Blastomere

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Blastomere
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Identifiers
Latin blastomerus
MeSH D001757
TE E7.0.1.2.0.0.2
FMA 72551
Anatomical terminology

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. [1]

Contents

Human blastomere characteristics

In humans, blastomere formation begins immediately following fertilization and continues through the first week of embryonic development. About 90 minutes after fertilization, the zygote divides into two cells. The two-cell blastomere state, present after the zygote first divides, is considered the earliest mitotic product of the fertilized oocyte. [2] These mitotic divisions continue and result in a grouping of cells called blastomeres. During this process, the total size of the embryo does not increase, so each division results in smaller and smaller cells. When the zygote contains 16 to 32 blastomeres it is referred to as a morula. These are the preliminary stages in the embryo beginning to form. Once this begins, microtubules within the morula's cytosolic material in the blastomere cells can develop into important membrane functions, such as sodium pumps. These pumps allow the inside of the embryo to fill with blastocoelic fluid, which supports the further growth of life. [3]

The blastomere is considered totipotent; that is, blastomeres are capable of developing from a single cell into a fully fertile adult organism. This has been demonstrated through studies and conjectures made with mouse blastomeres, which have been accepted as true for most mammalian blastomeres as well. Studies have analyzed monozygotic twin mouse blastomeres in their two-cell state, and have found that when one of the twin blastomeres is destroyed, a fully fertile adult mouse can still develop. Thus, it can be assumed that since one of the twin cells was totipotent, the destroyed one originally was as well. [4]

Relative blastomere size within the embryo is dependent not only on the stage of the cleavage, but also on the regularity of the cleavage amongst the cells. If the number of blastomeres in the cellular mass is even, then the sizes of the cells should be congruent. However, if the number of blastomeres in the cellular mass is not even, then the division should be asynchronous such that the sizes of the cells best support the mass's specific stage of differentiation. Blastomere size is typically considered uneven when one blastomere has a diameter over 25% larger than that of the other being compared. [5]

Blastomere differentiation

The division of blastomeres from the zygote allows a single fertile cell to continue to cleave and differentiate until a blastocyst forms. The differentiation of the blastomere allows for the development of two distinct cell populations: the inner cell mass, which becomes the precursor to the embryo, and the trophectoderm, which becomes the precursor to the placenta. These precursors typically appear when the blastomere differentiates into the 8- and 16-cell masses. [6]

During the 8-cell differentiation period, the blastomeres form adheren junctions, and subsequently polarize along the apical-basal axis. This polarization permanently changes the morphology of these cells, and starts the differentiation process. [6] After this, the 8-cell blastomere mass begins to compact by forming tight junctions between themselves, and cytosolic components of the cell accumulate in the apical region while the nucleus of each cell moves to the basal region. The adhesive lateral junction is then formed, and the blastomere is flattened to establish the apical cortical domain. Once the transition begins to a 16-cell mass, the apical cortical domain disappears, but elements of polarity are preserved. This allows for approximately half of the blastomeres to inherit polar regions that can rebuild the apical cortical domain. The other blastomeres that differentiate, then, will become apolar. Polar blastomere cells that differentiate will move to an outer position in the developing blastocyst and show precursors for the trophectoderm, while the apolar cells will move to an inner position and begin developing into the embryo. [7] The cells will then fully commit to their individual states in one of these two domains at the 32-cell stage. [7]

Models of differentiation

There are two main models for differentiation that determine which blastomere cells will divide into either the inner cell mass or the trophectoderm. The first conjecture is known as the "inside-outside model", and states that the cells differentiate based on their state in the 16-cell stage or later. This means that, under this model, blastomere cells do not differentiate based on cellular differences, but rather they do so because of mechanical and chemical stimuli based on where they are positioned at that time. [8]

