The branch of biology that studies the structure and function of cells. Cells are the basic units that make up the form and function of organisms, and are themselves composed of many parts. The study of structure requires not only knowing which parts it is made of, but also further clarifying the composition of each part. Correspondingly, regarding function, it is necessary to know not only the function of the cell as a whole, but also the functional interrelationship of the various parts.
The physiological functions of organisms and all life phenomena are expressed on the basis of cells. Therefore, cytology is crucial for understanding the genetics, development and physiological functions of organisms, as well as for pathology, pharmacology, etc., which are the basis of medical treatment, and agricultural breeding.
Foundation stage
Most cells are very tiny, beyond the limits of human vision. A microscope is required to observe cells. However, until the objective existence of cells is recognized, there is no way to know that the objects observed under the microscope are cells. Therefore, in 1677, when A. van Leeuwenhoek observed the "sperm" of animals with a simple microscope he made, he did not know that it was a cell. The word cell (derived from the Latin word cella, which originally means void, small chamber) was named after R. Hooke in 1667 when he observed slices of cork and saw that the cork contained small chambers. In fact, these chambers are not living structures, but gaps formed by cell walls, but the term cell has been used. In the enlightenment period of cytology, although many small objects, such as bacteria and ciliates, were observed with simple microscopes, the purpose was mainly to observe some developmental phenomena, such as the metamorphosis of butterflies, the structure of sperm and eggs, etc. Due to the limitations of the microscopes at the time and the imprecision of observations, coupled with the constraints of religious beliefs, these observations supported the dogma of preformation. Some people claim to have seen specific and tiny "little people" in sperm, and believe that they will develop into future individuals from this - spermists; others believe that the "little people" exist in eggs - eggists. The influence of preformation theory lasted for more than 100 years, preventing people from further understanding cells based on R. Hooke. It was not until 1827, when K.M. Bell discovered mammalian eggs, that the cells themselves began to be seriously observed. The development of achromatic objective lenses around this time, the introduction of carmine and hematoxylin as dyes for coloring cell nuclei, and the creation of microtome and slicing technology all created favorable conditions for more detailed observation of cells.
M.J. Schleiden and T.A.H. Schwan played a huge role in promoting the study of cells. The former described in 1838 that cells were produced in a mucus-like matrix through a crystallization-like process, and that nuclei were first produced (nucleoli were also discovered). He also regarded plants as homologous bodies of cells, just like a colony of hydrozoans. Inspired by him, Schwan firmly believed that animals and plants are composed of cells. He accumulated a large number of facts, pointed out the consistency between the two in structure and growth, and proposed the cell theory in 1839. At the same time, Czech animal physiologist J.E. Purkinje proposed the concept of protoplasm; German zoologist C.T.E. von Siebold (1845) concluded that protozoa are all single-celled. German pathologist R.C. Virchow (1855) put forward the famous saying "all cells come from cells" based on the study of connective tissue, and founded cytopathology. German zoologist M. Schulze defined a cell in 1861: "A cell is a mass of protoplasm with all the characteristics of life, in which the nucleus is located."
The above stage can be said to be cytology the foundation stage. The further development of cytology, first of all, deepens the understanding of cell structure. Because we must have a correct understanding of structures before we can proceed to explore their functions.
Research on morphological structure
From the mid-19th century to the early 20th century, research on cell structure, especially the nucleus, has made great progress.
The German botanist E.A. Strassburger first described the colored objects in plant cells in 1875 and concluded that each plant of the same species has a certain number of colored objects; in 1885, the German scholar C. Labour Propose the rule that the number of colored objects is constant.
In 1880, Baraniecki described the spiral structure of colored objects. The next year, Pfitzner discovered chromatin. It was not until 1888 that W. Walder officially named the colored objects in the nucleus chromosomes. German scholar H. Henkin observed the X chromosome in insect sperm cells in 1891, and in 1902 W.L. Stevens, E.B. Wilson and others discovered the Y chromosome.
