Tissues 1- epithelial cells

Question 1

Where does the variety of cells in the body come from

Answer 1

This variety arises as a result of differentiation of precursor cells- they all come from a single zygote

Question 2

Simply, what is meant by the epithelium

Answer 2

Multicellular sheets where the cells are joined together side by side

Question 3

Describe the contents of the eukaryotic nucleus

Answer 3

The contents of the nucleus, the nucleoplasm (Np) and the nucleolus (Nc), are enclosed by a double lipid bilayer, the nuclear envelope (Ne). The unravelled chromosomes (chromatin) are suspended in the nucleoplasm.

Question 4

What is the nucleolus associated with

Answer 4

The invagination of the nuclear envelope

Question 5

What does the nucleolus consist of

Answer 5

The nucleolus is an aggregate of the clusters of rRNA genes that are present at the ends of different pairs of chromosomes (5 pairs in humans: ~300 copies of the rRNA genes). The nucleolus also contains the synthesised rRNA and proteins being assembled to make the ribosome subunits.

Question 6

Why are there so many copies of the rRNA genes required

Answer 6

it is structural, mRNA can make many different proteins, alternate splicing and modifications, rRNA cannot

Question 7

What do ribosomes consist of

Answer 7

Ribosomes are composed of two distinct subunits, each of which contains ribosomal RNA (rRNA) and protein.

Question 8

Why are the values of S for each subunit not arithmetically additive

Answer 8

when the 60S large and 40S small subunits are assembled to make a ribosome, the ribosome is 80S (not 100S). This is because the density and shape of the assembled ribosome contributes to the lower sedimentation coefficient.

Question 9

Which organelle is continuous with the nuclear envelope

Answer 9

The RER

Question 10

Describe the role of the nuclear lamina in the nuclear envelope

Answer 10

The nuclear lamina is a specialised type of cytoskeleton formed on the internal surface of the nuclear envelope. Important in controlling the assembly/disassembly of the nuclear envelope in cell division.

Question 11

Describe the differences between the RER and the SER

Answer 11

SER-Agranular, more tubular sacs, no ribosomes, lipid metabolism
RER-Granular, protein synthesis and transport, ribosomes present

Question 12

Role of the RER

Answer 12

RER is the site of synthesis of membrane proteins and of proteins that are packaged in membrane-bound structures, e.g. secretory vesicles, lysosomes

Question 13

Role of the SER

Answer 13

SER specialises in detoxification and lipid metabolism. The enzymes involved are typically associated with the membranes. SER is also the site of Ca2+-storage in cells (which is important in many cell-signalling contexts).

Question 14

Describe the arrangement of the Golgi body and how this arrangement allows the Golgi to perform its function

Answer 14

The cis face of the Golgi apparatus is aligned to the endoplasmic reticulum, and the trans face is towards the cell periphery.

This arrangement allows the Golgi to receive vesicles from the ER and direct vesicles to the rest of the cell and the cell surface.

Question 15

What are peroxisomes

Answer 15

Peroxisomes are enclosed by a single membrane, and contain enzymes involved in lipid and oxygen metabolism, e.g. oxidases, catalases, peroxidases. Present in most eukaryotic cells.

Question 16

Describe the appearance of peroxisomes

Answer 16

The enzymes present in peroxisomes can be present at such high concentrations that they “crystallise” to form the cores that are observed in transmission EM.

These organelles are important in oxidation reactions. A by-product of many of these actions is peroxide, which is used in peroxidation reactions in many cell types.

Question 17

Describe the structure of microtubules

Answer 17

Polymers of a and b tubulin heterodimers, ~20nm thick.

Question 18

What is the role of microtubules

Answer 18

Involved in cell shape, and act as “tracks” for the movement of organelles and other cytoplasmic components within the cell.
Many accessory proteins involved in these functions.
They are also involved in the formation of the mitotic spindle, hence the microtubular mitotic spindle is the target for many antimitotic cancer drugs.

Question 19

Where are microtubules found

Answer 19

The major component of cilia and flagellae

Question 20

Where do the microtubules radiate from

Answer 20

MTs often radiate from a central structure in a cell, the microtubule organising centre (MTOC).

