mast cells, adipocytes, chondrocytes, osteocytes).

• Element
  Extracellular Matrix

  Located between widely spaced cells; composed of ground substance and protein fibers.

Ground Substance: Semifluid, gelatinous, or calcified material containing water and organic molecules (polysaccharides, proteins).

Protein Fibers:

• Fiber
  Collagen Fibers

  Strong, flexible fibers made of collagen protein.

• Fiber
  Elastic Fibers

  Branching fibers composed of elastin and fibrillin that provide strength and elasticity.

• Fiber
  Reticular Fibers

  Fine, branching collagen fibers (type III collagen) forming supporting frameworks (stroma).

Vascularity: Highly vascular (rich blood supply), except for cartilage (avascular) and tendons/ligaments (scant blood supply).

Figure: Connective Tissue Matrix. Connective tissue is defined by its extracellular matrix — ground substance plus collagen, elastic, and reticular fibers — surrounding widely spaced cells such as fibroblasts and macrophages.

3.2 Classification of Connective Tissues

I. Embryonic Connective Tissue

• Type
  Mesenchyme

  Consists of irregularly shaped mesenchymal cells embedded in semifluid ground substance containing delicate reticular fibers. Found under skin and along developing bones of the embryo; forms almost all other connective tissues.

• Type
  Mucous Connective Tissue

  Consists of widely scattered fibroblasts embedded in viscous, jellylike ground substance containing fine collagen fibers. Found in the umbilical cord of the fetus; functions in support.

II. Mature Connective Tissue

A. Loose Connective Tissue — fibers loosely intertwined between cells

• Loose
  Areolar Connective Tissue

  Contains collagen, elastic, and reticular fibers arranged randomly, plus various cells (fibroblasts, macrophages, adipocytes, etc.) in a semifluid ground substance. Location: subcutaneous layer deep to skin, mucous membranes, around blood vessels, nerves, and organs. Function: strength, elasticity, and support.

• Loose
  Adipose Tissue

  Consists of adipocytes specialized for triglyceride (fat) storage; nucleus and cytoplasm pushed to the periphery by a large fat droplet. Includes white adipose (energy store, insulation) and brown adipose (heat generation in newborns). Location: subcutaneous layer, around heart and kidneys, yellow bone marrow. Function: thermal insulation, energy reserve, support, and protection.

• Loose
  Reticular Connective Tissue

  Network of interlacing reticular fibers and reticular cells. Location: stroma of liver, spleen, lymph nodes, red bone marrow. Function: forms the stroma of organs, binds smooth muscle cells, filters blood (spleen) and lymph (lymph nodes).

B. Dense Connective Tissue — thicker, more numerous fibers, fewer cells

• Dense
  Dense Regular

  Collagen fibers regularly arranged in parallel bundles with fibroblasts in rows between them. Location: forms tendons (attach muscle to bone) and ligaments (attach bone to bone). Function: strong attachment; resists pulling stress along the fiber axis.

• Dense
  Dense Irregular

  Collagen fibers irregularly arranged with a few fibroblasts. Location: dermis of skin, periosteum of bone, perichondrium of cartilage, heart valves, fibrous capsules of organs. Function: tensile strength in multiple directions.

• Dense
  Elastic

  Branching elastic fibers with fibroblasts in the spaces between fibers. Location: lung tissue, walls of elastic arteries, trachea, bronchial tubes, vocal cords. Function: allows stretching of various organs and recoil to original shape.

C. Cartilage — chondrocytes in lacunae, embedded in chondroitin sulfate matrix; avascular

• Cartilage
  Hyaline Cartilage

  Bluish-white, shiny ground substance with fine collagen fibers and prominent chondrocytes in lacunae; most abundant cartilage. Location: ends of long bones, ribs, nose, larynx, trachea, bronchi, fetal skeleton. Function: smooth surfaces for joint movement, flexibility, and support.