The phenomenon of cell division had already been taken seriously at that time and analyzed carefully. The German botanist W. Hofmeister described indirect division in detail in plants in 1867 and A. Schneider in animals in 1873; the German cytologist W. Fleming discovered chromosomes in 1882. After longitudinal division, the name mitosis was proposed to replace indirect division, and E. Heuser described the chromosome distribution during indirect division; after him, E. A. Strassburger divided mitosis into two divisions that are still in common use today. Prophase, metaphase, anaphase, and telophase; he and other scholars also observed meiosis in plants, and after further research, they finally distinguished the haploid and diploid chromosome numbers.
The research on the cytoplasm is far less thorough than that on the nucleus. Although the German biologist O. Hertwig discovered the centrosome in 1875, a more detailed understanding of its evolution during mitosis was obtained through later studies on mitosis. As for the structure discovered by Golgi, which he called the Apparato reticulare interno (later called the Golgi apparatus) (1895), there has been controversy over its existence before the advent of the electron microscope. Because this structure needs to be fixed with a certain fixative and then stained with silver or osmotic acid before it can be displayed, some people think it is an artificial artifact; however, when observing living cells or using vital staining or frozen sections, at a certain position of the secretory cells You can definitely see this structure. Regarding mitochondria, since they were discovered and named by C. Benda in 1897, opinions on their existence have been relatively consistent. After being fixed with a certain fixative, some cells can be stained with a certain dye and can also be observed in vivo. However, under an optical microscope, its shapes are various, either linear, granular, or a string of granules. As for whether it exists in various cells of animals or in the cells of all living organisms, it has not yet been determined at that time.
Even less is known about the cytoplasm itself. Although there have been various theories, none of them reflect the real situation. For example, C. Froman believed in 1865 that it contained fibrous substances interwoven into a framework or network. In 1882, W. Fleming mistakenly generalized the mitochondria, spindles, and other fibrous structures seen in fixed samples and believed that the cytoplasm was composed of these filamentous components buried in the matrix. German histologist R. Altmann even believed in 1886 that certain small particles are the simplest, living, "basic organisms of cells" and constitute cells due to their special way of aggregation; this may also be due to misunderstanding. Mitochondria and secretory and storage granules. The one that is more easily accepted is the honeycomb or foam theory of the German zoologist O. Beachley in 1888: The cytoplasm is composed of a fine honeycomb structure formed by a relatively viscous substance (hyalopla-sm), which is filled with Another substance called enchylema. This theory is consistent with the actual situation to a certain extent, because Beachley proposed it based not on observations of fixed specimens, but on living observations of protozoa. The cytoplasm of protozoan sunworms is indeed foamy - the issue of whether protozoa are single-celled has been debated for almost half a century, and it was not confirmed until 1875 after Beachley studied ciliates - so the foamy theory is maintained The longest time.
Two situations should be discussed regarding the structure of the cytoplasm. In 1899, while studying various types of gland cells, Garnier discovered that the cytoplasm contained basophilic filamentous or rod-like structures that showed dynamic changes. He believed that these were not cytoplasmic inclusions, but components of the cytoplasm, hence the name. It is a moving substance, and this is described in detail. This was the endoplasmic reticulum, which was confirmed to be a real cytoplasmic structure under an electron microscope half a century later, but it did not receive the attention it deserved at the time. In 1902, Werrat described in detail the sarcoplasmic reticulum of striated muscles of different animals, which was also long forgotten. It was not until the application of electron microscopy that the accuracy of his observations was fully evaluated in 1960.
