Cytoplasm and Nucleus

BIO 408

Histology

D. D. L. Daley

 

Fundamental Types of Cells

n    Prokaryotic cells

n   Only in bacteria - typically small with a cell wall, no nucleus, no histones bound to the DNA and typically no membranous organelles

n    Eukaryotic cells

n   Larger cells with a distinct nucleus & nuclear envelope - numerous membranous organelles

Protoplasm

n    Living substance of a cell - two compartments

n   Cytoplasm - extends from cell membrane to the nuclear envelope

n   Karyoplasm - substance forming the contents of the nucleus

Cytoplasm

n    Bulk of the cytoplasm is water & the various organic and inorganic chemical dissolved or suspended in it - called the cytosol

n    Cytosol contains organelles that perform a variety of cell functions

n    Cytoskeleton - tubules and filaments that contribute the shape, motility and intracellular

n    Inclusions are also found in the cytoplasm - metabolic by-products, storage forms of various nutrients, inert crystals and pigments

Plasma Membrane

n    Also called the cell membrane or plasmalemma

n    Phospholipid bilayer & associated proteins

n    Typically in a 1:1 ratio by weight

n    Appears as trilaminar structure in electron micrographs

n   Inner leaflet - cytoplasmic side

n   Outer leaflet - extracellular side

n    Fluid mosaic model

Glycocalyx

n    Fuzzy coat on the outside of cell membranes

n    Usually composed of carbohydrate chains covalently bonded to transmembrane proteins and/or phospholipid molecules of of the outer leaflet

Membrane Transport Proteins

n    Channel proteins

n   Gated - can be open or closed

n   Voltage gated

n   Ligand gated - require ligand or signaling molecule to open or close the channel

n   Mechanically gated - hair cells in auditory system and mechanoreceptors of the skin

n   Ungated - always open channels

Membrane Transport Proteins

n    Carrier proteins are multipass proteins - possess binding sites for specific ions or molecules on both sides of the lipid bilayer

n    Maybe passive - electrochemical conc gradient driven movement or active - against a gradient

Active Transport

n    Uniport - single molecule transported up a conc gradient

n    Coupled - two different molecules transported

n   Symport - same direction

n   Antiport - opposite directions

Types of Transport

 

 

 

 

Cell Signaling

n    Communication that occurs between signaling cells and their target cells

n    Signaling molecule - ligand - maybe secreted and released by the signaling cell or may remain bound to its surface and be presented by the signaling cell to the target cell

n    The cell surface receptor is usually a transmembrane protein but receptors may even be intracellular receptors in the the cytosol or in the nucleus

 Signaling Molecules

n    Most are hydrophilic and cannot penetrate the cell membrane

n   e.g. acetylcholine

n    Hydrophobic molecules such as steroids or small nonpolar molecules like nitric oxide (NO) can diffuse through the cell membrane - bind intracellular receptors

 Cell-Surface Receptors

n    Most are glycoproteins - function in recognizing signaling molecules and in transducing the signal into an intracellular action

n    Classes

n   Ion Channel linked receptors

n   Enzyme -linked receptors

n   G-protein-linked receptors

G-Protein-Linked Receptors

 

 

 

 

Ribosomes

n    Small - 12nm by 25nm

n    Composed of rRNA and protein

n    Each composed of large subunit and small subunit

n    Small subunit - site for binding mRNA, P-site for binding peptidyl tRNAs and A-site for binding aminoacyl tRNAs

n    Large and small subunits do not come together until protein synthesis begins

Endoplasmic Reticulum

n    Smooth ER

n   Abundant in cells active in the synthesis of steroids, cholesterol, and triglycerides

n   Also in cells involved in detoxification & in muscle cells

n    Rough ER

n   Abundant in cells active in the synthesis of proteins that are to packaged and delivered to the plasma membrane

Golgi Apparatus

n    Mainly involved in the post-translational modification and packaging of of proteins received from the RER

n    Composed of one or more series of flattened, slightly curved membrane-bounded cisternae, the Golgi Stack - the periphery of each cisterna is dilated and rimmed with vesicles that are either budding or fusing

Golgi Apparatus

n    Each Golgi stack has three levels of cisternae

n   Cis-face - closest to the RER & convex

n   Mannose phosphorylation

n   Some removal of mannose also

n   Medial face (intermediate face)

n   Removal of mannose

n   Terminal glycosylation

n   Trans face - concave - exit face

n   Sulfation and phosphorylation of amino acids

Golgi Apparatus

n    Additional compartments

n   Endoplasmic reticulum/Golgi intermediate compartment (ERGIC) - formed from fusion of transfer vesicles from TER (transitional ER)

n   Trans golgi network (TGN) - distal side of the of the Golgi apparatus - sorting of proteins

Golgi and RER Vesicles

n    Vesicles have a protein coat and surface markers

n    Proteins known to elicit the formation of cargo-bearing vesicles -

n   Coatomer I - (COP I)

n   Coatomer II - (COP II)

n   Clathrin

Golgi & Trans Golgi Network

 

 

 

 

 

Endocytosis

n    Process whereby a cell ingests macromolecules or particulate matter from the extracellular space

n    Called phagocytosis when the vesicle is large >250nm

n   Vesicle - called a phagosome

n    Called pinocytosis when the vesicle is small < 150nm

n   Pinocytotic vesicle

Endocytosis

n    Receptor-mediated endocytosis

n   Cargo proteins - associate with particular macromolecules - form pinocytotic vesicle

n   Clathrin coat intercellularly

Endosomes

n    System of vesicles and tubules located in the cytosol near the cell surface

n   Early endosomes

n   These fuse with pinocytotic vesicles after they loose their clathrin coats

n   Late endosomes - situated deeper

n   Contents of early endosomes transferred to late endosomes when the entire contents of a pinocytotic vesicle requires degradation

