Histology

Muscle Tissue

• Function: Responsible for most body movement. Characterized by specialized cells whose primary role is contraction.

• Muscle Cells:

  • Elongated

  • Arranged in parallel arrays

  • Principal components related to contractility: myofilaments

Types of Myofilaments

• Actin (protein)

• Myosin (protein)

Classification of Muscle Tissue

• Striated Muscle

• Non-Striated (Smooth) Muscle

  • No cross-striations visible under light microscopy

Striated Muscle

• Exhibits cross-striations under light microscopy.

• Types of Striated Muscle:

  • Skeletal Muscle

  • Cardiac Muscle

• The arrangement of myofilaments is the same in both, but differences exist in:

  • Size of individual cells

  • Organization relative to each other

Skeletal Muscle

• Structure:

  • Composed of parallel striated muscle cells, known as fibers

  • These fibers are extremely long and multinucleated

  • The fibers are cross-striated and perform quick contractions, but fatigue quickly

  • Skeletal muscle is attached to bones and is highly adapted for locomotion

• Cellular Components:

  • Sarcoplasm: The cytoplasm of the muscle cell

  • Sarcolemma: The cell membrane surrounding the sarcoplasm

Connective Tissue in Skeletal Muscle

• Endomysium:

  • Delicate, loose connective tissue surrounding each individual muscle fiber

• Perimysium:

  • Covers bundles of muscle fibers (fascicles)

  • Provides support for blood and nervous supply to muscle fibers

• Epimysium:

  • Coats the entire muscle structure

Banding Patterns

The myofibrils in skeletal muscle are divided into units called sarcomeres.

Each sarcomere consists of alternating light and dark bands, which are due to the overlap of thick myosin filaments and thin actin filaments.

• H Zone:

  • Area with only myosin filaments

• A Band:

  • Dark band where both myosin and actin filaments overlap

• I Band:

  • Light band with actin filaments alone

• Z Lines:

  • Z-Z = 1 Sarcomere, marking the boundaries of a sarcomere. These lines are transverse membranes

Cardiac Muscle

• Heart muscle

• Network of branched, interconnected muscle fibers

• Intercalated discs (low resistance) between adjacent muscle cells allow the passage of excitation, leading to synchronous contraction

• Myogenic: exhibits rhythmical contractions originating from the muscle itself

Smooth Muscle

• Involuntary muscle

• Cells arranged in fascicles

• Composed of very thin, elongated cells

• No branching of cells

• Mainly found in blood vessels, as well as in the digestive, urogenital, and respiratory tracts

Connective Tissue

Connective tissue supports and holds organs and tissues in place, binding them together

Types:

• Areolar Type

• Collagen Type

• Elastic Type

• Adipose Type

Areolar Type

• Present all over the body, beneath the skin

• Connects and binds organs together

• Found in the spaces between adjacent tissues

• Made up of a gelatinous matrix, primarily composed of Hyaluronic acid

• Contains various cells and fibers embedded in the ground substance

Types of Cells - Areolar

• Fibroblasts: Long, flattened cells that synthesize collagen and elastic fibers

• Mast Cells: Secrete the ground substance and an anticoagulant

• Fat Cells: Store fat

• Macrophages: Large, amoeboid cells capable of phagocytosis and ingestion of foreign particles, dead cells, and infective particles

Types of Protein Fibers - Areolar

• Collagen (white): Flexible, strong, non-stretchable

• Elastic (yellow): Loose network of stretchable fibers

Collagen Type

• White fibrous tissue

• Matrix composed of densely-packed collagen fibers

• Provides increased tensile strength and inelasticity, found in tendons (muscle to bone)

Elastic Type

• Yellow elastic tissue

• Matrix composed of abundant elastic fibers

• Strong connective tissue with additional elastic properties, found in ligaments (bone to bone)

Adipose Type

• Fatty tissue

• Matrix packed with fat cells

• Adapted for insulation and lipid storage, found in the dermis of the skin

Skeletal Tissue

• Provides structural support and a rigid framework for the body

• Composed of cells embedded in a hard organic matrix

• Types:

