Lecture Outline: Histology
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- Cells
- Introduction to Cells
- Focus on human (animal) cells
- First unit contains introductory information, followed by organ system study
- Cell Types
- Eukaryotic cells: Have a true nucleus and other membrane-bounded organelles
- Humans are eukaryotes
- Prokaryotic cells: Do not have a true nucleus or other membrane-bounded organelles
- Only membrane is the plasma membrane
- Consist of a single compartment
- Organelles (Little Organs within a Cell)
- Nucleus
- Contains DNA
- Example of a doubly membrane-bounded organelle
- Double membrane called the nuclear envelope
- Nuclear envelope is perforated by nuclear pores for transport
- Outer membrane of nuclear envelope is continuous with endoplasmic reticulum
- Endoplasmic Reticulum (ER)
- Membrane is continuous with the nuclear envelope
- Two parts with different functions:
- Rough Endoplasmic Reticulum (Rough ER)
- Appearance: Studded with ribosomes, giving it a "rough" look
- Function: Modifies newly made proteins (e.g., sticking things on, cutting things off)
- Smooth Endoplasmic Reticulum (Smooth ER)
- Appearance: Lacks ribosomes, looks like tubes
- Functions: Produces lipids and is a storage place for substances like calcium ions (important for muscle function)
- Golgi Complex (Golgi Apparatus/Golgi)
- Structural similarity to rough ER but distinct function
- Function: Acts as a receiving and shipping center
- Receives proteins in vesicles (membrane-bound bags) from the rough ER
- Figures out destination of proteins (secretion, plasma membrane incorporation, or other organelles)
- Repackages proteins into new vesicles that bud off its membrane
- Mitochondria (Mitochondrion singular)
- Bean-shaped organelles
- Example of a multiply membrane-bounded organelle (has two membranes)
- Function: "Powerhouse of the cell" – where most ATP (energy storage from food) is made through cellular respiration
- Dismantles fuel molecules completely, producing carbon dioxide as a byproduct
- Lysosomes and Peroxisomes
- Lysosomes
- Meaning: "Splitting body" (lyse = split, some = body)
- Function: Site of digestion or breakdown of molecules using digestive enzymes
- Enzymes are contained to prevent damage to the rest of the cell
- Peroxisomes
- Function: Deal with harmful peroxides that naturally build up, converting them into safer substances
- Cytoskeleton
- Meaning: "Cell skeleton" (cyto = cell, skeleton = support structure)
- Function: Provides support and maintains order within the cell
- Made of three major kinds of fibrous proteins:
- Microtubules: Widest diameter
- Microfilaments: Narrowest diameter (thinnest)
- Intermediate filaments: Intermediate diameter
- Ribosomes
- Tiny organelles (brown dots in pictures)
- Not membrane-bounded organelles (present in prokaryotic cells as well)
- Function: Assemble proteins by stringing together amino acids in a specific order based on genetic code
- Cytosol
- The liquid part of the cytoplasm
- Cytoplasm is all the stuff inside the cell but outside of other organelles
- Site where much of the chemistry of life occurs
- Plasma Membrane
- Structure
- Main substructure: Two layers of phospholipids (phospholipid bilayer)
- Phospholipids have a polar head (likes water) and two non-polar tails (avoid water)
- Self-assemble in water to form the bilayer, with tails facing each other and heads facing outward
- Crucial for the formation of proto-cells
- Membrane-bound proteins: Embedded within the membrane, performing various jobs
- Allow substances that cannot directly pass through the phospholipid bilayer to enter or exit the cell
- Different from "membrane bounded" organelles, as they are part of the membrane itself
- Intercellular Junctions (Between Cells)
- Desmosomes
- Complexes of proteins connecting plasma membranes of adjacent cells
- Function: Act as "spot welds" to keep cells together and maintain tissue integrity
- Tight Junctions
- More complex protein complexes that go all the way around cells
- Function: Form blockades between cells, preventing particles from passing through the spaces between cells in a multi-cellular membrane (e.g., epithelium)
- Ensure substances must enter cells to pass through the tissue
- Gap Junctions
- Protein complexes forming little tunnels between cells
- Function: Allow for intercellular communication by permitting certain particles to pass directly between cells
- Important for synchronous actions (e.g., heart contraction through ion flow)
- Transport Across the Membrane
- Passive Processes (Energy already contained within the system; no additional energy needed)
- Diffusion
- The spreading out of particles from an area of higher concentration to lower concentration (down their gradient)
- Occurs due to random motion of particles
- Simple Diffusion
- Directly through the phospholipid bilayer
- Requires particles to be small enough and non-polar (or sufficiently non-polar)
- Osmosis
- A special case of diffusion where the diffusing particle is the solvent (water)
- Water moves through a membrane from where it's more watery to where it's less watery (down its water gradient)
- Examples with red blood cells and external solutions:
- Isotonic solution: Same wateriness as cell interior; water moves in and out at same rate, cell volume unchanged (normal red blood cell)
