1. What is Growth and Development?
Growth is a subset of development. Growth is often thought of as increase in size, but it is not an adequate definition. The size of a plant cell may increase as it takes up water by osmosis, but this process is easily reversible and thus cannot be considered growth. When a zygote divides repeatedly to form a ball of cells (early embryo), there is an increase in cell numbers without an increase in size (volume and mass). This is called cleavage and is the result of cell division without subsequent increase in size of daughter cells. This process is considered growth despite the fact that there is no increase in size.
Growth may be positive or negative. Positive growth occurs when synthesis of materials (anabolism) exceeds breakdown of materials (catabolism). Example of negative growth is in the course of germination of a seed and the production of a seedling, there is an increase in cell number, cell size, fresh mass, length, volume, complexity of form but the dry mass may actually decrease because the reserves are being used up. From this, germination is considered to include a period of negative growth which becomes positive only when the seedling starts to photosynthesize. The stored food is hydrolyzed to be used for respiration. Sugars are oxidized to carbon dioxide and water, causing a decrease in mass.
1.1 Growth
This involves an irreversible increase in the dry mass of an organism. During growth of a multi-cellular organism, there is an increase in the number of cells. This also does not include increases in water content and stored food material (e.g. fat) because these are not permanent. For example, when a plant cell is placed in a solution of higher water potential than itself, there is a net gain of water via osmosis and when placed in a solution oc lower water potential, there is a net loss of water.
Growth is a permanent, irreversible increase in dry mass of living material (normally accompanied by an increase in cell number).
1.2 Development
This is closely related to growth. Development involves differentiation of cells, an increase in dry mass (growth) and an increase in complexity of an organism e.g. development in flowering plants consists of growth of a zygote into an embryo within a seed, the process of germination and growth of a seedling into an adult plant.
The main difference between growth and development is that growth involves only a quantitative change whereas development involves both quantitative and qualitative changes.
Development is a progressive series of changes in form which is genetically programmed and maybe modified by the environment.
1.3 Why do we need Growth and Development?
In order to be successful in survival, multi-cellular organisms have to undergo various phases of growth and development.
2. How Cell Division and Cell Enlargement lead to Growth?
Growth and development includes 3 phases and are as follows.
2.1 Cell Division
One of the ways to achieve growth in an organism is by an increase in cell number as a result of mitosis and cell division. Individual multi-cellular organisms grow in size as their cells grow in number and size. An average adult human contains trillion of cells, all of which have grown from one original cell, the zygote.
2.2 Cell Enlargement
This is an increase in cell size. For example, an increase in length of stems and roots brought about by elongation of cells. Growing plant cells also produce additional organic material in their cytoplasm which contributes to the dry mass of the cells.
2.3 Cell Differentiation
In any multi-cellular organism, all the cells derived from the zygote by mitosis are genetically identical. Therefore a liver cell, for example, contains the sane set of genetic instructions as a kidney cell. As cells differentiate, different genes are switched on or off. The study of how this differing behavior of cells is controlled may lead to mew technique for treating a variety of medical disorders using stem cells which are undifferentiated cells found in young embryos able to develop into any organism‘s cell types. At any one time, a particular cell will have a variety of genes switched on or off in response to its environment without losing its identifying characteristics. This contributes more to development than growth.
2.4 Meristems and Plant Growth
A meristem is a group of plant cells that retain the ability to divide by mitosis. Once the very young embryo stage is past, growth in multi-cellular plants is confined to the meristems. Subsequent growth of the plant occurs by cell division at the meristems. There are 3 types of meristems.
2.4.1 Apical Meristems
These occur at the tips of the stem and root and are responsible for the primary growth of the plant. Growth at the apical meristems leads to increase in length of the stem and root.
2.4.2 Lateral Meristems
These occur as cylinders towards the outer part of the stem and are responsible for secondary growth of the plant, resulting in thickening.
2.4.3 Intercalary Meristems
These occur at the nodes of grasses. (A node is the stem at the point of attachment of a leaf.) These allow an increase in length in positions other than the tips; grass stems continue to grow even when the apical meristem has been cut off.
2.5 How Cell Division and Enlargement lead to Growth in Plants?
2.5.1 Cell Division
By a series of mitotic divisions, the plant zygote gives rise to the multi-cellular embryo within a seed. After germination, mitosis resumes, mostly in the apical meristems near the tips of roots and shoots contributing to increase in length.
