Review Sheet 6 Classification Of Tissues

Alright, let's dive deep into the fascinating world of tissues! Understanding the classification of tissues is fundamental to grasping how our bodies – and those of other animals – are structured and function. This comprehensive review will break down the four primary tissue types: epithelial, connective, muscle, and nervous tissue, exploring their subtypes, characteristics, functions, and locations within the body.

Epithelial Tissue: The Body's Protective and Functional Lining

Epithelial tissue forms coverings and linings throughout the body. Think of it as the body's versatile interface with the outside world and its internal compartments. It's characterized by closely packed cells with minimal extracellular matrix.

Key Characteristics of Epithelial Tissue

• Cellularity: Composed almost entirely of tightly packed cells connected by specialized junctions like tight junctions, adherens junctions, desmosomes, and gap junctions. These junctions provide structural integrity and regulate the passage of substances.
• Specialized Contacts: Tight junctions create a barrier that prevents leakage between cells. Adherens junctions and desmosomes provide strong adhesion between cells. Gap junctions allow for direct communication between cells via the passage of ions and small molecules.
• Polarity: Epithelial cells exhibit apical (free) and basal (attached) surfaces. The apical surface often faces a lumen or the external environment and may have specialized features like microvilli or cilia. The basal surface is attached to a basement membrane.
• Support by Connective Tissue: All epithelial sheets are supported by an underlying layer of connective tissue. This connective tissue provides nutrients and support. The basement membrane, composed of basal lamina (secreted by epithelial cells) and reticular lamina (secreted by connective tissue), reinforces the epithelial sheet and resists stretching and tearing.
• Avascular but Innervated: Epithelial tissue lacks blood vessels (avascular) but is supplied by nerve fibers (innervated). Nutrients are obtained by diffusion from underlying connective tissue.
• Regeneration: Epithelial tissue has a high regenerative capacity, allowing it to repair quickly. This is essential for tissues that are constantly exposed to damage, such as the skin and the lining of the digestive tract.

Classification of Epithelial Tissue

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

Based on the Number of Cell Layers:

• Simple Epithelium: Consists of a single layer of cells. Simple epithelia are typically found in areas where absorption, secretion, and filtration occur.
• Stratified Epithelium: Consists of two or more cell layers stacked on top of each other. Stratified epithelia are found in areas subject to abrasion and friction, where protection is important.
• Pseudostratified Epithelium: Appears to be stratified but is actually a single layer of cells. All cells are attached to the basement membrane, but not all reach the apical surface.

Based on the Shape of the Cells:

• Squamous Cells: Flattened and scale-like. The nucleus is flattened.
• Cuboidal Cells: Cube-shaped with a spherical, centrally located nucleus.
• Columnar Cells: Column-shaped with an elongated nucleus located near the base of the cell.
• Transitional Cells: Vary in shape, depending on the degree of distension.

Types of Epithelial Tissue and Their Locations

• Simple Squamous Epithelium: Single layer of flattened cells.
  • Function: Allows for diffusion and filtration.
  • Location: Air sacs of the lungs (alveoli), lining of blood vessels (endothelium), lining of body cavities (mesothelium).
• Simple Cuboidal Epithelium: Single layer of cube-shaped cells.
  • Function: Secretion and absorption.
  • Location: Kidney tubules, ducts of small glands, surface of ovaries.
• Simple Columnar Epithelium: Single layer of column-shaped cells.
  • Function: Absorption and secretion of mucus and enzymes.
  • Location: Lining of the stomach, small intestine, and large intestine.
  • Note: Often contains goblet cells (secrete mucus) and may have microvilli (increase surface area for absorption).
• Pseudostratified Columnar Epithelium: Single layer of cells of varying heights.
  • Function: Secretion, particularly of mucus, and propulsion of mucus by ciliary action.
  • Location: Lining of the trachea and upper respiratory tract.
  • Note: Often contains goblet cells and cilia.
• Stratified Squamous Epithelium: Multiple layers of flattened cells.
  • Function: Protection from abrasion.
  • Location: Epidermis of the skin (keratinized), lining of the mouth, esophagus, and vagina (non-keratinized).
  • Note: Keratinized epithelium contains keratin, a tough protective protein.
• Stratified Cuboidal Epithelium: Multiple layers of cube-shaped cells.
  • Function: Protection and secretion.
  • Location: Ducts of some larger glands (e.g., sweat glands, mammary glands).
• Stratified Columnar Epithelium: Multiple layers of column-shaped cells.
  • Function: Protection and secretion.
  • Location: Rare in the body; small amounts in the male urethra and some glandular ducts.
• Transitional Epithelium: Multiple layers of cells that can change shape.
  • Function: Allows for distension.
  • Location: Lining of the urinary bladder, ureters, and part of the urethra.

