Structural Organisation of Animals: Epithelial Tissues, Connective tissues, Muscular Tissues, Nervous Tissues
Organisation of Animals : A study material of all classes students of USA UK Canada Australia India and other countries
All multicellular organisms including the humans consist of an enormous number of tiny units called cells. Understanding cells is essential for the study of almost any functional system in the body. An aggregate of cells having a common origin, struc- ture and function is referred to as a tissue. Differ- ent types of tissues combine to fashion various organs.
The four major types of tissues are structurally and functionally distinct. They are epithelium, con- nective, muscular, and nervous.
(i) Epithelial Tissues: Epithelium is a sheet of cells that covers an external surface or lines an in- ternal part of the body. One of its surfaces is free and is exposed to either air or fluid. The other sur- face is lined by a non-cellular basement membrane binding the cells together. The epithelial cells are divided into three categories based on their shape in the side view in a vertical section. These are squamous, cuboidal and columnar. Depending upon the number of cell layers, the epithelium is broadly classified into two types; simple (one layer thick) and stratified (many layers thick).
Squamous component cells are thin flat plate joined to make a simple sheet. In surface view, the cell outline is hexagonal or irregular wavy. Cells, in the side view, appear as slender spindles be- cause the cell height is almost negligible as com- pared to other dimensions. Squamous epithelium is found in lung alveoli (simple), epidermis of skin and oesophagus lining (stratified).
The height of the cuboidal cell is almost equal to the width so that in the side view it gives the appearance of a square. These cells are also hex- agonal in the surface view. Cuboidal epithelium occurs in thyroid gland (simple), testis tubule (stratified) and germinal epithelium of ovary.
Columnar epithelium is like cuboidal epithelium except that the cell height greatly exceeds the width, and therefore, cells appear as tall rectangles in the side view. This type of epithelium lines the stomach, intestine, pharynx and larynx.
Epithelial tissues specialise to perform different kinds of functions such as protection, transport, secretion, excretion, absorption, lubrication, sen- sory reception and reproduction.
(ii) Connective Tissues: These tissues con- nect and anchor parts, and give support to the body and its organs. A typical connective tissue consists of cells, fibres and ground substance. The latter two combine to form the matrix. The component cells include the fibroblasts forming fibres and the ground substance, macrophages and white blood corpuscles performing the function of defence, and fat cells enclosing oil droplets.
The fibres are of three kinds-white, yellow and reticular. The white fibres are made of fine threads called fibrils having a wavy outline. The yellow fibres are solitary, elastic, straight and show ex- tensive branching to form a network. The reticular fibres share the properties of both white and yel- low fibres.
The matrix is the basis for further classification of connective tissues. Three main categories are recognised-proper (loose and dense), supporting and fluid.
Loose (Proper)
Areolar Dominated by white and yellow fibres, fibroblasts and macrophages. Fastens down the skin, provides pathways for blood vessels and nerves and fills up unused space in various organs as packing material.
Adipose Composed of groups of closely packed fat cells. Provides insulation against cold, stores nutritive reserves, forms elastic pads be- tween organs and parts of the body which act as shock absorbers.
Reticular Forms basal framework of a number of important organs such a liver, spleen, tonsils, bone marrow, mucous membranes of the alimentary and respiratory tracts.
Denser (proper) tissues comprise tendons and ligaments. Tendons are made of rope-like ca- bles from white fibres and attach the muscles to bones. The ligament represents a tissue that is pri- marily elastic and is distinct yellow in colour.
Supporting tissues include the cartilage and bone. Typically they also consist of cells, fibres and the ground substance. Cartilage cells are ar- ranged in groups of two, four or eight cells. They are housed in cavities called lacunae in the matrix. Bone cells are biconvex and possess a number of branching cytoplasmic processes fitting into lacu- nae and canaliculi (fine capillary tubes), respec- tively, in the hardened matrix. The hardness is due to the deposition of phosphates and carbonates of calcium and magnesium.
Fluid tissues include blood as an atypical con- nective tissue because of the absence of fibres. It consists of the fluid component, the plasma as the ground substance in which the corpuscles and platelets lie suspended. The plasma is a complex fluid forming 55 per cent of the total volume of blood. Its activities are related to respiration, co- agulation, temperature regulation, buffer mecha- nisms and fluid balance. Plasma also transports hormones, antibodies, nutrients and excretory and excretory wastes. Corpuscles are of two types- red and white. Red blood corpuscles (RBC) or erythrocytes of mammals are without nuclei. Their red colour is due to the presence of an iron pig- ment, haemoglobin. They play a vital role in the transport of oxygen. The white blood corpuscles (WBC) or leucocytes are amoeboid cells and per- form the function of defence for the body through their various cell types, viz. agranular leucocytes (lymphocytes and monocytes) and granular leucocytes (eosinophils, basophils and neutrophils).
(iii) Muscular Tissues: Muscles comprise flesh of the body and the walls of hollow organs. These tissues make possible movements of the body. The unit of muscle tissue is a cell that is usually called a muscle fibre. It represents a com- pound structure since it consists of fine threads called myofibrils. A myofibril is made up of still finer elements called myofilaments. Muscles are classified into three types-smooth, striped and cardiac.
Since the smooth muscle is deprived of any cross stripes, it is also called unstriped or non- striate and involuntary. It constitutes the walls of hollow organs except the heart. The component cells are in the form of long spindles tapering at two ends except in the middle where the elongated nucleus is present. They are significant in bringing about wave-like-peristaltic movements by the in- voluntary contraction and relaxation of the walls of the oesophagus, stomach and intestine.
The striped muscle is also called skeletal mus- cle by virtue of its being attached to the vertebrate skeleton. A muscle is made up of long, unbranched, parallel cylindrical fibres. Each fibre is a multinucleate structure in which nuclei are elongated and located peripherally. The muscle fibre bears cross stripes in the form of alternate dark and light bands. Striped muscles bring about powerful voluntary contractions affecting movements of limbs and other parts of the body.
The cardiac muscle is peculiar to the heart. Its structure is similar to the skeletal muscle except that (a) it shows branching, (b) the dark bands are less prominent than in the skeletal muscle due to abundant muscle protoplasm which prevents the heart from fatigue; and (c) some prominent irregu- lar stripes run across the cardiac fibre (intercalated discs).
(iv) Nervous tissues: A neuron is structural and functional unit of the nervous system. A large number of such neurons constitute the nervous sys- tem in mammals. The nervous tissues does not exclusively consist of neurons but is as well sup- ported by non nervous cells called neuroglia. A neuron consists of a cell body with a prominent central nucleus and a number of cytoplasmic proc- esses. One to several short processes that arise from the cell body show tree-like form and are called dendrites. They carry impulses toward the cell. One long and thin cytoplasmic process termed axon moves away from the cell body. It maintains a uniform diameter and branches profusely at its terminal end. Each tiny branch ends in a swollen structure called synaptic knob. The axon may be covered with two sheaths: (a) a fatty myelin sheath interrupted at intervals; and (b) a tough protoplasmic sheath-the neurolemma, surrounding the myelin shealth. It is the axon that is directly concerned with the conduction of nerve impulse away from the cell body.
As one neuron is not directly joined to the next, there is a small gap between the two, designated the synapse. It is an area between the terminal end- ing of an axon and the cell body or dendrites of the next neuron. The synaptic knobs that contain the chemical transmitter, acetylcholine, help in the transmission of the impulse across the synapse after it has travelled the entire course of an axon.