histology, lecture 3, the cell part 2

8/8/2019 Histology, Lecture 3, The Cell Part 2

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Hey guyz ^^I will continue talking about the cell. Last time I stopped talking about

the cytoskeleton, and I mentioned the three components of the cytoskeleton

which where Microtubules, Microfilaments, and Intermediate filaments. Somicrotubules are rod-like structures, having a dense wall and hollow interior,as shown in this electron micrograph.

Microtubules are composed of a protein called Tubulin. They function in cellform and intracellular transport. They are observed in the spindle of mitosis,since they form the basis for centrioles. You've already learnt about centrioles

in Biology, which are cylindrical structures. Now, centrioles are present in pairsclose to the nucleus of non-dividing cells forming what's called Centrosome.

The second cytoskeletal component is the microfilaments, which arealso known as the actin filaments since they are made of actin (a protein).Now, microfilaments form a thin sheath or network beneath the plasmamembrane - plasma membrane is also called plasmalemma. This network isinvoloved in cell shape changes, such as those occurring during endocytosisand excocytosis. Microfilaments are closely associated with cytoplasm,granules, vesicles and organelles, and they play a role in their movement orshifting, and this is called cytoplasmic streaming. This is an electron

micrograph of a part of the cytoplasm of a cell. You can see two types ofcytoskeletal components; the first one is the microfilaments, and the secondone is the microtubules. As you can see, the microtubules are thicker than themicrofilaments.

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The intermediate filaments have a size that's in between the microfilamentsand the microtubules; they are larger than the microfilaments and smaller

than the microtubules. The intermediate filaments are more stable than themicrotubules and the microfilaments, and they vary in their protein subunitstructure in different cell types, and they provide mechanical strength orstability to cells. This table (am afraid u guyz r gonna have to wait till we getthe slides) shows different types or examples of intermediate filaments. I saidin the previous slide that the intermediate filaments are composed of differentprotein subunit structure in different cells, and so these are examples of theintermediate filaments present in different cells. For example you have thecytokeratins; vimentin, desmin, and glial fibrillary acidic protein GFAP, andneurofilaments, and these are the cell types where you can see such kinds ofintermediate filaments. This is an electron micrograph of a part of a cellcytoplasm. You can see here these ones are the intermediate filaments.

Inclusions are cytoplasmic structures, they are not organelles, they areaccumulated metabolites or other substances. They last for a very short time,that's why they are described as being transitory! They are non-motile andthey have very little or no metabolic activity. These images show examples of

inclusions; the first one in "A" is an electron micrograph showing lipid dropletsor fat droplets, you can see those spherical structures with homogenousmatrices. In "B", you can see glycogen clusters, and they appear as electrondense structures in the electron microscope. Of course, glycogen is acarbohydrate polymer, so you expect it to be positive to PAS (Periodic AcidSchiff). The last one in "C" is for lipofuscin, which is a by-product of lysosomaldigestion that accumulates over time, so here, remember that it's pigmentedand shown under the light microscope, whereas those for the other examplesare shown under the electron microscope.

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Microtubules vs. microfilaments


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THE NUCLEUSThe nucleus regulates the cellular structure and activity by housing thegenetic material, which directs those activities and regulates the structure.Now, there's a production of ribosomal subunits in the nucleolus for exportinto the cytoplasm of the cell. The electron micrograph shown is of a cellnucleus. There are three components of the nucleus which are; the nuclearenvelope, which encloses the nucleus, then the chromatin, and then thenucleolus. Now, you can see that the chromatin is not completelyhomogeneous; you have heterogeneous clumps and then you have thehomogeneous or dispersed form of chromatin.

In the nucleolus as well, you can see two distinct regions; one is darker or

more electron dense than the other and called pars fibrosa pars meanslayer, and fibrosa is a greek way of naming things, so pars fibrosa meansfibrous layer. The second layer which is less electron dense is called parsgranulose, which means granular layer. The well developed nucleolus that ishighly basophilic, and you know that the nucleolus is highly basophilicbecause of it's RNA content.

