Chapter 5

Cells and Tissues

5.1 Cells Are Organized into Tissues

• Definition of Tissue: A group of similar cells with a common function.

• Study of Tissues: Known as histology.

• Four Major Types of Tissues in the Body:

  • Epithelial Tissue

  • Connective Tissue

  • Muscle Tissue

  • Nervous Tissue

5.2 Major Tissue Types

Table 5.1: Tissues: Four Major Tissue Types

• Epithelial Tissue

  • Function: Protection, secretion, absorption, excretion

  • Location: Covers body surface, covers and lines internal organs, composes glands

  • Distinguishing Characteristics: Lack blood vessels, cells readily divide, tightly packed cells

• Connective Tissue

  • Function: Bind, support, protect, fill spaces, store fat, produce blood cells

  • Location: Widely distributed throughout the body

  • Distinguishing Characteristics: Mostly have good blood supply, cells farther apart with an extracellular matrix in between

• Muscle Tissue

  • Function: Movement

  • Location: Attached to bones, in the walls of hollow internal organs, heart

  • Distinguishing Characteristics: Able to contract in response to stimuli

• Nervous Tissue

  • Function: Conduct impulses for coordination, regulation, integration, and sensory reception

  • Location: Brain, spinal cord, nerves

  • Distinguishing Characteristics: Cells communicate with each other and other body parts

5.3 Intercellular Junctions

Figure 5.1: Intercellular Junctions

• Types of Intercellular Junctions:

  • Tight Junctions: Fusing cell membranes to close spaces; e.g., cells that line the small intestine.

  • Desmosomes: Bind cells by forming "spot welds" between cell membranes.

  • Gap Junctions: Form tubular channels between cells that allow exchange of substances; e.g., muscle cells of the heart and digestive tract.

5.4 Epithelial Tissues

General Characteristics of Epithelial Tissues:

• Cover organs and body surfaces

• Line cavities and hollow organs

• Make up glands

• Features a free surface on one side and a basement membrane on the other

• Lack blood vessels (avascular), receiving nutrients through diffusion from underlying connective tissue

• Cells readily divide, allowing for rapid healing in case of injuries

• Cells are tightly packed with minimal intercellular space

Classification of Epithelial Tissues:

• Shapes of Epithelial Cells:

  • Squamous (flat)

  • Cuboidal (cube-shaped)

  • Columnar (tall)

• Layers of Epithelial Tissues:

  • Simple (single layer of cells)

  • Stratified (two or more layers of cells)

  • Pseudostratified (appears layered but is a single layer)

Simple Squamous and Simple Cuboidal Epithelia

• Simple Squamous Epithelium:

  • Description: Single layer of thin, flat cells.

  • Function: Substances pass easily through; locations include alveoli and capillaries.

  • Characteristics: Thin and delicate, easily damaged.

  • Locations: Walls of air sacs, capillaries, lining blood and lymphatic vessels.

• Simple Cuboidal Epithelium:

  • Description: Single layer of cube-shaped cells.

  • Function: Secretion and absorption.

  • Locations: Kidney tubules, thyroid follicles, ducts of some glands, and ovaries.

Simple Columnar and Pseudostratified Columnar Epithelia

• Simple Columnar Epithelium:

  • Description: Single layer of elongated cells; nuclei usually near basement membrane.

  • Features: May contain microvilli, cilia, goblet cells (secrete mucus).

  • Function: Secretion and absorption; lines uterus, stomach, intestines.

• Pseudostratified Columnar Epithelium:

  • Description: Appears layered; nuclei at different levels.

  • Features: All cells reach the basement membrane; may have cilia and goblet cells.

  • Function: Protection; lines respiratory passageways.

Stratified Squamous and Stratified Cuboidal Epithelia

• Stratified Squamous Epithelium:

  • Description: Many cell layers; outermost cells are squamous.

  • Function: Provides protection; new cells push older cells towards the surface.

  • Locations: Epidermis (keratinized), oral cavity, vagina, anal canal.

• Stratified Cuboidal Epithelium:

  • Description: 2 to 3 layers of cube-shaped cells.

  • Function: More protection than single-layered epithelium; lines ducts of mammary, sweat, salivary glands, and pancreas.

Stratified Columnar and Transitional Epithelia

• Stratified Columnar Epithelium:

  • Description: Several layers of cells; elongated cells on top.

  • Function: Protection, secretion; lines part of male urethra.

• Transitional Epithelium (Urothelium):

  • Description: Many layers; both cube-shaped and elongated cells.

  • Function: Changes shape and appearance under tension; provides a barrier; lines urinary bladder, ureters.

Table 5.2: Epithelial Tissues Summary

1. Simple Squamous Epithelium

   • Description: Single layer of flattened cells.

   • Function: Filtration, diffusion, osmosis.

   • Location: Air sacs of lungs, walls of capillaries.

2. Simple Cuboidal Epithelium

   • Description: Single layer of cube-shaped cells.

   • Function: Protection, secretion, absorption.

   • Location: Surface of ovaries, linings of kidney tubules.

