CIU Histo 3 PDF - Cell Biology & Histology Notes

Cytoskeleton, Intermediate filaments, Cytoplasmic inclusions Nucleus Used literature Junqueira’s Basic Histology 15th ed. 42-51; 53-58 BRS Cell Biology Histology 8th ed. Cytoskeleton Complex array of: microtubules, microfilaments (actin filaments) and intermediate filaments. Can only be seen via...

Cytoskeleton, Intermediate filaments, Cytoplasmic inclusions Nucleus Used literature Junqueira’s Basic Histology 15th ed. 42-51; 53-58 BRS Cell Biology Histology 8th ed. Cytoskeleton Complex array of: microtubules, microfilaments (actin filaments) and intermediate filaments. Can only be seen via electronic microscope. Determine shape of cells, play an important role in the movement of organelles and vesicles and even whole cells. Microtubules Organized in larger, more stable arrays called axonemes in the cytoplasmic extensions called cilia (“hair” almost all cells have one) and flagella. Microtubules are hollow, with an outer diameter of 25nm and a wall 5nm thick, which helps maintain cell shape. Can be linked by various proteins. Length is dynamic and variable and can be many micrometers long. The microtubules are polymers and are made of α and β tubulins (+&-). Their polymerization (assembly) is directed by microtubule organizing centers (MTOCs). This process happens when Ca and Mg Ions and various microtubule associated proteins are present. The dominant MTOC in most cells is the centrosome, it is organized around two cylindrical centrioles, each about 0.2μm in diameter and 0.3-0.5μm in length. Centrioles Each centriole is composed of nine highly organized microtubule triplets. Before cell division, specifically during the period of DNA replication, centrosomes duplicate and each centrosome has two pairs of centrioles. During mitosis centrosome divides in halves and each moves to the opposite end of the cell and become organizing centers for the of the mitotic spindle. Microtubule transport Microtubules also form part of the system for intracellular transport of membranous vesicles, macromolecular complexes, and organelles. Some examples: axoplasmic (axon cytoplasm) transport in neurons, melanin transport in pigment cells, chromosome movements by mitotic spindle and vesicle movements. These transport is carried out by motor proteins and utilizes ATP. Kinesins carry material away from the MTOC near the nucleus toward the plus end of microtubules (anterograde transport), while dyneins carry material along microtubules in the opposite direction (retrograde transport). (Some viruses such as Herpes and Rabies abuse this system) This system also extends ER from the nuclear envelope to plasmalemma and moves vesicles to and through the Golgi apparatus. Some drugs that can disrupt activity of mitotic spindle can be used in chemotherapy (vinblastine, vincristine, paclitaxel) Microfilaments (Actin Filaments) Abundant in all cells. Composed of actin sub-units and allow contraction of cells. Associated with myosin protein family. 5-7nm in diameter, polarized polymers, shorter and more flexible than microtubules, assemble in presence of K and Mg. Like microtubules 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. (exception myosin VI). This leads to migration of subunits through the polymer, which occurs rapidly in a process called treadmilling. Just as the molecular motors kinesin and dynein move structures along microtubules, various myosin motors use ATP to transport cargo along actin. Actin-myosin interactions are needed for: Transport of organelles, vesicles, and granules in the process of cytoplasmic streaming. Cytokinesis during cell splitting, endocytosis and forceful muscle contrations. Intermediate Filaments Intermediate in size between the other two. More stable and allows for mechanical stability of the cell. Some more specialized intermediate filaments include: Keratins: cytokeratins. Has acidic and basic forms, in all epithelial cells. Produce filaments with different chemical and immunologic properties. Some form large bundles (tonofibrils) and attach to certain junctions between epithelial cells. In some epithelial cells (skin) keratin accumulates (keratinization) and produces outer layer of dead cells for protection. Also produces various hard protective structures of skin, such as nails (as well as feathers, beaks, horns, and the scales of reptiles). Intermediate Filaments Vimentin: most common class III intermediate filament protein and is found in most cells derived from embryonic mesenchyme (connective tissue). Important proteins include desmin found in muscle