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A B C


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The following image is just a scanapick representation of the nucleus, so youcan see more details than the ones that are shown in the real electronmicrograph. You can see, as we previously mentioned, that the firstcomponent of the nucleusis the nuclear envelope. The nuclear envelope iscomposed of two membranes with the intervening space; there is the outernuclear membrane, the inner nuclear membrane and then the space as I saidbefore. There are sites where the outer and the inner nuclear membranes fuseforming pores, and these pores allow for the exchange of substances betweenthe nucleus and the cytoplasm since the nuclear envelope is completelyimpermeable to ions and molecules. The outer mitochondrial membrane hasbinding sites for ribosomes, so it has ribosomes attached to it, and it'scontinuous with the rough endoplasmic reticulum. You can see that, and evenif the electron micrograph is just inside the nuclear envelope, you can see athin electron dense layer called the nuclear lamina which is composed ofintermediate filaments called lamin. And the nuclear lamina helps to providestability to the nuclear envelope. Now closely associated with the nuclearlamina is the heterochromatin. Again in the nucleolus, you have pars fibrosaand pars granulosa. So again, the nuclear lamina is a lattice (means a mishwork or network), it's just inside the nuclear envelope, and it's formed by the

lamin, and it has the heterochromatin closely associated to it, that's why thenuclear lamina, and we said before it provides stability to the nuclearenvelope, also regulates chromatin by providing binding sites to it.

So Chromatin is composed of coiled strands of DNA that are bound to basicproteins called histones. The basic structural unit of chromatin is called

nucleosome. Chromatin is condensed into chromosome so it forms the

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microscopically visible structures called chromosomes during meiosis andmitosis.

One colleague asks a question (am sry I cdn't hear it) and the answer was:I want you to understand this really well, because you can benefit from it a lotin the lab, it's one of the bases! Now, this is the nucleus, and this is thenuclear envelope, of course it's not that clear under the light microscope,

however, it's a limiting structure of the nucleus. Now, surrounding thenucleolus is the chromatin, you can see, this first form of chromatin, it's calledEuchromatin, and then you can see these dense dark areas which are theclusters of the heterochromatin. In non-dividing cells, chromatin is moreeuchromatic. (plz refer to the images above)

The Dr. asks if the situation is well understood for us and the answer was no,so she further explains:Chromatin exists or is present in two forms; the first one is calledeuchromatin, and it's the disperse form of chromatin, and it's abundance inthe cell results in it having lightly stained nucleus, not the whole cell, thenucleus is lightly stained whenever chromatin is in the dispersed form calledeuchromatin. Euchromatin is the less coiled portion of the chromosome, andso it has dispersed form of DNA, which is more available for the transcriptionof RNA, therefore you expect euchromatin to be more available or present incells that are more active in protein synthesis.The second form is the heterochromatin. Heterochromatin has tightly coiledDNA, whenever you see the dark clumps in the nucleus these areheterochromatic clumps. So, the abundance of heterochromatin in the nucleusresults in it being darkly stained. Again the heterochromatin has the tightlycoiled DNA, so it has less access to transcription and therefore it's present in

cells that are less active in protein synthesis.

There's a chromatin that's called sex chromatin, why is it called so? Becauseit's one of the sex chromosomes which are the X chromosomes. Sex chromatinis present in females only, and again it's one of the two X chromosomes thatcondenses and remains heterochromatic and genetically inactive. It canappear in different forms in different cells. For example, you have this oralepithelial cell.You can see the plasma membrane here demarcating the cell,this is the nucleus with nuclear envelope, and this is the chromatin,heterochromatin and euchromatin, now you can see this small granuleadhering to the nuclear envelope and this is called sex chromatin. So it can

appear as a small granule adhering to the nuclear envelope. In neutrophils,this is a neutrophil because it has multi-loped nucleus, you can see the sexchromosome or the sex chromatin, as a drumstick projecting from the multi-loped nucleus. Sex chromatin is sometimes called as Barr body.

