Module
5
Icon module 5

Learning aims

  • Describing the development of the embryo from the first cells to the blastocyst
  • Knowing the important stages of the preimplantation phase
  • Describing the importance of these stages
  • Knowing the duration of the preimplantation phase

What you should already know

  • The processes during fertilization that lead to the formation of the zygote

Delving deeper

  • How does the early embryo get to the uterus; how does it find its way?
  • From what does the embryo nourish itself until its implantation?
  • How is the blastocyst formed?
Key concepts
Concepts that are important in the preimplantation phase..

 

Introduction

Following a successful fertilization the preimplantation period that lasts for around 6 days ensues.
While the fertilized oocyte wanders from the ampulla through the fallopian tube into the uterine cavity, an implantation-ready blastocyst develops through cell divisions. At the end of the sixth day after fertilization the blastocyst embeds itself in the endometrium.

Fig. 1 - Two cell stage
media/module5/e0a_zweizell.jpg

  1. Pellucid zone
  2. Blastomere (cleavage cell)
  3. Polar bodies

Fig. 2 - Blastocyst
media/module5/e0b_blastozyste.jpg

4
Inner collection of cells
(embryoblast)
5
Blastocyst cavity

Legend
Fig. 1

A two-cell embryo has been created through the division of the zygote. Picture made roughly 24 hours after the in-vitro fertilization

Fig. 2

This form of the embryo is called a blastocyst because the cells enclose a fluid-filled cavity on the inside. On the fifth day after fertilization.
© Dr. A. Senn et al, CHUV Lausanne

Fig. 3 - Fallopian tube
media/module5/e0c_wanderung01.gif

1
Ovary
2
Fallopian tube
3
Endometrium
4
Myometrium
5
Uterine cavity
A
Impregnated oocyte
B
Two-cell stage
C
Blastocyst

Legend
Fig. 3

The green line marks the path the embryo travels in the first six days until it reaches the implantation location in the uterine cavity. The images under A, B and C represent three snapshots of the evolution of the oocyte from a fertilized state (A), through a two cell stadium (B) to a blastocyst (C)