Chapter 4: Tissue – The Living Fabric (Anatomy & Physiology Study Notes)

Introduction to Tissues

Definition and Importance

Tissues are groups of cells that are similar in structure and perform a common or related function. The study of tissues is known as histology. Understanding tissues is fundamental to anatomy and physiology, as tissues form the building blocks of organs and organ systems.

• Four basic tissue types: epithelial, connective, muscle, and nervous tissue

• Each tissue type is specialized for distinct functions within the body

Preparing Tissue Samples

Steps in Tissue Preparation

To study tissues under a microscope, samples must be properly prepared. This process involves several key steps:

• Fixed: The tissue is preserved to prevent decay and maintain structure.

• Sectioned: The tissue is thinly sliced to allow light or electron transmission.

• Stained: Dyes are applied to enhance contrast and highlight specific structures. However, staining can introduce artifacts (distortions) that may not represent the tissue's appearance in the living state.

Microscopy Methods

Types of Microscopy

Different microscopy techniques are used to visualize tissues at various levels of detail:

• Light Microscopy: Reveals basic tissue structures using visible light.

• Transmission Electron Microscopy (TEM): Shows internal ultrastructure by passing electrons through thin sections.

• Scanning Electron Microscopy (SEM): Provides detailed 3D images of tissue surfaces.

Example: TEM can reveal the arrangement of organelles within a cell, while SEM can show the surface texture of cilia on epithelial cells.

Epithelial Tissue: Overview

General Characteristics

Epithelial tissue covers body surfaces, lines internal cavities, and forms glands. It serves several essential functions:

• Protection

• Absorption

• Filtration

• Secretion

• Sensation

Epithelial tissue is highly regenerative, allowing for rapid repair and renewal.

• Covering and lining epithelium: Forms the outer layer of the skin, lines open cavities, and covers the walls and organs of the ventral body cavity.

• Glandular epithelium: Forms the glands of the body.

Special Features of Epithelium

• Polarity: Has an apical (exposed) surface and a basal (attached) surface.

• Specialized contacts: Includes tight junctions (sealing) and desmosomes (strengthening).

• Supported by connective tissue: The basement membrane anchors the epithelium.

• Avascular but innervated: Contains no blood vessels but is supplied by nerves.

• High regeneration capacity: Rapid cell turnover for repair.

Classification of Epithelia

Naming System

Epithelia are classified based on the number of cell layers and the shape of the cells:

• Layers:

  • Simple: Single layer; found where absorption, secretion, and filtration occur.

  • Stratified: Multiple layers; found in areas subject to abrasion.

• Cell Shape:

  • Squamous: Flattened and scale-like.

  • Cuboidal: Box-like, as tall as they are wide.

  • Columnar: Tall and column-shaped.

Types of Epithelia

• Simple Squamous Epithelium: Thin, flat cells; allows rapid diffusion and filtration. Examples: Air sacs of lungs, lining of blood vessels.

• Simple Cuboidal Epithelium: Cube-shaped cells; functions in secretion and absorption. Examples: Kidney tubules, small glands.

• Simple Columnar Epithelium: Tall, closely packed cells; absorption and secretion of mucus and enzymes. May have microvilli or cilia. Examples: Digestive tract lining.

• Pseudostratified Columnar Epithelium: Appears layered but all cells touch the basement membrane; functions in secretion and absorption. Example: Ciliated lining of respiratory tract.

• Stratified Squamous Epithelium: Multiple layers for protection; keratinized (skin) or non-keratinized (mouth, esophagus).

• Stratified Cuboidal & Columnar Epithelium: Rare, found in ducts of large glands (cuboidal) and in parts of the pharynx and male urethra (columnar).

• Transitional Epithelium: Specialized to stretch; found in urinary system organs (bladder, ureters). Dome-shaped cells flatten when the organ fills.

Glandular Epithelium

Overview and Classification

Glands are composed of one or more cells that make and secrete a particular product, called a secretion. Glands are classified by:

• Release site: Endocrine (ductless, secrete hormones into blood/lymph) vs. Exocrine (secrete into ducts onto body surfaces or cavities).

• Cell number: Unicellular (e.g., goblet cells) vs. Multicellular (e.g., sweat glands).

Endocrine vs. Exocrine Glands

• Endocrine glands: Ductless, secrete hormones by exocytosis directly into blood or lymph. Example: Thyroid gland.

• Exocrine glands: Secrete products into ducts. Examples: Mucus, sweat, oil, and salivary glands. Can be unicellular or multicellular.

