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Chapter 17 Pre-natal Development. Fertilisation. The egg and sperm nuclei meet during fertilisation. The fertilised egg cell is now known as a zygote. Cleavage. CLEAVAGE. The number of cells doubles at each mitotic division and soon a solid ball of cells is formed.

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The egg and sperm nuclei meet during fertilisation

The fertilised egg cell is now known as a zygote



The number of cells doubles at each mitotic division and soon a solid ball of cells is formed

The solid ball of cells develops into a hollow ball with a fluid-filled interior



The solid ball of cells develops into a hollow ball with a fluid-filled interior

The group of cells to one side of the fluid-filled interior will become the fetus and is referred to as the embryonic area

The fluid-filled ball is surrounded by a thin outer layer of cells in the form of a membrane called the chorion

  • Implantation occurs about one week after fertilisation. This is the process by which the embryo becomes attached to the uterus wall with the side containing the embryonic area lying against the endometrium

3.As these projections burrow more deeply into the uterus wall, the embryo is drawn into the endometrium and becomes surrounded by it

1.The embryo’s cells start secreting an enzyme which digests away part of the endometrium

2.The embryo then grows rapidly amongst the maternal tissues forming finger-like projections which become part of the placenta

4.Once this process is complete, the embryo becomes successfully implanted. It receives food and oxygen from the endometrium until the placenta develops

  • The cells in the embryonic area continue to divide by mitosis forming a multi-layered mass of cells which undergoes differentiaion. This is the process by which unspecialised cells become altered and adapted to perform specific functions as part of permanent tissues

All the cells present in the embryo are derived by mitotic division from the original zygote and all contain exactly the same genes

During differentiation certain genes in different cells become ‘switched on’ or ‘switched off’

This is how specialised cells are formed

  • There are two types of twins-MonozygoticTwinsand Dizygotic Twins
  • Monozygotic Twins- are genetically identical to one another and originate from the same single egg fertilised by a single sperm. The embryo divides to form two separate embryonic areas within one fluid-filled ball. They possess separate amnions( water sacs) but share a common chorion and placenta
  • Dizygotic Twins- are non-identical to one another because they are formed when two eggs are released from the ovary and each is fertilised by a different sperm. Each embryo undergoes independent development and possesses its own amnion, chorion and placenta. Dizygotic twins maybe of the same or different sex. They are genetically dissimilar to one another.
harmful transfer
Harmful Transfer
  • The close proximity of the maternal and fetal circulations also allows harmful substances and pathogens to pass from mother to baby. The following are examples: Thalidomide, Alcohol, Nicotine, Heroin, Rubella, HIV
  • Carbon Monoxide- prevents your red blood cells from taking oxygen round the body. As a result babies are usually under-developed if mother’s smoke during pregnancy. They are also likely to have premature babies
  • Tar- coats the lungs. Contains nasty chemicals like rat poison, toilet cleaner, pesticides
  • Nicotine- the addictive drug in tobacco plants. Prevents babies from receiving adequate amounts of glucose.
  • Heroin is a narcotic drug which induces a temporary sense of relaxed detachment from pain and anxiety. It slows down bodily processes and makes the person feel contented and sleepy
placental hormones
Placental Hormones
  • Through Gestation, the uterus wall is maintained by oestrogen and progesterone secreted by the corpus luteum. After about two months the placenta takes over the job of secreting these hormones and the corpus luteum disentegrates
mammary glands
Mammary Glands
  • Oestrogen and Progesterone stimulate the mammary glands to prepare for lactation( milk production)

Each mammary gland is composed of lobes that secrete milk into sacs connected by ducts to the nipple

  • Milk production (lactation) is inhibited during pregnancy because it requires the activity of a hormone called PROLACTIN which only becomes active following :

Birth of the baby, expulsion of the placenta and the drop in oestrogen levels that occurs at this time. Prolactin is then secreted by the anterior pituitary gland which stimulates milk to be released into the sacs and tubules of the breast ready for breast-feeding. The milk is not released to the baby unless another hormone called OXYTOCIN is released

rhesus factor
Rhesus Factor

3.The mother is unaware that the baby is Rh+ as her body is unaware and doesn’t try to reject it

First Pregnancy

4.During birth however, the mother’s and baby’s blood becomes mixed and the mother starts to make anti-D antibodies. She is said to be sensitised.

1.The mother is Rh- but the baby is Rh+. The baby has antigen D on its red blood cell surface which is foreign to mum

2. The placenta prevents the blood of baby and mother mixing

rhesus factor1
Rhesus Factor

Subsequent Pregnancies

The mother now has red blood cells with no D antigen on the surface but she has Anti-D- antibodies.

If in subsequent pregnancies the baby is Rh+, the anti-D antibodies from the mother will cross the placenta and attack the baby’s red blood cells. The resulting condition is called Haemolytic Disease of the New-Born (HDNB)

treatment of haemolytic disease of the new born hdnb
Treatment of Haemolytic Disease of the New- Born (HDNB)
  • This can be treated by giving the baby massive transfusions of blood.
  • It can be prevented by injecting the mother with Anti-D Immunoglobinssoon after the birth of each Rh+ baby.
  • These antibodies destroy any D antigens from the fetus before the mother’s immune system has time to respond to them