Zoology Notes: 007 Chapter 4

Chapter 4. Tissues The bodies of multi-cellular organisms, particularly animals, are organized on the basis of tissues, organs, and organ systems. Organ systems are composed of organs, which are in turn composed of tissues. Tissues are aggregates of cells that are usually similar in both structure and function. The study of the structures and functions of tissues is called histology. Tissues are divided into four major categories: epithelial, connective, muscular, and nervous. During embryonic development, three germ layers differentiate into the four major categories of tissues by a process called histogenesis. The three germ layers are the ectoderm (outer), mesoderm (middle), and endoderm (innermost). Epithelial tissues differentiate from all three germ layers. Connective and muscular tissues differentiate from the mesoderm and nervous tissues differentiate from the ectoderm. Epithelial Tissue Epithelial tissue forms the covering or lining of all free body surfaces, both external and internal. The chief functions of epithelial tissues are: protection, absorption, secretion, and excretion. Epithelial tissues are avascular (lack blood vessels). Nutrition and waste removal are provided by the network of blood vessels in underlying connective tissues. In general, epithelial cells are attached to a specialized structure called basement membrane which serves as an anchor for the inner side of cells affording protection to the underlying connective tissues. Unique structures along the lateral surface of epithelial cells, called intercellular junctions, play essential functional roles in various kinds of epithelial tissues. Three distinct types of junctions have been identified. Tight junctions govern permeability. Gap junctions make intercellular communication possible by exchange of chemical substances. Desmosomes or adhering junctions provide strong mechanical linkage between cells, thereby preventing tissue disruption from stretching. Cells composing epithelial tissues are classified according to their shape, arrangement of cell layers, and function. Classification as to shape: 1. Squamous – flat and often serve as a protective layer. 2. Cuboidal – resembling small cubes. They are found in five regions of the body as lining tissues for ducts, secretory glands, renal tubules, germinal coveings of the ovaries, and pigmented layer of the retina of the eye. 3. Columnar – tall and often rectangular. They line ducts such as the urethra and are found in mucus-secreting tissues, mucosa of the stomach, bile ducts, villi of the intestines, uterine tubes, and upper respiratory tract. Classification as to arrangement of cell layers: 1. Simple – arrangement has one layer. 2. Stratified – arrangement has multiple layers. 3. Pseudo-stratified – arrangement seems to consist of several layers but is actually a single layer with all cells resting on the basement membrane. Fig. 4.1. Types of epithelial tissues. 25

4. Transitional – consists of several layers of closely packed, soft, pliable, and easily stretched cells. When the surface is stretched, the cells are flat but they appear saw-toothed when relaxed. They line the renal pelvis of the kidneys, the ureters, the urinary bladder, and the upper part of the urethra. Classification as to function: 1. Mucous membrane – serve four general functions: protection, support for associated structures, absorption of nutrients into the body, and secretion of mucus enzymes, and salts. They line the digestive, respiratory, urinary, and reproductive tracts. 2. Glandular epithelium – arise as involutions of epithelial cells, specializing in synthesizing and secreting certain special compounds. They are found in sweat glands, sebaceous glands, glands of the alimentary tract, pancreas, mammary glands, and large salivary glands. 3. Endothelium – serve as lining epithelium of lymphatic vessels, blood vessels and the lining of the heart (endocardium). Connective Tissue The second major division of tissues, connective tissues, which include the connective tissue proper and a number of specialized connective tissues, perform many functions including support and nourishment for other tissues, packing material in spaces between organs, and defense for the body by phagocytosis and antibody production. Connective tissues have fewer cells that are set apart due to an abundance of intercellular substances or ground substances that contain fibers except in blood. The ground substance may be fluid, gelatinous, or solid. Solid ground substances are often called matrix. General functions of connective tissues include: 1. Binding tissues and organs 2. Mechanical support 3. Storage of fats (in adipose tissue) and certain minerals (calcium in bones) 4. Exchange of metabolites between blood and tissue (lymph) 5. Play a significant role in the repair or healing of wounds, particularly the loose connective tissues 6. Protection against infection The Connective Tissue Proper 1. Loose connective tissue – fibers are loosely arranged in a meshwork a. Areolar tissue – most widely distribute connective tissue which is pliable and crossed by many delicate threads. The tissue resists tearing and is somewhat elastic. b. Adipose tissue – specialized connective tissue with fat-containing cells. Since fat is a poor conductor of heat, adipose tissue protects the body from excessive heat loss or excessive rise in temperature. c. Reticular – form the network of lymphoid tissue, the liver, and the bone marrow. 2. Dense connective tissue – has compact or loosely packed fibers a. Dense regular – fibers are arranged in parallel bundles (tendons, white ligaments, fascia, aponeuroses). b. Dense irregular – fibers are closely interwoven in a random way (dermis of the skin, capsules of organs, tendon sheaths). Specialized Connective Tissue

