Developmental Biology

Human Reproduction and Fertilisation

For the human species to continue surviving, it is essential that mature adults are capable of producing fertile offspring, to continue the existance of the species and pass on genetic information from generation to generation.

This is done via reproduction. The following is a step by step basis of how reproduction occurs from beginning to end.


Before the initial cell can develop into a mature adult, the building blocks to create that cell must be available before fertilisation can occur. Cells called gametes are produced by mature adults via meiosis which contain half the genetic information needed to produce the final cell capable of growth.

For a short time period every month, the female gamete is readily available to be fertilised by sperm, to form a zygote.


The gametes, both containing half the genetic information needed to produce the offspring, fuse together, meaning all the genetic information required for the offspring to grow is present. For this to occur, sexual intercourse must occur in order for the semen (sperm) to be ejaculated and have the potential to fuse with the ovum.

Millions of sperm are released at the point of ejaculation, and when ejected, they 'swim' towards the female egg with their thread-like tail. This race towards the egg is fuelled by a fuel tank of ATP that provides the energy for their efforts. 

After a long journey, many of the sperm will have died out in their efforts to reach the egg, though some still have to the potential to fertilise it. Each will attach itself to the ovum but only one should succeed in penetrating it. Enzymes contained in the acrosome (head) of the sperm break down the wall of the egg. When fertilised, the egg secretes various hormones to prevent it from being overwhelmed by the other millions of sperm attempting to fertilise it.

The Zygote

Within hours of conception the fused gametes, a zygote, undergoes cell division. The presence of a hormone called progesterone prevents further female eggs being produced. Within the first week after conception the fertilised egg travels towards the uterus, where the continued growth of the zygote will occur in the form of an embryo.

The continuation of the growing embryo is elaborated upon in the next page.

Growth and Development of a Human Baby

Continued from the initial human reproduction page, the previous page in this tutorial.

Upon arrival in the uterus, the zygote fuses itself to the uterine wall. At this point, cells of the zygote differentiate into two distinct types

The Placenta

The placenta is the life support machine of the developing embryo, providing oxygen and food for it and removing toxic material and CO2 from the developing embryo. The umbilical cord, a connection to the mothers' blood stream is responsible for this.

The Embryo

By the end of the third week this placenta provides a rich source of nutrients for the unborn baby that now is roughly the size of this letter o....

Towards the end of the first month, the heart starts to form while designated cells begin to form the basic structure of the limbs, spine, nervous and circulatory systems. After the initial formation of these major bodily areas, less essential characteristics begin to develop such as the eyes, ears and nose.

By the tenth week of pregnancy these facial characteristics become recognisable, and the overall silhouette of the embryo now resembles that of a human.

By the fourteenth week, all the major characteristics of the embryo have more or less developed, and possesses all the requirements of a fully functional being.

From this time onwards, the embryo will continue to grow in size rather in complexity via cell division. Forty weeks later, nine months since fertilisation, the embryo, now in the foetal stage is mature enough to enter the world.

The next page goes through the stages of birth, where the mothers body has to adapt to the harsh conditions of birth...

Birth of a Human Baby

Continued from the previous page looking at fertilisation and growth of an embryo.

The baby soon outgrows the energy requirements that the placenta previously met, and is time to be conceived into the open world by the mother. There are four main stages of birth process

Stage 1 - Between 8 to 10 Minutes

The release of oxytocin, a hormone, instructs the mothers body that the food supply for the baby can no longer be met, which bring about contractions in the mother and soften the tissue of the cervix where the baby will pass through.

The 'waters then break' caused by the pressure of the contractions, the amniotic fluid which once surrounded the foetus is released.

Stage 2 - Delivering the Baby - 30+ Minutes

This is where the baby is pushed down the cervix through the vagina, out to the open world. This is the hardest stage of labour, and it involves the mother 'pushing' with the contractions to force the baby out. This is a natural behavioural response in the mother due to the pain involved in getting the baby out.

