Histology. Morphology and functions of the nucleus. Cell reproduction

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Histology. Morphology and functions of the nucleus. Cell reproduction

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Topic 5. MORPHOLOGY AND FUNCTIONS OF THE NUCLEUS. CELL REPRODUCTION

The human body contains only eukaryotic (nuclear) cell types. Nuclear-free structures (erythrocytes, platelets, horny scales) are secondary formations, since they are formed from nuclear cells as a result of their specific differentiation.

Most cells contain a single nucleus, only rarely are binucleated and multinucleated cells. The shape of the nucleus is most often rounded (spherical) or oval. In granular leukocytes, the nucleus is subdivided into segments. The nucleus is usually localized in the center of the cell, but in the cells of the epithelial tissue it can be shifted to the basal pole.

Structural elements of the nucleus are clearly expressed only in a certain period of the cell cycle - in interphase. During cell division (mitosis or meiosis), pronounced changes in cell structures occur: some disappear, others are significantly transformed.

Structural elements of the core

The structural elements of the nucleus listed below are well expressed only in the interphase:

1) chromatin;

2) nucleolus;

3) karyoplasm;

4) karyolemma.

Chromatin is a dye-receptive substance (chromos), hence its name. Chromatin consists of chromatin fibrils 20–25 km thick, which can be loosely or compactly located in the nucleus.

On this basis, euchromatin can be distinguished - loose (or decondensed) chromatin, weakly stained with basic dyes, and heterochromatin - compact (or condensed) chromatin, well stained with basic dyes.

In preparing the cell for division in the nucleus, chromatin fibrils spiralize and chromatin is converted into chromosomes. After division in the nuclei of daughter cells, despiralization of chromatin fibrils occurs, and the chromosomes are again converted into chromatin. Thus, chromatin and chromosomes are different states of the same substance.

According to the chemical structure, chromatin consists of:

1) deoxyribonucleic acid (DNA) - 40%;

2) proteins - about 60%;

3) ribonucleic acid (RNA) - 1%.

Nuclear proteins are presented in two forms:

1) alkaline (histone) proteins - 80 - 85%;

2) acidic proteins - 15 - 20%.

Histone proteins are associated with DNA and form a deoxynucleoprotein, which is a chromatin fibrils, clearly visible under electron microscopy. In certain areas of chromatin fibrils, transcription from DNA to various RNA is carried out, with the help of which the synthesis of protein molecules subsequently takes place. Transcription processes in the nucleus are carried out only on free chromosomal fibrils, i.e., on euchromatin. In condensed chromatin, these processes are not carried out, therefore, heterochromatin is called inactive chromatin.

The ratio of euchromatin and heterochromatin is an indicator of the synthetic activity of the cell. DNA replication occurs on chromatin fibrils in the S-period of interphase. These processes can also occur in heterochromatin, but much longer.

The nucleolus is a spherical formation (1 - 5 microns in diameter), which perceives basic dyes well and is located among the chromatin. One nucleus can contain from 1 to 4 or even more nucleoli. In young and frequently dividing cells, the size of the nucleoli and their number are increased. The nucleolus is not an independent structure. It is formed only in interphase, in certain regions of some chromosomes - nucleolar organizers, which contain genes encoding a ribosomal RNA molecule. In the region of the nucleolar analyzer, transcription from DNA is carried out. In the nucleolus, ribosomal RNA combines with protein and the formation of a subunit of the ribosome.

Microscopically in the nucleolus distinguish:

1) fibrillar component (localized in the central part of the nucleolus and is a thread of ribonucleoprotein (RNP));

2) granular component (located in the peripheral part of the nucleolus and is an accumulation of ribosome subunits).

In the prophase of mitosis, when the spiralization of chromatin fibrils and the formation of chromosomes occur, the processes of RNA transcription and synthesis of the ribosome subunit cease, and the nucleolus disappears. At the end of mitosis, decondensation of chromosomes occurs in the nuclei of newly formed cells, and a nucleolus appears.

Karyoplasm (nucleoplasm or nuclear juice) consists of water, proteins and protein complexes (nucleoproteins, glycoproteins), amino acids, nucleotides, sugars. Under a light microscope, the karyoplasm is structureless, however, with electron microscopy, small granules (15 nm) consisting of ribonucleoproteins can be found in it. Karyoplasmic proteins are mainly enzyme proteins, including glycolysis enzymes that break down carbohydrates with the formation of ATP.

Non-histone proteins (acidic) form a structural network in the nucleus (nuclear protein matrix), which, together with the nuclear envelope, takes part in creating the internal environment.

With the participation of karyoplasm, the metabolism in the nucleus, the interaction of the nucleus and cytoplasm are carried out.

