{"id":4696,"date":"2019-06-24T14:24:43","date_gmt":"2019-06-24T14:24:43","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/9-3-3\/"},"modified":"2023-11-30T18:48:23","modified_gmt":"2023-11-30T18:48:23","slug":"9-3-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/9-3-3\/","title":{"raw":"7.3 Human Cells and Tissues","rendered":"7.3 Human Cells and Tissues"},"content":{"raw":" \r\n\r\n[caption id=\"attachment_2809\" align=\"alignright\" width=\"514\"]\"\" Figure 7.3.1 Dust mop or human cells?<\/em>[\/caption]\r\n\r\nDust Mop<\/span>\r\n\r\nThis photo (Figure 7.3.1) looks like a close-up of an old fashioned dust mop, and the object it shows has a somewhat similar function. The object, however, is greatly enlarged in the photo. Can you guess what it is? The answer may surprise you.\r\n\r\nIt is a scanning-electron\u00a0micrograph of human epithelial cells that line the bronchial passages. The floppy, dust-mop-like extensions are actually microscopic structures called cilia projecting from the outer surface of the epithelial cells. The function of the cilia is to trap dust, pathogens, and other particles in the air before it enters the lungs. The cilia also sway back and forth to sweep the trapped particles upward toward the throat, from which they can be expelled from the body.\r\n
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Human Cells<\/h1>\r\n<\/div>\r\nLike the ciliated bronchial cells in the micrograph above, many other cells in the\u00a0human body\u00a0are very distinctive and well-suited for special functions. To perform their special functions, cells may vary in a number of ways.\r\n

Variation in Human Cells<\/h2>\r\nSome cells act as individual cells and are not attached to one another. Red\u00a0blood\u00a0cells are a good example. Their main function is to transport oxygen to other cells throughout the body, so they must be able to move freely through the\u00a0circulatory system. Many other cells, in contrast, act together with other similar cells as part of the same tissue, so they are attached to one another and cannot move freely. For example, epithelial cells lining the respiratory tract are attached to each other to form a continuous surface that protects the\u00a0respiratory system\u00a0from particles and other hazards in the air.\r\n\r\nMany cells can divide readily and form new cells. Skin cells are constantly dying and being shed from the body and replaced by new skin cells, and bone cells can divide to form new bone for growth or repair. On the other hand, some other cells \u2014\u00a0like\u00a0certain nerve cells \u2014 can divide and form new cells only under exceptional circumstances.\u00a0Nervous system injuries (such as a severed spinal cord) generally cannot heal by the production of new cells,\u00a0which results\u00a0in a permanent loss of function.\r\n\r\nMany human cells have the primary job of producing and secreting a particular substance, such as a\u00a0hormone\u00a0or an\u00a0enzyme. For example, special cells in the\u00a0pancreas\u00a0produce and secrete the hormone insulin, which regulates the level of glucose in the\u00a0blood. Some of the epithelial cells that line the bronchial passages produce mucus, a sticky substance that helps trap particles in the air before it passes into the lungs.\r\n

Different, but Identical<\/h2>\r\nAll the different cell types within an individual human organism are genetically identical, so no matter how different the cells are, they all have the same genes. How can such different types of cells arise? The answer is differential regulation of genes. Cells with the same genes can be very different because different genes are expressed depending on the cell type.\r\n

Examples of Human Cell Types<\/h2>\r\nMany common types of human cells \u2014 such as bone cells and white blood cells\u00a0\u2014\u00a0actually consist of several subtypes of cells. Each subtype, in turn, has a special structure and function. A closer look at these cell types will give you a better appreciation for the diversity of structures and functions of human cells.\r\n