The understanding of cytoplasmic structure lags behind that of the nucleus or chromosomes, and this situation has not improved for a long time. Especially after the early 20th century, as cytogenetics studies the chromosomal basis of genetic phenomena such as segregation, recombination, linkage, and crossover, our understanding of chromosomes has deepened. H. Bauer discovered polytene chromosomes in the Malpighian duct cells of mosquitoes in 1933. This structure was also discovered in 1934 by T.S. Painter in Drosophila and by R.L. King and H.W. Beams in Chironomid. Polytene chromosomes are giant chromosomes found in some gland cells of Diptera larvae. In Drosophila, their length is about 100 times longer than normal chromosomes. Each chromosome consists of many (up to 400) chromosomes. It is composed of stained fibers, showing darkly stained band areas and lightly stained interband areas on the entire chromosome. It is formed due to intranuclear mitosis (only the chromosomes divide but not the nucleus), so each polytene chromosome is actually formed from many chromosomes. The large size of this chromosome facilitates analysis of its fine structure. In addition, the functional activity of polytene chromosomes can also be judged based on the swelling on them. But at the same time, regarding the cytoplasm, apart from understanding some of its physiological functions based on cell physiology, there has not been much progress in understanding the structure. This situation did not change significantly until the 1940s, when electron microscopes became widely used and the techniques for embedding and slicing specimens were gradually improved. Through a lot of work, not only have we clarified the structures of organelles that were previously visible but unclear under a light microscope, or were still controversial, such as mitochondria, Golgi apparatus, centrosomes, endoplasmic reticulum, cilia, flagella and other structures, but also Many structures that have never been seen before were also discovered, such as lysosomes, peroxisomes, ribosomes, various fibers that constitute the cytoskeleton, and supports composed of fibers with a thickness of 1 to 10 angstroms observed with a high-voltage electron microscope. The microbeam system covering various organelles, especially the various membranes of cells. In the past, cell membranes or nuclear membranes have never been seen under an optical microscope, and their existence was only judged based on interfaces or physiological conditions. However, under an electron microscope, it was concluded that all membranes are three-layer structures (called 75 to 100 angstroms thick). unit membrane). Not only that, the membranes of all parts of a cell are connected, the plasma membrane is connected to the endoplasmic reticulum, and the endoplasmic reticulum is connected to the Golgi apparatus or the nuclear membrane. The nuclear membrane is double-layered, consisting of an inner and outer membrane, and has a nuclear membrane pore with a certain structure, through which the cytoplasmic material and the nuclear material can communicate. Intercellular connections are also found on the plasma membrane: desmosomes, tight junctions, gap junctions, etc. These structures are related to cell-cell bonding or the exchange of materials between cells; using freeze-etching technology, they can be better observed.
In 20 to 30 years, I have gained a fairly in-depth understanding of the morphology of cytoplasm and organelles. Of course, in an era when electron microscopy is widely used, optical microscopy is still an indispensable and powerful tool. For example, the complete cytoskeleton can be observed under a light microscope using fluorescently labeled immune antibodies.
During this period, research on the cell nucleus made little progress. Although the structure of the nucleolus has been accurately described, as for the chromatin, only some colored spots can be seen when observing ultra-thin sections with an electron microscope - they should be the cut sections of the chromatin, and the complete chromatin cannot be seen. structure. Even if the chromatin is spread out by spreading, only fibers of different thicknesses can be seen. It was not until the 1970s that nucleosomes were observed under an electron microscope; soon thereafter, combined with biochemical extraction, it was observed that the chromosomes in metaphase have so-called scaffolding proteins as the core, from which DNA fibers extend out in a ring. But how chromatin condenses into chromosomes, although there are different assumptions - for example, some people think that it is due to the dye fibers spiraling again and again (so-called supercoiling), but to what extent this is consistent with the actual situation is difficult to judge.
Research on functions
Research in this area is promoted to a considerable extent by other disciplines. According to the influence of each discipline, it can be roughly divided into several stages. Of course, these stages are impossible Completely separate.
Impact of Embryology: For cell function, we cannot find a cell in a mass of tissue as the research object like studying structure. An egg is a cell. In an era when individual cells cannot be obtained for research, it is an extremely convenient material to use. Since eggs are used, studying the functions of each part of it must of course be judged based on the impact on development. This involves embryological issues.
However, if hybridization is used to study the function of heterogeneous sperm nuclei, it needs to be judged based on the appearance of heterogeneous traits, which involves genetic issues. Early work in this area was basically carried out by embryologists, which was characterized by comprehensive research. It did not simply study eggs from a cell perspective, but used eggs as cells to study issues related to development, heredity, etc. . Some major issues have been outlined, which will have a profound impact on future academic thinking. Brothers O. Hertwig and R. von Hertwig used sea urchins as materials in 1887. They first observed the fertilization of living eggs and conducted experimental analysis of fertilization. If the roles of the cytoplasm and the nucleus in development are considered separately, T. H. Boveri's analysis of the phenomenon of chromatin depletion found in Ascaris equine demonstrated that factors affecting depletion are present in the cytoplasm. In addition, cell lineage work that numbers blastomeres to track the ins and outs of each blastomere, and studies on the cleavage types of various eggs with different yolk contents, have pointed out that the distribution of cytoplasm in the egg affects the direction of the spindle. , determines the formation of the cleavage surface and determines the type of cleavage. Not only that, in some particularly suitable eggs, it can be seen that the materials for forming various organs have already been laid out in the egg. After cleavage, each blastomere has a certain correspondence with the organs to be formed. All of this suggests that the nuclei are identical in genetic potential and are only differentially regulated later in development through the cytoplasm or cell-to-cell interactions.
