Growth
Growth refers to a process in which size and weight of an organism increases. It is a characterstic of every living being. In the process of growth cells undergo wear and tear process upto maturity. The process of growth is fast. After maturity it becomes a gradual process and finally the organnism dies.
Auxetic Growth : The growth which is connected to the number of cells is known as auxetic growth. In an infant the number of cells is 2×1012 cells while an adult human has 6×1013 cells.
Accretionary growth : In this growth cells divide mitotically and growth takes place. This kind of growth takes place in post embryonic development and in adults.
Different phases of growth
(i) Lagperiod : In this the animals prepare for growth and adapts for the different environments.
(ii) Exponential growth : In exponential growth, initially the growth is slower and the rate increases gradually afterwards.
(iii) Deaccelerating growth phase : When exponential growth is over, further it gets completely checked up. After the age of 13 years in boys and 10 years in girls, the growth is very slow. The growth is controlled by the thyroxin hormone secreted by the thyroid gland. The growth from 14 to 18 years is very fast due to the activity of thyroxin and STH hormone but after attaining maturity at the age of 22-23 years the growth is completely controlled.
REGENERATION
Formation of new cells and organs in damaged parts is known as regeneration. It takes place only in some animals.
Types of regeneration
(i) Epimorphosis : In epimorphosis lytic activities are increased first at the site of injury, then the formation of epithelium like layer takes place. The simple epithelial cells of adjacent tissue divide repeatedly below the epithelium and form a group known as regeneration bud. The increase in the number and size and finally after tissue differentiation, they form complex tissue or organ. This is common in lizard, salamander etc.
(ii) Morphallaxis : In this type of regeneration the whole body can be reconstituted from a small fragment of body by reorganising existing body cells. This is common in hydra, planaria etc.
AGEING
In old age death takes place.
Once a person becomes matured the further changes are according to the age. Study of ageing is known as gerontology.
Gerontology
After maturity as animal grows old, the body undergoes certain changes which are as follows—
(i) Metabolic efficiency decreases.
(ii) Power of regeneration and repairing of damaged tissue also decreases.
The structural and physiological deterioration in the body takes place and it is called ageing.
(A) Morphological and physiological changes with ageing
1. Skin becomes wrinkled.
2. Decline in heart efficiency takes place.
3. Circulation of blood in lungs.
4. The number of functional nephrons and taste buds get reduced.
5. A reduction in RBC Production from bone marrow takes place.
6. There is a decrease in water retention capacity of cells.
7. Muscles become weak and bones become dry.
(B) Cellular changes with ageing
1. DNA structure alteration takes place.
2. Deposition of pigments in brain and muscle cells inceases.
3. Power of multiplication decreases.
(C) Extracellular changes with ageing.
1. The change is secretion of intercellular substances takes place.
2. An adverse affect takes place in the diffusion of substances from cell to cell.
Theories of Ageing : There are many theories of ageing. In these theories 11 are well known out of which some are given below :
(A) Molecular theories : The concept in molecular theories is about changes in the genetic programme. Changes in genetic programmes are
(a) Codon restriction (b) Somatic mutation (c) Error theory and (d) Gene regulation.
(B) Cellular theories : Cellular theories are based on changes which take place in the structural and functional elements of cells with the passage of time.
According to this theory wear and tear takes place in cellular parts due to ageing. Certain pigments get deposited in cells which are age dependent.
As age advances the Physical and chemical properties of molecules get changed.
(C) System Level theories : Adaptive mechamism declines which leads to ageing and death.
(D) Immunological theory : The immune system of the body declines by which ageing is fast.
(E) Stress theory : Adaptation to stresses decreases with age. External or internal stresses can not be tolerated by nervous and endocrine glands.
Apart from these theories, there exist several other theories also. But no theory is complete.
Death : When cells of the body stop working we call it death. This is one of the important biological cycles and takes place with every living organism.
When age becomes more heart, liver, kidneys become weak. Circulation of blood to brain and lungs is stopped which causes death. If the immune system of body fails it causes death.
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