Inner cell mass

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outer cell mass
Blastocyst English.svg
Blastocyst with an inner cell mass and trophoblast.
Details
Carnegie stage 3
Days6
Precursor blastocyst
Gives rise to epiblast, hypoblast
Identifiers
Latin embryoblastus; massa cellularis interna; pluriblastus senior
TE E6.0.1.1.2.0.4
FMA 86557
Anatomical terminology

In early embryogenesis of most eutherian mammals, the inner cell mass (abbreviated ICM and also known as the embryoblast in mammals or pluriblast) is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus. This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred. The ICM lies within the blastocoele (more correctly termed "blastocyst cavity," as it is not strictly homologous to the blastocoele of anamniote vertebrates) and is entirely surrounded by the single layer of cells called trophoblast.

The pluriblast is a pluripotent population of cells in the embryogenesis of marsupials, called the inner cell mass in eutherians. The pluriblast is distinct from the trophoblast, and gives rise to the germ layers of the embryo, as well as extra embryonic endoderm and extra embryonic mesoderm. Both the pluriblast and trophoblast arise from the totipotent cells of the early conceptus. By definition, the pluriblast does not give rise to trophoblast cells during normal development, although it may retain this potential under experimental conditions.

A fetus or foetus is the unborn offspring of an animal that develops from an embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilisation and continues until birth. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.

Contents

Further development

The physical and functional separation of the inner cell mass from the trophectoderm (TE) is a special feature of mammalian development and is the first cell lineage specification in these embryos. Following fertilization in the oviduct, the mammalian embryo undergoes a relatively slow round of cleavages to produce an eight cell morula. Each cell of the morula, called a blastomere, increases surface contact with its neighbors in a process called compaction. This results in a polarization of the cells within the morula, and further cleavage yields a blastocyst of roughly 32 cells. [1] In mice, about 12 internal cells comprise the new inner cell mass and 20 – 24 cells comprise the surrounding trophectoderm. [2] [3] There is variation between species of mammals as to number of cells at compaction with bovine embryos showing differences related to compaction as early as 9-15 cells and in rabbits not until after 32 cells. [4] There is also interspecies variation in gene expression patterns in early embryos. [5]

Morula embryo at an early stage

A morula is an early-stage embryo consisting of 16 cells in a solid ball contained within the zona pellucida.

Blastocyst

The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the placenta. The name "blastocyst" arises from the Greek βλαστός blastos and κύστις kystis.

The ICM and the TE will generate distinctly different cell types as implantation starts and embryogenesis continues. Trophectoderm cells form extraembryonic tissues, which act in a supporting role for the embryo proper. Furthermore, these cells pump fluid into the interior of the blastocyst, causing the formation of a polarized blastocyst with the ICM attached to the trophectoderm at one end (see figure). This difference in cellular localization causes the ICM cells exposed to the fluid cavity to adopt a primitive endoderm (or hypoblast) fate, while the remaining cells adopt a primitive ectoderm (or epiblast) fate. The hypoblast contributes to extraembryonic membranes and the epiblast will give rise to the ultimate embryo proper as well as some extraembryonic tissues. [1]

Hypoblast

The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.

Epiblast

In amniote animal embryology, the epiblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amnionic ectoderm and extraembryonic mesoderm also originate from the epiblast.

Regulation of cellular specification

Since segregation of pluripotent cells of the inner cell mass from the remainder of the blastocyst is integral to mammalian development, considerable research has been performed to elucidate the corresponding cellular and molecular mechanisms of this process. There is primary interest in which transcription factors and signaling molecules direct blastomere asymmetric divisions leading to what are known as inside and outside cells and thus cell lineage specification. However, due to the variability and regulative nature of mammalian embryos, experimental evidence for establishing these early fates remains incomplete. [2]

At the transcription level, the transcription factors Oct4, Nanog, Cdx2, and Tead4 have all been implicated in establishing and reinforcing the specification of the ICM and the TE in early mouse embryos. [2]

Early embryo apical and basolateral polarization is established at the 8-16 cell stage following compaction. This initial difference in environment strengthens a transcriptional feedback loop in either an internal or external direction. Inside cells express high levels of Oct4 which maintains pluripotency and suppresses Cdx2. Outside cells express high levels of Cdx2 which causes TE differentiation and suppresses Oct4. ICM signaling.jpg
Early embryo apical and basolateral polarization is established at the 8-16 cell stage following compaction. This initial difference in environment strengthens a transcriptional feedback loop in either an internal or external direction. Inside cells express high levels of Oct4 which maintains pluripotency and suppresses Cdx2. Outside cells express high levels of Cdx2 which causes TE differentiation and suppresses Oct4.

