Tissues, Intercellular Junctions, and Integumentary System – Comprehensive Exam Prep Notes: Lecture 5
Quick recap: transport mechanisms overview
Filtration
A form of passive transport that does not require ATP.
Driven by a physical force (e.g., gravity in lab settings) rather than a chemical concentration gradient.
Example from lab: large starch fragments are trapped by the filter, while smaller sugar molecules pass through.
Key characteristic: rate is determined by size (molecule size) rather than concentration gradient.
Summary: filtration is about molecular size; no gradient is involved.
Passive transport vs. active transport
Passive transport relies on gradients or other forces without using ATP.
Active transport requires ATP to move substances against their gradient.
Distinguishing passive mechanisms by gradient and protein involvement
If passive and does not require a concentration gradient: filtration.
If passive and requires a gradient but not a protein: simple diffusion.
If passive and requires a gradient and a protein: facilitated diffusion (protein helps transport).
Takeaway: organize transport mechanisms by gradient dependence and protein requirement to categorize the processes.
Osmosis and examples to build intuition
Freshwater fish example (hypotonic to the fish)
Freshwater is hypotonic relative to fish cells (low salt concentration).
Osmotic pressure in freshwater is relatively low; water tends to move into fish cells.
Fish handle excess water by expelling it to avoid swelling and bursting.
Concept: hypotonic environment → water moves into cells; osmotic pressure is related to solute concentration.
Ocean water example (hypertonic to the fish)
Ocean water is hypertonic relative to fish tissues (high solute concentration).
Higher osmotic pressure in seawater pulls water out of fish tissues.
Water movement follows solute gradients across membranes (osmosis).
Osmosis basics
Osmosis is the movement of water across a membrane, often with no need for membrane proteins.
Water moves toward higher solute concentration (higher osmotic pressure).
In the lab, we used gravity as the driving force but in biology, solute imbalances drive water movement.
Practical analogy: the freshwater puppy and the “wrinkle puppy” story
Freshwater environment can lead to water influx; saltwater environment can lead to water efflux from cells.
Visual metaphor: being in a salty environment causes cells to shrink (water leaves), analogous to folding and wrinkling when cells shrink; freshwater allows swelling.
Purpose: reinforce how osmosis affects cell volume and how cells respond to osmotic stress.
Summary note on osmotic pressure and movement
Osmotic pressure is the tendency of water to move toward a higher solute concentration.
Hypertonic environments draw water out of cells; hypotonic environments draw water into cells.
Osmosis is primarily about water, not solute transport through protein channels (though aquaporins can facilitate water movement).
Practical recap: use freshwater vs. saltwater fish and skin/bacteria context to reason about osmotic gradients.
Introduction to tissues and the skin
What is a tissue?
A tissue is a group of cells organized together, sometimes tightly or in a layer, to perform a common function.
Connects to the broader levels of organization (atoms, molecules, macromolecules, cells, tissues).
Four general tissue categories (nervous tissue discussed later):
Epithelial tissue
Connective tissue
Muscle tissue
Nervous tissue (to be covered later)
Key theme: intercellular connections help hold cells together and coordinate function.
Intercellular connections (junctions) and basement membrane
Tight junctions
Hold adjacent epithelial cells tightly so that substances cannot pass between cells.
Example: sweat duct lining should prevent leakage of sweat between duct cells.
Desmosomes (spot welds)
Provide strong adhesion at specific points between neighboring cells.
Allow cells to stay connected but permit some movement around the junction.
Analogy: a few Velcro-like dots (not a continuous seal) that keep cells connected at points.
Gap junctions
Form channels that connect adjacent cells, allowing cytoplasm (and small molecules) to pass directly between cells.
Not common in skin epidermis; will be discussed later in contexts like smooth muscle and bone.
Hemidesmosomes
Half-desmosome-like connections that attach a cell to the basement membrane (noncellular extracellular matrix).
Important for anchoring epithelial tissues to the underlying basement membrane.
Basal/ basement membrane
The noncellular layer to which epithelial cells attach via hemidesmosomes.
Osmotic/functional takeaways
These junctions allow tissues to maintain barriers, transmit signals, and maintain structural integrity while enabling controlled transport.
Four main tissue categories: key features and examples
Epithelial tissues
Characteristics: tightly packed cells forming layers; line cavities or cover organs; protective barrier; lack blood vessels; rapid turnover (example: stomach lining turns over every 3–4 days).
