Growth
Growth is a gradual increase in size or number.
Biological growth is usually considered as an increase in that part of the living system which is in its own turn capable of growth.
Growth is a permanent increase in size.
The growth of multi-cellular organisms is of three kinds with regard to the growth and multiplication of the body cells.
Multiplicative Growth or Embryonic Growth
Auxetic growth
Accretionary growth
What is cell growth?
The increase in cytoplasmic volume, as in cell development and cell reproduction.
The increase in size or population of cells, as in mitosis.
When used to describe the growth of cell populations, it refers to the growth and division of "mother cell“ into two "daughter cells“.
Why is cell growth important?
Mitosis is the process of cell division in eukaryotes. Mitosis is important as a form of reproduction in single celled organisms, like the amoeba. Mitosis regulates cell growth, development, and repair in multicellular organisms.
However, if mitosis is out of control, that will lead to cancer.
What is the role of growth factors?
Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes. Growth factors typically act as signaling molecules between cells. Examples are cytokines and hormones that bind to specific receptors on the surface of their target cells.
Types of growth
1. 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.
Example: Growth of embryos, prenatal growth in mammals
2. 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.
In the development of nematodes, cell division stop in the early stages of organogenesis.
The number of cells in the fully grown nematode is the same as in a young one which has just emerged from the egg. (959 cells in C.
elegans).
The number of cells in each organ rudiment may be definitely fixed in this type of development.
For instance, the whole excretory system of a nematode consists of only three cells.
Example: Auxetic growth is found in nematodes, rotifers and tunicates.
In certain tissues of higher animals like the body muscles, auxetic growth is seen.
3. 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.
Accretionary growth occurs due to mitotic multiplication of some special cell type of reserve cells (condriocytes, osteocytes and mesenchyme) occurring in specific location of the body, e.g. post-embryonic growth of animals.
Examples:
Bone marrow of vertebrates contains unspecialised cells that continuously produce blood cells to replace worn out ones.
The epidermis of terrestrial vertebrates may serve as an example of this type of growth.
In the outer layers of the epidermis, the cells do not divide and do not grow.
Their cytoplasm becomes keratinized, thus, forming a protective layer on the surface of the skin.
The fully keratinized cells are no longer vital and are perpetually being peeled off.
The Malpighian or generative layer of the epidermis consists of cells which are not keratinized and which possess the capacity to proliferate.
Later, the outermost cells of the layer replace the keratinized cells,
undergoing keratinization in their own turn.
Allometric Growth
Allometric growth is when an organism grows and the proportions of its body parts are different.
The word 'allometric' means different measure.
For example, at birth, a human's head may take up about 1/4 of its body but by the time that same human is 14 years old, the head may only take up about 1/8 of its body.
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.
Isometric Growth
Isometric growth is the exact opposite of allometric growth.
'Isometric' comes from the Greek for same measure.
When isometric growth occurs, the proportions of the organism stay relatively the same; the mature organism looks like a bigger version of the young version of the organism.
This is what happened with the sponge. One end of the sponge did not get wider than the other as it expanded or grew.
It simply looked like a bigger version of the pre-soaked sponge.
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.
Cell Reproduction and Cell Growth
At the cellular level, growth of multicellular organisms is governed by two main activities.
1. Reproduction of individual cells of body by mitotic cell divisions.
2. Growth of cells by synthesizing new protoplasm.
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 increase in size. The cells grow up to a limited extent after which these cells enter cell division.
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.
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.
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.
Stationa ry
Log or exponenti al
phase Lag
phase
Phases of growth
Growth of an organism can be differentiated into the following periods.
1. 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.
2. 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.
Stationa ry
Log or exponenti al
phase Lag
phase
3. 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.
Stationa ry
Log or exponenti al
phase Lag
phase
Growth rate in Animals
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.
Stages of growth period in Man
Growth period in man may be divided into 5 stages.
Prenatal stage - 9 months of embryonic life
Infantile stage - Birth to 10 months of age
Early childhood - 10 months to 5 years of age
Juvenile stage - 5 years to 14 years or the time of puberty
Adolescent and post adolescent stage - 14 years to 20 - 22 years
