Chap 3 Summary

Introduction to Tissues

• Tissues are collections of specialized cells and products organized for specific functions.

• Four primary tissue types:

  1. Epithelial tissue: Covers external surfaces, lines internal passageways/chambers

     • Aligns things

     • Produces glandular secretions

  2. Connective tissue: Supports, binds, and protects other tissues and organs, providing structural integrity and elasticity.

  3. Muscle tissue: Smooth, cardiac, and skeletal

     • All muscle tissue makes things move

  4. Nervous tissue: Neurons and glial cells produce electricity which elicits a response (chemical messenger, hormone)

• Histology: Study of tissues.

3.1 Epithelial Tissue

• Epithelial tissues: Layer of cells that cover an exposed surface or internal cavity/passageway

  • Epithelia (covers surfaces)

  • Glands (secretory structures from epithelia)

• An epithelium is an avascular sheet of cells forming a surface, lining, or covering.

• Epithelia consist mainly of tightly bound cells, with minimal extracellular material.

• Epithelial cells are continually replaced by stem cell activity.

Functions of Epithelial Tissue

1. Provides physical protection.

2. Controls permeability.

3. Provides sensation (contains sensory receptors)

4. Produces specialized secretions (gland cells).

Characteristics & Specializations of Epithelial Cells

• Characteristics unique to epithelial tissue:

  1. Cellularity: All epithelial cells are identical (same type of cell) that are packed closely together by CAMs, minimizing the space between them.

  2. Polarity: (different sides) Epithelial cells exhibit an apical surface that is exposed to the external environment or internal body cavity, while the basal surface is attached to the underlying connective tissue.

  3. Attachment: Basak surface is bound to a thin basement membrane— a union of epithelium (superficial) and CT (deep)

     • Lamina Lucida: secreted by the layer closest to the epithelium

     • Lamina Densa: Contains bundles of coarse protein fibers produced by the deeper CT

       • Combine to form the basal lamina, which plays a critical role in supporting and anchoring the epithelium to the underlying connective tissue.

  4. Avascularity: Lacks blood flow, relies on diffusion to transport gases and nutrients across apical or basal membranes

     • No blood flow to epithelium, makes for a better barrier (would be easier for pathogens to enter bloodstream if vascular)the

  5. Regeneration: Epithelia that are damaged or destroyed are continuously replaced through mitosis via basal cells in the deepest layer of epithelium, near the basement membrane, ensuring that the epithelial barrier remains intact and functional.

Maintaining the Integrity of the Epithelium

• The integrity of epithelial tissue depends on:

  1. Strong cell-to-cell adhesion

  2. Firm attachment to the basal lamina

  3. Continuous renewal by stem cells

• These mechanisms protect against mechanical damage, infection, and environmental stressors, ensuring that epithelial tissues maintain their critical roles in the body.

• Attachment/intercellular connections (CAMs): Cells are held together by:

  • Tight junctions: strongest due to partial fusion of adjacent membranes (found near the apical surface)

  • Hemidesosomes:

  • Desmosomes: Locks cells together, resists twisting and stretching— abundant in superficial layers of skin

  • Gap Junctions: Cells connected by the membrane proteins that produce a channel that let molecules, ions pass through— cardiac & smooth muscle

Classification of Epithelia

• Classified by:

  1. Number of cell layers:

     1. Simple- (one layer)

     2. Stratified- (multiple layers).

  2. Cell shape at the surface:

     • Squamous (thin/flat)

     • Cuboidal (short hexagonal boxes)

     • Columnar (tall and slender)

     • Pseudostratified columnar (appears stratified but isn’t)

     • Transitional (cuboidal and squamous, allows stretching).

• Simple Squamous Epithelia:

  • Best for rapid diffusion (diffusion does not work for thick barriers/long distanced)

  • found in alveoli of lungs (quick gas exchange) or serous membranes lining VBC and blood vessels, lining ventral body

• Stratified Squamous Epithelia:

  • Several layers thick

  • Found where there is mechanical stress (skin, oral, anal, esophagus)

• Simple Cuboidal Epithelia:

  • Provides limited protection

  • Found where secretion and absorption occur, such as in glands and kidney tubules.

• Stratified Cuboidal Epithelia:

  • Fairly rare

  • Sweat glands and some exocrine glands

• Transitional Epithelia:

  • Change in shape

  • Exclusively found in the urinary system

• Simple Columnar Epithelia:

  • Good for secretion/absorbtion

  • Found in the stomach & small intestine

• Stratified Columnar Epithelia:

  • Fairly rare, only superficial layer

  • Salivary ducts

• Pseudostratified Columar Epithelia:

  • Stratified in some places but not in others, may be different heights & may not reach apical surface but are attached to basement membrane

  • Nasal cavity, trachea, bronchi, respiratory

Glandular Epithelia

• Glands classified by secretion type, structure, and mode:

  • Exocrine secretions: Discharged through ducts to external surfaces, leaves the body.

  • Endocrine secretions: Hormones released into interstitial fluid, stay in the body.

  • Exocrine classification:

    1. Serous (watery secretions with enzymes)

    2. Mucous (viscous, sticky mucus)

    3. Mixed (both types)

• Gland types:

  • Unicellular: single secretory cells.

  • Multicellular: aggregates of gland cells.

Secretion Mechanisms

• Merocrine (Eccrine): most common, product released by exocytosis.

