cytology

• Cytology


  The study of cells
• Histology


  The study of tissues
• Cells are the structural units of all living organisms
• Types of cells


  • Prokaryotic
  • Eukaryotic
• Prokaryotic cells


  • Found only in bacteria
  • Small (1–5 micro meter long)
  • Have a cell wall outside the plasma lemma
  • Lack a nuclear envelope
  • Have no histones bound to their DNA
  • Have no membranous organelles
• Eukaryotic cells


  • Larger and have a distinct nucleus surrounded by a nuclear envelope
  • Histones are associated with the genetic material
  • Have numerous membrane-limited organelles in the cytoplasm
• Size of human cells

  • Most human cells are so small they can only be seen with the aid of a microscope and are measured in units called micrometers
  • One micrometer is 1/1,000,000 of a meter or 1/25,000 of an inch
  • The human ovum or egg cell is about 1 millimeter in diameter, just visible to the unaided eye
  • Some nerve cells, although microscopic in diameter, may be quite long (at least 2 feet or 60 cm long)
• Shape of human cells

  • Round or spherical
  • Rectangular
  • Irregular
  • White blood cells even change shape as they move
• Based on function, there are more than 200 different kinds of human cells
• Cellular differentiation


  The process where the cells synthesize specific proteins, change their shape, and become very efficient in specialized functions
• The human organism has about 200 different cell types, all derived from the zygote (a single cell formed by fertilization of an oocyte by a spermatozoon)
• The first cellular divisions of the zygote originate cells called blastomeres that are able to form all cell types of the adult
• Specialized cell functions


  • Movement (muscle cell)
  • Synthesis and secretion of enzymes (pancreatic acinar cells)
  • Synthesis and secretion of mucous substances (mucous-gland cells)
  • Synthesis and secretion of steroids (some adrenal gland, testis, and ovary cells)
  • Ion transport (cells of the kidney and salivary gland ducts)
  • Intracellular digestion (macrophages and some white blood cells)
  • Transformation of physical and chemical stimuli into nervous impulses (sensory cells)
  • Metabolite absorption (cells of the intestine)
• Organismal activity depends on individual and collective activity of cells
• Biochemical activities of cells are dictated by sub-cellular structure
• Continuity of life has a cellular basis
• Cell ecology


  The same cell type can exhibit different characteristics and behaviors in different regions and circumstances
• Macrophages and neutrophils (both of which are phagocytic defense cells) will shift from oxidative metabolism to glycolysis in an inflammatory environment
• Cells that appear to be structurally similar may react in different ways because they have different families of receptors for signaling molecules (such as hormones and extracellular matrix macromolecules)
• Breast fibroblasts and uterine smooth muscle cells are exceptionally sensitive to female sex hormones because of their diverse library of receptors
• Cell components


  • Nucleus
  • Cytoplasm (cytosol, cytoskeleton, organelles, inclusions)
  • Protoplasm (nucleus and cytoplasm)
• Plasma membrane


  • Also known as plasma lemma or cell membrane
  • About 7.5 nm thick
  • The outermost component of the cell
  • Separates the cytoplasm from its extracellular environment
  • Composed of a lipid bilayer and associated proteins called integrins that are linked to cytoplasmic cytoskeletal filaments and to extracellular molecules
• Functions of the plasma membrane


  • Envelops the cell and aids in maintaining its structural and functional integrity
  • Functions as a semi-permeable membrane between cytoplasm and external environment
  • Acts as a sensory device that permits the cell to recognize (and be recognized by) other cells and macromolecules
  • Exhibits a trilaminar structure (called the unit membrane) when thin sections are examined by transmission electron microscopy
• Components of the plasma membrane

  • Phospholipids
  • Cholesterol
  • Proteins
  • Chains of oligosaccharides covalently linked to phospholipids and protein molecules
• Fluid mosaic model of membrane structure

  • The membrane consists of a phospholipid double layer with proteins inserted in it or bound to the cytoplasmic surface
  • Integral membrane proteins are firmly embedded in the lipid layers, some of which completely span the bilayer and are called transmembrane proteins
  • Many of the proteins and lipids have externally exposed oligosaccharide chains
• Lipid bilayer


