![]() Cells have been studied, classified, and precisely characterized since the seventeenth century when Robert Hooke first identified them under the microscope. Classically, estimates are that there are almost 200 different types of cells in an adult human body based on a histological or morphological perspective. Nonetheless, our knowledge of the cells that make up the human body, how they vary from person to person, or throughout development and in health or disease, is still quite limited. They are classified into different types and intended to perform specialized activities like nerve cells, muscle cells, etc. The cell size varies significantly with the diameter, ranging from 7.5 µm (RBC) to 150 µm (Ovum). The human brain itself is estimated to contain around a hundred billion neurons and the same amount of supporting glial cells. ![]() ![]() They show high diversity in their sizes, structure, number, and function. Ī human body is composed of close to fifty to a hundred trillion cells. The total number of cells, their type, size, and shape finally determines the size, structure, and function of each particular organism. A combination of these tissues is present in an organ. The tissues basically classify as four different types, namely, the epithelial, connective, muscular, and nervous tissues. ![]() Prokaryotic cells are some bacteria and blue-green algae which do not contain a clear and distinct nucleus, but the nuclear material is spread within the cytoplasm. The cells with a similar structure and function come together to form tissue. All living things are composed of cells, which then further subdivide based on the presence or absence of the nucleus, into two types: eukaryotic cells (Greek, Eu=true, karyo=nut, nucleus) - these cells are present in all the human, animal and plants with a clear, distinct nucleus. The cell is the fundamental organizational unit of life.
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