Chapter 5: Human Tissues Vocabulary

Overview of Human Tissues and Histology

A tissue is defined as a group of similar cells working together to perform a specific, specialized function. The microscopic study of tissues and their cellular structures is known as histology.

All organs in the body are composed of combinations of four primary tissue classes:

Primary Tissue Classes

• Epithelial Tissue: Consists of layers of closely spaced cells that cover organ surfaces, line body cavities, form glands, and serve essential roles in protection, secretion, and absorption.

  • Representative Locations: Epidermis, inner lining of the digestive tract, liver, and other glands.

• Connective Tissue: Tissue in which the extracellular matrix usually occupies more volume than the cells themselves. Specialized to support, protect organs, and bind other tissues and organs together.

  • Representative Locations: Tendons, ligaments, cartilage, bone, and blood.

• Nervous Tissue: Contains highly excitable cells specialized for the rapid transmission of coded electrical information to other cells.

  • Representative Locations: Brain, spinal cord, and peripheral nerves.

• Muscular Tissue: Composed of elongated, excitable muscle cells specialized for contraction and movement generation.

  • Representative Locations: Skeletal muscles, heart (cardiac muscle), and walls of hollow internal organs/viscera (smooth muscle).

Embryonic Origin of Tissues

During early embryonic development, tissues arise from three distinct primary germ layers:

1. Endoderm (Innermost Germ Layer):

   • Gives rise to the inner epithelial lining of the digestive tract and reproductive tract, as well as many associated visceral organs.

2. Mesoderm (Middle Germ Layer):

   • Gives rise to muscle tissue, bone tissue, blood, and most other connective tissues.

3. Ectoderm (Outermost Germ Layer):

   • Gives rise to the epidermis of the skin, hair, nails, and the entire nervous system.

Epithelial Tissue Principles and Classification

Characteristics of Epithelial Tissue

• Composed of tightly packed cells with minimal extracellular matrix between them.

• Covers body surfaces, lines internal body cavities, and forms the secretory units and ducts of most glands.

• Avascular: Epithelia lack a direct blood supply; they rely on nutrients diffusing from blood vessels located in the underlying connective tissue.

• Cellular Polarity: Epithelial cells possess defined structural orientation:

  • Apical surface: Faces an exposed exterior surface or an internal cavity/lumen.

  • Basal surface: Rests upon and adheres to a basement membrane.

  • Lateral surface: Contacts neighboring epithelial cells through specialized cell junctions.

Classification Principles

Epithelia are categorized based on two main criteria: the number of cell layers and the shape of the surface cells.

• Layer Classification:

  • Simple Epithelia: Contain a single layer of cells. Every cell rests directly on and touches the basement membrane.

  • Stratified Epithelia: Contain two or more layers of cells. Surface cells rest upon deeper layers and do not touch the basement membrane.

  • Pseudostratified Epithelium: Appears multilayered because cell nuclei are positioned at different heights, but it is technically a simple epithelium because all cells contact the basement membrane.

• Cell Shape Classification (determined by the shape of the cells at the apical surface):

  • Squamous: Thin, flat, scale-like cells.

  • Cuboidal: Cube-shaped or square cells with equal height and width.

  • Columnar: Tall, rectangular, column-like cells.

Specific Epithelial Tissue Types

Simple Squamous Epithelium

• Structure: Single layer of thin, flattened cells.

• Functions: Allows rapid diffusion or transport of substances across membranes; secretes lubricating serous fluid.

• Locations: Air sacs (alveoli) of lungs, kidney glomeruli, inner lining of blood vessels (endothelium), and serous membranes (serosa).

Simple Cuboidal Epithelium

• Structure: Single row of square or cube-shaped cells with centrally located round nuclei.

• Functions: Facilitates absorption and secretion; produces mucus.

• Locations: Kidney tubules, thyroid gland, liver, mammary glands, salivary glands, and bronchioles.

Simple Columnar Epithelium

• Structure: Single row of tall, narrow, slender cells. Nuclei are oval-shaped and situated in the basal half of the cells. Features an apical brush border of microvilli and often contains mucus-secreting goblet cells.

• Functions: Active absorption and secretion; secretion of mucus.

• Locations: Lining of the gastrointestinal (GI) tract (stomach to intestines), uterus, uterine tubes, and kidney collection ducts.