Together these transcription factors function in a positive feedback loop that strengthens the ICM to TE cellular allocation. Initial polarization of blastomeres occurs at the 8-16 cell stage. An apical-basolateral polarity is visible through the visualization of apical markers such as Par3, Par6, and aPKC as well as the basal marker E-Cadherin. [2] The establishment of such a polarity during compaction is thought to generate an environmental identity for inside and outside cells of the embryo. Consequently, stochastic expression of the above transcription factors is amplified into a feedback loop that specifies outside cells to a TE fate and inside cells to an ICM fate. In the model, an apical environment turns on Cdx2, which upregulates its own expression through a downstream transcription factor, Elf5. In concert with a third transcription factor, Eomes, these genes act to suppress pluripotency genes like Oct4 and Nanog in the outside cells. [2] [8] Thus, TE becomes specified and differentiates. Inside cells, however, do not turn on the Cdx2 gene, and express high levels of Oct4, Nanog, and Sox2. [2] [3] These genes suppress Cdx2 and the inside cells maintain pluripotency generate the ICM and eventually the rest of the embryo proper.

Although this dichotomy of genetic interactions is clearly required to divide the blastomeres of the mouse embryo into both the ICM and TE identities, the initiation of these feedback loops remains under debate. Whether they are established stochastically or through an even earlier asymmetry is unclear, and current research seeks to identify earlier markers of asymmetry. For example, some research correlates the first two cleavages during embryogenesis with respect to the prospective animal and vegetal poles with ultimate specification. The asymmetric division of epigenetic information during these first two cleavages, and the orientation and order in which they occur, may contribute to a cell’s position either inside or outside the morula. [11] [12]

Stem cells

Blastomeres isolated from the ICM of mammalian embryos and grown in culture are known as embryonic stem (ES) cells. These pluripotent cells, when grown in a carefully coordinated media, can give rise to all three germ layers (ectoderm, endoderm, and mesoderm) of the adult body. [13] For example, the transcription factor LIF4 is required for mouse ES cells to be maintained in vitro. [14] Blastomeres are dissociated from an isolated ICM in an early blastocyst, and their transcriptional code governed by Oct4, Sox2, and Nanog helps maintain an undifferentiated state.

One benefit to the regulative nature in which mammalian embryos develop is the manipulation of blastomeres of the ICM to generate knockout mice. In mouse, mutations in a gene of interest can be introduced retrovirally into cultured ES cells, and these can be reintroduced into the ICM of an intact embryo. The result is a chimeric mouse, which develops with a portion of its cells containing the ES cell genome. The aim of such a procedure is to incorporate the mutated gene into the germ line of the mouse such that its progeny will be missing one or both alleles of the gene of interest. Geneticists widely take advantage of this ICM manipulation technique in studying the function of genes in the mammalian system. [1] [13]

Additional images

See also

Related Research Articles

Chimera (genetics) single organism composed of two or more different populations of genetically distinct cells

A genetic chimerism or chimera (/kɪˈmɪərə/ ky-MEER or /kaɪˈmɪərə/ kə-MEER, also chimaera is a single organism composed of cells with distinct genotypes. In animals, this means an individual derived from two or more zygotes, which can include possessing blood cells of different blood types, subtle variations in form and, if the zygotes were of differing sexes, then even the possession of both female and male sex organs. Animal chimeras are produced by the merger of multiple fertilized eggs. 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.

Blastula embryogenesis

The blastula is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoele 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.

Mammalian embryogenesis is the process of cell division and cellular differentiation during early prenatal development which leads to the development of a mammalian embryo.

In biology, a blastomere is a type of cell produced by cleavage of the zygote after fertilization and is an essential part of blastula formation.

Blastocoel

A blastocoel is a fluid-filled cavity that forms in the animal hemisphere of early amphibian and echinoderm embryos, or between the epiblast and hypoblast of avian, reptilian, and mammalian blastoderm-stage embryos.

Embryonic stem cell pluripotent stem cells derived from the inner cell mass of blastocysts

Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre-implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells. Isolating the embryoblast, or inner cell mass (ICM) results in destruction of the blastocyst, a process which raises ethical issues, including whether or not embryos at the pre-implantation stage should have the same moral considerations as embryos in the post-implantation stage of development. Researchers are currently focusing heavily on the therapeutic potential of embryonic stem cells, with clinical use being the goal for many labs. Potential uses include the treatment of diabetes and heart disease. The cells are being studied to be used as clinical therapies, models of genetic disorders, and cellular/DNA repair. However, adverse effects in the research and clinical processes such as tumours and unwanted immune responses have also been reported.

Oct-4 protein-coding gene in the species Homo sapiens

Oct-4, also known as POU5F1, is a protein that in humans is encoded by the POU5F1 gene. Oct-4 is a homeodomain transcription factor of the POU family. It is critically involved in the self-renewal of undifferentiated embryonic stem cells. As such, it is frequently used as a marker for undifferentiated cells. Oct-4 expression must be closely regulated; too much or too little will cause differentiation of the cells.

Embryonic development also embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to 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 embryo. The process follows fertilization, with the transfer being triggered by the activation of a cyclin-dependent kinase complex. 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.

Homeobox protein NANOG protein-coding gene in the species Homo sapiens

Homeobox protein NANOG is a transcriptional factor that helps embryonic stem cells (ESCs) maintain pluripotency by suppressing cell determination factors. Therefore NANOG deletion will trigger differentiation of ESCs. There are many different types of cancer that are associated with NANOG. In humans, this protein is encoded by the NANOG gene.