Naming: based on the number of layers and the shape of the cells.
Simple = 1 layer; Stratified = 2 or more layers.
Shapes: squamous (flat), cuboidal (cube-like), columnar (tall).
Functions: covering/lining, protection, secretion, absorption.
Attachments: always attached to a basement membrane via hemidesmosomes.
Epidermis (skin) is composed of epithelial tissue.
Example tissue types in skin context:
Simple cuboidal epithelium (duct lining of sweat glands)
Stratified squamous epithelium (epidermis)
Glandular epithelium (glands; specialized secretory epithelia)
Connective tissues
Characteristics: cells are spread out within an extracellular matrix (ECM or intercellular matrix); ECM includes ground substance and protein fibers; can be fluid (e.g., blood) to solid (e.g., bone).
Common cells: fibroblasts (build fibers), macrophages (engulf debris/foreign material).
ECM components:
Ground substance: gel-like or solid depending on tissue (e.g., fluid in blood; mineralized in bone).
Protein fibers: collagen (collagenous), elastin (elastic), reticular fibers (thin, supportive nets).
Examples:
Areolar connective tissue (in dermis)
Adipose tissue (fat storage; adipocytes with large lipid droplets; nucleus pushed to the side; stores energy, insulates, cushions)
Irregular dense fibrous connective tissue (collagen fibers randomly arranged; dermis; strength in multiple directions)
Dense regular connective tissue (tends/ligaments; fibers aligned in one direction)
Blood as connective tissue (ground substance is plasma; fluid matrix)
Muscle tissue
Characteristics: cells specialized for contraction via protein filaments (inside cells).
Distinctions:
Muscle proteins are inside the cells and organized for contraction.
Connective tissue proteins (collagen, elastin) are outside the cells in the ECM.
Types and notes:
Skeletal muscle: voluntary, typically requires neural input for contraction.
Heart muscle: some contractions can occur without direct neural input; reason transplants require careful consideration of nerves.
Smooth muscle: involuntary; found in certain contexts like arrector pili; spindle-shaped cells; contains gap junctions; often arranged in bundles.
Nervous tissue
Mentioned as a topic to be discussed later; will cover neurons and supporting cells and their signaling functions.
Integumentary system (skin) overview
Major roles of the skin
Protection: barrier against external environment and pathogens; maintains water balance.
Exteroception: sensory receptors for touch, temperature, and pain.
Thermoregulation: sweat glands help regulate body temperature.
Vitamin D production: important for calcium absorption; linked to bone health.
Excretion: minor waste removal through sweat.
Sweat and antimicrobial defense
Sweat is salty (contains salt; influenced by dermal glands).
Sweat creates a hypertonic surface relative to bacteria; water moves out of bacteria toward the sweat (osmotic effect), contributing to antimicrobial defense.
Skin layers and tissue composition
Epidermis: primarily stratified squamous epithelium; avascular (lacks blood vessels); rapid turnover (e.g., stomach lining turnover cited as an example of rapid mitosis in epithelia).
Dermis: contains connective tissue (areolar and dense irregular), blood vessels, nerves, and appendages like hair follicles and sweat glands.
Hypodermis (subcutaneous layer): rich in adipose tissue; main site of fat storage that cushions and insulates; fat cells can shrink but number remains constant during weight loss/gain.
Integumentary system components related to transport and barriers
Sweat glands produce sweat that travels through ducts to the skin surface; ducts are lined by appropriate epithelial tissue to prevent leakage.
Basement membrane and epithelial attachments (hemidesmosomes) anchor epidermal cells to the basement membrane.
Desmosomes in the epidermis allow cells to stay linked while permitting movement at intercellular junctions; tight junctions help maintain barrier integrity in ducts.
Functional consequences and practical implications
The epidermis lacks blood vessels, explaining limited bleeding from superficial cuts.
The epidermis renews rapidly to maintain barrier function in a harsh external environment.
Vitamin D synthesis occurs in the skin and supports calcium absorption elsewhere in the body; this links skin health to bone health.
Salty sweat and the osmotic environment contribute to antimicrobial defenses on the skin surface.
Specific tissue types in the skin context
Simple cuboidal epithelium (illustrative duct model)
Structure: single layer of cube-shaped cells; lines ducts (e.g., sweat glands).
