Citologia, Citology

Cytology

It is useful to understand how the cell is arranged in order to complete its function. Normally, when we speak about cells we have to consider that all around us, we are surrounded by 2 kinds of cells:

• Eukaryotic: are less numerous if compared to prokaryotic cells.
• Prokaryotic

Prokaryotic cells vs eukaryotic cells

Origin of names

Why do we call them prokaryotic and eukaryotic? From Greek, Eu- means "well"; Pro- indicates something that must be complete, because they are not already completed in their organization. This is related to the carion which is the nucleus.

Characteristics of prokaryotic cells

• Prokaryotic: "pro" "karyon" - it lacks nucleic acid.

Difference with eukaryotic: the prokaryotic cell has:

• Single compartment
• Absence of a nucleus covered by a membrane → darker under TEM → They present the nucleoid, the region where the nucleic acid can be found
• Morphology of membrane: prokaryotic is always thicker → 2 layers of membrane à Gram positive à Gram negative
• Do not have structures with membrane as eukaryotic cells
• Presence of cell wall è We do not find organelles surrounded by membranes

Similarities with eukaryotic cells

• They have ribosomes rRNA (similarity)
• They have a cytoskeleton

Presence of the nucleus

Eukaryotic cells do have a nucleus: the nucleus is not an organelle, but it is a compartment that appears darker under TEM. This doesn’t mean that prokaryotic cells don’t provide genetic material. They have it, but the organization is different. In histology, in cytology, the carion is the nucleus; so eukaryotic are cells provided with a cytoplasm and a nucleus. Prokaryotic cells are those in which it is not possible to demonstrate a structured nucleus, a nucleus covered by a membrane. It doesn’t mean that all prokaryotic cells do not have a nucleus; they have nucleic acid, but their organization is completely different from a eukaryotic cell. So we do not find the nucleus because DNA, RNA and so on are not organized in a single structure, but they are dispersed in the cytoplasm.

• In prokaryotic cells we have a single compartment: organelles and nucleic acid.
• In eukaryotic cells, we have a cytoplasm which surrounds the other compartment which is the nucleus.

Morphologically, the nucleus is not an organelle, it is one of the 2 compartments of a eukaryotic cell: cytoplasm and nucleus. They are well separated: there is a double membrane which completely covers genetic material and so the nucleus, forming a different compartment to cytoplasm. → Clearly this is not the only difference with prokaryotes.

Prokaryotic membrane

We know that the cell is surrounded by a membrane which divides the internal part of a cell from the external environment; so in a eukaryotic cell, we have 1 membrane; instead, in prokaryotic cells, we have an external limit particularly constituted. The prokaryotic membrane is:

• Thicker than the eukaryotic one.
• The presence of 2 layers of membrane, which can be similar or completely different.

Prokaryotic cells do not have structures with membranes as eukaryotic cells; however, they present some organelles.

Prokaryotic organelles

Bacteria are prokaryotic cells. In our body, we can identify a huge diversity of microbiome, characterized by gram-positive and gram-negative bacteria (really thick membranes that differ in specific components that allow a different coloration with Gram stain).

In the image:

• In the central part: we see a darker zone but we do not see another membrane à this is called nucleoid; resembles a nucleus but is not surrounded by a membrane.
• We also have ribosome, RNA, nucleic acids;
• The components of the cytoskeleton: microfilaments, microtubules, intermediate filaments.

In prokaryotic cells, we find only structures without a membrane, so in a prokaryotic cell, we do not find, at the moment, organelles provided by a membrane.

Wikipedia says: prokaryotic cells lack a membrane covering the nucleus, mitochondria, and other organelles with membranes. It’s wrong. Prokaryotic organelles can be generally divided into 2 major groups based on the composition of the membrane layer surrounding them. → So there can be membranes surrounding organelles, something similar to the eukaryotic cells.

