Cells and Human Body Tissues Study Guide

Cellular Structure and Anatomy

Foundational Biological Units:
- Cells: The fundamental structural and functional units of all living organisms; responsible for executing all chemical and metabolic activities necessary to sustain life.
- Tissues: Collections of specialized cells that share similar structural characteristics and perform coordinated functions.
Anatomical Regions of the Human Cell:
- 1. Nucleus: The command center of the cell that stores genetic information in the form of DNA.
  - Nuclear Membrane (Envelope): A double phospholipid membrane barrier that separates the nuclear material from the cytoplasm; perforated by nuclear pores that facilitate chemical exchange between the nucleus and cytosol.
  - Nucleolus: One or more dense spherical bodies located inside the nucleus; functions as the specific site of ribosome assembly and production.
  - Chromatin: A loose network of DNA bound to protein dispersed throughout the nuclear matrix; condenses into distinct rod-like chromosomes prior to cell division (mitosis).
- 2. Cytoplasm: The region situated between the nuclear membrane and the plasma membrane.
  - Cytosol: The semitransparent fluid environment suspending intracellular components.
  - Organelles: Specialized sub-cellular structures that carry out specific metabolic tasks.
- 3. Plasma Membrane: The outer boundary enclosing the cell.

Plasma Membrane Structure and Specializations

Structural Composition:
- Consists of a double phospholipid layer (lipid bilayer) composed of polar, hydrophilic ("water-loving") heads oriented toward the fluid environment and nonpolar, hydrophobic ("water-fearing") tails pointing inward.
- Contains embedded proteins, cholesterol molecules (which maintain membrane stability), and carbohydrate chains forming glycoproteins and glycolipids.

Plasma Membrane Specializations:
- Microvilli: Tiny, finger-like projections of the apical plasma membrane that increase surface area for cellular absorption.
- Membrane Junctions:
  - Tight Junctions: Impermeable junctions that form leakproof seals between adjacent cells, preventing substances from passing through extracellular space.
  - Desmosomes: Anchoring junctions composed of internal protein filaments that mechanically link neighboring cells, preventing cellular separation under mechanical stress.
  - Gap Junctions: Communicating junctions containing hollow protein cylinders called connexons that allow direct transport of chemical molecules and ions between adjacent cell cytoplasms.
  - Cell-Matrix Junctions: Structural attachments such as hemidesmosomes and actin-linked junctions that bind cell bases to the underlying extracellular matrix.

Cytoplasmic Organelles and Cellular Dynamics

Diagram of generalized human cell organelles

Organelle Profiles:
- Ribosomes:
  - Composed of ribosomal RNA (rRNA) and proteins.
  - Function as the primary sites of protein synthesis within the cell.
  - Found floating freely in the cytoplasm or bound to the rough endoplasmic reticulum.
- Endoplasmic Reticulum (ER):
  - A network of fluid-filled tubules and sacs involved in intracellular material transport.
  - Rough ER: Studded with ribosomes; acts as the primary assembly site where building materials for cellular membranes and secreted proteins are manufactured.
  - Smooth ER: Lacks ribosomes; functions in cholesterol synthesis and breakdown, lipid/fat metabolism, and detoxification of drugs and poisons.
- Golgi Apparatus:
  - Consists of flattened membranous sacs (cisternae) that modify, package, and direct proteins produced by the rough ER.
  - Vesicle Secretion Pathways:
    - Pathway 1: Secretory vesicles containing modified proteins fuse with the plasma membrane to release products outside the cell via exocytosis.
    - Pathway 2: Golgi vesicles containing membrane components fuse directly with and replenish the plasma membrane.
    - Pathway 3: Vesicles packed with hydrolytic digestive enzymes remain inside the cell as functional lysosomes.
- Lysosomes:
  - Membranous sacs originating from the Golgi apparatus that contain potent hydrolytic enzymes.
  - Digest worn-out or nonusable cellular materials and foreign matter ingested by the cell.
- Peroxisomes:
  - Membranous sacs containing specialized oxidase and catalase enzymes.
  - Detoxify harmful chemical substances (including alcohol and formaldehyde) and neutralize free radicals (highly reactive chemicals).
  - Replicate independently by pinching in half.
- Cytoskeleton:
  - An internal structural framework composed of a network of protein structures that provides shape, mechanical support, and intracellular transport pathways.