  • Cartilage

  • Bone

Cartilage - Skeletal Tissue

• Forms the skeleton of primitive fish, dogfish, sharks, and rays

• Present at joints and in intervertebral discs

• Matrix is made of chondrin, a hard ground substance secreted by chondroblasts

• Basic type: Hyaline cartilage

  • Found in the larynx, bronchus, and trachea

• Other types derived from hyaline cartilage:

  1. White fibrocartilage

     • Matrix rich in collagen fibers

     • Provides additional strength

     • Found in joint capsules and intervertebral discs

  2. Yellow fibrocartilage

     • Matrix rich in elastic fibers

     • Provides flexibility and elasticity

     • Found in the nose and pinna of the ear

• Contains no blood or lymphatic vessels

• Nutrients diffuse through the matrix to reach the cells

Bone - Skeletal Tissue

• Harder than cartilage due to the presence of calcium salts (mainly calcium phosphate) in the matrix

• Function:

  • body support

  • organ protection

  • muscle attachment

  • blood cell production

  • blood calcium regulation

Forms of Bone - Skeletal Tissue

• Compact bone: composed of long, strong bones

• Spongy bone: contains marrow cavities

Osteoblasts - Bone - Skeletal Tissue

• Secrete the bone matrix

• Reside in lacunae with slender projections called canaliculi

• Arrange themselves in concentric rings (lamellae) around blood vessels and nerves in Haversian canals

• Canaliculi connect the cells to the blood supply

• Each structural unit is called an osteon or Haversian system

Histology

microscopic study of the normal tissues of the body

Tissues

• Formed by collections of cells arranged in an orderly manner to maintain body functions

• This organization reflects the cooperation of cells to perform shared functions

Cells

Functional units of the body

Basic types of tissue

• Epithelial

• Connective

• Muscular

• Nervous

These tissues combine in varying proportions to form organs and systems

Epithelia

• Lining tissue

• Composed of closely packed cells with strong adhesions

• Covers all internal and external surfaces

• No direct blood supply

  • Oxygen diffuses from blood vessels in underlying tissue via basement membrane

• May be arranged in single or multiple layers

Single layer Epithelia

• Example: intestinal lining

• Function:

  • Protection

  • Lubrication

  • Absorption

  • Secretion

Multiple layered Epithelia

• Example: skin

• Function:

  • Protection

  • Impermeability

  • Prevents water loss

  • Helps in heat insulation

Classification of epithelia

• Based on arrangement and shape of cells

• Related to structure and function

• Simple → one layer of cells

• Compound → two or more layers

Simple epithelium (by shape)

• Squamous

• Cuboidal

• Columnar Striated / Ciliated

• Glandular

• Pseudostratified

Squamous epithelium

• Flat pavement cells

• Small round nuclei

• Large homogenous polygonal cytoplasm

• Examples: alveolar lining, blood vessel lining, mesothelium

Cuboidal epithelium

• Square-shaped cells

• Central round or oval nuclei

• Examples: kidney tubules, thyroid follicles

Columnar epithelium

• Rectangular cells

• Oval or elongated nuclei near base

• Types:

  • Striated / Brush border cells

    • Adapted for absorption

    • Microvilli increase surface area

    • Example: intestinal villi

  • Ciliated cells

    • Have cilia (hair-like structures)

    • Longer than microvilli

    • Beat rhythmically to propel mucus

    • No absorptive function

    • Example: fallopian tube

Glandular

• Secretory columnar epithelium

• Mucus-secreting cells

• Cytoplasmic mucus distention (goblet cells)

• Examples: stomach lining, respiratory tract

Pseudostratified

• Columnar epithelium that may be ciliated

• Closely packed cells that appear multinucleated

• All cells anchored to basement membrane

• Examples: bronchi (ciliated), salivary gland ducts (non-ciliated)

Compound epithelium

• Stratified squamous epithelium

• Stratified columnar / cuboidal epithelium

• Transitional epithelium (urothelium)

