Tissues: Structure, Function, and Classification (Chapter 4 Study Notes)

Tissues: An Overview

Definition and Importance

Tissues are groups of cells that are similar in structure and perform a common or related function. Cellular specialization allows the body to carry out complex functions efficiently. The loss or destruction of one cell type can be catastrophic, highlighting the importance of tissue integrity.

• Definition: Any grouping of cells that are similar in structure and carry out a similar or common function.

• Benefit: Cellular specialization enables the body to perform complicated functions.

• Drawback: Destruction of one cell type can be detrimental to overall function.

Four primary types of tissue:

• Nervous tissue: control tissue

• Muscle tissue: movement tissue

• Epithelial tissue: covering tissue

• Connective tissue: support tissue

Nervous Tissue

Structure and Function

Nervous tissue regulates and controls various body functions. It is found in the brain, spinal cord, and nerves, and is essential for communication throughout the body.

• Neurons: Specialized cells capable of creating and transmitting electrical impulses throughout the body. They respond to various stimuli and are the primary functional units of nervous tissue.

• Supporting cells (neuroglia): Nonconducting cells that protect, insulate, and support neurons.

Example: Neurons transmit signals from sensory organs to the brain, allowing perception and response to the environment.

Muscle Tissue

Types and Functions

Muscle tissue is responsible for movement, both voluntary and involuntary. It is classified into three types based on structure and control.

• Skeletal muscle: Voluntary control, striated appearance. Attached to bones and responsible for body movement.

• Cardiac muscle: Involuntary control, striated. Found only in the walls of the heart, responsible for pumping blood.

• Smooth muscle: Involuntary control, non-striated. Located in the walls of hollow organs (e.g., intestines, blood vessels) to move substances through the body.

Example: Skeletal muscles contract to move limbs, while smooth muscle in the digestive tract propels food.

Epithelial Tissue (Epithelium)

Functions and Types

Epithelial tissue forms boundaries between different environments, protects, secretes, absorbs, and filters substances. It covers body surfaces and lines cavities.

• Boundary-forming: Separates one area/region of the body from another.

• Covering epithelium: Forms the covering of all internal and external surfaces, lines body cavities and organs.

• Glandular epithelium: Composes some glands responsible for secretion.

Example: Skin epithelium protects against pathogens; glandular epithelium in the pancreas secretes digestive enzymes.

Characteristics of Epithelial Tissue

• Apical polarity: The apical (free) surface is exposed to the exterior or cavity. May have microvilli (increase surface area) or cilia (propel substances).

• Basal surface: Attaches to underlying connective tissue.

• Cell junctions: Neighboring cells are joined by tight junctions and/or desmosomes, forming sheets that prevent leakage and resist mechanical stress.

Additional info: Microvilli are common in absorptive tissues (e.g., intestines), while cilia are found in respiratory epithelium.

Support Structures

• Basement membrane: Defines the boundary between epithelium and underlying connective tissue. Reinforces the epithelial sheet and resists stretching/tearing.

• Layers:

  • Basal lamina: Filters substances entering the epithelium.

  • Reticular lamina: Contains connective tissue fibers.

Other Characteristics

• Avascular: No direct blood supply; nourishment occurs by diffusion from underlying tissues.

• Innervated: Supplied with nerve fibers.

• Regeneration: High capacity for cell reproduction, especially in areas subject to abrasion.

Example: Skin epithelium rapidly regenerates after injury.

Classification of Epithelial Tissue

Criteria for Naming

Epithelial tissues are classified based on the number of cell layers and the shape of cells.

• Number of layers:

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

  • Stratified epithelium: Two or more layers; found in areas subject to abrasion.

• Cell shape:

  • Squamous: Flattened, scale-like cells.

  • Cuboidal: Cube-shaped cells.

  • Columnar: Tall, column-shaped cells.

Additional info: The nucleus shape often matches the cell shape (flattened in squamous, round in cuboidal, elongated in columnar).

Types of Simple Epithelia

• Simple squamous epithelium: Thin and permeable; ideal for diffusion and filtration. Endothelium lines blood vessels; mesothelium lines body cavities.

• Simple cuboidal epithelium: Single layer of cube-shaped cells; functions in secretion and absorption.

• Simple columnar epithelium: Single layer of tall cells; may have microvilli or cilia; functions in absorption and secretion.

• Pseudostratified columnar epithelium: Appears stratified but all cells touch the basement membrane; often ciliated; functions in secretion and absorption.

Types of Stratified Epithelia

• Stratified squamous epithelium: Multiple layers; protects against abrasion; surface cells are constantly replaced.

• Stratified cuboidal and columnar epithelium: Rare; usually two layers; found in some glands.

• Transitional epithelium: Apical cells vary in shape (cuboidal/columnar); found in urinary bladder, allows stretching.

