Tissue Level of Organization: Anatomy & Physiology Study Notes

Introduction to Tissues

Definition and Importance

Tissues are groups of cells and cell products that perform specific, limited functions. In multicellular organisms, tissues combine to form organs, which are organized into organ systems.

• Tissue: Structure with discrete structural and functional properties.

• Organs can be grouped into 11 organ systems.

Types of Tissues

Main Categories

There are four primary types of tissues in the human body, each with distinct roles:

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Neural tissue

Epithelial Tissue

Functions of Epithelial Tissue

Epithelial tissue covers exposed surfaces, lines internal passageways, and forms glands. Its main functions include:

• Physical protection

• Control permeability

• Provide sensation

• Produce specialized secretions (glandular epithelium)

Characteristics of Epithelia

• Cellularity (cell junctions)

• Polarity (apical and basal surfaces)

• Attachment (basement membrane or basal lamina)

• Avascularity (lack of blood vessels)

• Regeneration (replaced by division of germinative cells)

Specializations of Epithelial Cells

• Move fluids over the epithelium (protection)

• Move fluids through the epithelium (permeability)

• Produce secretions (protection and messengers)

Maintaining Integrity of Epithelia

• Intercellular connections (cell adhesion molecules, transmembrane proteins, intercellular cement)

• Attachment to the basement membrane

• Epithelial maintenance and repair

Intercellular Connections

• Cell junctions:

  • Tight junctions: Prevent passage of water and solutes, isolate wastes in the lumen.

  • Gap junctions: Allow rapid communication, coordinate contractions in heart muscle.

  • Desmosomes: Allow bending and twisting, attach cells to the basement membrane.

Attachment to Basement Membrane

• Clear layer (lamina lucida): Thin layer, secreted by epithelia, barrier to proteins.

• Dense layer (lamina densa): Thick fibers, produced by connective tissue, strength and filtration.

Classification of Epithelia

Based on Shape

• Squamous epithelia: Thin and flat

• Cuboidal epithelia: Square shaped

• Columnar epithelia: Tall, slender rectangles

Based on Layers

• Simple epithelium: Single layer of cells

• Stratified epithelium: Several layers of cells

Types and Examples

• Simple squamous epithelium: Absorption and diffusion (e.g., mesothelium lining body cavities, endothelium lining heart and blood vessels)

• Stratified squamous epithelium: Protection against attacks, keratin protein adds strength and water resistance

• Simple cuboidal epithelium: Secretion and absorption

• Stratified cuboidal epithelium: Sweat ducts and mammary ducts

• Transitional epithelium: Tolerates repeated cycles of stretching and recoiling (e.g., urinary bladder)

• Simple columnar epithelium: Absorption and secretion

• Pseudostratified columnar epithelium: Cilia movement

• Stratified columnar epithelium: Protection

Glandular Epithelium

• Endocrine glands: Release hormones into interstitial fluid, no ducts

• Exocrine glands: Produce secretions onto epithelial surfaces through ducts

Modes of Secretion

• Merocrine secretion: Produced in Golgi apparatus, released by vesicles (exocytosis), e.g., sweat glands

• Apocrine secretion: Produced in Golgi apparatus, released by shedding cytoplasm, e.g., mammary glands

• Holocrine secretion: Released by cells bursting, e.g., sebaceous glands

Types of Exocrine Secretions

• Serous glands: Watery secretions

• Mucous glands: Secrete mucins

• Mixed exocrine glands: Both serous and mucous

Gland Structure

• Unicellular glands: Mucous (goblet) cells scattered among epithelia

• Multicellular glands:

  • Simple (undivided) or compound (divided)

  • Shape: Tubular, alveolar, or branched

Connective Tissue

Characteristics

• Specialized cells

• Solid extracellular protein fibers

• Fluid extracellular ground substance

• Matrix = fibers + ground substance

Functions of Connective Tissue

• Establishing a structural framework for the body

• Transporting fluids and dissolved materials

• Protecting delicate organs

• Supporting, surrounding, and interconnecting other types of tissue

• Storing energy reserves (triglycerides)

