Jethalal S. Zaveri
A. CELLS
Unit of Life
The human body and its parts are made up of (i) trillions of microscopic structures called cells, (ii) inter-cellular material (matrix) that the cells produce, and (iii) body fluids. Of these three ingredients, only the cells have the characteristics which distinguish life from non-life viz., growth, metabolism, response to stimuli and reproduction. Cells are thus the smallest living units of the body.
Since cells are the units from which the body is built, we shall commence our study of the body systems with them. The cell is often called the basic element of life. In fact, it is life itself. Can you imagine to compress the functions of a big city viz, transport system, power stations, communication set-up, factories and waste disposal system, all in a tiny sphere of about one-hundredth of a millimeter in diameter? This is exactly what a cell is. It is hard to believe that a structure too small to see with the naked eye should be as complex as a city.
Size and Shape
Cells are microscopic units with great diversity of size and shape. There are 60 billion (60,000,000,000,000) cells in a human body. Though they are of different sizes, nearly all human cells need a high magnification microscope to be seen and a super microscope to peep inside its body. The smallest cells (certain brain cells) are about 1/200 mm and the largest ones (ova) are about 1/4 mm in diameter. They come in a variety of shapes. Some cells are nearly spherical, others are like tiny cubes and still others are cylindrical in shape. Nerve cells are long, thread-like structures Red blood cells are only 7.5 microns [1] in diameter. A muscle cell may be more than 2 to 3 cms long but only 50 microns in diameter. Yet with all these differences nearly all the cells share certain features in common. This is the rationale for the "typical cell" we shall now discuss. Most cells carry out specialized functions. Cells of similar forms and functions group together to form tissues.
Structure of the Cell
The development of the electron microscope and other new techniques for probing into their depths has revealed finer details of cell structure for the first time. The cell is found to have one of the most complex structures in the universe. Within its confinement, thousands of chemical reactions take place. Each cell of the body leads a life, that in some respect mirrors, in miniature, the life of the body as a whole.
A typical cell is bounded by a delicate cell membrane that encloses the living substance protoplasm. Roughly at the centre of the cell is the nucleus enclosed in its own membrane. This is the control centre without which it cannot exist. The protoplasm of the nucleus is referred to as nucleoplasm, while that outside the nucleus is called cytoplasm.
Numerous tiny structures or organelles are dispersed in the cytoplasm. Some, such as mitochondria, have their own membrane covering. The number, appearance and arrangement of the various organelles vary with the type of cell and are co-related with the function of the cell as a whole. When a cell divides in two, yielding two smaller copies of itself, all its characteristic organelles are reproduced.
The organelles are specialised "departments" organelles where specific jobs are done. The cytoplasm is highly structured and compartmentalized and contains numerous organelles. The actual assortment of organelles may vary from one type of cell to another; however the functions of each organelle are the same in all cells.
The Nucleus
The nucleus, as its name indicates, is a very important structure. Without it, cell cannot divide and eventually it dies. The nucleus is surrounded by an envelope which is a site of active interaction with the cytoplasm. Each nucleus contains a complete set of hereditary information, encoded in the DNA blue-prints, for all of the structures and functions of the whole body. These are found in a characteristic set of 46 thread-like structures called the chromosomes, grouped into 23 matching pairs.
The cell membrane has a 3-layered protein-lipid-protein structure. Its thickness is not uniform. It is far from a passive 'bag' holding in the cell's contents. In fact, it is a scene of frenetic activity. It is semi-permeable and permits certain substances to pass into or out of the cell, while barring the way to others. This external membrane of a cell is as remarkable as its internal structure. It is a bare.0000001 millimetre thick. Acting as a watchman, it decides what shall be admitted or excluded. It seems to have a communication system to talk to other ceils. Hormones, secreted by the endocrines or ductless glands and neurohormones are the chemical messengers carrying work orders for regulating the production of various cells.
For its operation, the cell requires a lot of energy which is generated in hundreds of super-minute power stations called mitochondria. The process is the familiar combustion process in which sugar is the fuel which combines with oxygen producing energy and leaving behind carbondioxide (and water). During this chemical reaction, they synthesize adenosine triphosphate (ATP) which is the universal source of power for every living being and which can be stored until required. Whenever energy is needed—to think, to speak or to lift a load—(ATP) breaks down into simpler substances releasing energy in the process. All cells have mitochondria except the red blood cells; since they do no manufacturing, they have no need for power.
Reproduction and Division of Cells
Living organisms perpetuate their kind from one generation to another through reproduction. It is a careful duplication and transmission of characteristics from parent to offspring. Reproduction at the cellular level occurs by a process of cell division in which the original cell splits into two cells. We have dealt with the process of reproduction, in detail, in a later chapter. Here we shall give a brief account of a process of normal cell division called mitosis.