  • Is freely permeable to small non-polar lipid soluble molecules but, is impermeable to charged ions
  • Consists of phospholipids, cholesterol, and glycolipids
  • Glycolipids are lipids with bound carbohydrate
  • Phospholipids have hydrophobic tail and hydrophilic head
• Molecular structure of the lipid bilayer

  1. Phospholipids (such as phosphatidylcholine and phosphatidylethanolamine) are amphipathic, with a polar head and two non-polar fatty acyl tails

  2. The polar heads face the membrane surface, while the tails project into the interior of the membrane

  3. The inner and outer leaflets form weak bonds that attach the leaflets to one another

  4. Glycolipids are present in the outer leaflet only, with polar carbohydrate residues extending into the extracellular space

  5. Cholesterol is located in both leaflets, constituting approximately 2% of the plasma lemma lipids and assisting in maintaining the structural integrity of the plasma membrane
• Fluidity of the lipid bilayer

  • Is crucial to cellular activities like exocytosis, endocytosis, membrane trafficking and membrane biogenesis
  • Is increased by a rise in temperature and by greater unsaturation of the hydrocarbon (fatty acyl) tails
  • Is decreased by an increase in the membrane's cholesterol content
• Membrane proteins


  • Integral proteins (dissolved in the lipid bilayer, may extend to both leaflets or into the inner leaflet only, include transmembrane proteins which span the entire plasma lemma and function as membrane receptors and/or transport proteins)
  • Peripheral proteins (do not extend in the lipid bilayer, located on the cytoplasmic aspect of the inner leaflet, bound to the polar groups of membrane phospholipids or to integral membrane proteins)
• Functional characteristics of membrane proteins

  • Ratio of lipid to protein in plasma membrane varies from 1:1 (by weight) in most cells to 4:1 in myelin
  • Some membrane proteins can diffuse laterally in the lipid bilayer, whereas others are immobile, being held in place by interactions with cytoskeletal constituents
• Glycocalyx


  • The sugar coat commonly associated with the extra-cytoplasmic aspect of the outer leaflet of the plasma membrane
  • May measure as much as 50 nm in thickness
  • Also known as the cell coat
• Composition of glycocalyx


  • Polar oligosaccharide side chains linked covalently to most protein and some lipid (glycolipid)
  • Contains cell surface proteoglycans which consist of membrane integral proteins to which are bound glycosaminoglycans (negatively charged polysaccharides)
• Functions of the glycocalyx


  • Aiding in cellular attachment to extracellular matrix components
  • Binding of antigens and enzymes to the cell surface
  • Facilitating cell-cell recognition and interaction (e.g. sperm-egg adhesion)
• Cytoplasm


  • All materials inside the cell and outside the nucleus
  • Composed of cytosol (intracellular fluid, a jelly-like fluid containing dissolved materials like nutrients, ions, proteins, and waste products) and organelles
• Cytoplasmic organelles


  • Membranous (mitochondria, peroxisomes, ER, Golgi apparatus, lysosomes, endosome)
  • Non-membranous (cytoskeleton, centrioles, cilia, filagella, ribosomes)
• Mitochondria


  • Membranous organelles that are spherical or filamentous, 0.5–1 micro meter wide and up to 10 micro meter long
  • Tend to accumulate in parts of the cytoplasm with high energy utilization
  • Transform the chemical energy of metabolites into energy stored as high-energy phosphate bonds in ATP molecules
• Mitochondrial structure


  • Outer membrane
  • Inner membrane (highly folded to form cristae, location of enzymes and electron carriers for electron transport and oxidative phosphorylation)
  • Intermembranous space
  • Intercristal space (matrix space)
• Functions of mitochondrial components

  • Outer membrane: lipid synthesis, fatty acid metabolism
  • Inner membrane: ATP production
  • Matrix: TCA cycle
  • Intermembranous space: nucleotide phosphorylation (i.e. ADP to ATP)
• The number of mitochondria and the number of cristae in each mitochondrion are related to the energetic activity of the cells in which they reside
• Mitochondrial configurations


  • Orthodox configuration (cristae are prominent, matrix occupies large part, low level of oxidative phosphorylation)
  • Condensed configuration (cristae are not easily recognized, matrix is concentrated, intermembranous space is increased in volume, high level of oxidative phosphorylation)