3. Simple Columnar Epithelium

   • Description: Single layer of elongated cells.

   • Function: Protection, secretion, absorption.

   • Location: Linings of uterus, stomach, intestines.

4. Pseudostratified Columnar Epithelium

   • Description: Appears layered with elongated cells.

   • Function: Protection, secretion, movement.

   • Location: Linings of respiratory passages.

5. Stratified Squamous Epithelium

   • Description: Many layers; top cells flattened.

   • Function: Protection.

   • Location: Superficial layer of skin, lining of oral cavity, vagina.

6. Stratified Cuboidal Epithelium

   • Description: 2 or 3 layers of cube-shaped cells.

   • Function: Protection.

   • Location: Linings of ducts of mammary glands.

7. Stratified Columnar Epithelium

   • Description: Top layer elongated; lower layers cuboidal.

   • Function: Protection, secretion.

   • Location: Part of male urethra.

8. Transitional Epithelium

   • Description: Many layers of varying shapes.

   • Function: Stretchability, protection.

   • Location: Lining of urinary bladder.

Glandular Epithelium

• Definition: Composed of cells that produce and secrete substances into ducts or body fluids.

• Types of Glands:

  • Endocrine Glands: Secrete into tissue fluid or blood.

  • Exocrine Glands: Secrete into ducts that open onto a surface.

• Structural Types of Exocrine Glands:

  • Unicellular: Composed of one cell (e.g., goblet cell).

  • Multicellular: Composed of many cells; can be simple or compound (e.g., sweat and salivary glands).

Features of Exocrine Glands

• Simple: Duct does not branch.

• Compound: Duct branches before reaching secretory portion.

• Tubular: Consist of epithelial-lined tubes.

• Alveolar: Terminal portions form sac-like dilations.

Table 5.4: Types of Exocrine Glands

1. Unicellular Glands: A single secretory cell; e.g., mucous-secreting cell.

2. Multicellular Glands: Composed of many cells.

3. Simple Glands: Communicate with the surface by means of non-branching ducts.

   • Types include:

     • Simple tubular gland: Straight and opens directly onto the surface (e.g., intestinal glands).

     • Simple branched tubular gland: Branched with short or absent ducts (e.g., gastric glands).

     • Simple coiled tubular gland: Long coiled structure with a long duct (e.g., merocrine glands of skin).

     • Simple branched alveolar gland: Secretory portions expand into saclike compartments along the duct (e.g., sebaceous gland).

4. Compound Glands: Ducts branch before reaching the secretory portion.

   • Types include:

     • Compound tubular gland: Secretory portions extend from branches (e.g., bulbourethral glands).

     • Compound alveolar gland: Irregularly branched tubules with numerous saclike outgrowths (e.g., mammary glands).

Types of Glandular Secretion

• Merocrine Glands: Secrete fluid products by exocytosis; e.g., salivary and sweat glands.

• Apocrine Glands: Lose small part of cell during secretion; e.g., mammary glands.

• Holocrine Glands: Release entire cells filled with product; e.g., sebaceous glands.

5.5 Connective Tissues

General Characteristics of Connective Tissues

• Most abundant tissue type by weight.

• Cells are farther apart than in epithelial tissues.

• Contains an abundant extracellular matrix consisting of protein fibers and a ground substance.

• The consistency of connective tissues varies from fluid to solid.

• Major categories:

  • Connective tissue proper

  • Specialized connective tissues

• Most connective tissues have a good blood supply; well-nourished.

• Most cells possess the ability to divide.

Functions of Connective Tissues

• Bind structures together.

• Provide support and protection.

• Serve as frameworks.

• Fill spaces and store fat.

• Produce blood cells.

• Protect against infections.

• Help repair tissue damage.

Major Cell Types of Connective Tissue

• Fibroblasts:

  • Most common fixed cell; large star-shaped cells.

  • Function: Secrete fibers into the extracellular matrix.

• Macrophages (Histiocytes):

  • Attached to fibers but can detach and wander.

  • Function: Conduct phagocytosis and defend against infection.

• Mast Cells:

  • Large cells distributed widely; release heparin (prevention of blood clotting) and histamine (inflammatory response).

Connective Tissue Fibers

• Collagen Fibers:

  • Thick threads, the main structural protein in the body with great tensile strength and flexibility.

  • Found in ligaments and tendons.

• Elastic (Yellow) Fibers:

  • Composed of elastin protein; can stretch and return to original shape but are not as strong as collagen.

  • Found in vocal cords and respiratory air passages.

• Reticular Fibers:

  • Thin, branching fibers of collagen; form delicate supportive networks.

  • Found in the spleen and liver.

Table 5.6: Components of Connective Tissue

Categories of Connective Tissue

1. Connective Tissue Proper:

• Loose Connective Tissues: Fewer collagen fibers (e.g., Areolar, Adipose, Reticular).

• Dense Connective Tissues: Abundant in collagen fibers (e.g., Dense Regular, Dense Irregular, Elastic).

