Growth ,Ageing , Repair

An embryo and offsprings body gradually enlarges and assumes the form and size characterstic for the adult of its species (growth). The animals carries on the various vital processes to maintain health and keep alive. In its body, the cell organelles are constantly renewed worn out cells are healed up (repair). In certain animals, even the lost organs of the body are regrown (regeneration). Since the animals have limited life span, their body starts undergoing degenerative changes showing sign of old age (ageing). The last events of which is death.

                         Differences between Protoplasmic and Aprotoplasmic substances.

S.No.	Characters	Protoplasmic structures	Aprotoplasmic structures
(1)	Nature of structures	Living	Non-living.
(2)	Location	Intracellular	Extracellular.
(3)	Examples	Cytoplasm, cell organelles and nucleus	Matrix, fibres, minerals, etc.
(4)	Growth	Grow	Do not grow.
(5)	Division power	Can divide	Cannot divide

                                           Differences between growth and degrowth

S.No.	Characters	Growth	Degrowth
(1)	Rate of metabolism	Anabolism faster than catabolism.	Catabolism faster than anabolism.
(2)	Fate of living matter	Both protoplasmic and aprotoplasmic structures are synthesized so there is increase in the living matter.	Food reserves (fats and glycogen) are catabolised to provide the required energy so there is decrease in the living matter.

                         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.

                                    Differences between auxetic and multiplicative growth

S.No.	Characters	Auxetic growth	Multiplicative growth
(1)	Nature of growth	Body growth occurs only due to increase in size of the body cells.	Body growth occurs due to increase in number of the body
(2)	Number of cells	Number of cells remains same, but the size of cells increases.	Number of cells increases but size of cells remains nearly same.
(3)	Occurrence	Nematodes, tunicates, etc.	During embryonic period of development in most of vertebrates.

                                Differences between Isometric growth and Allometric growth

S.No.	Characters	Isometric growth	Allometric growth
(1)	Nature of growth of organs	At same mean rate.	At different rates during different periods.
(2)	Body form and size	Remain proportional.	Becomes unproportional as body parts grow at different rates.
(3)	Examples	Fish and insects (e.g. locust).	Birds and mammals.

Differential growth of human body parts :

Changes in the weights of human body parts from birth to adulthood

S.No.	Body parts	Weight in kilograms
		New-born baby	Adult male
(1)	Muscles	0.8	30
(2)	Skeleton	0.4	10
(3)	Fat	0.8	10
(4)	Brain	0.4	1
(5)	Rest of body	0.9	19
	Total	3.3	70

Hormonal control of human growth rate :

Approximate ages of sexual maturity in some mammals

S.N0.	Mammals	Age of Maturity
(1)	Human being	11-16 years
(2)	Asiatic elephant	8-16 years
(3)	White-handed gibbon	8 years
(4)	Fin whale	3 years
(5)	Rhesus monkey	2-4 years
(6)	Horse	1 year
(7)	Cat	6-15 months
(8)	Dog	6-8 months
(9)	Rabbit	6 months
(10)	House mouse	35 days

                           Differences between reparative and restorative regeneration

S.No.	Reparative regeneration	Restorative regeneration
(1)	It is restricted to healing of injuries or replacement of cells.	It can replace the lost part of the body, or produce a complete organism from a fragment.
(2)	It involves minor cell proliferation and migration.	It involves large-scale cell reproduction and differentiation.
(3)	It occurs in all animals.	It is possible mainly in lower animals.
(4)	Examples : repair of a cut, replacement of skin cells, intestinal cells, blood cells etc.	Examples : regrowth of starfish's arm, wall lizard's tail, salamander's limb, etc.

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

                              Differences between regeneration in Hydra and of limb in salamander.

S.No.	Characters	Regeneration in Hydra	Regeneration of limb in salamander
(1)	Extent of regeneration.	The whole body can be reconstituted from a small fragment of body by reorganizing existing body cells.	It involves redevelopment of lost body parts.
(2)	Regeneration blastema	Not formed	Formed.
(3)	Dedifferentiation	Not involved.	Dedifferentiation is involved.
(4)	Mechanism	Occurs by morphallaxis.	Occurs by epimorphosis.

                                    Differences between Morphallaxis and Epimorphosis

S.No.	Morphallaxis	Epimorphosis
(1)	It is production of an entire animal from a small fragment.	It is regeneration of a body part by growth at the injured surface.
(2)	Regenerated animal is far smaller than the normal one.	Regenerated organ may be different from the original one.
(3)	It occurs in lower forms (Sponge, Hydra, Planaria).	It occurs in higher forms (arm of starfish, tail of lizard, limb of salamander).

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. The regenerated liver resembles the original liver in volume but not in shape. Similarly, if one kidney is lost, the other enlarges and takes over the function of the missing kidney also. Such a reparative regeneration is known as compensatory hypertrophy.

                             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).

                                  Regeneration and embryonic development

                   Differences between regeneration and embryonic development.

S.No.	Characters	Regeneration	Embryonic development
(1)	Occurrence	Both in larva and adult forms.	Only in embryonic stages.
(2)	Controlled by	Neural and hormonal control.	Not under neural or hormonal control.
(3)	Nature of process	Dedifferentiation of differentiated cells.	Cells are derived from zygote and undergo differentiation.