• Bounded by non-unit membrane protein shell, lipid monolayer → lipid bodies (also eukaryotes) → polyhydroxybutyrate granules (also in eukaryotes) → carboxysomes (no eukaryotes) → gas vacuole

In the images at the microscope, polyhydroxy butyrate granules are surrounded by a membrane (nothing similar to our eukaryotic cells).

• Lipid-bilayer membrane non as eukaryotic → magnetosomes of magnetic bacteria → Photosynthetic membrane → planctomycetes (the internal membrane structure of Plankton.)

Formation of the eukaryotic cells

Eukaryotic cells derive from prokaryotic cells. In the passage from prokaryotes to eukaryotes, the cell membrane had become a bit simpler, losing certain specializations that were not required anymore in the economy of a eukaryotic cell. The most accredited theory is the endosymbiotic theory:

Overview

How does a prokaryotic cell change its morphology to become a eukaryotic cell? It was demonstrated that the prokaryotic cells are more ancient than the eukaryotic ones, so the change was from prokaryotes to eukaryotes (a cell with nucleus and with all the organelles which are located into the cytoplasm). A lot of theories had been proposed. This kind of process seems to be the most possible:

1. Starting from a cell, which has the genetic material in its internal part, first of all, the cellular membrane simplifies: loses 1 of its 2 layers, generally the external part.
2. Owing to the presence of ribosomes and nucleic acid, there was a process of growing of the membrane, so the membrane modifies its dimensions.
3. It folds, comes into the cell and "simply" digests the nucleoid, the genetic material. In this way, we started from a cell without a nucleus and slowly we obtained a cell provided with the nucleus.

We have to demonstrate that in a living eukaryotic cell similar processes can occur, so that it is possible that a prokaryotic cell becomes a eukaryotic one. Up to now we were able to show classical organelles into a prokaryotic cell and in particular we were able to demonstrate the presence of mitochondria, which is an energy-producing organelle. They have a double membrane à internal and external. They are provided with their own DNA, genetic material. è So the endosymbiotic theory, which says that aerobic prokaryotic cells penetrate into the prokaryotic cell becoming eukaryotic, is possible. So our eukaryotic cells are derived from a prokaryotic, but due to the morphological and functional changes, which occur during this period, the cell receives another prokaryotic cell which was able to stay in the cell, to have a symbiotic relation with the cell and showing characteristics typical of the prokaryotic cells. Just to have an idea, a prokaryotic cell doesn’t divide by mitosis, but by creating a double structure from a single, like mitochondria.

