GROWTH REGENERATION AND AGEINGFlame Institute

Growth

Meaning and definition of growth : Growth is an important properties of all living organisms. All organisms grow from a young stage to an adult stage. Growth is a permanent increase in dimensions of the body and its parts. It results from the addition to the body tissues.

Growth at different levels

(1)Molecular level : At molecular level, the growth involves synthesis of new molecules and their aggregation into organelles and storage products in the cells.

(2) Cellular level : At the cellular level, the growth involves.

(i) Cell expansion (hypertrophy) : Increase in the size of the cells due to addition of new cell material, called protoplasm.

(ii) Cell division (hyperplasia) : Increase in the number of cells by cell division.

(iii) Cell differentiation : Specialisation of cells for specific roles, in its broad sense, growth includes.

(iv) Matrix formation : Addition of intercellular materials, termed apoplasmatic substances, secreted by the cells between them. The apoplasmatic substances include the matrix of connective tissues and intercellular fluid.

(3) Individual level : At individual level, the growth is the visible increase in the body, dimension, size volume and weight. Increase in weight will show that the growth has taken place.

Physiological condition for growth : A variety of chemical reaction occur all the time in the living organisms. These are collectively referred to as metabolism. Metabolism has two phases building up phases or anabolism and break down phase or catabolism. Variation in the rates of the metabolic phases result in three types of growth.

(1) Positive growth : Anabolism normally out weighs catabolism and this brings about growth during the growing period of the organism and maintain the body thereafter. This is called positive growth.

(2) Zero growth : If the anabolic and catabolic processes are balanced, there is no addition to the bulk of the body and no increase in body size. This is referred to as zero growth.

(3) Negative growth : If catabolism occurs at a faster rate than anabolism, as happens in fasting the organism gradually becomes weak and may finally die. At this time, first the food reserves (glycogen, fat) and then body’s own protein are used as sources of energy to run the body machine. This depletes the living material, causing negative growth. Due to this the reserve food and living material decrease in amount and is called degrowth.

States of growth : Two states of growth are :

(1) Pre-functional state of growth : It is the early embryonic stage during which the developmental processes transform a zygote into an embryo. During this growth period the organ rudiments are established but are not functional.

(2) Functional state of growth : It is the late embryonic and post embryonic developmental stage during which organ rudiments become functional and organogenesis begins.

Table : 8.4-1 Differences between Embryonic growth and Post-embryonic growth

S.No.	Characters	Embryonic growth	Post-embryonic growth
1.	Period of occurrence	During pre-natal (before birth) period, e.g., during blastulation and gastrulation.	During post-natal period.
2.	Cell growth	Does not occur.	Occurs.
3.	Nature of cells	Cells only divide so size of blastomeres becomes smaller and smaller.	Cell division occurs after the cell growth so size of cells remains nearly same.
4.	Nature of organs	Only organ rudiments are formed but are non-functional.	Organs have been fully developed and are functional.

Biological activities of growth : Growth of a multicellular organism is governed by two main biological activities.

(1) Cell growth : Growth and division of a cell occur in three cyclic phases :

(i) G₁-Phase : It involves, pooling of amino acids and nucleotides for the synthesis of protein and nucleic acids. A newly formed cell grows by synthesizing carbohydrates, lipids, proteins, RNAs, ATP and enzymes to loosen and unfold the DNA.

(ii) S-phase : It involves, replication of DNA, so each chromosome now consist of two sister chromatids joined at the centromere and carries a duplicate set of genes. A diploid cell (2n), thus, becomes tetraploid (4n) at the end of S-phase. Synthesis of histone protein of the chromosome.

(iii) G₂-phase : It involves, the cell grows further, synthesizing more protein and RNAs and doubling the organelles such as centrioles, mitochondria, Golgi apparatus. The G₂ phase prepares the cell for its division.

The growth of individual cells is most essential factor of growth in all multicellular animals.

