Human Anatomy - H. I. Koliadenko 2009

Structure of the Human Body
Cell Division

A universal property of all living systems is the capacity for self-reproduction, which enables organismal growth as well as the replacement of dead and damaged Tissues. The lifespan of different Cells varies. For instance, intestinal epithelial cells live up to 24 hours, Skin cells from 5 to 35 days, erythrocytes 120 days, and Liver cells 180 days. Throughout their entire lifespan, cells continually multiply by division.

The Regulation of Cell division occurs at THE TISSUE LEVEL and is a complex physiological process of cell regeneration.

Cells reproduce through two primary modes: Direct and Indirect division. The majority of body cells reproduce via indirect division (mitosis). The sequence of events occurring in a cell before and during division is known as the mitotic cycle, which comprises four phases: 1) mitotic division proper; 2) the postmitotic phase; 3) METABOLISM/36.html">DNA Replication; and 4) energy accumulation.

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Fig. 3. Mitosis (karyokinesis)

Division of a chorionic cell from a six-week human fetus: 1, 2, 3 — prophase; 4, 5 — metaphase; 6, 7 — anaphase; 8 — telophase; 9 — newly formed cells

The mitotic division phase proper is the shortest, accounting for no more than 5% of the cycle duration. During this period, The Cell divides into two daughter cells. In the postmitotic phase, which may last several hours, cell mass increases. This is followed by the DNA replication phase, characterized by enhanced Protein Synthesis AND DNA doubling, thereby producing an exact copy of the DNA molecules that will supply the hereditary material for two new daughter cells during subsequent division. In the final, fourth phase, energy required for subsequent processes is accumulated. Only young cells retain the capacity for division. The duration of the mitotic cycle varies among different cells, ranging from a few minutes to 2–30 hours, depending on cell type, environmental conditions, etc. As cells age, their proliferative capacity declines.

Alongside mitosis, direct Cell Division (amitosis) can also occur, in which a cell gradually constricts in half to yield two daughter cells. Occasionally, such division is incomplete—The Nucleus divides into two, but the protoplasm does not, resulting in The formation of multinucleated cells.

Mitosis (karyokinesis) proceeds in four stages: prophase, metaphase, anaphase, and telophase (Fig. 3).

During prophase, the cell's nucleus swells, and Chromatin transitions from granular particles into a continuous strand, condensing into a mobile skein.

Subsequently, Chromosomes emerge from the chromatin strands, each splitting into two daughter halves. During this phase, the karyotheca (nuclear envelope) dissolves, the nucleolus disappears, and the centrosome divides into two centrioles that migrate toward opposite poles of the cell.

In metaphase, the centrioles take up fixed positions at the cell poles, after which the achromatic spindle forms, composed of numerous filaments derived from the achromatic substance of the nucleus. One end of these filaments attaches to the centriole, while the other attaches to the midpoint of the chromosome. Meanwhile, the chromosomes arrange themselves in the central region of the cell, forming a star-like configuration. Toward the end of this phase, chromosomes become shorter and thicker, and begin to split longitudinally into two daughter chromosomes, though they remain joined at the equator.

In anaphase, the daughter chromosomes are pulled toward opposite poles by the winding of the achromatic spindle fibers around the centrioles, forming a double-star configuration.

In telophase, the daughter chromosomes congregate, condense, and form new nuclei within which the karyotheca and nucleolus reappear. Concurrently, the cell body undergoes constriction in the equatorial zone, giving rise to two independent daughter cells.

The total duration of these phases ranges from 1 to 1.5 hours. The interval between cell divisions is termed interphase. Within the mitotic cycle, it encompasses three periods: the postmitotic phase, DNA replication, and energy accumulation.

According to D.S. Sarkisov, Two Types of regeneration are distinguished: cellular and intracellular. Cellular regeneration is characterized by an increase in cell number while cell size remains unchanged. This type of regeneration is typical of skin epithelial cells, mucous membranes, and all Connective Tissues. Intracellular regeneration is characterized by an increase in the size of cells and their components, accompanied by heightened activity of intracellular structures. This type of regeneration is typical of ganglionic Cells of the Central Nervous system and, as some researchers believe, also occurs in The Development of cardiac and skeletal Muscles.

Alongside Cellular forms of Organization of living matter in The Human Body, non-cellular forms also exist, such as the intercellular ground substance. Furthermore, cellular structures may exhibit simplified (e.g., Blood erythrocytes lacking nuclei, Blood Platelets) or complex (e.g., symplasts, syncytia) forms of existence. An example of a symplast is a striated Muscle fiber; it cannot be classified as a single cell because it contains numerous nuclei. A syncytium consists of structures that form a continuous network via cytoplasmic processes. For instance, in reticular tissue, the protoplasm of certain cells merges with that of neighboring cells through such extensions.