Cleavage and Blastocyst Formation

Cleavage is the series of rapid mitotic divisions that follow fertilization, dividing the zygote into progressively smaller cells called blastomeres without overall growth. These divisions produce the morula and then the blastocyst, the stage at which the first distinct cell lineages of the embryo become apparent.

Definition

Cleavage is the set of mitotic divisions that partition the zygote into blastomeres, and blastocyst formation is the stage at which the embryo becomes a fluid-filled sphere comprising an outer trophoblast and an inner cell mass.

Scope

This topic covers the cleavage divisions, compaction, formation of the morula, cavitation to form the blastocyst with its fluid-filled cavity, and the segregation of the inner cell mass from the trophoblast. It is reference educational material in developmental anatomy and does not provide clinical or assisted-reproduction guidance.

Core questions

• How do the cleavage divisions partition the zygote without increasing its overall size?
• How does the morula compact and cavitate to form the blastocyst?
• How are the first cell lineages, the inner cell mass and the trophoblast, established?

Key concepts

• Blastomeres
• Cleavage without growth
• Compaction
• Morula
• Blastocoel (blastocyst cavity)
• Inner cell mass (embryoblast)
• Trophoblast
• First lineage specification

Mechanisms

After fertilization the zygote undergoes successive mitotic cleavage divisions within the zona pellucida, so the cell number rises while the total volume stays roughly constant and the blastomeres become smaller. Around the eight-cell stage the embryo compacts, with blastomeres maximizing cell-cell contact and becoming polarized, which underlies the first cell fate decisions. Continued division produces the morula, and fluid accumulation creates a cavity, the blastocoel, converting the morula into a blastocyst. By this stage the cells are segregated into an outer trophoblast layer, which will contribute to the placenta, and an inner cell mass, which will form the embryo proper. Differential gene expression and signaling distinguish these lineages, and the blastocyst subsequently hatches from the zona pellucida in preparation for implantation.

Clinical relevance

The blastocyst stage is central to assisted reproduction and to the understanding of preimplantation development and early pregnancy. This topic is reference background on how the early embryo is organized and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

The morphology and timing of cleavage and blastocyst formation are consolidated in standard embryology textbooks, while the lineage specification events of the preimplantation embryo are summarized in peer-reviewed reviews of mouse and human development.

History

The morphological stages of cleavage, morula, and blastocyst were described by classical embryology, and the cell lineage decisions of the preimplantation embryo were clarified by later molecular and genetic studies in model organisms and, more recently, in human embryos.

Seminal works

• chazaud-yamanaka-2016
• niakan-2012

Frequently asked questions

Why do the cells get smaller during cleavage?

Cleavage divides the zygote by mitosis without overall growth, so each division produces smaller blastomeres while the embryo as a whole stays about the same size.

What are the two cell populations of the blastocyst?

The outer trophoblast, which contributes to the placenta, and the inner cell mass, which gives rise to the embryo proper.