Chapter summary

Understanding tissue structure and organization requires the ability to study a two-dimensional view in a microscope or a photomicrograph, but imagine how multicellular structures (ducts, tubes, layers, etc.) are organized.

Epithelial cells are classified based on the number cells in the layer. A layer one cell thick is called simple. If there are multiple layers of cells, it is called stratified. Cells are also classified based on shape: squamous, cuboidal, or columnar. When the tissue is stratified, the outer layer of cells is used to determine shape characteristics. Other specializations are also used to distinguish types of epithelium,that is,the presence of keratin or cilia, for example. Epithelial cells are often the "functional" cells within an organ or tissue and, in those cases, are referred to as the tissue parenchyma. Epithelial tissues typically have many cells that are very closely packed. Examples include the islet cells of the pancreas that produce insulin and glucagon, or the glandular and ductular cells that synthesize and secrete digestive enzymes. Epithelial cells also often create effective barriers between regions or compartments, for example, the barrier between the gut lumen and internal organs, or the lung surface and the internal body. Specialized structures between the cells (tight junctions, desmo- somes, and gap junctions) act to control anchoring of the epithelial cells, movement of molecules between cells, and communication between cells. Epithelial cells that are part of glands are arranged in several different structures including simple tubular, simple alveolar, compound tubular, or compound tubule- alveolar.

Connective tissues have many fewer cells than equivalent areas of epithelial tissues. A common cell type is the fibroblast, but other cells (wandering WBCs, macrophages, adipocytes, etc.) are frequent. In the regions between cells, there are a variety of extracellular elements that are produced primarily by the fibroblasts. These include collagen, elastin, reticular fibers, and proteoglycans. The density of fibers and extracellular materials allows for classifi­cation of connective tissues as loose or dense. In more compact tissues (tendons or ligaments), fibers are very densely arranged but are described as regular or irregular. Adipose tissue is a specialized connective tissue dedicated to the storage of triglyc­erides. Bone and cartilage is unique. In these tissues, the principal cells, osteocytes and chondrocytes, respectively, have become essentially trapped in the extracellular materials they have secreted. Cartilage occurs in three variations: hyaline, fibrocartilage, and elastic cartilage. Their properties are determined by variations in proteoglycans and fibers (collagen vs. elastin). Familiar as the white glistening cover at the ends of long bones, hyaline cartilage is the most abundant of the three types. Bone is particularly well organized. In a long bone, the outer compact bone is composed of circular layers of bone matrix arranged in concentric circles that become evident if the tissue is cut in cross section. Each of these structures is called a Haversian system or osteon. In the center, there is a canal Ihatprovides for passage of blood vessels and nerves. The appearance is much like the rings of a tree with the osteocytes residing in small spaces at the boundaries between lamellae or layers. In the center of the long bone, there are areas of ossi­fication by also spaces filled by bone marrow. Lastly, blood is also classified as connective tissue, that is, cells surrounded by a matrix (the blood plasma).

Muscle tissue appears in three versions: skeletal, cardiac, and smooth. All muscle cells are character­ized by their ability to contract and thereby to elicit movement and do work.However, the contractile ele­ments are most evident in cross sections of skeletal or cardiac muscle. In these views, alternating bands of light and dark occur along the entire length of the muscle cell. Groups of cells create bundles called fascicles and groups of fascicles to create muscles. The structure responsible for contraction, the sarco­mere, becomes clearly evident in transmission elec­tron microscope images. Individual sarcomeres are bounded on either end at Z-Iines where proteins that make up the thin filaments (primarily filamentous actin) are anchored. The center region of the sarco­mere has the thick filaments (primarily myosin). Con­traction involves the sliding of the thin filaments over the thin filaments. Since all the sarcomeres are arranged in sequence, as the individual sarcomeres shorten, the entire muscle shortens. Cardiac muscle cells also have sarcomeres, but muscle cells tend to be branched and muscle fibers are much shorter than in skeletal muscle. There are also specialized "connec­tors" between ends of cells called intercalated disks. In these areas, gap junctions are abundant. This allows for much greater coordination between groups of cardiac muscle cells. Finally, although calcium is central to the initiation of contraction in both skeletal and cardiac muscles, skeletal muscle depends on release of calcium for storage within the sarcoplasmic reticulum, whereas cardiac muscle contraction is more greatly impacted by extracellular calcium con­Centrations. Skeletal muscle is voluntary; that is, it requires neural input to initiate contractions. Cardiac muscle also depends on neural activity, but there is an increased degree of inherent control, that is, the pacemaker cells of the sinoatrial node.

Smooth muscle cells do not exhibit striations or sarcomeres.

Neural or nervous tissue is composed of neurons, but the majority of the cells are actually supporting cells called neuroglial cells. The supporting cells of the central nervous system include astrocytes, microglia, ependymal cells, and oligodendrocytes. In the peripheral nervous system, Schwann cells func­tion much like oligodendrocytes where they produce the myelin wrap around most axons.

Review questions and answers are available online.

References

Bacha, WJ., Jr. and L.M. Bacha. 2012. Color Atlas of Veteri­nary Histology, 3rd edition. Wiley-Blackwell, Chichester, West Sussex, UK.

Bartol, F.F., A.A. Wiley, DJ. Miller, AJ. Silva, K.E. Roberts, M.L.P. Davolt, J.C. Chen, A. Frankshun, M.E. Camp, K.M. Rahman, J.L. Vallet, and C.A. Bagnell. 2013. Lactocrine signaling and developmental programming. J. Animal Sci. 91: 696-705.

Ellis, S., R.M. Akers, A.V. Capuco, and S. Safayi. 2012. Bovine mammary epithelial cells lineages and parenchymal development. J. Animal Sci. 90: 1666-1673.