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Cell structure

Cell structure. Fields of study Cytology Histology Pathology. Levels of Organization Cells Tissues Organs Organ systems Individual. Cells vary in size and shape according to location and function.

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Cell structure

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  1. Cell structure

  2. Fields of study Cytology Histology Pathology Levels of Organization Cells Tissues Organs Organ systems Individual

  3. Cells vary in size and shape according to location and function. Cells vary in internal structure depending upon their function. Cells vary in their life history, e.g., rates of cell renewal. Neuron Lining cell Macrophage Muscle cell Neuron Enterocyte

  4. Nucleus Nuclear envelope Cytoplasm Organelles Plasma membrane Plasma membrane Major compartments of the cell Cytoplasm - provides an aqueous matrix containing the internal structures of the cell and the cytosolic metabolic pathways, e.g., glycolysis Nucleus – houses the DNA, production of mRNA, initial ribosome assembly

  5. Cell membranes • General concepts • a. Intracellular – forms organelles • b. Cell surface – plasma membrane • c. All membranes have a similar structure, • referred to as unit membranes • d. Appears tri-laminar by electron microscopy • due to the deposition of osmium Unit membrane • Fluid mosaic model of membrane structure • a. Two phospholipid layers (phospholipid bilayer) • b. Associated proteins • i. Integral • ii. Peripheral

  6. I I I I P P Plasma membrane Integral membrane proteins Insert into one leaflet Transmembrane (channels, receptors) – Span both leaflets Peripheral membrane proteins

  7. Freeze fracture Plasma membrane

  8. Freeze fracture Exposed membrane surface is “replicated” by coating with a thin layer of metal. Replica is view with a transmission EM

  9. EM 150,000x Plasma membrane LM 1,000x EM 10,000x

  10. Glycocalyx (PAS positive)

  11. Functions of the plasma membrane • Transport • Diffusion - depends on the hydrophobicity, size, charge, presence of channel proteins. Passive diffusion Facilitated diffusion – uses transmembrane proteins • Active transport (energy dependent, e.g., ATP) • Vesicular transport (macromolecules/cell debris/microbes)

  12. Vesicular Transport Endocytosis - internalization of small membrane vesicles from the plasma membrane; involves vesicles < 1.0 micron a) Pinocytosis (“cell drinking”) - uptake of fluid by a continuous process; membrane recycling. b) Receptor-mediated endocytosis – requires receptor-ligand binding for vesicle formation and internalization.

  13. Phagocytosis Pinocytosis vs phagocytosis Pinocytosis 100,000x Phagocytosis (“cell eating”) Ingestion of large particles,e.g., bacteria; restricted to macrophages and leucocytes; involves particles > 1.0 micron

  14. Receptor-mediated endocytosis Triskelion LDL and the LDL receptor Transferrin Clathrin Clathrin-coated vesicle

  15. Vesicular Transport Exocytosis - Fusion of cytoplasmic vesicles with the plasma membrane and release of the vesicle contents to the outside of the cell a) Constitutive exocytosis - continuous process, e.g., renewal of plasma membrane, continuous secretion b) Regulated exocytosis - requires extracellular signal for vesicle fusion and release, e.g., hormone secretion and neurotransmitter release (“secretion on demand”)

  16. Transcytosis Transcytosis - Uptake of material on one side of a cell followed by transport and release from the opposite surface. Vessel lumen Endothelial cell Connective tissue Common in endothelial cells which line blood vessels. Also a mechanism used by some cells for protein trafficking to apical or basal membrane domains.

  17. Cell junctions Cell contacts established by integral membrane proteins Best developed in epithelial tissues which are present at body surfaces 1 1. Seal adjacent Tight junctions cell membranes establish apical and basal domains 2. Attach and anchor cells Adherent junctions 3. Provide channels for Gap junctions ionic continuity between cells 2 2 2 3 On the CD these structures are included with Epithelium

  18. Tight junction (zonula occludens) Belt-like barrier around the apex of the cell Close apposition of membrane proteins, exclusion of intercellular space

  19. Zonula occludens - Excludes luminal contents from intercellular space - Prevents migration of proteins within the cell membrane, maintains apical and basolateral domains - Maintains cell polarity

  20. Adherent junctions - Anchor cells to each other or to the basal lamina • No membrane fusion • Types of adherent junctions • Spot desmosome (macula adherens) • Hemidesmosome • Belt desmosome (zonula adherens)

