Overview of Tissues: Structure, Types, and Functions in Anatomy & Physiology

A) Histology

Definition and Study of Tissues

Histology is the scientific study of tissues, which are groups of cells with similar structure and function. Understanding tissue organization is fundamental in anatomy and physiology.

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

• Cells of most tissues are surrounded by extracellular fluid called interstitial fluid (mostly water and ions).

• Major tissue types:

  1. Epithelial Tissue

  2. Connective Tissue

  3. Muscle Tissue

  4. Nervous Tissue

B) Cell Junctions (in Some Tissues)

Types and Functions of Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, especially in epithelial, nervous, and muscle tissues. They are formed by cell membrane proteins and serve various functions.

• Tight Junctions:

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

  • Form ring-like tight seals.

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

  • Location depends on tissue type.

• Anchoring Junctions (e.g., desmosomes):

  • Proteins that fasten cells to each other and/or extracellular material (act as "rivets").

• 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 (allows synchronization of contractions).

  • Also found in epithelial tissue.

C) Major Tissue Types (Overview)

Classification and Functions

The four major tissue types in the human body are classified based on their structure and function.

1. Epithelial Tissue: Covers body surfaces, lines body cavities, and forms glands.

2. Connective Tissue: Supports, connects, and protects other tissues.

3. Muscle Tissue: Responsible for movement and contraction.

4. Nervous Tissue: Specialized for sensation and signaling.

D) Epithelial Tissue

Structure and Characteristics

Epithelial tissue covers body surfaces (e.g., skin), lines body cavities, and forms glands. It is characterized by closely packed cells with minimal extracellular material.

• Organ cavity: Lumen (open space inside tissue).

• No free surface on the basement membrane.

• Cells are avascular (no blood vessels).

• Basement membrane anchors epithelium to underlying connective tissue.

Classification of Epithelia

• Classified by number of cell layers:

  • Simple: One cell layer.

  • Stratified: Two or more cell layers.

• Classified by cell shape:

  • Squamous: Flattened cells.

  • Cuboidal: Cube-shaped cells.

  • Columnar: Tall, rectangular cells.

Types of Epithelial Tissues

• Simple Epithelia: One layer

  • Simple squamous: Single layer of flat cells (e.g., lungs, blood vessels).

  • Simple cuboidal: Single layer of cube-shaped cells (e.g., kidneys).

  • Simple columnar: Single layer of column-shaped cells (e.g., stomach, small intestine).

• Stratified Epithelia: More than one layer

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

  • Predominantly stratified squamous (cells squished flat).

• Pseudostratified Epithelium: Appears layered but all cells touch the basement membrane (e.g., trachea).

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

Glandular Epithelium

• Exocrine glands: Secrete products onto body surfaces or into body cavities.

  • Unicellular (e.g., goblet cells).

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

• Endocrine glands: Secrete hormones into the extracellular fluid (interstitial fluid) and then into the blood (e.g., thyroid gland).

Functions of Epithelia

• Protection: Often stratified squamous (e.g., epidermis).

• Secretion: Glandular epithelium (e.g., thyroid, sweat glands).

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

E) Connective Tissue (CT)

General Features and Cell Types

Connective tissue supports, connects, and protects other tissues. It is characterized by cells embedded in an extensive extracellular matrix.

• Some tissues have high vascularity; others have low vascularity.

• Cell types often end in -blast (create matrix), -cyte (maintain matrix), -clast (break down matrix).

• Examples:

  • Osteoblast, chondroblast, fibroblast

  • Osteocyte, chondrocyte, fibrocyte

  • Osteoclast

Matrix Composition

• Fibers (proteins):

  • Collagen fibers: Provide strength.

  • Elastic fibers: Contain elastin, allow stretch and recoil.

  • Reticular fibers: Form networks, support soft tissue.

• Ground Substance: Water, minerals, and large organic molecules (proteins, carbohydrates).

Classification of Connective Tissue

• Connective Tissue Proper:

  • Loose Connective Tissue:

    • Areolar: Loosely arranged collagen and elastin fibers.

    • Adipose: Fat tissue, very little matrix.

  • Dense Connective Tissue:

    • Many fibers, poorly vascularized.

    • Dense regular: Collagen fibers running in the same direction.

• Cartilage:

  • Cells: Chondrocytes in lacunae.

  • Matrix: Collagen, elastic fibers, ground substance (mostly water).

  • Avascular: Heals slowly.

  • Types:

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

    • Elastic cartilage: Many elastic fibers (e.g., epiglottis, pinna).

    • Fibrocartilage: High in collagen fibers (e.g., intervertebral discs, pubic symphysis).

• Bone:

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

  • Matrix: Collagen fibers, ground substance (calcium phosphate salts).

  • Very vascular.

• Blood:

  • Fluid connective tissue.

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

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

F) Muscle Tissue

Types and Functions

Muscle tissue is specialized for contraction and movement. It is classified based on structure and function.

• Skeletal muscle: Striated, voluntary (e.g., biceps).

• Cardiac muscle: Striated, involuntary (e.g., heart).

• Smooth muscle: Non-striated, involuntary (e.g., stomach, blood vessels).

Summary Table: Major Tissue Types and Key Features

Tissue Type	Main Function	Key Features	Examples
Epithelial	Protection, secretion, absorption	Closely packed cells, avascular, basement membrane	Skin, lining of gut, glands
Connective	Support, connection, protection	Cells in matrix, fibers, ground substance	Bone, cartilage, blood, adipose
Muscle	Contraction, movement	Elongated cells (fibers), contractile proteins	Skeletal muscle, heart, stomach
Nervous	Sensation, signaling	Neurons, glial cells	Brain, spinal cord, nerves

Key Equations and Terms

• Interstitial Fluid: Fluid surrounding tissue cells, important for nutrient and waste exchange.

• Basement Membrane: Thin layer anchoring epithelium to connective tissue.

• Lacunae: Small spaces in cartilage and bone housing cells.

• Matrix: Extracellular material in connective tissue.

• Equation for Diffusion Rate (Fick's Law):

$J = -D \frac{dC}{dx}$

Where $J$ is the diffusion rate, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: Some context and definitions have been expanded for clarity and completeness.