Untitled Quiz

Main Title: Histology

• Main topic is Histology
• Subtitle: The Ultrastructure of Cell
• Lecturer: Prof. Yeşim ULUTAŞ UĞUR M.D

Plasma Membrane (Cell Membrane)

• Essential for cell viability
• Dynamic structure
• Organized as a bilayered lipid
• Contains two electron-dense (dark) and one electron-lucent (light) layers
• 8-10 nanometers thick
• Contains embedded integral membrane proteins
• Has attached peripheral membrane proteins
• Defined as a modified fluid-mosaic model

Components of Plasma Membrane

• Cholesterol
• Protein
• Phospholipid
• Glycoprotein (forms glycocalyx)
• Glycolipid (forms glycocalyx)
• Hydrophobic (central) and hydrophilic parts

Glycocalyx

• Acts as a receptor
• Lipid rafts are areas with high concentrations of cholesterol & glycosphingolipids
• Lipid rafts facilitate protein movement within the membrane
• Lipid rafts support intercellular communication (signaling)

Integral Membrane Protein Types

• Receptor
• Linker
• Pump
• Canal (channel)
• Enzyme
• Constitutive protein

Transport Types at the Plasma Membrane

• Simple diffusion
• Structures constructed by proteins (voltage-gated, ligand-gated, mechanically gated channels)
• Mechanisms functioning through mobile molecules (passive transport (glucose), active transport (ion pumps))
• Endocytosis (taking into cytoplasm): pinocytosis (water & small molecules), phagocytosis (large molecules & structures), receptor-mediated (by binding to specific ligand)
• Exocytosis (giving out of cytoplasm): constitutive (works continuously), regulated (works only when needed)

Endocytosis at the Plasma Membrane

• Pinocytosis & phagocytosis are clathrin-independent
• Receptor-mediated endocytosis is clathrin-dependent
• A coated vesicle is formed
• Receptor interacts with clathrin via ADAPTIN
• DYNAMIN is the enzyme of the act

Exocytosis at the Plasma Membrane

• Sorting & packing in the Golgi apparatus
• Cargo is coated by a "coatomer"
• Targeting provided by COP I or COP II.

Transcytosis

• Diagram shows the process involves endocytosis, passing through the cell, and exocytosis, involving vesicle formations.

Endosome

• Early endosome: constructed by fusion of plasma membrane-originated vesicles, located near the plasma membrane, sorts internalized proteins, environment isn't acidic.
• Late endosome: acidic environment, located near Golgi apparatus & nucleus.

Lysosome

• Originates from endosomal structures
• Contains enzymes for degrading or destroying
• Lysosome-specific membrane proteins & lysosomal enzymes are added to the endosomal structure
• Endosomes & lysosome have mannose-6-phosphate receptors
• Mannose-6-phosphate is added to cargo before leaving Golgi apparatus

Lysosomal Membrane

• Cholesterol, lysobisphosphatidic acid
• Proton (H+) pumps
• Transport proteins
• LIMP (lysosomal integral membrane protein)
• LAMP (lysosome-associated membrane protein)
• LGP (lysosomal membrane glycoprotein)

Proteasome

• Ubiquitin added to cargo to target proteasome
• Contains ATP-dependent protease complexes
• Degrades abnormal proteins
• Recycles short-lived normal regulatory proteins for reuse

Peroxisome (microbody)

• Provides detoxification (hydrogen peroxide is toxic)
• Contains oxidative enzymes (peroxidases)
• Surrounded by membrane
• Provides fatty acid degradation

Endoplasmic Reticulum

• Granular endoplasmic reticulum (GER)
• Smooth endoplasmic reticulum (SER)
• Protein synthesis occurs in granular endoplasmic reticulum & ribosomes
• GER-containing cytoplasm is defined as ergacytoplasm
• Organized as membrane-limited continuous stacks
• Stacks have spaces called cisternae

Granular Endoplasmic Reticulum

• Cytoplasmic surfaces of cisternae contain ribosomes
• Ribosomes are protein synthesizing factories
• A single ribosome has small & large subunits
• Each subunit has ribosomal RNA (rRNA)
• A group of ribosomes is defined as a polysome
• Ribosomes are attached to Messenger RNA (mRNA) threads
• GER is continuous with the outer leaflet of the nuclear envelope
• Ribosomes can also be found freely within the cytoplasm
• Ribosomes bind to mRNA
• Peptide synthesis occurs