Basement membrane: dark pink layer; cells sit on this surface.
Intercellular connections:
Between adjacent duct cells: tight junctions to prevent leakage between cells.
Between cell and basement membrane: hemidesmosomes attach to the basement membrane.
When rolled into a duct-like tube and sectioned longitudinally, cross-section views illustrate the continuity of the duct through stacked cuboidal cells.
Stratified squamous epithelium (epidermis-focused)
Structure: multiple cell layers; cells flatten toward the surface.
Basal layer attaches to the basement membrane via hemidesmosomes.
Surface layers held together by desmosomes; desmosomes allow the duct-like path to reach surface while maintaining integrity of the epidermis.
Glandular epithelium
Specialized epithelial cells that form glands and secrete substances.
Often discussed as a subset of glandular tissue in contexts beyond simple layered naming.
Connective tissue examples in the skin context
Areolar connective tissue
Found in the dermis; contains fibroblasts; extracellular matrix with loose arrangement of fibers and ground substance.
Adipose tissue (hypodermis)
Adipocytes store fat in droplets; nucleus pushed to the side; tissue provides insulation and cushioning; fat cell number remains constant while cell size changes with fat storage or loss.
Irregular dense fibrous connective tissue
Dense network of collagen fibers arranged irregularly; provides strength in multiple directions; found in the dermis.
Dense regular connective tissue (mentioned for contrast)
Fibers aligned in one direction (tendons/ligaments); high tensile strength in a single direction; more prone to tears if stressed in non-preferred directions.
Muscle context relevant to skin
Erector pili muscles
Smooth muscle associated with each hair follicle; involuntary control; causes hair to stand on end when stimulated (e.g., goosebumps).
Smooth muscle cells are spindle-shaped; arranged in bundles; contain gap junctions that facilitate impulse spread.
Difference in muscle protein location
In connective tissue, proteins (collagen/elastin) are outside cells in the ECM.
In muscle tissue, contractile proteins (actin/myosin) are inside the cells.
Quick glossary and key takeaways for exam prep
Simple vs. stratified: one layer vs two or more layers of epithelial cells.
Epithelial cell shapes: squamous (flat), cuboidal (cube-like), columnar (tall).
Hemidesmosomes: anchor epithelial cells to the basement membrane.
Tight junctions: seal between epithelial cells to prevent paracellular leakage.
Desmosomes: spot-like junctions for mechanical stability between cells.
Gap junctions: cytoplasmic channels linking adjacent cells (present in smooth muscle contexts).
Ground substance: non-fiber component of the ECM that fills space between cells and fibers; can be fluid (e.g., blood plasma) or solid (mineralized in bone).
Collagen fibers: provide tensile strength; thick, strong fibers.
Elastic fibers: provide elasticity and recoil after stretch.
Areolar connective tissue: loose connective tissue in the dermis; flexible framework.
Adipose tissue: energy storage, insulation, cushioning; adipocytes store fat as droplets; cell number remains constant during weight changes.
Dense irregular connective tissue: collagen fibers arranged irregularly for multi-directional strength (dermis).
Dense regular connective tissue: collagen fibers aligned in one direction (tendons/ligaments).
Epidermis: outer skin layer composed of stratified squamous epithelium; avascular; rapid turnover.
Dermis: connective tissue layer containing fibers, glands, hair follicles, and nerves.
Hypodermis: subcutaneous layer rich in adipose tissue; insulation and cushioning.
Involvement of the integument in health: Vitamin D production, calcium absorption, antimicrobial defense through sweat.
Key example links to prior lectures: levels of organization (cells -> tissues), transport mechanisms (filtration, diffusion, osmosis), and intercellular junctions as structural-functional motifs in epithelia and connective tissues.
ext{Osmotic pressure (conceptual)}: \
\Pi = iMRT
i: van't Hoff factor (number of particles into which a solute dissociates)
M: molar concentration
R: gas constant
T: absolute temperature
Note: This is a standard formula often used to quantify osmotic pressure in solutions; the lecture emphasizes the qualitative idea that osmotic pressure drives water movement toward higher solute concentration.
Closing reminder
Focus on understanding how these tissue classes differ, how intercellular junctions function, and how skin structure supports its protective and physiological roles.