UNIT 2801 – LECTURE 3 – HISTOLOGY (Cell Structure and Function II)
STRUCTURE & FUNCTION OF:
(1) Cytoplasmic Matrix
(2) Cytoskeletal Components
(3) Endoplasmic Reticulum
(4) Golgi Apparatus
Cytoplasmic Matrix
- the matrix – one of the most impt. and complex parts of the cell – it is the environment of the organelles and location of many impt. biochemical processes
- matrix activity – results in physical changes in cells – viscosity changes, cytoplasmic steaming etc
-
Light Microscope  amorphous (shapless), no structure
soluble electrolytes & molecules
Electron Microscope  3-D network thin trabeculae
FUNCTION
Provides order for reactions occurring within ground substance
Cytoskeleton
- major component of matrix
= internal framework of interconnected filaments & tubules
FUNCTION
1. structural support – cell shape
2. intracellular movement of organelles & metabolites
COMPONENTS
1. microtubules (25nm diam)
- thin cylindrical shape
- constructed of 2 spherical protein subunits – α-tubulin and β tubulin (4-5nm diam)
- these subunits are assemble in a helical arrangement to form a cylinder – aprox. 13 Subunits along the circumference
2. microfilaments (4-7nm diam)
- Minute protein filaments scattered within matrix or organised into networks and parallel arrays
- composed of actin protein – similar to that of actin contractile protein of muscle tissue
-involved in cell motion and shape changes such as the motion of pigment granules, amoeboid movment, and protoplasmic steaming in slime molds.
3. intermediate filaments (10nm diam)
- hereterogenous elements of cytoplasmic matrix
- assembled from a group of proteins that can be divided into several classes
- intermediate proteins having dif. functions are assembled form one or more of these classes of proteins

- tole is unclear – some have been shown to form the nuclear lamina (structure that provides support of nuclear envelope) + helps links cells together to form tissues.
Microtubules
A minute filament (a slender strand or fibre of a material) in living cells that is composed of the protein tubulin and occurs singly, in pairs, triplets or bundles.
Microtubules help cells to maintain their shape; they also occur in cilia, flagella and the centrioles, and form the spindle during nuclear division.
LM visulaize with anti-tubulin antibody staining
EM tubulin
hollow, non-branching cylinders
22nm diameter
5nm wall thickness
STRUCTURE tubulin proteins arranged as dimmers 13 dimers arranged in a circle
FUNCTION
1. cell elongation
2. movement of cilia & flagella
3. maintain cell shape
4. intracellular transport of secretory granules
5. movement of chromosomes during mitosis & meiosis (mitotic spindle)

Microfilaments
 a thin thread of protein found in muscle and the cytoplasm of all cells
Monomers of the protein actin polymerize to form long, thin fibers. fibroblast - An elongated connective tissue cell with cytoplasmic extensions that is capable of forming collagenous fibers or elastic fibers.
STRUCTURE
• Actin
• Linear, helical array
• Flexible
• 5-7nm diameter

• most abundant close to plasma membrane
FUNCTION
• movement of plasma membrane during endocytosis & exocytosis
• movement of microvilli
• cytokinesis
• extension of cell processes
• cytoplasmic streaming
Intermediate filaments
STRUCTURE
• 8-10nm diameter
• heterogeneous
4 MAJOR SUBCLASSES
1) cytokeratins (epithelial cells)
2) vimentin (muscle cells & neurons)
3) neurofilaments (axons & dendrites)
4) lamins (nuclear envelope of most cells)
FUNCTION
1) structural
2) provide mechanical strength & resistance to extracellular forces

rough ENDOPLASMIC RETICULUM (rER)
Morphology:
- Cytoplasmic leaflet of rER- studded with ribosomes and polyribosomes = rough appearance  structures extend throughout the cytoplasm as a series of flattened membrane stacks of cisternae.
- the rER membrane is continuous with the membrane of the nuclear envelope
Distribution:
- present in all cells
- but abundant in cells specialized for protein secretion
- it is prominent in the basal cytoplasm of some polarized secretory cells e.g. exocrine pancreas and in the perikarya (the part of a nerve cell that contains the nucleus) of large nueons where is called Nissl substance.
LM
•can be viewed by staining with basic dyes and with metachromatic dyes. – because of the net anionic charge of phosphate groups in the ribonucleic acids.
• intense basophilia (property of of being readily stained with basic dyes) in cytoplasm close to nucleus
EM
• Cisternae = flattened interconnected membrane-limited sacs
• external surface covered with ribosomes
FUNCTION a) location of protein synthesis
- responsible for protein biosynthesis  e..g secretory proteins + lysosomal hydrolases + membrane proteins of the ER, golgi apparatuse and lysosomes and proteins of plasma membrane.
b) translocation of proteins
- As translation of the mRNA continues on the rER membrane bound polysomes
- newly synthesised proteins are transferred across the rER membrane into the rER cisternae
Secretory proteins
- When protein is cleaved by rER resident protein signal peptidase – secretory proteins are released into cisternae
- protein is then transported within vesicles to the golgi apparatus c) modification of proteins
- proteins synthesis undergo a variety of modifications

