Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

was first observed in 1855 by German researcher Rudolph Virchow.10 Mitosis defines division of eukaryotic nuclei, although the term is used broadly in defining cellular duplication.

      Cellular division differs between eukaryotes and prokaryotes. Prokaryotes (single-celled organisms) rely on asexual reproduction, producing offspring with the same genetic makeup of the parent through binary fission.

      Figure 2-10 Cellular mitosis.

      Eukaryotic cells undergo mitosis or cellular division and production by manufacturing identical copies that duplicate their DNA sequences through specific phases known as the cell cycle.

      Cellular mitosis is most simplistically defined as reproduction. Technically, mitosis is recognized as the division of nuclear cells in the production of two identical “daughter” cells during numerous phases (cell cycle):

      •Interphase – generally defined as a ‘pre-mitosis’ preparation activity

      •Prophase

      •Prometaphase

      •Metaphase

      •Anaphase

      •Telophase

      •Cytokinesis

      Each of these phases involved in mitosis is a vital aspect of growth and rejuvenation or replacement of older or damaged, or otherwise literally “worn out” cells.

      During mitosis, the cell divides and creates identical copies of itself. The process involves a parent cell that divides and produces identical daughter cells.

      This process enables the parent cell to translate or pass on its genetic coding to each daughter cell.

      Before this happens, the cells duplicate their DNA, and mitosis is defined as the process through which the cell separates identical copies or duplications of its nucleus.

      In many circles, Interphase is not technically defined as the first stage or phase of mitosis, but is generally further broken down into three separate yet distinct stages: G1 (first gap), S (synthesis phase), and G2 (second gap phase).

      During Gap 1 stage, the cells that will divide perform a number of metabolic activities, including growth. During the synthesis phase (S. phase) the cell effectively duplicates its DNA. Each chromosome creates its own copy known as a sister chromatid. The two chromatids fuse together in the shape of a X, with the intersection known as the centromere.

      During the second gap phase (G2), the cell grows and manufactures proteins necessary for mitosis.

      In Prophase, some structures inside the cell dissolve, while others are formed. Chromosomes will condense and mitotic spindles start to form. The spindle is responsible for organizing chromosomes, growing as the centrosomes gradually move apart. During this phase, the nucleolus also dissolves, triggering the next stage, typically called late prophase or prometaphase. As the nuclear envelope breaks down, the chromosomes are released. At this point, some microtubules bind to patches of protein (kinetochore) on the centromere of each sister chromatid.

      During metaphase, the spindle has lined up the chromosomes in the middle of the cell in preparation for actual division. At this time, chromosomes are aligned with kinetochores and attached to microtubules. This process is vital in order to ensure that the sister chromatids divide evenly between the two daughter cells. If chromosomes are not aligned properly, the cell will trigger this division process to stop until they are properly arranged.

      Anaphase occurs when the sister chromatids actually separate from one another. They drift to opposite sides of the cell, each now becoming its own chromosome. Each pair of chromosomes drift to opposite sides of the cell.

      This movement activity is compelled by motor proteins as defined earlier. The motor proteins literally transport chromosomes during this phase.

      Telophase is the point in time when the cell is nearly finished with the division process and once again reestablishes its normal structure. The division of the cellular contents actually takes place in a process known as cytokinesis. Cytokinesis is defined as the actual point at which division of the cytoplasm occurs, creating two new cells.

      At this point, the mitotic spindle is disassembled and two new nuclei form; one nuclei for each set of chromosomes. Following that, nuclear membranes reappear.

      When cells don’t divide properly, abnormal cellular growth processes go awry, creating cancer.

       ■Abnormal Cell Division

      Cancer is defined as the abnormal growth of a single cell or group of cells that have lost their ability to control their growth. A malignant cancer cell can appear in any tissue or organ in the body. As that cancer cell develops, grows, and then multiplies, it forms a mass. This mass of cancerous tissue is called a tumor.

      Figure 2-11 Abnormal cell development.

      Tumors can attack, invade, and then destroy adjacent tissues that are otherwise normal and healthy. Tumors, in most cases referring to abnormal growth or masses, are either classified as cancerous (malignant) or noncancerous (benign). Cancer cells that migrate from their initial point of origin can spread through a number of ways through the body. This is known as metastasizing.

      Development of cancer cells

      A normal and healthy cell can turn into a cancerous cell through a molecular process known as transformation. Transformation occurs in steps that include:

      •Initiation

      •Promotion

      Figure 2-12 Skin cancer.

      Initiation occurs when an alteration in the genetic material of the cell occurs. This alteration can be triggered in the chromosomal structure or the DNA. This will cause the cell to become cancerous or “abnormal”. The alteration of cellular genetic material is typically caused by carcinogens, or environmental agents, or spontaneously. The most common carcinogens that contribute to cancerous growth include:

      •Tobacco

      •Over-exposure to sunlight

      •Radiation

      •Chemicals

      Not everyone is susceptible to carcinogens. For example, a smoker may never develop lung cancer, although a non-smoker can. In many cases, it’s a matter of genetics as to whom is more susceptible to carcinogens.

      Promotion defines the latter step of cancerous development. Any agent that triggers promotion is called a promoter. In many cases, these are environmental substances or even drugs. Unlike carcinogens, a promoter doesn’t necessarily cause cancer all by itself, but it will “promote” or enable a cell that has undergone initiation to develop into a cancerous cell. Some cancers develop without the influence of promotion.

      Cancers can develop and grow by spreading directly into adjacent tissues or organs. It can also travel great distances within the body through the lymphatic system, which is the more typical in metastatic cancers.

      As an example, breast cancer typically migrates (metastasizes) into the lymph nodes nearby, and through the lymph, spreads through the body.

      Cancers can also spread through the bloodstream, typical of most sarcomas.

       ■Different Types of Cancer

      Malignant cancers are typically divided into either blood-forming tissue (such as lymphoma or leukemia) or a solid tumor such as with sarcoma or carcinoma.

      Cancers such as those that form in the blood or blood-forming tissues such as lymphomas or leukemias don’t form a mass or lump, but often remain as individually separated abnormal cells. However,