The other, more widely accepted model is known as the "cell-polarity model". This model states that the orientation of the cleavage plane at the 8-cell and 16-cell stages determines their later differentiation. [9] There are two main ways in which blastomeres typically divide: symmetrically, meaning perpendicular to the apical-basal axis, or asymmetrically, meaning horizontal to the apical-basal axis. Many potential hypotheses and conjectures that attempt to explain why these cells orient themselves the way that they do. Some researchers have stated that early-dividing blastomeres tend to divide asymmetrically, [10] while others have proposed that the orientation of 8-cell stage blastomeres is random and cannot be predicted on a larger scale. [11] One study in particular states that the position of the nucleus in each blastomere can be used to indicate how the cell will divide: if the nucleus is in the apical region then the cell will likely divide symmetrically, while if the nucleus is located in the basal region then the cell will likely divide asymmetrically. [12]

It is possible for errors to occur during this process of repetitive cell division. Common among these errors is for the genetic material to not be divided evenly. Normally, when a cell divides each daughter cell has the same genetic material as the parent cell; if the genetic material does not split evenly between the two daughter cells, an event called "nondisjunction" occurs. Since this event occurs in only one of the several cells that exist at this point, the embryo will continue to develop but will have some normal cells and some abnormal cells. This disorder is called "numerical mosaicism". [13]

This mosaicism, especially of diploidy and polyploidy, can lead to the failure of cell cleavage and mitosis. When these necessary early cell divisions do not occur, the embryo can begin to form polyploid giant cancer cells that function very similarly to blastomere cells in order to grow and evolve in response to mechanical and chemical signals just like blastocyst precursors do. Studies have shown that these giant cancer cells are often also the genetic equivalent to somatic blastomeres. [14]

Diagnostics

Oftentimes, clinicians and researchers will use blastomere biopsies in at-risk pregnant women as a way to test for genetic disorders. These biopsies are invasive, however, and have a major disadvantage when compared to other forms of invasive genetic testing in that only a few number of cells can be extracted at a time. Over time many specialists have switched to blastocyst biopsies, which provide a lower level of mosaicism, but blastomere biopsies can still be used for earlier-stage studies and genetic diagnostics. [15]

Related Research Articles

<span class="mw-page-title-main">Ontogeny</span> Origination and development of an organism

Ontogeny is the origination and development of an organism, usually from the time of fertilization of the egg to adult. The term can also be used to refer to the study of the entirety of an organism's lifespan.

<span class="mw-page-title-main">Zygote</span> Diploid eukaryotic cell formed by fertilization between two gametes

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.

<span class="mw-page-title-main">Embryo</span> Multicellular diploid eukaryote in its earliest stage of development

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 cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres are arranged as a solid ball that when reaching a certain size, called a morula, takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocyst in mammals.

<span class="mw-page-title-main">Chimera (genetics)</span> Single organism composed of two or more different populations of genetically distinct cells

A genetic chimerism or chimera is a single organism composed of cells with more than one distinct genotype. In animals and human chimeras, this means an individual derived from two or more zygotes, which can include possessing blood cells of different blood types, and subtle variations in form (phenotype). Animal chimeras are produced by the merger of two embryos. In plant chimeras, however, the distinct types of tissue may originate from the same zygote, and the difference is often due to mutation during ordinary cell division. Normally, genetic chimerism is not visible on casual inspection; however, it has been detected in the course of proving parentage. In contrast, an individual where each cell contains genetic material from two organisms of different breeds, varieties, species or genera is called a hybrid.

<span class="mw-page-title-main">Blastulation</span> Sphere of cells formed during early embryonic development in animals

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 is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell 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.

<span class="mw-page-title-main">Blastocyst</span> Structure formed around day 5 of mammalian embryonic development

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 trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. In the late blastocyst the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion and the underlying uterine tissue of the mother.

