Developmental Biology Growth GROWTH Growth defined as the

Developmental Biology: Growth

GROWTH • Growth defined as the increase in size of an organism or of its parts due to the synthesis of Protoplasm (includes both cytoplasm and nucleus) or Apoplasmatic substances (substance produced by cells and forms a part of the tissues eg. fibers, matrix etc. ) • Protoplasm synthesized with no cell division and as a result the cells becomes larger, which is basically for the synthesis of cell components like mitrochondria, cell membranes, enzymes and other proteins. • In multicellular bodies, growth of individual cell is essential but the measurement of the growth of individual tissue cell is difficult because of their size, only the rhythm of cell multiplication invitro in tissue cultures can be observed.

• There are 3 basic types of the growth of multicellular animals related to the growth of its cells: A) Auxetic growth B) Multiplicative growth C) Accretionary growth • A) Auxetic growth: - Volume of the body increases due to growth of individual cells without any increase in number of cells. (in early chordates) Eg. Nerve cells which do not divide and lost the ability to divide by mitosis retain the capacity for growth, after division stop increase in size to a very considerable degree becoming the largest cells in the vertebtare body. • B) Multiplicative growth: - Growth results due to proliferation of the constituent cells without much increase in size. (in case of embryos) Eg. The embryos and parental growth of the higher vertebrates. • C) Accretionary growth: - Special type of undifferentiated cells, which reinforce and replace the worn out differentiated cells, i. e. cells are added on like cartilage and bone growth. Eg. The Osteoblasts (bone cells) do not grow and proliferate, but the growth of the bone is dependent on the osteoblasts of the Periosteum.

GROWTH PATTERN • Growth can be additive or multiplicative. • Additive- is confined to bones, teeth etc. • Multiplicative- exponential growth seen in growing organism. Allometric growth When organ grows at a different rate from the rest of the body. This produces change in size, along with change in shape of organism. Characteristic of mammals. Isometric growth When an organ grows at the same mean rate as the rest of the body, it is called isometric growth. The proportion of the structures remain the same. Eg in fishes, and insects like locust.

Allometric Growth Allometry in humans Eg. The embryo produced a head which is larger in proportion to the rest of the body, after the embroyonic period the head grows slower than the hands, legs and torso. Allometric growth found in social insect Eg. The growth regulators in social insects is juvenile hormones, important in determing which of the female becomes the given in ant & bee colonies.

Isometric Growth

Proportional (Isometric) and disproportional growth (allometric) of organs To compare the growth of different organs their rate of growth may be calculated (rate of growth changes with age) According to Huxley 1932, if two parts of animal grow at different rates than their sizes at any given moments are in relationships to each other which is expanded by the formula Y= bx k formula for allometric growth Y=size of one of the organ X size of the other organ b= constant k=growth ratio If k =1 than two organs grow proportionally (Isometrically) If k≠ 1 the growth is disproprtionate or allometric If k>1 organ y grows quicker pace than organ x (Positively allometric) If k< 1 organ y grows slowly than organ x (negatively allometric)

Phases of Growth of an organism can be differentiated into the following periods. 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. 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. 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.

Growth Rate • the most common graph plotted for growth • the curve is of sigmoid shape / S-shaped • obtained if any suitable parameter, e. g. dry mass (m), is plotted against time (t) • applicable in the study of growth of microbe population or any large plant/animal

Growth Rate The Absolute Growth Curve

The Absolute Growth Curve • it shows the overall growth pattern and • the overall growth period • for most organisms, the growth pattern are the same i. e. initially, initially the growth is slow then, it grows faster . finally, finally it slows down again

The Absolute Growth Curve • can be divided into five phases: The lag phase (AB) The log phase (BC) The retardation phase (CD) The stationary phase (DE) The negative growth (EF)

The Absolute Growth Curve D C E F B A Lag phase Logarithmic phase Stationary phase Retardation phase Death phase

The Absolute Growth Curve The lag phase (AB) • very little growth D E • cell division, enlargement s F C • the organism is adapting the new sources/ environm B A AB BC CD DE EF

The Absolute Growth Curve The log phase (BC) D • exponential growth E F C • and remains for a certain period of time B A AB • the growth accelerates BC CD DE EF • until the sources become limited

The Absolute Growth Curve The retardation phase (CD) • the growth slows down D E • the growth is limited by: F C B • external factors: A AB • internal factors: • organism maturity BC CD DE EF • food supply � • space � • competition �

The Absolute Growth Curve The stationary phase (DE) • equilibrium / stable D E F C • the parameters studied becomes constant B • the organism matures A AB • no further growth • nett growth rate = 0 BC CD DE EF • the rate of cell division = the rate of cell death

The Absolute Growth Curve The negative growth phase (EF) • ageing stage D E F C • or the ability to get nutrients or to compete for spaces worsens B A AB • badly deficient in nutrient sources and spaces BC CD DE EF • growth rate falls drastically

Exponential Growth 1. The J-shaped exponential growth curve has two phases. a. Lag phase (growth is slow because population is small) b. Exponential growth phase (growth is accelerating)

SIZE OR WEIGHT J- shaped curve / /

EXPONENTIAL GROWTH Exponential growth produces a J-shaped curve. The growth defined in geometrical progression, the increase proportional to the initial gravity of growing substance. Geometrical progression means exponential, thus growth is an exponential process. Exponential Growth Rate V: dw x 1= dt w V = rate of growth w = size or weight t = Given time So the rate of growth is measured as the increase related to the initial mass of growing substance. Formula for the Exponential growth: W= evt W= Weight of the animal at any given time t. V= Obesrved rate of growth. e= base of natural logarithm. • The formula applied if the internal gravity of growing matter is infinitley small or intial size is negleable, if the intial size taken into account than the formula changed as W= bevt b= constant which is equal to the initial size of the growing organism.