Cleavage Divisions

Timeline of Human Pregnancy

• Overview of Pregnancy Timeline:

  • Pregnancy lasts about nine months or approximately thirty-eight weeks from fertilization until birth.

  • Fertilization marks the beginning of gestational time where the female carries the conceptus (fertilized zygote) through various developmental stages.

Cleavage Divisions

• Definition of Cleavage Divisions:

  • Cleavage divisions refer to the series of mitotic cell divisions that a zygote undergoes after fertilization.

  • Cleavage is responsible for increasing cell number without increasing the overall size of the embryo, occurring within the first four days after fertilization.

Timeline of Cleavage Divisions

• Day of Fertilization to Day 4:

  • Day 0: Fertilization occurs, forming the zygote, a two-nucleus (2n) cell with 46 chromosomes (23 from mother, 23 from father).

  • Day 1 (around 36 hours post-fertilization): Zygote divides into two daughter cells (the two-cell stage).

  • Day 1.5 (approx. 48 hours post-fertilization): Two cells divide to form four cells (the four-cell stage).

  • Day 2 (around 60 hours post-fertilization): Four cells divide to form eight cells (the eight-cell stage).

  • Day 2.5 (approx. 72 hours post-fertilization): Eight cells divide to form sixteen cells (the 16-cell stage or morula).

Characteristics of Cleavage Divisions

• Zygote:

  • Fertilized egg; the union of male and female pronuclei creates a diploid cell.

  • Contains all necessary genetic material (2n = 46 chromosomes).

• Two-Cell Stage:

  • Occurs at approximately 36 hours after fertilization.

  • Contains two identical somatic cells, still enclosed in a zona pellucida.

• Four-Cell Stage:

  • Occurs approximately 12 hours after the two-cell stage.

  • Further divided and still enclosed in the zona pellucida.

• Eight-Cell Stage:

  • Occurs approximately 12 hours after the four-cell stage.

  • Cells remain identical, with no differentiation yet, giving potential for identical twins if cells separate.

  • At this stage, cells are still within the zona pellucida, becoming weaker due to immune system response.

• Sixteen-Cell Stage (Morula):

  • Formation of the morula occurs 12 hours after the eight-cell stage, now consisting of approximately 16-32 cells (blastomeres).

  • Known as a "little berry", it is characterized by the potential for individual cells to still separate and form a unique human.

Importance of Cleavage Divisions

• Increase Surface to Volume Ratios:

  • Goal of cleavage divisions: to maintain effective cell functioning by increasing surface area to volume ratios, allowing for better nutrient absorption and waste removal.

  • As cells divide, they decrease in size, improving their surface area relative to their volume.

Transition to Blastocyst

• Formation of the Blastocyst:

  • Approximately day 4, the morula transitions into a blastocyst which consists of 64 cells or more.

  • The blastocyst will emerge into the uterine cavity during this period.

Extraembryonic Membranes

• Concept of Extraembryonic Membranes:

  • These membranes support the embryo but are not part of the embryo itself.

  • Each cellular stage (two-cell to blastocyst) contributes to the formation of these extraembryonic membranes and the developing embryo.

  • The textbook considers the embryonic period to start two weeks after fertilization, while the preembryonic stage encompasses these initial cellular stages.

• Contribution of Stages to Extraembryonic Membranes:

  • All stages (zygote, two-cell, four-cell, eight-cell, morula, blastocyst) are responsible for forming the four extraembryonic membranes and the entire embryo.

Conclusion and Forward Look

• The development of the blastocyst and its implications for the continuing stages of embryonic development will be discussed in subsequent material.

• Key Terms to Remember:

  • Zygote, Cleavage, Morula, Blastocyst, Blastomere, Differentiation, Surface to Volume Ratio, Extraembryonic Membranes, Preembryonic Stage, Embryonic Period.