  21. Desmosome (Macula adherens) Adherent junctions

  22. ZO ZA D Junctional complex • Components • Zonula occludens (ZO) • Zonula adherens (ZA) • Desmosome (D) • Epithelial tissues only

  23. Gap junctions Composed of six transmembrane proteins clustered like a rosette called a connexon. Adjacent connexons form a pore between cells Provide metabolic and electrical continuity between cells (coupling)

  24. Cell compartments Nucleus Cytoplasm Endoplasmic reticulum Golgi apparatus Lysosomes Mitochondria Peroxisomes

  25. Nucleus Eukaryotic cells Provides sequestered compartment for DNA Produces pre-ribosomal particles Site of messenger RNA synthesis

  26. Nucleus Electron microscopy 5,000x Light microscopy (H & E) 1,000x

  27. Nuclear envelope

  28. EM 18,000x (Freeze fracture) Nuclear envelope and nuclear pores EM 65,000x

  29. Electron micrograph Nuclear pores Regulate passage of materials in and out of the nucleus Corbett & Krebber, 2004 Nuclear pore proteins are common targets in autoimmune diseases, e.g., biliary cirrhosis and inflammatory bowel disease

  30. Nucleolus Ribosomal RNA synthesis Ribosomal subunit assembly Not surrounded by a membrane LM 1000x EM 12,000x

  31. Regions of the nucleolus 1. Nucleolar organizing centers (fibrillar centers) Site of DNA encoding rRNA 2. Pars fibrosa (dense fibrillary component) Contains rRNA transcripts 3. Pars granulosa Site of ribosomal particle assembly EM 40,000x

  32. ChromatinDNA + Protein (mostly histones) Euchromatin Transcriptionally active Dispersed Pale staining Heterochromatin Transcriptionally inactive Condensed Dark staining LM 1,000x

  33. Light microscopy (H & E, 1,000x) Electron microscopy (6,000x) Most nuclei show a mixture of heterochromatin and euchromatin

  34. Endoplasmic reticulum Rough endoplasmic reticulum (RER) Smooth endoplasmic reticulum (SER)

  35. Rough endoplasmic reticulum Composed of flattened membranous sacs (unit membranes) Possesses ribosomes on the outer cytoplasmic surface Continuous with the nuclear envelope Site of protein synthesis and some phospholipid synthesis

  36. Rough endoplasmic reticulum Light microscopy (H&E, 1,000x) Electron microscopy 15,000x

  37. 65,000x

  38. Smooth endoplasmic reticulum Tubular membranes continuous with RER No ribosomes Functions: - Site of triglyceride, cholesterol and steroid hormone synthesis - Detoxification - Calcium storage

  39. Smooth endoplasmic reticulum 30,000x 10,000x

  40. Ribosomes Two subunits, composed of rRNA and protein Site of protein synthesis (translation) Distribution - Free in the cytoplasm (polysomes) - Attached to the endoplasmic reticulum (RER) Polysomes RER EM 75,000x; 250,000x EM 20,000x

  41. Sites of protein synthesis Polysomes Cytoplasmic proteins, e.g., glycolytic enzymes and cytoskeletal proteins Mitochondrial proteins Peroxisomal proteins Nuclear proteins, e.g., transcription factors ribosomal proteins and histones Rough endoplasmic reticulum Proteins for secretion Lysosomal enzymes Integral membrane proteins Resident proteins for ER and Golgi

  42. Sites of protein synthesis Polysomes RER

  43. Golgi apparatus Flattened membranous sacs Usually located near the nucleus No direct continuity with ER Site of post-translational processing and vesicle packaging Receives products synthesized in the ER

  44. Golgi apparatus Cis face (forming face) Trans Golgi network Mid-Golgi region Trans face (maturing face)

  45. Golgi apparatus

  46. Golgi apparatus – Classical model Transition/transfer vesicles Forming face/cis face Maturing face/trans face Trans-Golgi network EM 75,000x

  47. Golgi apparatus – Classical model Transition/transfer vesicles Forming face/cis face Maturing face/trans face Trans-Golgi network EM 75,000x

  48. Golgi apparatus – Classical model Transition/transfer vesicles Forming face/cis face Maturing face/trans face Trans-Golgi network EM 75,000x

  49. Golgi apparatus – Classical model Transition/transfer vesicles Forming face/cis face Maturing face/trans face Trans-Golgi network EM 75,000x

  50. Cis-golgi network Trans-golgi network Cisternal progression model

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