Chapter 5: The Human Tissues - Anatomy & Physiology Study Notes
Introduction to Tissues
The human body comprises approximately 50 trillion cells, categorized into 200 different cell types.
Tissues are groups of similar cells and cell products that work together to perform specific roles within an organ.
Organs are structures with discrete boundaries, composed of two or more tissue types.
Histology (microscopic anatomy): The study of tissues and how they are arranged into organs.
The four broad categories of tissues are:
Epithelial tissue
Connective tissue
Nervous tissue
Muscular tissue
Primary Tissue Classes
Tissues differ in:
Types and functions of their cells
Characteristics of the matrix (extracellular material)
Relative amount of space occupied by cells and matrix
Matrix (extracellular material) is composed of:
Fibrous proteins (e.g., collagen)
A clear gel called ground substance, also known as tissue fluid, extracellular fluid (ECF), or interstitial fluid.
Ground substance contains water, gases, minerals, nutrients, wastes, and hormones.
Embryonic Tissues
Human development begins with a fertilized egg that divides to produce the first tissue layers.
Three primary germ layers are formed:
Ectoderm (outer layer):
Gives rise to the epidermis (outer skin layer) and the nervous system.
Endoderm (inner layer):
Gives rise to the mucous membrane lining of the digestive and respiratory tracts, digestive glands, and other internal structures.
Mesoderm (middle layer):
Becomes a gelatinous tissue called mesenchyme, which consists of wispy collagen fibers and fibroblasts in a gel matrix.
Mesenchyme gives rise to cartilage, bone, and blood.
Interpreting Tissue Sections
Not all tissues able to be cut:
Smear: Tissue is rubbed or spread across a slide (e.g., blood, spinal cord).
Spread: Tissues are laid out on a slide (e.g., membranes, areolar tissue).
Epithelial Tissue
Properties and Structure
Epithelial tissue: A sheet of closely adhering cells, one or more cells thick, with an upper surface exposed to the environment or an internal space.
Covers body surfaces and lines body cavities; constitutes most glands.
Avascular: Lacks blood vessels; usually nourished by the underlying connective tissue.
High rate of mitosis in cells near the connective tissue, allowing for regeneration.
Rests on a basement membrane, which is a layer between an epithelium and underlying connective tissue.
Composed of collagen, glycoproteins, and other protein-carbohydrate complexes.
Anchors the epithelium to the connective tissue below it.
General structure of epithelial cells' surfaces:
Basal surface: Faces the basement membrane.
Apical surface: Faces away from the basement membrane (exposed surface).
Lateral surface: The "sidewall" surface between the basal and apical surfaces.
Classification by number of cell layers:
Simple epithelia: Consist of a single layer of cells, with each cell anchored to the basement membrane.
Stratified epithelia: Consist of multiple layers of cells, where cells are stacked on top of each other, meaning not all cells contact the basement membrane.
Functions
Protection: Protects deeper tissues from injury and infection.
Secretion: Produces and releases substances like mucus, sweat, enzymes, and hormones.
Excretion: Voids wastes from the tissues.
Absorption: Absorbs chemicals, such as nutrients.
Filtration: Selectively filters substances leaving the body.
Sensation: Contains nerve endings to detect stimuli.
Simple Epithelia Types
Composed of a single layer of cells.
Simple Squamous Epithelium:
Description: Single row of thin, scaly cells.
Function: Permits rapid diffusion or transport of substances, secretes serous fluid.
Locations: Alveoli of lungs, glomeruli of kidneys, endothelium (lining of blood vessels), serosa (outer covering of organs).
Simple Cuboidal Epithelium:
Description: Single layer of square or round cells.
Function: Absorption and secretion, mucus production and movement.
Locations: Liver, thyroid, mammary and salivary glands, bronchioles, kidney tubules.
Simple Columnar Epithelium:
Description: Single row of tall, narrow cells with oval nuclei typically in the basal half of the cell. Often has a brush border of microvilli, sometimes ciliated, and may possess goblet cells.
