Inner cell mass in the context of Endometrium

In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell in a cell lineage. They are found in both embryonic and adult organisms, but they have slightly different properties in each. They are usually distinguished from progenitor cells, which cannot divide indefinitely, and precursor or blast cells, which are usually committed to differentiating into one cell type.

In mammals, roughly 50 to 150 cells make up the inner cell mass during the blastocyst stage of embryonic development, around days 5–14. These have stem-cell capability. In vivo, they eventually differentiate into all of the body's cell types (making them pluripotent). This process starts with the differentiation into the three germ layers – the ectoderm, mesoderm and endoderm – at the gastrulation stage. However, when they are isolated and cultured in vitro, they can be kept in the stem-cell stage and are known as embryonic stem cells (ESCs).

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 (from Greek βλαστός (blastos meaning sprout)) 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.

A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, 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.

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.

The trophoblast cells form tight junctions between them making the structure leakproof. Trophoblast cells have sodium pumps on their membranes, and pump sodium into the centre of the morula. This draws fluid in through osmosis causing a cavity to form inside the morula, and to increase in size. The cavity is the blastocoel. Following the formation of the blastocoel, the inner cell mass positions itself in one portion of the cavity, while the rest of the cavity is filled with fluid, and lined with trophoblasts.

The blastocoel (/ˈblæstəˌsiːl/), 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 is called the blastocyst, which consists of an outer epithelium, the trophectoderm, enveloping the inner cell mass and the blastocoel.

It develops following cleavage of the zygote after fertilization. It is the first fluid-filled cavity or lumen formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula. In the Xenopus a very small cavity has been described in the two-cell stage of 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 or lumen 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 (the portion of the chorion that develops villi). The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called the blastula.

In humans, blastocyst formation begins about five days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells.The blastocyst has a diameter of about 0.1–0.2 mm and comprises 100-200 cells following 7-8 rounds of cleavage (cell division without cell growth). About seven days after fertilization, the blastocyst undergoes implantation, embedding into the endometrium of the uterine wall where it will undergo further developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, the egg coat that prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus.

Embryonic stem cells (ESCs) 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 inner cell mass (embryoblast) using immunosurgery results in destruction of the blastocyst, a process which raises ethical issues, including whether or not embryos at the pre-implantation stage have the same moral considerations as embryos in the post-implantation stage of development.

Researchers focus heavily on the therapeutic potential of embryonic stem cells, with clinical use being the goal for many laboratories. Potential uses include the treatment of diabetes and heart disease. The cells are 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 tumors and unwanted immune responses have also been reported.

The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. The inner of these membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois, and via the umbilical cord, the placenta.

The yolk sac, amnion, chorion, and allantois are the four extraembryonic membranes that lie outside of the embryo and are involved in providing nutrients and protection to the developing embryo. They form from the inner cell mass; the first to form is the yolk sac followed by the amnion which grows over the developing embryo. The amnion remains an important extraembryonic membrane throughout prenatal development. The third membrane is the allantois, and the fourth is the chorion which surrounds the embryo after about a month and eventually fuses with the amnion.

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.

In metatherians (marsupials), the pluriblast forms part of the blastocyst wall and no structure exists that can be described as an inner cell mass. "Inner cell mass" is thus a morphological term peculiar to eutherian mammals, whereas "pluriblast" is a functional term more widely applicable to conserved aspects of mammalian development.

Immunosurgery is a method of selectively removing the external cell layer (trophoblast) of a blastocyst through a cytotoxicity procedure. The protocol for immunosurgery includes preincubation with an antiserum, rinsing it with embryonic stem cell derivation media to remove the antibodies, exposing it to complement, and then removing the lysed trophoectoderm through a pipette. This technique is used to isolate the inner cell mass of the blastocyst. The trophoectoderm's cell junctions and tight epithelium "shield" the ICM from antibody binding by effectively making the cell impermeable to macromolecules.

Immunosurgery can be used to obtain large quantities of pure inner cell masses in a relatively short period of time. The ICM obtained can then be used for stem cell research and is better to use than adult or fetal stem cells because the ICM has not been affected by external factors, such as manually bisecting the cell. However, if the structural integrity of the blastocyst is compromised prior to the experiment, the ICM is susceptible to the immunological reaction. Thus, the quality of the embryo used is imperative to the experiment's success. In addition, when using complement derived from animals, the source of the animals matters. They should be kept in a specific-pathogen-free environment to increase the likelihood that the animal has not developed natural antibodies against the bacterial carbohydrates present in the serum (which can be obtained from a different animal).