A morula is one of the earliest stages in embryonic development, occurring before the embryo has implanted but after it is fertilized. This stage is usually reached at about four to five days after fertilization, and it is followed by the development of the blastula, a cluster of cells surrounding a fluid-filled cavity. The morula is an important state of development, and it can be easily identified on a high power microscope used to monitor embryonic development.
Embryonic development starts with fertilization to create a zygote. The zygote starts to replicate and divide, still staying within the confines of the zona pellucida, the membrane that surrounds the outside of the egg. When around 12 to 30 cells have developed, the growing embryo becomes a morula. The cells have a slightly blurred appearance and look as though they are running together. They are also very small, because they are still inside the zona pellucida. Thus the number of cells increases, but the overall size stays the same.
Once at the morula stage, the cells begin to differentiate and arrange themselves into the blastula shape. This also marks the beginning of the disintegration of the zona pellucida, allowing the embryo to grow and implant, connecting the embryo with the uterine wall so that the placenta can develop. These are all critical landmarks in embryonic development and each landmark also represents a stage where development can go wrong or stop, sometimes with no apparent cause.
Viewed under magnification, this tight ball of cells resembles the fruit of the mulberry tree. This is referenced in the name "morula," Latin for "mulberry." The number of cells involved can change because, as the cells start dividing, they can divide at different rates. Developing embryos do not follow an exponential progression of two, four, eight, 16, and 32 cells, in other words; at any given time the number of cells in the embryo can vary.
When people are treated with in vitro fertilization for infertility problems, physicians aim to transfer the embryos after the morula stage so that they can implant inside the uterus. If the developing embryo is still a morula after five days, this raises concerns that it may not develop any further and is no longer viable, although it may be perfectly healthy and just a little slower than usual. Some physicians like to wait to transfer until they are confident that cell division and development are still occurring, while others may go ahead and transfer a morula.