• Glandular epithelium

• Metaplastic Epithelium

Stratified squamous epithelium

• Mainly in skin

• Multilayered squamous cells

• Cells mature from basal cuboidal cells upward

• Nucleus size decreases as cytoplasm surface area increases

• Types:

  • Keratinzed

  • Non-keratinized

Keratinized Stratified squamous epithelium

• Example: skin epidermis

• Dry

• Protection

• Prevents water loss

• Dead, anucleated surface cells

• Cornified upper layer

Non-keratinized Stratified squamous epithelium

• Examples: oesophagus, vagina, mouth

• Moist

• Protection

• Prevents water loss

• Associated with secretory elements

Stratified columnar / cuboidal epithelium

• Found in few locations

• 2–3 layers of cells

• Examples: larger ducts of glands, urethra, pancreas, salivary gland

Transitional epithelium (urothelium)

• Multilayered

• Withstands distention and stretching

• Very strong cell junctions

• Flattened superficial cells

• Lower cuboidal layers

• Examples: ureter, bladder

Glandular epithelium

• Endocrine glands

  • Secrete directly into the vascular system

  • Arranged around capillaries

  • Classified by arrangement

    • Tubular formation → example: thyroid gland

    • Cluster formation → example: pancreas

• Exocrine glands

  • Secrete into ducts

  • Unicellular → example: goblet cells

  • Multicellular → classified by duct arrangement

    • Simple tubular → gland of Lieberkühn

    • Simple coiled tubular → sweat gland

    • Simple branched tubular → gastric gland

    • Simple branched acinar → sebaceous gland

    • Compound tubular → oral cavity mucus gland

    • Compound acinar → salivary gland

Metaplastic Epithelium

• A normal cell lineage growing in an abnormal location

• Occurs only in cells that have the potential to divide

  • Examples: stem cells, reserve cells

• Multipotent cells can form different cell types depending on the stimulus

• These new cells are better equipped to withstand the specific stimulus

• Metaplasia is reversible

• Metaplasia does not lead to neoplasia

• Can be both a physiological and a pathological condition

Examples:

• Columnar to squamous

  • Cervix

  • Lung

• Squamous to columnar

  • Barrett's oesophagitis

• Transitional to squamous

  • Bladder

Protoplasm

• Basic substance of all living things

• Colloidal in nature

• Consists of:

  • Water

  • Protein

  • Carbohydrates

  • Lipids

  • Inorganic salts

• These make up the cell constituents

• Contained within small units called cells, and these act as:

  • Structural units

  • Functional units

  • Reproductive units

  • All chemical reactions occur within cells

Cell membrane

• Surface covering

• Boundary wall

• Regulates transport of substances in and out of the cell

Nucleic acids

Coded hereditary information

Nucleus

Nucleic acids can be contained within a special membrane-surround region

Organelles

• Small structures which perform specific functions and specialized chemical reactions

• Known as “little organs” responsible for the cell’s machinery, i.e.:

  • Energy release

  • Protein synthesis

  • Packaging of storage products

  • Transportation of substances throughout the cell

Prokaryotic cells

• Primitive

• Lack a distinct nucleus

• Lack complexity

• Limited organelles

• Unable to form complex interactions for development of multi-cellular organisms

• Found in bacteria and blue-green algae

Eukaryotic cells

• Larger

• Complex

• Contain many more types of organelles

• Well-defined nucleus

• Found in higher plants and animals

Cell / Plasma Membrane

• Encloses a mass of protoplasm (cytoplasm)

• Organelles are situated in the cytoplasm

• Contains a well-defined nucleus bounded by a nuclear membrane

Functions of the Cell Membrane

• Structural function

  • Provides a boundary that keeps cell components together

• Transport function

  • Serves as a doorway through which substances enter and leave the cell

Selective Doorway

• Substance passage takes place in two general ways:

  • Passive transport

    • Based on principles of diffusion

  • Active transport

    • Movement of materials against a concentration gradient

    • Requires the expenditure of energy

Cell Membrane Structure

Not a passive envelope – it:

• Regulates the internal environment of the cell

• Participates actively in the transport of substances

• Recognizes the external environment

• Determines the immunologic makeup of the cell and tissues

Molecular Structure

Composed of a double layer of lipids (fatty acid chains)

• Hydrophilic (water-soluble) portions face outwards

• Hydrophobic (water-insoluble) fatty acid chains point inward toward the membrane center

• Cholesterol molecules

• Protein molecules

• Carbohydrate molecules

• Peripheral proteins (on the inner membrane)

Cholesterol molecules

Add structural rigidity to the membrane

Protein molecules

• Embedded within the lipid bilayer (trans-membrane)

• Provide communication between the cell membrane and the cell interior

Carbohydrate molecules

• Attached to lipids (glycolipids) or proteins (glycoproteins)

• Possibly responsible for the immune status of the cell

Peripheral proteins (on the inner membrane)

• Function structurally to maintain the cell membrane

• Help keep integral proteins in position

Cytoplasmic Organelles

• Endoplasmic Reticulum (ER)

• Golgi Apparatus

• Lysosomes

• Mitochondria

• Centrioles

Endoplasmic Reticulum (ER) - Cytoplasmic Organelles

• Organized network of fine branching tubules called ER

• Tubules lined by a membrane

• Newly synthesized proteins are discharged into cisternal spaces

• Products exported in membrane-enclosed spaces to the Golgi apparatus

Rough ER - Cytoplasmic Organelles

• Coated with granules of ribonucleoprotein (ribosomes)

• Associated with protein synthesis

Smooth ER - Cytoplasmic Organelles

• Not coated with ribosomes

• Associated with synthesis of fats and lipids

• Detoxification of drugs and toxic substances

• Substances are transported or broken down

Golgi Apparatus - Cytoplasmic Organelles

• Stack of connected, flat, smooth membranous vesicles

• Located close to the nucleus and connected with the rough ER

Process:

• Raw materials from Rough ER enter Golgi vesicles

• Molecules added or eliminated

• Final packaged products transported and released in special vesicles

• Products stored or transported to cell membrane for discharge

Lysosomes - Cytoplasmic Organelles

• Sac-like structures

• Contain strong digestive enzymes

• Catalyze breakdown of fats, proteins, and nucleic acids

• Rupture of lysosome membrane causes enzyme release and autolysis

• Abundant in leucocytes and macrophages (phagocytosis and intracellular digestion)

Mitochondria - Cytoplasmic Organelles

• Spherical or sausage-shaped structures

• More numerous in regions of high metabolic activity (e.g., liver, muscle cells, sperm tail)

• Involved in energy-producing oxidative processes

• Formation of Adenosine Triphosphate (ATP) from phosphorus and ADP

Mitochondria Structure - Cytoplasmic Organelles

• Double membrane

• Inner membrane folded into cristae

• Oxidative enzymes present on cristae (high surface area for energy production)

• Contain some DNA, RNA, and ribosomes

• Possibly originated from previously incorporated microorganisms

Centrioles - Cytoplasmic Organelles

• Small, deeply staining particles near the nuclear membrane

• Hollow cylindrical bodies made of microtubules

• Arranged at right angles to each other

• Key role in cell division

• Form/assemble spindle fibres during mitosis

Mitosis

• Two pairs of centrioles positioned at opposite sides

• Each pair anchors the mitotic spindle

Cytoplasmic Inclusions

• Non-living substances

• Stored nutrients, ingested particles, or cell products

• Serve as energy reserves or building materials

Cytoplasmic Inclusions Types

• Glycogen

• Fat

• Secretory granules

• Pigments

Glycogen - Cytoplasmic Inclusions

• Fine granules or large amorphous globules

• Food storage

• e.g., liver, muscle cells

Fat - Cytoplasmic Inclusions

Stored in fat cells as minute globules that can fuse and displace cytoplasm or nucleus

Secretory granules - Cytoplasmic Inclusions

Contain products of cell synthesis; secretory function; discharge materials

Pigments - Cytoplasmic Inclusions

• Endogenous (produced by body)

  • e.g., melanin, haemosiderin

• Exogenous (foreign particles)

