BMED 2801 – Lecture 6 – Histology: Cell structure and Function III
Objectives:
• Endocytosis & Exocytosis
• Lysosomes & extracellular digestion
• Structure & function of mitochondria and nucleus
• Structure, function & organization of chromatin
How do we recognize these structures with the light & electron microscope?
ENDOCYTOSIS
- The process by which cells absorb material (molecules such as proteins) from outside the cell by
engulfing it with their cell membrane via vesicular transport.
- It is used by all cells of the body because most substances important to them are large polar
molecules that cannot pass through the hydrophobic plasma membrane or cell membrane.
Vesicular transport process by which substances ENTER the cell
Clathrin-mediated endocytosis
-
The major route for endocytosis in most cells, and the best-understood, is that mediated by the
molecule clathrin.
Clathrin-mediated endocytosis is the specific uptake of large extracellular molecules such as
proteins, membrane localized receptors and ion-channels.
These receptors are associated with the cytosolic protein clathrin, which initiates the formation of
a vesicle by forming a crystalline coat on the inner surface of the cell's membrane.
This large protein assists in the formation of a coated pit on the inner surface of the plasma
membrane of the cell.
This pit then buds into the cell to form a coated vesicle in the cytoplasm of the cell.
In so doing, it brings into the cell not only a small area of the surface of the cell but also a small
volume of fluid from outside the cell.
Pinocytosis
-literally, cell-drinking. This process is concerned with the uptake of solutes and single molecules such as
proteins.
-small particles are brought into the cell suspended within small vesicles which subsequently fuse with
lysosomes to hydrolyze, or to break down, the particles.
- In contrast to phagocytosis, generates very small vesicles.
- Unlike receptor-mediated endocytosis, pinocytosis is unspecific in the substances that it transports.
Budding off of plasma
membrane containing
fluid/protein molecules.
Cytoplasm
Nucleus
• Ingestion of fluid, protein molecules
• Via small membrane bound vesicles <150nm diameter
• Constitutive
• Clathrin molecule-Independent
Receptor-mediated
- Takes place in regions of the cell membrane known as clathrin-coated pits – indentions where the
cytoplasmic side of the membrane has high conc. of the protein clathrin.
- extracellular ligands brought into cell to bind to their membrane receptors  receptor-ligand
complex migrates along cell surface until it encounters a coated pit  membrane draws inwards
(folding)/invaginates pinches of cell membrane – and becomes a cytoplasmic vesicle
- in vesicle receptor + ligand sep, leaving ligand inside (endosome)  endosome moves into
lysosome to be destroyed or golgi complex if it is to be processed
- Vesicle with receptor – undergo membrane recycling – via exotyosis it is incorporated back into
the membrane.
• Select molecules
• Clathrin (proteins within cytoplasm-create polarity to plasma membrane- allowing for budding off of
plasma membrane for vesicle transport ) – dependant
• Form clathrin coated vesicles
E.g. of receptor mediated endoxytosis – at synapses – release of acetylcholine at end of terminal boutons
of axon towards specific receptors at the dendrite membrane.
Phagocytosis
- literally, cell-eating of foreign particles – mediated by immune system
- Phagocytes undertake Phagocytosis  granulocyte, the macrophage, neutrophils, monocytes,
the dendritic cell etc.
- Phagocytosis (actin-mediated process) is the cellular process of Phagocytes (WBC) and Protists
- engulfing solid particles e.g. cells which have undergone apoptosis, bacteria, or viruses by the
cell membrane to form an internal phagosome, which is a food vacuole, or pteroid.
- The membrane folds around the object (engulfs), and the object is sealed off into a large vacuole
known as a phagosome  phagaosome pinches off from the cell membrane and moves to the
interior of the cell - fuses with a lysosome – whose digestive enzymes destroy the bacteria.
• Ingestion of large particles
• Bacteria, cell debris, carbon, asbestos fibres
• Phagosome vesicles ~250nm created
• Clathrin-independant
• Actin cytoskeleton filaments on apical surface of the membrane interacts with plasma membrane to
create large phagosomes (vesicles)
• Usually receptor-mediated
EXOCYTOSIS
Vesicular transport process by which substances LEAVE the cell
- Process by which a cell directs the contents of secretory vesicles out of the cell membrane.
- These membrane-bound vesicles contain soluble proteins (large lipophonic molecules) to be
secreted to the extracellular environment, as well as membrane proteins and lipids that are sent
to become components of the cell membrane + wastes left in lysosome from intracellular
digestion.
- Goblet cells- in intestine – continuously release mucus via exotyosis and fibroblasts in connective
tissue release collagen.
-
constitutive pathway- responds to continuous and immediate demand - relies heavily on the
cytoskeletal elements to align vesicles to reach the surface plasma membrane e.g. immune cells
producing immunoglobulin all the time - go down constitutive pathway
regulated secretory pathway- secretory vesicles can remain in cell cytoplasm – until stimulus is
produced stimulating its release e.g. pancreas – islet cells continuously produce insulin but only
secret insulin only in response to increase in blood sugar levels via hormonal or neuronal
receptor response stimulus.
