Chapter 3 Notes: Compartmentation—Cells and Tissues

3.1 Functional Compartments of the Body

  • Three major cavities (anatomical compartments):
    • Cranial cavity
    • Thoracic cavity
    • Abdominopelvic cavity
  • Fluid-filled (functional) compartments that separate body fluids and organs:
    • Circulatory system
    • Eyes
    • Cerebrospinal fluid (CSF)
    • Pleural and pericardial sacs
  • The lumens of some organs lie outside the body (external environments through hollow organs):
    • Hollow organs include heart, lungs, blood vessels, and intestines
    • Lumen: interior of any hollow organ; may be wholly or partially filled with air or fluid
    • For some organs, the luminal space is an extension of the external environment
  • Functional body fluids (three compartments):
    • Extracellular fluid (ECF)
    • Plasma
    • Interstitial fluid
    • Intracellular fluid (ICF)
  • Size/scale illustration (from Fig. 3.1b):
    • Red blood cell: 7.5 ext{ μm} ext{ (labeled as }7.5 ext{ mm in the figure)}
    • White blood cell: 15 ext{ μm}
    • Smooth muscle cell: 15{-}200 ext{ μm}
    • Fat cell: 50{-}150 ext{ μm}
    • Ovum: 100 ext{ μm}
    • Note: the units in the figure appear inconsistent; typical cell sizes are in micrometers
  • Blood plasma is the extracellular fluid inside blood vessels; cells subdivide into intracellular compartments; interstitial fluid surrounds most cells
  • Concept: Compartments can be separated by membranes and organized into a hierarchy of functional spaces

3.2 Biological Membranes

  • The cell membrane separates the cell from its environment
    • Physical isolation
    • Regulation of exchange with the environment
    • Communication between the cell and its environment
    • Structural support and secretion (cell releases substances into extracellular space)
  • Membranes are primarily composed of lipids and proteins; described by the Fluid Mosaic Model
  • Table 3.1: Composition of Selected Membranes
    • Red blood cell membrane: ext{Protein}=49 ext{%}, ext{Lipid}=43 ext{%, Carbohydrate}=8 ext{%}
    • Myelin membrane around nerve cells: ext{Protein}=18 ext{%, Lipid}=79 ext{%}, ext{Carbohydrate}=3 ext{%}
    • Inner mitochondrial membrane: ext{Protein}=76 ext{%, Lipid}=24 ext{%}, ext{Carbohydrate}=0 ext{%}
  • The Fluid Mosaic Model components (Fig. 3.2b):
    • Phospholipid bilayer with hydrophilic heads facing the aqueous sides and hydrophobic tails forming the interior
    • Cholesterol interspersed within the lipid bilayer
    • Membrane proteins: integral (transmembrane) and peripheral proteins; lipid-anchored proteins (e.g., GPI anchors)
    • Glycoproteins and glycolipids contribute to the glycocalyx on the extracellular surface
    • Cytoskeleton proteins attach to the membrane, providing structural support
  • Membrane organization details:
    • Integral vs peripheral proteins; transmembrane proteins cross the lipid bilayer (e.g., the membrane-spanning protein shown crossing seven times)
    • Peripheral proteins can be removed without disrupting membrane integrity
    • Lipid rafts: microdomains rich in sphingolipids and lipid-anchored proteins; involved in signaling and trafficking
  • Lipids create a hydrophobic barrier: major lipid classes include
    • Phospholipids
    • Sphingolipids
    • Cholesterol
  • Phospholipid organization in aqueous solutions: bilayer, micelle, liposome
    • Bilayer forms a sheet with nonpolar tails inward and polar heads outward
    • Micelles are droplets with hydrophobic cores; liposomes have an aqueous center
  • Membrane proteins:
    • Categorized as Integral vs Peripheral; Transmembrane; Lipid-anchored (e.g., GPI anchors)
    • Lipid rafts concept and functional implications
  • Membrane carbohydrates and glycocalyx:
    • Glycocalyx is on the external surface and serves protective functions
    • Composed of glycolipids and glycoproteins
  • Functional map (Fig. 3.2c) emphasizes:
    • Structural stability
    • Cell recognition
    • Immune response
  • Lipid rafts (Fig. 3.3): microdomains enriched in sphingolipids