The apical meristem of the stem is composed of an outer coat (tunica) of 2 or 3 rows of regularly arranged meristematic cells, surrounding a central mass of irregularly arranged meristematic cells (corpus). During mitosis of the cells of the tunica, the plane of division is at right angles to the surface and cells of the corpus divide haphazardly in any plane.
2.5.2 Cell Enlargement / Expansion
Increase in length of stems and roots is mainly brought about by elongation of cells. In the zone of elongation, cells increase in size, mainly by osmotic uptake of water into the cytoplasm and into the vacuoles (accounts for 90% of a plant cell’s expansion). Growing plant cells also produce additional organic material in their cytoplasm which contribute to the dry mass.
Small vacuoles increase in size, eventually fusing to form a large vacuole. This stretches the thin cell walls and the orientation of cellulose microfibrils in the walls help to determine the final shape of the cell. As enlargement nears completion, may cells develop additional thickening of the cell walls, either of cellulose or lignin which contributes to dry mass.
2.6 Advantages of Multi-Cellularity
Cells differentiate to perform different functions.
Specialized cells performing one particular function lead to greater efficiency.
It is possible to store more materials and so be better able to withstand periods when resources are scarce.
Some processes require a range of conditions e.g. digestion often has an acid and an alkaline phase. It is easier to separate regions of opposing conditions in a multi-cellular organism than it is in a single cell.
Larger organisms may have a competitive advantage e.g. large plants compete better for light than small ones.
Large size may provide protection from predators because they are simply too large to ingest.
2.7 Terminologies
Morphogenesis is the generation of form and structure during development of an individual organism. It is the collective process of growth and development. Differentiated cells come together to perform a similar function, forming tissues and organs, which give overall form and structure to the mature organism. It is influenced by both genes and the environment.
Determination refers to the fixed pathway a cell may take eventually. This depends on the potency of the cell.
Potency is the number of options open to the cell in its subsequent development.
Totipotency is the inherent capability of a single cell to provide the genetic program required to direct the development of an entire individual. In mammals, totipotent cells have the potential to become any type in the adult body. The only totipotent cells are the fertilized egg and the first 4 or so cells produced by its cleavage.
Pluripotency / Multipotency is the property of a pluripotent cell which can give rise to many cell phenotypes of the organism to which it belongs when suitably challenged but lacks complete totipotency. The cell can follow many possible pathways but less than a totipotent cell.
Unipotent describes only one possible developmental pathway possible to a cell.
2.8 Cleavage
After fertilization, the zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells.
The first cleavage occurs shortly after the zygote nucleus forms. A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the grey crescent. This divides the egg into 2 halves forming the 2 cell stage. The second cleavage forms the 4 cell stage. The cleavage furrow again runs through the poles but at right angles to the first furrow.
The furrow in the third cleavage runs horizontally but the plane closer to the animal than to the vegetal pole. It produces the 8 cell stage.
The next few cleavages also proceed in synchrony, producing a 16 cell and then a 32 cell embryo. However, as cleavage continues, the cells in the animal pole begin dividing more rapidly than those in the vegetal pole and thus becoming smaller and more numerous. By the next day, continued cleavage has produced a hollow ball of thousands of cells called the blastula. A fluid-filled cavity, the blastocoel, forms within it.
3. Growth Curves and Growth Rates
Before looking in detail at ways of measuring growth, we shall consider how best to describe growth and show it in the form of graphs. Graphs showing growth are known as growth curves. (The graphs will not be shown here.)
3.1 Absolute Growth Curves
Absolute growth is an increase in size or mass with time (actual growth). A graph which shows absolute growth has time on its x axis and size on its y axis and is called an absolute growth curve.
The absolute growth of an organism is the cumulative increase in size over a period of time. Absolute growth curves are useful for showing the overall pattern of growth and how much growth has occurred. Absolute growth curves often have a sigmoid shape.
3.1.1 Lag Phase
Very little growth occurs during the lag phase because there are very few cells initially. Even if these are rapidly dividing, the actual increase in size is small. In this period, the organisms are adapting to available resources.
3.1.2 Log Phase
The growth proceeds at an exponential rate of increase. The rate of growth increases and at any point, it is proportional to the number of cells present. As the number of cells increase, more cells undergo mitosis and the size of the organism increases greatly. This is a time of no apparent constraints on growth; supplies of nutrients are adequate and waste products have not accumulated.