(2) Cell reproduction : It occurs during the M-phase of the cell cycle during which a fully formed adult cell undergoes mitosis to produce two genetically similar daughter cells which repeat the process.

Strategies of growth : Growth is accompalished by three strategies :

(1) Cell proliferation : The growth of a structure by cell multiplication due to cell division e.g., growth of lens.

(2) Cell enlargement : In this, cell do not divide but their size increase due to synthesis of more cytoplasm e.g., growth in cardiac muscles, neurons and skeletal muscles.

(3) Growth by accretion : In this, growth occurs due to secretion of large amount of extra-cellular materials e.g., growth of cartilage and bones.

Types of growth : In animals body four basic types of cellular growth are recognised.

(1) Auxetic growth : In some organisms growth occurs as a result of increase in the size of their cells. The number of cells remains the same. It is a rare type of growth and is found in a few nematodes (Ascaris) rotifers, tunicates (Herdmania) etc. Muscular hypertophy is also the auxetic growth.

(2) Multiplicative growth : In this, the growth occurs due to an increase in the number of cells of the body by rapid mitosis division an appreciable growth of the cells. It involves both cell growth and cell reproduction. It is found in the embryo.

(3) Accretionary growth : During postembryonic growth and also in the adult, all the body cells are incapable of undergoing division. The differentiated or specialized cells of organ and tissues lose the ability to divide. The undifferentiated cells (reserve cells) present at specific location in the body divide mitotically and help in growth. This kind of growth is called accretionary growth

(4) Appositional growth : It involves the addition of new layers on the previously formed layers. For example, the addition of lamellae in the formation of bone. It is characteristic mode of growth in rigid materials.

Growth rate in animals : Growth is the perceptible and measurable increase in the mass of living materials and can be confirmed by an increase in weight of an animal. All higher animals, including man, grow at a specific rate and rhythm. The growth rate is not uniform but is different at different periods of life, so the growth is differential.

Growth period in human may be divided into 5 stages :

(1) Prenatal stage : It comprises about nine months of embryonic life.

(2) Infantile stage : It extends from birth to 10 months of age.

(3) Early childhood stage : It extends from 10 months to 4 or 5 years of age.

(4) Juvenile stage : It extends from 4 or 5 years to about 14 years of age, i.e., upto the time of puberty.

(5) Adolescent plus post adolescent stage : It extends from 14 years to 20 or 22 years of age.

Growth curve : The growth rate in an individual at different periods of life can be represented in a curve by plotting the weight of individual at different time intervals (in years) on graph paper.

(1) Sigmoid curve : Growth curve of higher animals, including man, is S-shaped and is called sigmoid growth curve. This growth curve proves that :

(i) First rises very slowly, showing a low rate of growth.

(ii) Then rises steeply, indicating fast rate of growth.

(iii) Its rise again slow down

(iv) Finally it starts running horizontally, depicting stoppage of growth.

Its 4 phases are respectively called lag phase, exponential (log) phase, senescent (decelerating) phase and steady (plateau) phase. The point where the exponential growth begins to slow down is known as inflexion point.

(2) Absolute growth : The difference between the initial and final weight (or size) of an individual in a given period of time is called absolute growth.

(3) Variation in steady phase : The nature of the curve during the steady phase may vary in different species. In some cases (many invertebrates, fish and certain reptiles) the curve may continue to rise slightly till the animal dies. This is a case of positive growth. In some cnidarians, the curve flattens out showing stoppage. In many mammals including humans, the curve slowly tails off, showing degrowth or negative growth due to physical weakness caused by ageing.

Differential growth of human body parts : In human being, similar to other animals, different body organs or body parts (head, neck, thorax and limbs etc.) do not grow at the same rate. The growth rate of different body parts is different.

If we keenly observe the growth of these body parts by comparing their photographs from birth for a number of years till these attain their final shape, size and weight e.g., head of a newly born human baby is proportionately larger than the rest of its body.