Unicellular and Multicellular Exocrine Glands

• Unicellular: Mucous and goblet cells; secrete mucus to respiratory and digestive linings.

• Multicellular: Consist of a duct and a secretory unit. Classified by duct structure (simple vs. compound) and mode of secretion:

  • Merocrine: Secrete by exocytosis (e.g., sweat, pancreas).

  • Holocrine: Rupture and release contents (e.g., sebaceous glands).

Connective Tissue: Overview

General Characteristics

Connective tissue is the most abundant and widely distributed tissue type in the body. It provides binding, support, protection, insulation, and transportation.

• Four main classes: Connective tissue proper, cartilage, bone, blood

• Common origin: All connective tissues arise from mesenchyme (an embryonic tissue)

• Extracellular matrix: Non-living material (ground substance + fibers) separates living cells

• Varying vascularity: Some types are highly vascular (bone), others are avascular (cartilage)

Structural Elements of Connective Tissue

• Ground substance: Unstructured material that fills the space between cells; contains fluid, proteins, and proteoglycans

• Fibers:

  • Collagen: Strongest and most abundant; provides tensile strength

  • Elastic: Long, thin fibers that allow for stretch and recoil

  • Reticular: Short, fine fibers that form delicate networks

• Cells: Each major class has a resident cell type (e.g., fibroblasts in connective tissue proper, chondroblasts in cartilage, osteoblasts in bone). "Blast" cells are immature and secrete matrix; "cyte" cells are mature and maintain the matrix.

Types of Connective Tissue

Connective Tissue Proper

• Loose connective tissue:

  • Areolar: Binds and cushions organs; most widely distributed

  • Adipose: Stores nutrients (fat); provides insulation and protection

  • Reticular: Forms a soft internal skeleton for certain organs (e.g., lymph nodes)

• Dense connective tissue:

  • Dense regular: Closely packed collagen fibers; found in tendons and ligaments

  • Dense irregular: Irregularly arranged collagen fibers; found in dermis of skin

  • Elastic: Contains elastic fibers; found in large arteries

Cartilage

• Avascular, lacks nerve fibers, flexible yet strong

• Types:

  • Hyaline cartilage: Most common; supports and reinforces (e.g., nose, trachea)

  • Elastic cartilage: Maintains shape while allowing flexibility (e.g., external ear)

  • Fibrocartilage: Resists compression; found in intervertebral discs

Bone (Osseous Tissue)

• Highly vascularized, hard, mineralized matrix for support and protection

• Osteoblasts: Build bone matrix

• Osteocytes: Maintain bone matrix; reside in lacunae

• Osteons: Structural units of bone tissue with central canal for nerves and blood vessels

Blood

• Fluid connective tissue; consists of blood cells suspended in plasma

• Red blood cells (RBCs): Transport oxygen

• White blood cells (WBCs): Defense and immunity

• Platelets: Blood clotting

• Functions: Transport, immunity, regulation

Muscle Tissue Overview

General Characteristics

Muscle tissue is highly vascularized and specialized for contraction. It contains actin and myosin filaments that generate force and movement.

• Skeletal muscle: Voluntary, striated, multinucleated, attached to bones

• Cardiac muscle: Involuntary, striated, branched, joined by intercalated discs; found only in the heart

• Smooth muscle: Involuntary, non-striated, spindle-shaped; found in walls of hollow organs (e.g., intestines, blood vessels)

Nervous Tissue

Structure and Function

Nervous tissue regulates and controls body functions. It is composed of two main cell types:

• Neurons: Specialized cells that generate and conduct electrical impulses; consist of dendrites (receive signals) and axons (send signals)

• Supporting cells (neuroglia): Insulate, protect, and support neurons

Tissue Repair

Process of Tissue Repair

Tissue repair involves the restoration of tissue architecture and function after injury. The process occurs in three main steps:

1. Inflammation: Prepares the area for repair by removing debris and pathogens

2. Organization: Replaces the blood clot with granulation tissue and restores blood supply

3. Regeneration and/or Fibrosis: Damaged cells are replaced with the same type (regeneration) or with fibrous connective tissue (fibrosis)

The regeneration capacity varies by tissue type.

Developmental Aspects of Tissues

Embryonic Development

• Three primary germ layers form during early embryonic development:

  • Ectoderm: Forms skin and nervous tissue

  • Mesoderm: Forms muscle and connective tissue

  • Endoderm: Forms the lining of the digestive and respiratory tracts

• These layers specialize to form the four primary tissue types.

In adults, epithelia and blood-forming tissues remain highly mitotic (renew rapidly). Other tissues regenerate via mature cell division or activation of stem cells.