    *   **Microfilaments**: Made of actin subunits; thin structures measuring approximately  $7\,\text{nm}$  in diameter; participate in cell mobility and cell-surface changes.
    *   **Intermediate Filaments**: Composed of fibrous subunits; tough, rope-like structures measuring approximately  $10\,\text{nm}$  in diameter; resist pulling forces on the cell.
    *   **Microtubules**: Composed of tubulin protein subunits; hollow cylindrical tubes measuring approximately  $25\,\text{nm}$  in diameter; determine overall cell shape and guide organelle movement.
    *   **Centrioles**: Specialized rod-shaped microtubule complexes that generate the mitotic spindle during cell division (mitosis).
*   **Mitochondria**:
    *   The "powerhouses" of the cell; double-membraned, highly adaptable organelles that continuously alter their shape.
    *   Carry out aerobic cellular respiration reactions utilizing oxygen to break down nutrients and synthesize adenosine triphosphate ( $\text{ATP}$ ) for energy.
*   **Cellular Projections**:
    *   Specialized surface structures used for movement (absent in certain cell types).
    *   **Cilia**: Multiple hair-like projections that sweep materials across the external cell surface.
    *   **Flagellum**: A single, elongated tail-like projection that propels the entire cell through fluid (e.g., sperm).

Specialized Cell Types and Structural Diversity

Cell diversity examples connecting, covering, moving, and storing

Cell diversity examples gathering information and reproduction

Functional Categories of Specialized Human Cells:
- 1. Cells that Connect Body Parts or Connect Cells:
  - Fibroblast: Elongated cell containing abundant rough ER and a prominent Golgi apparatus; secretes protein fiber building blocks into the extracellular matrix.
  - Erythrocyte (Red Blood Cell): Biconcave disc devoid of organelles and a nucleus at maturity; packed with hemoglobin to transport oxygen through the blood.
- 2. Cells that Cover and Line Body Organs:
  - Epithelial Cell: Hexagonal-shaped cell tightly packed in sheets; rich in internal intermediate filaments to resist mechanical tearing.
- 3. Cells that Move Organs and Body Parts:
  - Skeletal Muscle Cell and Smooth Muscle Cell: Elongated cells packed with contractile filaments (actin and myosin) that shorten forcefully to produce movement.
- 4. Cells that Store Nutrients:
  - Fat Cell (Adipocyte): Spherical cell containing a massive central lipid droplet housed in a vacuole, which displaces the cytoplasm and nucleus to the cell perimeter.
- 5. Cells that Fight Disease:
  - Macrophage Cell: Phagocytic cell that extends long pseudopods to crawl through tissue spaces; rich in lysosomes to ingest and destroy pathogens.
- 6. Cells that Gather Information and Control Body Functions:
  - Nerve Cell (Neuron): Cell featuring long extensions (dendrites and axons) specialized for receiving stimuli and conducting high-speed electrical signals across the body.
- 7. Cells of Reproduction:
  - Ovum (Egg Cell): Unusually large female gamete containing extensive cytoplasmic reserves, organelles, and DNA to nourish an early embryo.
  - Sperm: Motile male gamete equipped with a head carrying genetic material and a long flagellum for swimming toward the egg.

Overview and Classification of Body Tissues

Primary Tissue Categories:
- Epithelial Tissue: Covering, lining, and glandular tissue.
- Connective Tissue: Supporting, binding, and protective tissue.
- Muscle Tissue: Movement-producing contractile tissue.
- Nervous Tissue: Control and communication tissue.

Epithelial Tissue Characteristics and Types

General Characteristics:
- Cells are bound closely together in tight sheets with very little extracellular material.
- Always possesses one unattached "free edge" (apical surface).
- The lower surface rests upon a structural basement membrane.
- Avascular: Contains no direct blood supply; relies entirely on diffusion from underlying connective tissue capillaries.
- Regenerates easily if properly nourished.
Functional Roles: Protection, Absorption, Filtration, Secretion.

