At the cell stage,during which a fluid-filled cavity begins to form, the two layers of cells formed in the morula become more defined as the outer cells, which become committed to the trophoblast trophectoderm lineage and surround the cavity, and the inner layer, which form the inner cell mass ICM, embryoblast , a compact mass on one side of the cavity defined as the embryonic pole, vs.
The blastocyst is therefore comprised of an inner cell mass, which subsequently forms the embryo, and a surrounding outer layer of cells, the trophoblast, which later forms the outer chorionic sac and the fetal component of the placenta.
The human blastocyst comprises cells, but cell numbers and stage names remain variable in the scientific community. Between days of development, the human embryo reaches the uterus. Embryos with an odd number of cells e. Soon after development of the 8-cell or cell embryo depending on the species , the blastomeres begin to form tight junctions with one another, leading to deformation of their round shape and formation of a mulberry-shaped mass of cells called a morula.
This change in shape of the embryo is called compaction. It is difficult to count the cells in a morula; the embryo shown here probably has between 20 and 30 cells. Formation of junctional complexes between blastomeres gives the embryo and outside and an inside. The outer cells of the embryo also begin to express a variety of membrane transport molecules, including sodium pumps. One result of these changes is an accumulation of fluid inside the embryo, which signals formation of the blastocyst.
An early blastocyst, containing a small amount of blastocoelic fluid, is shown to the right. As the blastocyst continues to accumulate blastocoelic fluid, it expands to form - you guessed it - an expanded blastocyst.
The blastocyst stage is also a landmark in that this is the first time that two distinctive tissues are present. The increase in oxygen consumption reflects the considerable energy required for the formation and maintenance of the blastocoel, but the increase in glucose utilization reflects an increased demand for biosynthetic process. PEP, phosphoenolpyruvate. In in vitro fertilization IVF , the blastocyst culture was important to increase the success rate of IVF because of better embryo selection after better genomic activation and endometrial receptivity [ 6 ].
The blastocyst comprises two cell types: the inner cell mass ICM, from which the fetal tissues develop and the trophectoderm which will form mostly extraembryonic tissues such as the placenta [ 1 ]. This morphological differentiation was thought to represent the developmental capability of the blastocyst [ 6 , 7 ]. The fluid accumulation presence between cells at the morulae stage is the phase that determines embryonic development. An increase in fluid volume and the number of cells in the blastocyst causes an enhancement in the size cavity of the blastocyst and its cavity with depletion of the zona pellucida ZP [ 6 ].
The stages of blastocyst embryo development are divided into four grades. A grade of 3 was given to the embryo that had a blastocoel cavity which had fulfilled that all embryos and zona pellucida ZP appeared to be thinner than embryo on day 3.
A grade of 4 is given to the embryo that has successfully hatched from ZP [ 6 , 9 ]. A collection of cells located within the blastocoel in one pole of the blastocyst cavity is called ICM. ICM will develop into fetal tissue. The morphological form of ICM is assessed based on how much the cell compaction is until there is no cell clot at all. A grade of 1 is given to ICM with a very large and dense form of cell clots. A grade of 2 is given to ICM with a slightly diffuse cell form.
A grade of 3 is given to ICM with very few cells and which does not even form clots. However, the best ICM grade A contains tightly packed and many cells; the middle ICM grade B is composed of loosely grouped and several cells, and the worst grade C describes an ICM that contains very few cells that are loosely bound [ 6 , 9 ].
The outer cells of the blastocyst, forming the blastocyst structure itself, are called the trophectoderm TE cells. TE cells play a role in the formation of fluid accumulation in the blastocoel which can be a key in ICM determination, but in the early stages of the blastocyst stage, the role of TE cells is unclear.
TE cells will develop into extraembryonic tissue such as the placenta. In addition, TE cells are thought to play a role in the implantation process which the TE cells have contributed in the phase of apposition, adhesion, and invasion of the endometrium which can support the implantation of the blastocyst in the uterus.
Molecular factors are also produced by TE cells that play a role in the embryo implantation process [ 6 , 7 , 8 , 9 ]. According to blastocyst grading by Gardner and Schoolcraft, the best TE cell grade A contains many cells that form a cohesive epithelium; the middle TE category B is composed of few cells forming a loose epithelium, and the worst category C describes a TE that contains very few large cells that struggle to form a cohesive epithelium. However, other publications have found no relationship between TE grade and viability Figure 4 [ 6 , 8 , 9 ].
Blastocyst morphology. Gardner and Schoolcraft in introduced the blastocyst grading system which is adopted by the majority of IVF laboratories in the world. In the blastocyst grading system, they classified the degree of blastocyst into three category, i. Embryo, according to the Istanbul consensus, is given a grade based on three things; the first thing is the embryo development stage, the second is ICM, and the third is TE.
So that the order of the fifth day embryo assessment is based on the stages of development, then based on ICM morphology, and the last is based on TE. The following is a presentation of the fifth day embryo according to the Istanbul consensus Figures 5 , 6 , 7 , 8 , 9 ; Table 1 [ 6 , 9 ]. Blastocyst grade 4: A: A means that the blastocyst has been expanded grade 4 , has a large ICM and compact grade A , and has TE which consists of many cells which are mutually binding grade A.
Blastocyst grade 4: C: C means that blastocyst embryo has been expanded grade 4 , has ICM that is difficult to see and has very few cells grade C , and has TE which has very little cell grade C. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the chorion and play an important part in the development of the placenta. On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and quickly assumes the form of a small sac, called the yolk sac.
Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic cavity is gradually developed. The floor of this cavity is formed by the embryonic disk, which is composed of a layer of prismatic cells called the embryonic ectoderm.
This layer is derived from the inner cell mass and lies in opposition to the endoderm. Learning Objectives Outline the process of blastocyst formation.
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