Endosymbiotic theory

1. The plasma membrane enfolded/invaginated into the cell. Prokaryotic cells have the nucleoid at its center. The cellular membrane is able to make infoldings for example during the process of phagocytosis or in general, endocytosis, so that some material which is external can be put into the cell owing to the unfolding of the membrane. So in this moment, we have the unfolding of the membrane, many infoldings. (Differently, the cell membrane of a mitochondrion, is more sticky, blocked)
2. The invaginations became vesicles that flattened and moved towards the center of the cell, thereby surrounding the genetic material and causing a reduction of the cell’s size. This round vesicle, formed by the membrane, loses the connection with the membrane, migrates towards the nucleoid, the genetic material, and changes its appearance. During this movement, they became longer and flatter. Which is the consequence? The cell membrane has become smaller because it has lost many parts. This flattened vesicles continue the migration, so that they are located all around the nucleoid. But is it true? Possibly.
3. A non-continuous nuclear envelope was formed, characterized by a double bi-layer. The vesicles at the beginning have a round form, but going towards the internal part of the cell, they tend to become flat, cisternae, and they would tend to get close to the genetic material forming the nuclear envelope. The nuclear envelope it’s not a continuous ring, but is filled with nuclear pores, a system of connection between the nucleus and the cytoplasm. Material can pass from the nucleus to the cytoplasm and vice versa. No nuclear membrane, but Envelope, because the covering is formed by a double membrane, so there are the external nuclear membrane, the internal one, and the nuclear cisternae, the space between the 2 membranes. A lot of these cisternae are able to cover the nucleus. è Now we have the characteristics of eukaryotic cells. We have a nuclear compartment and a cytoplasmic compartment. The cytoplasm lacks membrane pores, those which characterized the prokaryotic cells.
4. The nuclear envelope grew and forms the ER. After the interaction between ER and ribosomes, a distinction among RER and SER became clearer. From the nuclear envelope, from the membrane of the nucleus, there is a synthesis of new membrane, which grows towards the cytoplasm. The new membrane divides into:
• P SER: Some of them maintain the morphology
• P RER: some of them instead are rich of ribosomes → so the ribosomes which are free in the cytoplasm adhere to the newly formed membrane. → In this way we obtain the SER and RER → So the first structure formed are the reticules. There is a growing of the reticular. We know that in the reticula there is either the production of proteins, or the production of lipids, glucids and so on.
5. Following the same process, Golgi apparatus, vesicles, and other components were formed. To form proteins, glycoproteins… so we have the growing of these membranous structures, so that the Golgi apparatus is formed.
6. The cell received another prokaryotic cell that was able to create an endosymbiotic relationship with the larger cell and give energy to it. At this point the cell was provided with membranous organelles but was still not able to utilize O₂, and anaerobiosis does not provide the cell enough energy to evolve. Aerobiotic bacteria penetrated the cell, and instead of being digested, they started a symbiotic relationship.
7. The cell regained its size: Parts of RE and Golgi tend to detach from the structure under them so the vesicles can:
• Remain in the cytoplasm becoming transfer vesicles →
• Going to the surface of the cell becoming secretory vesicles

The secretory vesicles returned to the plasma membrane and exit the cell by exocytosis → So these vesicles go towards the membrane, fuse its membrane with the cellular membrane, empty its content into the external environment, but the membrane remains attached to the cellular membrane à growing of the cell. → This is an important passage in the maintenance of an equilibrium in order to maintain the right size of the cells. In the newly formed eukaryotic cell, nucleus and cytoplasm communicate and are filled with filaments (nucleoskeleton and cytoskeleton). The growth of a cell is not only due to the exocytosis, but due to the increasing of structures which are connected to the cell, like RER, SER, Golgi. The appearance of them enlarges the cell. This is a hypothetical process. This occurs regularly into a cell, because a cell is able to bring substances into the cytoplasm, elaborate them, and then secrete them in the external environment. → These are the basis of the Secretory process.

Incongruences of the theory nuclear envelope

• The nuclear envelope, in reality, has no continuity with the membrane, and it has nuclear pores.
• Nuclear envelope is formed by a doubled membrane → One is internal and the other is external, they are separated by a space called cisternae.

When a cell divides, the nuclear envelope "disappears", because DNA changes its organization and becomes chromosomes. Chromatin of chromosome must migrate to the opposite pole of the cell. The presence of the nuclear envelope here is impossible, so it becomes rough endoplasmic reticulum. At the end of the process, the RER goes around the DNA, so that the nuclear envelope appears again. So the nuclear envelope is not a static structure and it is not correct to say that it disappears. After the cytodieresis, the RER goes around the DNA, so that the nuclear envelope re-appears. So the nuclear envelope is not a static structure. This is the demonstration that membranes can have different forms, organization.

Images of an eukaryotic cell - TEM

1. In the internal part, there is the nucleus:
• Surrounded by a double membrane (discontinuous due to the pores that are a connection between cytoplasm and nucleus in a double way).
• Nucleus formed only by 2 structures: genetic material (DNA) called chromatin (mix of proteins and nucleic acid) and a second part which is not always evident in the cell, which is the nucleolus formed by RNA.
2. The external part is formed by the cytoplasm:
• Surrounded by a single membrane (continuous-no pores),
• Characterized by the presence of many organelles. è 2 compartments (nucleus and cytoplasm) that are morphologically completely different from one another.