The karyolemma is a nuclear envelope that separates the contents of the nucleus from the cytoplasm (barrier function), while at the same time ensuring a regulated metabolism between the nucleus and the cytoplasm. The nuclear envelope is involved in the fixation of chromatin.

The karyolemma consists of two bilipid membranes, the outer and inner nuclear membranes, separated by a perinuclear space 20–100 nm wide. The karyolemma has pores 80–90 nm in diameter. In the pore region, the outer and inner nuclear membranes pass into each other, and the perinuclear space is closed. The lumen of the pore is closed by a special structural formation - the pore complex, which consists of fibrillar and granular components. The granular component is represented by protein granules 25 nm in diameter, arranged along the edge of the pore in 3 rows. Fibrils depart from each granule and unite in a central granule located in the center of the pore. The pore complex plays the role of a diaphragm that regulates its permeability. The pore size is stable for a given cell type, but the number of pores may change during cell differentiation. There are no pores in the nuclei of spermatozoa. Attached ribosomes can be localized on the outer surface of the nuclear membrane. In addition, the outer nuclear membrane may continue into the EPS channels.

Functions of somatic cell nuclei:

1) storage of genetic information encoded in DNA molecules;

2) repair (restoration) of DNA molecules after their damage with the help of special reparative enzymes;

3) reduplication (doubling) of DNA in the synthetic period of interphase;

4) transfer of genetic information to daughter cells during mitosis;

5) implementation of the genetic information encoded in DNA for the synthesis of protein and non-protein molecules: the formation of an apparatus for protein synthesis (information, ribosomal and transfer RNA).

Functions of germ cell nuclei:

1) storage of genetic information;

2) the transfer of genetic information during the fusion of female and male germ cells.

Cellular (life) cycle

The cell (or life) cycle of a cell is the time of existence of a cell from division to the next division or from division to death. The cell cycle is different for different cell types.

In the body of mammals and humans, the following types of cells are distinguished, localized in different tissues and organs:

1) frequently dividing cells (poorly differentiated cells of the intestinal epithelium, basal cells);

2) rarely dividing cells (liver cells - hepatocytes);

3) non-dividing cells (nerve cells of the central nervous system, melanocytes, etc.).

The life cycle of these cell types is different.

The life cycle of frequently dividing cells is the time of their existence from the beginning of division to the next division. The life cycle of such cells is often called the mitotic cycle.

This cell cycle is divided into two main periods:

1) mitosis (or division period);

2) interphase (cell life span between two divisions).

There are two main methods of reproduction (reproduction) of cells.

1. Mitosis (karyokenesis) - indirect cell division, inherent mainly in somatic cells.

2. Meiosis (reduction division) is characteristic only for germ cells.

There are also descriptions of the third method of cell division - amitosis (or direct division), which is carried out by constriction of the nucleus and cytoplasm with the formation of two daughter cells or one binuclear one. However, it is currently believed that amitosis is characteristic of old and degenerating cells and is a reflection of cell pathology.

These two methods of cell division are divided into phases or periods.

Mitosis is divided into four phases:

1) prophase;

2) metaphase;

3) anaphase;

4) telophase.

Prophase is characterized by morphological changes in the nucleus and cytoplasm.

The following transformations take place in the kernel:

1) condensation of chromatin and the formation of chromosomes consisting of two chromatids;

2) disappearance of the nucleolus;

3) disintegration of the karyolemma into individual vesicles.

The following changes occur in the cytoplasm:

1) reduplication (doubling) of centrioles and their divergence to opposite poles of the cell;

2) formation of a fission spindle from microtubules;

3) reduction of granular ER and also a decrease in the number of free and attached ribosomes.

In metaphase, the following happens:

1) the formation of a metaphase plate (or parent star);

2) incomplete separation of sister chromatids from each other.

Anaphase is characterized by:

1) complete divergence of chromatids and the formation of two equivalent dipole sets of chromosomes;

2) divergence of chromosome sets to the poles of the mitotic spindle and divergence of the poles themselves.

Telophase is characterized by:

1) decondensation of chromosomes of each chromosome set;

2) formation of the nuclear membrane from the bubbles;

3) cytotomy, (constriction of a binuclear cell into two daughter independent cells);

4) the appearance of nucleoli in daughter cells.

Interphase is divided into three periods:

1) I - J1 (or presynthetic period);

2) II - S (or synthetic);

3) III - J2 (or postsynthetic period).

In the presynthetic period, the following processes occur in the cell:

1) enhanced formation of the synthetic apparatus of the cell - an increase in the number of ribosomes and various types of RNA (transport, informational, ribosomal);

2) increased protein synthesis necessary for cell growth;

3) preparation of the cell for the synthetic period - the synthesis of enzymes necessary for the formation of new DNA molecules.

