Histology Wk 7 Flashcards
General function of cytoskeleton
- Structural: Provides structural support to cell; stabilizes junctions between cells
- Movement: Assists with cytosol streaming and cell motility; helps move organelles and materials throughout cell; helps move chromosomes during cell division
Function of microtubule
Maintain cell’s shape and polarity; provide tracks for organelle and chromosome movement; move cilia and flagella
Structure of microtubules
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
Polymersiation of microtubules
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
Dynamic instability
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
Polarity of microtubules
+ AND - ENDS
Centrosomes
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
What are motor proteins
Transport along microtubules is under the control of proteins called motor proteins, which use ATP in moving the larger structures
What are kinesins
Kinesins carry material away from the MTOC near the nucleus toward the plus end of microtubules (anterograde transport)
What are cytoplasmic dyeneins
carry material along microtubules in the opposite direction (retrograde transport), generally toward the nucleus.
What are the functions microfilaments
Contract and move cells; change cell shape; cytokinesis; cytoplasmic transport and streaming
Structure of microfilaments
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.
What are they composed of
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.
Polarity of microfilaments
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
What is the cytoplasmic region called
Cell cortex
What are stress fibres
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
What do actin binding proteins do
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
What do myosin motors do
Use ATP to transport cargo along F actin
What are the basic cell movements
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
Function of intermediate filaments
Strengthen cell and tissue structure; maintain cell shape; maintain nuclear shape (lamins)
Subunits of IFs, structure and monomers
-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
Polarity and general locations of IFs
No apparent polarity
Stable
Arrayed throughout cytoplasm; at desmosomes; inside nuclear envelope
Intermediate filament proteins with particular biological, histological, or pathological importance include the following:
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
Vimentin
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.
Neurofilament
Neurofilament proteins of three distinct sizes make heterodimers that form the subunits of the major inter- mediate filaments of neurons.
Lamins
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