Glandular Epithelium

Glandular epithelium is specialized for secretion. A gland consists of one or more cells that make and secrete a particular product called a secretion.

• Endocrine Glands: Ductless glands that secrete hormones directly into the bloodstream.

  • Examples: Thyroid gland, adrenal glands, pituitary gland.

• Exocrine Glands: Glands that secrete their products onto a surface or into a duct.

  • Examples: Sweat glands, salivary glands, mammary glands.

  Exocrine glands are further classified based on their mode of secretion:

  • Merocrine Glands: Secrete their products by exocytosis. The cell is not damaged during secretion.
    • Example: Salivary glands
  • Apocrine Glands: Accumulate their products just beneath the apical surface. The apex of the cell pinches off, releasing the secretion and a small amount of cytoplasm.
    • Example: Mammary glands (controversial if truly apocrine in humans)
  • Holocrine Glands: Accumulate their products within the cell until the cell ruptures, releasing the secretion and cellular debris.
    • Example: Sebaceous (oil) glands

Connective Tissue: Support, Connection, and More

Connective tissue is the most abundant and widely distributed tissue type in the body. As its name suggests, it connects, supports, and protects other tissues and organs. Unlike epithelial tissue, connective tissue has abundant extracellular matrix.

Key Characteristics of Connective Tissue

• Extracellular Matrix: Connective tissue is largely nonliving extracellular matrix, which separates the living cells of the tissue. The matrix is composed of ground substance and fibers.
• Ground Substance: The unstructured material that fills the space between cells and contains the fibers. It's composed of interstitial fluid, cell adhesion proteins, and proteoglycans.
• Fibers: Provide support. There are three types of fibers found in connective tissue:
  • Collagen Fibers: Strongest and most abundant type of fiber. Provide high tensile strength.
  • Elastic Fibers: Contain elastin, which allows them to stretch and recoil.
  • Reticular Fibers: Short, fine, collagenous fibers that form delicate networks.
• Cells: Different types of cells are found in connective tissue, including:
  • Fibroblasts: Secrete the proteins that make up the extracellular matrix.
  • Chondroblasts: Secrete the matrix of cartilage.
  • Osteoblasts: Secrete the matrix of bone.
  • Hematopoietic Stem Cells: Found in bone marrow; give rise to blood cells.
  • Adipocytes: Store fat.
  • White Blood Cells: Involved in immune response.
  • Mast Cells: Involved in inflammation.
  • Macrophages: Phagocytize foreign materials and debris.
• Vascularity: Most connective tissues are well vascularized (have a rich blood supply). Cartilage is avascular, and tendons and ligaments are poorly vascularized.

Classification of Connective Tissue

Connective tissue is classified into four main classes: connective tissue proper, cartilage, bone, and blood.

1. Connective Tissue Proper:

• Loose Connective Tissue:
  • Areolar Connective Tissue:
    • Function: Wraps and cushions organs, plays an important role in inflammation.
    • Location: Widely distributed under epithelia of the body; surrounds organs and capillaries.
    • Note: Contains all three types of fibers (collagen, elastic, and reticular) and various types of cells.
  • Adipose Tissue:
    • Function: Provides reserve food fuel, insulates against heat loss, and supports and protects organs.
    • Location: Under the skin, around kidneys and eyeballs, within abdomen, in breasts.
    • Note: Adipocytes are the predominant cell type.
  • Reticular Connective Tissue:
    • Function: Forms a soft internal skeleton (stroma) that supports other cell types.
    • Location: Lymphoid organs (lymph nodes, bone marrow, spleen).
    • Note: Contains reticular fibers.
• Dense Connective Tissue:
  • Dense Regular Connective Tissue:
    • Function: Attaches muscles to bones or to muscles; attaches bones to bones.
    • Location: Tendons, ligaments.
    • Note: Primarily parallel collagen fibers; fibroblasts are the main cell type.
  • Dense Irregular Connective Tissue:
    • Function: Withstands tension exerted in many directions; provides structural strength.
    • Location: Dermis of the skin, fibrous capsules of organs and joints.
    • Note: Irregularly arranged collagen fibers; fibroblasts are the main cell type.
  • Elastic Connective Tissue:
    • Function: Allows recoil of tissue following stretching; maintains pulsatile flow of blood through arteries; aids passive recoil of lungs following inspiration.
    • Location: Walls of large arteries, some ligaments of the vertebral column, within the walls of the bronchial tubes.
    • Note: Predominantly elastic fibers.

