Cytology

BASICS OF CELLULAR STUDY

OUR STA T G STARTING POINT Whatt I’m going Wh in to t assume you already know!

WHERE DO WE GO FROM HERE? • General principles of cell & tissue architecture & development • Non-typical and mechanistic approach – Developing the “why” behind the “what” – Correlate structures with function – Overlaps with embryology & physiology

• This Thi will ill be b the h most useful f l course you take! k !

“There is one universal priniciple of development for the most elementary parts of organisms however they may differ: and that principle is the formation of cells.” ─ Theodor Schwann (1810-1882)

Microscopic Investigations (1839)

• THE ANIMAL IS THE SUM OF ITS PARTS &

THEIR FUNCTIONS – Cells, tissues & organs are specialized – All animals start as single cells and undergo changes to achieve final form – Morphogenesis & growth are common to cells, tissues & organs

THE FIRST KEY DEFINITION • CELL: A mass of protoplasm

surrounded by a membrane and containing a nucleus and organelles – The fundamental unit of life

– CELLS DO NOT EXIST AS

INDEPENDENT ENTITIES (No, not even in blood…as we’ll see later!)

THE SECOND KEY DEFINITION • TISSUE: An aggregation of cells and

intercellular materials specialized p for specific functions – Cells are always y p part of a tissue – Tissue’s function determines what cells are p present – Cells make tissue function possible – Structure of cells often p predictable based on tissue function & vice versa

FOUR “BASIC TISSUES” • EPITHELIUM • CONNECTIVE TISSUE • MUSCLE • NERVOUS TISSUE – Organs g made up p of these – At least 2 basic tissues in any organ in

THE THIRD KEY DEFINITION ORGANS: • Aggregations of cells,

tissues, and intercellular materials specialized for specific functions – Tissues are not autonomous – Always integrated with other g tissues to form organs

• Separation of tasks and of cells & tissues is a hallmark of organs

The animal…is to be viewed as a complex organism made up of an assemblage of simple microscopic organisms, the cells…Each…makes its own highly specific contribution to life…Similar kinds of cells doing a similar job tend to be grouped together within the tissues…A single…organ carries out multiple functions because it has multiple tissues and multiple kinds of cells in it. it ─Sherwin B Nuland, MD

Doctors: A Biography of Medicine (1988)

Limits ts on o Cellular Ce u a Growth G owt • Must have enough cytoplasm to function • Can’t have too much for diffusion, etc. – Some structural “leeway” leeway in size – Ability to compensate isn’t infinite

A Single Cell!

SIZE RANGE IN ANIMAL CELLS • Smallest: 3-4 μm – Some cells of blood, e g quiescent e.g. lymphocytes

• Largest: g 100-150 μ μm – Some neurons – Monocytes – Skeletal muscle

SMALL CELLS HAVE LIMITATIONS

• Typically very little cytoplasm – – – –

Limits functions May have inactive inclusions Nuclear material condensed M be May b “transformed” “ f d” to an active i state

LARGE CELLS HAVE LIMITATIONS

• Greater size limits internal diffusion rate of materials • Have compensatory mechanisms – Compensation isn’t unlimited

CELL DEATH • Many cells are pre-programmed to die – Major mechanism of morphogenesis – Shapes and sculpts limbs, etc. – Timing is exact and preprogrammed

• Many embryonic structures only temporary – Removes the h “scaffold” ff ld ffrom the h “building”

• Examples: – Formation of paws – Wound healing

Differentiation: How We Get From….

Here

to

Here

Differentiation • Process by which cells come to have p different characteristics & capabilities • Differentiated cells produce different

proteins than its p p progenitors g – Not all capabilities expressed

• Not limited in time – Can continue throughout life, e.g., wound healing & hemopoiesis

• Usually a precursor or “stem” cell type is involved – Stem cells often present in tissues/organs

Stem Cells • A general term • A pop population lation of “reserve” cells – Quiescent – Can be stimulated – Undergo g differentiation – One stem may produce d severall cell lines

STRUCTURAL S UC U ADAPTATIONS ONS • Remember: Anatomy = Physiology! • Applies at all levels – Cells – Tissues – Organs g – Whole animal

• A camel is a camel because he has to be a camel to survive in a camel’s world!