Hormonal control of human growth rate : Throughout the developmental period from birth to adulthood, the growth is controlled by hormones secreted by endocrine glands in the blood. But different periods of growth are under different hormones e.g.,

(1) Growth rate in early childhood period and juvenile period (from 10 months to 14 years) is very slow and is controlled by thymosine hormone secreted by thymus gland. It is a pinkish coloured, bilobed gland located in front of heart.

(2) During the late childhood period, growth rate becomes faster as along with thymosine, two more hormones start operating. Thyroxine hormone of thyroid gland and somatotrophic hormone (STH) or Growth hormone (GH) of anterior pituitary.

Repair and Regeneration

Definition : It is that post-embryonic morphogenetic phenomenon which when temporarily stimulated brings about repair of the damaged cells/Tissues, or replacement or redevelopment of severed body parts or reconstruction of whole body from a small body fragment.

Capacity for regeneration : Among animals, power of regeneration was first discovered in Hydra by Trambley, in 1740. The capacity of repeated regeneration, though, present throughout the animal kingdom, but to varying degree. It is more marked in the lower animal than in the higher animals. Among invertebrates, protozoans, sponges and coelenterates, the regeneration capacity is very high. In higher animals, regenerative ability is much greater in the embryonic and larval stages than in the adult. In man, it is restricted to healing of injured tissues such as skin, muscles, bones, blood vessels and nerves; the lost organs cannot be regenerated.

Types of regeneration : Regeneration is of two main type – Reparative and Restorative.

(1) Reparative regeneration : In this, multicellular organism has the power only to repair certain damaged cells of the body. It is a common phenomenon observed in both invertebrates as well as the vertebrates.

(2) Restorative regeneration : In this, a multicellular organism can redevelop the severed body parts or the whole body can be formed from a body segment. It is very common in invertebrates. It may occur by epimorphosis or morphallaxis. The power of restorative regeneration varies in different groups of organisms e.g.,

(i) Autotomy power in some animals, some part of the body is broken off the body on being threatened by the enemy or predator. This phenomenon of self mutilation of body is called autotomy. The lost part may be tail, limb, viscera or arm e.g.,,

Crabs break of their leg on approaching the enemy.
Lizards throw off their tail.
Holothurians (Echinoderm) throw off their internal viscera (respiratory tree etc.). It is called Evisceration.
Starfish (Echinoderm) can regenerate the whole arm.
Autotomy is a special adaptation for escaping the danger of attack by enemy or predator.

(ii) The climax of regeneration in which whole body can be developed from a body fragment is found in Hydra among the coelenterates; Scypha among the sponges and Planaria among flat worms.

Mechanism of regeneration : T.H. Morgan recognized two primary mechanism of regeneration in animals.

(1) Morphallaxis : It is the reconstruction of an entire animal from a small fragment by reorganizing the existing cells. The regenerated animal is far smaller than the original one after the completion of the process. It grows to attain the normal size. e.g., Hydra

(2) Epimorphosis : It replaces a lost organ of the body by proliferating new cells from the surface of the injured part. Regeneration of an appendage in an arthropod, arm in a starfish, limb in a salamander and tail in a lizard occurs in this manner.

Regeneration of a limb of a newt or salamander : Newt/salamander has very high power of regenerating their lost limb by the process of restorative regeneration. It involves the following steps :

(1) Wound healing : The epidermal cells from the edges of the cut migrate and spread over the exposed surface. This is known as wound healing.

(2) Blastema formation : A few days after the healing of the cut, the undifferentiated cells accumulate inside the epidermis. Due to this cellular aggregation, a stumpy outgrowth or bulge is formed. This is known as regeneration bud or blastema.

(3) Redifferentiation and morphogenesis : The blastema develops rudiments of digits by indentation at the free edge. These grow out into new digits.

(4) Growth : The regenerated limb increases till it attains the size of a normal limb.

Control of regeneration : Though exact control mechanism for the regeneration of a lost limb in a salamander / newt is not known but a number of experiments have confirmed its dependency upon nerves, hormones and epithelial cover.

