1 of 47© Boardworks Ltd 2007. 2 of 47© Boardworks Ltd 2007

1 of 47 © Boardworks Ltd 2007



Your first photograph was probably taken when you were10-12 weeks old.

You already looked human and you were about to start gaining mass faster than you ever will again.

Your first photo?

Your head diameter was measured to check your age and make sure you were growing properly.


What is growth?

Growth is an irreversible increase in the size or mass of an organism as it develops.

Different organisms grow at different rates, and these growth rates vary at different stages of development.

Mammalian development begins in the womb. This is called the gestation period and is different for each species.

The rate of growth in the womb is fixed but changes during development.


Gestation periods


Bonny, bouncing babies!

What is a ‘normal-sized’ baby?


Growing fast


Average-sized babies – around 3.3 kg – have the best chance of surviving birth and developing into healthy adults.

It’s best to be average

What could stop a baby growing properly?

Babies that are small for their age are much more likely to die just before, during or just after birth.

Those that survive are more likely to have health and developmental problems later in life.


What affects the size of babies?

What factors affect the size of babies?

genetics – many genes have an affect on body size, such as the gene for growth hormone

diet of mother – this affects the type and amount of nutrients available for growth

disease – infections and abnormalities can reduce growth

drug-taking by mother (e.g. nicotine, alcohol) – smoking is associated with a smaller birth size

size of mother – this affects the size of the uterus and therefore the space in which the foetus can develop


Genes or environment?


Why measure growth?

As a baby develops, its growth is usually checked against average measurements called growth curves.

They take into account the range of measurements that are considered ‘normal’.

Comparing a baby to an average as it grows can help detect any changes that could indicate a medical problem.

Why is it important to measure a baby’s growth?

These growth curves show how long/tall and heavy a baby should be for its age.


Ways of measuring growth


True or false?


Changing growth rates


Underweight babies?

The old ones were based on bottle-fed babies. Was this valid data to use for breast-fed infants? What problems might have arisen from these old growth curves?

In 2006, the World Health Organisation (WHO) issued new growth curves for monitoring child development.

Breast-feeding mothers may have been worried the growth of their babies and switched to bottle-feeding. This can lead to ‘bottle-baby disease’ – a combination of dehydration, diarrhoea, and malnutrition caused by unsafe bottle-feeding.


Breast is best

After eight months, the average bottle-fed baby has consumed 20 Mars bars’ worth of surplus energy compared with the average breast-fed baby.

Compared with bottle-fed babies, breast-fed babies have a lower risk of:

acute infections, such as diarrhoea, pneumonia and meningitis

developing chronic conditions such as diabetes, allergies and inflammatory bowel disease

high blood pressure and obesity.


Different parts of the body grow at different rates. This is called allometric growth.

Changing body proportions

For example, the head and brain grow quickly at the start of development. The rest of the body then catches up after birth.

fetus newborn 2 years 6 years 12 years 20 years


The stages of human life



By the time you finish growing, you will have an estimated 50 trillion cells. That’s 50,000,000,000,000!

You started out as a single cell. How did one cell turn into 50 trillion?

How did you get like this?


A slow start

For the first week, humans don’t actually grow – the fertilized egg cell (zygote) splits in two every day to make a bundle of smaller cells.

cell division

After about 1 week, the zygote is called an embryo.


Exponential growth


What are stem cells?

The first cells are stem cells. These are unspecialized cells capable of developing into many different types of cell.

If stem cells continued to divide as they were, humans would end up as a large jelly-like blob!

Stem cells found in embryos are called embryonic stem cells and develop into all the different types of cell in the body.


Changing cells

When the embryo contains about 500 cells, the cells stop being the same and they stop getting smaller with each division. They start to differentiate into different types of cell.

At this point, stem cells no longer form two new stem cells when they divide. Instead, one of the two daughter cells becomes a progeny or tissue cell.

stem cell

tissue cellstem cell


Becoming specialized

Tissue cells continue to divide and differentiate, each time becoming more and more specialized.

Some will become nerve cells, others will become blood cells, muscle cells, bone cells, etc.

nerve cells red bloodcells

stem cell

tissue cells

cardiac musclecells


Taking shape

After 6 weeks of life, a human weighs about 1 gram and measures 13 mm, but cell division and differentiation has given the embryo a visibly human shape.

Between 6 weeks and 6 months ofpregnancy, new cells continue to be madeand the mass of the embryo rapidly increases.

cell division

After about 8 weeks of life, the embryo is called a fetus.


The final stretch

Towards the end of pregnancy, growth slows down as space in the uterus becomes limited.