In almost every tissue, cells wear out and must be replaced. Even after overall growth of the body has ceased, growth still occurs at the cell and tissue levels, providing for the replacement of worn out or damaged structures. New cell formation occurs through cell division. Human life itself begins with the division of a single cell, the ovum (female egg) fertilized (by a male sperm) in the mother's womb. This single cell divides into two cells which in turn divide into four.
Cell-division (Mitosis)
Mitosis is a continuous process with characteristic sequences of events as one cell becomes two. It begins with the replication of the DNA (deoxyribonucleic acid) of the nucleus. The division continues till there are millions of cells of the human body. Even this wonderful phenomena of multiplication is nothing when compared to the transmission of enormous amount of information stored within the fertilized egg. This tiny fragment of life contains the genes—the messengers of heredity. They store complete blueprints for building complex chemical plants like liver and coded information on colour, texture and size of the body.
Genes are strung into long thin chains called chromosomes. There are 46 (23 pairs) chromosomes in each human cell. The genetic material is a mass, consisting of long thin tangled strands. In the next phase the tangled strands become shortened and thickened into discrete, rod-like structures. Each of these actually consists of two separate strands jointed together by a small body. In the next phase these chromosomes begin to move and align themselves. When the alignment is complete, each double stranded chromosome divides producing two single-stranded daughter chromosomes. One of each pair of formerly jointed chromosomes are dragged outward in the opposite directions. In the next phase the chromosomes lose their discrete shapes, reforming the tangled mass. A pinching in the cell membrane along the equator of the old cell appears and deepen progressively until the old cell separates into two replicas, each surrounded by its own complete cell membrane. Various organelles have been distributed also. Thus when division is completed, two fully equipped, functioning cells have been produced, ready to grow and to divide again.
The deoxyribonucleic acid (DNA) is the dictator of all cells controlling their behaviour by ordering their constituents what to make, what to seek and what to avoid. It can be compared to an architect who designs, draws up plans and prepares blueprints for a building. Actual construction is carried out by contractors ribonucleic acid - RNA
The division that commenced in the womb continues throughout life. Millions of cells die and millions are born by the process of division, each producing two new ones which are exact duplicates of the mother cell. The exception to this constant replacement are brain cells called neurons. Worn out and damaged neurons keep dying without replacement. We shall learn more about neurons in the next chapter.
Chemistry of Life
Each living cell contains thousands of different kinds of chemicals. These chemicals are not an inert mixture but are constantly interacting with one another. The blue-prints of heredity are encoded in chemical from. The structures of the body are built up from chemical constituents, and differences in chemical composition distinguish one type from another.
To sum up, cells participate in every function of the body from birth to death. It is really a supreme wonder how 60 billions of them live in such harmony, each one performing its own assigned duty.
B. TISSUES
Types of Tissues
Groups of cells with a similar structure and function together with the non-living material (inter-cellular substance) between them, form a tissue. Tissues are of many types. They differ in the structure of cells that form them and also in the inter-cellular substance-They can be grouped together into the following categories:
- Epithelial or covering tissue
- Connective tissue
- Bone and cartilage
- Muscle tissue, and
- Nerve tissue.
Tissues which cover and protect all external and internal surfaces of the body are called epithelia. The outer portion of the skin, the lining of the body cavities and of the digestive, urinary and reproductive tracts are epithelia. Epithelial tissue provides protection from microbes, from physical injury, from various irritants and from drying out. In kidneys, it acts as a membrane for filtration and dialysis, permitting selective passage of certain types of molecules while retaining others. There are many varieties of epithelial tissues, from thick tough skin to the delicate lining of the alveoli in the lungs. They consist of sheets of closely packed cells on a basement membrane of connective tissue. The simplest epithelia are formed of a single layer of flattened cells. They cover the tubes of the kidney, the inner side of the ear-drums, blood vessels, etc. The lining of the alimentary tract, on the other hand, is much thicker, because they have to secrete the enzymes and mucus. Multi-layered epithelia cover the outside of the body, outer ear, mouth, throat, etc. A special water-proof variety covers the internal surfaces of the bladder and other parts of the urinary tract.
The skin is the thickest epithelial tissue in the body. It is an organ of protection as well as heat regulation. It has two parts: tough epidermis and soft dermis.
(ii) Connective Tissues
Connective tissue is the most widespread and abundant tissue in the body, and also the most varied. The functions of connective tissues are just as varied as their structure. Indeed the implication of their name may be somewhat misleading. Although they do connect, e.g. muscles to bones, connective tissues [also support the body, serve as depots for fat storage and help to nourish the tissues they support, surround or permeate.
Connective tissues such as tendons and ligaments provide a mobile supporting framework for more specialised tissues. Capillary blood vessels and nerves pass through connective tissues.
