Human Anatomy - Kotsan I. Y. 2009

Cells and Living Matter of Non-Cellular Structure

The discovery and Study of the Cell began with Robert Hooke's invention of the Microscope in 1665. In 1839, the German scientist Theodor Schwann formulated Cell Theory, which is based on the premise that The Cell is the fundamental unit of life in both PLANT AND ANIMAL organisms. The branch of science that studies the development, Structure, and Functions of Cells is called Cytology (from the Greek kytos — cell, and logos — science).

The cell (cellula) is the elementary, structurally defined living system that serves as the basis for the structure, development, and vital activity of All living organisms (Fig. 4).

Class="center">

Fig. 4. Diagram of the ultramicroscopic structure of a cell

1 — cytolemma; 2 — pinocytotic vesicles; 3 — centrosome (cell center); 4 — hyaloplasm; 5 — Endoplasmic reticulum: a — membranes of The endoplasmic reticulum; b — Ribosomes; 6 — nucleus; 7 — connection between the perinuclear space and the cavities of the endoplasmic reticulum; 8 — nuclear pores; 9 — nucleolus; 10 — intracellular reticular apparatus (Golgi apparatus); 11 — secretory vacuoles; 12 — Mitochondria; 13 — Lysosomes; 14 — three consecutive stages of phagocytosis; 15 — connection of The cell membrane (cytolemma) with the membranes of the endoplasmic reticulum

The Human Body contains a vast number of cells (approximately 10¹⁴), ranging in size from 5–7 to 80–120 µm. The largest cells are female sex cells (ova) and Nerve Cells, while the smallest are Blood Cells, known as lymphocytes.

Cell shape, much like size, varies greatly. Cells can be flat, cuboidal, rounded, stellate, spherical, or spindle-shaped, which is determined by their functions and living conditions. For instance, blood cells are rounded; cells that conduct impulses are stellate with branching processes; and cells responsible for moving PARTS OF THE body or its Organs are elongated and spindle-shaped.

Although cells vary in shape and size and perform diverse functions, they share a similar internal structure. The main parts of a cell are the Cytoplasm and The Nucleus. The cytoplasm comprises the cytolemma, hyaloplasm, Organelles, and inclusions (Table 1).

Table 1 Diagram of Cell Structure

The cytolemma (Plasma Membrane) is a universal biological membrane that separates the cell from its external environment. It consists of three layers: outer, middle, and inner. The outer and inner layers are composed of protein molecules, while the middle layer is formed by lipid molecules. The cell cytolemma is semipermeable; it regulates METABOLISM between the cell and its environment and ensures the constancy of the cell's internal environment. The cytolemma participates in the uptake of large solid particles (phagocytosis) and liquids (pinocytosis), and also facilitates the expulsion of Metabolic waste products from the cell (exocytosis).

The hyaloplasm (Cytosol) is the major volumetric component of the cell. It is a semi-fluid mass with a colloidal structure that houses the nucleus, organelles, inclusions, and intracellular metabolic products. The hyaloplasm contains Proteins, fats, CARBOHYDRATES, inorganic substances, Water, Lipids, Nucleic Acids, and Enzymes. Proteins account for 5 to 8%, carbohydrates for 1–5%, fats for 5–9%, lipids for 2–3%, and water for 75–85% of the cell's mass. Inorganic substances are represented by potassium, sodium, calcium, and magnesium salts; nucleic acids include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Proteins perform a structural (plastic) function, forming the framework of cellular structures; Introduction/36.html">Carbohydrates and lipids serve as sources of energy. Water and salts determine the PHYSICOCHEMICAL PROPERTIES OF cells, establishing osmotic pressure and electrical charge. The most critical biological function of nucleic acids is their involvement in Protein Biosynthesis and the transmission and Replication of hereditary traits. Thus, the hyaloplasm integrates all Cellular Structures and facilitates their chemical interaction with one another.

All biochemical processes within the cell take place in ordered cellular structures known as organelles (a diminutive of 'organ'), which perform highly specialized functions. They are categorized into general organelles and special organelles.

General organelles include the endoplasmic reticulum, ribosomes, mitochondria, lysosomes, Microbodies, the Golgi apparatus (intracellular reticular apparatus), and the centrosome (cell center).

