Cell
Cell English name: CELL, abbreviated as C in the article
It is composed of protoplasm containing the nucleus (or nucleoid) surrounded by a membrane. The basic unit of structure and function of living organisms, and also the basic unit of life activities. Cells can proliferate through division, which is the basis of ontogeny and phylogeny of organisms. Cells are either independent units of life, or multiple cells form a cell group or tissue, or an organ or body; cells can also divide and reproduce; cells are the basic unit of heredity and have genetic totipotency (plants)
Animal cell nuclei are totipotent
Cytology is a branch of biology that studies cell structure and function.
Cells are the basic units that make up the form and function of organisms, and they themselves are composed of many parts. Therefore, the study of cell structure requires not only knowing what 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.
The vast majority of cells are very tiny, beyond the limits of human vision, and a microscope must be used to observe cells. So in 1677, when Leeuwen Hook used a simple microscope he made to observe the "sperm" of animals, he did not know that it was a cell. The word cell was named after Robert Hooke in 1665 when he observed slices of cork and saw that the cork contained cells. 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, ciliates, etc. - were also observed with simple microscopes, the purpose was mainly to observe some developmental phenomena, such as the metamorphosis of butterflies, sperm and The structure of the egg, etc. It was not until Bell discovered the mammalian egg in 1827 that serious observation of the cells themselves began. The development of achromatic objective lenses around this time, the introduction of magenta and hematoxylin as dyes for coloring cell nuclei, and the creation of microtome and sectioning technology all created favorable conditions for more detailed observation of cells.
It was the German biologists Schleiden and Schwan who played a huge role in promoting the study of cells. The former described in 1838 that cells are produced in a mucus-like matrix through a crystal-like process, and regarded plants as homologous bodies of cells. Inspired by him, Schwan firmly believed that animals and plants are composed of cells, and pointed out the consistency of their structure and growth, and proposed the cell theory in 1839.
At the same time, Czech animal physiologist Purkinje proposed the concept of protoplasm; German zoologist Siebold concluded that protozoa are all single-celled. German pathologist Virchow put forward the famous saying "all cells come from cells" based on the study of connective tissue, and founded cytopathology.
From the mid-19th century to the early 20th century, research on cell structure, especially the nucleus, has made great progress. The German botanist 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 1880, Baraniecki described the spiral structure of the colored objects. In the following year, Pfitzner discovered chromosomal particles, and it was not until 1888 that Walder officially named the colored objects in the nucleus chromosomes. German scholar Henkin observed the X chromosome in insect sperm cells in 1891, and in 1902 Stevens, Wilson and others discovered the Y chromosome.
The German botanist Hofmeister described indirect division in detail in 1867 for plants and Schneider in 1873 for animals; the German cytologist Fleming discovered the invention in 1882. After the longitudinal division of chromosomes, the name mitosis was proposed in place of indirect division, and Hoyzel described the distribution of chromosomes during indirect division; after him, Strassburger divided mitosis into the still common term 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 understanding of cytoplasmic structure lags behind that of the nucleus or chromosomes, and this situation will eventually improve. 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. 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.
After the rediscovery of Mendel’s research achievements in 1900, genetic research strongly promoted the progress of cytology. Morgan, an American geneticist and embryologist, 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 emergence of white-eyed males and began to explain genetic phenomena from cells. , 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 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 It may or may not be inherited according to Mendel's laws, which foreshadows the concept of linkage and deepens the research on mature divisions, especially chromosome pairing and chromosome exchange.
In addition, after discovering that radiation 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, all have their basis in chromosomes. By crossing the mutant type with the wild type and statistically processing the offspring, the genetic arrangement of the chromosome can be derived. Extensive research on sex chromosome morphology has also provided a cytological basis for determining male and female gender.
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 Brachet started from embryological issues and used specific staining methods to study the significance of nucleic acids in development. At about the same time, Swedish biochemist 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. 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.
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 - mechanical grinding of cells in a suitable solution - and differential centrifugation were used. In addition to the nucleus, several parts such as mitochondria, microsomes and hyaline could be obtained. Study them separately to understand the existence and distribution of some substances and enzymes and where certain metabolic processes occur. Studies such as those 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.
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 issues that cannot be studied as a whole, such as cell nutrition, movement, behavior, and the relationship between cells. Almost every kind of tissue, including some invertebrates, has 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. 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 entire 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 after the use of electron microscopy, and the in-depth study of functions after the application of biochemical techniques, have already contributed to cell biology-in 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.