Blastulation is the stage in early
animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
embryonic development that produces the blastula. In mammalian development the blastula develops into the
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 t ...
with a differentiated
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 ...
and an outer
trophectoderm. The blastula (from Greek ''
βλαστός'' ( meaning ''sprout'')) is a hollow sphere of
cell
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 w ...
s known as
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 surrounding an inner fluid-filled cavity called the
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 mammal ...
.
Embryonic development begins with a
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, wh ...
fertilizing an
egg cell
The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...
to become a
zygote, which undergoes many
cleavages to develop into a ball of cells called a
morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the
gastrula in which the
germ layers of the embryo form.
A common feature of a
vertebrate blastula is that it consists of a layer of blastomeres, known as the
blastoderm A blastoderm (germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the fo ...
, which surrounds the blastocoel. In
mammals, the blastocyst contains an
embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the
fetus, and a
trophoblast which goes on to form the extra-embryonic tissues.
During blastulation, a significant amount of activity occurs within the early embryo to establish
cell polarity
Cell polarity refers to spatial differences in shape, structure, and function within a cell. Almost all cell types exhibit some form of polarity, which enables them to carry out specialized functions. Classical examples of polarized cells are desc ...
,
cell specification,
axis formation, and to regulate
gene expression.
In many animals, such as ''
Drosophila'' and ''
Xenopus
''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...
'', the
mid blastula transition (MBT) is a crucial step in development during which the
maternal mRNA is degraded and control over development is passed to the embryo.
Many of the interactions between blastomeres are dependent on
cadherin
Cadherins (named for "calcium-dependent adhesion") are a type of cell adhesion molecule (CAM) that is important in the formation of adherens junctions to allow cells to adhere to each other . Cadherins are a class of type-1 transmembrane proteins, ...
expression, particularly
E-cadherin
Cadherin-1 or Epithelial cadherin (E-cadherin), (not to be confused with the APC/C activator protein CDH1) is a protein that in humans is encoded by the ''CDH1'' gene. Mutations are correlated with gastric, breast, colorectal, thyroid, and ovari ...
in mammals and EP-cadherin in
amphibian
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arborea ...
s.
The study of the blastula, and of cell specification has many implications in
stem cell research
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
, and
assisted reproductive technology
Assisted reproductive technology (ART) includes medical procedures used primarily to address infertility. This subject involves procedures such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), cryopreservation of gametes ...
.
In ''Xenopus'', blastomeres behave as
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 ...
stem cells which can migrate down several pathways, depending on
cell signaling
In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
.
By manipulating the cell signals during the blastula stage of development, various
tissues can be formed. This potential can be instrumental in
regenerative medicine for disease and injury cases.
In vitro fertilisation involves the
transfer of an embryo into a
uterus for
implantation.
Development
The blastula stage of early embryo development begins with the appearance of the blastocoel. The origin of the blastocoel in ''Xenopus'' has been shown to be from the first
cleavage furrow
In cell biology, the cleavage furrow is the indentation of the cell's surface that begins the progression of cleavage, by which animal and some algal cells undergo cytokinesis, the final splitting of the membrane, in the process of cell division. ...
, which is widened and sealed with
tight junctions to
create a cavity.
In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore exclusively from the mother.
Midblastula transition
In many organisms including ''Xenopus'' and ''Drosophila'', the
midblastula transition usually occurs after a particular number of cell divisions for a given species, and is defined by the ending of the synchronous cell division cycles of the early blastula development, and the lengthening of the
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
s by the addition of the
G1 and
G2 phase
G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G2 phase ends with th ...
s. Prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle.
The addition of the two growth phases into the cell cycle allows for the cells to increase in size, as up to this point the blastomeres undergo reductive divisions in which the overall size of the embryo does not increase, but more cells are created. This transition begins the growth in size of the organism.
The mid-blastula transition is also characterized by a marked increase in
transcription of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point, either by proteins such as
SMAUG in ''Drosophila'' or by
microRNA
MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRN ...
.
These two processes shift the control of the embryo from the maternal mRNA to the nuclei.
Structure
A blastula (
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 t ...
in
mammals), is a sphere of cells surrounding a fluid-filled cavity called the
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 mammal ...
. The blastocoel contains
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s,
proteins,
growth factors, sugars, ions and other components which are necessary for
cellular differentiation
Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular ...
. The blastocoel also allows blastomeres to move during the process of
gastrulation.
