Citologia, Citology

INTRACELLULAR ORGANELLES

If I look at this image, I know that this part of a cell is seen under TEM.

• N= nucleus;
• Eu= euchromatin;
• ET= heterochromatin;
• SER= small endoplasmic reticulum;
• RER= rough endoplasmic reticulum;
• M= mitochondrion;
• L= lysosome;
• V= vacuole;
• R= ribosomes.

Cytoplasm = is formed by Hyaloplasm + Organelles→Hyaloplasm: it’s not a cytoplasmic organelle it’s a compartment of the cell

Organelles are divided into:

1. Organelles enclosed by a membrane

The cellular membrane is able to form invaginations into the cytoplasm, so that these organelles are formed. Among these organelles, there is a continuous exchange of material: for example from the RER to the Golgi, from the Golgi to the lysosomes or secretory vesicles and so on. So it’s not a static organization, but it’s dynamic.

• Nuclear envelope
• ER
• Golgi
• lysosomes
• peroxisome
• mitochondria
• vesicles and inclusion bodies
2. Organelles formed by microfilaments
• Cytoskeleton
• centrioles

Others

Nucleolus (formed by a highly dense part and a less dense part – easy to see with TEM)

Ribosomes → they do not share with others either the membranous structure or filamentous structure.

Hyaloplasm

The hyaloplasm (cytosol) is the cytoplasmic ground, so it is a portion of cytoplasm which surrounds organelles.

Organelles swim into the hyaloplasm. Organelles are not free to move, because they are reciprocally connected, so only specific movements are allowed.

Water is the main constituent of hyaloplasm.

Hyaloplasm is also the medium by which the substances which have been absorbed by the cell are driven all along the cells to reach their destination.

For example, in protein synthesis (cell absorbs the amino acid from the hyaloplasm for synthesizing new polypeptide chains), or when amino acids move into hyaloplasm, because they have to reach the ribosomes; or the mRNA it’s not brought into the cytoplasm through vesicles, but is able to move into the hyaloplasm, because

it has no relations with other organelles. So the hyaloplasm is a kind of main road into which free substances (not enclosed in membranes) are able to move. It is also used to take and drive material by the cell (the amino acids during protein synthesis)

It consists of:

• Water (main constituent)
• Salts
• Macromolecules
• Low molecular substances (amino acids, mono and oligosaccharides)
• Ions
• Phosphate + chloride ions

During dehydration, the water and the fixative which are contained into the organelles, but mainly into the hyaloplasm are pulled away so that ethanol can enter into the cell, into the hyaloplasm and into the organelles.

The hyaloplasm is considered the main structure located into the cell. So if a cell possesses a well hyaloplasm organization, this means that the cell is well structured.

1. ORGANELLES SURROUNDED BY A MEMBRANE

NUCLEUS

The nucleus is not a cellular organelle, otherwise it is a cellular compartment, because it is separated from the cytoplasm through a particular membrane.

nuclear envelope

Studying image:

• It comes from a TEM microscope
• In the center we find the nucleus
• →In the center, the nucleus is formed mainly by euchromatin high cellular activity
• Nucleolus: is expression of high cellular activity
• Small concentration of heterochromatin in the periphery

How to recognize euchromatin and heterochromatin

• → →very dark part heterochromatin
• → →lighter part euchromatin

Nuclear envelope

Surrounded by cytoplasm, formed exclusively by RER in this cell. If I make an highermagnification of the nuclear envelope, first of all:

• It is formed by 2 membranes
• →It is not a continuous layer because it contains nuclear pores they allow→substances to pass from the cytoplasm to the nucleus and vice versa thesemolecules are found in the hyaloplasm
• very plastic structure, in fact we can describe nuclear envelope only when the cell isnot dividing, because during cellular division the structure of the nucleus iscompletely changed: there is

The appearance of the chromosomes, and as the chromosomes are pushed in different ways, the presence of a nuclear envelope would make the process of migration of chromosomes into the daughters cells impossible so the cell needs a structure that is able to change the nuclear envelope organization. In the image, the cell is under metaphase of a mitotic division and the chromosomes (in the center) are not surrounded by nuclear envelope. The nuclear envelope changes over the different phases of cellular division and interphase.

We can notice that there is a tight relationship between the nuclear envelope and the transformation from the chromatin into chromosomes, so if we have chromatin we need a nuclear envelope, if in the nucleus chromosomes appear, the presence of the nuclear envelope is no longer necessary. It is compulsory that the envelope disappears during cellular division because otherwise the proper division and organization of the chromosomes would not be.

anastomosis. anastomosis in anatomy means that blood vessels are connected together → so in general the SER is formed by tubules connected one to the other that form a 3D structure

Surfaces: →We do not find any other structure the surface has a smooth structure

Functions:

• Steroidogenesis: Synthesis of lipids, phospholipids, steroids and sex hormones.
• →Glycogenesis and gluconeogenesis Hydrolysis of glycogen into glucose in liver
• Detoxification of drugs and poisons in the liver
• Storage and control the movement of calcium ions.Ca2+
• Pumping of in the sarcoplasm of muscle fibers.

In muscle cells, where it is called sarcoplasmic reticulum, SER is involved in the storage and release of calcium ions that activate the contractile mechanism

The SER is well developed in muscle fibers (skeletal muscle and cardiac muscle, where it is called sarcoplasmic reticulum), in steroids synthesizing cells, such as seminiferous tubules in testis, and liver, where it has a detoxifying function.→

From the function of an organelle and organ, we can derive if that specific organelle is more or less present. For example, Leydig cells (in the image above) that produce testosterone have a large smooth endoplasmic reticulum (SER) image.

In the upper left part, we have a histological section under Light Microscopy. We have the cells producing gametes, but between the tubules where spermatogenesis occurs, we find groups of cells called Leydig cells that produce testosterone. Under TEM, I see a large number of tubules of the SER.

Rough endoplasmic reticulum (RER): we see elongated structures. It is not an anastomosis structure, but the RER is formed by flat structures. The nuclear envelope is also flat, and the internal space is called perinuclear cisternae. Cisternae are flat structures formed by the cellular membrane. In general, the RER is formed by flat structures called cisternae, where on their surfaces, we find ribosomes.

Surface: Ribosomes attached to the external face of cisternae (black dots).