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The cell is the basic organizational unit of life. All living organisms consist of cells, which are categorized into 2 types based on the presence or absence of a nucleus. Eukaryotic cells (derived from Greek, with "eu" meaning true and "karyo" referring to the nucleus) possess a well-defined nucleus and are found in humans, animals, and plants (see Image. Animal Cell). In contrast, prokaryotic cells, such as certain bacteria and blue-green algae, lack a distinct nucleus, with nuclear material dispersed within the cytoplasm. Cells with similar structures and functions assemble to form tissue. Tissues are classified into 4 primary types: epithelial, connective, muscular, and nervous. Organs consist of combinations of these tissues. The total number, type, size, and shape of cells collectively determine an organism’s size, structure, and function.[1] Current estimates indicate that the average adult male body has approximately 36 trillion cells, the average adult female body has around 28 trillion cells, and the body of a 32-kg child has about 17 trillion cells.[2] The human brain alone is estimated to contain around 100 billion neurons and an equal number of supporting glial cells.[3] Cell size varies widely, with diameters ranging from 7.5 µm in red blood cells to 150 µm in ova. Cells are classified into different types, each specialized for distinct functions. Classical estimates suggest that the adult human body contains nearly 200 distinct cell types based on histological and morphological characteristics. Despite extensive research, knowledge remains limited regarding the composition of human cells, their variability between individuals, and how they change during development or in response to health and disease. Since the 17th century, when Robert Hooke first observed cells under a microscope, researchers have studied, classified, and characterized them in detail. However, the complete molecular composition of cells and their interactions within tissues and organ systems are not yet fully understood. Many undiscovered cell types, cellular modifications, and interactions likely remain unidentified. In 2017, the International Human Cell Atlas Initiative was established to create a comprehensive reference map of all human cells. The goal of this initiative is to enhance our understanding of human health and improve disease diagnosis and treatment.

Despite extensive research, knowledge remains limited regarding the composition of human cells, their variability between individuals, and how they change during development or in response to health and disease. Since the 17th century, when Robert Hooke first observed cells under a microscope, researchers have studied, classified, and characterized them in detail. However, the complete molecular composition of cells and their interactions within tissues and organ systems are not yet fully understood. Many undiscovered cell types, cellular modifications, and interactions likely remain unidentified. In 2017, the International Human Cell Atlas Initiative was established to create a comprehensive reference map of all human cells. The goal of this initiative is to enhance our understanding of human health and improve disease diagnosis and treatment. A deep understanding of cell histology enables healthcare professionals to recognize normal and pathological cellular structures, facilitating accurate diagnosis and targeted treatment of diseases. Mastery in this field enhances participants' ability to interpret microscopic findings, improving clinical decision-making and patient outcomes.

Oxygen, as a biradical, reacts with various metal ions and biological molecules in a process known as oxidation. Mitochondrial respiration generates reactive oxygen species (ROS) such as superoxide anion radical (O2·?), hydrogen peroxide (H2O2), and hydroxyl radical (·OH). All biological molecules, including DNA, are vulnerable to oxidative damage. To counteract oxidation-induced stress, the body employs antioxidant mechanisms involving superoxide dismutases, peroxidases, peroxiredoxins, glutathione, and glutaredoxins. DNA constantly sustains damage from cellular metabolites and external mutagens, which can cause strand breakage during replication. Cyclin-dependent kinases (CDKs), which regulate the cell cycle, also contribute to DNA repair.[35] Double-stranded breaks are the most toxic DNA lesions. If left uncorrected or incorrectly repaired, these breaks may result in loss of heterozygosity or extensive chromosomal rearrangements. Other types of DNA damage include single-stranded breaks, depurination, depyrimidination, O6-methylguanine formation, and cytosine deamination. These alterations may lead to disease if not corrected by the DNA repair system. The DNA repair system includes nonhomologous DNA end joining (NHEJ), base excision repair (BER), single-strand break repair (SSBR), homologous recombination, and interstrand cross-link (ICL) repair. Defects in homologous recombination and ICL repair can contribute to Fanconi anemia, familial breast cancer, and ovarian cancers. Insufficient NHEJ function is associated with severe combined immunodeficiency. Pathologies linked to BER and SSBR deficiencies include hyper-immunoglobulin M syndrome and colorectal carcinomas. Defective SSBR also manifests with ataxias. DNA damage response pathways are crucial for maintaining genome stability, preventing neurodegeneration and malignant transformation, and supporting normal growth, immune development, and neurogenesis.[36] Cellular Adaptations Cells undergo various adaptations in response to external stimuli or environmental demands. These changes can be physiological or pathological and sometimes contribute to disease progression. Cellular adaptations generally fall into 5 categories.

The DNA repair system includes nonhomologous DNA end joining (NHEJ), base excision repair (BER), single-strand break repair (SSBR), homologous recombination, and interstrand cross-link (ICL) repair. Defects in homologous recombination and ICL repair can contribute to Fanconi anemia, familial breast cancer, and ovarian cancers. Insufficient NHEJ function is associated with severe combined immunodeficiency. Pathologies linked to BER and SSBR deficiencies include hyper-immunoglobulin M syndrome and colorectal carcinomas. Defective SSBR also manifests with ataxias. DNA damage response pathways are crucial for maintaining genome stability, preventing neurodegeneration and malignant transformation, and supporting normal growth, immune development, and neurogenesis.[36] Cellular Adaptations Cells undergo various adaptations in response to external stimuli or environmental demands. These changes can be physiological or pathological and sometimes contribute to disease progression. Cellular adaptations generally fall into 5 categories. Hypertrophy refers to an increase in cell size without an increase in cell number, leading to an overall enlargement of a structure. This adaptation is evident in the pregnant uterus and in the muscles of bodybuilders. The increase in muscle mass is attributed to a protein growth factor called "insulin-like growth factor 1" (IGF-1).[37] Hyperplasia involves a rapid increase in cell number, resulting in an overall enlargement of a tissue or organ. This process can be physiological or pathological. A physiological example is the expansion of the pregnant uterus. Pathologically, hyperplasia may be either benign or malignant. Benign prostatic hyperplasia (BPH) is a common example of benign hyperplasia. In contrast, endometrial hyperplasia, which can be a precursor to endometrial carcinoma, is a pathological condition characterized by excessive proliferation of the endometrial glandular tissue and stroma.[38] Atrophy is the opposite of hypertrophy, and it involves a reduction in cell size that leads to a decrease in the overall size of a tissue or organ. A classic example of physiological atrophy is the shrinkage of the thymus after middle adulthood. Disuse atrophy occurs when a specific tissue or organ diminishes in size due to prolonged inactivity. The process results from a loss of cell organelles, proteins, and cytoplasm.[39]

Atrophy is the opposite of hypertrophy, and it involves a reduction in cell size that leads to a decrease in the overall size of a tissue or organ. A classic example of physiological atrophy is the shrinkage of the thymus after middle adulthood. Disuse atrophy occurs when a specific tissue or organ diminishes in size due to prolonged inactivity. The process results from a loss of cell organelles, proteins, and cytoplasm.[39] Metaplasia is an adaptive response in which one type of healthy cell is replaced by another within a tissue or organ. This change occurs in response to an abnormal stimulus.[40] A common example is the transformation of cells in the lower esophagus due to chronic gastroesophageal reflux, giving rise to a condition known as Barrett esophagus. Dysplasia refers to the abnormal arrangement of cells caused by alterations in their typical growth patterns. This condition often represents a precancerous change, increasing the risk of progression to malignancy if left untreated.[41]