2. Specialized Connective Tissues:

• Cartilage

• Bone

• Blood

Areolar Connective and Adipose Tissues

• Areolar Connective Tissue:

  • Forms thin, delicate membranes; mostly fibroblasts in a gel-like ground substance; contains both collagenous and elastic fibers.

  • Location: Subcutaneous layer beneath most epithelia, nourishing nearby epithelial cells.

• Adipose Tissue (Fat):

  • Contains adipocytes that store fat, pushing their nuclei to one side, and crowding out other types of cells.

  • Functions: Cushions and insulates the body.

  • Locations: Beneath skin, around kidneys, behind eyeballs, around the heart, in spaces between muscles.

Reticular and Dense Regular Connective Tissues

• Reticular Connective Tissue:

  • Composed of networks of thin reticular fibers.

  • Function: Supports walls of internal organs; locations include walls of the liver and spleen.

• Dense Regular Connective Tissue:

  • Closely packed collagenous fibers; consists of fibroblasts.

  • Very strong; binds body parts together; found in tendons and ligaments.

  • Characteristics: Poor blood supply, slow to heal.

Specialized Connective Tissues

• Cartilage:

  • Rigid, supportive tissue with a unique matrix containing collagen in gel-like ground substance.

  • Contains Chondrocytes (cartilage cells) in lacunae surrounded by matrix; lacks blood supply which leads to slow healing.

  • Covered by perichondrium (connective tissue) which provides some nutrients.

Types of Cartilage:

1. Hyaline Cartilage:

   • Most common; contains fine collagen fibers.

   • Locations: Ends of bones in joints, nose, respiratory passages, embryonic skeleton.

2. Elastic Cartilage:

   • Flexible due to elastic fibers.

   • Locations: External ear, larynx.

3. Fibrocartilage:

   • Very tough; contains many collagen fibers.

   • Locations: Intervertebral discs, meniscal pads of knee, pelvic girdle.

Bone as Specialized Connective Tissue

• Bone (Osseous Tissue):

  • Most rigid connective tissue due to a solid matrix made of mineral salts and collagen.

  • Functions: Supports and protects structures, produces blood cells, stores/releases calcium and phosphorus, serves as attachment sites for muscles.

  • Contains Osteocytes in lacunae; two types exist: compact and spongy.

Types of Bone Tissue

1. Compact Bone:

   • Osteoblasts deposit matrix in lamellae (layers), forming cylindrical units called osteons.

   • Characteristics: Well-nourished, heals quickly due to a good blood supply.

2. Spongy Bone:

   • Composed of bony plates containing osteocytes; lighter than compact bone.

   • Contains spaces for marrow.

Blood as Specialized Connective Tissue

• Blood:

  • Composed of formed elements (cells and fragments) suspended in plasma (fluid matrix).

  • Functions:

  • Red Blood Cells: Transport gases.

  • White Blood Cells: Defense against infection.

  • Platelets: Aid in blood clotting.

• Location: Circulates throughout the body in a closed system of blood vessels and heart chambers.

Table 5.7: Connective Tissues Summary

5.6 Membranes

• Definition of Membranes: Sheets of cells; may be composed of epithelial and connective tissues.

• Types of Epithelial Membranes:

  • Serous Membranes: Line body cavities that do not open to the outside; secrete serous fluid for lubrication.

    • Made up of simple squamous epithelium and areolar connective tissue.

  • Mucous Membranes: Line cavities and tubes that open to the outside; contain goblet cells that secrete mucus.

  • Cutaneous Membranes: Commonly called skin; covers body surface; part of the integumentary system.

  • Synovial Membranes: Composed entirely of connective tissue; line joint cavities.

5.7 Muscle Tissues

General Characteristics

• Muscle tissues are excitable; can respond to nervous stimulation.

• Muscle cells are also called muscle fibers.

• Three Types of Muscle Tissue:

  • Skeletal Muscle Tissue:

  • Characteristics: Striated, long, cylindrical, multinucleated cells, voluntary control, attached to bones.

  • Cardiac Muscle Tissue:

  • Characteristics: Found only in the heart, branched cells, involuntary control, striated, intercalated discs (specialized intercellular junctions).

  • Smooth Muscle Tissue:

  • Characteristics: Non-striated, spindle-shaped fibers; walls of hollow organs and blood vessels; involuntary control.

Table 5.8: Muscle Tissue Summary

5.8 Nervous Tissue

• Nervous Tissue: Found in the brain, spinal cord, and peripheral nerves.

• Neurons: Main cells specialized for communication via conduction of nerve impulses (sensory reception, motor control).

  • Function: Coordinate, integrate, and regulate body functions.

  • Structure: Composed of cell body (soma), dendrites (receive impulses), and an axon (transmits impulses).

• Neuroglia: Support and nourish neurons; assist in the overall functioning of the nervous system.

Table 5.8: Nervous Tissue Summary

Conclusion

• Knowledge of tissues is essential in anatomy and physiology to understand how cells come together to form organs and systems that perform intricate body functions. Various tissue types play distinct roles vital for maintaining the body's homeostasis and overall health. Their study offers insights into disease processes and healing.