Pseudostratified Columnar Epithelium

• Structure: Appears stratified due to nuclei positioned at multiple levels, but all cells remain in contact with the basement membrane. Surface cells commonly bear hair-like cilia and are interspersed with goblet cells.

• Functions: Secretes and propels mucus across surfaces via ciliary movement.

• Locations: Upper and lower respiratory tract (trachea, bronchi) and portions of the male urethra.

Keratinized Stratified Squamous Epithelium

• Structure: Multiple layers of cells. Basal cells are active and cuboidal or columnar, while superficial cells are flattened, dead, and packed with the durable protein keratin.

• Functions: Resists mechanical abrasion; retards water loss through skin; prevents penetration by pathogenic microorganisms.

• Locations: Epidermis of the skin, particularly thick skin on the palms of hands and soles of feet.

Non-keratinized Stratified Squamous Epithelium

• Structure: Similar to keratinized stratified squamous epithelium, but lacks the superficial layer of dead, keratinized cells. Apical cells remain alive and moist.

• Functions: Resists mechanical abrasion and invasion by pathogens while maintaining a moist surface.

• Locations: Esophagus, vagina, tongue, and oral mucosa.

Stratified Cuboidal Epithelium

• Structure: Two or more layers of cells, with the surface cells possessing a distinct square or round shape.

• Functions: Secretes sweat; contributes to sperm production and ovarian hormone synthesis.

• Locations: Ducts of sweat glands, developing ovarian follicles, and seminiferous tubules of the testes.

Urothelium (Transitional Epithelium)

• Structure: Multilayered epithelium featuring rounded superficial umbrella cells that transform and flatten into squamous-like forms when stretched.

• Functions: Accommodates rapid stretching and volume changes during filling of the urinary tract.

• Locations: Urinary bladder, ureters, and part of the urethra.

Connective Tissue Structure and Components

Connective tissue is the most abundant and structurally diverse tissue class in the human body. Cells are widely separated by an abundant extracellular matrix, and most cells do not touch one another. Connective tissues perform critical roles in binding organs, providing structural support, physical protection, energy storage, and thermal insulation.

Vascularity varies dramatically across connective tissues, ranging from highly vascularized loose tissue to completely avascular cartilage.

Cellular Components of Connective Tissue

• Fibroblasts: Large, flat cells that produce protein fibers and ground substance of the matrix.

• Macrophages: Phagocytic cells that engulf and destroy bacteria, cellular debris, and foreign material.

• Leukocytes: White blood cells (such as neutrophils and lymphocytes) that enter from blood vessels to provide immune defense.

• Adipocytes: Fat cells specialized for storing triglycerides within large internal lipid droplets.

• Mesenchymal Cells: Stem cells that give rise to various connective tissue cell types.

Extracellular Matrix Fibers

• Collagenous Fibers: Made of collagen protein; extremely tough, flexible, and resistant to stretching. They form coarse, bright white fibers in fresh tissue.

• Reticular Fibers: Very thin, branched collagen fibers coated with glycoprotein. They create a fine sponge-like framework (stroma) for organs.

• Elastic Fibers: Thinner fibers made of the protein elastin. They allow tissues to stretch under tension and recoil back to original shape without damage.

Ground Substance

The ground substance is a featureless medium filling the space between cells and fibers. Depending on the tissue type, its consistency ranges from watery or gelatinous to rubbery or calcified and solid.

Types of Fibrous, Cartilaginous, Bone, and Fluid Connective Tissues

Fibrous Connective Tissues

• Areolar Tissue:

  • Structure: Loosely organized collagen and elastic fibers running in random directions, abundant ground substance, and numerous blood vessels.

  • Locations: Upper dermis of skin, underlying serous membranes, surrounding blood vessels and nerves, and in spaces between muscles.

• Reticular Tissue:

  • Structure: Meshwork of thin reticular fibers interspersed with abundant fibroblasts and leukocytes.

  • Functions: Forms the supportive internal framework (stroma) of lymphatic organs.

  • Locations: Spleen, lymph nodes, bone marrow, and thymus.

• Dense Regular Connective Tissue:

  • Structure: Densely packed, parallel bundles of collagen fibers with flattened fibroblast nuclei squeezed between them. Contains minimal open space and few blood vessels. Elastic forms create wavy sheets.

  • Functions: Provides high tensile strength in a single direction.

  • Locations: Tendons (attaching muscle to bone) and ligaments (holding bone to bone).