Question 21

Describe the structural arrangement of the microtubules in flagellae

Answer 21

9 microtubule doublets and 2 central microtubules make up the core of cilia and flagellae: the 9+2 arrangement.
ATP-dependent motor proteins (blue in the diagram above) distort the cilium or flagellum to produce movement.
Not pairs- hence not fully tubular

Question 22

How does the flagella give bacteria motility

Answer 22

Motor proteins bind to microtubules, ATP deforms the microtubules, allowing the motor proteins to walk along it, as the microtubules are fixed in the flagellum, they bend, giving the bacteria motility

Question 23

What are the intermediate filaments

Answer 23

A group of polymers of filamentous proteins which form rope-like filaments, with diameter in the range 10-15 nm.

Question 24

Describe the role of the intermediate filaments

Answer 24

IFs give mechanical strength to cells. Desmosome cell-cell adhesions are connected by intermediate filaments (cytokeratins in epithelia; desmin in cardiac muscle cells). 
Nuclear lamins (mentioned earlier) are intermediate filaments found forming a network on the internal surface of the nuclear envelope, being involved in stabilising the envelope.

Question 25

Describe the different types of intermediate filaments found in different cell types

Answer 25

epithelia have cytokeratins;
mesenchymal cells have vimentin;
neurones have neurofilament protein.
muscle cells have desmin.

Question 26

What are the microfilaments

Answer 26

Polymers of the globular protein, actin; associate with adhesion belts in epithelia and endothelia, and with other plasma membrane proteins.
5-9nm diameter

Question 27

Describe the role of the microfilaments

Answer 27

Involved in cell shape and cell movement (crawling of cells; cell contractility esp. muscle).
Accessory proteins, e.g. myosin, act with actin to control actin organisation and cell movement.
In addition to being a major component of the contractile apparatus of muscle. Actin microfilaments are involved in the contraction of non-muscle cells.

Question 28

What are the monomers in actin and microfilament

Answer 28

Monomer = globular actin, G-actinMicrofilaments = filamentous actin, F-actin

Question 29

Where is the actin normally found

Answer 29

Typically present in the cortical (peripheral) regions of a cell.

Question 30

Describe the main cell type groups

Answer 30

Connective tissue cells: fibroblasts (many tissues), chondrocytes (cartilage), osteocytes (bone).
Contractile tissues: skeletal muscle, cardiac muscle, smooth muscle.
Haematopoietic cells: blood cells, tissue-resident immune cells, and the cells of the bone marrow from which they are derived.
Neural cells: cells of the nervous system having two main types; neurones (carry electrical signals) and glial cells (support cells).
Epithelial cells: cells forming continuous layers, these layers line surfaces and separate tissue compartments and have a variety of other functions.

Question 31

Describe the different types of tumour that arise from different cell types

Answer 31

epithelial cancers are carcinomas

mesenchymal (connective tissue and muscle) cancers are sarcomas

haematopoietic cancers are leukaemias (from bone marrow cells) or lymphomas (from lymphocytes)

neural cell cancers are neuroblastomas (from neurones) or gliomas (from glial cells)

Question 32

What is a tissue

Answer 32

a group or groups of cells whose type, organisation and architecture are integral to its function
tissues are made up of cells, extracellular matrix and fluid

Question 33

What is the ECM

Answer 33

material deposited by cells which forms the “insoluble” part of the extracellular environment
generally composed of fibrillar (or reticular) proteins (e.g. collagens, elastin) embedded in a hydrated gel (proteoglycans or “ground substance”)
may be poorly organised (e.g. loose connective tissue) or highly organised (e.g. tendon, bone, basal lamina)

Question 34

Describe epithelial organisation

Answer 34

epithelial cells make organised, stable cell-cell junctions to form continuous, cohesive layers

epithelial layers line internal and external body surfaces and have a variety of functions, e.g. transport, absorption, secretion, protection

cell-cell junctions key to the formation and maintenance of epithelial layers

Question 35

Describe simply how continuous layers of epithelial cells can form

Answer 35

continuous epithelial layers can form because cells make stable cell-cell junctions which give the epithelia mechanical integrity and act to seal the intercellular pathways of the layer.