(1) Epithelium : C.S. Thornton (1960) has shown that the presence of the wound epithelium which covers the amputated surface is necessary for blastema formation. This epidermal cap acts as a stimulus for the aggregation of blastemal cells of the mesenchyme. This establishes an epithelium-mesenchymal interaction. It was reported that if the epidermal cap is placed eccentrically, an eccentric blastema is formed while if it is continually removed, blastema formation can be prevented.

(2) Neural trophic factor : It has been shown that if a limb is first denervated and then amputated, or if nerves are by any means blocked from penetrating the epidermis, no regenerative blastema is formed. But if the amputated limb is denervated after the initiation of blastema formation, regenerative process continues and a new limb is formed.

(3) Hormones : Adrenal glands and pituitary gland have been found to influence the regenerative process considerably.

Examples of regeneration in different animal groups : The regeneration was first discovered in Hydra by trembley in 1740. Later, it was also found in other animal groups but to a varying degree.

(1) Invertebrate : Power of regeneration are found in following phylum of invertebrate.

(i) Protozoa : Among the protozoans, very high power of regeneration was found in Amoeba and it was confirmed that the presence of nucleus is essential for regeneration as anucleate part finally dies.

(ii) Sponges : Sponges have remarkable power of regeneration. Any part of the body injured or cut off is readily repaired or replaced. Small fragments of sponges grow into complete individuals.

(iii) Coelentrates : Coelenterates too have a remarkable power of regeneration. Hydra shows regeneration to an amazing degree. Trambley (1740) reported that if Hydra is cut transversely into two or more parts, then each fragment, as small as 0.004 mm, can grow into a complete organism. In Hydra, regeneration occurs by morphallaxis. Hydra has a unique capacity of regenerating its hypostome (oral end) again and again.

(iv) Flatworms (Platyhelminthes) : Very high power of regeneration has been reported in planaria among the flat worms. Like Hydra, a small fragment of Planaria can also develop into a complete animal though of smaller size than the parental animal. Internal organs used up during starvation are also fully regenerated if food becomes available.

(v) Nematodes : The power of regeneration is poor in nematodes. Superficial wounds are, however, healed up.

(vi) Annelids : Annelids have less power of regeneration than the planarians. If an earthworm or other oligochaete is cut in two halves, then each half may regenerate the lost parts. But in majority of the annelids, the regeneration power is restricted and only 4 or 5 segments at either end or both ends of the body can be regenerated.

(vii) Arthropods : Certain insects, crabs, lobsters and spiders can regenerate a lost leg. Crayfish regenerates any of the appendages and the eyes when removed. Regeneration is faster in the young than in the adult. The regenerated part may not always be similar to the lost one.

(viii) Molluscs : Molluscs have low power of regeneration. Gastropods are capable of regenerating certain body parts only like eyes, eye stalks, the parts of head and foot. The cephalopods (e.g., cuttlefish) can also regenerate their arms only.

(ix) Echinodermates : The power of regeneration is high in the echinoderms. Almost all the echinoderms have good power of autotomy and regeneration e.g., starfish can lose and regenerate upto 4 arms; Holothurion (sea cucumber) can lose its respiratory tree and visceral organs (called evisceration) in self defence.

(2) Vertebrates : Many vertebrates also possess a good power of regeneration.

(i) Fishes : The ammocoetes larva of lamprey can regenerate the lost tail. Some fishes are known to regenerate parts of fins.

(ii) Amphibians : The salamanders, newts and axolotl larvae are outstanding in their regenerative capacity among the vertebrates. They can regenerate a severed arm or leg. They can also regrow tail, jaws, external gills, intestine and retina. Tadpole of frog and toad can regenerate tail and hind limbs. Adult frog and toad are unable to regenerate limbs.

(iii) Reptiles : Certain lizards can regenerate a lost tail. The wall lizard, when threatened, can sever its tail near the base, leaving the moving tail to detract the predator while it escapes, and later regenerates a new tail.

