Week 1: Ovulation to Implantation

There are four main steps to know about here:

  1. Fertilisation (0-24 hours)
  2. Morula formation (1-4 days)
  3. Blastocyst formation (4-5 days)
  4. Implantation (5-6 days)
This image summarises the process of the first week


  • First, the oocyte (egg) is released during ovulation in the ovarian cycle
    • This enters into the fallopian tube/oviduct via fimbriae
  • The sperm meets the oocyte 
    • It penetrates the corona radiata then the zona pellucida
      • The zona pellucida is a glycoprotein membrane covering the oocyte that  provides nutrients
      • The corona radiata surrounds the oocyte and the zona pellucida
  • The membranes of the sperm and the egg fuse to form the zygote
The oocyte and associated structures.

Morula Formation

  • The totipotent zygote undergoes rapid cleavage to form 2, 4, 8, and then 16 cells
    • At the 16 cell stage it is collectively known as the morula (no cystic cavity).
      •  The individual cells are called blastomeres
      • NB size of the individual cells remains the same
    • The cells continue to divide in the morula
  • The zona pellucida still remains and is important in protecting and providing nutrients to the zygote via diffusion
  • This all occurs while the morula is travelling toward the uterus for implantation

Blastocyst formation (4-5 days)

  • At this stage, the zona pellucida degenerates
    • Therefore implantation into the uterus is necessary to provide nutrients
  • Two separate pluripotent cell types develop from the cells of the morula to develop the blastocyst (this name is used because it has a fluid filled cavity)
    1. Trophoblast develops at the top
      • Cells flatten and form epithelial wall to form blastocyst cavity.
      • These will produce the placenta
      • Also secrete fluid inwards to form the blastocyst cavity
    2. Inner cell mass (aka embryoblast)
      • Produce the embryo proper
      • The side of the ICM facing blastocele will be ventral surface of embryo – thus is the first axis formed (i.e. the ventral vs dorsal orientation of the embryo)


Implantation (5-6 days)

  • The endometrium is the lining of the uterus
    • It is a single layer of columnar epithelium overlying loose connective tissue
    • It is rich in blood vessels
    • It is maintained by progesterone and later HCG release from syncytiotrophoblasts of the embryo
  • Implantation should occur in the anterior or posterior uterus
    • Ectopic pregnancies (outside the uterus) can cause many problems
      • E.g. placenta previa if the cervical os is blocked,
      • E.g. abdominal haemorrhage if in Douglas’ pouch
  • Implantation occurs when the blastocyst’s syncitiotrophoblasts adhere to the epithelium and then invade
    • See Week 2 for more detail