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 of ...

, ''morus'':

mulberry ''Morus'', a genus of flowering plants in the family Moraceae, consists of diverse species of deciduous trees commonly known as mulberries, growing wild and under cultivation in many temperate world regions. Generally, the genus has 64 identi ...

) 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 sp ...

consisting of a solid ball of

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

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 In ...

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 ap ...

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 multicellu ...

, 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. Proc ...

, the morula at about the 32–cell stage begins to take in fluid. The fluid comes about from sodium-potassium pumps (on

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

s) that pump sodium into the morula, drawing in water from the maternal environment to become

blastocoel The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mamma ...

ic fluid. Hydrostatic pressure of the fluid creates a large cavity in the morula called a

or blastocyst cavity. Embryoblast cells also known as the

inner cell mass The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of t ...

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 The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the ...

Formation

The morula is produced by a series of cleavage

divisions Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication * Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting ...

of the early embryo, starting with the single-celled zygote. Once the embryo has divided into 16 cells, it begins to resemble a

, hence the name morula (

, ''morus'':

). Within a few days after fertilization, cells on the outer part of the morula become bound tightly together with the formation of

desmosomes A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin for ''adhering spot''), is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adh ...

and

gap junctions Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulat ...

, becoming nearly indistinguishable. This process is known as compaction. The cells on the outside and inside become differentially fated into

(outside) and

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

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 distinguis ...

s, and the

blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fr ...

in other animals. The blastocyst's outer cells will become the first embryonic

epithelium 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 interce ...

(the

trophectoderm The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

). Some cells, however, will remain trapped in the interior and will become the

(ICM), and are

pluripotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...

. 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 that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mat ...

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

Formation

References

Further reading