The morula (from the Latin morum) is a mass that originates as a consequence of the consecutive division of an embryo, beginning with a single-celled zygote, during the fertilization process.
After the embryo divides into 16 cells, it begins to take the shape of a blackberry, from which it gets its name. This mass forms a solid ball within the zona pellucida (outer lining of the oocyte in mammals) and divides into multiple blastomeres, which are undifferentiated embryonic cells.
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A morula differs from a blastocyst, in that the former is a spherical mass made up of 16 cells that appears 3 or 4 days after fertilization.
The blastocyst, on the other hand, has an opening within its zona pellucida, with a mass inside, and appears 4 or 5 days after fertilization. In other words, if the morula remains implanted and intact, it will later transform into a blastocyst.
A few days after fertilization, compaction begins. In this procedure the outer cells are tightly bound by desmosomes, which are the structures that hold the cells together.
Within the morula, a cavity arises due to the active transport of sodium ions from trophoblastic cells and the process of water osmosis.
As a consequence of this transformation, a hollow ball made up of cells is formed, called a blastocyst. The outer cells of the blastocyst will be the first embryonic epithelium called the trophectoderm.
Some cells remain inside the blastocyst, will transform into the inner cell mass (ICM) and are pluripotent, that is, they are stem cells capable of forming all the cells of the body.
In mammals, with the exception of the monotreme species, the internal cell mass will be what will form the embryo as such. The trophectoderm (outer cells) will give rise to the placenta and extra-embryonic tissues.
In reptiles, the inner cell mass is different and the stages of formation are spread out and divided into four parts.
Early development of the embryo
The fertilized egg is carried down the fallopian tube by ciliary and muscular activity. The first division or excision occurs at 30 hours after fertilization, the second will occur at right angles to the first.
After the egg is fertilized, a series of mitotic divisions called cleavages begin. After 40 to 50 hours of fertilization, the cell has already divided into four cells.
At the end of the 8-cell phase, the ovule presents microvilli, and the cellular organelles are located at the apex of them. After this cellular subdivision, differentiation occurs in the embryo.
The embryo reaches the uterine cavity when it is in the 8-cell phase. The splits happen every 12 hours and are timed. The next division produces a 16-cell ball: the morula.
Upon reaching 16 cells, and already in the uterine wall, it grows and develops a cavity (coelom) in which it maintains a supply of nutrients.
This cavity allows the formation of: the inner cell mass on one side of the morula and the outer cell mass that covers the cell.
The inner cell mass will originate the embryo tissues and the outer mass will originate the trophoblast tissues. Later, fluids will be stored and the morula will grow and become a blastocyst.
The total size of the blastocyst is equal to that of the secondary oocyte, approximately 100 µm millimicrons in diameter.
The daughter cells originating from the excised embryo are called blastomeres. This first division is controlled by the RNA transcribed from the oocyte DNA, which remains isolated in the zona pellucida until just before implantation.
Polarity
The concept of polarity is pretty straightforward. The female cell ovulate and then fertilized ovum, could be conceived as a world with its own geography in which the location of all its structures is predetermined according to its functionality.
For more than 20 years of research, Van-Blerkom has devoted himself to the study of the phenomenon called polarity.
This wonder known as polarity, could clarify how the path of an embryo can be modified and predicted by biological events that precede conception and that prevail days, weeks or months later.
These findings would raise the possibility that the viability of life can be determined even before fertilization.
The way the embryo divides, compacts, leaves the zona pelcida, produces molecules that allow it to implant in the uterine wall, and later locates blood vessels to nourish the placenta and the fetus, is one of the most impressive transformations of the nature.
Importance of the morula
Research has determined how to obtain stem cells from a four-day-old embryo in the morula stage. Until now, the technique used was to use older blasts, but they were destroyed in the procedure.
However, the research took a new turn, when it was decided to use a single cell from a morula and it was observed that it was capable of transforming into a normal embryo.
There would then be the possibility that the parents could decide, the extraction of a cell from its morula to give rise to the development of a stem cell line. These could be stored for use in therapy or research.
Parallel to this, the morula could continue its development process and become an embryo suitable for implantation.
References
- Boklage, C. (2010). How new humans are made. Greenville: World scientific.
- Cardozo, L. and Staskin, D. (2001). Textbook of female urology and Urogynaecology. London: Isis Medical Media.
- Chard, T. and Lilford, R. (1995). Basic sciences dor obstetrics and gynaecology. London: Springer.
- Hall, S. (2004). The Good Egg. Discover.
- Zimmer, C. (November 3, 2004). The Loom. Retrieved from Discover magazine: blogs.discovermagazine.com