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. However, the destruction of one cell type can be catastrophic for tissue function.

• 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: Loss of one cell type can be detrimental to tissue function.

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.

• Neurons: Specialized cells capable of creating and transmitting electrical impulses throughout the body. They respond to various stimuli.

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

Muscle Tissue

Types and Functions

Muscle tissue enables mobility, both voluntary and involuntary. There are three main types of muscle tissue:

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

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

• Smooth muscle: Involuntary control, non-striated. Located in the walls of hollow organs to move substances through the body.

Epithelial Tissue (Epithelium)

Functions

Epithelial tissue serves as a boundary-forming tissue, separating different regions of the body and controlling the passage of substances.

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

• Glandular epithelium: Composes some glands in the body.

Characteristics of Epithelial Tissue

• Apical polarity: The apical (free) surface is exposed to the organ's exterior or the outside of the body.

• Microvilli: Increase surface area for absorption.

• Cilia: Create currents to propel substances across the surface.

• Basal surface: Attaches to underlying connective tissue.

Cell Junctions

• Tight junctions: Prevent leakage of material between cells.

• Desmosomes: Provide mechanical strength and resist stress.

Support Structures

• Basement membrane: Defines the boundary between epithelium and underlying connective tissue.

• Functions: Reinforces epithelial sheet, resists stretching and tearing.

• Layers:

  • Basal lamina: Filters what can/cannot enter epithelium.

  • Reticular lamina: Contains fibers from connective tissue.

Other Characteristics

• Avascular: No direct blood supply; nourishment by diffusion.

• Innervated: Supplied with nerve fibers.

• Regeneration: High capacity for cell reproduction, especially after damage.

Classification of Epithelial Tissue

Based on Layers

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

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

Based on Cell Shape

• Squamous cells: Flattened, scale-like.

• Cuboidal cells: Cube-shaped.

• Columnar cells: Tall, column-shaped.

Special Classes

• Pseudostratified columnar epithelium: Appears stratified but all cells touch the basement membrane; often ciliated.

• Transitional epithelium: Apical cells vary in shape; found in areas subject to stretching (e.g., bladder).

Glandular Epithelia

Definition and Types

Glands are one or more cells that produce and secrete a particular product (secretion).

• 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).

Number of Cells

• 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).

Mode of Secretion

• Merocrine glands: Secrete via exocytosis; cell remains intact.

• Holocrine glands: Secretion released by cell rupture.

• Apocrine glands: Secretion released by partial cell rupture (cell does not completely die).

Connective Tissue

General Characteristics

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

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

• Origin: Arises from embryonic mesenchyme.

Components of Connective Tissue

• Ground substance: Fills space around cells; composed of interstitial fluid, cell adhesion proteins, and proteoglycans.

• Fibers:

  • Collagen fibers: Strong, flexible, resist pulling forces.

  • Elastic fibers: Stretch and recoil; important in tissues subject to frequent stretching.

  • Reticular fibers: Branched, form supportive networks.

• Cells:

  • Immature cells (-blast): Actively mitotic, produce ground substance and fibers.

  • Mature cells (-cyte): Maintain ground substance and fibers.

Types of Connective Tissue

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

• Cartilage: Chondroblasts/chondrocytes; avascular and tough.

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

• Blood: Erythrocytes, leukocytes, thrombocytes; functions in transport, regulation, and hemostasis.

Connective Tissue Disorders

• Marfan Syndrome: Genetic disorder affecting connective tissue due to mutation in the FBN1 gene (fibrillin protein). Causes weakness in elastic fibers, affecting multiple body systems (joints, blood vessels, respiratory tract).

Covering and Lining Membranes

Types of Membranes

• Cutaneous membrane: Skin; composed of keratinized stratified squamous epithelium.

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

• Serous membrane (serosa): Lines closed body cavities; named according to location and organ association.

Tissue Repair and Regeneration

Process of Repair

1. Inflammation: Injury causes bleeding and release of inflammatory chemicals; clot forms and dries to a scab.

2. Restoration: Blood supply restored; granulation tissue forms, bringing in fibroblasts (lay down new fibers) and macrophages (clean debris).

3. Regeneration and fibrosis: Surface epithelium regenerates; scar tissue may remain visible or invisible.

Homeostatic Imbalance: Cancer

Unregulated Cell Division

• Neoplasm: Mass of cells; can be benign or malignant.

• Metastasis: Malignant cells can spread to other tissues.

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

• Treatment: Chemotherapy and radiation.

Summary Table: Major Tissue Types and Functions

Tissue Type	Main Function	Location
Nervous	Control, communication	Brain, spinal cord, nerves
Muscle	Movement	Skeletal muscles, heart, hollow organs
Epithelial	Covering, protection, secretion	Body surfaces, cavities, glands
Connective	Support, binding, transport	Throughout body (bone, blood, cartilage, fat)

Key Equations and Concepts

• Diffusion (for epithelial nourishment): $J = -D \frac{dC}{dx}$ Where J is the diffusion flux, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: Some details, such as the full process of tissue repair and the molecular basis of Marfan syndrome, were expanded for academic completeness.