Morula- Stage, Development, Significance

Within twenty-four hours after fertilization, the zygote initiates a rapid series of mitotic cell divisions called cleavage.
These divisions are not accompanied by cell growth, so they subdivide the large zygote into many smaller daughter cells called blastomeres.
The first cleavage division divides the zygote to produce two daughter cells.
The second division, which is complete at about forty hours after fertilization, produces four equal blastomeres.
By three days, the embryo consists of eight to sixteen cells, and by four days, it consists of sixteen to thirty-two cells.
The embryo at this stage is called a morula.

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Morula Stage

An early stage in post-fertilization development when cells have rapidly mitotically divided to produce a solid mass of cells (16 or more) with a “mulberry” appearance is called the morula stage.
The morula stage is the final stage prior to the formation of a fluid-filled cavity called the blastocoel cavity.
It is distinct from a blastocyst in that a morula (3–4 days after fertilization) is a mass of totipotent cells in a spherical shape whereas a blastocyst (4–5 days after fertilization) has a cavity inside the zona pellucida along with an inner cell mass.
A morula, if untouched and allowed to grow, will eventually develop into a blastocyst.

A key event prior to morula formation is “compaction”.
During compaction, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable.
These changes in cell morphology and cell-cell adhesion initiates the formation of the solid ball of cells.
With time, the cells on the outside and inside become differentially fated into trophoblast (outside) and inner cell mass (inside) progenitors.

The morula is the first embryonic stage where mammalian cells can be categorized as being either internal or external.
The morula reaches the uterus between three and four days of development and greatly absorbs nutrients and fluid from the surrounding in preparation for the implantation process.
In Assisted Reproductive Technology, the morula stage is when one of the earliest prenatal diagnostic tests can be carried out, by removing a single cell (blastomere) and carrying out genetic diagnosis on its DNA.

Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen’s human embryology (5th ed.). New York; Edinburgh: Churchill Livingstone.
Sadler, T. W., & Langman, J. (2004). Langman’s medical embryology. Philadelphia, Pa: Lippincott Williams & Wilkins.
Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2008). The developing human: Clinically oriented embryology. Philadelphia, PA: Saunders/Elsevier.
https://embryology.med.unsw.edu.au/embryology/index.php/Morula_Development