SMOOTH ENDOPLASMIC RETICULUM (sER)
- Smooth endoplasmic reticulum is found in a variety of cell types and it serves different functions in each.
- In the case of smooth endoplasmic reticulum in muscle cells, the vesicles and tubules serve as a store of calcium which is released as one step in the contraction process.
Calcium pumps serve to move the calcium.
- In liver cells- The smooth endoplasmic reticulum serves to metabolize glycogen (which are deposited as black rosettes over the area), and it contains enzymes needed to detoxify drugs.
- In adrenal cortical cells (as well as steroid producing cells in the gonads), the smooth endoplasmic reticulum serves to metabolize the steroids and produce the final steroid hormone.
Morphology:
- It consists of tubules and vesicles that branch forming a network.
- In some cells there are dilated areas like the sacs of rough endoplasmic reticulum.
- The network of smooth endoplasmic reticulum allows increased surface area for the action or storage of key enzymes and the products of these enzymes.
- has an extensive membrane network of cisternae (sac-like structures) held together by the cytoskeleton – but lack ribosomes
Distribution:
- because its role in phosphlipid synthesisis essential for membrane maintenance – all cells contain some sER
- however, it is prominent in certain cell types
- e.g.
- hepatocytes – sER plays an impt. role in glycogen metabolism
- striated muscle cells – sER removes calcium ions – maintains low conc. of calcium in sacroplasm and thus impt. in regulation of muscle contraction
- steroid hormone-producing cells – such as adrenal cortical cells
LM distinct cytoplasmic eosinophilia (RED)
EM short, anastomosing tubules
No ribosomes
enzymes associated with sER
FUNCTION
• Lipid metabolism
• Synthesize & secrete steroids
• Isolates Ca 2+
• Detoxification
• Membrane formation
• Glycogen metabolism

Versicular transport from the rER to the Golgi apparatus a. Transport vesicles containing newly synthesized proteins pinch off the smooth surfaced regions of the rER. These regions are called transitional elements of the rER b. The vesicles move to and fuse with membranes of the cis Golgi
Versicular transport to other membranous organelles
- Proteins that begin their synthesis in the rER move from the rER to the Golgi apparatus, move between and among Golgi stacks, and move from the trans-Golgi network to their final destinations by way of vesicles.
- The vesicles pinch off from one membrane, move to the next membranous compartment
(target) in the pathways, fuse with it and deliver their content.
- They then pinch off from the target membrane and return to and fuse with the original membrane to pick up more cargo.
Golgi apparatus
Morphology
The Golgi apparatus is an organelle that appears as a stack of 6-8 plate like membranous compartments associated vesicles and vacuoles, often located near the centrosome.
1. Vesicles. Large numbers of small vesicles are associated with the Golgi stacks. They are clustered on the cis face of the Golgi apparatus, which is the side nearest the rER. Vesicles are responsible for the directed movement of molecules from the rER to the Golgi apparatus. a. Vesicles are also arrayed along the outer rims of the Golgi cisternae b. These small vesicles are responsible for the directed movement of molecules between the sub compartments of the Golgi apparatus.
2. Vacuoles. Vacuoles are located on the trans face of the Golgi apparatus, which is the side furthest away from the rER. vacuoles are responsible for concentration and transport if finished products to their final destinations
3. Functional Compartments of the Golgi Apparatus. The Golgi apparatus can be separated into four functionally distinct compartments: cis Golgi stacks, medial Golgi stacks, trans Golgi stacks, and the trans Golgi network (TGN). The first three are involved in post translational modification of molecules transisting the stacks. The TGN is involved in sorting the molecules to their final destination (i.e. to lusosomes, to secretory vesicles or to the plasma membrane)
Function.

Packaging (receive and transport) materials and prepares them for secretion
- Each Golgi body within a cell has a cis face- Here, the Golgi body receives macromolecules synthesized in the endoplasmic reticulum encased within vesicles.
- The trans face of the Golgi body - the site from which modified and packaged macromolecules are transported to their destinations.


The further modificiation of proteins and lipids sunthesixed in the ER
- Within the Golgi body, various chemical groups are added to the macromolecules so ensure that they reach their proper destination.
- e.g. example, cells called goblet cells in the lining of the intestine secrete mucous. 
The protein component of mucous, called mucin, is modified in the Golgi body by the addition of carbohydrate groups. From the Golgi body, the modified mucin is packaged within a vesicle. The vesicle containing its mucous cargo fuses with the plasma membrane of the goblet cell, and is released into the extracellular environment
LM
• Clear region in the cytoplasm
EM
• Stacks of flattened cisternae plus tubular extensions
• Embedded in a microtubule network
• Vesicles associated with it
Polarized morphologically & functionally
‘Forming face’ is closest to rER of convex shape
‘Maturing face’ is near secretory granules & concave