Histology: The Study of Cells and Tissues

Histology: The Study of Cells and Tissues

Bio 201 Unit 1

Key Terms to Know

  • Histology: The study of tissues and how they form organs.

  • Tissue: A group of similar cells and their products.

  • Organ: A structure composed of 2 or more tissue types; almost always consists of connective and epithelial tissue.

Four Primary Tissue Types

  1. Connective tissue

  2. Epithelial tissue

  3. Muscle tissue

  4. Nervous tissue

Specific Features of Tissues

  • Categorization Factors:

    • Type and function of cells

    • Characteristics of the extracellular matrix (ECM)

    • Amount of space occupied by cells versus matrix

Epithelial Tissues

  • General Characteristics:

    • Comprised of one or more layers of closely adhering cells.

    • Contains very little extracellular matrix (ECM).

    • Forms flat sheets with an upper surface exposed to the environment or an internal body cavity, with one side exposed to free space.

    • Avascular: Depends on the underlying connective tissue for oxygen and blood supply.

    • Sits on a basement membrane: A thin layer of collagen and adhesive proteins that anchors the epithelium to the underlying connective tissue.

Types of Epithelial Tissues

Classification

  1. Covering and Lining Epithelium

  2. Glandular Epithelium

    • Functions:

      • Protection

      • Absorption

      • Filtration

      • Excretion

      • Secretion

      • Sensory reception

Layers of Epithelial Tissues

  • Simple Epithelium:

    • Contains one layer of cells, named by the shape of the cells.

  • Stratified Epithelium:

    • Contains more than one layer, named by the shape of the apical (surface) cells.

    • The deepest cells (basal cells) sit on the basement membrane.

  • Pseudostratified Epithelium:

    • A simple epithelium that appears stratified due to the arrangement of nuclei that look “stacked,” but it is not truly stratified.

Shapes of Epithelial Tissues

  1. Squamous (flat/scale-like):

    • Allows rapid diffusion of substances; secretes serous fluid.

  2. Cuboidal (square):

    • Functions in absorption and secretion; produces mucus.

  3. Columnar (rectangular):

    • Also functions in absorption and secretion; secretes mucus.

    • Cilia may be present.

Simple Squamous Epithelial Cells

  • Location:

    • Air sacs of the lungs

    • Lining of heart, blood vessels, and lymphatic vessels

  • Functions:

    • Allows materials to pass through via diffusion and filtration.

    • Secretes lubricating substances.

Simple Cuboidal Epithelial Cells

  • Location:

    • Ducts and glands

    • Kidney tubules

  • Functions:

    • Secretion

    • Absorption

Simple Columnar Epithelial Cells

  • Location:

    • Ciliated: Bronchi, uterus

    • Smooth: Digestive tract, bladder

  • Functions:

    • Enzyme and mucous secretion

    • Absorption

Pseudostratified Columnar Epithelial Cells

  • Location:

    • Ciliated: Trachea, upper respiratory tract

  • Functions:

    • Mucous secretion

    • Movement of particles

Stratified Squamous Epithelial Cells

  • Location:

    • Mouth

    • Esophagus

    • Lower part of the GI tract

  • Functions:

    • Protection against abrasions

Glands: Types of Epithelial Tissues

  • Glands Classification:

    • Glands are classified as epithelial tissues even if they are not associated with free surfaces.

    • Exocrine Glands: Maintain connection to surface through a duct.

    • Endocrine Glands: Have no ducts; secrete products/chemical messages (hormones) directly into the bloodstream.

Types of Glandular Secretions

  1. Serous Glands: Produce thin, watery secretions; e.g. sweat, milk, tears, digestive juices.

  2. Mucous Glands: Produce a glycoprotein called mucin that absorbs water to form mucus.

  3. Cytogenic Glands: Release whole cells, such as sperm and egg cells (produced in testis and ovary).

Intercellular Junctions

  • Present in all cells except blood cells.

  1. Tight Junction: Encircle the cell, joining it to surrounding cells, acting as a barrier to prevent substances from passing between cells.

  2. Desmosome: Connects two cells, providing mechanical strength against stress, sometimes referred to as adhesion junctions.

  3. Gap Junction: Forms pores that allow for direct communication between cells.

Connective Tissue Overview

  • Structure: Consists of widely spaced cells separated by fibers and ground substance called the Extracellular Matrix (ECM).

  • Characteristics: Most abundant and variable tissue type.

  • Functions:

    • Connects structures to each other.

    • Provides support and protection (both physical and immune).

    • Stores energy.

    • Facilitates movement and transport of materials.

Structures within Connective Tissues

  1. Cells

  2. Fibers

  3. Ground Substance:

    • Ground substance and fibers are collectively called the matrix or ECM.

    • The ECM can be rubber-like, fluid-like, or stone-like in appearance.

Connective Tissue: Ground Substance

  • Description: Gelatinous or rubbery material found between cells.

  • Components:

    • Large molecules called proteoglycans: protein-carbohydrate complexes embedded in plasma membranes.

    • Adhesive Glycoproteins: Protein-carbohydrate complexes that bind plasma membrane to collagen or proteoglycans outside the cells.

Connective Tissue: Fibers

  1. Collagen Fibers:

    • Composed of the protein collagen.

    • Extremely tough (stronger than steel fibers of the same diameter).

    • Resistant to stretch, yet flexible; found in tendons, ligaments, and deep layer of the skin (dermis).

  2. Reticular Fibers:

    • Thin collagen fibers coated with glycoprotein.

    • Form a framework for spleen and lymph nodes.