Function: Absorption and secretion, secretion of mucus.
Locations: Lining of the gastrointestinal (GI) tract, uterus, kidneys, and uterine tubes.
Goblet cells: Wineglass-shaped mucus-secreting cells found in simple columnar and pseudostratified epithelia.
Pseudostratified Columnar Epithelium:
Description: Appears multilayered because some cells are taller than others, and nuclei are at various levels, but all cells reach the basement membrane (though not all reach the surface). Characterized by cilia and goblet cells.
Function: Secretes and propels mucus.
Locations: Respiratory tract, portions of the male urethra.
Stratified Epithelia Types
Composed of 2 to 20 or more layers of cells; only the deepest layer attaches to the basement membrane.
Exfoliation (Desquamation): Daughter cells from the mitotically active stem cells in the deepest layer are pushed toward the surface, flatten, die, and flake off.
Stratified Squamous Epithelium: Most widespread stratified epithelium.
Deepest cells: Cuboidal to columnar, include mitotically active stem cells.
Keratinized (Cornified) Stratified Squamous Epithelium:
Description: Multiple cell layers; cells become flat and scaly toward the surface, with a surface layer of dead cells containing keratin.
Function: Resists abrasion, retards water loss through the skin, resists penetration by pathogenic organisms.
Locations: Epidermis of the skin; heavily keratinized in palms and soles.
Nonkeratinized Stratified Squamous Epithelium:
Description: Similar to keratinized epithelium but lacks the surface layer of dead cells.
Function: Resists abrasion and penetration of pathogens.
Locations: Tongue, oral mucosa, esophagus, vagina.
Stratified Cuboidal Epithelium:
Description: Two or more cell layers; surface cells are square or round.
Function: Secretes sweat, produces sperm, produces ovarian hormones.
Locations: Sweat gland ducts, ovarian follicles, seminiferous tubules.
Stratified Columnar Epithelium: Rare; found only in transition zones where two other epithelial types meet.
Urothelium (Transitional Epithelium):
Description: Multilayered epithelium with surface cells that change from round to flat when stretched.
Function: Allows for filling and stretching of the urinary tract.
Locations: Ureter and bladder.
Connective Tissue
Overview and General Properties
Connective tissue: The most abundant, widely distributed, and variable type of tissue.
Characteristic feature: Cells occupy less space than the matrix.
Most cells are not in direct contact with each other.
Vascularity: Highly variable; loose connective tissues have many blood vessels, while cartilage is avascular (no blood vessels).
Functions of Connective Tissues
Binding of organs: Connects bones to bones (ligaments), muscles to bones (tendons), skin to muscle, and holds organs in place.
Support: Supports the body and its organs, forming internal frameworks.
Physical protection: Protects and cushions delicate organs.
Immune protection: Connective tissue cells (e.g., macrophages, WBCs) attack foreign invaders.
Movement: Bones provide a lever system for body movement; cartilages are involved in speech.
Storage: Maintains stores of fat, calcium, and phosphorus.
Heat production: Metabolism of brown fat generates heat.
Transport: Blood transports gases (O2, CO2), nutrients, wastes, hormones, and blood cells.
Fibrous Connective Tissue
Components
Cells:
Fibroblasts: Produce the fibers and ground substance of the matrix.
Macrophages: Arise from monocytes; phagocytize foreign material and activate the immune system when sensing foreign antigens.
Leukocytes (White Blood Cells - WBCs): Function in immune defense. Examples include:
Neutrophils: Attack bacteria.
Lymphocytes: React against bacteria, toxins, and other foreign agents.
Plasma cells: Arise from lymphocytes and synthesize antibodies.
Mast cells: Secrete heparin (inhibits clotting) and histamine (dilates blood vessels).
Adipocytes (fat cells): Clustered in some fibrous tissues; when dominant, they form adipose tissue.