CELLULAR STRUCTURE IS AN ENGINEERING PROBLEM! • If it isn’t designed right it won’t work right! • Applies A li at all ll levels of structural study The Tacoma Narrows Bridge Collapse, 1940

PLASMA MEMBRANE • Defines cell’s limits • Controls passage of materials • Site of receptors, markers, etc. • Can be inferred but not

demonstrated with LM • Stereotypical appearance in EM

“FLUID MOSAIC” MODEL

Singer & Nicholson, 1972

PLASMA MEMBRANE

PM ADAPTATIONS FOR ABSORPTION/SECRETION • Process involves transfer across plasma membrane • Must optimize transfer based on: – Flow rate – Length and diameter of tubule • May be quite limited

– – – –

Degree g Of permissible p variation Rate of transfer per unit of area “Best case” and “worst case” “Safety Factor”

• Assumptions: rate is predictable, stable, and limited – Examples: E l kid kidney tubules, b l intestinal cells, epididymal cells

MICROVILLI • LM seen as “brush border” – Individually not resolvable – Uniform length & height – Intestine, kidney, some other sites

MICROVILLI

• • • •

Filled with cytoplasm, surrounded with PM Mayy contain actin filaments May be arranged for maximum # per unit area Fairly small structures

PM ADAPTATION FOR ABSORPTION/SECRETION • BASAL FOLDS • “Reverse” of microvilli – Basal end of cell, not apex – Infolds of PM containing y p cytoplasm

• Often contain mitochondria • Associated with active transport – Slower transfer rate – Transporting “finished goods”

ADAPTATIONS FOR MOVEMENT • Cell migration • Movement of materials on cell surface • Always involves microtubules – Cilia and flagellae g – Amoeboid motion

• Entire cell or only parts of it mayy be affected • Directional • Energy from ATP • Interact with aqueous environment – Respiratory, reproductive systems

• An ancient development – The only solution!

CILIA & FLAGELLAE

STRUCTURE OF CILIA

Don’t Confuse Cilia & Microvilli! • An order of magnitude g difference in size • Cilia can be 10100 μm long, and at least 10 μm thick • Microvilli rarely exceed d 11.00 μm thick and 10 μm long

ADAPTATIONS FOR MAINTAINING SHAPE • Odd shapes crucial to function – Internal “scaffolding” scaffolding – May serve other needs • Internal routing of materials • “Wiring” for information transfer

• Disruption causes problems – Chemotherapy agents • e.g. Colchicine

MICROTUBULES & MICROFILAMENTS • Vital to movement normal architecture • Ubiquitous and variable in makeup – May be contractile • Cilia, flagellae, and amoeboid movement

– May be “stiff” – May be for internal transport – Polymeric structures

• Shorten & lengthen g byy adding dimers • Principal component is

tubulin

• May contain ATPase,

dynein

MICROTUBULES • 20-50 nm (200-500 Å) • Cytoskeleton – Mitotic spindle, cilia, flagella

CYTOSKELETAL MICROTUBULES • Maintenance of shape of odd cells, e.g. neurons

MICROFILAMENTS • Intermediate filaments – Internal structural “scaffold” – Anchor nucleus – Connect cytoskeleton to PM – Maintain shape of nuclear envelope

• Thin microfilaments – Mainly actin for intracell lar intracellular contractility • Amoeboid motion, division etc division, etc.

– Myosin usually involved as well – Gel-like network in cytosol of other thin filaments

MICROFILAMENTS & INTERMEDIATE FILAMENTS • Smaller than microtubules (6-10 nm) & associated with contractility – Actin & myosin

• May be involved with adhesion structures – Tonofilaments of desmosomes

• Also a cytoskeletal element •

Variable in size, related to function

MICROFILAMENTS

Astroglial fibers in an astrocyte

MICROFILAMENTS

• Microfilaments Mi r fil nt & secretory r t r vesicles, i l rat r t ovarian ri n granulosa cells

MICROFILAMENTS

Actin & myosin in skeletal muscle

ADAPTATION FOR SURFACE PROTECTION • Cells are susceptible p to damage – May be unavoidable, e.g. Intestinal cells eroded by digestive juices – Strategy is to delay it

• Glycocalyx – Expendable & renewable surface covering – Resistant to erosion – Can be “sacrificed”