• Cartilage
  Fibrocartilage

  Chondrocytes scattered among clearly visible, thick bundles of collagen fibers; lacks perichondrium. Location: pubic symphysis, intervertebral discs, menisci of knee. Function: support and joining structures; shock absorption.

• Cartilage
  Elastic Cartilage

  Chondrocytes located in a threadlike network of elastic fibers within matrix; perichondrium present. Location: epiglottis, external ear, auditory tubes. Function: support while maintaining shape and elasticity.

D. Bone Tissue (Osseous Tissue)

Living tissue consisting of cells (osteoblasts, osteocytes, osteoclasts) and extracellular organic matrix (osteoid) calcified by calcium phosphate crystals.

• Bone
  Compact Bone

  Organized into repeating structural units called osteons (Haversian systems), each with a central canal, concentric lamellae, lacunae, and canaliculi.

• Bone
  Spongy Bone

  Lacks osteons; consists of slender columns called trabeculae containing lamellae, osteocytes, lacunae, and canaliculi. Spaces between trabeculae are filled with red bone marrow.

Figure: The Osteon. Compact bone is organized into osteons: concentric lamellae of calcified matrix surround a central (Haversian) canal carrying blood vessels and nerves. Osteocytes sit in lacunae between the lamellae, connected by tiny canaliculi that allow nutrients and waste to diffuse through the hard matrix.

E. Liquid Connective Tissue (Blood)

Description: Consists of blood plasma (liquid extracellular matrix) and formed elements:

• Blood
  Red Blood Cells

  Erythrocytes transport oxygen and carbon dioxide.

• Blood
  White Blood Cells

  Leukocytes carry out phagocytosis and immune responses.

• Blood
  Platelets

  Thrombocytes are essential for blood clotting.

Location: Within blood vessels (arteries, arterioles, capillaries, venules, veins) and chambers of the heart. Function: Transport of gases, nutrients, wastes, hormones; protection via immunity and clotting.

4. Nervous Tissue

Nervous tissue consists of two principal cell types:

• Cell Type
  Neurons (Nerve Cells)

  Sensitive to stimuli, convert stimuli into nerve impulses (action potentials), and conduct impulses to other neurons, muscle fibers, or glands.

• Cell Type
  Neuroglia

  Supportive, protective, and nutrient-supplying cells that do not generate or conduct nerve impulses. Comprise more than half the volume of neural tissue.

Structure of Neurons

A neuron consists of three main structural components:

• Part
  Cell Body (Soma)

  Contains the nucleus, cytoplasm, and organelle machinery.

• Part
  Dendrites

  Highly branched, short processes that act as the receiving or input portions of the neuron.

• Part
  Axon

  Single, thin, cylindrical process that conducts nerve impulses away from the cell body toward another neuron, muscle fiber, or gland.

Axon Hillock: Cone-shaped elevation joining the cell body to the axon. Myelin Sheath: Multilayered lipid and protein covering that insulates the axon and increases the speed of impulse conduction. Node of Ranvier: Unmyelinated gap in the myelin sheath along the axon. Axon Terminals & Synaptic Knobs: Fine axon branches ending in swollen bulbous structures that store neurotransmitters in synaptic vesicles.

Figure: Basic Neuron Structure. Dendrites receive signals and pass them to the cell body; the axon hillock initiates the impulse that travels along the myelin-insulated axon, jumping between nodes of Ranvier, before reaching the axon terminals and synaptic knobs where neurotransmitters are released.

5. Muscular Tissue

Muscular tissue consists of elongated cells called muscle fibers or myocytes that use ATP to generate force. It is classified into three types based on location, structural, and functional features.

Comparison of Muscle Tissue Types

Figure: The Three Types of Muscle Tissue. Skeletal muscle fibers are long, unbranched, striated, and multinucleated. Cardiac fibers branch and join at intercalated discs, each with one central nucleus. Smooth muscle cells are spindle-shaped with tapered ends and a single central nucleus, lacking striations.

6. Organ Systems of the Human Body

The human body contains 11 major organ systems that work together to maintain homeostasis (a stable internal environment).