  3. Elastic Fibers:

    • Thin branching fibers made of elastin.

    • Provide stretch and recoil like a rubber band, found in skin, lungs, and arteries.

Types of Cells Found in Connective Tissues

  • Blast Cells: Mitotic cells that secrete ground substance and fibers for the matrix.

    • Fibroblasts: Produce protein fibers and ground substance.

    • Chondroblasts: Produce cartilage.

    • Osteoblasts: Form bony matrix.

    • Hemocytoblasts: Form blood cells, specifically located in bone marrow.

  • White Blood Cells: Fight against infections/pathogens.

  • Mast Cells: Secrete heparin that inhibits clotting and histamine for blood vessel dilation.

  • Adipocytes: Fat cells that store triglycerides.

Types of Connective Tissue

  1. Connective Tissue Proper:

    • Dense: Regular and irregular

    • Loose: Adipose, areolar, reticular

  2. Cartilage:

    • Types: Hyaline, elastic, fibrocartilage

Dense Connective Tissue

  • Description: Fibers fill spaces between cells, with two types varying in fiber orientation.

    • Dense Regular Connective Tissue:

    • Aligned collagen fibers, found in tendons and ligaments; it is the strongest connective tissue.

    • Dense Irregular Connective Tissue:

    • Multidirectional collagen fibers, providing 3D strength; found in the dermis of the skin.

Loose Connective Tissue

Adipose Tissue

  • Structure: Large, empty-looking cells with thin margins. Nucleus pressed against cell membrane.

  • Functions: Energy storage, insulation, cushioning; found beneath the skin and surrounding organs.

Areolar Connective Tissue

  • Description: Most widely distributed connective tissue.

  • Functions:

    • Support and binding of tissues together.

    • Holds body fluids (amount of fluid in areolar tissue equals that in blood).

    • Defends against infections; fibroblasts are the most common cell type.

Reticular Tissue

  • Structure: Reticular fibers in loose ground substance.

  • Function: Filtration, providing soft structure that supports other cells.

  • Locations: Bone marrow, lymph nodes, spleen.

Connective Tissue: Cartilage

  • Description: Supportive connective tissue with a rubbery matrix; chondroblasts produce the matrix, and once surrounded by the cartilage, become chondrocytes.

  • Characteristics: Lacks blood vessels, hence nutrients must diffuse in for healing, making it heal slowly.

  • Major Types of Cartilage depend on the fiber types:

    • Hyaline

    • Fibrocartilage

    • Elastic Cartilage

Fibrocartilage

  • Description: Contains extensive parallel collagen fibers.

  • Function: Resists compression and absorbs shock in certain joints.

  • Locations: Pubic symphysis and intervertebral discs.

Hyaline Cartilage

  • Description: Most common type; appears clear and glassy with fine dispersed collagen fibers.

  • Features: Chondrocytes in small clusters enclosed in lacunae.

  • Locations: Forms joints, ends of bones, mid part of the nose; a supportive material in larynx, trachea, bronchi.

Elastic Cartilage

  • Comparison: Similar to hyaline cartilage but contains more elastic fibers.

  • Function: Maintains the shape of structures, allowing for greater flexibility.

  • Locations: Pinna/auricle of ear, end of nose, epiglottis.

Muscular Tissue

  • Characteristics: Composed of elongated cells that are irritable (respond to stimuli) and contractile (exert physical force).

  • Functions:

    • Moves bones.

    • Pushes substances through vessels (blood, urine, food, etc.).

    • Pumps blood.

    • Important source of body heat.

  • Types:

    1. Skeletal Muscle

    2. Cardiac Muscle

    3. Smooth Muscle

Skeletal Muscle

  • Description: Long, cylindrical, multinucleate, unbranched, voluntary cells with striations.

  • Functions: Movement, facial expression, posture, breathing, speech, swallowing, and excretion.

  • Control: Voluntary.

Cardiac Muscle

  • Description: Short branched, mononucleated cells with striations and intercalated discs.

  • Function: Pumps blood through arteries and veins.

  • Control: Involuntary.

Smooth Muscle

  • Description: Short, non-striated, mononucleated cells; forms sheets in viscera, iris, hair follicles, and sphincters.

  • Functions: Swallowing, GI tract functions, labor contractions, control of airflow, erection of hairs, control of pupil.

  • Control: Involuntary.

Nervous Tissue

  • Description: Comprised of large electrical conduction cells called neurons, with long cell processes surrounded by much smaller support cells called glial cells (or glia) that lack dendrites and axons.

  • Locations: Found in the brain, spinal cord, nerves, and ganglia.

  • Functions: Communication and control of bodily functions.

Mucous Membranes

  • Structure: Consists of epithelium, connective tissue, and underlying muscular tissues.

  • Functions: Lines passageways that open to the exterior; includes digestive, respiratory, urinary, and reproductive tracts. Mucous coating and ciliary movement help trap and remove foreign particles and bacteria from internal surfaces of the body.

Other Membranes

  1. Cutaneous Membrane (Skin): Stratified squamous epithelium resting on connective tissue; serves a protective function and is relatively dry.

  2. Synovial Membrane: Lines joints (involves only connective tissue) and secretes synovial fluid rich in hyaluronic acid into the joints.

  3. Serous Membrane (Serosa): An internal epithelial membrane that lines major body cavities (thoracic & abdominal) and covers organs; produces serous fluid for organ lubrication.

Questions?

Inspirational Quote

  • "Believe in yourself and all that you are. Know that there is something inside you that is greater than any obstacle." – Christian D. Larson