(iv) Birds : Certain birds may regenerate beak.

(v) Mammals : Mammals are unable to regenerate any of the external parts, but can readily regenerate the liver. This organ has the maximum capacity of regeneration. Removal of over half of the liver is fully replaced.

Table : 8.4-2 Different animal groups and their regenerative body parts

S.No.	Animal group	Regenerated body part
	(A) Invertebrates
1.	Coelenterates (e.g., Hydra), Flatworms (e.g., Planaria) and Sponges (e.g., Sycon)	Fragmented body parts.
2.	Arthropoda (e.g., Insects, Spiders, Crustaceans)	Limbs.
3.	Annelida (e.g., Earthworm)	Body segments.
4.	Mollusca (e.g., Snails)	Parts of the head, foot, eye, eyestalk.
5.	Echinodermata (e.g., Starfish, Sea cucumber)	Arms.
	(B) Vertebrates
1.	Pisces (e.g., Fishes)	Fins.
2.	Amphibia (e.g., Salamander)	Limbs, tail.
3.	Reptilia (e.g., Lizards)	Tail.
4.	Aves	Beak.
5.	Mammals (e.g., Man)	Skin, body parts, kidney, liver (only reparative).

Ageing

Definition : Ageing is the slow deterioration in the structure and function of body cells tissues and organs of an animal and starts after the adulthood.

Gerontology : The field of developmental biology that deal with the process and problems of ageing is known as gerontology – (Gr. geron = old man; logos = discourse). The scientists involved in the science of ageing are called gerontologist.

Life cycle and life span : In all metazoan animals, the life cycle includes two developmental period; embryonic period (pre-natal developmental period) which extends from zygote to offspring till hatching or birth, and post embryonic period (post-natal developmental period)- which includes growth, adulthood, reproduction, ageing. Thus, the life cycle comprises five main events : birth, growth, maturity, old age and death, that follow in the sequence named. Maximum life span is the maximum number of years survived by any member of a species, while average life span is the number of years survived by members of a population. Life expectancy is the age at which half the population still survives.

Table : 8.4-3 Life span in different organisms

S.No.	Animals name	Life span
1.	Mayfly	24 hours
2.	Silk moth	2-3 days
3.	Mouse	3-5 years
4.	Rats	4-6 years
5.	Humming bird	8 years
6.	Rabbits	13 years
7.	Monkeys	26 years
8.	Dog	20-30 years
9.	Bullfrog and Lion	30 years
10.	Toads	36 years
11.	Cat	28 years
12.	Chimpanzee	45 years
13.	Horses	60 years
14.	Man	60 years (during 1988-95 period – WHO report)
15.	Elephant	70 years
16.	Turkey	118 years
17.	Parrots	140 years
18.	Tortoise and banyan tree	200 years
19.	Sequoia

Why old age ends in a natural death : Though it is difficult to give a categorical answer, certain factors that lead to death in old age are known. During the growth period, new cells are formed faster than the rate of death of old cells. But after the maximum growth, the metabolic rate declines and rate of formation of new cells is lower than the rate of death of body cells. So the repair of damaged cells is not complete and a slow deteriorative process starts. In human beings, it starts after the age of 30 years. But in certain cases, deterioration in structure and function of cells may start during childhood or even during prenatal life e.g., hearing efficiency of ear and atresia in ovary. As the organism grows older.

Deterioration data of an old person : A 75 years old man, for instance, has, as compared to a 30 years old person, about 64% less taste buds, about 44% less renal glomeruli about 20% less nerve cells in the brain, and about 37% less axons in the spinal nerves. His heart pumps 35% less blood and sends 20% and 58% less blood to the brain and the kidneys respectively. His lungs have 44% less vital capacity and provide about one-third less oxygen to the blood per minute. His kidneys have 31% lower rate of glomerular filtration. His nerve impulses are propagated at a rate about 10% slower. At the age of 30, the height starts decreasing indetectibly, it decreases by 0.3 cm. at 40, by 2 cm. at 50 and by 2.5 cm. at 70.

