Cells are classified into four types of primary tissues in higher animals including man. They are epithelial, connective, muscular and nervous tissues.
They are covering tissues. They serve as a protective covering and form a continuous layer on the entire body surface and cavities inside the body and its parts. The skin, surface layers of mouth, the alimentary canal and lungs are made of epithelial tissues. The cells in epithelial tissue vary in their shape. Depending upon the shape and function of the cells, epithelial tissues are classified as squamous (flattened), cuboidal (cubical), columnar, ciliated and glandular epithelium. A simple squamous epithelium is shown in Figure 5.5. The epithelial tissues are mainly protective in function. They also help in absorption and secretion.
Surface View of Simple Squamous Epithelium
Connective tissues vary widely in their form and function, but they are characterised by the presence of extracellular matrix or bedding substance. The classification of these tissues is as shown in Figure .
Classification of Connective Tissues
The extracellular matrix is a non-living material composed of protein fibres and ground substance. The protein fibres are composed of collagen (which provide strength) or elastin (which provides flexibility). The number and type of fibres differs between the various types of connective tissue.
The ground substance fills the spaces between the cells and the fibres. It contains interstitial fluid (tissue fluid) and large polysaccharide molecules. The consistency of the ground substance can vary from liquid to gel-like to a solid.
- Dense Connective Tissues: The layer of skin that lies deep to the epidermis is called the dermis and is composed of dense connective tissue. This tissue contains densely packed bundles of irregularly arranged collagen fibers. It is found in areas of the body that are subject to tension from many different directions. Nuclei of the connective tissue cells are scattered throughout the collagen fibers.
Areolar Tissues: It consists of adipocytes, or fat storage cells. It functions in energy storage, insulation, and cushioning. Small pockets of adipose tissue can be found all over the body, but accumulates under the skin (subcutaneous fat) and around certain organs, such as kidneys. Unlike other connective tissues, it has very little matrix and the cells are closely packed together. Each cell contains a large fat droplet, which pushes the nucleus to the side.
Hyaline cartilage: It is the most abundant type of cartilage in the body and is found in the rib cage, the nose, the trachea, and the ends of long bones. It provides structural support (but is more flexible than bone) and has cushioning properties. Hyaline cartilage has a firm matrix with abundant collagen fibers, but the individual fibers cannot be seen under the microscope. The cells, which are known as chondrocytes, reside in small cavities within the matrix called lacunae.
Bone tissue: It forms the skeletal system. It functions in structural support, protection, and mineral (calcium) storage. The extracellular matrix of bone tissue contains abundant collagen fibers as well as a hard, calcified ground substance. Mature bone cells, called osteocytes, reside in cavities within the matrix called lacunae. As bone tissue is formed, channels remain in the hardened matrix that provides passageways for blood vessels and nerves. The larger channels are called central canals (Haversian canals). Bone tissue forms in rings (lamellae) around these canals, creating a structure called an osteon.
- Blood: Blood consists of plasma and the formed elements.
- Plasma: It is the liquid portion of the blood. It consists mostly of water along with suspended proteins, and dissolved substances such as gases (oxygen and carbon dioxide), ions, food molecules, and hormones.
- Formed elements : These elements are the solid portions of the blood that are suspended in the plasma. They are as follows:
- Red blood cells (erythrocytes): They transport oxygen to the cells of the body using the red pigment, haemoglobin.
- White blood cells (WBCs or leukocytes): Although there are five different kinds of WBCs (see Figure 5.7), overall, it can be said that the role of the white blood cells is to fight infection.
- Platelets: They are involved in the clotting of the blood.
Classification of White Blood Cells
- The lymph is a colourless fluid tissue. The lymph consists of two parts, namely a clear, colourless fluid matrix, the plasma, and floating amoeboid cells, the white corpuscles mostly lymphocytes.
- It differs from the blood in lacking red corpuscles, platelets and some blood proteins.
- It also has less calcium and phosphorus than the blood. It can clot like blood.
- The lymph carries materials from the tissues into the bloodstream and vice versa and also destroys the invading microorganisms.
Types of Blood Cells
Muscles of the body are made of muscle cells. Being elongated in structure they are called muscle fibres. There are three types of muscle fibres.
- Striated muscles (skeletal or voluntary muscles).
- Unstriated muscles (smooth or involuntary muscles).
- Cardiac muscles.
(i) The skeletal muscles are attached to the bones and help in body movement (see Figure). The main characteristic features of this type of tissue are as follows:
- Cells of the tissue are long cylindrical, non-tapering and unbranched.
- Transverse alternate light and dark bands or striations can be seen.
- There are many nuclei which are situated towards the periphery of the muscle fibre.
Striated Muscle Cells
(ii) Smooth or unstriated muscle tissues are found within the walls of all the tubular organs such as stomach, intestine, ureter, bronchi, etc. These muscles help in the contraction of the organs in which they are present (see Figure). They have the following features:
- The cells are long with pointed ends (spindle shape).
- The cells have only one nucleus situated in the centre.
- It does not show any stripes or striations across the muscle.
Unstriated Muscle Cells
(iii) Cardiac muscle tissues are exclusively present in the heart. They help in the contraction of heart (see Figure). Their chief features are as follows:
- They are composed of non-tapering cells with faint cross-striations.
- Each cell contains one nucleus situated in the centre.
- The function is rhythmic contraction and relaxation throughout life without fatigue under normal conditions.
- The cells are cylindrical and branched.
- Partitions between the two cells are known as inter-related discs.
Cardiac Muscle Cells
Brain, spinal cord and nerves are all composed of nervous tissues (see Figure 5.12). The cells of the nervous tissue are called neurons. These are highly specialised to conduct impulses (signals). The impulse travels from one neuron to another neuron. Neurons are the structural and functional unit of nervous tissue.