<span class="mw-page-title-main">Preimplantation genetic diagnosis</span> Genetic profiling of embryos prior to implantation

Preimplantation genetic diagnosis is the genetic profiling of embryos prior to implantation, and sometimes even of oocytes prior to fertilization. PGD is considered in a similar fashion to prenatal diagnosis. When used to screen for a specific genetic disease, its main advantage is that it avoids selective abortion, as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation. Embryos are generally obtained through blastomere or blastocyst biopsy. The latter technique has proved to be less deleterious for the embryo, therefore it is advisable to perform the biopsy around day 5 or 6 of development.

<span class="mw-page-title-main">Blastocoel</span> Fluid-filled or yolk-filled cavity that forms in the blastula

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 mammals the blastula develops into the blastocyst containing an inner cell mass, and outer trophectoderm.

A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis.

<span class="mw-page-title-main">Animal embryonic development</span> Process by which the embryo forms and develops

In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula, or of the blastocyst in mammals.

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.

<span class="mw-page-title-main">Inner cell mass</span> Early embryonic mass that gives rise to the fetus

The inner cell mass (ICM) or embryoblast 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 the fetus. The inner cell mass forms in the earliest stages of embryonic development, before implantation into the endometrium of the uterus. The ICM is entirely surrounded by the single layer of trophoblast cells of the trophectoderm.

An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to symmetric cell divisions which give rise to daughter cells of equivalent fates. Notably, stem cells divide asymmetrically to give rise to two distinct daughter cells: one copy of the original stem cell as well as a second daughter programmed to differentiate into a non-stem cell fate.

<span class="mw-page-title-main">Human embryonic development</span> Development and formation of the human embryo

Human embryonic development, or human embryogenesis, is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form the single cell zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. The eight weeks has 23 stages.

<span class="mw-page-title-main">Cavitation (embryology)</span>

Cavitation is a process in early embryonic development that follows cleavage. Cavitation is the formation of the blastocoel, a fluid-filled cavity that defines the blastula, or in mammals the blastocyst. After fertilization, cell division of the zygote occurs which results in the formation of a solid ball of cells (blastomeres) called the morula. Further division of cells increases their number in the morula, and the morula differentiates them into two groups. The internal cells become the inner cell mass, and the outer cells become the trophoblast. Before cell differentiation takes place there are two transcription factors, Oct-4 and nanog that are uniformly expressed on all of the cells, but both of these transcription factors are turned off in the trophoblast once it has formed.

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Sir Richard Lavenham Gardner, FRSB, FRS is a British embryologist and geneticist. He is currently an Emeritus Professor at the University of York, and was previously a Royal Society Research Professor.

Morphokinetics (‘morpho’’ form/shape and ‘kinetics’ movement) refers to time specific morphological changes during embryo development providing dynamic information on a fertilized egg. The detailed information eases morphological selection of embryos with high implantation potential to be used in In-Vitro Fertilisation treatment.

This glossary of developmental biology is a list of definitions of terms and concepts commonly used in the study of developmental biology and related disciplines in biology, including embryology and reproductive biology, primarily as they pertain to vertebrate animals and particularly to humans and other mammals. The developmental biology of invertebrates, plants, fungi, and other organisms is treated in other articles; e.g. terms relating to the reproduction and development of insects are listed in Glossary of entomology, and those relating to plants are listed in Glossary of botany.