3.3 Intracellular Compartments

  • Cellular differentiation depends on selective gene expression; cells are organized into compartments
  • Core compartments of the cell:
    • Cell membrane
    • Cytoplasm (cytosol, inclusions, cytoskeleton, organelles)
    • Nucleus
  • Inclusions (non-membrane-bound structures) are in direct contact with the cytosol and serve storage/other roles:
    • Nutrient storage: glycogen granules, lipid droplets
    • Non-nutrient storage/processing: ribosomes (made of protein and RNA); ribosomes synthesize proteins
    • Ribosomes exist as fixed (attached to rough ER) and free ribosomes; polyribosomes can synthesize multiple copies
  • Cytoplasmic protein fibers form the cytoskeleton and come in three sizes:
    • Microfilaments (Actin): d ext{(microfilaments)} = 7 ext{ nm}
    • Intermediate filaments (Keratin, neurofilaments): d = 10 ext{ nm}
    • Microtubules (Tubulin): d = 25 ext{ nm}
    • These fibers function with motor proteins to provide structural support and enable movement
  • Cytoplasm and organelles (Fig. 3.4):
    • Inclusions include lipid droplets and glycogen granules
    • Cytoskeleton provides mechanical support and forms networks for organelle positioning and transport
  • Cytoskeleton and microvilli (Fig. 3.4b):
    • Microvilli increase cell surface area; supported by microfilaments (actin)
  • Major organelles and their functions:
    • Mitochondria: double membrane; matrix inside; intermembrane space; cristae; site of most ATP production
    • Endoplasmic reticulum (ER): network of membrane tubules
    • Rough ER (RER): ribosome-studded; protein synthesis for secretion and organelle lumen
    • Smooth ER (SER): lipid/fatty acid synthesis; steroid synthesis; calcium storage in some cells
    • Golgi apparatus: sorts, modifies, and packages proteins into vesicles
    • Lysosomes: digestive enzymes for breakdown of bacteria and old organelles
    • Peroxisomes: breakdown of fatty acids and detoxification of toxic materials
  • Transport and protein processing pathway (protein synthesis focus; Fig. 3.4i sequence):
    • mRNA is transcribed from DNA in the nucleus and exits to cytosol
    • Cytosolic ribosomes translate cytosolic proteins
    • Some proteins are directed to organelles via Rough ER ribosomes; these proteins are processed in the ER lumen
    • Transport vesicles carry proteins from the ER to the Golgi apparatus
    • Golgi cisternae migrate toward the plasma membrane; vesicles bud off in retrograde or forward directions
    • Some vesicles become lysosomes or storage vesicles; others become secretory vesicles that release contents outside the cell

3.4 Tissues of the Body

  • Histology: study of tissue structure and function
  • Extracellular matrix (ECM) has multiple roles:
    • Synthesized and secreted by cells
    • Composition varies by tissue
    • Components include proteoglycans, glycoproteins (e.g., laminin, fibronectin), and insoluble protein fibers (collagen, fibronectin, laminin)
  • Cell junctions and CAMs (cell-adhesion molecules):
    • CAMs types include
    • Cadherins (cell–cell; calcium-dependent; adherens junctions, desmosomes)
    • Integrins (cell–matrix junctions; signaling roles)
    • Immunoglobulin superfamily CAMs (e.g., NCAMs)
    • Selectins (temporary cell–cell adhesions)
    • Gap junctions (communicating junctions) allow direct cytosol-to-cytosol communication
    • Tight junctions (occluding junctions) prevent paracellular movement
    • Anchoring junctions include adherens junctions, desmosomes (cell–cell); hemidesmosomes and focal adhesions (cell–matrix)
  • Table 3.3 Major CAMs: examples and roles
    • Cadherins: cell–cell junctions (adherens, desmosomes); calcium-dependent
    • Integrins: cell–matrix junctions; signaling roles
    • Immunoglobulin superfamily CAMs: NCAMs (nerve cell adhesion molecules)
    • Selectins: transient adhesions
  • Figure 3.8 outlines junction categories and components: gap (communicating), tight (occluding), adherens junctions, desmosomes, focal adhesions, hemidesmosomes
  • Four tissue types (Table 3.4): epithelial, connective, muscle, nerve
  • Epithelia: provide protection and regulate exchange
    • Structure: one or more layers of epithelial cells; basal lamina (basement membrane) separates epithelia from underlying tissue; tight vs. leaky epithelia
  • Epithelia: structural classification
    • Layers: simple (one layer) vs. stratified (multiple layers)
    • Shapes: squamous, cuboidal, columnar
  • Five functional categories of epithelia (Fig. 3.9b):
    • Exchange
    • Transporting
    • Ciliated
    • Protective
    • Secretory
  • Characteristics per category (summary from Fig. 3.9 and related text):
    • Exchange epithelium: very thin, flat; simple squamous; lines blood vessels and lungs; endothelium in heart and vessels
    • Transporting epithelium: cuboidal or columnar; apical and basolateral membranes; tight junctions; high mitochondria content; regulated transport
    • Ciliated epithelium: cilia for movement of fluids/particles
    • Protective epithelium: multiple layers; high turnover; protection against mechanical/chemical stress
    • Secretory epithelium: produce and secrete substances; goblet cells secrete mucus; exocrine glands secrete to external environment; endocrine glands secrete hormones into blood
  • Secretory epithelium (Fig. 3.10e) and goblet cells illustrate mucus production; TEM images show secretory pathways
  • Development of glands (Fig. 3.11):
    • Exocrine glands: epithelium infolds into connective tissue to form ducts; lumen forms; secretions released onto surface
    • Endocrine glands: lose connection to the surface; secretions released into bloodstream
  • Table 3.4 (characteristics of epithelial, connective, muscle, nerve):
    • Matrix: minimal (epithelial, muscle); extensive (connective); minimal (nerve)
    • Matrix type: basal lamina (epithelial); varied connective tissue matrix; external lamina for muscles; external lamina for nerves
    • Unique features: no direct blood supply (epithelial); cartilage lacks blood supply (connective); electrical signaling (nerve); electrical signaling and movement (muscle)
    • Surface features: microvilli and cilia (epithelial)
    • Locations and organization: coverage and lining roles; connective tissue supports and connects; muscle forms skeletal or hollow organ walls; nerve distributed throughout body, concentrated in brain/spinal cord