Stage 3 - Afterbirth

This is where the placenta is excreted after the newly born baby, as it is no longer required in the body and has served its function. The newly born baby must survive in the open world with its maternal parents there to help them learn, grow up healthily and care for.

The next page investigates contraception, used to prevent pregnancy which although is morally disputed, can be beneficial depending on your point of view. Contraception is not accepted in all parts of the world due to cultural and religious beliefs, but are a way to prevent intercourse leading to pregnancy.

Birth Control and Contraception

Birth Control

Some religious beliefs see methods of contraception unethical while some political bodies chose birth control as a means of determining pregnancies, by making it against the law to have more than a designated amount of children. In fact, some governments actually pay their citizens not to have any children.


Any type of barrier that prevents contact between egg and sperm after ejaculation. Possible barriers include the female and more popular male condom, which also contain spermicide chemicals to nullify the activity of the sperm.


Hormonal contraception in its best instance in 'the pill' used by women. It controls the production of gametes in the female by taking a pill for every day in the cycle. Their is the option of coming off it to have the period as some feel this is more comfortable. Older versions of the pill were not so accurate in their function which increased chances of breast cancer in users.

Recent medical research has been looking at the option of the male pill, This looks like 


This involves either

  1. blocking the fallopian tubes in women by either cutting or tying them, stopping the availability of gametes for pregnanc
  2. In males it involves cutting the vas deferens, the sperm transport duct from the testes that transport sperm out the penis

Both are fairly simple procedures that effectively end either patients ability to produce offspring.


The fertility of the female fluctuates at certain points of the month, meaning there is a 'good' and 'bad' time to get pregnant. The part of the cycle where the female is least likely to be impregnated is a possible way of maximising the possibilities of not getting pregnant

Ultra Uterine Device

Also known as the coil, this device prevents the formation of the embryo and placenta by fusing to the uterus. This practice of contraception is generally outdated as it is associated with painful experiences by the user during periods and leads to complications in the longer term

However, for those children who are born, they have a long life ahead of them, and continue to grow in the outside world..

The next page investigates the physical development of humans through a lifetime, and relates to information associated with this process.

Physical Development in Humans

The Newly Born Child

Depending on the nutrients available to the child within pregnancy and the genetic make up of the child, most healthy babies weight between five and eight pounds. Since the placenta is absent, which previously provided the baby with nutrients, food is required for the metabolic processes and the continued growth of the baby.

Its nutrients derive from feeding of liquids by the parents to begin with, as the young baby's digestive system cannot cope with more complex solid foods. This provides the nutrients to begin with for continued growth and development

The Toddler

Through learning and development , by the end of the first year the baby can feed on solids and perform basic functions such as crawling and walking. The dependence on growth from food from the mother is now switched to the production of hormones in the young child, which now has a developed endocrine system for hormone secretion.

At this stage the young child is at a period of accelerated growth, which will continue into early childhood

Depending on the genetics of the baby, the hormones secreted will determine the height of the child in its future years. As a general rule of thumb, the size of the child at 2 years is roughly proportional to its final height barring unusual factors.

Puberty and Adolescence

After early childhood, the child continues to grow steadily in its single figure years up until it becomes a teenager. At this point, puberty begins. 

Puberty is the point in time where the development of sexual characteristics begin, and will allow these humans to become sexually active and be able to produce gametes for reproduction.

Puberty in Females

On average, most girls tend to reach puberty before boys.

Puberty in Males

On average, most boys tend to reach puberty after most girls have developed, though puberty lasts longer

By the end of the teenage years puberty ceases and maturity has been reached. At this point, no more true growth occurs (an increase in cell number) and new cells are solely used for regeneration purposes.

Women cease to be fertile on average in their 50's where no more eggs are produced by the ovaries. It is interesting to know that a female baby already has over 1 000 000 follicles for eggs to develop in, which decreases to 100 000 by puberty. By the 6th decade of life in the female, this supply will have run out.