• Apocrine: Portion of cell are lost w secretory product

  • Sweat, smelly, hormonal related

• Holocrine: destroys the cell, which bursts with secretions.

  • Sebaceous glands

3.2 Connective Tissues

• All connective tissues consist of:

  1. Specialized cells

  2. Extracellular protein fibers

  3. Ground substance

     1. The gel-like material in connective tissue that fills the space between cells and fibers. It helps cushion and support the tissue, allows nutrients and waste to move between cells and blood vessels, and keeps the tissue hydrated. It’s important because it helps connective tissue stay strong, flexible, and able to resist pressure.

• Connective tissues varies becasue there are many types of CT that all do different things, they are identified by their extracellular matrix, .

• Functions include:

  • Structural framework— bones

  • Transportation of fluids/dissolved materials— blood

  • Protection of organs— fat

  • Support and interconnection of tissues— tendons

  • Energy storage— adipose

  • Defense against microorganisms— WBs

Classification of Connective Tissues

1. Connective tissue proper: “properly connects things”

   1. Varied cell populations and fiber types in viscous ground substance.

2. Fluid connective tissues: Blood & lymph

   1. Unique cells in watery ground substance (e.g., blood, lymph).

3. Supporting connective tissues: “supports doesn’t connect”

   1. Less diverse cell population with dense fibers (e.g., cartilage, bone).

Connective Tissue Proper

• Composed of extracellular fibers, ground substance, and two cell types (fixed and wandering).

• Three types of fibers:

  • Collagen fibers— the strongest and most common fiber

    • Tensile strength (resists tension)

    • Not flexible

  • Reticular fibers— single unit of collagen protein

    • Tough but flexible, thinner and branched into interwoven framework

  • Elastic fibers— Contain elastin

    • Branching, wavy can stretch up to 150%

• Includes:

  • Loose connective tissues:

    • Areolar— Most common & least specialized. Has a loose, random arrangement with lots of space for ground substance

    • Adipose— Fat tissue, consists mostly of adipocytes

    • Reticular— Consists of reticular fibers, forms CT of liver

      • The ground substance takes up most volume.

  • Dense connective tissues:

    1. Dense regular— Usually found in:

       • Tendons (connect muscle to bone)

       • Aponeuroses (connect muscle to muscle)

       • Ligaments (connect bone to bone)

    2. Dense irregular— Usually found in

       • Nerve and muscle sheaths, provides strength

Fluid Connective Tissues

• Blood is a fluid CT containing a matrix called plasma and various types of cells

• Lymph is formed as interstitial fluid and collected into lymphatic vessels to be brought back to the blood vessels

Supporting Connective Tissues

• Cartilage: Matrix includes chondroitin sulfates, produced by chondroblasts and maintained by chondrocytes.

  • Types of cartilage:

    1. Hyaline— The strongest & most common (serves as fetal skeleton), surrounds synovial joints, trachea

    2. Elastic— More flexible & less supportive (think ears) something that can move

    3. Fibrous— Very dense and great for shock absorption (verbal disks)

• Bone (osseous tissue): Matrix includes collagen fibers and calcium salts, housing osteocytes in lacunae.

  • Nutrition through diffusion in intercellular connections.

  • Periosteum covers bone surfaces for attachment and repair.

3.3 Membranes

• Membranes: combine epithelia and connective tissues to cover and protect structures.

• Four types of membranes:

  1. Mucous

  2. Serous

  3. Cutaneous

  4. Synovial.

Mucous Membranes

• Line passageways to the exterior (e.g., digestive, respiratory) and moistened by mucus; contain areolar tissue (lamina propria).

Serous Membranes

• Line internal cavities, moist and permeable, producing transudate (e.g., pleural, peritoneal, pericardial membranes).

Cutaneous Membrane (Skin)

• Covers body surface, thick, waterproof, and dry.

Connective Tissue Framework of the Body

• Connective tissue proper (e.g., superficial fascia, deep fascia, subserous fascia) interconnects organ systems.

3.5 Muscle Tissue

• Specialized cells for contraction; three types:

  • Skeletal Muscle

  • Cardiac Muscle

  • Smooth Muscle.

Skeletal Muscle Tissue

• Large cylindrical fibers, striated, controlled voluntarily by the nervous system.

  • Contains myosatellite cells for fiber regeneration.

Cardiac Muscle Tissue

• Found in the heart; short branched cells, involuntary control, striated muscle.

Smooth Muscle Tissue

• Short tapered cells, non-striated, involuntary, regenerative capability, found in blood vessels and hollow organs.

3.6 Nervous Tissue

• Specialized for conducting electrical impulses; consists of two cell types:

  • Neurons (transmit electrical impulses)

  • Neuroglia (support neurons and provide nutrients).

• Neurons have a cell body (soma), dendrites (receive messages), and an axon (conducts messages).

Cutaneous Membrane (Skin)

• Covers the body surface; unlike other membranes, it is thick, waterproof, and usually dry.

3.7 Synovial Membranes

• Located in synovial joints, produces synovial fluid for lubrication and smooth movement.

Embryonic Connective Tissues

• All connective tissues derived from embryonic mesenchyme.

Tissues and Aging

• Tissues undergo changes with age; repair becomes less efficient, and structure/chemical composition alters.

3.8 Summary of Early Embryology

• Early embryology encompasses:

  • Tissue formation

  • Development of epithelia

  • Development of connective tissues

  • Development of organ systems.