Introduction to Tissues: Histology, Cell Junctions, and Major Tissue Types

Histology

Definition and Overview

• Histology is the study of tissues.

• Tissue: A group of cells with similar structure and function.

• Most tissues are surrounded by an extracellular fluid called interstitial fluid (mainly water and ions).

Major Tissue Types

1. Epithelial Tissue

2. Connective Tissue

3. Muscle Tissue

4. Nervous Tissue

Cell Junctions (in Some Tissues)

Overview

• Points of contact between adjacent cells, especially in epithelial tissue, some nervous and muscle cells.

• Formed by cell membrane proteins.

Types of Cell Junctions

1. Tight Junctions

   • Partial fusion of specific proteins on the lateral surface of the cell membrane.

   • Prevents material from passing between cells (e.g., bacteria, proteins, or fluids).

   • Separates apical (lumen-exposed) and basolateral (attached) surfaces of the cell.

   • Example: Lining of the small intestine.

2. Anchoring Junctions (Desmosomes)

   • Fasten cells to each other and/or extracellular material ("rivets" cells together).

3. Gap Junctions

   • Open channels (formed by proteins) through adjacent cell membranes.

   • Allow ions/small molecules to pass from one cell to another.

   • Important in cardiac and smooth muscle (synchronization of contractions).

   • Also found in epithelial tissue.

Major Tissue Types (Overview)

Epithelial Tissue

• Lines body surfaces and cavities.

• Functions: protection, absorption, secretion, and sensation.

Connective Tissue

• Supports and connects other tissues.

• Contains an extracellular matrix that gives it unique properties.

Muscle Tissue

• Specialized for contraction and movement.

Nervous Tissue

• Specialized for sensation and signaling.

Epithelial Tissue

Characteristics

• Covers body surfaces and lines body/organ cavities (organ cavity = lumen).

• Has a free surface.

• Little extracellular space between cells.

• Avascular (no blood vessels).

• Has a basement membrane (extracellular layer attaching epithelium to underlying connective tissue).

Classification of Epithelia

• Based on:

  1. Number of cell layers:

     • Simple = 1 layer

     • Stratified = more than one layer

  2. Shape of cells in the top layer:

     • Squamous = flat

     • Cuboidal = cube-shaped

     • Columnar = rectangular

Types of Epithelial Tissue

1. Simple Epithelia (1 layer)

   • Allows exchange of molecules (gases, nutrients, ions) – absorption/secretion.

   • Types:

     • Simple squamous (lungs)

     • Simple cuboidal (kidneys)

     • Simple columnar (stomach, small intestine)

2. Stratified Epithelia (>1 layer)

   • Protective (e.g., epidermis of skin).

   • Predominant subtype: stratified squamous (apical cells squished flat).

3. Pseudostratified Epithelium

   • Appears stratified, but all cells touch the basement membrane.

4. Transitional Epithelium

   • Cell shape and layering varies with stretching (e.g., urinary bladder).

   • Only in urinary system.

   • Cells appear cuboidal to squamous when stretched.

5. Glandular Epithelium

   • For secretion.

   • If epithelial cells form a gland, the cell layer/shape classification is not used.

   • Subtypes:

     • Exocrine glands: Secrete products onto body surface or into a body cavity.

       • Unicellular (e.g., goblet cells – secrete mucus into a cavity).

       • Multicellular (e.g., sweat, sebaceous, mammary glands).

     • Endocrine glands: Secrete hormones into extracellular fluid, then into blood for transport to target cells/tissues (e.g., thyroid gland secretes thyroid hormone).

Functions of Epithelia

• Protection (e.g., epidermis of skin).

• Secretion (e.g., glandular epithelium, sweat glands).

• Selective passage of materials (nutrients, water, ions) across membranes (e.g., kidney, intestine, capillaries).

Connective Tissue (CT)

Overview

• Mainly supports, connects, and protects tissues.

• Cells are separated by an extracellular matrix that forms the bulk of the tissue.

• The matrix gives connective tissue its unique characteristics.

Matrix Composition

1. Fibres (proteins)

   • Collagen fibres: provide strength.

   • Elastic fibres: contain elastin, allow stretch and recoil.

   • Reticular fibres: form networks (e.g., in basement membrane).

2. Ground Substance

   • Water, ions, and large organic molecules (proteins, carbohydrates).

Classification of Connective Tissue

• Based mainly on matrix composition.

• Types:

  1. Connective Tissue Proper

     • Cells: fibroblasts/fibrocytes (except adipose tissue: adipocytes).

     • Subtypes:

       • Loose Connective Tissue

         • Areolar: loosely arranged collagen and elastic fibres, highly vascular (e.g., lamina propria).

         • Adipose: large adipocytes, stores triglycerides, highly vascular (e.g., under skin).

       • Dense Connective Tissue

         • Mainly fibres, less ground substance, poorly vascular.

         • Types:

           • Dense regular: fibres run in same direction (e.g., tendons, ligaments).

           • Dense irregular: fibres arranged irregularly (e.g., dermis of skin).

  2. Cartilage

     • Cells: chondrocytes (in lacunae), chondroblasts.

     • Matrix: firm but flexible, avascular.

     • Types:

       • Hyaline: most abundant (e.g., trachea, ribs, ends of long bones).

       • Elastic: many elastic fibres (e.g., epiglottis, ear pinna).

       • Fibrocartilage: high in collagen fibres, shock absorber (e.g., intervertebral discs, pubic symphysis).

  3. Bone

     • Cells: osteocytes (in lacunae), osteoblasts, osteoclasts.

     • Matrix: abundant collagen fibres, inorganic calcium salts (Ca2+), very vascular.

  4. Blood

     • Fluid connective tissue.

     • Cells: red blood cells (RBC), white blood cells (WBC), platelets.

     • Matrix: plasma (mostly water with ions, proteins).

Muscle Tissue

Overview

• Specialized for contraction and movement.

• Subtypes:

  • Skeletal (striated, voluntary)

  • Cardiac (striated, involuntary)

  • Smooth (non-striated, involuntary)

Nervous Tissue

Overview

• Specialized for communication and control.

• Cell types:

  • Neurons: conduct electrical impulses.

  • Glial cells: support and protect neurons.