Epithelial cell shapes and layer arrangements

Classification System:
- By Cell Shape:
  - Squamous: Flattened, scale-like cells.
  - Cuboidal: Cube-shaped cells.
  - Columnar: Tall, column-like cells.
- By Number of Layers:
  - Simple: Consists of a single layer of cells.
  - Stratified: Consists of multiple stacked layers of cells.
Specific Epithelial Types:
- 1. Simple Squamous Epithelium:
  - Description: Single layer of flat, thin cells resting on a basement membrane.
  - Functions: Rapid diffusion, filtration, and lining membrane formation.
  - Locations: Air sacs (alveoli) of lungs, capillary walls, and ventral body cavity linings.

*   2. **Simple Cuboidal Epithelium**:
    *   Description: Single layer of cube-shaped cells.
    *   Locations: Glands and their excretory ducts, kidney tubule walls, and ovary surface.
*   3. **Simple Columnar Epithelium**:
    *   Description: Single layer of tall, column-like cells; commonly interspersed with mucus-secreting **goblet cells**.
    *   Locations: Lines the gastrointestinal tract from the stomach to the anus.

*   4. **Stratified Squamous Epithelium**:
    *   Description: Multiple layers of cells; cells at the apical free edge are flattened, while basal cells are cuboidal or columnar.
    *   Functions: Provides a protective barrier against friction and mechanical wear.
    *   Locations: Skin (epidermis), esophagus, mouth, and lungs.
*   5. **Pseudostratified Columnar Epithelium**:
    *   Description: Single layer of cells resting on a basement membrane, but varying cell heights and nucleus levels give a double-layered appearance.
    *   Functions: Absorption and secretion; frequently ciliated in the respiratory tract to propel mucus away from lungs.

Pseudostratified ciliated columnar epithelium

*   6. **Transitional Epithelium**:
    *   Description: Specialized stratified epithelium capable of stretching; cell appearance alters depending on the degree of tissue distension.
    *   Locations: Lines organs of the urinary system (urinary bladder, ureters, urethra).
*   7. **Stratified Cuboidal and Stratified Columnar Epithelium**:
    *   Description: Rare tissue configurations consisting of two or more layers of cuboidal or columnar cells.
    *   Locations: Restricted primarily to the ducts of large glands.

Glandular Epithelium

Gland Definition: Consists of one or more specialized cells that synthesize and secrete chemical products (such as sweat or thyroid-releasing hormone [TRH]).

Major Glandular Classifications:
- 1. Endocrine Glands:
  - Ductless glands ("endo" = enter).
  - Secrete regulatory products (hormones) directly into the blood or interstitial fluid.
- 2. Exocrine Glands:
  - Duct-retaining glands ("exo" = exit).
  - Empty secretions through epithelial ducts onto internal or external body surfaces.
  - Examples include sweat glands and oil (sebaceous) glands.

Connective Tissues

General Characteristics:
- Found everywhere in the human body; represents the most abundant and widely distributed tissue type.
- Functions: Binds body tissues together, supports organ systems, provides structural protection.
- Vascularization: Ranges from rich blood supply (vascularized, like bone) to completely avascular (like cartilage).
Extracellular Matrix Structure:
- Non-living structural material surrounding connective tissue cells; allows tissue to withstand mechanical stress and weight.
- Ground Substance: Hydrated matrix composed mostly of water, cell adhesion proteins, and polysaccharides.
- Matrix Fibers: Produced by resident cells:
  - Collagen Fibers: Provide high tensile strength.
  - Elastic Fibers: Provide stretch and elastic recoil.
  - Reticular Fibers: Provide fine, internal structural networks.
Connective Tissue Types:
- 1. Bone Tissue (Osseous Connective Tissue):
  - Composition: Osteocytes situated inside matrix cavities called lacunae; embedded within a hard matrix of calcium salts and dense collagen fibers.
  - Functions: Protects vital internal organs and provides structural support to the body.
- 2. Cartilage Connective Tissue:
  - Hyaline Cartilage: Most common cartilage type; composed of abundant collagen fibers embedded in a rubbery matrix; makes up the entire fetal skeleton, larynx, and rib-sternum attachments.