Tendons are one type of dense connective tissues having great tensile strength. Tendons anchor muscles to bones and must withstand the tremendous forces generated by muscle contraction. They are composed of parallel bundles of fibres.
Ligaments bind bones to other bones. Here the bundles of elastic fibres predominate.
Aponeuroses are flat sheets of white fibrous tissues that anchor muscles to other structures. Elastic tissues are found in the trachea, larger arteries and spinal ligaments. They are tough but springy. Most of the body fat is stored in the connective tissues inside special cells, forming a uniform appearance and soft texture.
Blood and Lymph
We generally visualize tissue as a solid mass. Yet blood and lymph are also tissues. Here the matrix is entirely fluid, without suspended fibres. Since, in a sense, blood and lymph connect all the regions of the body, they are grouped under connective tissue. Unlike the other tissues of the body, the blood is in constant motion, the movement occurring within fixed channels—the blood vessels.
Lymph is mostly water. It is formed by the continual draining of fluid from the intercellular spaces of the cells into the lymph vessels.
(iii) Bone and Cartilage
Two types of firm tissues—bone and cartilage—form the inner skeleton of the human body. In early life all bones are made of cartilage which is more flexible than bony tissue. Later on, it is replaced in all weight-bearing parts of the body Cartilage is a tough but resilient, pliable form of compact connective tissue
In the adults, cartilage is found in the nose, outer ear, larynx and air passage in the adults. It is also found in the front parts of ribs and the moving surfaces of some joints. The transformation of cartilage into bone begins in later foetus—life, when calcium is deposited on a matrix made by the bone-forming cells. Until adolescence, a plate of cartilage cells remains near the ends of bones, enabling them to lengthen.
Bone, commonly thought of as a solid and inert substance, is really a living tissue. The hardness and rigidity of bone results from the deposits of inorganic calcium salts. There is a compact outer layer and a porous inner part in most of the bones. Some bones are hollow with a central cavity containing marrow. Periosteum is a fibrous covering containing cells which can form new bones to mend fractures.
Bones are joined together in a variety of diffetent ways. Some permit no movement, while some like the vertebral joints (with intervening discs) allow limited bending and rotation. Joints with a lubricated membrane and cartilagenous discs on moving surfaces permit free movement.
(iv) Muscles
Muscles make up the bulk of soft tissues in the human body. Close to half of the body-mass is muscle. Three types of muscles are:
- Skeletal or striated muscle
- Smooth muscle and
- Cardiac muscle.
Muscle cells have perfected contractility to an unparalleled degree. The contraction of muscle cells move body parts and the forces they can exert are phenomenal.
Skeletal muscles of the head, trunk and limbs are known as voluntary muscles. They are generally anchored at both ends in the skeleton and produce movement by contracting and relaxing in response to conscious efforts of the will. They are also called striped muscles, because their long, thin fibres have fine, dark and light cross markings called the H band and the I band. Muscle fibre is built up from fibrils aligned together.
The basis of the bodily movement is the ability of the muscular tissues to contract and relax in response to nervous messages. An innumerable variety of movements can be produced because of the complexity of the muscle arrangement in the body. In fact, almost every muscle has an antagonist which produces the opposite action. This enables one to control every movement in force and range, while the intricate arrangement of the muscles across joints allow the greatest possible diversity of action. The biceps and triceps provide an instance of muscular antagonism. While the biceps is contracted, the triceps is relaxed and vice versa.
The muscles of the intestines and certain other structures like blood vessels and the uterus are called smooth muscles, because unlike the limb muscles they have no cross markings. They contract involuntarily or automatically and are relatively slow in action. Normally the smooth muscles do their work silently and we are unaware of their action.
The heart or cardiac muscle is a unique type found nowhere else in the body. It is intermediate between striated and smooth muscles. It is unique because its contractions are initiated within the muscle itself.
Energy is required to contract the muscle. It comes from glucose which is derived from glycogen stored in the muscles. Lactic acid which is formed during muscular action accumulates in muscles which are used for any length of time and excess amount of it causes cramp.
'Muscle tone or tonus' is the residual degree of contraction in muscles imparting firmness and resistance to stretching. It is important in keeping the muscles in a state of readiness to respond to stimuli,
(v) Nerve Tissue
The cells of never tissue are specialized in two key areas: irritability or responsiveness to stimuli and conductivity, the ability to transmit impulses. The cell unit of nerve tissue are called neurons. They extend to almost every region of the body. They differ somewhat in structure and ranging in size from a few thousandths of an inch to several feet long. However, each one is composed of a cell body, an extension called the axon which carries impulses away from the cell body and several processes called dendrites which carry impulses to the cell body.
In addition to the neurons, nerve tissue includes intestinal cells, the neuroglia, which support and nourish the neurons. We shall discuss the intricate method of the transmission and impulses by the nerves in the section 'Nervous System'.