After about 6 months, division stops in most tissues and the bulk of fetal growth is due to cell elongation.

cell elongation


How many divisions?

Stem cells are special because they can carry on replicating indefinitely. However, cells that are fully differentiated (tissue cells) only replicate a certain number of times before dying.

This number of divisions is the Hayflick limit and, in humans, it is about 50. Bacterial cells have no Hayflick limit.

Cancer cells are tissue cells that have retained the ability to divide indefinitely and have no Hayflick limit. These cells grow exponentially, producing a tumour.


Growth terms


How is the growth of plants different to the growth of animals?

Many plants can continue growing throughout their lifetime.

Most plant growth occurs by cell elongation, not cell division.

Many plants cells retain the ability to differentiate into any cell type. This is why cuttings can be used to create new plants.

Plant vs. animal growth

Growth in plants only occurs in certain tissues.


How do plants grow?

zone ofcell division

zone ofcell elongation

zone ofcell differentiation

Cell division only takes place in regions of the plant called meristems. These tissues are found in the tips of the roots and shoots.

Plants only get taller when cells in meristems elongate following cell division. After this, cells differentiate into their final form.

meristem


Animals, plants or both?



Super starfish!

What do plants and starfish have in common?

Chop the stem of a plant and it will grow back. What’s more, the cut stem can grow into a completely new plant.

The same is true of the starfish! If it loses an arm to a predator, it will grow back.

Even better, if the predator manages to eat everything but the arm, a new individual can grow out of the remaining arm!


Animal regeneration

Why can a starfish regenerate missing limbs?

Humans and most other animals cannot do this, but scientists are studying how stem cells could be used to create new tissue and even new organs to treat disease and injury.

The starfish contains stem cells that can divide, differentiate and produce new tissues to replace those that are lost.

What type of stem cells should be used?


A controversial source?

Most stem cell research has used cells obtained from embryos left over from fertility treatment.

These are called embryonic stem cells (ESCs) and are the most powerful type because they can turn into any type of cell.

Their use in research is strongly criticized by people who believe it is unethical to kill embryos for their cells.

Stem cells from fetuses are almost as powerful, and these have also been found in the blood of umbilical cords and amniotic fluid of pregnant women.


If you ran the country…


Therapeutic cloning


What are adult stem cells?

Adults also contain stem cells, where they are found in small numbers in many organs, including bone marrow, brain, skin and muscle.

Research has shown that some adult stem cells can be manipulated to produce many different cell types (e.g. brain stem cells can make muscle cells). This is called plasticity.

Adult stem cells normally maintain and repair damaged tissue, and can usually only make a small number of cell types. For example, adult stem cells in the skin only normally form skin cells.


What are bone marrow transplants?

Stem cells found in bone marrow have been used for many years to treat diseases that affect blood cells, such as leukaemia, by a bone marrow transplant.

Bone marrow is harvested from a suitable donor under general anaesthetic.

The donated bone marrow is injected into the patient.

Stem cells in the injected bone marrow circulate in the blood and end up in the patient’s bone marrow, where they start to divide and produce new blood cells.

The patient’s abnormal blood cells are destroyed by chemotherapy and/or radiotherapy.


Advantages of using adult stem cells

Adult stem cells are not as powerful as embryonic stem cells, but they do have other advantages.

They come from volunteers so they are more ethically acceptable.

They are less likely to become cancerous.

A patient’s own stem cells could be used to treat their own disease, avoiding the problem of immune rejection.

There is a great deal of research into using adult stem cells to treat diseases. They could reduce the need for organ transplants, which have long waiting lists.

It might be easier to guide their development into specific cell types.


How could adult stem cells be used?



Glossary (1/2)

bone marrow – The spongy tissue in the centre of bones that is a rich source of adult stem cells.

differentiation – The process by which a cell specializes into a specific type.

division – The process by which one cells splits into two new cells.

elongation – The process by which a cell increases in length.

embryo – The early stage of animal development, when it is just a cluster of cells.

fetus – The stage of animal development after 8 weeks of development.


Glossary (2/2)

gestation – The period of mammalian development in the uterus.

growth – An irreversible increase in mass of an organism.Hayflick limit – The number of divisions a cell can undergo

before dying.meristem – An undifferentiated region of a plant in which

cells divide.puberty – The stage of human development characterized

by sexual development and growth spurts.regeneration – The replacement of lost tissue or organs by

new ones grown from stem cells.stem cell – An unspecialized cell that can potentially

become any type of cell.


Anagrams


Multiple-choice quiz

Documents

1 of 47© Boardworks Ltd 2007. 2 of 47© Boardworks Ltd 2007