Post-translational Modification

• Within GER cisterna modifications occur through enzymes (folding by chaperons, glycosylations)
• After modifications, products move towards Golgi apparatus

Transport between GER & Golgi Apparatus

• Transport between GER & Golgi apparatus is provided by coatomers
• Anterograde transport is from GER towards Golgi apparatus (COP II)
• Retrograde transport is from Golgi apparatus backwards to GER (COP I)
• Cargo dissociates from its coatomer when it reaches its target
• Abnormal protein is carried back by COP I.

Free Ribosomes

• Some functional cytoplasmic elements are produced by free ribosomes residing within cytoplasmic matrix

Smooth Endoplasmic Reticulum (SER)

• Lacks ribosomes
• Stacks are tube-shaped
• Prominent in steroid-synthesizing cells, involved in lipid & glycogen metabolism; detoxification
• Neutralizes toxic substances by conjugation
• Named as sarcoplasmic reticulum in skeletal & cardiac muscles
• Stores calcium
• Regulates intracytoplasmic calcium concentration

Golgi Apparatus

• Organized as membranous stacks
• A polarized organelle
• GER-facing side is called cis-Golgi network (CGN)
• Cargo-releasing side is called trans-Golgi network (TGN)
• Between them is the medial-Golgi network
• Transport vesicles transfer cargo between cisternae
• Well-developed in secretory cells
• Functions of Golgi Apparatus: sorting & packing, secretion, membrane addition, post-translational modifications, adding phosphate/sulfate groups.

Mitochondrion

• Produces energy (ATP)
• Can be stained & displayed by Janus Green (vital dye)
• Divides & proliferates; its division cycle isn't synchronized with the cell cycle.
• Changes location & shape
• Acidophilic staining pattern
• Erythrocytes lack mitochondria
• Mitochondrion's evolution: aerobic bacteria, own genome, ribosomes, some constitutive proteins by itself, closed-type circular DNA, rRNA & tRNA provide translation of mRNA, via its DNA produces 13 self-enzymes, other mitochondrial proteins produced by free ribosomes.

Mitochondrial Membranes

• Outer & inner membranes
• Space between is intermembrane space
• Medium enclosed by inner membrane is matrix
• Inner membrane surrounds the matrix
• Transport traffic between mitochondrial membranes, needs energy & chaperon proteins.
• TOM complex: translocase of outer mitochondrial membrane
• TIM complex: translocase of inner mitochondrial membrane

Outer Mitochondrial Membrane

• Smooth membrane
• Contains canals (channels) named porins
• Porins are open to large uncharged molecules
• Contains receptors for proteins & polypeptides to be carried into the intermembrane space
• Has enzymes

Inner Mitochondrial Membrane

• Thinner than outer membrane
• Has folds defined as cristae
• Cristae extend towards matrix
• Cristae increase surface area; tubular-shaped in steroid-synthesizing cells; impermeable to ions
• Cardiolipin (a phospholipid) provides impermeability function

Inner Mitochondrial Membrane proteins

• Respiratory electron transport chain extends towards the matrix
• Contains tennis racket-shaped structures defined as elementary particles
• Elementary particles contain enzymes and provide ATP synthesis via oxidative phosphorylation.

Intermembrane Space of Mitochondrion

• ATP produced at inner membrane is used at intermembrane space
• Contains enzymes: cytochrome c (initiates apoptosis), kinases

Mitochondrial Matrix

• Contains matrix granules (calcium stores)
• Contains enzymes of citric acid cycle (Krebs) & beta oxidation of fatty acids
• Contains mitochondrial DNA, tRNA, & ribosomes
• Matrix reactions produce CO2 & NADH
• Functional configurations: orthodox (Cristae prominent, enlarged matrix, low oxidative phosphorylation) and condense (Cristae not prominent, condensed matrix, wide intermembrane space, high oxidative phosphorylation)

Cytoskeleton

• Microfilament
• Intermediate filament
• Microtubule

Microfilament

• Actin is the microfilament
• Actin has globular (ball-like) & filamentous (thread-like) forms
• When there is no need, it is in globular form within cytoplasm
• When in need, it polymerizes and changes into its filamentous form
• Filamentous actin is polarized structure
• Rapidly-growing end is called as positive end
• Slow-growing end is called as negative end.