    *   **Elastic Cartilage**: Rich in elastic fibers; provides flexibility and elasticity; located in the external ear.
    *   **Fibrocartilage**: Highly compressible tissue with dense fibers; forms cushion-like intervertebral discs between spinal vertebrae.
*   3. **Dense Connective Tissue (Dense Fibrous)**:
    *   Composition: Matrix packed with parallel collagen fibers and active cells called **fibroblasts**.

    *   *Tendons*: Connect skeletal muscle to bone.
    *   *Ligaments*: Connect bone to bone at joints.
*   4. **Areolar Connective Tissue**:
    *   Description: The most widely distributed connective tissue; soft, pliable, gel-like tissue containing all fiber types.
    *   Functions: Cushions and protects body organs; acts as a fluid reservoir soaking up excess fluid during inflammation (edema).
*   5. **Adipose Connective Tissue**:
    *   Description: Modified areolar tissue dominated by fat globules; contains massive lipid deposits.

    *   Functions: Insulates the body, cushions internal organs, and stores long-term energy fuel.
*   6. **Reticular Connective Tissue**:
    *   Description: Delicate network of interwoven reticular fibers.
    *   Functions: Forms the internal supporting framework (**stroma**) of lymphoid organs.
    *   Locations: Lymph nodes, spleen, red bone marrow.
*   7. **Blood (Vascular Connective Tissue)**:
    *   Description: Fluid connective tissue consisting of blood cells surrounded by a non-living liquid matrix called plasma.
    *   Functions: Transports nutrients, gases, respiratory products, and wastes throughout the cardiovascular system.

Muscle Tissues

Primary Function: Contract and shorten to produce physical movement.
Muscle Tissue Types:
- 1. Skeletal Muscle Tissue:
  - Control: Voluntary control.
  - Morphology: Cylindrical cells attached to bones or connective tissue; distinct striations; multinucleated (contains more than one nucleus per cell).

*   2. **Cardiac Muscle Tissue**:
    *   Control: Involuntary control.
    *   Location: Found exclusively in the heart.
    *   Function: Pumps blood through heart chambers and blood vessels.
    *   Morphology: Striated cells containing a single nucleus; attached end-to-end at specialized membrane junctions called **intercalated disks** (unique structural features of cardiac muscle).

Cardiac muscle tissue with intercalated disks

*   3. **Smooth Muscle Tissue**:
    *   Control: Involuntary control.
    *   Location: Found in walls of hollow organs (stomach, intestines, bladder, blood vessels).
    *   Morphology: Spindle-shaped cells attached to neighboring smooth muscle cells; non-striated (no visible striations); single nucleus per cell.

Nervous Tissue

Composition: Composed of functional nerve cells (neurons) and specialized supporting cells (neuroglia).
Function: Transmits high-speed electrical impulses throughout the central and peripheral nervous systems.

Core Functional Properties:
- Irritability: Ability to perceive and respond to physical or chemical stimuli.
- Conductivity: Ability to transmit electrical impulses across distances to other cells.
Anatomical Distribution: Brain, spinal cord, and peripheral nerves.

Developmental Aspects and Aging of Tissues

Developmental germ layers and tissue origins

Primary Embryonic Germ Layer Lineages:
- Ectoderm (External Layer): Gives rise to nervous tissue, epidermis of the skin, pigment cells, and germ cells.
- Mesoderm (Middle Layer): Gives rise to all muscle tissue types (cardiac, skeletal, smooth), connective tissues (bone, blood, cartilage, adipose, dense/areolar), and kidney tubule cells.
- Endoderm (Internal Layer): Gives rise to mucosal linings of internal organs, lung alveolar cells, thyroid cells, and pancreatic cells.
- Epithelial Tissue Derivation: Arises from all three primary germ layers (ectoderm, mesoderm, endoderm).
Tissue Aging Effects: With advancing age, most body tissues undergo a gradual reduction in mass, functional efficiency, cellular repair rate, and viability.