5.1 The cleavage divisions and the migration of the embryo through the tube

The emergence of the blastocyst (hatching)


Quiz 04

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 Fig. 12 - Blastocyst  Legend


Pellucid zone
Trophoblast (outer cell mass)
Hypoblast (part of the inner cell
Blastocyst cavity
Epiblast (part of the inner cell mass)

Fig. 11, Fig. 12
The embryo emerges from the pellucid zone, leading with the abembryonic pole first.


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


Development from the morula up to hatching.
(260 kB)

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  Legend

Fig. 13
Characteristic for the volume increase are the rhythmic expansion and contractions as seen in the figure.
After M.P. Primi, CHUV, Lausanne

Previous page | Next page