A new organism begins as a single cell, the zygote, formed when an egg is fertilised by a sperm. In humans, the early stages of development follow a set pattern.Cleavage
The zygote divides repeatedly by mitosis as it moves along the oviduct towards the uterus. These divisions are called cleavage. The embryo does not grow in size, so each new cell is smaller than the one it came from. After about 3 to 4 days the embryo is a solid ball of around 16 cells called a morula. The cells of the very early embryo, up to about the 8-cell stage, are totipotent.The blastocyst
By about day 5, a fluid-filled cavity forms and the embryo becomes a blastocyst, a hollow ball of about 100 or more cells. It has two distinct groups of cells:
• the trophoblast, an outer layer of cells that will form part of the placenta
• the inner cell mass, a cluster of cells on the inside that will form the embryo itself. These cells are pluripotent.
At about day 6 to 7, the blastocyst attaches to and sinks into the lining of the uterus. This is implantation. The trophoblast cells help to form the placenta.Gastrulation
During the third week, cells of the inner cell mass move and rearrange to form three germ layers: the ectoderm (which forms skin and the nervous system), the mesoderm (muscle, bone and blood) and the endoderm (the lining of the gut and organs such as the liver).As development continues, cells become more and more specialised and lose the ability to form any cell type.
The zygote divides repeatedly by mitosis as it moves along the oviduct towards the uterus. These divisions are called cleavage. The embryo does not grow in size, so each new cell is smaller than the one it came from. After about 3 to 4 days the embryo is a solid ball of around 16 cells called a morula. The cells of the very early embryo, up to about the 8-cell stage, are totipotent.The blastocyst
By about day 5, a fluid-filled cavity forms and the embryo becomes a blastocyst, a hollow ball of about 100 or more cells. It has two distinct groups of cells:
• the trophoblast, an outer layer of cells that will form part of the placenta
• the inner cell mass, a cluster of cells on the inside that will form the embryo itself. These cells are pluripotent.
At about day 6 to 7, the blastocyst attaches to and sinks into the lining of the uterus. This is implantation. The trophoblast cells help to form the placenta.Gastrulation
During the third week, cells of the inner cell mass move and rearrange to form three germ layers: the ectoderm (which forms skin and the nervous system), the mesoderm (muscle, bone and blood) and the endoderm (the lining of the gut and organs such as the liver).As development continues, cells become more and more specialised and lose the ability to form any cell type.