26

1. Cartilage – tissue whose intercellular substances contain fibers, firm but pliable in consistency. Cells of cartilage, called chondrocytes, are large and rounded with spherical nuclei and are clustered in small cavities called lacunae. Cartilages are covered with a dense connective tissue called the perichondrium. Since cartilages are avascular, chondrocytes are nourished by diffusion through the matrix of substances from perichondrial blood vessels. a. Hyaline cartilage – somewhat elastic, semi-transparent with an opalescent bluish-gray tint. They are found in the nose, larynx, trachea, bronchi, ends of ribs, and surfaces of bones. b. Elastic cartilage – yellowish with greater opacity, flexibility, and elasticity. They are found in the external ear, walls of external auditory and eustachian tubes, and epiglottis. c. Fibro-cartilage – most resistant type. They are found in the intervertebral disks, in the pubis synthesis, in the mandibular joints and in sites of attachment of certain tendons to bones. This type of cartilage has no perichondrium. 2. Bone – firm tissue formed by the impregnation of intercellular material with organic salts. It is a living tissue supplied by blood vessels and nerves and is constantly being remodeled. The two types of bones are compact, forming the dense outer layer, and cancellous or spongy, forming the inner, lighter tissue. 3. Dentin – closely related to bone. The crown of the tooth is covered by enamel, the hardest substance in the body. Enamel is secreted onto the dentin by epithelial cells of the enamel organ. Dentin resembles bone but is harder and denser. 4. Blood and Hematopoetic Tissue – red bone marrow is the blood-forming (hematopoetic) tissue. Blood is a fluid tissue circulating through the body, carrying nutrients to cells and removing waste products. Solid or formeed elements of the blood are: red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (thrombocytes). 5. Lymphoid Tissue – found in lymph nodes, thymus, spleen, and tonsils. Reticular tissue forms its framework, and lymphocytes lie within the reticular tissue. Lymphoid tissue plays a role in immunity. Muscular Tissue Muscular tissues are primary tissues of motion responsible for locomotion and movement of the different parts of the body. They are composed of muscle fibers and intercellular substances (loose areolar connective tissue). Three Types of Muscle Tissue 1. Skeletal muscle – striated, voluntary – muscle has cross-striations and can be controlled at will. 2. Smooth muscle – non-striated, involuntary – without striations and is under the control of the autonomic nervous system. 3. Cardiac muscle – striated, involuntary – found exclusively in the heart.

Nervous Tissue Fig. 4.2 Types of muscle tissue. From left: skeletal muscle, smooth muscle, and cardiac muscle.

27

The fourth type of tissue, nervous tissue, is divided into two categories: nervous tissue proper (neurons) and accessory cells (neuroglia). Nervous tissue is the most highly organized tissue in the body initiating, controlling, and coordinating the body’s ability to adapt to its environment. In nervous tissue proper, the specialized conducting cells are neurons, linked together to form nerve pathways. A neuron is composed of dendrites, a cell body (soma), and an axon.

Fig. 4.3. A typical neuron with dendrites, a body, and an axon.

28