Animal Tissues

7.1 Introduction — Structural Organisation in Animals

All complex animals consist of only four basic types of tissues. These tissues are organised in specific proportions and patterns to form organs like stomach, lung, heart and kidney. When two or more organs perform a common function by their physical and/or chemical interaction, they together form organ system, e.g., digestive system, respiratory system, etc.

Cells, tissues, organs and organ systems split up the work in a manner that exhibits division of labour and contribute to the survival of the body as a whole. We will study the structure and function of these tissues in the context of the body plan of organisms.

7.2 Animal Tissues — The Four Basic Types

The structure of the cells vary according to their function. Therefore, the tissues are different and are broadly classified into four types:

1. Epithelial tissue — covers and lines surfaces.
2. Connective tissue — supports, binds, and protects.
3. Muscular tissue — produces movement by contraction.
4. Neural (nervous) tissue — receives stimuli and conducts impulses.

7.2.1 Epithelial Tissue

An epithelial tissue has a free surface, which faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body. The cells are compactly packed with little intercellular matrix. There are two types of epithelial tissues namely simple epithelium and compound epithelium. Simple epithelium is composed of a single layer of cells, while compound epithelium consists of two or more cell layers.

A. Simple Epithelium

Simple epithelium is further divided into three types based on cell shape and function:

When columnar or cuboidal cells bear cilia on their free surface, they are called ciliated epithelium. Their function is to move particles or mucus in a specific direction over the epithelium. They are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes.

B. Glandular Epithelium

Some of the columnar or cuboidal cells get specialised for secretion and are called glandular epithelium. They are mainly of two types:

• Unicellular, consisting of isolated glandular cells (goblet cells of the alimentary canal).
• Multicellular, consisting of cluster of cells (salivary gland).

On the basis of the mode of pouring of their secretions, glands are divided into two categories:

• Exocrine glands — secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products. These products are released through ducts or tubes.
• Endocrine glands — do not have ducts. Their products are called hormones, secreted directly into the fluid bathing the gland (blood).

C. Compound Epithelium

Compound epithelium is made of two or more cell layers (multilayered) and thus has limited role in secretion and absorption. Their main function is to provide protection against chemical and mechanical stresses. They cover the dry surface of the skin, the moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and of pancreatic ducts.

Cell Junctions in Epithelia

All cells in an epithelium are held together with little intercellular material. In nearly all animal tissues, specialised junctions provide both structural and functional links between its individual cells. Three types of cell junctions are found in the epithelium and other tissues:

• Tight junctions help to stop substances from leaking across a tissue.
• Adhering junctions perform cementing to keep neighbouring cells together.
• Gap junctions facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules.

7.2.2 Connective Tissue

Connective tissues are most abundant and widely distributed in the body of complex animals. They are named so because of their special function of linking and supporting other tissues/organs of the body. They range from soft connective tissues to specialised types, which include cartilage, bone, adipose, and blood. In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or elastin. The fibres provide strength, elasticity and flexibility to the tissue. These cells also secrete modified polysaccharides, which accumulate between cells and fibres and act as matrix (ground substance).

Connective tissues are classified into three types:

1. Loose connective tissue
2. Dense connective tissue
3. Specialised connective tissue (cartilage, bone, blood)

A. Loose Connective Tissue

Loose connective tissue has cells and fibres loosely arranged in a semi-fluid ground substance. There are two types:

• Areolar tissue is present beneath the skin and acts as a support framework for epithelium. Contains fibroblasts, macrophages and mast cells.
• Adipose tissue stores fats; located beneath the skin. Cells (adipocytes) are specialised to store fats. Excess of nutrients which are not used immediately are converted into fats and stored.

B. Dense Connective Tissue

Fibres and fibroblasts are compactly packed in dense connective tissues. Orientation of fibres show a regular or irregular pattern:

• Dense regular — collagen fibres are present in rows between many parallel bundles of fibres. Tendons (attach skeletal muscles to bones) and ligaments (attach one bone to another) are examples.
• Dense irregular — fibroblasts and many fibres (mostly collagen) oriented differently. Present in skin (dermis).

C. Specialised Connective Tissue — Cartilage, Bone, Blood

Cartilages are specialised connective tissues. The intercellular material of cartilage is solid and pliable and resists compression. Cells of this tissue (chondrocytes) are enclosed in small cavities within the matrix secreted by them. Most of the cartilages in vertebrate embryos are replaced by bones in adults. Cartilage is present in the tip of nose, outer ear joints, between adjacent bones of the vertebral column, limbs and hands in adults.

Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres which give the bone its strength. They are the main tissues that provide structural frame to the body. Bones support and protect softer tissues and organs. Bone cells (osteocytes) are present in the spaces called lacunae. Limb bones, such as the long bones of the legs, serve weight-bearing functions. They also interact with skeletal muscles attached to them to bring about movements.

Blood is a fluid connective tissue containing plasma, red blood cells (RBC), white blood cells (WBC) and platelets. It is the main circulating fluid that helps in the transport of various substances.

7.2.3 Muscle Tissue

Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. These fibres are composed of numerous fine fibrils, called myofibrils. Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion. Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body. In general, muscles play an active role in all the movements of the body. Muscles are of three types:

Skeletal muscle is closely attached to skeletal bones. In a typical muscle such as the biceps, striated (striped) muscle fibres are bundled together in a parallel fashion. Smooth muscle fibres taper at both ends (fusiform) and do not show striations. Cell junctions hold them together and they are bundled together in a connective tissue sheath. The wall of internal organs such as the blood vessels, stomach and intestine contains this type of muscle tissue. Smooth muscles are 'involuntary' as their functioning cannot be directly controlled.

Cardiac muscle tissue is a contractile tissue present only in the heart. Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together. Communication junctions (intercalated discs) at some fusion points allow the cells to contract as a unit, i.e., when one cell receives a signal to contract, its neighbours are also stimulated to contract.

7.2.4 Neural Tissue

Neural tissue exerts the greatest control over the body's responsiveness to changing conditions. Neurons, the unit of neural system are excitable cells. The neuroglial cells which constitute the rest of the neural system protect and support neurons. Neuroglia make up more than one-half the volume of neural tissue in our body.

When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma membrane. Arrival of the disturbance at the neuron's endings, or output zone, triggers events that may cause stimulation or inhibition of adjacent neurons and other cells.

7.3 Worked Examples