EMBRYO STEM CELLS
Fertilisation
Cleavage
Blastocyst formation
Gastrulation
Organogenesis
ADVICE AND GUIDANCE FOR PRACTITIONERS (NATIONAL 5, BIOLOGY)
© Crown copyright 2012
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EMBRYO STEM CELLS
A male sperm and female egg fuse to form a zygote (fertilised egg).
The zygote can produce all the cell types needed to make a complete human .
When? 0–24 hours
The zygote starts dividing (cleaving) into identical cells as it passes down the
oviduct towards the uterus. By the time it gets there, the process has repeated
sufficiently to form a ball of around 100 cells.
When? 1–4 days
The ball of cells begins to specialise, forming an outer layer of cells with a
cluster of cells inside (inner cell mass), which can form most cell types of the
human body.
When? 3–8 days
The cells in the inner cell mass move to form three layers: the ectoderm,
mesoderm and endoderm. The ectoderm forms the skin, brain and nervous
system; the mesoderm forms muscle and the skeletal and circulatory systems;
and the endoderm forms the gut lining and many of the internal organs.
When? Week 3
The formation of body organs. The embryo is called a foetus at 8 weeks, by
which time the structures that will give rise to all the major organs are
present.
When? Week 3–8
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ADVICE AND GUIDANCE FOR PRACTITIONERS (NATIONAL 5, BIOLOGY)
© Crown copyright 2012
ADVICE AND GUIDANCE FOR PRACTITIONERS (NATIONAL 5, BIOLOGY)
© Crown copyright 2012
3