Blastocyst Formation

Blastocyst formation is the structural transition that converts the solid morula into the blastocyst. The blastocyst is a fluid-containing, structurally organised conceptus with two clearly distinct cell populations.

It is the final major stage of preimplantation development, before the conceptus begins implantation into the uterine endometrium.

Blastocyst formation begins with cavitation, the appearance of a fluid-filled cavity, the blastocoele, within the interior of the morula. It typically becomes apparent around day 4 to 5 after fertilisation.

Cavitation is driven by active fluid transport. The outer cells of the morula, already connected by tight junctions established during compaction, actively pump sodium ions, and water follows osmotically. Fluid moves from the outer environment into the intercellular spaces between the inner cells of the morula.

The tight junctions of the outer cell layer prevent this fluid from leaking back out. It progressively accumulates and coalesces into a single, expanding central cavity, the blastocoele.

The blastocoele pushes the inner cells of the morula toward one pole of the conceptus. It displaces them from a central position to an eccentric, polar position against the inner surface of the outer cell layer.

The inner cell mass, also termed the embryoblast, is the cluster of cells displaced to one pole of the expanding blastocoele by the process of cavitation, corresponding directly to the inner cell population first distinguished from the outer population at compaction.

The embryoblast is the cell population from which the embryo proper will subsequently derive. Further development of the embryoblast, over the following days, produces the bilaminar embryonic disc, epiblast and hypoblast, from which every tissue of the eventual fetus, via the three germ layers established at gastrulation, originates.

The embryoblast retains pluripotency at this stage, capable of giving rise to any cell type of the embryo proper, though not to the trophoblastic, placental tissue that the outer cell population is now committed to forming.

The trophoblast is the outer, single-cell-thick layer of the blastocyst, corresponding directly to the outer cell population first distinguished at compaction, and it forms the entire outer surface, or wall, of the blastocyst, enclosing both the blastocoele and the eccentrically positioned embryoblast.

The trophoblast is the cell population from which the placenta and the fetal membranes will subsequently derive, and, unlike the embryoblast, it contributes no tissue at all to the embryo or fetus itself; its entire developmental contribution is extraembryonic, forming the interface tissue required for implantation and for subsequent exchange between the conceptus and the mother, and its own further differentiation, into cytotrophoblast and syncytiotrophoblast, begins shortly after the blastocyst stage as implantation gets underway.

As the blastocyst continues to expand, driven by ongoing accumulation of blastocoelic fluid, it outgrows the fixed physical capacity of the surrounding zona pellucida. Around day 5 to 6 the blastocyst sheds the zona layer entirely, a process termed hatching.

Hatching is an essential prerequisite for the next stage of development. The zona pellucida serves important protective functions during the cleavage and morula stages. It would otherwise physically prevent the trophoblast from making the direct contact with the uterine endometrial epithelium that implantation requires.

Only a hatched blastocyst, with its trophoblastic surface fully exposed, can adhere to and subsequently invade the endometrium.

By the time hatching is complete, around day 6, the blastocyst has every structural feature required to begin implantation:

• an outer trophoblastic layer, now directly exposed and able to establish physical contact with the endometrial surface
• a fluid-filled blastocoele providing the overall structural framework of the conceptus
• an eccentrically positioned embryoblast, protected within the blastocyst interior, from which the embryo proper will develop

The blastocyst orients so that its embryonic pole is the first region to contact the endometrium. This is the pole at which the embryoblast lies closest to the trophoblastic wall.

Initial attachment and the subsequent invasive stages of implantation take place at this pole, beginning around day 6 to 7.