Leukemia inhibitory factor protein-coding gene in the species Homo sapiens

Leukemia inhibitory factor, or LIF, is an interleukin 6 class cytokine that affects cell growth by inhibiting differentiation. When LIF levels drop, the cells differentiate.

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

Induced pluripotent stem cell

Induced pluripotent stem cells are a type of pluripotent stem cell that can be generated directly from adult cells. The iPSC technology was pioneered by Shinya Yamanaka’s lab in Kyoto, Japan, who showed in 2006 that the introduction of four specific genes encoding transcription factors could convert adult cells into pluripotent stem cells. He was awarded the 2012 Nobel Prize along with Sir John Gurdon "for the discovery that mature cells can be reprogrammed to become pluripotent."

SOX2 protein-coding gene in the species Homo sapiens

SRY -box 2, also known as SOX2, is a transcription factor that is essential for maintaining self-renewal, or pluripotency, of undifferentiated embryonic stem cells. Sox2 has a critical role in maintenance of embryonic and neural stem cells.

TEAD4 protein-coding gene in the species Homo sapiens

Transcriptional enhancer factor TEF-3 is a protein that in humans is encoded by the TEAD4 gene.

Rex1 protein-coding gene in the species Homo sapiens

Rex1 (Zfp-42) is a known marker of pluripotency, and is usually found in undifferentiated embryonic stem cells. In addition to being a marker for pluripotency, its regulation is also critical in maintaining a pluripotent state. As the cells begin to differentiate, Rex1 is severely and abruptly downregulated.

The Cdx protein family is a group of the transcription factor proteins which bind to DNA to regulate the expression of genes. In particular this family of proteins can regulate the Hox genes.

Cell potency

Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum, begins with totipotency to designate a cell with the most differentiation potential, pluripotency, multipotency, oligopotency, and finally unipotency.

References

  1. 1 2 3 Wolpert, Lewis. Principles of Development: Third Edition. 2007. Oxford University Press.
  2. 1 2 3 4 5 6 Marikawa, Yusuke, et al. Establishment of Trophectoderm and Inner Cell Mass Lineages in the Mouse Embryo. Molecular Reproduction & Development 76:1019–1032 (2009)
  3. 1 2 Suwinska A, Czołowska R, Ozdze_nski W, Tarkowski AK. 2008. Blastomeres of the mouse embryo lose totipotency after the fifth cleavage division: Expression of Cdx2 and Oct4 and developmental potential of inner and outer blastomeres of 16- and 32-cell embryos. Dev Biol 322:133–144.
  4. Koyama et al Analysis of Polarity of Bovine and Rabbit Embryos by Scanning Electron Microscopy Biol of Reproduction, 50, 163-170 1994
  5. Kuijk, et al Validation of reference genes for quantitative RT-PCR studies in porcine oocytes and preimplantation embryos BMC Developmental Biology 2007, 7:58 doi:10.1186/1471-213X-7-58
  6. 1 2 Nichols J, Zevnik B, Anastassiadis K, Niwa H, Klewe-Nebenius D, Chambers I, Sch€oler H, Smith A. 1998. Formation of pluripotent stem cells in the mammalian embryo depends on the POU transcription factor Oct4. Cell 95:379–391.
  7. Rodda DJ, Chew JL, Lim LH, Loh YH, Wang B, Ng HH, Robson P. 2005. Transcriptional regulation of nanog by OCT4 and SOX2. J Biol Chem 280:24731–24737.
  8. 1 2 Strumpf D, Mao CA, Yamanaka Y, Ralston A, Chawengsaksophak K, Beck F, Rossant J. 2005. Cdx2 is required for correct cell fate specification and differentiation of trophectoderm in the mouse blastocyst. Development 132:2093–2102.
  9. Nishioka N, Yamamoto S, Kiyonari H, Sato H, Sawada A, Ota M, Nakao K, Sasaki H. 2008. Tead4 is required for specification of trophectoderm in pre-implantation mouse embryos. Mech Dev 125:270–283.
  10. Nishioka N, et al. 2009. The Hippo signaling pathway components Lats and Yap pattern Tead4 activity to distinguish mouse trophectoderm from inner cell mass. Dev Cell 16: 398–410.
  11. Bischoff, Marcus, et al. Formation of the embryonic-abembryonic axis of the mouse blastocyst: relationships between orientation of early cleavage divisions and pattern of symmetric/asymmetric divisions. Development 135, 953-962 (2008)
  12. Jedrusik, Agnieszka, et al. Role of Cdx2 and cell polarity in cell allocation and specification of trophectoderm and inner cell mass in the mouse embryo. Genes Dev. 2008 22: 2692-2706
  13. 1 2 Robertson, Elizabeth , et al. Germ-line transmission of genes introduced into cultured pluripotential cells by retroviral vector. Nature 323, 445 - 448 (2 October 1986)
  14. Smith AG, Heath JK, Donaldson DD, Wong GG, Moreau J, Stahl M and Rogers D (1988) Inhibition of pluripotential embryonic stem cell differentiation by purified polypeptides. Nature, 336, 688–690