The role of the cell nucleus has also been fully evaluated. In 1887, German experimental embryologist T.H. Boveri fertilized sea urchin eggs with two sperms. Based on the distribution of chromosomes in each blastomere and the development of each blastomere, he believed that each chromosome was qualitatively different and that chromosomes were Personal. Using sea urchin eggs, T.H. Morgan completed artificial parthenogenesis in 1896 - eggs can develop without fertilization. Egg masses without nuclei were fertilized or fertilized with heterogeneous sperm, and the respective roles of cytoplasm and nucleus in development were studied. It was observed that the larvae produced showed paternal characteristics. These all illustrate the importance of the cell nucleus. To sum up the achievements at that time, in 1883 the German embryologist W. Lu once expressed this idea: "Not only chromosomes, but also various parts of each chromosome may be important in determining the development, physiology and morphology of the individual." In 1887 German zoologist A. Weismann proposed the hypothesis of germ plasm. Although this hypothesis was overturned by subsequent experimental studies, there are some ideological connections between the determinants proposed in the hypothesis and later genes.
Except for the impact of academic thought, in order to solve problems in embryology, it also provides an important experimental method for cytology, which is tissue culture. The American embryologist R.G. Harrison created the in vitro culture method in 1907 to study the growth of nerve fibers. It was later taken over by the American physiologist A. Carrel and developed into a specialized technology. After the 1930s, its importance became more and more obvious. Today, it is not only used to study all aspects of living cells, but is also an indispensable technology for many other disciplines.
The influence of genetics After the rediscovery of G.J. Mendel’s research achievements in 1900, genetic research has strongly promoted the progress of cytology. American geneticist and embryologist T.H. Morgan studied the inheritance of fruit flies and found that the occasional white-eyed individuals were always male. Combined with the existing knowledge about sex chromosomes, he explained the appearance of white-eyed males and began to explain inheritance from cells. Phenomenon, genetic factors may be located on chromosomes. Cytology and genetics are connected, quantitative and physiological concepts are obtained from genetics, qualitative, material and narrative concepts are obtained from cytology, and cytogenetics is gradually produced.
In 1920, American cytologist W.S. Sutton further pointed out that the parallel phenomenon between genetic factors and chromosome behavior must mean that genetic factors are located on chromosomes, and mentioned that if two pairs of factors are located on the same chromosome, They may or may not be inherited according to Mendelian laws, which foreshadows the concept of linkage and deepens research on mature divisions, especially chromosome pairing and chromosome exchange.
In addition, after it was discovered that radiation phenomena (X-rays, radium radiation, ultraviolet rays) and temperature can cause mutations in fruit flies, the high frequency of mutations is more conducive to experimental research on chromosomes. Various mutations caused by radiation, including gene translocations, inversions and deletions, can be found in chromosomes.
By crossing the mutant type with the wild type and statistically processing their offspring, the genetic arrangement of the chromosomes can be calculated.
The discovery of polytene chromosomes opened up new avenues for chromosome research. After concluding that polytene chromosomes are thickened and paired chromosomes, on the one hand, we conducted a detailed study of its structure and discovered the chromatin grains on the dye line. Many adjacent chromatin grains gathered into bands, and the dye line Although it is difficult to see clearly, if it is properly dyed or under ultraviolet light, you can see that they are not arranged straight and parallel, but in a very loose spiral shape. On the other hand, the gene arrangement diagram on the chromosome calculated based on the linkage group can be matched with the morphological chromosome diagram using the so-called salivary gland method; the hybridization experiment and the morphological observation of the cells can perfectly corroborate each other, and can be used in multiple lines. The arrangement of genes can be seen more specifically on the chromosomes. Each band actually contains more than one gene. Not only that, some mutations are due to gene position effects. For example, the bar-eye mutation was first evidenced on polytene chromosomes.