• Defending the body from invading microorganisms

Classification of Connective Tissues

• Connective tissue proper: Connect and protect

• Fluid connective tissues: Transport

• Supporting connective tissues: Structural strength

Categories of Connective Tissue Proper

• Loose connective tissue: More ground substance, fewer fibers (e.g., adipose tissue)

• Dense connective tissue: More fibers, less ground substance (e.g., tendons)

Connective Tissue Proper Cell Populations

• Fibroblasts: Most abundant, secrete proteins and hyaluronan

• Fibrocytes: Maintain fibers

• Adipocytes: Fat cells, store a single, large fat droplet

• Mesenchymal cells: Stem cells, differentiate into other cell types

• Macrophages: Large, amoeba-like, eat pathogens and damaged cells

• Mast cells: Stimulate inflammation, release histamine and heparin

• Lymphocytes: Specialized immune cells, may develop into plasma cells

• Microphages: Phagocytic blood cells (e.g., neutrophils, eosinophils)

• Melanocytes: Synthesize and store melanin

Connective Tissue Fibers

• Collagen fibers: Most common, long, straight, unbranched, strong and flexible, resist force in one direction (e.g., tendons, ligaments)

• Reticular fibers: Network of interwoven fibers (stroma), strong and flexible, resist force in many directions, stabilize functional cells and structures (e.g., sheaths around organs)

• Elastic fibers: Contain elastin, branched and wavy, return to original length after stretching (e.g., elastic ligaments of vertebrae)

Ground Substance

• Clear, colorless, viscous

• Fills spaces between cells and slows pathogen movement

Embryonic Connective Tissues

• Not found in adults

• Mesenchyme: Embryonic stem cells

• Mucoid connective tissue: Loose embryonic connective tissue

Loose Connective Tissues

• "Packing materials" of the body

• Three types in adults:

  • Areolar: Least specialized, open framework, holds blood vessels and capillary beds (e.g., under skin)

  • Adipose: Contains many adipocytes, two types: white fat (most common, stores fat, absorbs shocks, slows heat loss) and brown fat (more vascularized, breaks down fat, releases energy)

  • Reticular: Provides support, complex network, supports functional cells (parenchyma), found in spleen, liver, lymph nodes, bone marrow

Dense Connective Tissues

• Connective tissues proper, tightly packed with high numbers of collagen or elastic fibers

• Dense regular connective tissue: Tightly packed, parallel collagen fibers, tendons attach muscles to bones, ligaments connect bone to bone, aponeuroses attach in sheets to large, flat muscles

• Dense irregular connective tissue: Interwoven networks of collagen fibers, layered in skin, around cartilages (perichondrium), around bones (periosteum), form capsules around some organs (e.g., liver, kidneys)

Type of Tissue	Main Function	Example
Epithelial	Protection, secretion, absorption	Skin, lining of GI tract
Connective	Support, transport, storage	Tendons, adipose tissue
Muscle	Contraction, movement	Skeletal muscle, heart
Neural	Electrical signaling	Brain, nerves

Key Terms and Concepts

• Matrix: Combination of fibers and ground substance in connective tissue

• Basement membrane: Layer that anchors epithelial tissue to underlying connective tissue

• Apical surface: Exposed surface of epithelial cell

• Basolateral surface: Attached surface of epithelial cell

• Germative cells: Stem cells responsible for regeneration of epithelium

Formulas and Equations

• Matrix composition: $\text{Matrix} = \text{Fibers} + \text{Ground Substance}$

Summary Table: Modes of Glandular Secretion

Mode	Description	Example
Merocrine	Exocytosis, no cell damage	Sweat glands
Apocrine	Loss of cytoplasm with secretion	Mammary glands
Holocrine	Cell ruptures to release product	Sebaceous glands

Additional info:

• These notes cover the foundational concepts of tissue organization, including the structure, function, and classification of epithelial and connective tissues, as well as key cell types and extracellular components.