GLYCOCALYX • Cell surface coat – Carbohydrate in nature • Glyco = “sweet” sweet calyx = “husk”

– Term coined by John Luft in 1965

• F Found d on all ll cells ll to some extent • Usually heaviest at free surface • Not quite “basement membrane” • Functions usually p protective otective • Some enzymatic activity • PAS+ / RR+ in EM • May not be obvious in routine preps

GLYCOCALYX

• LM image courtesy of Dr. Ihab El-Zhogby

ADAPTATIONS FOR TISSUE INTEGRITY & FUNCTIONAL COHESION • Tissues are INTEGRATED both structurally & functionally • Cells are not independent p units • Cells must communicate • Cells must maintain contact with each other • A whole h l series i off PM specializations i li i

Occluding (“Tight”) Junctions • Adjacent PM’s are fused together

OCCLUDING JUNCTION • Function to separate “inside” “i id ” from “outside” • Control C t l passage off materials; forces

them to go through a cell • Used to control osmotic pressure, ion flux, etc.

DESMOSOMES • Most common membrane specialization p • Found in all types of tissues & organs • Similar Si il tto adhering dh i junction – Also for mechanical int it integrity – Distinguished by dense filamentous component – Anchor cells to each other

• A “spot weld” versus a bead weld” weld “bead

SPECIALIZATIONS FOR COMMUNICATION • Cells have to know what’s going on around them • Tissue function depends on this – – – –

Smooth muscle Cardiac muscle Glandular epithelium Many other examples

Gap Junction • Limited area of plasma membrane • Found in all tissues • Not for adhesion but for

communication • Site of lowered or variable resistance to passage of ions • Membrane gap is 20Å or so • “Pores” on either side • Cell-to-cell C ll t ll communication

Gap Junctions

NUCLEUS & NUCLEAR ENVELOPE • “Command & Control” center of cell • All eukaryotic cells have the entire “blueprint” – MOST of it isn’t used – Degree of specialization affects how much is accessible – Physiological state determines appearance

• Broken down & reconstructed each cycle • Nuclear N l r morphology rph l varies with function & state • Envelope continuous with RER

NUCLEUS & RER • Nuclear envelope & RER are continuous – Ribosomes found on NE outer t surface

• Nuclear pores p located at turnbacks of the NE

NUCLEAR PORES • Openings in nuclear envelope • Allow passage p g of RNA • Complex structure to control movement

NUCLEAR PORES

SYNTHESIS & SECRETION • MOST cells have some synthetic capability • SOME cells are p for it specialized • ALL use the same structures to do it – Endoplasmic reticulum – Golgi apparatus

ENDOPLASMIC RETICULUM • Two types: – Rough g ER functions for protein p synthesis y • Described many times from LM studies • EM reveals true nature • Porter coined term in 1950’s

– Smooth ER functions in various ways • Lipid synthesis • Enzymatic degradation pathways • Special role in muscle

RIBOSOME • Functional unit of RER • Bound & free types exist – Identical structure – Large L & small ll subunits – Entire ribosome complex about 300Å

ROUGH E.R. • Prominent feature in secretory cells – Pancreatic cells – Plasma cells – Peptic cells

• Amounts vary with cell function • Usually some present, may be minor amount • Accounts for LM visible BASOPHILIA

BASOPHILIA & THE RER

GOLGI APPARATUS • Known since 19th Centuryy – Visible in LM – Nature & existence d b t d until debated til 1960’s 1960’

• Functions to modify & package products of RER for release

• Camillo Golgi (1843-1926)

GOLGI APPARATUS

GOLGI APPARATUS

• Contains enzymes for attaching y & lipid p moieties to peptides p p carbohydrate

SMOOTH ENDOPLASMIC RETICULUM • Visible only in EM • Prominent in cells making steroids or lipids – Leydig cells – Luteal cells

• Role R l iin d detoxification t ifi ti – Large amounts in hepatocytes

• C Collection ll i off interconnected tubules & vesicles • Membranous but not “studded” with ribosomes

Anatomy and physiology form but the vestibule of medical education. They teach the normal structure of the body, the normal function of the parts, fluids, organs, and the conditions under which they operate. — Abraham Flexner (1866-1959)

Medical Education in the United States and Canada (1910)