5.0 Learning aims, what you should already know, introduction, delving deeper

• Learning aims
  
• What you should already know
  
• Introduction
  
• Delving deeper

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

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
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
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).

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?