Changes in ageing or symptoms of ageing : Gerontologist have worked out a large number of changes that accompany ageing. These are discussed below under three heads –

(1) Changes at organ level : During ageing, different organs and organ systems show different rates of decline e.g.,

(i) Heart : With increasing age the efficiency of heart decreases. In a man of 70 years, the heart pumps only 65 per cent blood per minute as compared to a 30 years old man. Consequently, the blood going to the brain and kidney is reduced to 80 percent and 40 percent respectively.

(ii) Oxygen uptake by blood : At the age of 20 blood takes about 4 litres of oxygen per minute, while in a man of 75 years, it takes only about 1.5 litres of oxygen in the same period.

(iii) Decrease of blood volume : The production of new RBCs from the bone marrow declines and consequently the volume of blood also decreases.

(iv) Kidney : The number of kidney tubules is found to reduce to half in the old age. As a result the volume of urine decreases. This creates lots of other urinary troubles and also causes body ache, low back and difficulty in passing urine.

(v) Lungs : The capacity of lungs for intake of air decreases. This leads to reduction in the oxygen supply to different tissues. Therefore, old persons suffer from breathlessness and inflammation of mucous membrane.

(vi) Digestive system : The number of taste buds on tongue reduces to about one-third. The secretion of digestive juices also decreases with old age. This may result in indigestion. loss of appetite, dyspepsia, constipation and gas formation.

(vii) Retention of water : The capacity of body cells to retain water also decreases with the result, the skin in old persons is dry and wrinkled.

(viii) Nerve impulse : The rate of nerve impulse propagation reduces with age. The decline is about ten percent in man of 75 years as compared to that of 50 years old person.

(ix) Eyes : Accommodation power of eye starts declining in the 40s; ability to distinguish fine details may begin to decline in the 70s while there is increased susceptibility of eyes to glare and more difficulty in detecting moving objects from 50 years onward.

(2) Cellular changes : Cellular changes are of two types

(i) Morphological changes

(a) Accumulation of exhaustion pigments : The exhaustion pigment lipofuscin, yellow pigment and brown deposits are byproducts of unsaturated lipid oxidation. It is especially obvious in nerve and heart muscle cells but is present in almost all other cells of the body, though to a lesser degree.

(b) Appearance of lipid vacuoles : Small lipid vacuoles appear in the cytoplasm.

(3) Physiological changes :

(i) Accumulation of chromosomal aberrations and gene mutation in the nuclei with advancing age. These change the transcribed RNA which leads to the synthesis of defective proteins. These also retard the replication of DNA.

(ii) Decrease in semipermeability of cell membrane due to deposition of calcium in the peripheral part of cytoplasm.

(iii) Decrease in the rate of metabolism due to decreased number of mitochondria with advancing age.

(iv) Decreased rate of protein synthesis is due to decrease in RER in cells.

(v) Increased inactivity of aldolase enzyme in the liver cells with advancing age.

(vi) Decreased rate of cell mitosis. The non-dividing nerve cells and muscle cells start ageing earlier than the dividing cells of spleen and liver.

(vii) Size of the nucleus decreases.

(viii) Breakdown of cellular membrane. With advancing age, the lipids of biological membranes, that surround the cells and certain cell organelles, breakdown, forming a fatty, brown pigment called lipofuscin. The lipofuscin granules accumulate in ageing muscle and nerve cells, and interfere in their functioning.

Theories of ageing : Biological phenomena leading to ageing are not fully known. Several theories have been proposed to explain various aspects of ageing. These are following types –

(1) Immunity theory : This theory suggests a link between ageing and disappearance of the thymus gland by late middle age in man. Thymus stimulates the proliferation of lymphocytes, increasing resistance to infection. Absence of this gland affects in two ways –

(i) It weakens the body’s natural defence against foreign germs.

(ii) It increases the number of abnormal (defective or harmful) cells formed in the body itself. This destroys the tissues. The neuro-hormonal and immunity theories are collectively called pacemaker theories of ageing.