Bone Cells<\/h3>\r\nThere are four main subtypes of bone cells, as shown in Figure 7.3.2. Each type has a different form and function:\r\n
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  1. Osteogenic cells<\/strong>\u00a0are undifferentiated stem cells that differentiate to form osteoblasts in the tissue that covers the outside of bone.<\/li>\r\n \t
  2. Osteoblasts<\/strong>\u00a0are immature bone cells that are involved in synthesizing new bone. They develop into osteocytes, or mature bone cells.<\/li>\r\n \t
  3. Osteocytes<\/strong>\u00a0are star-shaped bone cells that make up the majority of bone tissue. They are the most common cells in mature bone.<\/li>\r\n \t
  4. Osteoclasts<\/strong>\u00a0are very large, multinucleated cells responsible for the breakdown of\u00a0bones\u00a0through resorption. The breakdown of bone is very important in\u00a0bone health, because it allows for bone remodeling.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_2812\" align=\"aligncenter\" width=\"807\"]\"\" Figure 7.3.2 <\/em>Four subtypes of bone cells in the human skeletal system.[\/caption]\r\n\r\n
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    White Blood Cells<\/h3>\r\n<\/div>\r\nWhite blood cells (also called leukocytes) are even more variable than bone cells. Five subtypes of white blood cells are shown in Figure 7.3.3. All of them are immune system cells involved in defending the body, but each subtype has a different function. They also differ in the normal proportion of all leukocytes they make up.\r\n
      \r\n \t
    1. Monocytes<\/strong> make up about five per cent of leukocytes. They engulf and destroy (phagocytize) pathogens in tissues.<\/li>\r\n \t
    2. Eosinophils<\/strong> compose\u00a0about two per cent of leukocytes. They attack larger parasites and set off allergic responses.<\/li>\r\n \t
    3. Basophils<\/strong> make up less than one per cent of leukocytes. They release\u00a0proteins\u00a0called histamines that are involved in inflammation.<\/li>\r\n \t
    4. Lymphocytes<\/strong> make up about 30 per cent of leukocytes. They include B cells and T cells. B cells produce antibodies against nonself antigens, and T cells destroy virus-infected cells and\u00a0cancer\u00a0cells.<\/li>\r\n \t
    5. Neutrophils<\/strong> are the most numerous white blood cells, making up about 62 per cent of leukocytes. They phagocytize single-celled\u00a0bacteria\u00a0and\u00a0fungi\u00a0in the blood.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_2813\" align=\"aligncenter\" width=\"800\"]\"\" Figure 7.3.3 Five subtypes of human white blood cells in the human immune system.<\/em>[\/caption]\r\n\r\n
      \r\n\r\nTissues<\/span>\r\n\r\n<\/div>\r\nGroups of connected cells form tissues. The cells in a tissue may all be the same type, or they may be of multiple types. In either case, the cells in the tissue work together to carry out a specific function. There are four main types of human tissues: connective, epithelial, muscle, and nervous tissues.\u00a0 We will be learning about each of the four tissue types in more detail in the next few sections, but here is a summary:\r\n

      Epithelial Tissue<\/h2>\r\n[pb_glossary id=\"2814\"]Epithelial tissue[\/pb_glossary]<\/strong>\u00a0is made up of cells that line inner and outer body surfaces, such as the skin and the inner surface of the digestive tract. Epithelial tissue that lines inner body surfaces and body openings is called\u00a0mucous membrane.<\/strong>\u00a0This type of epithelial tissue produces\u00a0mucus<\/strong>, a slimy substance that coats mucous membranes and traps pathogens, particles, and debris. Epithelial tissue protects the body and its internal organs, secretes substances (such as hormones) in addition to mucus, and absorbs substances (such as nutrients).\r\n\r\nEpithelial tissue is identified and named by shape and layering.\u00a0 Epithelial cells exist in three main shapes: squamous, cuboidal, and columnar.\u00a0 These specifically-shaped cells can, depending on function, be layered several different ways: simple, stratified, pseudostratified, and transitional.\r\n

      Connective Tissue<\/h2>\r\nBone and blood are examples of connective tissue.\u00a0[pb_glossary id=\"2815\"]Connective tissue[\/pb_glossary]<\/strong> is very diverse. In general, it forms a framework and support structure for body tissues and organs. It's made up of living cells separated by non-living material, called an [pb_glossary id=\"6005\"]extracellular matrix[\/pb_glossary], which can be solid or liquid. The extracellular matrix of bone, for example, is a rigid mineral framework. The extracellular matrix of blood is liquid plasma. Connective tissues such as bone and cartilage generally form the body's structure.\r\n\r\nThere are three main categories of connective tissue, based on the nature of the matrix. They \u00a0look very different from one another, which is a reflection of their different functions:\r\n
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      1. Fibrous connective tissue: is characterized by a matrix which is flexible and is made of protein fibres including collagen, elastin and possibly reticular fibres.\u00a0 These tissues are found making up tendons, ligaments, and body membranes.<\/li>\r\n \t
      2. Supportive connective tissue: is characterized by a solid matrix and is what is used to make bone and cartilage.\u00a0 These tissues are used for support and protection.<\/li>\r\n \t
      3. Fluid connective tissue: is characterized by a fluid matrix and includes both blood and lymph.<\/li>\r\n<\/ol>\r\n