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There's something called karyotype, which is the number and type ofchromosomes in an individual, and you know that most of the cells in the bodyare somatic, and they have diploid number of chromosomes; so they havepairs of chromosomes (23 pairs of chromosomes), 22 pairs of thosechromosomes are called autosomes, and then the remaining pair is called sexchromosomes. Remember that the chromosomes of each of these pairs havethe same forms of genes and therefore they are called homologous. Spermcells and mature oocytes have half the diploid number present in somatic cellsand therefore they are called haploid, they do not have pairs, they haveindividual chromosomes, they have 22 autosomes and one sex chromosome;because each pair of chromosomes has been separated during meiosis.

Interphase is the period between mitoses (-sz/ plural of mitosis), mitosis is thecell division and so interphase is the period between cell divisions to preparethe cell for the division. So we can expect that during interphase DNA isreplicated, centrosomes and centrioles are duplicated, and then, it's just anobservation, that the nucleolus is commonly seen in the cells duringinterphase.

Mitosis can be subdivided into four phases; prophase, metaphase, anaphase

and telophase. These are important for the lab part; next time in the lab youwill be seeing them under the light microscope. Now, in prophase,centrosomes - we talked about them in the intermediate filaments - they arepairs of centrioles in non-dividing form, so they have duplicated in theinterphase, and in the prophase they are ready now to move and migrate toopposite poles of the cell. Now, what is the aim of mitosis or cell division? It'sto divide the cell in two cells, so keep in mind that all of these phases are nowgoing on in order to make a product of two cells. So originally there was onecentrosome, in the interphase it duplicated, in the prophase those twocentrosomes moved to opposite poles of the cell.So this is the cell, this is one pole and this is the other pole, microtubules of

the mitotic spindle appear, nucleolus disappears because there's no need forthe transcriptional activity which's stopped, and then the nuclear envelopefragments as a result of the nuclear lamina and pore complexes disassembly.Chromosomes condense and become visible, of course each of them consistsof two sister chromatin.

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Now, in metaphase, meta is like mida, so now the chromosomes that youalready have will be aligned in the mida of the cell, which is the equator of thecell. So chromosomes are first attached to the mitotic spindle's microtubulesby protein complexes called kinetochores which are located at a restrictedarea or region of the chromatin called centromere. Now chromosomes arealigned at the equatorial plane of the cell, which is now more spherical.Remember, this is centromere, this is another one, these are the microtubulesof the mitotic spindle and these are the chromosomes. Now the microtubulesof the spindle are attached to the chromosomes by protein complexes calledkinetochores. This is just a scanapick representation, so it's just a drawing.

Under the light microscope you can see the cell, this is a centrosome and thisis another one, these are the chromosomes, these are the microtubules andthis is the mitotic spindle. Of course you cannot see the kinetochores.

In anaphase, ana means separation, so the chromosomes will separate now bybeing pulled by the kinetochores to opposite poles of the mitotic spindle or thecell. At the same time the kinetochores are pulled away toward the poles, thespindle poles also move away from each other.

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Telophase is the last phase, so now we can see the real division. First, themicrotubules depolymerize. The microtubules connect the chromosomes tothe poles of the mitotic spindle, so now they depolymerize because there's noneed for them anymore, right? We have two sets of chromosomes! Then thecell pinches in two by-constructions, there's a belt like contractive ring here atthe equator of the cell, it's construction results in the formation of thecleavage furrow. Now transcription resumes, nucleoli appear, and nuclearlamina and nuclear envelope reassemble.

Here, you have this under the light microscope; it's like two daughter cells thatare connected to each other, and you can recognize the cleavage furrow.

Cell renewal, is the continuous cell division and death of cell in the differenttissues. It occurs in most tissues except for nerve tissue and cardiac musclecells.

Stem cells are important for tissue renewal. They are small population ofundifferentiated cells to serve to renew the differentiated cells of tissue as

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needed. Stem cells divide asymmetrically, producing two cells; the first one isanother stem cell, like the parent cell, and the other one is called progenitor ortransit amplifying cell, which is committed to differentiation. This progenitorcell deposes few more times at more rapid rate, eventually stopping dividingand becoming fully differentiated.

That's all :D

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I had no access to the slides, so I've tried googlin' sm images

Good luck ^^Done by Hadeel Husam Al-Deen

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histology, lecture 3, the cell part 2

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