Ageing (= Gerontology)

        Ageing is the unabated deterioration in the structure and function of the body cells, tissues and organ; first described by Huxley.

        There is gradual decline in metabolic activities, water content, average body weight and the power of regeneration.

         This ultimately justifies entropy which prevails in a system.

Life Span

It is the period from birth to death, longer in women than man.

Maximum life span : The maximum age reached by any member of a species.

Average life span : The number of years lived by average members of the species

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	35-40 years
(12)	Chimpanzee	45 years
(13)	Horses	60 years
(14)	Man	60.4 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	About 3000-4000 years (longest life span)

CHANGES IN AGEING

Individuals age at different rates. Even within same person the organs show different rates of decline. Some generalization are made though do not apply to all persons. Changes at 70 years age are :

Heart :

           Increases in size pumping efficacy decreases upto 65%, and hence blood supply to some vital organs also decreases.

Lungs :

           Vital capacity (normal 3000-4600 ml) decline by 44%.

Kidney :

             Number of nephron decreases upto 56% and rate of glomerular filtration drops upto 69% leading to oliguria.

Brain :

20% of neurons die, weight decreases upto 56%, number of dendrites and axon increases to increase the number of synapse, rate of nerve conduction slows.

Sight :

           Presbyopia begins at the age of 40, increase in susceptibility to glare, difficulty of vision at low illumination, problem in detecting moving objects, formation of cataract.

Hearing :

              Difficult to detect high frequency sound waves, hearing declines more quickly in men than in women.

Skin :

              Loses its elasticity and develops wrinkles.

Muscle :

              Mass degenerate by 22% in women and 23% in men, contractility decrease (can be prevented by exercise).

Body Fat :

               No much loss of fat but is redistributed from below skin to deeper parts of the body. Women have more deposited in lower parts while men have more in abdominal areas.

                Ageing is greatly influenced by genetic factors, diet, social and environmental conditions.

                 Other changes include occurrence of diseases like diabetes, atherosclerosis, arthritis etc.

                 Increases in cholesterol and globulin levels in blood.

CELLULAR AGEING

A clear mechanism is still unknown but is believed that it starts with the accumulation of worn out cell organelles and waste molecules. Due to continuous differentiation and maturation cells lose its functional ability which ultimately leads to cell death.

Changes Include

Chromosomal aberration and gene mutation

Deterioration of cytoplasmic content and

Breakdown of membranous components.

Nuclei become irregularly lobed, with condensed chromatin, dehydrated, darkly stained nucleoplasm, called as pyknotic nuclei.

Distorted ER, Golgi apparatus and vacuolated mitochondria.

The enzyme aldolase become more and more inactive.

Causes of Ageing

There is no consensus on the causes but some cause effect relationship can be ascertained. There are two groups of theories :

Programmed (endogenous) theories

Damage or error (exogenous) theories

Death

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

(ii) Characteristics :

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

 (b) 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.

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

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

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

(iii) Causes of death : Causes of death are many. These can be separated into following main categories –

(a) 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.

(b) 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.

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

(iv) 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.

(v) 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.

(vi) 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.

(vii) 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.

• 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).

• Average life span of women is longer than men. It is so as biological process of ageing is faster in human male than in human female.

• Huxley : First described ageing.

• Vladimor Korenchevsky : Father of Gerontology.

• Biological death : When all the body cells stop functioning.

• Clinical death : When the heart beat, pulse and respiration stop. Pupils become fixed and dilated.

• Werner’s syndrome : A disorder with signs of advanced ageing in their 20s.

• 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.

• Wilson’s diseases. Due to defective metabolism caused by gene mutation.

• On average, cells stop dividing and wither out after they have divided about 50 times.

• H.W. Wilson (1907) : Reported morphollaxis regeneration in Scypha.

• Regeneration involves : Cell migration; cell proliferation, cell-differentiation and dedifferentiation.

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

• Basic human body plan is laid down during first two months of pregnancy period.

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

• Head forms about half of the body at two month old foetal stage; one-fourth of body at four month foetal stage and one-eight of body in an adult person.

• In human beings, muscles show maximum growth, while brain shows minimum growth.

• Growth occurs at a rate of 2 cm per month during first year after birth.

• Growth of individual cells is the most essential component of the growth of multicellular organisms.

• Heart of infant has same number of cells as that of adult but is only 6% of its body weight. It undergoes auxetic growth.

• Archaeocytes of the sponges and interstitial cells of coelenterates act as totipotent cells.

• Inflexion point : Point where the exponential growth begins to slow down.

• Arthropods show discontinuous growth. They grow only after moulting or ecdysis.

• Committing suicide by the cells (autolysis) by activating internal death programme is called apoptosis.

• Hypertrophy : Increase in the size of cells due to addition of protoplasm.

• Hyperplasia : Increase in number of cells by cell division.

• In human infant, there are about 2 × 10¹² cells, while an average adult man has about 60 × 10¹⁵ cells.

• Auxetic growth : Found in a few nematodes (e.g. Ascaris), rotifers, tunicates (e.g. Herdmania) and muscles fibres.

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