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