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

• The cleavage divisions up to the morula stage
  
• How a blastocyst is engendered
• The emergence of the blastocyst (hatching)
• The polarity of the embryo
• The migration of the embryo through the fallopian tube

The cleavage divisions up to the morula stage

Approximately 24 hours after fertilization the impregnated oocyte begins with the first cleavage division.

Fig. 4
Zygote (approximately 16-20 hours after insemination).

Fig. 5
Two-cell embryo (approximately 24 hours after insemination).



© Laboratoire de Biologie de la Reproduction; Lausanne

Fig. 6
Four-cell embryo (approximately 45 hours after insemination).

Fig. 7
Eight-cell embryo (approximately 72 hours after insemination).



© Laboratoire de Biologie de la Reproduction; Lausanne

The morula, a collection of around 30 cells (blastomere), is created at about 96 hours. Because these cells arise only through the cleavage of the zygote and all are found inside the pellucid zone, which cannot expand, no growth is seen. Every new cell is thus only half as large as the cell from which it derives. The name of this stage comes from its resemblance to a mulberry, since it really looks like a collection of spherical cells.

Fig. 4
Morula (approximately 96 hours after insemination).


© Laboratoire de Biologie de la Reproduction; Lausanne

The cleavage divisions of the individual cells do not have to happen synchronously and thus phase shiftings can occur. The result is that the cell divisions do not happen according to the sequence 4,8,16,32,64,128,... , but rather at any given moment any number of cells can be present in the embryo.