in many epithelia these are found at the apical region of cell-cell contact as a junctional complex.

generally in 2 forms: zonulae (belts) or maculae (spots)

Question 36

Describe cell-cell junctions in epithelia

Answer 36

typically arranged as an apical junctional complex containing a tight junction nearest the apex, then an adhesion belt, then, scattered throughout the lateral membrane, desmosomes (spot adhering junctions)

other important junctions are gap junctions, which act as regions of direct communication between adjacent cells

Question 37

Describe the formation of tight junctions

Answer 37

The tight junction is formed by proteins called claudins and occludins, which are arranged in strands along the lines of the junction to create the seal.zonula occludens (belt junction)
points on adjacent membranes form close contacts at apical lateral membranes
form a network of contacts, the more elaborate the network, the tighter the seal

Question 38

What are the roles of the tight junction

Answer 38

act to seal paracellular pathways (i.e. between cells)- without junctions to prevent leakage, the pumping activities of absorptive cells such as those around the gut would be futile, and the composition of fluids on both sides of the epithelium would be the same

The tight junctions also play a key part in maintaining the epithelial polarity of the epithelial cells

The tight junctions around the apical region prevents the diffusion of proteins within the plasma membrane and so keeps the apical domain biochemically different from the basolateral domain.

The tight junctions are also sites for the assembly of proteins which separate the apical and basolateral domains.

Question 39

Describe the adhesion belt

Answer 39

zonula adherens (belt junction, adherens junction (AJ))
usually formed just basal to the apical tight junction
transmembrane adhesion molecule is a cadherin (family of Ca2+-dependent cell adhesion molecules)
Cadherin of one cell binds directly to an identical cadherin molecule in its neighbour,
cadherins associate with the microfilament (actin) cytoskeleton- via several linker proteins
this junction controls the assembly of the other junctions (“master junction”)

Question 40

What is the role of the adhesion belt

Answer 40

Bundles of actin filaments are connected from cell to cell across the epithelium, this network can contract, giving the epithelial sheet to develop tension and change its shape. By shrinking the epithelial sheet along one axis, the sheet can roll into a tube. Alternatively, by shrinking the apical surface locally along all axes at once, the sheet can invaginate into a cup and eventually create a vesicle by pinching off from the rest of the epithelium.- important in the development of the embryo

Question 41

Describe the desmosomes

Answer 41

macula adherens (spot desmosome)
found at multiple spots between adjacent cell membranes
transmembrane cell adhesion molecule is a cadherin-like molecule
linked to the intermediate filament cytoskeleton- keratin filaments
provides good mechanical continuity between cells

Question 42

Describe the gap junction

Answer 42

macula communicans (spot junction)
made up of clusters of pores formed from 6 identical subunits in the membrane - these pores are continuous with pores in adjacent cell membrane
allows passage of ions and small molecules between cells
pH, Ca2+ conc, voltage, and some signalling molecules can affect passage, i.e. can open and close pores thereby controlling intercellular communication
also known as the electrical synapse; important in the passage of electrical signals in some tissues

Question 43

Describe how gap junctions appear

Answer 43

As a region where the membranes of two cells lie close together and are parallel, with a very narrow gap between them.

Question 44

What does the gap consist of

Answer 44

It is spanned by the protruding ends of identical, trans-membrane complexes called connexons, which are aligned end to end to form water-filled channels across the two plasma membranes. These channels allow the passage of inorganic ions and small water-soluble substances below a molecular mass of 1000, to move directly between the cytosol of the two cells, creating an electrical and metabolic coupling between the cells.

Question 45

Describe the importance of the gap junction in cardiac cells

Answer 45

Allows the electrical waves of excitation to spread synchronously throughout the heart, triggering a coordinated contraction

Question 46

Describe the role of the gap junction in synapses

Answer 46

synapses mainly in neural tissue (i.e. not epithelial)
button-like junctions formed between neurones or between neurones and target cells (e.g. muscle)
information passed one-way via a chemical signalling system
a variety of chemical signals and receptors are utilised at synapses
Gap junctions can be open or closed in response to extracellular or intracellular signals.