Cell shape (eukaryotic)

• Round shape: Cells isolated in a fluid environment, not interacting with other cells (such as blood cells, morula)
• Geometrical shape: Cells that are brought together and form junctions (such as epithelial cells) and have a space between them no lesser than 200 nm
• Some cells have not a defined structure (such as neurons).

We can describe them based on its parts:

• Ex. Neuron: formed by a round part in which the nucleus is present; in the center we have the nucleus and the nucleolus (a small white dot). → This is not a description of shape, but a morphological description of the cell.
• Skeletal muscle fibers: it is a cylindrical structure. In general, cells near one another acquire a polyhedral morphology
• Polygonal appearance: if the cells are close together, so that one is compressed →by others, distance between cells is lesser than 200 nm

Cell size

Normally, an animal cell size is between 10 and 100-120 micrometers. No human cell can be observed with our eyes, even the oocytes. The bare eye has a resolution of 0.2 mm/200 μm, so no cell can be seen without a microscope. There isn’t a normal size for a cell; every type of cell has its specific size.

Why are most eukaryotic cells from 10 mc and 100 mc?

• 10 microm: lower limit: under this size, something essential will not fit (The example is the red blood cell, which loses the nucleus, becomes smaller than 10 micrometers but has a lifelong 120 days after which it dies. When a cell is smaller, there is no possibility to survive)
• 100 microm: Upper limit: without enough surface area for its size, such a large cell will not be able to supply its own needs → If the cell is particularly large, the membrane is large too, but the organelles are not enough to elaborate on the material which is brought from the outside. So, the cell would suffer from too much material which is not possible to be organized, produced, and so on.

Examples:

• Oocyte (largest cell in the human body) = 120/150 μm; The largest cell such as unfertilized eggs is metabolically suppressed, so the metabolism is very low. In fact, when they are fertilized, and have to produce the embryo, they became smaller, to obtain the ratio between nucleus and cytoplasm. → but within minutes after fertilization, the single large frog egg cell begins to divide and divide. Soon the original volume of the egg has been divided into hundreds of cells no larger than 100 microm
• Epithelial cells = 10/15 μm;
• Sperm cells = 4*60/80 μm.
• Elements such as Platelets and Erythrocytes = 2/4 μm and 7.5/10 μm; (A properly-said Cell contains both a nucleus and a cytoplasm, otherwise is called an element) è Red blood cell, is not a cell, as it lacks the nucleus, is 7 micrometers, which is less than 10 micrometers. è Platelets are not cells, are part of the cytoplasm of other cells, so they’re 3-5 micrometers is possible because they are not cell.

Ratio between nucleus and cytoplasm

Volume of the cytoplasm and of the nucleus must be 1:1, → nearly the same quantity of nuclear material and cytoplasmic material. This ratio regulates many processes in our organism:

• Immediately after fertilization, so when the zygote is formed, the zygote has the same volume of the oocyte and a small nucleus. So the process that occurs nearly after fertilization is regulated by the necessity of obtaining the same quantity of nucleus and cytoplasm. The oocyte, the largest one, even if the nucleus is large enough, is nothing compared to the cytoplasm. → So the next division has to bring a cell in which nucleus and cytoplasm have the same ratio (or dimension).

→ Eukaryotic cell observed with TEM. Cells of the connective tissue.

• Collagen: All around the cells we find collagen fibers which are the fibrous part of the connective tissue.
• Nucleus surrounded by membrane: We see the nuclear material, what we called chromatin, morphologically, DNA + proteins. Chromatin, (imagine) can appear differently:
  • There is a clear chromatin (euchromatin): is active in synthesizing chromatin
  • Darker part (heterochromatin): is inactive genetic material
• Cytoplasm filled with organelles
• The cell has structures all around it, so it’s not a polygonal structure, but has some material located.