The endoplasmic reticulum is a system of tubules, microvesicles, cisternae, and channels that ensure the Transport of substances from the environment into the cell and their movement within the cell. There are two forms of the endoplasmic reticulum: smooth (agranular) and rough (granular). The membranes of the rough endoplasmic reticulum are studded with numerous granules called ribosomes. The agranular endoplasmic reticulum is involved in the synthesis of carbohydrates and lipids, whereas the granular type participates in Protein Synthesis.

Ribosomes are the smallest cellular organelles, appearing as tiny granules. They are located on the membranes of the rough endoplasmic reticulum, on the nuclear envelope, or freely within the cytoplasm. Composed of approximately 40% RNA and 60% structural protein, ribosomes synthesize proteins specific to each cell type. They act as highly efficient protein factories, capable of producing protein mass exceeding their own within a single hour.

Mitochondria appear as small granules, rods, or filaments. Each mitochondrion is bounded by an outer and an inner membrane. The inner membrane forms inward folds known as cristae. Embedded in the cristae are numerous oxidative enzymes—specialized protein molecules that act as catalysts in oxidation reactions. The precise functional role of mitochondria remained unclear for a long time. Initially, they were thought to actively participate in forming various cellular structures, then believed to accumulate secretions in glandular cells, and hypothesized to take part in general metabolism. Today, the active role of mitochondria in cellular Respiration and energy supply is well established. This energy is synthesized and stored within the cell in the form of a high-energy compound: adenosine triphosphate (ATP). ATP is generated through The transfer of electrons along a chain of specific proteins known as Cytochromes, located on the mitochondrial cristae. This process is called Oxidative Phosphorylation, with The breakdown of fats and carbohydrates in the mitochondrial matrix serving as the electron source.

Lysosomes are rounded bodies approximately 0.4 µm in size, enclosed by a cytoplasmic membrane. The lysosomal matrix contains a rich assortment (20–40) of hydrolytic enzymes that participate in the intracellular Digestion of incoming nutrients, damaged cellular components, and foreign particles. Consequently, lysosomes are particularly abundant in cells engaged in phagocytosis, such as leukocytes, hepatocytes, and Cells of the Small Intestine.

Microbodies are small, membrane-bound vesicles. Among them, Peroxisomes are the most thoroughly studied; they contain the enzyme catalase, which breaks down hydrogen peroxide—a toxic byproduct of cellular metabolism.

The Golgi apparatus (intracellular reticular apparatus) is named after the Italian scientist Camillo Golgi, who first described it. It appears as a collection of vesicles, lamellae, tubules, and saccules bounded by Cytoplasmic membranes, and is typically located near the nucleus. It synthesizes Polysaccharides that interact with proteins and participates in the segregation and export of cellular metabolic products.

The centrosome (cell center) consists of two centrioles, usually positioned near the nucleus or the Golgi apparatus. Centrioles play a crucial role in The formation of the mitotic spindle, ensuring the proper orientation and movement of Chromosomes.

Special organelles are structures adapted to perform specific, specialized cellular functions. These include myofibrils, neurofibrils, tonofibrils, flagella, cilia, microvilli, and synaptic vesicles.

Myofibrils are contractile filaments found within Muscle cells and fibers, responsible for the contraction function.

Neurofibrils are located in the cell body and processes of nerve cells. Composed of complexes of microtubules and microfilaments, they are responsible for Intracellular Transport and maintaining the shape of the neuron.

Tonofibrils are found in stratified Epithelial Tissues and perform a supportive function.

Flagella and cilia are designed to move specialized cells (spermatozoa) or induce fluid movement near cells (epithelial cells of the Trachea and Bronchi). At the base of flagella and cilia lies a basal body, which structurally resembles a centriole. The length of flagella reaches 120—150 µm, and cilia 5—10 µm.

Microvilli are located On the surface of intestinal epithelial cells in the simple columnar epithelium (1500—3000 on The surface of a single cell) covering the intestinal villi. Thanks to microvilli, the absorptive surface of these cells is increased; furthermore, the villi contain A large number of active enzymes that break down nutrients (membrane digestion).

Synaptic vesicles are located in nerve cells (Neurons) in the region of the presynaptic membrane. They store specific chemical substances—mediators (Neurotransmitters)—which ensure the transmission of a Nerve Impulse from neuron to neuron, from neuron to muscle elements, or to secretory cells.

