Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

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Multicellular Organisms
Types of Animal Cells and Tissues

Tissue Cells, even highly differentiated ones, tend to rapidly lose their differentiation when grown in culture and transform into one of three main Cell types: epitheliocytes, mechanocytes, or amoebocytes. Epitheliocytes are tightly packed cells of epithelial origin; they are thought to derive from the two superficial layers of the embryonic blastula. Mechanocytes, often referred to as fibroblasts or fibrocytes, are cells that grow during the cultivation of Muscle or Connective Tissue. Like amoebocytes, they originate from embryonic mesenchymal cells that migrate inward from the lower layer of the blastula (Chapter 16, Section B.2). Neurons, neuroglia, and lymphocytes are also regarded as distinct cell types.

a. Tissues

By combining, cells form four MAIN TYPES OF tissue. Epithelium lines the body surfaces and Internal Organs, including the Skin, digestive tract, urogenital tract, and glands. There are two primary types of epithelium: squamous and columnar. Epithelial cells are constituents of glands (sweat, sebaceous, mammary, and Endocrine glands) as well as certain Sensory Organs. These cells are polarized: one surface faces outward toward an air or Water medium, while the other typically adheres directly to the basement membrane.

Connective (supporting) tissue includes adipose, cartilaginous, and Bone Tissues. The latter two tissue types contain a large amount of intercellular matrix, known as the ground substance, which consists predominantly of complex Polysaccharides. Embryonic fibroblasts differentiate into two cell types: white fibroblasts produce the protein Collagen, and yellow fibroblasts form Elastin. Both Proteins accumulate in the extracellular space and become incorporated into the ground substance. Osteoblasts form bone by depositing calcium phosphate and carbonate salts (in layers 3— 7 мкм thick), along with organic cementing substances.

The third type is Muscle tissue. A distinction is made between striated (voluntary skeletal Muscles) and smooth (involuntary) muscles, with cardiac (involuntary striated) muscle standing apart as a distinct category.

The fourth tissue type, Nervous Tissue, consists primarily of two main cell types. Neurons are the principal elements of The Nervous System responsible for the Generation and Conduction of impulses, whereas glial cells lie between the neurons and provide structural support for them.

b. Blood Cells

Blood and the cells lining Blood Vessels are sometimes considered a fifth tissue type. In humans, 1 ml of blood contains 5∙10⁹ erythrocytes (red blood cells), which translates to a total of 2,5∙10¹³ cells based on the total blood volume in the body (5 l). Erythrocytes are rapidly synthesized in the Bone Marrow. Mature erythrocytes in humans and other mammals lack a Nucleus and are almost entirely filled with Hemoglobin. The average lifespan of these cells is 125 days; they are destroyed by leukocytes in the Spleen and Liver.

Leukocytes (white blood cells) number about 7 ∙ 10⁶ per 1 ml, which is nearly a thousand times fewer than erythrocytes. Leukocytes comprise three cell types: lymphocytes (~26% of the total leukocyte count), monocytes (~7%), and polymorphonuclear leukocytes, or granulocytes (~70—75%). Lymphocytes are roughly the same size as erythrocytes, but they originate in lymphoid tissue. The lifespan of individual lymphocytes can reach up to 10 years. Their function is to produce Antibodies, and it is thanks to these cells that long-term Immunity is maintained. Monocytes are twice the size of lymphocytes and are capable of engulfing bacterial cells. Granulocytes, measuring 9—12 мкм in diameter, are produced in the Cytology/practical/86.html">Red bone marrow and perform various Functions. Staining with different Dyes reveals Three types of granulocytes: neutrophils, eosinophils, and basophils. Neutrophils, like monocytes, engulf bacterial cells; the function of the other two granulocyte types is less clear. It is known that the number of eosinophils increases in hay fever, asthma, and in the presence of certain parasitic organisms, whereas the number of basophils rises sharply in leukemia and during inflammatory processes. Blood Platelets (thrombocytes) are tiny (2—3 мкм in diameter), cell-like corpuscles formed from the Cytoplasm of bone marrow megakaryocytes; a single mature megakaryocyte produces up to 3,000 platelets, and the total platelet count is 1—3∙10⁸ per 1 ml of whole blood. Platelets play a vital role in blood clotting.

c. Cell Culture

The cultivation of isolated animal cells under laboratory conditions is gaining increasing importance in biochemical research. In some cases, this is necessary to obtain a sufficient quantity of cells with as closely matching Genetic information as possible. For Bacteria, such cells are obtained by plating and selecting a small colony grown from a single cell, which represents a "pure strain." Similarly, a tissue culture can be grown from a single Introduction/5.html">Eukaryotic Cell to yield a clone of genetically identical cells, which will remain identical until Mutations alter them.

An important breakthrough in this field was The Development of embryonic fibroblast culture techniques for Prenatal Diagnosis of inherited Metabolic Disorders (Box 1-G). Embryonic or Cancer cells are the easiest to cultivate, but cultures of many other tissues can be established under specific conditions. It should be noted that the cells that grow best are not entirely normal; for example, the widely known HeLa cell line (human cancer cells that have been cultivated for many years in laboratories worldwide) contains 70—80 Chromosomes instead of the usual 46.