(2) Wear-and-Tear or stress theory : It states that the cells and tissues of the body continuously wear out due to internal and external stress factors. This coupled with the fact that the regenerative capacity progressively declines with age, causes ageing and finally death.

(3) Cross linkage theory : According to this theory ageing is caused by the increase of bonds between protein and nucleic acid molecules in the cell. These bonds alter the functional characteristic of these important cellular components leading to non-availability of certain functional proteins and resulting in malfunctioning of the cell.

(4) Waste product theory : According to this theory the accumulation of waste products are considered to poison the cell gradually, resulting in their ageing and death.

(5) Clinker’s theory : It states that ageing is due to accumulation of metabolic wastes inside the body cells. These wastes, beyond some limit, poison the body cells and decline the metabolic rate and induce ageing.

(6) Error catastrophy theory : It was proposed by Orgel (1963 A.D.). It states that errors in reading genetic code results in defective proteins which form defective enzyme leading to catastrophic damage to cells, tissues and organs, so induce senescence.

(7) Free radical theory : A free radical is a molecule with an unpaired and highly reactive electron. The oxygen-free radicals are formed as the by-product of normal cellular respiration. These free radicals take electrons from other molecules of a biological system so making them unstable and combine readily with other molecules. These free radicals initiate a chemical instability.

Brain and Ageing : The brain consumes more oxygen than any other organ so more free radicals of oxygen are generated. Free radicals cause damage of cell membranes and DNA with each collision. These damages cause ageing skin, ageing of brain and even cancer. Vitamin E and vitamin A are natural anti-oxidants of the body. Many antioxidants are found in fruits and vegetables such as – lycopene in tomatoes, isoflavones in soybean, sulphoraphane in broccoli, ellagic acid in strawberries and polyphenols found naturally in leaves of green and black tea.

(8) Metabolic or living theory : It states that those organisms (e.g., rats, mice, insects, birds, etc.) which have higher metabolic rate, mature, age and die earlier than those organisms (e.g., human beings) which have low metabolic rate and take years to mature and years to age.

(9) Collagen theory : It was proposed by F. Verzar (1964). It states that ageing is induced by changes in the collagen protein in the interstitial fluid surrounding the body cells.

Death

Definition : Death may be defined as the permanent cessation of all the vital function in an organism.

Characteristics

(1) It is the last event in the degenerative processes of ageing.

(2) Death of an organism involves the death of the body cells. But all the cells of the body do not die at the same rate e.g., ciliated cells lining the respiratory tract of mammals continue to beat their cilia for a long time even after animal’s death. Brain cells of body are last to die.

(3) There is no natural death in the protozoans e.g., Amoeba.

(4) Death involves widespread cell breakdown and cell death.

(5) It usually occurs due to lack of oxygen supply to body tissues.

Causes of death : Causes of death are many. These can be separated into following main categories :

(1) The weakening of the body tissues and of vital organs like heart, lungs, liver, kidneys, etc. which cause physiological and metabolic disorders of permanent nature leading to death. Death, in some cases, occurs due to sudden stoppage of the circulation of blood, food and oxygen to heart and brain leading to immediate death.

(2) The immune system (A system that provide resistance against disease – causing microbes) of the body is gradually impaired with advancing age. This increases the chances of infection in old age. Many old persons die of infectious diseases.

(3) Sudden blockage in the circulation of blood to heart, lungs and brain. This causes instantaneous death.

Brain or cerebral death : In the presence of cardiac activity, the permanent loss of cerebral functions, manifested clinically by absence of responses to external stimuli, lack of breath, and absence of cerebral reflexes is called brain death.

The only truth of life : The death is an inevitable reality of life, and should be gladly accepted. It is a biological necessity for the maintenance of the balance of nature. Old organisms must make room for new ones.

