Growth
It is an autonomic process by which the synthesis of protoplasm results in the increase in size and weight. It is a basic property of all of living beings. The anabolic activities are far more than the catabolic activities. The cells of every living being undergo wear and tear. In some animals, some cells and tissues are replaced and renewed. After attaining the maturity the organisms gradually grow and die at last.
All these processes (growth, repair, regeneration, ageing) are post-embryonic developmental phenomenon.
Auxetic growth
Increase in the number of cells is a multiplicative growth. An adult human has 6 × 1013 cells whereas the infant has only about 2 × 1012 cells. In some animals increase in cell size gives the growth in size. e.g. ascaris, nematodes, rotifers and tunicates. This type of growth is known as auxetic growth.
Accretionary growth
In post-embryonic development of adults some specialized tissues and cells divide mitotically and growth takes place. This type of growth is known as accretionary growth.
During the interphase of cell division, the synthesis of nucleic acids and proteins takes place in the G1, S and G2 phases. The animal growth curve is a sigmoid curve.
Phases of growth
(i) Lag period : The animals prepare for growth and adapt for the different environment.
(ii) Exponential growth : Initially the growth is slower but the growth rate increases gradually afterwards.
(iii) Deaccelerating growth phase: After the exponential growth, the growth is almost stopped.
After the age of 13 years in boys and 10 years in girls, the growth becomes slow. The growth is controlled by the thyroxin hormone secreted by the thyroid gland. The growth from 14 to 18 years of age becomes very fast due to the activity of thyroxin but after attaining full maturity at the age of 22-23 years the body growth is almost stopped.
Regeneration
Regeneration is the process of formation of new organs in damaged parts e.g. starfish.
Types of regeneration
(i) Epimorphosis : At the site of injury the lytic activities are increased first, 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 a regeneration bud. The increase in the number and size and finally after tissue differentiation, they form complex organ. Dedifferentiation is involved. e.g. amphibians, taenia, salamander, lizard etc.
(ii) Morphallaxis : The blastema formation does not take place. The whole body can be reconstituted from a small fragment of body by reorganising existing body cells. Dedifferentiation is not involved. e.g. hydra, planaria.
Ageing
Death comes to all at the end of old age. Once the physical maturity is obtained, the further changes of decline are according to the body age. Study of ageing is known as Gerontology.
As animal glows old after maturity, the body undergoes certain gradual declines which are as follows :
(A) Metabolic efficiency decreases.
(B) Power of regeneration and repairing of damaged tissue also decreases; even the vital organs such as heart, kidneys, brain, liver etc. become functionally week.
Thus structural and physiological decline in the body is called ageing.
(A) Morphological and physiological changes with ageing
1. Thin, shrivelled and stooping body with dry and wrinkled skin.
2. Heart efficiency goes down.
3. Circulation of blood amount in lungs decreases.
4. Functional nephrons and taste buds get reduced in number.
5. Production of RBC from bone marrow is reduced consideralbly.
6. Water retention capacity of cells and tissues also decreases.
7. Muscles become weak and bones become dry.
(B) Cellular changes with ageing
1. Chromosomal abnormalities and gene mutation. e.g. DNA structure alteration takes place and enzyme aldolase synthesis in mice liver cells becomes inactive gradually.
2. Pigment deposition in brain and muscle cells inceases.
3. Multiplication power decreases.
(C) Extracellular changes with ageing
1. Secretion of intercellular substances such as polysaccharides and fibrous proteins (Collagen fibres) changes. The permeability of collagen fibres is remarkably decreased.
2. Diffusion of substances from cell to cell is also affected adversely due to the collagen substances present in the extracellular substances.
Death : Death is one of the most important biological facts of life cycle of an organism. The cells cease their functioning.
1. Due to ageing heart, liver, kidneys etc. become weak in function and the metabolic rate decreases fast.
2. Sudden stop of blood circulation to the brain and lungs causes death.
3. The immune system becomes week causes infections, resulting in death.
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