3.5 Tissue Remodeling

  • Cell death types:
    • Necrosis: death from injury
    • Apoptosis: programmed cell death (cell suicide)
  • Stem cells and mitosis:
    • Totipotent, pluripotent, multipotent
    • Plasticity (ability to diversify into multiple cell types depending on signals)

3.6 Organs

  • Organs are groups of tissues with related function
  • Example: Skin incorporates all four tissue types and has multiple functions
  • Skin structure overview (Fig. 3.15):
    • Layers of the skin:
    • Epidermis: multiple layers; provides protective barrier
    • Dermis: loose connective tissue; contains exocrine glands, blood vessels, muscles, nerve endings
    • Hypodermis (subcutaneous layer): adipose tissue for insulation
    • Hair follicles and arrector pili muscles; goose bumps when arrector pili contract
    • Blood vessels extend into the dermis
    • Sensory receptors monitor external conditions
    • Sweat glands secrete dilute salt fluid for cooling; sebaceous glands secrete lipid mixtures
    • Apocrine glands in specific areas secrete waxy or viscous secretions in response to fear or sexual excitement
  • Epidermis and its components (Fig. 3.15b):
    • Melanocytes produce melanin
    • Surface keratinocytes produce keratin fibers
    • Desmosomes anchor epithelial cells; basal lamina at the boundary with connective tissue
  • Connection between epidermis and dermis (Fig. 3.15c):
    • Hemidesmosomes tie epidermal cells to basal lamina
    • Basal lamina is an acellular layer separating epithelia from dermis

Summary and Key Takeaways

  • The body is organized into functional compartments (cavities and fluid compartments) separated by membranes and organized into tissues and organs
  • Biological membranes employ a fluid mosaic with lipids, cholesterol, and a diverse set of proteins that regulate transport, signaling, and cell identity
  • Intracellular compartments include cytosol, organelles, cytoskeleton, and non-membrane inclusions; the nucleus acts as control center for genetic information
  • The cytoskeleton provides structural support, enables movement, and organizes intracellular transport via microfilaments, intermediate filaments, and microtubules
  • Organelles (mitochondria, ER, Golgi, lysosomes, peroxisomes) perform specialized functions and coordinate protein synthesis, processing, and trafficking
  • Tissues are built from epithelial, connective, muscle, and nerve tissues; their ECM, cell–cell adhesion, and junctions regulate tissue integrity and function
  • Epithelia are classified by layers and shapes and are organized into five functional categories: exchange, transporting, ciliated, protective, and secretory; glands arise from epithelial tissue and may be exocrine or endocrine
  • Tissue remodeling involves controlled cell death (necrosis and apoptosis) and stem cell differentiation (totipotent, pluripotent, multipotent) with plasticity
  • Organs such as the skin illustrate multi-tissue integration and the complex architecture of barriers, glands, and sensory structures
  • Key numerical references:
    • Cytoskeletal fiber diameters: d{microfilaments}=7~ ext{nm},\, d{intermediate}=10~ ext{nm},\ d_{microtubules}=25~ ext{nm}
    • Table 3.1 membrane compositions: ext{RBC membrane: }(49\%, 43\%, 8\%)\;\text{(Protein, Lipid, Carbohydrate)}
    • Myelin membrane: (18\%, 79\%, 3\%)
    • Inner mitochondrial membrane: (76\%, 24\%, 0\%)
  • Conceptual links to real-world relevance:
    • Understanding compartments helps in diagnosing and treating fluid balance disorders, edema, and membrane transporter defects
    • Membrane composition and protein organization underlie many pharmacological targets (receptors, transporters, signaling pathways)
    • Tissue organization explains how organ systems function together (skin as a multi-tissue organ; epithelia in vessels, lungs, glands)