3.1.3 Decelerating Phase (Linear Growth Phase)
Growth now proceeds ay a steady, relatively constant rate. Grrowth is limited to some extent by internal (e.g. genotype of an individual which specifies a certain maximum size) or external factors (environment) or both. This third phase is not always present or apparent, it can be absent from the growth curve of certain organisms like bacteria.
3.1.4 Stationary Phase
This is a plateau on the curve. Cells are still dividing during the stationary phase but only at a rate the replaces the cells that have died. The size of the organism remains constant during this phase. When replacement of cells is slower than the death of the cells, the curve slopes downward and senescence (growing old) sets in.
4. Absolute and Relative Growth Rates
4.1 Absolute Growth Rate Curves
The absolute growth rate is a measure of increase in size over a series of equal time intervals. Plotting these increments against time produces an absolute growth rate curve. An absolute growth rate curve rate is bell shaped. The absolute growth rate increases up to a maximum, after which it decreases. The peak of the curve is the inflexion point of the absolute growth curve.
4.2 Relative Growth Rate
Relative growth rate takes into account the existing size and is a measure of specific growth. Relative growth rate is also known as specific growth rate. This is useful for showing how efficiently an organism is growing. It is a meaningful measure of growth because the amount of growth occurring at any stage depends on the tissue already present, especially when the growth of 2 organisms, with rather different initial masses, is compared.
4.3 Relative Growth Rates of Boys and Girls
The relative growth rates of boys and girls are highest at 6 months of age and are similar in the initial period i.e. from 6 months to 9 years. However, there are distinct differences. After puberty, the maximal growth rate is higher for boys than for girls. The growth spurts also occurs later in boys than in girls. The difference could be due to girls genotype are XX and boys are XY and gonadotrophin-releasing hormone (stimulating the production of FSH and LH) is released earlier in girls than in boys. It could also be because boys and girls produce different hormones. Girls produce estrogen and progesterone and boys produce testosterone which promotes muscular growth that results in greater growth during puberty.
5. Patterns of Growth
The sigmoid curve forms the basis of most growth curves. However, various patterns of growth occur among organisms. These include isometric growth, allometric growth, limited growth, unlimited growth and intermittent growth.
5.1 Isometric Growth
Isometric growth is growth in which the various organs within an organism grow at the same rate as the rest of the body. Therefore, an organism increases in size without changing its shape. The relative proportions of any 2 structures remain the same in isometric growth.
In isometric growth, there is a simple relationship between the length, area, volume and mass of the organism. The area increases with the square of the length and the volume increases with the cube of the length. Isometric growth occurs in the grasshopper.
5.2 Allometric Growth
Allometric growth is growth in which the various organs within an organism grow at different rates that are also different from the growth rate of the organism as a whole. The organism increases in size as well as changes its shape. Allometric growth occurs in mammals.
In humans, the head, lymphoid tissue and reproductive organs grow at very different rates. The head of the human grows rapidly in the first 5 years after birth and thereafter, it does not grow much. The lymphoid tissues grow rapidly from birth to early adolescence and thereafter it decreases to half its maximum size by adult age. The reproductive organs grow very little in early life but rapidly during puberty.
5.3 Limited Growth
Limited growth is also known as definite growth or determinate growth. Organisms that show limited growth up to certain predetermined size and then stop growing. The growth curve is similar to a sigmoid shaped curve. Limited growth occurs in animal, plants, insects, birds and mammals.
5.4 Unlimited Growth
It is also known as indefinite growth of indeterminate growth. There is no maximum adult size and when conditions are favourable, growth continues, only tending to slow during old age. Growth curve is a combination of a series of sigmoid curves.
5.5 Intermittent Growth
It is also known as discontinuous growth. The growth curve consists of a step like pattern. For arthropods, they undergo ecdysis (moulting), so before the new exoskeleton fully hardens, it must take up air or water to expand the exoskeleton as much as possible. After it hardens, the insect decreases to its original size to allow space for more actual growth. If dry mass was used to measure growth of arthropods, the absolute growth curve would be a smoother S-shaped curve because actual growth in arthropods is continuous.
6. Parameters of Growth
Growth can be measured at various levels of biological organization such as growth of a cell, organism or population. At the level of the organism, various parameters can be measured over a period of time to show growth. The various parameters are as follows.
