Histology Wk 7

Question 1

General function of cytoskeleton

Answer 1

1. Structural: Provides structural support to cell; stabilizes junctions between cells
2. Movement: Assists with cytosol streaming and cell motility; helps move organelles and materials throughout cell; helps move chromosomes during cell division

Question 2

Function of microtubule

Answer 2

Maintain cell’s shape and polarity; provide tracks for organelle and chromosome movement; move cilia and flagella

Question 3

Structure of microtubules

Answer 3

Microtubules are also organized into larger, more stable arrays called axonemes

Diameter of 25 nm

the protein subunit of a micro- tubule is a heterodimer of α and β tubulin, each with a molecular mass of about 50 kDa

Question 4

Polymersiation of microtubules

Answer 4

The tubulin subunits align lengthwise as protofilaments, with 13 parallel protofila- ments forming the circumference of each microtubule wall.

Polymerization of tubulins is directed by microtubule organizing centers (MTOCs), which contain short assem- blies of tubulin that act as nucleating sites for further polym- erization. Microtubules are polarized structures, with growth (polymerization) occurring more rapidly at the (+) end

Question 5

Dynamic instability

Answer 5

Microtubules show dynamic instability, with continuous cycles of polymerization and depolymerization at steady-state conditions, which depend on concentrations of tubulin, Ca2+, Mg2+, and the presence of various microtubule- associated proteins (MAPs). Energy for assembly is derived from GTP bound to incoming tubulin subunits

Question 6

Polarity of microtubules

Answer 6

+ AND - ENDS

Question 7

Centrosomes

Answer 7

The dominant MTOC in most cells is the centrosome, which is organized around two cylindrical centrioles, each about 0.2 μm in diameter and 0.3-0.5 μm in length

Question 8

What are motor proteins

Answer 8

Transport along microtubules is under the control of proteins called motor proteins, which use ATP in moving the larger structures

Question 9

What are kinesins

Answer 9

Kinesins carry material away from the MTOC near the nucleus toward the plus end of microtubules (anterograde transport)

Question 10

What are cytoplasmic dyeneins

Answer 10

carry material along microtubules in the opposite direction (retrograde transport), generally toward the nucleus.

Question 11

What are the functions microfilaments

Answer 11

Contract and move cells; change cell shape; cytokinesis; cytoplasmic transport and streaming

Question 12

Structure of microfilaments

Answer 12

Microfilaments are composed of actin subunits and allow motility and most contractile activity in cells, using reversible assembly of the actin filaments and interactions between these filaments and associated myosin family proteins.

Question 13

What are they composed of

Answer 13

They are composed of globular G-actin monomers that assemble in the presence of K+ and Mg2+ into a double-stranded helix of filamentous F-actin

Another nucleating factor, a complex of polypeptides called Arp2/3, also binds to the side of preexisting actin fila- ments and induces a new F-actin branch, a process which can lead to formation of a microfilament network.

Question 14

Polarity of microfilaments

Answer 14

actin filaments are highly dynamic. Monomers are added rapidly at the (+) or barbed end, with ATP hydrolysis at each addition; at the same time monomers dissociate at the (−) or pointed end. This leads to migration of subunits through the polymer, which occurs rapidly in a pro- cess called treadmilling

Question 15

What is the cytoplasmic region called

Answer 15

Cell cortex

Question 16

What are stress fibres

Answer 16

Cell movements on firm substrates involve sheet-like protrusions, or lamel- lipodia, in which the concentrated actin filaments are con- tinuous with deeper parallel F-actin bundles

Question 17

What do actin binding proteins do

Answer 17

Actin-binding proteins, such as formin and others just mentioned, change the dynamic physical properties of micro- filaments, particularly their lengths and interactions with other structures, and this determines the viscosity and other mechanical properties of the local cytoplasm

Question 18

What do myosin motors do

Answer 18

Use ATP to transport cargo along F actin

Question 19

What are the basic cell movements

Answer 19

Transport of organelles, vesicles, and granules in the pro- cess of cytoplasmic streaming
■ Contractile rings of microfilaments with myosin II constricting to produce two cells by cytokinesis during mitosis

Membrane-associated molecules of myosin I whose movements along microfilaments produce the cell sur- face changes during endocytosis

Question 20

Function of intermediate filaments

Answer 20

Strengthen cell and tissue structure; maintain cell shape; maintain nuclear shape (lamins)

Question 21

Subunits of IFs, structure and monomers

Answer 21

-Antiparallel tetramers of two rod-like dimers
-Cable of four intertwined protofibrils, each consisting of bundled tetramers associated end-to-end
-8-10 nm
-Various α-helical rod-like proteins (~55 kDa, Table 2–5)
-No apparent polarity

Question 22

Polarity and general locations of IFs

Answer 22

No apparent polarity
Stable
Arrayed throughout cytoplasm; at desmosomes; inside nuclear envelope

Question 23

Intermediate filament proteins with particular biological, histological, or pathological importance include the following:

Answer 23

Keratins or cytokeratins - Intermediate filaments of keratins form large bundles (tonofibrils) that attach to certain junctions between epithelial cells. In skin epidermal cells, cytokeratins accumulate during differentiation in the process of keratinization, producing an outer layer of nonliving cells that reduces dehydration. Keratiniza- tion of skin made terrestrial life possible in the course of evolution. Keratinization also provides some protec- tion from minor abrasions and produces various hard protective structures of skin, such as nails

Question 24

Vimentin

Answer 24

Vimentin is the most common class III intermediate
filament protein and is found in most cells derived from embryonic mesenchyme. Important vimentin-like pro- teins include desmin found in almost all muscle cells and glial fibrillar acidic protein (GFAP) found espe- cially in astrocytes, supporting cells of central nervous system tissue.

Question 25

Neurofilament

Answer 25

Neurofilament proteins of three distinct sizes make heterodimers that form the subunits of the major inter- mediate filaments of neurons.

Question 26

Lamins

Answer 26

Lamins are a family of seven isoforms present in the cell nucleus, where they form a structural framework called the nuclear lamina just inside the nuclear envelope