References

  1. ↑ "Blastomere". Encyclopædia Britannica Online. Retrieved May 21, 2022.
  2. ↑ Casser, E.; Israel, S.; Witten, A.; Schulte, K.; Schlatt, S.; Nordhoff, V.; Boiani, M. (December 2017). "Totipotency segregates between the sister blastomeres of two-cell stage mouse embryos". Scientific Reports. 7 (1): 8299. Bibcode:2017NatSR...7.8299C. doi:10.1038/s41598-017-08266-6. ISSN   2045-2322. PMC   5557898 . PMID   28811525.
  3. ↑ "Cleavage and Blastocyst Formation". Colorado State University. March 2000. Retrieved May 21, 2022.
  4. ↑ TOGASHI, Mamoru; SUZUKI, Hiroshi; MIYAI, Tatsuya; OKAMOTO, Michio T (1987). "Production of monozygotic twins by splitting of 2-cell stage embryos in mice". The Japanese Journal of Animal Reproduction. 33 (2): 51–57. doi: 10.1262/jrd1977.33.51 .
  5. ↑ "C. Blastomere size". Atlas of Human Embryology. 2016. Retrieved May 21, 2022.
  6. 1 2 Li, Chao-Bo; Wang, Zhen-Dong; Zheng, Zhong; Hu, Li-Li; Zhong, Shu-Qi; Lei, Lei (2010-08-25). "Number of blastomeres and distribution of microvilli in cloned mouse embryos during compaction". Zygote. 19 (3): 271–276. doi:10.1017/s0967199410000377. ISSN   0967-1994. PMID   20735894. S2CID   23902185.
  7. 1 2 Johnson, M (April 1981). "The foundation of two distinct cell lineages within the mouse morula". Cell. 24 (1): 71–80. doi:10.1016/0092-8674(81)90502-x. ISSN   0092-8674. PMID   7237545. S2CID   22263112.
  8. ↑ TARKOWSKI, ANDRZEJ K. (October 1959). "Experiments on the Development of Isolated Blastomeres of Mouse Eggs". Nature. 184 (4695): 1286–1287. Bibcode:1959Natur.184.1286T. doi:10.1038/1841286a0. ISSN   0028-0836. PMID   13836947. S2CID   4148223.
  9. ↑ Johnson, M (April 1981). "The foundation of two distinct cell lineages within the mouse morula". Cell. 24 (1): 71–80. doi:10.1016/0092-8674(81)90502-X. PMID   7237545. S2CID   22263112.
  10. ↑ Spindle, Akiko (1982-02-20). "Cell allocation in preimplantation mouse chimeras". Journal of Experimental Zoology. 219 (3): 361–367. doi:10.1002/jez.1402190311. ISSN   0022-104X. PMID   7061978.
  11. ↑ Pickering, Susan J.; Johnson, Martin H.; Braude, Peter R.; Houliston, Evelyn (November 1988). "Cytoskeletal organization in fresh, aged and spontaneously activated human oocytes". Human Reproduction. 3 (8): 978–989. doi:10.1093/oxfordjournals.humrep.a136828. ISSN   1460-2350. PMID   3204153.
  12. ↑ Ajduk, Anna; Biswas Shivhare, Sourima; Zernicka-Goetz, Magdalena (August 2014). "The basal position of nuclei is one pre-requisite for asymmetric cell divisions in the early mouse embryo". Developmental Biology. 392 (2): 133–140. doi:10.1016/j.ydbio.2014.05.009. PMC   4111899 . PMID   24855000.
  13. ↑ Hastings RJ, Cavani S, et al. Cytogenetic Guidelines and Quality Assurance: a common European framework for quality assessment for constitutional and acquired cytogenetic investigations. Eur J Hum Genet. 2007 May;15(5):525-7.
  14. ↑ Niu, N; Mercado-Uribe, I; Liu, J (2017-04-24). "Dedifferentiation into blastomere-like cancer stem cells via formation of polyploid giant cancer cells". Oncogene. 36 (34): 4887–4900. doi:10.1038/onc.2017.72. ISSN   0950-9232. PMC   5582213 . PMID   28436947.
  15. ↑ Gianaroli, Luca; Ferraretti, Anna P.; Crippa, Andor; Valerio, Marzia; Cafueri, Giulia; Pomante, Alessandra (March 2016). "Preimplantation genetic testing: polar bodies, blastomeres, trophectoderm cells, or blastocoelic fluid?". Fertility and Sterility. 105 (3): 676–683. doi: 10.1016/j.fertnstert.2015.11.018 . ISSN   0015-0282. PMID   26658131.

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