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 growth

                                                 

growth in living beings can be defined as the irreversible increase in overall size and weight of an organism due to the synthesis of new protoplasm.

forms of growth

In living organisms, growth can occur in two following forms.

Intussusception: It is an internal type of growth. In this form, the surface area of the cell increasing via deposition of any external material, that is synthesized by the existing body cell.

Apposition It is an external type of growth in which the surface area of the cell increase via deposition of any external material, this is not the constituent of the cell. Non-living, like mountains, sand mouth, and snow also grow by apposition, if the increase in body mass is the criterion. however, this growth is by the accumulation of material on the surface only.

Features of growth

At the cellular level, growth involves

1. Increase in size of the cell due to the synthesis of protoplasmic structure (hypertrophic) 

2. increase in the number of cells by cell division ( hyperplasia )

3. Increase in the amount of apoplastic structure like the fibres, material, etc.

Different cells may grow differently in humans for examples

1. Lens cells grow by multiplication.

2. Cardiac and skeleton muscle cells grow by the increase in volume.

3. Newron growth by extension and growth of the axons and dendrites.

4. Cartilage and bone cell growth by the secretion of extracellular Matrix.

Pattern of growth

On the basis of changes in body proportions, there are two types of growth patterns.

(1) Isometric growth It is a type of growth in which all the parts grow at the same rate and same time Thus, there is no change in the form and body proportion, e.g. fish, locust, and grasshopper.

(2) Allometric growth It is a type of growth, in which different organs grow at a different rate and at different time. Thus, the external form and body proportion change which the stage of growth, e.g. mammals.

Types of growth

The different types of growth in the bodies of a living organism are 

(1) Auxetic growth It is the growth due to the increase in the size of cells (not a number), e.g. nematodes, rotifers, and tunicates (early chordates).

(2) Multiplicative growth It is the growth of the body due to the increase in the no of cells, e.g. embryonic development of higher vertebrates.

(3) Accretionary growth   It is the growth in which undifferentiated reserve cells get differentiated to perform a specific function, e.g. formation of erythrocytes from erythropoietic tissue, replacement of worn-out cells of the skin, secretion of extracellular matrix by chondrocytes and bone osteocytes.

Growth curve

It is a graphic representation of growth against time. The sigmoid curve or S-shaped growth curve is the characteristic growth curve of all higher animals including humans. In the initial phase, The curve rise at a very slow rate.

In the middle part, the curve rises steeply indicating a very fast rate of growth (acceleration phase). Finally, in the last part, the rate of growth again slows down till the curve becomes horizontal (Plateau phase).

It signifies that no further growth is taking place and the individual is simply itself. Hence, animals show a differential growth rate.

An absolute growth rate curve shows how much the rate of growth changes with time, whereas a relative growth rate curve is taken into account the existing size.

Positive and negative growth 

Positive growth occurs when the synthesis (anabolism) of the material exceeds the crackdown (catabolism) of the material. in some cases, continuous positive growth is non-vertebrates, fish, and certain some reptiles. in such cases, growth may continue to rise slightly until the organism dies.

Negative growth occurs when catabolism exceeds anabolism. It is during the germination of seeds. Germination of seeds includes a period of negative growth, which becomes positive when seedlings start to photosynthesis and make their own food. In certain cnidarians, the growth curve flattens out, indicating zero growth.

Growth in human

the human body undergoes the following pre-birth phase of growth.

                                                    

1. cellular growth

The unicellular organism grows only through cell growth. In a multicellular organism, growth is the sum total of growth shown by their cells. The steps involved in cellular growth initiate with cell division following by cell enlargement and cell differentiation finally leading to maturation. cell growth occurs during the post-mitotic phase and interphase.

2. Embryonic growth

In humans, the early embryonic development stage constitutes transitional growth. During cleavage and blastula formation, the cells exhibit little growth.

During gastrulation (early embryonic state), rudiments of primary organs develop, but they are in their functional state. with further development, the rudimentary organs grow, differentiate, and become functional.

The overall growth of the fetus occurs mainly in the period after the basic morphological plane is established, i.e. 

Post-embryonic growth

East animals have a specific rate and rhythm of growth. There is no uniformity in the rate of growth. Animals grow at different rates at different periods of life. The human embryo is about 150-micro meters at the time of implantation, which grow about 50 cm over the 9 months of gestation. embryos can not grow considerably during the first 2 month after implantation as it is a time for the formation of basics human body structure. after four months, the embryo grows 10 cm/ month and during the first year, 2 cm/month and it stops after 22-33 yrs of age. 

Control of growth

Growth depends on growth factors and hormones. The various growth factors involved in the control of growth in the human body are discussed in afters.

Author : scienceinfo

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