Fibers:
Collagenous fibers: Made of collagen protein.
Properties: Tough, flexible, and stretch-resistant. Appear as "white fibers" in fresh tissue.
Locations: Tendons, ligaments, and the deep layer of the skin (mostly collagen); less visible in the matrix of cartilage and bone.
Reticular fibers: Thin collagen fibers coated with glycoprotein.
Properties: Form a delicate framework.
Locations: Framework (stroma) of the spleen and lymph nodes; part of basement membranes under epithelia.
Elastic fibers: Thinner than collagenous fibers; made of the protein elastin.
Properties: Allows stretch and recoil, similar to a rubber band.
Ground Substance:
Description: Featureless substance surrounding cells, usually gelatinous or rubbery.
Function: Absorbs compressive forces.
Components:
Glycosaminoglycans (GAGs): Long polysaccharides composed of amino sugars and uronic acid.
Functions: Regulate water and electrolyte balance of tissues.
Examples: Chondroitin sulfate, heparin, hyaluronic acid.
Proteoglycans: Gigantic molecules shaped like bottle brushes.
Function: Form gravy-like colloids that hold tissues together.
Adhesive glycoproteins: Protein-carbohydrate complexes.
Function: Bind components of a tissue together.
Types of Fibrous Connective Tissue
Loose Connective Tissue: Characterized by abundant ground substance in the space surrounding cells.
Areolar Tissue:
Description: Loosely organized fibers, abundant blood vessels. Possesses all six cell types and all fiber types.
Locations: Underlies epithelia, in serous membranes, between muscles, passageways for nerves and blood vessels.
Reticular Tissue:
Description: Mesh of reticular fibers and fibroblasts.
Function: Forms the supportive stroma (framework) for lymphatic organs.
Locations: Lymph nodes, spleen, thymus, and bone marrow.
Dense Connective Tissue: Characterized by a predominance of fibers in the space surrounding cells.
Dense Regular Connective Tissue:
Description: Densely packed, parallel collagen fibers, with compressed fibroblast nuclei. Elastic tissue forms wavy sheets in some locations.
Function: Great tensile strength in one direction.
Locations: Tendons (attach muscles to bones) and ligaments (hold bones together).
Dense Irregular Connective Tissue:
Description: Densely packed, randomly arranged collagen fibers with few visible cells.
Function: Withstands unpredictable stresses from multiple directions.
Locations: Deeper layer of the skin (dermis), capsules around organs.
Adipose Tissue (Fat)
Adipose tissue: A connective tissue dominated by adipocytes (fat cells).
The space between adipocytes contains areolar tissue, reticular tissue, and blood capillaries.
Primary energy reservoir of the body.
Quantity of stored triglyceride and number of adipocytes are relatively stable; fat is continuously recycled.
Two types in humans:
White Adipose Tissue (WAT or white fat):
Most abundant and significant type in adults.
Functions: Thermal insulation, cushions organs (e.g., eyeballs, kidneys), contributes to body contours (e.g., female breasts and hips), secretes hormones that regulate metabolism.
Description: Empty-looking cells with thin margins; nucleus pressed against cell membrane due to large lipid droplet.
Brown Adipose Tissue (BAT or brown fat):
Found mainly in fetuses, infants, and children; smaller deposits in adults.
Color: Derived from abundant blood vessels, mitochondria, and mitochondrial enzymes.
Function: Heat-generating tissue, especially important for thermoregulation in infants.
Cartilage
Cartilage: A stiff connective tissue with a flexible, rubbery matrix.
Gives shape to structures like the ear, tip of the nose, and larynx.
General features:
Chondroblasts: Cartilage cells that produce the matrix.
Chondrocytes: Mature cartilage cells that become trapped in lacunae (cavities) within the matrix.
Perichondrium: Sheath of dense irregular connective tissue that surrounds elastic and most hyaline cartilage (absent in articular cartilage).