2. Cartilage:

• Hyaline Cartilage:
  • Function: Supports and reinforces; serves as resilient cushion; resists compressive stress.
  • Location: Forms most of the embryonic skeleton; covers the ends of long bones in joint cavities; forms costal cartilages of the ribs; cartilages of the nose, trachea, and larynx.
  • Note: Contains chondrocytes in lacunae (small cavities) within an amorphous matrix.
• Elastic Cartilage:
  • Function: Maintains the shape of a structure while allowing great flexibility.
  • Location: External ear (pinna), epiglottis.
  • Note: Similar to hyaline cartilage, but with more elastic fibers.
• Fibrocartilage:
  • Function: Tensile strength allows it to absorb compressive shock.
  • Location: Intervertebral discs, pubic symphysis, menisci of the knee.
  • Note: Less firm than hyaline cartilage; thick collagen fibers predominate.

3. Bone (Osseous Tissue):

• Function: Supports and protects; provides levers for muscles to act on; stores calcium and other minerals; marrow inside bones is the site for blood cell formation (hematopoiesis).
• Location: Bones.
• Note: Hard, calcified matrix containing many collagen fibers; osteocytes reside in lacunae. Well vascularized.

4. Blood:

• Function: Transports respiratory gases, nutrients, wastes, and other substances.
• Location: Contained within blood vessels.
• Note: Red blood cells (erythrocytes), white blood cells (leukocytes), and platelets in a fluid matrix (plasma).

Muscle Tissue: Movement is the Key

Muscle tissue is highly specialized to contract, generating force that produces movement. There are three types of muscle tissue: skeletal, cardiac, and smooth.

Key Characteristics of Muscle Tissue

• Excitability: Muscle cells can respond to stimuli (e.g., nerve impulses).
• Contractility: Muscle cells can shorten and generate force.
• Extensibility: Muscle cells can be stretched beyond their resting length.
• Elasticity: Muscle cells can recoil to their resting length after being stretched.

Types of Muscle Tissue

• Skeletal Muscle:
  • Function: Voluntary movement; locomotion; manipulation of the environment; facial expression; voluntary control.
  • Location: Attached to bones or skin.
  • Note: Long, cylindrical, multinucleate cells with striations.
• Cardiac Muscle:
  • Function: As it contracts, it propels blood into the circulation; involuntary control.
  • Location: Walls of the heart.
  • Note: Branching, striated, uninucleate cells with intercalated discs.
• Smooth Muscle:
  • Function: Propels substances or objects (foodstuffs, urine, a baby) along internal passageways; involuntary control.
  • Location: Walls of hollow organs (e.g., stomach, intestines, bladder, uterus, blood vessels).
  • Note: Spindle-shaped, uninucleate cells without striations.

Nervous Tissue: Communication and Control

Nervous tissue is the main component of the nervous system, which controls and coordinates many bodily functions. It is composed of two main types of cells: neurons and neuroglia.

Key Characteristics of Nervous Tissue

• Neurons: Specialized cells that generate and conduct electrical signals. They have a cell body, dendrites (receive signals), and an axon (transmits signals).
• Neuroglia (Glial Cells): Supporting cells that surround and support neurons. They provide nutrients, insulation, and protection.

Location and Function of Nervous Tissue

• Function: Transmits electrical signals from sensory receptors to the brain and spinal cord, and from the brain and spinal cord to effectors (muscles and glands).
• Location: Brain, spinal cord, and nerves.

Summarizing the Four Tissue Types

Review Questions and Answers

Let's test your knowledge with some review questions!

Question 1: Which type of epithelial tissue is best suited for diffusion and filtration?

Answer: Simple squamous epithelium. Its thin, single layer of flattened cells allows for rapid passage of substances.

Question 2: What are the three types of fibers found in connective tissue?

Answer: Collagen fibers, elastic fibers, and reticular fibers.

Question 3: Which type of muscle tissue is responsible for voluntary movement?

Answer: Skeletal muscle.

Question 4: What are the two main types of cells found in nervous tissue?

Answer: Neurons and neuroglia.

Question 5: Where is transitional epithelium found, and what is its function?

Answer: Transitional epithelium is found in the lining of the urinary bladder, ureters, and part of the urethra. Its function is to allow for distension.

Conclusion: Mastering Tissue Classification

Understanding the classification of tissues is essential for anyone studying biology, anatomy, or physiology. This review has provided a comprehensive overview of the four primary tissue types, their characteristics, functions, and locations in the body. By mastering these concepts, you'll gain a deeper appreciation for the complexity and elegance of living organisms. Keep practicing, keep exploring, and keep learning!