Along with the listed structures, the Cell Cytoplasm contains isolated accumulations of various substances known as inclusions. Unlike organelles, inclusions can alternately appear and disappear. Inclusions are classified into:

1. trophic (protein, lipid, and carbohydrate);

2. excretory (harmful metabolic waste products that need to be removed from the cell);

3. secretory (products of glandular cell activity; unlike excretions, these are substances necessary for the Organism that accumulate in the cell and are subsequently released from it);

4. pigmented (substances that, upon accumulating in the cell, determine its color).

The nucleus (nucleus, caryon) is the second major component of the cell. Human body cells typically contain a single nucleus, but binucleated cells (in The Liver and Kidneys) or multinucleated cells (in vascular endothelium) also occur, as well as anucleated cells such as erythrocytes. In most cases, the nucleus occupies a central position, though this is not an absolute rule, as it is very often displaced toward the periphery.

The shape of the nucleus can vary: spherical, oval, elongated, rod-like, or lobulated. Most often, it depends on the shape of the cell itself: in round, oval, and cuboidal cells, the nucleus is spherical; in tall, prismatic, and spindle-shaped cells, it is strongly elongated. The size of the nucleus depends on the volume of the cytoplasm—it is larger in large cells than in small ones. However, this does not imply a strict proportionality between nuclear and cellular dimensions. Nevertheless, for any given cell type, the ratio between the nuclear and cytoplasmic volumes remains a relatively constant parameter (index).

The nucleus consists of the nuclear envelope (nucleolemma, karyolemma), Chromatin, 1—2 nucleoli, and nuclear sap (nucleoplasm, karyoplasm).

The nucleolemma consists of two membranes separated by a perinuclear space. The outer nuclear membrane is continuous with the membranes of the endoplasmic reticulum, connecting with its tubules, while the inner nuclear membrane is in close contact with the karyoplasm. The nuclear envelope is perforated by pores that facilitate close communication between the nucleoplasm and the cytoplasm. The nuclear envelope not only separates the nucleus from the cytoplasm but also plays an active role in Metabolic exchange between them.

Nuclear chromatin appears as clumps and threads, containing proteins and nucleic acids. During Cell Division, the chromatin structures of the nucleus spiralize and become clearly visible as chromosomes, each carrying hereditary determinants—genes. Two Types of chromatin are distinguished: euchromatin (despiralized), which is synthetically active, and heterochromatin (spiralized), which is less active.

The nucleus contains one or two nucleoli, which are composed of RNA and Phosphoproteins. The nucleolus participates in the synthesis of RNA molecules that pass into the cytoplasm and, within ribosomes, take part in protein synthesis.

Nucleoplasm (nuclear sap) fills the spaces between nuclear structures, containing proteins, enzymes, and RNA granules, and ensures the interaction of nuclear components.

The Significance of the nucleus in cellular life processes is immense. It directs all cellular activities: metabolism, movement, and reproduction. The nucleus houses the bulk of DNA, which carries hereditary information. Cells cannot survive for long without a nucleus; they lose their ability to divide and perish (for instance, human erythrocytes).

The cell, as the fundamental unit of living matter, exhibits specific functional properties: metabolism, irritability, movement, development, and reproduction.

Metabolism is the totality of Chemical Reactions that form The basis of cellular activity. It encompasses assimilation, or anabolism—the uptake and utilization of substances entering the cell—and dissimilation, or Catabolism—the breakdown of substances accompanied by the release of energy required for cellular functions.

Irritability refers to the ability of cells to respond to changes in environmental factors: Temperature, light, humidity, chemical agents, osmotic pressure, X-radiation, and others. The response to a stimulus may manifest as the displacement of cellular structures, an increase in metabolic rate, the secretion of substances, Muscle contraction, and Other forms of excitation.

The movement of living cellular matter can be amoeboid, accomplished via flagella and cilia, or driven by specialized myofibrils.

Amoeboid movement is independent of specialized cellular structures and occurs through the formation of pseudopodia (false feet). This type of movement is characteristic of certain leukocytes and many Connective Tissue cells. Through amoeboid movement, leukocytes migrate across vessel walls and enter the tissues. They move particularly actively toward sites of inflammation, where Bacteria accumulate in large numbers. The formation of pseudopodia in leukocytes and other cells is important not only for locomotion; pseudopodia also serve to capture various particles from the surrounding environment. Leukocytes and connective tissue cells engulf harmful foreign particles that need to be eliminated using their pseudopodia. This cellular property was studied by I.I. Mechnikov and termed phagocytosis, while the cells themselves were named phagocytes.

