A morula (

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

, ''morus'': mulberry) is an early-stage

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

consisting of a solid ball of cells called

blastomere In biology, a blastomere is a type of cell produced by cell division (cleavage) of the zygote after fertilization; blastomeres are an essential part of blastula formation, and blastocyst formation in mammals. Human blastomere characteristics ...

s, contained in mammals, and other animals within the

zona pellucida The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first a ...

shell. The blastomeres are the daughter cells of the

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...

, and when the blastomeres number from 16–32 the ball of cells is called a morula. At the 8-cell stage the blastomeres are round and only loosely adhered. With further division they begin to become flattened, and develop an inside-out polarity that optimises the cell to cell contact. In the human embryo by day four after

fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...

, the morula at about the 32–cell stage begins to take in fluid. The fluid comes about from sodium-potassium pumps (on trophoblasts) that pump sodium into the morula, drawing in water from the maternal environment to become blastocoelic fluid. Hydrostatic pressure of the fluid creates a large cavity in the morula called a blastocoel or blastocyst cavity. Embryoblast cells also known as the inner cell mass form a compact mass of cells at the ''embryonic pole'' on one side of the cavity. The trophoblast is organized into a thin sheet of tightly adhered

epithelial cell Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...

s. The embryo is now termed a blastocyst.

Formation

The morula is produced by a series of cleavage divisions of the early embryo, starting with the single-celled zygote. Once the embryo has divided into 16 cells, it begins to resemble a mulberry, hence the name morula (

, ''morus'': mulberry). Within a few days after fertilization, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable. This process is known as

compaction Compaction may refer to: * Soil compaction, for mechanically induced compaction near the ground surface * Compaction of ceramic powders * Compaction (geology), part of the process of lithification involving mechanical dewatering of a sediment by ...

. The cells on the outside and inside become differentially fated into trophoblast (outside) and inner cell mass (inside) progenitors. A cavity forms inside the morula, by the active transport of sodium ions from trophoblast cells and

osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...

of water. This results in a hollow ball of cells known as the blastocyst in

placental mammal Placental mammals ( infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguish ...

s, and the blastula in other animals. The blastocyst's outer cells will become the first embryonic epithelium (the trophectoderm). Some cells, however, will remain trapped in the interior and will become the inner cell mass (ICM), and are pluripotent. In mammals (except

monotremes Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals ( Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their bra ...

), the ICM will ultimately form the "embryo proper", while the trophectoderm will form the

placenta The placenta is a temporary embryonic and later fetal organ (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...

and extra-embryonic tissues. However, reptiles have a different ICM. The stages are prolonged and divided in four parts.

Formation

References

Further reading