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Embryo culture definition
Human embryo development begins in relative transcriptional silence with an oocyte to embryo transition that lasts for ~3 days and encompasses fusion of the egg and sperm, migration and fusion of the germ cell pronuclei, genetic and epigenetic reprogramming and a series of cleavage divisions that culminate with a major wave of embryonic genome activation (EGA) between the 4- and 8-cell stages.
Following EGA, the embryo subsequently undergoes compaction to form a morula that marks the first morphological indication of a break in radial symmetry. As human embryos continue to progress despite lysis or fragmentation of one or more blastomeres, morphological changes during human pre-implantation development, such as compaction or cavitation, are a function of the timing of development, rather than cell number. Subsequent cell divisions lead to the development of a blastocyst that comprises a fluid-filled blastocyst cavity and an inner cell mass, surrounded by trophectoderm cells.
Early human embryo development
1 day after fertilization, 0.1 mm in diameter
The oocyte is 120-150 m in diameter and is surrounded by the zona pellucida. The second maturation division of the oocyte completes as the sperm penetrates the egg (fertilization). The sperm head and the nucleus of the oocyte then swell to form the male and female pronuclei, respectively. Once they unite, the resultant diploid cell is called the zygote. The first mitotic division soon begins.
1.5-3 days after fertilization, 0.1-0.2 mm in diameter
The conceptus is composed of two to 16 cells but has no blastocystic cavity yet. The cell division at this stage is called cleavage since furrows (clefts) appear as the cytoplasm divides. The daughter cells are called blastomeres. An embryo with 16-32 cells is called a morula.
4 days after fertilization, 0.1-0.2 mm in diameter
The conceptus is a free (unattached) blastocyst. The blastocyst is a hollow mass of cells characterized by the blastocystic cavity. The blastocystic cavity begins by the coalescence of intercellular spaces when the embryo has acquired about 32 cells. The blastomeres segregate into an internally situated inner cell mass and an outer trophectoderm.
Implantation, which in humans occurs at approximately day 7 of development, is required for further development of the embryo proper.