Contains a reserve population of chondroblasts that contribute to cartilage growth throughout life.
Avascular: No blood capillaries. Nutrients are brought in and wastes removed by diffusion, leading to slow healing.
Matrix: Rich in GAGs and contains collagen fibers.
Types of cartilage vary with fiber composition:
Hyaline Cartilage:
Description: Clear, glassy appearance due to the fineness of its collagen fibers.
Function: Eases joint movement, holds airways open, moves vocal cords, provides for growth of juvenile long bones.
Locations: Articular cartilage (covers ends of bones at joints), costal cartilage (attaches ribs to sternum), trachea, larynx, fetal skeleton.
Elastic Cartilage:
Description: Contains an abundance of elastic fibers.
Features: Covered with perichondrium.
Function: Provides flexible, elastic support.
Locations: External ear and epiglottis.
Fibrocartilage:
Description: Contains large, coarse bundles of collagen fibers.
Function: Resists compression and absorbs shock.
Locations: Pubic symphysis, menisci of the knee, and intervertebral discs.
Bone (Osseous Tissue)
Bone: A calcified connective tissue that composes the skeleton.
Two forms of osseous tissue:
Spongy bone: Delicate slivers and plates give it a spongy appearance.
Locations: Heads of long bones and in the middle of flat bones (e.g., sternum).
Compact (dense) bone: Denser, calcified tissue with no visible spaces.
Locations: Forms the external surfaces of all bones.
Features of compact bone:
Arranged in cylinders called osteons.
Central (osteonic) canal: Passage running longitudinally along the bone shaft; contains blood vessels and nerves.
Concentric lamellae: Ring-like layers of bone matrix surrounding the central canal.
Osteon: A central canal and its surrounding lamellae.
Osteocytes: Mature bone cells, located within small cavities called lacunae between lamellae.
Canaliculi: Delicate canals radiating from each lacuna to its neighbors, allowing osteocytes to communicate and exchange nutrients.
Periosteum: Tough fibrous connective tissue covering the entire bone.
Blood
Blood: A fluid connective tissue that travels through tubular blood vessels.
Function: Transports cells and dissolved matter (gases, nutrients, wastes, hormones) from place to place.
Contains formed elements suspended in a liquid ground substance called blood plasma.
Formed elements include:
Erythrocytes (red blood cells, RBCs):
Appearance: Pale pink discs with light centers.
Function: Transport O2 and CO2.
Leukocytes (white blood cells, WBCs):
Appearance: Slightly larger than RBCs and have variously shaped nuclei.
Function: Defend against infection and disease.
Examples: Neutrophils, eosinophils, basophils, lymphocytes, monocytes.
Platelets: Small cell fragments with no nuclei.
Function: Involved in blood clotting.
Nervous and Muscular Tissues: Excitable Tissues
Excitability
Excitability: The ability to respond to stimuli by changing the membrane potential.
This property is developed to the highest degree in nervous and muscular tissues, hence they are called excitable tissues.
Excitation is founded on a charge difference (voltage) across the cell membrane, known as the membrane potential.
In nerve cells, changes in voltage result in rapid transmission of signals to other cells.
In muscle cells, changes in voltage result in contraction and shortening of the cell.
Nervous Tissue
Nervous tissue: Specialized for communication via electrical and chemical signals.
Consists of:
Neurons (nerve cells):
Function: Detect stimuli, respond quickly, and transmit coded information rapidly to other cells.
Parts of a neuron:
Cell body (soma): Houses the nucleus and other organelles; controls protein synthesis.
Dendrites: Short, branched processes that receive signals from other cells and transmit messages to the cell body.
Axon (nerve fiber): A long process that sends outgoing signals to other cells; can be more than a meter long.
Neuroglia (glial cells):
Function: Protect and assist neurons; often referred to as the "housekeepers" of the nervous system.
Muscular Tissue
Muscular tissue: Specialized to contract when stimulated, exerting a physical force on other tissues, organs, or fluids.