Better living conditions have led to people in modern societies living to a ripe old age. In light of this, increased study has went into longevity in humans (how long we live) and how we can combat the degenerative nature of aging.

Over time our bodies become less efficient, and the homeostasis of an efficient body becomes less able to maintain the favourable conditions. This leads to degeneration of our body, until death.

The previous 5 pages have investigated growth from an embryo to adult. The next pages will study the internal operations of the human body more deeply, beginning with what food we need to survive from a healthy diet and continuing on how our body works with this supply of energy

Human Biology - Food and Digestion

Food is what is required by humans to grow and survive, and provide a 'fuel' for the energy needed in our biological reactions

As opposed to plants that can acquire their food from the soil and sunlight, animals are required to eat foodstuffs in order to create the energy they need to survive. These foods contain many compounds and elements that are useful to our body and our daily functions.

This food is entered into the body via the mouth, where enzymes such as lysozyme begin to breakdown the food while it is chewed. When this food is broken down into smaller pieces it can then be swallowed, where it travels down the oesophagus towards the stomach.

The stomach then continues to break down the foodstuff. The stomach at this point is filled with a type of acid that further breaks down the food, until it is in a semi-liquid state. Note that the stomach only acts as a storage point at this stage, until the food slowly passes into the duodenum, which is part of the small intestine. It is here where the food is completely broken down, in the following manner:

From here, you can see that a complex product (the food you eat) is stripped bare, until the substances that they are broken down to can be used by the body in some way. By breaking down these foodstuffs to their most basic nature, the body can then rebuild these substances as and when required, for example, to create enzymes.

The body can then use these substances when they are in demand, such as the raising blood sugar levels in the blood stream so that energy can be created in the cells around the body via respiration.

The Physiological Homeostasis page in the Regulation of Biological Systems tutorial looks at how the body determines when a particular action is required, such as the increase of blood sugar concentration. Also, the Cell Biology Tutorial page ADP and ATP introduces ATP, the body's biological energy, and how it is created, which is continued on the following page.

In order for the body to successfully utilise the energy available within the food and use it properly, humans must intake the required amount of nutrients and minerals that our body requires; this can be done by having a balanced diet. A good diet will consist of the right measurement of the following compounds.

The right measurement of the above compounds in our daily diet leads to an optimally functioning body, which has all the building blocks it needs to execute cellular processes on a day to day basis.

Such a balanced diet also helps combat disease and provide the optimal conditions for our body to operate and grow. If this is not the case, a deficiency in a particular substance can lead to disease.

Some of the diseases associated with an unbalanced diet are investigated on the next page.

A Balanced Vitamin Diet. Vitamins A - K

As mentioned in the tutorial, a balanced diet is essential to a healthy organism. Deficiencies in particular elements can lead to a decline in health, and exhibit symptoms that are not desirable at all. The following information looks at some of these deficiencies and why they happen

Vitamin A

Vitamin A is essential for night vision and improves effectiveness of the immune system. It can be found in many dairy products, and especially in carrots.

Vitamin B12

This vitamin is essential in the formation of red blood cells and lack of this may cause anaemia.

Vitamin C

Scurvy is caused by a lack of vitamin C in the diet. Vitamin C is essential in the formation of red blood cells, antibodies and a healthy circulatory system. Symptoms of scurvy include bleeding gums and dizziness caused by the deficient blood supply.

Vitamin D

Essential part of the diet required in the absorption of minerals in food, where a lack of vitamin D in the diet leads to a condition called rickets, where softening of the bones cause them to bend from the lack of calcium. Humans have the ability to synthesise vitamin D from sunlight

Vitamin E

Important in preventing the oxidation of fatty acids in cells, and is commonly found in cereals and green vegetables

Vitamin K

Vitamin K is important for the blood clotting process and can be synthesised in the human gut, and found in green vegetables.

To continue more about human development and functionality, click to go to the human biology tutorial.

A Balanced Diet - Carbohydrates and Fat

Alongside the num