• Dense Irregular Connective Tissue:

  • Structure: Thick bundles of collagen fibers arranged randomly in unpredictably oriented directions with few visible cells.

  • Functions: Withstands mechanical stresses applied in multiple directions.

  • Locations: Deeper layer (reticular layer) of the dermis and protective capsules around internal organs.

• Adipose Tissue:

  • Structure: Dominated by adipocytes—cells that appear empty with thin rims of cytoplasm and nuclei pressed tightly against the outer cell membrane due to large central lipid droplets.

  • Functions: Primary energy storage site, thermal insulation, and protective cushioning for delicate organs.

Cartilage Tissues

Cartilage features a firm, flexible matrix produced by chondroblasts, which become trapped in tiny matrix spaces called lacunae and are then called chondrocytes.

• Hyaline Cartilage:

  • Structure: Clear, glassy, bluish-white matrix containing very fine, invisible collagen fibers.

  • Functions: Eases joint movement, holds airways open, moves vocal cords, and provides the template for long bone growth in juveniles.

  • Locations: Articular surfaces of synovial joints, costal cartilages of ribs, trachea, larynx, and fetal skeleton.

• Elastic Cartilage:

  • Structure: Contains a dense network of dark elastic fibers embedded in matrix, surrounded by a distinct fibrous layer called the perichondrium.

  • Functions: Provides flexible, elastic support while maintaining structural shape.

  • Locations: External ear (pinna) and epiglottis.

• Fibrocartilage:

  • Structure: Matrix contains coarse, visibly dense parallel bundles of collagen fibers with rows of chondrocytes in lacunae; lacks a perichondrium.

  • Functions: Resists high compression forces and absorbs shock.

  • Locations: Intervertebral discs, pubic symphysis, and knee joint menisci.

Bone Tissue (Compact Bone)

• Structure: Calcified matrix arranged in concentric rings called concentric lamellae centered around a central canal containing blood vessels and nerves. This structural unit is called an osteon.

• Cellular Elements:

  • Osteocytes: Mature bone cells housed within tiny cavities called lacunae.

  • Canaliculi: Tiny canals radiating from each lacuna to connect neighboring lacunae, allowing osteocytes to extend cell processes and exchange nutrients.

Fluid Connective Tissue (Blood)

Blood is a liquid connective tissue responsible for transporting cells, nutrients, wastes, and chemical signals.

• Plasma: The liquid extracellular matrix.

• Formed Elements:

  • Erythrocytes (Red Blood Cells / RBCs): Disk-shaped cells lacking nuclei; transport oxygen ($O_2$) and carbon dioxide ($CO_2$).

  • Leukocytes (White Blood Cells / WBCs): Nucleated immune cells (including neutrophils, lymphocytes, and monocytes) that defend against pathogens and disease.

  • Thrombocytes (Platelets): Membrane-bound cell fragments essential for blood clotting.

Excitable Tissues: Nervous and Muscle Tissues

Nervous and muscular tissues are categorized as excitable tissues because their cells respond to environmental stimuli by altering their membrane potential (electrical voltage across the plasma membrane).

• In nerve cells, membrane potential changes trigger rapid transmission of electrical signals to other cells.

• In muscle cells, membrane potential changes stimulate cell contraction and shortening.

Nervous Tissue

Specialized for rapid intercellular communication using electrical and chemical signals.

• Neurons (Nerve Cells): Functional units that detect stimuli, process information, and transmit signals.

  • Neurosoma: Cell body housing the nucleus and organelles.

  • Dendrites: Short, branched processes receiving incoming signals.

  • Axon: Long single process conducting outgoing electrical impulses to other cells.

• Neuroglia (Glial Cells): Supportive cells that protect, nourish, and assist neurons; act as essential "housekeepers" of the nervous system.

Muscular Tissue Types

• Skeletal Muscle:

  • Structure: Composed of long, cylindrical, unbranched threads called muscle fibers. Possesses multiple oval nuclei located just under the plasma membrane. Displays prominent light and dark cross-stripes called striations.

  • Control: Voluntary (under conscious control).

  • Locations: Skeletal muscles attached to bones.

• Cardiac Muscle:

  • Structure: Composed of short, branched cells called cardiomyocytes, each containing a single centrally located nucleus. Displays striations and is joined end-to-end by complex junctions called intercalated discs.

  • Control: Involuntary (not under conscious control).

  • Location: Heart wall.