The synthetic period is characterized by doubling (reduplication) of DNA, which leads to a doubling of the ploidy of diploid nuclei and is a prerequisite for subsequent mitotic cell division.

The postsynthetic period is characterized by increased synthesis of messenger RNA and all cellular proteins, especially tubulins, necessary for the formation of the fission spindle.

The cells of some tissues (for example, hepatocytes), upon exiting mitosis, enter the so-called J0 period, during which they perform their numerous functions for a number of years without entering the synthetic period. Only under certain circumstances (when a part of the liver is damaged or removed) do they enter the normal cell cycle (or synthetic period), synthesizing DNA, and then mitotically divide. The life cycle of such rarely dividing cells can be represented as follows:

1) mitosis;

2) J1-period;

3) J0-period;

4) S-period;

5) J2-period.

Most of the cells of the nervous tissue, especially the neurons of the central nervous system, do not further divide after leaving mitosis in the embryonic period.

The life cycle of such cells consists of the following periods:

1) mitosis - I period;

2) growth - II period;

3) long-term functioning - III period;

4) aging - IV period;

5) death - V period.

Over a long life cycle, such cells constantly regenerate according to the intracellular type: protein and lipid molecules that make up various cellular structures are gradually replaced by new ones, i.e., cells are gradually renewed. During the life cycle, various, primarily lipid inclusions accumulate in the cytoplasm of nondividing cells, in particular lipofuscin, which is currently considered as an aging pigment.

Meiosis - a method of cell division, in which there is a decrease in the number of chromosomes in daughter cells by 2 times, is characteristic of germ cells. In this method of division, there is no DNA reduplication.

In addition to mitosis and meiosis, endoreproduction is also released, which does not lead to an increase in the number of cells, but contributes to an increase in the number of working structures and an increase in the functional ability of the cell.

This method is characterized by the fact that after mitosis, the cells first enter the J1- and then the S-period. However, such cells, after DNA duplication, do not enter the J2 period and then mitosis. As a result, the amount of DNA becomes doubled - the cell becomes polyploid. Polyploid cells can re-enter the S-period, as a result of which they increase their ploidy.

In polyploid cells, the size of the nucleus and cytoplasm increases, the cells become hypertrophied. Some polyploid cells enter mitosis after DNA replication, but it does not end with cytotomy, since such cells become binuclear.

Thus, during endoreproduction, there is no increase in the number of cells, but the amount of DNA and organelles increases, and, consequently, the functional ability of a polyploid cell.

Not all cells have the ability to endoreproduce. Endoreproduction is most characteristic for liver cells, especially with increasing age (for example, in old age, 80% of human hepatocytes are polyploid), as well as for acinar cells of the pancreas and bladder epithelium.

Cell response to external influence

This cell morphology is not stable and constant. When the body is exposed to various adverse environmental factors, various changes occur in the structure of the cell. Depending on the impact factors, the change in cellular structures occurs differently in the cells of different organs and tissues. At the same time, changes in cellular structures can be adaptive and reversible or maladaptive, irreversible (pathological). It is not always possible to determine the boundary between reversible and irreversible changes, since adaptive ones can turn into maladaptive ones with further action of the environmental factor.

Changes in the nucleus under the influence of environmental factors:

1) swelling of the nucleus and its displacement to the cell periphery;

2) expansion of the perinuclear space;

3) the formation of invaginations of the karyolemma (the invagination of individual sections of its membrane into the nucleus);

4) chromatin condensation;

5) pycnosis (wrinkling of the nucleus and compaction (coagulation of chromatin));

6) karyorrhexis (disintegration of the nucleus into fragments);

7) karyolysis (dissolution of the nucleus).

Changes in the cytoplasm:

1) thickening and then swelling of mitochondria;

2) degranulation of granular ER (desquamation of ribosomes and fragmentation of tubules into separate vacuoles);

3) expansion of cisterns and disintegration of the lamellar Golgi complex into vacuoles;

4) swelling of lysosomes and activation of their hydrolases;

5) increase in the number of autophagosomes;

6) disintegration of the fission spindle and the development of pathological mitosis during mitosis.

Changes in the cytoplasm may be due to:

1) structural changes in the plasmalemma, which leads to an increase in its permeability and hydration of the hyaloplasm;

2) metabolic disorders, which leads to a decrease in the content of ATP;

3) a decrease in splitting or an increase in the synthesis of inclusions (glycogen, lipids) and their excessive accumulation.

After the elimination of adverse environmental factors, adaptive changes in structures disappear and the cell morphology is completely restored. With the development of non-adaptive changes, even after the elimination of the action of adverse environmental factors, the changes continue to grow, and the cell dies.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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