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| Types of Growth |
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| The growth of multi-cellular organisms is of four kinds with regard to the growth and multiplication of the body cells. |
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| Multiplicative Growth or Embryonic Growth |
| In multi cellular organisms, growth occurs by an increase in the number of cells of the organism. The increase in the number of cells is due to mitotic cell division. In this type of growth, the average cell size remains the same, or increases insignificantly. |
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| Example: |
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| Growth of embryos, prenatal growth in mammals |
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| Auxetic growth |
| In some organisms like Ascaris, growth occurs as a result of increase in the size of their cells. The number of cells remains the same. The body grows in size because of the enlargement of its cells. |
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| Auxetic growth is found in nematodes, rotifers and tunicates. In certain tissues of higher animals like the body muscles, auxetic growth is seen. |
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| Accretionary growth |
| During post embryonic growth, and also in the adult, all the body cells are not capable of undergoing division. This is because they have become differentiated. But at some locations, undifferentiated cells are present which divide mitotically and replace the worn out differentiated cells as and when needed. These cells are called reserve cells. |
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| Example: |
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| Bone marrow of vertebrates contains unspecialised cells that continuously produce blood cells to replace worn out ones. |
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| Cells of Malpighian layer of epidermis of human skin produce new cells which replace the worn out cells of the outer layers of epidermis. |
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| Appositional growth |
| It is the addition of new layers on the previously formed layers. If is the characteristic type of growth, seen in rigid materials |
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| Example: |
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| Addition of lamellae in the formation of bone |
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| Cell Reproduction and Cell Growth |
| At the cellular level, growth of multi cellular organisms is governed by two main activities |
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 Reproduction of individual cells of body by mitotic cell divisions |
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 Growth of cells by synthesizing new protoplasm. |
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| The interphase stage of the cell cycle is differentiated into G1, S and G2 phases. During these phases new materials such as nucleic acids and proteins are synthesized and accumulated in the cells so that cells and their nuclei increases in size. The cells grow up to a limited extent after which these cells enter cell division. |
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| The growth of the individual cells comprising the body is the most important factor of growth in all multi cellular animals. After attaining a specific nuclear cytoplasmic ratio, the cells divide and multiply adding to the size of the organism. |
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| Animals Growth Curve |
| The growth rate of an individual at different periods of life can be represented in a growth curve by plotting the weight of individual at different time intervals (in years) on a graph paper. For example a human baby can be weighted from birth till adult hood when growth stops. Plot the weight in kilograms against time in months or years. This gives a growth curve. This is a simple S-shaped sigmoid curve. |
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| The S - Shaped sigmoid curve is characteristic of all higher animals including man. The difference between the initial and final weight or initial and final size of an individual for any period of time is the absolute increase. |
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| Growth of an organism can be differentiated into the following periods. |
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| Lag period |
| It is the first period during growth phase, where the curve rises gradually. The organism is getting prepared for growth by synthesizing enzymes and accumulating substances to metabolize protoplasmic components. |
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| Exponential period |
| During this period growth begins slowly at first and becomes rapid later on. Hence the curve rises steeply. As a result the organism enlarges doubling and redoubling in size. This phase is also called as logarithmic phase. |
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| Deaccelerating growth period |
| The exponential growth does not continue indefinitely. It is followed by a period when growth proceeds more slowly and finally ceases altogether. The curve therefore rises slowly and these become horizontal, signifying limit of growth. During this phase, the rate of acceleration is exactly equal to catabolism. |
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| Growth in all higher animals including man, follow a specific rate and rhythm of growth, and stops long before death. The rate of growth is not uniform throughout life. If varies during different periods of life. |
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| Stages of growth period in Man |
| Growth period in man may be divided into 5 stages. |
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 Prenatal stage - 9 months of embryonic life |
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 Infantile stage - Birth to 10 months of age |
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 Early childhood - 10 months to 5 years of age |
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 Juvenile stage - 5 years to 14 years or the time of puberty |
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 Adolescent and post adolescent stage - 14 years to 20 - 22 years |
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| Rapid growth is seen in the prenatal and puberty periods. Little or no tissue is added after the post adolescent period. |
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| (above) Development in fish - an example of isometric growth. The external structures retain their shape and partial relationships as a result of a proportional growth rate. |
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| (below) Development in Man - an example of allometric growth. To show the relative rates of growth from the age of two months to 25 years each stage has been given a constant height. |
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