• Smooth Muscle:

  • Structure: Made of short, spindle-shaped (fusiform) cells that taper at both ends. Lacks striations and contains a single central nucleus.

  • Control: Involuntary.

  • Locations: Walls of hollow visceral organs (stomach, intestines, blood vessels, bladder).

Intercellular Junctions

Cell junctions are specialized structural connections formed between adjacent cells that hold cells together, resist mechanical stress, and control the movement of substances between cells.

Types of Junctions

• Tight Junction:

  • Outer plasma membranes of adjacent cells are bound tightly together by interlocked cell-adhesion proteins.

  • Forms an impermeable seal preventing fluids and dissolved molecules from passing between cells.

• Desmosome:

  • A patch-like cell junction that holds cells tightly together against mechanical tearing.

  • Consists of cell-adhesion proteins linking thick protein plaques attached to cytoskeleton intermediate filaments inside each cell.

• Gap Junction:

  • Formed by a ring-like protein complex called a connexon, consisting of six transmembrane proteins surrounding a central water-filled pore.

  • Allows ions, glucose, amino acids, and small solutes to pass directly from the cytoplasm of one cell into the next for electrochemical synchronization.

• Hemidesmosome:

  • Half-desmosome structures that anchor the basal surface of epithelial cells to the underlying basement membrane.

Glandular Structure, Secretion Types, and Secretion Modes

A gland is a cell or organ that secretes substances for use elsewhere in the body or for elimination as waste.

General Gland Classification

• Exocrine Glands:

  • Possess epithelial ducts that convey their secretions directly onto an internal or external body surface (e.g., sweat glands, salivary glands).

• Endocrine Glands:

  • Lack ducts; secretes products (called hormones) directly into adjacent capillary networks and bloodstreams to alter target cell activities throughout the body (e.g., thyroid gland, adrenal glands).

Types of Secretions

• Serous Secretions: Thin, watery fluids such as perspiration, tears, milk, and digestive juices.

• Mucous Secretions: Glycoproteins called mucins that absorb water after release to form thick, sticky mucus; secreted by unicellular goblet cells or multicellular mucous glands.

• Mixed Secretions: Glands (such as the submandibular salivary gland) containing both serous and mucous cells, producing a mixture of fluid types.

Modes of Exocrine Secretion

• Eccrine (Merocrine) Secretion:

  • Secretory products are released cleanly via exocytosis without any damage to or loss of host cell cytoplasm (e.g., eccrine sweat glands, pancreas, salivary gland proteins).

• Apocrine Secretion:

  • Secretory product (such as lipid droplets) accumulates at the cell apex; the apical portion of cytoplasm buds off enclosed by plasma membrane (e.g., milk fat secretion in mammary glands).

• Holocrine Secretion:

  • Secretory cells accumulate product until the entire cell disintegrates and dies, turning the cell itself into the secretion (e.g., sebaceous oil glands of skin). Basal stem cells undergo rapid mitosis to continuously replace disintegrated cells.

Tissue Growth, Degeneration, and Repair

Mechanisms of Tissue Growth

Tissue growth occurs through changes in cell quantity or individual cell dimensions.

• Hyperplasia: Tissue growth through an increase in total cell number via active mitotic cell division (e.g., embryonic growth, tissue regeneration).

• Hypertrophy: Tissue growth due to enlargement of pre-existing cells without cell division (e.g., skeletal muscle enlargement from weight training).

Tissue Degeneration and Death

• Atrophy: Shrinkage and reduction of tissue mass due to a loss in cell size or cell number.

  • Causes: Normal biological aging or disuse/inactivity (e.g., muscular atrophy following limb immobilization).

• Necrosis: Premature, pathological cell and tissue death resulting from physical trauma, hypoxia, toxins, or infection.

  • Example: Gangrenous necrosis (dry or wet gangrene) caused by severe restriction of arterial blood supply to peripheral tissues.

Tissue Repair Mechanisms

When damaged, tissues repair themselves through two primary processes:

1. Regeneration:

   • Replacement of dead or damaged cells with exact functional cells of the same type.

   • Fully restores normal tissue structure and functional capacity (e.g., healing of minor skin cuts, liver regeneration).

2. Fibrosis:

   • Replacement of damaged, functional tissue with non-functional collagenous scar tissue produced by fibroblasts.

   • Binds the injured organ together but fails to restore physiological function (e.g., healing of severe deep burns, cardiac scar tissue formation post-infarction, lung scarring in tuberculosis).