- Hypertonic solution: Higher solute concentration / less watery than cell; water leaves cell, causing crenation (shrinkage)
- Hypotonic solution: Lower solute concentration / more watery than cell; water enters cell, causing swelling or lysis (bursting)
- Facilitated Diffusion
- Still diffusion (passive, down gradient) but requires transport proteins embedded in the membrane
- For particles too big, polar, or charged to pass directly through the phospholipid bilayer
- Active Processes (Requires additional energy, often ATP, to force movement)
- Active Transport
- Moves substances against their concentration gradient (from lower to higher concentration)
- Performed by pumps (transport proteins that force movement)
- Creates and maintains gradients, stockpiling substances
- Example: Sodium-potassium exchange pump
- Pumps sodium ions out of the cell and potassium ions into the cell simultaneously
- Crucial for nervous and muscle cell function; accounts for a significant portion of body's energy budget
- Vesicular Transport (Transport using vesicles)
- Exocytosis (Exo = out, Cyto = cell)
- Movement of substances out of the cell via a vesicle
- Vesicle fuses with the plasma membrane, spilling its contents outside
- Often called secretion (e.g., how glands work)
- Endocytosis (Endo = in, Cyto = cell)
- Movement of substances into the cell via a vesicle
- Different kinds based on what is brought in:
- Pinocytosis: "Cell drinking," taking a sip of liquid from the exterior
- Phagocytosis: "Cell eating," bringing in a solid chunk from the exterior
- Receptor-mediated endocytosis: Cell forms a vesicle only when specific substances bind to cell-surface receptors
- DNA Functions
- Replication
- The exact copying of DNA
- Necessary for cell division (new cells come from pre-existing cells)
- Before a cell splits, it doubles its DNA to provide a full copy to each daughter cell
- Based on DNA's double-stranded nature and complementarity (A with T, C with G)
- Each strand serves as a template for building the complementary new strand
- The original cell does not survive after division; it splits into two new cells
- Cell division involves two components:
- Mitosis: Division of the nucleus
- Cytokinesis: Division of everything else in the cell
- Gene Expression
- Process by which information from a gene (instructions for a protein within DNA) is used to make a protein
- Consists of two main parts:
- Transcription
- Occurs in the nucleus (where DNA is located)
- DNA's code (gene sequence) is used to make an RNA molecule (a copy of the instructions)
- RNA carries the code out of the nucleus via nuclear pores
- Translation
- Occurs at a ribosome (in the cytoplasm)
- The ribosome reads the RNA code to assemble the specific protein (stringing amino acids together)
- Major Cell Functions/Types
- Cells that Connect Body Parts (e.g., fibroblasts, red blood cells – functionally connecting by transporting oxygen)
- Cells that Cover and Line Body Organs (e.g., epithelial tissues like epidermis for protection)
- Cells that Move Organs and Body Parts (e.g., muscle cells, which shorten or contract)
- Cells that Store Nutrients (e.g., adipocytes/fat cells, specialized for fat storage due to high energy density and for padding/insulation)
- Cells that Fight Disease (e.g., white blood cells/leukocytes, involved in immunity, produce antibodies, basis for vaccinations)
- Cells that Gather Information and Control Body Functions
- Nervous system (e.g., neurons: send electrochemical signals via axons and dendrites)
- Endocrine system (e.g., cells that release hormones – chemical signals into bloodstream)
- Cells of Reproduction (e.g., gametes: sperm (male) and egg (female); fusion forms a zygote, the beginning of a new life)
- Tissues
- Introduction to Tissues
- Histology: The study of tissues
- Tissues are collections of cells that come together to perform a specific job
- Humans have four major tissue types:
- Epithelial tissue
- Connective tissue
- Nervous tissue
- Muscle tissue
- Epithelial Tissue (Epithelia plural)
- Function: Line surfaces of organs or the body (exteriorly or interiorly)
- Characteristics:
- Polarity: One side is attached, the other is free
- Basal surface: Attached to an underlying basement membrane
- Apical surface: Free or unattached surface (e.g., surface of skin)
- Classification by Layers (Specifies how many layers of cells)
- Simple epithelium: One layer of cells thick
- Stratified epithelium: More than one layer of cells (layered)
- Pseudostratified epithelium: "Falsely stratified"; appears layered but is actually a simple epithelium where all cells touch the basement membrane (cells vary in height)
- Transitional epithelium: Special category found in organs like the urinary bladder; can change from multiple layers to fewer layers as it stretches, and cell shape changes (e.g., columnar to squamous)
- Classification by Cell Shape (Determined by the shape of cells in the apical layer)
- Squamous: Cells are much thinner/shorter than they are wide (scale-like, like floor tiles)
- Cuboidal: Cells are roughly as thick/tall as they are wide (cube-shaped)
- Columnar: Cells are thicker/taller than they are wide (column-shaped)
- Examples of Epithelia:
- Simple Squamous Epithelium