Also an important source of body heat.
Three types:
Skeletal Muscle:
Description: Made of long, thin cells called muscle fibers. Contains multiple nuclei adjacent to the plasma membrane. Exhibits striations (alternating dark and light bands).
Control: Voluntary (under conscious control).
Locations: Most skeletal muscles attach to bones.
Cardiac Muscle:
Description: Limited to the heart wall. Composed of cardiomyocytes which are short, branched cells with one centrally located nucleus. Joined end-to-end by intercalated discs.
Control: Striated and involuntary (not under conscious control).
Location: Heart wall.
Smooth Muscle:
Description: Made of short, fusiform-shaped cells (tapered at ends). Cells have one central nucleus and no striations.
Control: Involuntary function.
Locations: Most is visceral muscle, forming parts of the walls of hollow organs (e.g., digestive tract, blood vessels, bladder).
Cellular Junctions, Glands, and Membranes
Cellular Junctions
Cellular junctions: Connections between two cells, anchoring them to each other or their matrix.
Functions: Cells communicate, resist mechanical stress, and control what moves through the gaps between them.
Three types:
Tight junctions:
Description: Zipper-like, interlocking linkage between two adjacent cells formed by transmembrane cell-adhesion proteins.
Features: Encircles an epithelial cell near its apical surface, joining it with neighboring cells.
Function: Seals off intercellular space, making it difficult or impossible for substances to pass between cells.
Example: In stomach and intestines, prevent digestive juices from seeping between epithelial cells and digesting underlying connective tissue.
Desmosomes:
Description: A patch that holds cells together, resembling a clothing snap.
Features: Hook-like, J-shaped proteins arise from the cytoskeleton.
Function: Keeps cells from pulling apart, resisting mechanical stress.
Locations: Common in the epidermis and other epithelia.
Hemidesmosome: A "half-desmosome" that anchors basal cells of an epithelium to an underlying basement membrane, preventing the epithelium from easily peeling away.
Gap (communicating) junctions:
Description: A channel between cells formed by a ring-like connexon.
Features: A connexon consists of six transmembrane proteins arranged like segments of an orange around a water-filled channel.
Function: Allows ions, nutrients, and other small solutes to pass directly between cells.
Locations: Found in cardiac and smooth muscle, embryonic tissue, lens, and cornea, facilitating rapid communication and coordinated activity.
Glands
Gland: A cell or organ that secretes substances for use elsewhere in the body or releases them for elimination from the body.
Usually composed of epithelial tissue within a connective tissue framework and capsule.
Secretion: A product useful to the body.
Excretion: A waste product.
Types of Glands
Exocrine glands:
Description: Maintain their contact with the surface of the epithelium by way of a duct (tube).
Surfaces: Can be external (e.g., sweat, tear glands) or internal (e.g., pancreas, salivary glands).
Endocrine glands:
Description: Have no ducts but possess many blood capillaries; they secrete hormones directly into the blood.
Hormones: Chemical messengers that stimulate cells elsewhere in the body.
Examples: Thyroid, adrenal, and pituitary glands.
Unicellular glands:
Description: Secretory cells found in an epithelium that is predominantly non-secretory.
Can be exocrine or endocrine.
Examples: Mucus-secreting goblet cells in the trachea or endocrine cells of the stomach.
Exocrine Gland Structure Classification
By Duct Shape:
Simple: Unbranched duct.
Compound: Branched duct.
By Secretory Portions:
Tubular: Narrow secretory portion, where the duct and secretory portion are of uniform diameter.
Acinar (Alveolar): Secretory cells form a dilated sac (acinus or alveolus).
Tubuloacinar (Tubuloalveolar): Secretory cells are present in both tubular and acinar portions.
Types of Secretions
Serous glands: Produce thin, watery secretions, such as perspiration, milk, tears, and digestive juices.