The use of length as a parameter for measuring growth. Examples include measurements of height of plant, length of root, length of internode and length of body of insect. The disadvantage is that it does not take into account of growth in other directions which may be considerable.
The use of area as a parameter for measuring growth. An example is the measurement of leaf area.
The use of volume as a parameter for measuring growth. An example is the measurement of volume of a tree. The disadvantage is it is difficult to measure if the organism is irregular in shape.
The use of fresh mass as a parameter for measuring growth. Fresh mass refers to the mass of an organism under normal conditions. The advantage is that it is easy to measure since it requires little preparation and it does not cause injury to the organism therefore this method can be used to monitor the growth of an organism over time. The disadvantage is that fresh mass does not measure true growth because it may give inconsistent readings due to fluctuations in water content.
The use of dry mass as a parameter for measuring growth. Dry mass refers to the mass of an organism after all its water content has been removed by drying. Dry mass determination is performed by killing the organism and drying it in an oven at 100 oC. This causes the water to evaporate without burning the carbohydrate, lipid and protein. The organism is then cooled in a desiccator before weighing. The process of drying and weighing are repeated until the dry mass is constant. It is more accurate to obtain dry masses of a large sample of individuals and calculate the mean dry mass, rather than the dry mass of a single organism. The disadvantages are a follows. Organisms will be killed during drying and this method cannot be used to monitor the growth of an organism over time. The sample size must be large enough to obtain a representative reflection of growth and therefore many organisms would have to be killed.
Other difficulties are as follows. There is a problem with allometric growth. Different growth rate of body parts will not be considered with mass of height of entire organism is used for measuring growth. There is a problem with fat accumulation. An increase in fat content is not considered as growth because it is reversible. There is a problem of irregular growth due to fluctuations in diet or the environment.
7. Measurement of Growth – microorganism, plants and animals
7.1 Measuring Growth of Unicellular Organisms
In the study of the growth of a culture of unicellular organisms, it is population growth that is measured by counting the numbers of individuals present in the culture at specific intervals.
The following tells us how to do it. Inoculate a sterile nutrient medium with a small number of microorganisms. Incubate the liquid culture at a constant temperature of 30 – 37 oC with shaking. Withdraw a small volume of sample at regular intervals. Two types of cell count are done – total and viable.
7.1.1 Total Cell Count
Direct counting using a haemocytometer under a microscope. A microscopic slide that is designed to contain a fixed volume of sample covering a rule grid is used. The total number of cells (dead or alive) per ml of culture can be obtained.
Indirect counting using turbidity measurements (spectrophotometry / colorimetry) The higher the number of bacteria in the culture, the more turbid it appears. In the assay, turbidity is measured by the colourimeter which passes a beam of light through the sample of microbial culture. The light absorbed by the microbial culture is then recorded. The more number of microorganisms there are, the more turbid the liquid medium becomes and the greater the absorbance of light by the microbial culture.
There are problems associated with total cell count method. The technique requires practice. Slides must be very clean. Small particulate matter may be mistaken as bacteria. Uneven distribution of bacteria in stock culture may lead to error in final count especially in small samples.
7.1.2 Viable Count
It takes into consideration the total number of living microorganisms. It can be done by sample count or to measure a product of metabolism such as gas (e.g. carbon dioxide from respiration for yeast of bacteria). The application is the pasteurization of milk. The viable cell count can be taken to compare with the count before pasteurization to determine if significantly lass bacteria are alive. If there are too many bacteria in the original solution, serial dilutions can be made before the plating and counting of colonies. Replicates must be done and controls must be set up to achieve a reliable and accurate viable count.
There are problems associated with the viable cell count method. Contamination occurs easily and aseptic procedures require special apparatus and techniques. The presence of more than one type of bacteria will cause one to be more favoured than the others and growth will be an inaccurate measure since culture conditions have placed a constraint on it. Culture conditions will not favour different types of bacteria equally. Bacteria are rarely homogeneously present throughout a sample, but found in clumps as reproducible results may not be obtained. Some bacteria are pathogenic so care is required during handling.
7.2 Measuring Growth in Plants
Being able to measure and analyze growth has a number of important applications. Growth of crops and domestic livestock such as cattle is of commercial significance. If techniques are available for measuring growth, the effects of important variables such as light, water, minerals, sowing density of crops and diet and shelter for livestock can be investigated. Optimum growth conditions can then be found.