- Characteristics: Thinnest possible epithelium (one layer, squamous cells)
- Ideal for: Rapid diffusion over tiny distances
- Example: Lining of the interior of the lungs (for gas exchange)
- Simple Cuboidal Epithelium
- Characteristics: One layer of cuboidal cells
- Ideal for: Functions requiring internal machinery for secretion or absorption
- Example: Tubules in the kidneys
- Simple Columnar Epithelium
- Characteristics: One layer of columnar cells (thickest simple type)
- Ideal for: Functions requiring extensive internal machinery for absorption and transport proteins
- Example: Lining of the gut
- Pseudostratified Epithelium
- Characteristics: Appears stratified due to varied cell heights, but all cells contact basement membrane (simple)
- Stratified Squamous Epithelium
- Characteristics: Multiple layers, with apical cells being squamous
- Ideal for: Areas undergoing a lot of abrasion, providing protection by having many layers to lose
- Examples: Lining of the throat, epidermis of the skin, vagina
- Transitional Epithelium
- Example: Urinary bladder (stretches, layers slip past each other and cells change shape)
- Connective Tissue
- General Characteristics:
- Catch-all category for tissues not classified as epithelial, nervous, or muscle tissue
- Has two major components:
- Cellular component: Various types of cells specific to the tissue
- Extracellular matrix: Stuff outside the cells, varying greatly among connective tissues
- Examples/Types of Connective Tissue:
- Bone
- Cellular component: Osteocytes (bone cells) living in spaces called lacunae
- Extracellular matrix: Hard, mineralized material
- Structural unit: Osteon (concentric layers around a central canal containing blood vessels)
- Cartilage
- Tough but not as hard as bone
- Cellular component: Chondrocytes (cartilage cells) living in lacunae
- Types:
- Hyaline Cartilage: "Glassy" appearance, e.g., between sternum and ribs
- Fibrocartilage: Contains many protein fibers in extracellular matrix, making it strong
- Example: Intervertebral discs (pads between vertebrae of the spine), made of tough protein fibers like collagen, to hold vertebrae together
- Dense Fibrous Connective Tissue
- Characteristics: Extracellular matrix contains many protein fibers packed closely together ("dense")
- Example: Tendons
- Connect skeletal muscle to bone
- Fibers are arranged in parallel ("regular") because they are pulled in one direction
- Do not stretch; provide strong, direct pull
- Areolar Tissue (Loose Connective Tissue)
- Characteristics: Few fibers packed loosely, with much space between them
- Function: Allows for free movement, acts as "packing material"
- Example: In the hypodermis beneath the dermis of the skin, allowing skin to move freely
- Adipose Tissue
- Composed of adipocytes (fat cells), specialized to contain fat
- Function: Efficient energy storage (higher energy density than carbohydrates/proteins), padding delicate organs, thermal insulation
- Fat is dissolved during slide preparation, making cells appear empty
- Reticular Tissue
- Characteristics: Fibers arranged in a weblike network ("reticulum")
- Function: Acts as a filter
- Examples: In lymph nodes (filtering harmful substances) and the spleen (filtering out old, worn-out red blood cells)
- Red blood cells lose nucleus and mitochondria during development, limiting their lifespan and ability to repair; wear out squeezing through capillaries
- Blood
- A mobile tissue and a connective tissue
- Cellular component: Red blood cells (erythrocytes) (most numerous) and five types of white blood cells (leukocytes)
- Extracellular matrix: Plasma (liquid component, remaining after cells are centrifuged out)
- Muscle Tissue
- General Characteristics:
- Excitable tissue: Able to create and transmit electrochemical signals (like nervous tissue)
- All types contract (get shorter) actively; cannot push or actively get longer
- Three Major Kinds:
- Skeletal Muscle
- Component of the muscular system
- Cells (myofibers/muscle fibers) are long, non-branching cylinders that run the entire length of the muscle
- Characterized by very regular striations (banding patterns) that are all oriented in parallel
- Contraction is initiated by signals from the nervous system (voluntary movement)
- Cardiac Muscle
- Makes up the heart; part of the circulatory system
- Cells are also striated but individual cells are branched, leading to less regular striations
- Autorythmic: Contracts on its own, over and over (involuntary)
- Smooth Muscle
- Found in many organ systems, usually lining hollow organs (e.g., intestines)
- Cells are spindle-shaped (thicker in the middle, tapering at ends)
- Non-striated: Lacks the regular striations seen in skeletal and cardiac muscle
- Autorythmic: Contracts on its own (involuntary), forcing contents through organs
- Nervous Tissue
- Components:
- Neurons
- The excitable cells that create and transmit electrochemical signals
- Have a large cell body, input structures called dendrites, and a long output structure called an axon
- Form the basis of thought and personality in the brain
- Neuroglia (Glial Cells)
- A category of cells within nervous tissue that are not excitable
- Function: Support the neurons so they can live and perform their job
- Functions: Gather information and control body functions (nervous system controls other systems)