Driven by the search for the material basis of heredity, the study of chromosomes has spread across the world, not only for genetic research materials, but also for many other animal and plant species (some statistics include about 12,000 vascular species). Plants and more than 500 mammal species) cell division (meiosis), chromosome behavior, and chromosome maps have been studied. Species in the same genus often have the same number of chromosomes; however, species in the same family may have different numbers, or one genus may have multiples of another genus (polyploidy). The various chromosomes of the same individual may not seem to be very different at first glance, but they are different upon closer inspection. Therefore, the number, shape, and size of each chromosome of a species can be accurately stated, and they can be numbered and lined up. The chromosomes of different species that are closely related can be compared to find the evolutionary relationship of the species; the study of karyotype points out that the number of chromosomes of similar species may be exactly the same, but there may also be very obvious differences. In the latter case Careful study always reveals the original form, and the various forms derived from it. Three types of mutations are known in plants: polyploidy, breakage of one chromosome into several smaller ones or conversely the assembly of several smaller chromosomes into one larger one, and the doubling of a certain pair of chromosomes. These three mutations are sometimes associated with the formation of subspecies and species. In addition, research on plant polyploidy has led to the use of various methods, such as chemicals, temperature, radiation, etc., to induce the generation of polyploidy, which has gained application value in some plants.
Extensive research on sex chromosome morphology has also provided a cytological basis for determining male and female gender. Some animals are XX and XY types, and some are ZZ and ZW types.
Influence on cell physiology. At this stage, experimental methods were used to study the functions of other parts of the cell, but no satisfactory results were obtained. The cell membrane cannot be observed with a microscope, and its existence can only be judged based on the exchange of substances between the cytoplasm and the outside world, as well as the penetration of certain substances, so as to judge some of its functions. Because generally speaking, fat-soluble substances easily enter cells, it has been speculated that the cell membrane may be composed of lipids or lipid pores. It was also believed that substances with different molecular weights have different difficulty in entering cells - the larger the molecular weight, the more difficult it is to enter. It was speculated that the cell membrane is like a filter layer, and its pores prevent large molecules from entering the cell. In addition, based on the permeability of cells by electrolytes, such as positive ions and negative ions, and the acidity of the cell environment, which can affect or change the permeability of positive and negative ions, a charge hypothesis has been proposed to explain the extremely complex problem of cell permeability. process. As for the phagocytosis of solid particles, through simulation experiments, such as the phagocytosis of chloroform drops by amoebas, it is believed that this is because cells have greater adhesion to the surface of foreign bodies than to the surrounding environment, and local changes in the surface tension of the cell membrane are caused by adhesion. causing foreign objects to be swallowed.
The above assumptions, even at that time, the cell membrane was passive in terms of permeability; but cells can also actively take in or out certain substances against the diffusion gradient or concentration gradient. Therefore, it was also thought that there might be energy-requiring processes in the cell membrane that were of great significance to these processes, but there was no data at that time.
At that time, the understanding of cellular respiration was mainly limited to the heat generated by food through the action of various enzymes.
Because we know several enzymes in this process, such as certain dehydrogenases, oxidases, cytochromes a, c, b, etc., we understand that the burning of food in cells does not involve a sudden oxidation of all energy into It is released in the form of heat, but gradually passes through small stages, obtaining and utilizing small amounts of energy step by step. This process can proceed and be finely regulated because many enzymes are added to the overall respiration process as oxygen transfer, hydrogen acceptance, redox systems, etc.
The influence of other disciplines In the early 1940s, technical methods from other disciplines were successively used in the study of cytology, opening up a new situation and forming some new fields. First, the application of electron microscopy gave rise to ultramicroscopic morphology.