        Muscular Tissue<\/h2>\r\n[caption id=\"attachment_4170\" align=\"alignleft\" width=\"217\"]\"12.3 Figure 7.3.4 These magnified images show (a) skeletal muscle tissue, (b) smooth muscle tissue, and (c) cardiac muscle tissue.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"2817\"]Muscular tissue[\/pb_glossary] <\/strong>is made up of cells that have the unique ability to contract. There are three major types of muscle tissue, as pictured in Figure 7.3.4: skeletal, smooth, and cardiac muscle tissues.\r\n
          \r\n \t
        1. Skeletal muscles<\/strong>\u00a0are striated (or striped) in appearance, because of their internal structure.\u00a0Skeletal muscles\u00a0are attached to\u00a0bones\u00a0by tendons, a type of connective tissue. When they pull on the bones, they enable the body to move. Skeletal muscles are under voluntary control.<\/span><\/li>\r\n \t
        2. Smooth\u00a0muscles<\/strong>\u00a0are nonstriated\u00a0muscles. They are found in the walls of\u00a0blood vessels\u00a0and in the reproductive, gastrointestinal, and respiratory tracts. Smooth muscles are not under voluntary control.<\/li>\r\n \t
        3. Cardiac muscles<\/strong>\u00a0are striated and found only in the\u00a0heart. Their contractions cause the heart to pump blood. Cardiac muscles are not under voluntary control.<\/li>\r\n<\/ol>\r\n

          <\/h2>\r\n

          Nervous Tissue<\/h2>\r\n[pb_glossary id=\"2818\"]Nervous tissue[\/pb_glossary]<\/strong> is made up of neurons and other types of cells, generally called glial cells. Neurons transmit messages \u2014 usually through an electrochemical process \u2014\u00a0with support from the other cells. Nervous tissue makes up the\u00a0central nervous system\u00a0(mainly the brain and spinal cord) and\u00a0peripheral nervous system\u00a0(the network of nerves that runs throughout the rest of the body).\r\n
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          Feature: My Human Body<\/h1>\r\n<\/div>\r\nIf you are a blood donor, then you have donated tissue. Blood is a tissue that you can donate when you are alive. You may have indicated on your driver\u2019s license application that you wish to be a tissue donor in the event of your death. Deceased people can donate many different tissues, including skin, bone, heart valves, and the corneas of the eyes. If you are not already registered as a tissue donor, the information from the BC Transplant link below may help you decide if you would like to register.\r\n\r\nAs of April 2020, there were over 1.5 million donors registered in the BC Organ Donor Registry.\u00a0 In 2019, 480 lives were saved in BC as a result of organ donation, both through deceased donors and [pb_glossary id=\"2820\"]living donors[\/pb_glossary].\u00a0 Over the years, organ transplants have saved the lives of over 5,000 British Columbians.\u00a0 In 2019, 331 kidney transplants were conducted, the most common transplant needed, and of these, 120 kidneys were from living donors \u2014 people who donated their kidney and are still walking around to tell the tale!\u00a0 Despite these encouraging statistics, there are still over 640 British Columbians on the waiting list for a kidney transplant.\r\n\r\nUnlike organs \u2014 which generally must be transplanted immediately after the donor dies \u2014 donated tissues can be processed and stored for a long time for later use. Donated tissues can be used to replace burned skin and damaged bone, and to repair ligaments. Corneal tissues can be used for corneal transplants that restore sight in blind people. Unfortunately, there are not enough tissues to go around, and the need for donated tissues keeps rising.\r\n\r\nFor more information on organ and tissue transplants, you can visit the BC Transplant<\/a> website.\r\n
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          7.3 Summary<\/span><\/h1>\r\n<\/header>\r\n
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