Glandular Epithelia

Structure and Function

Glandular epithelia consist of cells that produce and secrete products (e.g., hormones, enzymes, mucus). Glands are classified by their mode of secretion, number of cells, and location.

• Endocrine glands: Release hormones directly into the bloodstream (e.g., thyroid, pancreas).

• Exocrine glands: Release products onto surfaces or into cavities (e.g., sweat, salivary glands).

• Unicellular glands: Single cells (e.g., goblet cells in digestive and respiratory tracts).

• Multicellular glands: Composed of many cells; have ducts and secretory units (acini).

Gland Type	Secretion Method	Example
Merocrine	Exocytosis; cell remains intact	Salivary glands
Holocrine	Cell ruptures to release secretion	Sebaceous (oil) glands
Apocrine	Partial cell loss; cell repairs itself	Mammary glands

Connective Tissue

General Characteristics

Connective tissue is the most abundant and widely distributed tissue type. It supports, binds, and protects other tissues and organs.

• Extracellular matrix (ECM): Nonliving material that separates living cells; consists of ground substance and fibers.

• Origin: All connective tissue arises from embryonic mesenchyme.

Components of Connective Tissue

• Ground substance: Fills space around cells; made of interstitial fluid, cell adhesion proteins, and proteoglycans. Viscosity increases with more proteoglycans.

• Fibers:

  • Collagen fibers: Strong, flexible, resist pulling forces; made of collagen protein.

  • Elastic fibers: Contain elastin; can stretch and recoil, important in tissues that undergo frequent stretching (e.g., lungs, skin).

  • Reticular fibers: Short, branched fibers forming supportive networks in organs (e.g., lymph nodes).

• Cells: Immature cells (-blast) build matrix; mature cells (-cyte) maintain it.

Types of Connective Tissue

• Connective tissue proper: Includes loose (areolar, adipose, reticular) and dense (regular, irregular, elastic) types.

• Cartilage: Chondroblasts/chondrocytes; avascular, tough but flexible.

• Bone (Osseous tissue): Osteoblasts/osteocytes; contains inorganic calcium salts, highly vascularized.

• Blood: Erythrocytes (RBCs), leukocytes (WBCs), thrombocytes (platelets); functions in transport, regulation, and hemostasis.

Connective Tissue Type	Main Cell Type	Function	Location
Areolar	Fibroblast	Support, bind, nourish	Almost everywhere
Adipose	Adipocyte	Energy storage, insulation	Subcutaneous tissue
Reticular	Reticular cell	Support for blood cells	Lymph nodes, bone marrow
Dense regular	Fibroblast	Resist tension (one direction)	Tendons, ligaments
Dense irregular	Fibroblast	Resist tension (multiple directions)	Dermis, joint capsules
Elastic	Fibroblast	Recoil after stretching	Large arteries, lungs

Homeostatic Imbalance: Marfan Syndrome

Marfan syndrome is a genetic disorder affecting connective tissue due to a mutation in the FBN1 gene (codes for fibrillin). This leads to weakened elastic fibers, impacting multiple body systems, especially joints, blood vessels, and the respiratory tract.

Covering and Lining Membranes

Types and Functions

Membranes are simple organs composed of epithelial and connective tissues. They cover and line body surfaces and cavities.

• Cutaneous membrane (skin): Keratinized stratified squamous epithelium; protects the body.

• Mucous membrane (mucosa): Lines organs that open to the exterior; moist, involved in absorption and secretion.

• Serous membrane (serosa): Lines closed body cavities; named by location (e.g., pleura, pericardium).

Tissue Repair and Regeneration

Process of Repair

When injury occurs, tissue repair involves regeneration (replacement of lost cells) or fibrosis (formation of scar tissue).

1. Step 1: Inflammation

   • Damaged cells release inflammatory chemicals.

   • Blood vessels become more permeable; clot forms and scab develops.

2. Step 2: Organization

   • Granulation tissue replaces clot; highly vascularized.

   • Fibroblasts lay down new fibers, forming scar tissue.

   • Macrophages remove debris and dead cells.

3. Step 3: Regeneration and Fibrosis

   • Epithelium regenerates; scab detaches.

   • Scar tissue may remain, depending on severity.

Additional info: Vascularization is crucial for bringing nutrients and cells needed for repair.

Homeostatic Imbalance: Cancer

Unregulated Cell Division

Cancer results from uncontrolled division of tissue cells, forming a mass called a neoplasm. Neoplasms can be benign or malignant; malignant tumors may become metastatic (spread to other tissues).

• Cause: Mutations in genes regulating cell division (oncogenes, proto-oncogenes, tumor suppressor genes).

• Treatment: Chemotherapy and radiation target rapidly dividing cells.

Additional info: Proto-oncogenes promote normal cell growth; oncogenes are mutated forms that drive cancer. Tumor suppressor genes inhibit cell division and prevent tumor formation.