Mucous glands: Produce a glycoprotein called mucin, which absorbs water to form mucus.
Goblet cells: Unicellular mucous glands.
Mixed glands: Contain both serous and mucous cell types and produce a mixture of both types of secretions (e.g., the two pairs of salivary glands in the chin).
Modes of Secretion
Eccrine (merocrine) secretion:
Mechanism: Release their products by exocytosis (vesicles fuse with the cell membrane to expel contents).
Examples: Tear glands, pancreas, gastric glands.
Apocrine secretion:
Mechanism: A lipid droplet covered by membrane and cytoplasm buds from the cell surface.
Example: Mode of milk fat secretion by mammary gland cells (though "apocrine glands" in skin are actually modified eccrine glands).
Holocrine secretion:
Mechanism: Cells accumulate a product and then the entire cell disintegrates, releasing a mixture of cell fragments and synthesized substances.
Examples: Oil glands of the scalp and skin (sebaceous glands), and glands of the eyelids.
Membranes
Membranes can be composed of epithelial tissue only, connective tissue only, or a combination of epithelial, connective, and muscular tissues.
Cutaneous membrane (the skin):
Largest membrane in the body.
Composition: Stratified squamous epithelium (epidermis) resting on a layer of connective tissue (dermis).
Function: Relatively dry layer that serves a protective function.
Mucous membrane (mucosa):
Description: Lines passages that open to the external environment (e.g., mouth, digestive tract, respiratory tract, urinary, and reproductive tracts).
Sublayers:
Epithelium (e.g., simple columnar, pseudostratified columnar, stratified squamous)
Lamina propria: A layer of areolar connective tissue.
Muscularis mucosae: A thin layer of smooth muscle.
Functions: Absorptive, secretory, and protective functions; often contains goblet cells to secrete mucus.
Serous membrane (serosa):
Description: Lines some internal body cavities (e.e., peritoneal, pleural, pericardial cavities).
Composition: Simple squamous epithelium (called a mesothelium) resting on a layer of areolar tissue.
Function: Produces serous fluid (which arises from blood plasma) to lubricate organs and reduce friction.
Examples: Pleurae (around lungs), pericardium (around heart), peritoneum (in abdominopelvic cavity).
Tissue Repair
Damaged tissues can be repaired in two ways:
Regeneration: The replacement of dead or damaged cells by the same type of cell as before.
Restores normal function.
Examples: Repair of minor skin injuries, liver regeneration after injury.
Fibrosis: The replacement of damaged cells with scar tissue.
Scar tissue holds organs together but does not restore normal function.
Examples: Repair of severe cuts and burns, scarring of lungs in tuberculosis or severe pneumonia.
Four Stages in the Healing of a Skin Wound
Bleeding and Inflammation:
Severed vessels bleed into the cut.
Mast cells and damaged cells release histamine, which dilates blood vessels and increases capillary permeability.
Blood plasma seeps into the wound, carrying antibodies and clotting proteins.
Scab Formation and Macrophage Activity:
A blood clot forms, knitting the edges of the cut together and inhibiting the spread of pathogens.
The clot forms a scab that temporarily seals the wound and blocks infection.
Macrophages phagocytize and digest tissue debris.
Formation of Granulation Tissue (Fibroblastic/Reconstructive Phase):
Begins 3-4 days after injury and lasts up to 2 weeks.
New capillaries sprout from nearby vessels.
Deeper portions of the clot become infiltrated by capillaries and fibroblasts, transforming into a soft mass called granulation tissue.
Macrophages continue to remove the blood clot.
Fibroblasts deposit new collagen.
Epithelial Regeneration and Connective Tissue Fibrosis (Remodeling/Maturation Phase):
Begins several weeks after injury and may last up to 2 years.
Epithelial cells around the wound multiply and migrate beneath the scab; the epithelium regenerates.
The underlying connective tissue undergoes fibrosis.
Scar tissue may or may not be visible through the regenerated epithelium.