Electron microscope
look at eosinophil - a granular white blood cell (leukocyte) that stains with the dye eosin and is thought to
play a part in allergic reactions and the body's response to parasitic diseases - stain with haemotoxylin
and eosin  will see bright pink little circles within cytoplasm = vesicles that contain histamine and
heparin – allergic response - heparin and histamine granules, exocytose out of the cell.
LYSOSOMES - DIGESTIVE ORGANELLES
- membrane bound vesicle filled containing a variety of hydrolytic enzymes
FUNCTIONS
1. Degrade molecules that enter cell via endocytosis
2. Degrade old cell organelles (Autophagy)
3. Metabolize fatty acids
lysosomes – also recycle  e.g. plasma components may be reincorporated into plasma membrane or
produce other organelles.
-
lysosomes compartmentalise enzymes – because free floating enzymes will destroy the cell.
APPEARANCE
Stain = acid phosphatase (bright pink) method common in macrophages & neutrophils
RESIDUAL BODIES- sign of cell ageing = cell debris filled vacuole cell ageing
v. electron dense (electron density is given
by what is being broken down and digested),
v.dark
MITOCHONDRIA
Functions = generate ATP & regulate ion concentration via Krebs Cycle & Oxidative
Phosphorylation that occur within the mitochondria.
-
Present in all cells except red blood cells.
elliptical membrane bound organelles with unusual double wall
outer membrane – gives shape
inner membrane – folded into leaflets/tubules called cristae
= 2 sep compartments
there is a true space between them – intermembrane space
Between outer and inner mem – intermembrane space  impt. role in production of ATP
has mitochondrial DNA + matrix RNA = can produce their own protein
Mitochondria have 2 membranes
• Outer membrane
o permeable with large pores
o smooth
o 6nm thick
• Inner membrane
o thinner
o Forms Cristae (sheet-like folds) – increased surface area – to increase functional ability.  holds
the matrix
o functional proteins
o 4nm thick
o elementary particles – contain enzymes on attached to cristae surface of inner membrane – part
of electron transport system – is where ATP is produced
rER
Cristae
Intermembrane Space (space between the 2 membranes)
• contains apoptosis (normal programmed cell death) enzymes
Matrix
• Enzymes of Krebs Cycle & fatty acid oxidation
• Granules store Ca2+
• Mitochondrial DNA, ribosomes and tRNA
UNDERGO MORPHOLOGICAL CHANGES RELATED TO THEIR FUNCTIONAL STATE
Low level activity
o Cristae – prominent (less density in matrix – therefore, greater contrast between the two)
o Matrix - largest volume
High level activity
o Cristae (structure - length and folding does not change) – not easily recognized  due to
increased conc. of matrix – hard to resolve diff. between high density matrix and cristae.
o Matrix – concentrated
o Intermembrane space – 50% volume
THE NUCLEUS – CHROMATIN (EXAM!!)
Nucleus contains chromosomes – chromosomes are chromatin – chromatin are called chromosomes
when aligned or arrange in a particular way.
- scattered granular structures
Chromatin contains - DNA and Histones (proteins)
Depending on activity level of cell – arrangement and appearance of chromatin varies
-Nucleus of lymphocyte and neutrophil – have alternative light/dark patches
- The neutrophil is more active – lower electron density – therefore contains more euchromatin
than heterochromatin
- Heterochromatin tends to lie closer to the nuclear membrane.
Light/dark patches- are different forms
of chromatin
Condensed chromatin – called
heterochromatin – the entire DNA is
coiled and folded up = more electron
dense  w/ light microscope stain
DARK blue w/ electron microscope –
v. black region. The chromatin is
inactive
Extended/ Euchromatin  DNA
uncoiled/unfolded and active – ready
for replication, less electron dense
w/ light microscope does not stain
pale white areas in between purple
region , w/ electron microscope paler
region – the white.
Nucleosomes: smallest unit in Chromatin
8 Histones plus 2 DNA loops – this structure is the smallest functional unit in chromatin called
nucleosomes.
histones
DNA
Long strands of coiled nucleosome = Chromatin Fibril  because they are long – they coil.
In Dividing Cells
- chromosomes are only seen in dividing cells
Condensed and organised Chromatin = Chromosomes
THE NUCLEUS – NUCLEOLOUS
-
Contain the genes and proteins that control the synthesis of RNA for ribosomes  the nucleolus
is the site of synthesis and assembly of ribosomes. It aids in transcription.
large dark staining bodies within the nucleus
= site of rRNA synthesis initial ribosomal assembly
• 1 or more/cell
• Non-membranous
• spherical
• RNA, proteins, DNA
LM appearance
• stains with Haematoxylin (blue)
THE NUCLEUS - NUCLEAR ENVELOPE
- 2 membrane structure that sep. nucleus from the cytoplasmic compartment
- outer membrane is connected with the endoplasmic reticulum
- both membranes are porous
- communication between eh nucleus and cytosol occurs through the nuclear pore complexes
(large protein complexes with central channel)
2x parallel membranes perinuclear space
Nuclear pores
• 70nm diameter
• fusion of the 2 membranes
Outer membrane
• continuous with rER membranes
• polyribosomes
Inner membrane
• supported by intermediate filaments (Lamin)
THE NUCLEUS
NUCLEOPLASM
• material enclosed by nuclear envelope
• proteins + metabolites
• excludes chromatin and the nucleolus
THE CELL - THE NUCLEUS