Belgian zoologist J. Brachet started from the problem of embryology and used a specific staining method (Unna, Feulgen) to study the significance of nucleic acids in development. At about the same time, Swedish biochemist T.O. Caspersson created an ultraviolet cell spectrophotometer based on the absorption of certain wavelengths by various substances to detect the presence of proteins, DNA, and RNA in cells. If the former can be qualitative based on dyeing, the latter can be quantitative based on absorption. In essence, their work drew attention to the role of nucleic acids in cell growth and differentiation. On the basis of their work, cytochemistry was developed. The study of the chemical composition of cells can complement the study of morphology and add some understanding of cell structure.
Analysis using polytene chromosomes and taking pictures under ultraviolet light show that the chromatin and nucleolus contain DNA, whereas the chromatin contains very little or even no chromatin. Digestion with a (possibly impure) protease can dissolve them, leading to the mistaken belief that the stained lines were made of protein. In addition, the percentage of certain amino acids in chromosome segments (euchromatin and heterochromatin) can be precisely determined based on UV absorption spectra. Segments of euchromatin appear to contain more high-molecular-weight globulin-type proteins, whereas segments of heterochromatin contain more low-molecular-weight histone-type proteins.
In the 1940s, work began to be carried out to study the functions of various parts of cells from a biochemical perspective, giving rise to biochemical cytology. First, homogenization—mechanically grinding cells in a suitable solution—and differential centrifugation were used. In addition to cell nuclei, mitochondria, microsomes, hyaline and other parts could also be obtained. Study them separately to understand the existence and distribution of some substances and enzymes and where certain metabolic processes occur. Mitochondria have been isolated more successfully because their size has been measured using electron microscopy and the biochemical processes carried out in this organelle have been roughly understood, and their importance to energy metabolism has been recognized. Microsomes were once mistaken for an organelle. It was later learned that this was a product under the isolation conditions at that time, and was a complex composed of ribosomes and a small amount of endoplasmic reticulum. Nonetheless, studies on mitochondria and microsomes indicate that many basic biochemical processes occur in the cytoplasm rather than in the nucleus. Such approaches combined with in-depth morphological studies have led to an increasingly profound understanding of processes in cells.
The application of radioactive isotopes has opened up new ways to study metabolic processes in cells. Their participation can accurately track the synthesis, transportation, and storage utilization of intracellular substances. This method has shown, for example, that phosphorus compounds are incorporated not during mitosis but during interphase, shortly before the start of division, and are then distributed to the daughter cell nuclei. From the results obtained with these and other isotopes, it is possible to infer the movement of some important substances in the cell.
Although tissue culture has developed greatly in the 1930s, it can only culture tissue blocks, but cannot culture single cells of normal tissues, and its importance has not been fully demonstrated. Cultured cells can be used to study many problems that cannot be studied in the whole body (in situ), such as cell nutrition, movement, behavior, and the relationship between cells. Almost all kinds of tissues, including certain invertebrates (cuttlefish, ascidians, fruit flies, etc.), have been cultured. Various cells grown from tissue blocks under good culture conditions have different growth conditions. Morphologically, it can be basically divided into three types: epithelium, connective tissue, and migratory cells (such as lymphocytes, monocytes, and macrophages).
Sometimes cultured cells will show characteristics that normal tissues do not show in organisms. For example, if the culture medium contains substances that enhance surfactant, cells of various tissues can acquire the ability to phagocytose. However, they retain their unique properties and potential because they can continue to grow if the culture environment is changed or if they are moved back to their original location in the animal's body.
It is worth mentioning that the growth of fibroblasts in culture is also affected by the substrate. Under normal circumstances they grow radially and aimlessly from the tissue mass. However, if the culture medium is artificially placed under tension in a certain direction, or traces are artificially made on the substrate, the cells will grow along the direction of the tension or along the traces. This phenomenon may be used to explain the functional adaptation of connective tissue and tendons in the whole body-they always grow and differentiate in the direction of tension.
It can be seen that the study of cells, the in-depth study of submicroscopic structures through the use of electron microscopy, and the in-depth study of functions through the application of biochemical techniques, have already contributed to the advancement of cell biology─ It creates conditions for studying the life phenomena of cells at the molecular level - the formation of cells. So later, under the influence of the outstanding achievements of molecular genetics and molecular biology, the new discipline of cell biology was quickly formed.