Pre-embryonic Stage

The pre-embryonic stage is the first two weeks of development after fertilisation and involves three major processes: a) cleavage, b) implantation, c) embryogenesis.

 Cleavage divisions

a) Cleavage refers to the rapid mitotic divisions within the first 3 days. After fertilisation of the egg, it’s known as a zygote and the divisions within the zygote are very rapid, producing up to 100 cells but there is no significant growth.

Cleavage can be holoblastic (full) or meroblastic (partial) and depends on how much yolk is present within the egg. Humans and other mammals have little yolk as they receive nourishment from the mother (placenta or milk) during the embryonic stages and therefore undergo holoblastc cleavage.

Each cell produced is called a blastomere and the collective term for over 100 blastomeres is a blastula. 

The blastula then forms a blastocyst, characterised by it's inner cell mass (pale blue lump on the inside of the cell differentiation image).

Establishment of the different layers after gastrulation in the embryob) Within the blastocyst there’s an outer layer of simple squamous cells called the trophoblast that becomes the placenta and there’s an inner cell mass or embryoblast that’s destined to become the embryo itself. 

When the blastocyst attaches to the endometrium, this is the beginning of implantation. 

c) During implantation, the embryoblast undergoes gastrulation, which is the arrangement of the three germ layers: endoderm, mesoderm and ectoderm. Cells migrating inwards into the blastocyst form these.  

After gastrulation, a process called organogenesis occurs. This starts with neurulation, when the neural plate (ectoderm) forms the neural tube, precursor of the central nervous system.

These germs layers will continue to form the digestive and respiratory tracts, muscle related tissues and nervous system respectively.

Implantation occurs in three stages:
(1) Apposition
(2) Adhesion
(3) Invasion


Cleavage image courtesy of Gray's Anatomy under public domain:


Cell Differentiation image courtesy of NCBI under public domain:


Fast Facts

Endometriosis is a condition that affects 2 million women in the UK alone and is when the cells of the endometrial lining are found elsewhere in the body, especially on the ovary or in the pelvis.  As the breakdown of the lining cannot be released from the body like the lining in the uterus, it causes pain and inflammation.  Sometimes adhesion is increased between the cells and if this is on the ovaries it can impair fertility.

Fast Facts

Implantation of the fertilised egg can occur outside of the uterus, such as the abdomen, ovaries, fallopian tubes or cervix.  Massive tissue damage can occur as the embryo tries its hardest to secure a good blood supply.  This is known as an ectopic pregnancy and will result in a miscarriage or the embryo will have to be be aborted as it can cause serious health risks for the mother.

For more information see:

  • Juan Manuel Corp

Ectopic pregnancy in animals and humans.

Reproduction (2006) 131, 631-640