Microfilament-Associated Proteins

• Actin-binding proteins
• Actin-bundling proteins
• Actin-severing proteins
• Actin-capping proteins
• Actin cross-linking proteins
• Actin motor proteins

Microfilament

• Mostly located near plasma membrane
• Form a terminal web immediately beneath apical cell surface to construct & stabilize cell shape
• Intracytoplasmic attachment point of junctional units
• Via polymerization provides movement of the cell.

Intermediate Filament

• Acts as cytoskeleton
• NOT polarized
• Specific to tissue
• Provides linkage between cytoplasm & extracellular environment (desmosomes).
• Intermediate Filament Types:
  • Type 1 & Type 2: Acidic & basic cytokeratins (epithelial cells)
  • Type 3: Vimentin (fibroblasts), Desmin (myocytes)
  • Type 4: Neurofilaments
  • Type 5: Lamins
  • Type 6: Beaded-filaments

Intermediate Filament-Associated Proteins

• In links between neighboring cells, cells & extracellular matrix (desmosomes)

Centriole

• Found as two short sticks arranged at an angle of 90 degrees to each other
• Made up of 9 microtubule triplets
• Centriole pair is located within "pericentriolar material"
• Together, they are termed centrosome or microtubule-organizing center (MTOC)
• Centrioles give rise to basal bodies, sources of cilia & flagella
• Centrioles form mitotic & meiotic spindles during cell division
• Each microtubule within a centriole is composed of 13 globular dimeric tubulins
• Each microtubule triplet has 3 types of microtubules (A, B, & C)
• Neighboring microtubules share common parts
• A microtubule is a complete ring; B & C are incomplete, half moon-shaped
• They share some part of their neighbors' rings
• Basal Bodies & CilioGenesis: Cilium formation ("ciliogenesis")
• Basal bodies are involved in ciliogenesis
• Kinocilium is a movable process of the cell.
• Stereocilium is a non-moveable process of the cell & does not contain axoneme.
• Cilium has 2 central separated & 9 peripheral conjoined paired microtubules
• Structure is called as axoneme
• Axoneme originates from A & B microtubules of basal body.

Nucleus, Nucleolus, Nuclear Envelope

•  Nucleoplasm: Material within the nuclear envelope excluding chromatin & nucleolus.
•  Nucleolus: Not surrounded by a membrane. rRNA genes are found; site where ribosome production is initiated; rRNA synthesis takes place; ribosomal subunits are produced; can be more than one per nucleus. - Nucleolus Parts: Fibrillar center (DNA-containing rRNA genes), Fibrillar material (pars fibrosa) (Ribosomal genes are in the process of transcription), Granular material (pars granulosa) (site of ribosomal subunits formation) -The nucleolar content is totally called as nucleolonema
• Nucleolus is the site where the cell cycle is controlled
• Inclusion: remnants of metabolic activity, pigment granules (surrounded by plasma membrane), lipofuscin pigment, lipid droplets, glycogen granules, hemosiderin, crystalline inclusions
• Cytoplasmic matrix: electrolytes, metabolites, RNA, proteins
• Nuclear envelope: separates cytoplasm from nucleoplasm; composed of two membranes; perinuclear cisternal space is between outer & inner membranes, continues with GER cisternae; selective barrier – Nuclear Pores: Nuclear pore complex (NPC) is a cylinder (with 8 proteins-nucleoporins) through which ribosome subunits pass while moving into the cytoplasm.
• Outer nuclear membrane: structure is similar to endoplasmic reticulum membrane; has ribosomal docking proteins at the cytoplasmic face; polyribosomes are attached to these dockers
• Inner nuclear membrane: nucleoplasmic face is supported by intermediate filament network ("nuclear fibrous lamina"); lamina contains nuclear lamins & lamin-associated proteins; which are involved in DNA replication & transcription; during cell division lamina is disrupted, but reformed after the event.