Locomotion via flagella and cilia is carried out by specially differentiated cell projections: they are called flagella when few and long, and cilia when numerous. Spermatozoa—Male Germ Cells—move with the aid of flagella. Ciliary movement (beat) is analogous to flagellar movement, differing only in that flagellar propulsion is effected by a single flagellum, whereas ciliary movement relies on the coordinated action of an entire system of cilia.

The most advanced form of movement is muscular contraction, driven by specialized intracellular structures known as myofibrils, which are capable of shortening and thickening or lengthening and thinning to produce motion.

Cell growth refers to the process by which cellular structures increase in size, resulting in an overall increase in cell volume.

Cell Differentiation is the process by which a cell acquires specific functions.

Cell reproduction, or the capacity for self-replication, is fundamental to the preservation and development of cells and, by extension, the entire organism, as well as the replacement of Aging and dead cells, tissue regeneration, and somatic growth. All of these processes are driven by cell division. There are three distinct forms of cell division: mitosis, or indirect division; amitosis, or direct division; and Meiosis, or reductional division.

Mitosis (mitosis cellularis), or indirect cell division, is the most common form of cell division, as it ensures the equal distribution of hereditary material between newly formed daughter cells. A cell undergoing mitosis goes through four consecutive stages: prophase, metaphase, anaphase, and telophase, each characterized by a specific state of the nucleus and cytoplasm. The new cells resulting from mitotic division enter a state of relative rest known as interphase, during which substances essential for cellular survival and subsequent division are synthesized. The time elapsed from one cell division to the next is termed its life cycle.

Mitotic cell division ensures the self-renewal of tissues during either physiological regeneration (characteristic of the normal life cycle) or reparative regeneration (following tissue injury).

Amitosis (amitosis cellularis) is the simplest mode of division, wherein the nucleus divides first, followed by the cytoplasm. However, nuclear division is not always accompanied by cytoplasmic division. In such cases, binucleated cells are formed, or even multinucleated cells upon repeated nuclear division, which occasionally attain large sizes.

The Biological Significance of amitosis remained a subject of debate for a long time. Following the discovery of mitotic division, its existence was initially denied altogether. Subsequently, the prevailing view gained widespread acceptance that amitosis indicated a degenerative state of the cell, reflecting a loss of capacity for the complex transformations that accompany karyokinesis. However, it was later discovered that direct cell division occurs alongside more complex forms and is quite widespread. Amitosis is observed particularly frequently during regeneration and is characteristic of cells in certain tissues during their development (such as Cartilage and muscle), as well as cells nearing the end of their life cycle and pathological cells.

Meiosis (mejosis) is the specialized division of germ cells: male spermatozoa and female oocytes.

All somatic cells possess a double (diploid) set of chromosomes (23 pairs, or 46 chromosomes). In contrast, germ cells contain a single (haploid) set of chromosomes (23 chromosomes).

Following the fusion of male and Female Germ Cells—that is, Fertilization—the diploid chromosome set is restored, containing Genetic information contributed by both parents. Cell division via mitosis maintains this diploid (double) chromosome number. If germ cells were formed through the same mechanism, the chromosome number would double with every fertilization event. However, this does not occur because germ Cell Formation begins with a reductional division, which halves the chromosome number, followed by an equational division that preserves the haploid (single) set of chromosomes. Together, these two divisions—reductional and equational—constitute meiosis. Meiosis is a universal process characteristic of both plants and animals. It consists of a series of phases proceeding according to a uniform pattern. Both the reductional and equational divisions comprise four phases: prophase, metaphase, anaphase, and telophase. The Phases of the reductional division are designated by the numeral I, while those of the equational division are designated by the numeral II.

The bodies of humans and animals contain not only cells but also non-cellular structures. Non-cellular structures represent one of the forms of living matter. Non-Cellular forms of living substance include the symplast and Extracellular matrix.

Unlike cells, a symplast contains a large number of nuclei. A symplast is formed either through the fusion of cells or through nuclear division without subsequent cytoplasmic division. A classic example of a symplast is a striated muscle fiber.

The extracellular matrix is located in the spaces between cells. It can exist in liquid, gelatinous, or solid consistency.

Main Latin Terms

Cell — cellula

Cytolemma — cytolemma

Hyaloplasm — hyaloplasma

Nucleus — nucleus

Amitosis — amitosis

Mitosis — mitosis

Meiosis — mejosis