Icon module 5

The emergence of the blastocyst (hatching)

Around the end of the fifth day the embryo frees itself from the enveloping pellucid zone. Through a series of expansion-contraction cycles the embryo bursts the covering. This is supported by enzymes that dissolve the pellucid zone at the abembryonic pole. The rhythmic expansions and contractions result in the embryo bulging out of and emerging from the rigid envelope. This "first birth" is called hatching.

Fig. 11 - Hatching scheme

  1. Pellucid zone
  2. Trophoblast
    (outer cell mass)
  3. Hypoblast
    (part of the inner cell mass)
  4. Blastocyst cavity
  5. Epiblast
    (part of the inner cell mass)

Fig. 12 - Blastocyst

Fig. 11, 12

The embryo emerges from the pellucid zone, leading with the abembryonic pole first.


Development from the morula up to hatching.

Fig. 12

The cleavage divisions up to hatching.
(Overview of the pictures)

The volume of an embryo begins to grow as soon as the blastocyst cavity within the morula starts to form.

Fig. 13 - Contractions of the blastocyst

Fig. 13

Characteristic for the volume increase are the rhythmic expansion and contractions as seen in the figure.
After M.P. Primi, CHUV, Lausanne