In ''Xenopus'' embryos, the blastula is composed of three different regions. The animal cap forms the roof of the blastocoel and goes on primarily to form
ectodermal derivatives. The equatorial or marginal zone, which compose the walls of the blastocoel differentiate primarily into
mesodermal tissue. The vegetal mass is composed of the blastocoel floor and primarily develops into
endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastru ...
al tissue.
In the mammalian blastocyst there are three lineages that give rise to later tissue development. The
epiblast
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is ...
gives rise to the fetus itself while the trophoblast develops into part of the
placenta and the primitive endoderm becomes the
yolk sac
The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
.
In the mouse embryo, blastocoel formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient which is the result of
sodium–potassium pump
The sodium–potassium pump (sodium–potassium adenosine triphosphatase, also known as Na⁺/K⁺-ATPase, Na⁺/K⁺ pump, or sodium–potassium ATPase) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal c ...
s that produce a high sodium gradient on the basolateral side of the trophectoderm. This movement of water is facilitated by
aquaporin
Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a ...
s. A seal is created by tight junctions of the
epithelial cells that line the blastocoel.
Cellular adhesion
Tight junctions are very important in embryo development. In the blastula, these cadherin mediated cell interactions are essential to development of epithelium which are most important to
paracellular transport
Paracellular transport refers to the transfer of substances across an epithelium by passing through the intercellular space ''between'' the cells. It is in contrast to transcellular transport, where the substances travel ''through'' the cell, pass ...
, maintenance of cell polarity and the creation of a permeability seal to regulate blastocoel formation. These tight junctions arise after the polarity of epithelial cells is established which sets the foundation for further development and specification. Within the blastula, inner blastomeres are generally non-polar while epithelial cells demonstrate polarity.
Mammalian embryos undergo compaction around the 8-cell stage where
E-cadherins as well as
alpha
Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...
and
beta
Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...
catenins are expressed. This process makes a ball of embryonic cells which are capable of interacting, rather than a group of diffuse and undifferentiated cells. E-cadherin adhesion defines the
apico-basal axis in the developing embryo and turns the embryo from an indistinct ball of cells to a more polarized
phenotype which sets the stage for further development into a fully formed blastocyst.
''Xenopus'' membrane polarity is established with the first cell cleavage. Amphibian EP-cadherin and XB/U cadherin perform a similar role as E-cadherin in mammals establishing blastomere polarity and solidifying cell-cell interactions which are crucial for further development.
Clinical implications
Fertilization technologies
Experiments with implantation in mice show that
hormonal induction,
superovulation Controlled ovarian hyperstimulation is a technique used in assisted reproduction involving the use of fertility medications to induce ovulation by multiple ovarian follicles. These multiple follicles can be taken out by oocyte retrieval (egg coll ...
and
artificial insemination
Artificial insemination is the deliberate introduction of sperm into a female's cervix or uterine cavity for the purpose of achieving a pregnancy through in vivo fertilization by means other than sexual intercourse. It is a fertility treatmen ...
successfully produce preimplantation mouse embryos. In the mice, ninety percent of the females were induced by mechanical stimulation to undergo pregnancy and implant at least one embryo.
These results prove to be encouraging because they provide a basis for potential implantation in other mammalian species, such as humans.
Stem cells
Blastula-stage cells can behave as pluripotent stem cells in many species. Pluripotent stem cells are the starting point to produce organ specific cells that can potentially aid in repair and prevention of injury and degeneration. Combining the expression of
transcription factors and locational positioning of the blastula cells can lead to the development of induced functional organs and tissues. Pluripotent ''Xenopus'' cells, when used in an in vivo strategy, were able to form into functional
retinas. By transplanting them to the eye field on the
neural plate, and by inducing several mis-expressions of transcription factors, the cells were committed to the retinal lineage and could guide vision based behavior in the ''Xenopus''.
See also
*
Polarity in embryogenesis
In developmental biology, an embryo is divided into two hemispheres: the animal pole and the vegetal pole within a blastula. The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. In some cases, the ...
*
Diploblasty
Diploblasty is a condition of the blastula in which there are two primary germ layers: the ectoderm and endoderm.
Diploblastic organisms are organisms which develop from such a blastula, and include cnidaria and ctenophora, formerly grouped togeth ...
*
Triploblasty
Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organo ...
References
Bibliography
*
*
*
{{Authority control
Animal developmental biology
Cloning