  • ingested by phagocytosis or absorbed

  • e.g., coal, dust, carbon

Nucleus

• Main information and control centre of cellular activity

• Centre of reproductive activities and regulation of metabolic processes

Nucleus Structure

• Usually spherical or oval

• Separated from cytoplasm by nuclear membrane/envelope (two layers)

• Filled with homogeneous ‘nuclear sap’

• Contains basophilic dark granules called chromatin

Chromatin - Nucleus

• Basophilic dark granules

• Composed of protein and nucleic acid

• DNA (deoxyribonucleic acid) forms chromosomes involved in reproduction

Nucleus Functions

• Interacts with cytoplasm via various forms of RNA

• Proteins and RNA synthesized in nucleus pass through nuclear pores to cytoplasm

• Responsible for hereditary information transmission

Nuclear Membrane

• Double membrane

• Inner side contains a layer of filaments

• Outer side shows ribosomes, similar to Rough ER membrane

• Contains distinct pores for exchange of nuclear material

• Pores have occluding diaphragm (no free transport)

• Behaves as a closed compartment

• Continuous with the ER

Nucleolus

• Small spherical bodies in nucleus

• Single or multiple; variable in size and shape

• Contains high proportion of ribonucleic acid (rRNA)

• Thought to be involved in protein synthesis

Respiratory System

• Consists of the Lungs + a series of air passages to and from the lungs

• Branching progressively into smaller tubes until reaching the alveoli (smallest air spaces where gas exchange occurs)

Respiratory Tract portions

• Conducting Portion

• Respiratory Portion

Conducting Portion Function

• Air is conditioned

• Air is warmed

• Air is moistened

• Air is cleaned

Conducting Portion Structures

• Nasal Cavity

• Olfactory Mucosa

• Nasopharynx

• Oropharynx

• Larynx

• Trachea

Nasal Cavity - Conducting Portion

• Non-keratinizing stratified squamous epithelium

• Contains hair

Olfactory Mucosa - Conducting Portion

Specialized sensory cells for smell

Nasopharynx - Conducting Portion

Pseudostratified ciliated columnar epithelium with goblet cells

Oropharynx - Conducting Portion

Connected via the epiglottis

Larynx - Conducting Portion

• Cartilaginous framework

• Pseudostratified ciliated columnar epithelium

• Vocal cords lined with stratified squamous epithelium

Trachea - Conducting Portion

• Lined by respiratory mucosa: pseudostratified ciliated columnar epithelium with goblet cells

• 16–20 C-shaped rings of hyaline cartilage maintain open airway

• Bounded by connective tissue and smooth muscle

Branching of Trachea

Divides into Two Bronchi

Bronchus - Conducting Portion

• Pseudostratified columnar ciliated epithelium

• Contains elastic fibers, spirally arranged smooth muscle, and irregular hyaline cartilage plates

Bronchiole - Conducting Portion

• Pseudostratified columnar ciliated epithelium with few goblet cells

• No cartilage present

• Smooth muscle spirals remain

• Epithelium simplifies to simple columnar or cuboidal in smaller bronchioles

Terminal Bronchioles - Conducting Portion

• No goblet cells

• Contain Clara cells:

  • Non-ciliated

  • Dome-shaped apex

  • Secrete granular surface-acting agents

• Surrounded by smooth muscle and elastic fibers

Respiratory Portion Function

Gas exchange

Respiratory Portion Structures

• Respiratory Bronchioles

• Alveolar Ducts

• Alveolar Sacs

• Alveoli

Respiratory Bronchioles - Respiratory Portion

• First part where gas exchange begins

• Walls of low columnar/cuboidal epithelium

• Clara cells present

• Interrupted by numerous saccular alveoli

• Contains small amounts of smooth muscle and elastic connective tissue

Alveolar Ducts - Respiratory Portion

• Lined by simple squamous epithelial cells

• More alveoli present

• Supported only by a rich matrix of elastic and collagen fibers

Alveolar Sacs - Respiratory Portion

Contain abundant elastic and reticular fibers