Growth in plants is most commonly measured by increase in height, length, dry mass or fresh mass.
7.2.1 Measurement by Increase in Length and Height
It is relatively easy to measure the length of structures such as leaves or fruits or to measure the overall height of a plant form a convenient point such as soil level.
There are problems associated with measurement by increase in length and height. Both shoots and roots may vary in length in response to environmental conditions. Shoots may grow longer in search of light without necessarily increasing in mass and roots may grow longer in search of water. This leads to inaccuracy in measuring growth. A full definition of growth may include cell division and cell differentiation as well as cell elongation whereas method measures only cell elongation. Also, there may be more variations in linear dimensions than in mass. Mass is more representative of true growth than length.
7.2.2 Measurement by Increase in Mass
It is more difficult and time consuming to measure changes in mass than changes in length. Measurement of mass is generally regarded as a better guide to growth than measurements of a single dimension like height and length because it is more representative of the whole structure and is a better geode to the eventual yield.
Fresh mass is easy to determine, by weighing the whole plant. The problem of removing soil form the roots can be avoided under lab conditions by growing the plants in nutrient solutions.
Measuring dry mass is much better than measuring fresh mass because the weight of the plant may be affected by fluctuations in water uptake and loss and such changes in weight do not represent true growth.
Dry mass measurements are more difficult and time-consuming to obtain. They are destructive too which means a different sample of plants much be used each time. Efforts must be made to make samples truly representative, which means the mean value of about 30 plants should be taken and all of them must be grown under identical conditions.
7.2.2.1 Experiment to Measure Growth by changes in Mass
Standardized conditions: seeds chosen (e.g. peas or wheat) can be germinated in commercial compost (mixture of decaying substances) or in a sterile medium and watered with a standard culture solution containing the required mineral salts.
Fresh mass can be measured after blotting dry the material to remove excess liquid.
Dry mass can be obtained by drying in an oven at 110 oC for 24 hours, cooling in a dessicator and weighing. This procedure is repeated until the dry mass is constant.
Start with 300 seeds soak for 24 hours, take a sample of 30 seeds for fresh and dry mass determination and plant the remainder. Further samples of 30 can be taken at fixed regular intervals e.g. every 2 days. When the seedling stage is reached, the masses of whole plant, including roots, should be determined. Graphs of means of mass, or other measurements, versus time can be plotted after 20 days.
7.2.2.2 Problems associated with Measurement by Mass
It is difficult and tedious. Measuring dry mass will destroy plants. Large sample size to be used to derive representative results.
7.3 Measuring Growth in Animals
Fresh mass, height or length of the entire body can be used to monitor the growth of animals. Body parts may also be used but they must displat isometric growth with the rest of the body.
7.3.1 Measuring Growth in Humans
Mass is a convenient growth parameter to use. The mass of a baby can be recorded and charted throughout life till he / she dies. The growth curve will not have a sigmoid-shaped graph, as humans have 2 growth spurts, one at infancy and the other at puberty.
Height is another parameter that can be used. A relative growth rate curve may be plotted. However, upon comparison, mass would be a better growth measurement as height only takes into account one dimension of growth.
There are problems associated with measurement of growth in humans. Sample size must be large and large sample size is hard to come by for humans. Growth period is very long e.g. measurement over a 6 month period and this will take a long time to study.
7.3.2 Measuring Growth in Insects
Measuring growth in most animals present a whole new host of problems because many of them live in the wild and move about, so trapping them for measurement purposes is difficult.
Insects would be more convenient subjects since they are easy to keep and grow relatively quickly. Most undergo metamorphosis which is the change in form from a larval stage to an adult stage during the life cycle. This involves the hormome-controlled breakdown of existing tissues by enzymes released from lysozymes, and the formation of new tissues.
There are 2 kinds of metamorphosis. Complete metamorphosis is a total change of form that occurs with the result that the adult is very different in form from the lava. Incomplete metamorphosis is the gradual change in form from larva to adult where a series of moults occurs. Each successive stage is called a nymph or instar and is large and more like the adult, though only the adult has functional wings.
If the body length of the insects were measured at fixed regular intervals, the growth curve would take place in spurts since the rigid exoskeleton does not allow expansion of the insect, so growth in length occurs only after moulting and before the new exoskeleton forms.
Measuring dry mass of insects at fixed regular intervals yield a smooth sigmoid curve reflecting the true continuous growth of the insect based on living material.