cells and glial fibrillar acidic protein (GFAP) common in astrocytes (CNS supporting cells). Neurofilament: proteins of three distinct sizes make heterodimers that form the sub-units of the major intermediate filaments of neurons. Lamins: present in the cell nucleus, where they form a structural framework called the nuclear lamina just inside the nuclear envelope. Inclusions Are accumulated metabolites or other substances but have little to no metabolic activity. Most are transitory and have no membrane. Commonly seen variants are: Lipid droplets: accumulations of lipid filling adipocytes (fat cells) and present in various other cells. Glycogen granules: aggregations of glucose polymer, common in hepatocytes. Melanin: dark brown granules which in skin serve to protect cells from UV radiation. Lipofuscin: age pigment. Pale brown granule found in many cells, especially in stable nondividing cells (eg, neurons, cardiac muscle), containing a complex mix of material partly derived from residual bodies after lysosomal digestion. Hemosiderin: a dense brown aggregate of denatured ferritin proteins with many atoms of bound iron, prominent in phagocytic cells of the liver and spleen, where it results from phagocytosis of red blood cells. Nucleus; Nuclear envelope Nucleus is main structure of cell as it contains DNA (a code for all the cell proteins). Components include: Nuclear envelope, chromatin, nucleoli. Envelope: Selectively permeable barrier, between the nuclear and cytoplasmic compartments. Electronic microscopy shows 2 membranes, separated by 30-50nm perinuclear space. Outer membrane is continuous with RER, while Inner membrane is closely associated with meshwork of proteins - nuclear lamina (for stabilization). Major components of this layer are intermediate filament proteins called lamins. These two layers are bridged by 3000-4000 nuclear pore complexes made of nucleoporins. Proteins have specific import and export sequences that bind to transport proteins (importins, exportins), which in turn interact with pore complex proteins and use GTP. Chromatin Consists of DNA and all the associated proteins involved in its organization and function. Divided into 23 pairs of chromosomes (except gametes), each consisting of 2 identical chromatids connected by cohesin proteins. DNA is approx. 2m long with 3.2 billion base pairs and must be packaged in nucleus. This is done firstly by wrapping 150bp of DNA around the histone proteins and forming nucleosome, which has 4 histone paiir core (H2A,H2B,H3 and H4) and H1 outside. In electronic microscope nucleosomes an 50-80bp linker DNA (between them) have “beads on a string” appearance. They are dynamic, and their rearrangement allows DNA to be “unwrapped” and transcribed. Chromatin DNA in nucleosomes is further coiled with multiple steps and become visible via light microscopy during mitosis. 2 types of chromatin exist, euchromatin and heterochromatin. First being the gene rich “open” version of chromatin, that can be easily transcribed – “gene rich”, while heterochromatin has little to no transctiptional activity and is “gene poor”. Two types of heterochromatin can be observed in the chromosomes: Constitutive – mainly similar in all cells and contain repetitive sequences (centromeres, telomeres). While facultative is inactivated but can be “opened” and transcribed if necessary. Chromatin The ratio of heterochromatin to euchromatin seen with nuclear staining can provide a rough indicator of a cell’s metabolic and biosynthetic activity. Euchromatin is abundant in active cells – neurons, while cells with little synthetic activity, like circulating lymphocytes, have more heterochromatin. Facultative heterochromatin also occurs in small, dense “sex chromatin” or Barr body which is one of two X chromosomes present only in females. Recent studies have shown that heterochromatin tends to be located near the nuclear lamina, and that different chromosomes occupy different chromosomal territories, with more active domains located deeper. Karyotype is a microscopic image of chromosomes that allows to see any numerical abnormalities. Nucleolus A generally spherical, highly basophilic subdomain of nuclei in cells actively engaged in protein synthesis. This basophilic nature comes from high amounts of rRNA that is transcribed, processed, and assembled into ribosomal sub-units. Common cells with intensive protein synthesis and therefore high rRNA demand. The rRNA sub-units are matured and quickly associated with proteins, after which newly organized sub-units are exported in the cytoplasm via nuclear pores.