Habit that influence life span : Life-style habits can influence life span considerably. Although a healthful diet does not guarantee immortality, regular exercise and avoiding of alcohol, smoking and drugs, contentment and freedom from stress can make a person’s last years more pleasant.

Significance : Death is an essential and inescapable biological phenomenon which helps in maintaining ecological balance or homeostasis in nature. It prevents overcrowding of the members of a specific species and justifies the ‘continuity of life’ on earth.

? Maximum growth in human foetus occurs at the age of 4^th month.

? Eutyly : When number of cells is constant both for entire animal and specific organs e.g., nematodes like Ascaris.

? With increasing age, the enzyme aldolase synthesized by the mice liver cells becomes more and more inactive.

? Vitamins A, C and E act as anti-oxidants so can be used to neutralise the volatile and unstable oxygen-free radicals.

? Hayflick limit : Uppermost limit of cells to divide. At this limit, cells stop dividing, go quiet for a while and then die. It was discovered by Dr. Hayflick in 1960s. He stated that telomere of chromosome acts as a molecular clock so determines the life span of a cell. With each cell division, telomeres become a little shorter.

? Human embryo is about 150 mm at the time of implantation which grows to about 50 cm over the nine months of gestation period.

? According to latest world development report, Japanese have longest life span (average life expectancy is 76.3 years while that of female is 82.5 years).

? In India, male life expectancy in Kerala is 69 years while that of female is 74 years. Average life span of an Indian is 60 years.

? World Day for Elderly People : 8^th October.

? Geriatics : Diagnosis and treatment of diseases which affect the elderly persons.

? Vladimir Korenchevsky : Father of Gerontology.

? Shock (1962) : Reported Ca²⁺-accumulation in aged cells.

? Minot (1971) : Suggested that a change in nucleo-cytoplasmic ratio acts as an important index for natural senescence and ageing.

? Cross-linking of biomolecules has a close relationship to diabetes.

? In certain cases, age-related symptoms and pathological symptoms are similar. e.g., in osteoporosis (characterized by weakening and fracture of bones especially in postmenopausal period). Similarly impairment in body temperature control may be due to ageing process or may be due to cerebrovascular disease or the dementing process, called Alzheimer’s disease.

? Turnover time for skin cells is of 1-2 weeks while it is of 2-3 days for intestinal cells.

? The correct sequence of events during regeneration are dedifferentiation, cell division, cell movement and tissue differentiation.

? Gross (1968) described “Planarian is immortal under the sharp edged knife.”

? Binding of glucose molecules to protein molecule set a chain of chemical reactions that ends in cross linking of proteins.

? Cross linked proteins are called advanced glycosylation end products (AGEs).

? Advanced glycosylation end products are also found in younger persons suffering with diabetes mellitus and having continuous high levels of blood sugar.

? Cells must receive proper growth signals through growth factors or specific proteins for growth and division.

? Biological process of ageing is faster in human males than in females.

? Werner syndrome is a rare disorder in which those affected show signs of advanced ageing in their 20s. Werners cells have an excess of collagen and fibronectin, as do ageing cells in general.

Growth

The phase of the cell cycle is signified by the synthesis of [AIIMS 1991]

(a) mRNA only (b) rRNA only

Growth curve in animals is

(a) Delta curve (b) Alpha curve

Mitotic division in chondriocytes, osteocytes and mesenchyme cells in an adult is the example of

(a) Multiplicative growth (b) Auxetic growth

Growth curve indicates

(a) Growth rate

(b) A growth parameter at various intervals

(d) Absolute increase

T–cells are found in

(a) Thymus (b) Thyroid

Which one of these is body building material

[MP PMT 1994]

(a) Protein (b) Sugar

Between the ages of 1 and 2 years, a baby boy’s weight increases from 10 to 12 kg and in the same period of time the weight of a teenage boy goes up from 50 to 55 kg. The percentage growth rate of [MP PMT 1994]

(a) The teenager is higher than that of the baby boy by 5 to 5.5%

(b) The baby boy is higher than that of the teenager

(d) (a) and (b) both