Lysosomes

n    The contents of late endosomes are delivered for enzymatic digestion into the lumina of specialized organelles called lysosomes

n    Contain 40 different acid hydrolases

n    e.g. sulfatases, proteases, nucleases, lipases and glycosidases

n    Hydrolytic enzymes from the trans golgi network - arrive from different vesicles

Peroxisomes

n    Small (0.2-1.0 um) membrane bound organelles - self replicating

n    Contain 40 oxidative enzymes

n    Especially urate oxidase, catalase and D-amino acid oxidase

n    Function in the catabolism of long-chained fatty acids (beta oxidation) forming Acetyl-CoA and H2O2

n   AcetylCoA - used for energy

n   H2O2 - detoxifies (e.g. ethanol) and kills microorganisms - excess H2O2 destroyed by catalase

Mitochondria

n    Produce ATP via oxidative phosphorylation

n    Contain their own DNA

n    Self replicating - come from preexisting mitochondria

n    Enlarge in size, replicate their DNA and undergo fission

n    Average life span about 10 days

Inclusions

n    Glycogen - abundant in liver and muscle cells

n    Lipids - not only in adipocytes but also as individual droplets in hepatocytes

n    Pigments - hemoglobin in RBCs, melanin in melanocytes of the skin, hair, substantia nigra of the brain

n    Crystals - not common in cells however crystals of Charcot-Bottcher in Sertoli cells and crystals of Reinke in the interstitial cells of the testes are more common - crystalline forms of proteins

Cytoskeleton

n    3d meshwork of proteins responsible for maintaining cell morphology

n    Components

n    Thin filaments -  long chains (f actin)of g-actin

n   Involved in intracellular a& cellular movement

n    Intermediate filaments - (8-10 nm)

n   Provide structural support, anchor nucleus in place, forms adaptable connection between the cell membrane and the cytoskeleton, provide framework for for maintenance of the nuclear envelope

Cytoskeleton

n    Components

n   Microtubules - long straight, rigid tubular-appearing structures

n   Act as intracellular pathways for movement of organelles, maintain cell shape, allow for ciliary and flagellar motion and establish intracellular compartments

n   Microtubule-Associated proteins - motor proteins - assist in intracellular movement of organelles

Centrioles

n    Small cylindrical structures

n    Usually paired and arranged at right angles to each other

n    Near the Golgi apparatus

n    Composed of 9 microtubule triplets

n    The core of the microtubule organizing center or centrosome

n    Involved in formation of the spindle during cell division

n    Act as basal bodies that guide the formation of cilia and flagella

Nucleus

n    Largest organelle - contains most of the DNA and mechanisms for making RNA

n    Bounded by two phospholipid membranes - nuclear envelope

n    Generally spherical & centrally located but may be vary different

n    Generally only one - osteoclasts and skeletal muscle fiber are multinucleate

n    Size and shape constant for a given cell type - useful for clinical diagnosis of cancerous cells

Nuclear Components  

n    Chromatin - genetic material of the cell

n    Nucleolus - center for ribosomal RNA synthesis

n    Nucleoplasm - macromolecules and nuclear particles involved in the maintenance of the cell

Nuclear Envelope

n    Two phospholipid membranes

n   Outer nuclear membrane - continuous with RER - covered with a meshwork of intermediate fibers (vimentin) & ribosomes

n   Inner nuclear membrane - in contact with nuclear lamina (meshwork of intermediate fibers

n    Perinuclear cisterna - space between

n    Nuclear pores

Nuclear Pores

n    Number ranges from dozens to several thousand - correlates with the metabolic activity of the cell

n    Bidirectional traffic between the nucleus and the cytoplasm

n   Exportins - proteins that transport macromolecules like RNA form nucleus to the cytoplasm

n   Importins - proteins that transport cargo (e.g. protein subunits of ribosomes) from cytoplasm to the nucleus

Nuclear Pore Complex

n    The pore plus associated glycoproteins

n    80 to 100nm & spans both membranes

n    Cytoplasmic ring - 8 subunits - cytoplasmic aspect

n    Middle ring - 8 transmembrane proteins - project into lumen of the pore & perinuclear cistern

Nuclear Pore Complex

n    Transporter - center of the middle ring hourglass shape - coupled to the spokelike proteins of the middle ring

n    Middle believed to be a gated channel that restricts passive diffusion

n    Nucleoplasmic ring - analogous to the cytoplasmic ring

n    Nuclear basket - suspended from the nucleoplasmic ring & protrudes into the nucleoplasm

n    Becomes deformed during the process of nuclear export

Chromatin

n    Complex of DNA and proteins

n    Represents the relaxed and uncoiled form of chromosomes

n    Depending on the degree of transcriptional activity it maybe condensed as heterochromatin (inactive form located mostly on the periphery of the nucleus) or extended as euchromatin

Chromosomes

n    Chromatin fibers that are condensed and tightly coiled

n    Visible with the light microscope

n    Genome - number of chromosomes in somatic cells - specific for the species - total genetic makeup

n   In humans - composed of 22 pairs of autosomes and one pair of sex chromosomes

Chromatin Packaging

 

 

 

 

 

Nucleoplasm

n    Interchromatin granules (IGs) - contain ribonucleoprotein particles (RNPs) (precursors of ribosomal large and small subunits) & enzymes - function unclear

n    Perichromatin granules  (PCGs) - located at the margins of the heterochromatin - function unknown

Nucleolus

n    Darkly staining non-membrane bounded structure within the nucleus

n    Involved in rRNA synthesis and in the assembly of small and large ribosomal subunits