Common terms you should familiarize with: here's an overview of CATI basics.
An activity graph identifies and displays the dynamic of cell shape changes.
Apoptosis is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include membrane blebbing, cell shrinkage and lysis, nuclear and chromosomal DNA fragmentation.
Cells undergoing apoptosis
Cells that don’t exhibit division are considered arrested. Cell-cycle checkpoints can arrest the cells irreversibly (no further development) or reversibly (delay in the cell’s division) due to DNA damage, limited nutrients, alteration of pH, osmolality, etc. Some of the non-dividing cells are undergoing karyokinesis without cytokinesis and contain multiple nuclei and DNA (tetraploid, etc.) - these cells are not considered arrested.
When all embryonic cells stop in development, we can consider the whole embryo as arrested - no progress in development can be achieved. The term “arrested” is used in cleavage stages.
Blastocoel is the fluid-filled cavity of the blastocyst
The collapse of the blastocoel can occur due to the leakage of fluid from the blastocoel cavity.
Fluid build-up in the blastocoel cavity increases its volume, causing blastocoel expansion and blastocyst growing, i.e. increase of the embryo’s diameter.
Blastocyst hatching is a natural proces of embryo escaping from its envelope - zona pellucida (ZP) after 5 - 6 days of development. This is a prerequisite step before implantation. In time‑lapse records we see the initial phases of hatching as herniation of trophoectodermic (TE) cells occuring frequently on the place of ICSI needle penetration of ZP.
The nucleated cells of an embryo
Cell cycle checkpoints are control mechanisms in eukaryotic cells which ensure proper division of the cell. Metaphase cell cycle checkpoint does not operate in early human embryos. Due to limited number of the cells probably even aneuploid cells are tolerated in early phases of embryo development.
The process of cell division into two (2) and more daughter cells.
An interval between the cleavage of the first and the last daughter cells of the same mitotic sequence. Abbreviated as CI, this is the duration of the mitotic sequence expressed in hours. The more synchronous the cell cleavage is, the more homogeneous developmental competencies of the resulting daughter cells are achieved. Synchronous cell cleavage in a given mitotic sequence is exhibited by a short cleavage interval, and vice-versa.
The time (t) interval (hrs) between ICSI fertilization and the occurrence of the cleavage. The cleavage of the cells occurs within a specific time frame and is influenced by specific processes of early embryo development.
The cluster means the group of embryos having the same, or similar, morphokinetic parameters e.g. hatching blastocysts, expanded blastocysts, expanding blastocysts, blastocysts, early blastocysts, blastocysts with collapses, morulas, degenerated or arrested embryos, etc.
Cytokinesis divides the cytoplasm, organelles, metabolites and cell membrane into two new daughter cells containing roughly equal amounts of the cellular components.
Two identical copies of the former cell resulting from mitotic division.
When all cells undergo necrosis or apoptosis, the whole embryo can be considered as degenerated - no progress in development can be achieved. In blastocyst stages, the degeneration is characterized by the total collapse of the blastocoel cavity without re expansion.
Within a few days after fertilization, cells on the outer part of the morula bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable. This process is known as compaction.
Embryonic events The term “events” describes the embryo developmental processes which are not cleavage related, i.e. appearance of the pronucleus (PN), PN fading - syngamy, cell compaction, blastocoel expansion/collapse, hatching, etc.
The process during which the embryonic genome is activated, i.e. when transcription is evident (day 3 of human embryo development, at the 4- to 8-cell stage). EGA is a crucial event of transition from maternal to embryonic (zygotic) genome products.
Fertilization is the fusion of gametes to create the embryo and initiate the development of a new individual organism.
Forming of fragments during the embryo development.
Anuclear parts of the embryo which are not considered cells
Genetic constitution of embryos Genetic constitution of an embryo correlates with the embryo’s competence to ensure the development of a genetically healthy fetus/child. Genetic constitution of embryos is influenced by the three (3) main factors: 1. inherited diseases, 2. meiotic errors, 3. mitotic errors.
Grading means sorting embryos based on their implantation potential (IP). In our grading system, the embryos’ IP is suggested assuming that the embryos are ideally transferred into an ideal in-vivo environment.
Implantation is the stage of pregnancy at which the embryo adheres to the wall of the uterus.
Implantation potential, or IP, is an expression of the embryo’s capability to achieve successful implantation. Because the implantation is a multifactorial event, in our grading system, the embryos’ IP is suggested assuming that the embryos are ideally transferred into an ideal in-vivo environment.
Hereditary genetic disorders caused by one or more abnormalities in the genome which are passed down from the parents’ genes. Monogenic (X-linked, dominant, recessive) gene disorders and translocations are the most frequent inherited disorders. The global incidence of simple-gene diseases at birth is very close to 8 in every 10,000 liveborn infants.
After compaction some cells remain trapped inside and will become the inner cell mass (ICM). In mammals, the ICM is pluripotent and will ultimately form the fetus, while the trophectoderm will form the placenta and extra-embryonic tissues.
Interphase is the longest phase of the cell cycle in which the cell copies its DNA and other metabolites and prepares the nucleus (karyokinesis) and cytoplasm for the cleavage in mitosis (cytokinesis). The stages of interphase are: G1 (Gap 1), S (Synthesis - DNA replication) and G2 (Gap 2). In some cells there is also G0 (Gap zero).
Intracytoplasmic sperm injection, or ICSI, is an in vitro fertilization (IVF) procedure in which a single sperm cell is injected directly into the cytoplasm of an egg.
Karyokinesis is a division of the nuclear content during mitosis to ensure genetic identity of new daughter cells
An interval (hrs) between two subsequent mitotic sequences measured as the period between the cleavages of the first cells in a given mitotic sequence.
A number of TE cells that is lower than expect can be caused by two main factors. The embryo undergoes a lower number of mitotic cycles in delayed embryos. Or, the embryo lost its cells due to cell arrest, fusion, an excess fragmentation, apoptosis, etc.
Meiotic errors occurring de-novo during gametogenesis are caused by chromosomal/chromatid non-disjunctions, anaphase lack, etc. resulting in nuli-, mono-, tri-somy, etc. Meiotic errors correlate with increased maternal age and alteration of spermiogenesis e.g. severe OTA syndrome or non-obstructive azoospermia in men. Some meiotic errors can develop to term (Down, Turner, Edwards, etc.).
Mitosis is a division of the nucleus - karyokinesis. Mitosis is preceded by the S-stage of interphase and is accompanied by cytokinesis. Cytokinesis divides the cytoplasm, organelles and cell membrane into two new daughter cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of an animal cell cycle - the division of the mother cell into two daughter cells genetically identical to each other. Stages of M-phase: prophase, prometaphase, metaphase, anaphase and telophase.
Mitotic cycle is a process in which the cells duplicate themselves to create genetically identical copies - daughter cells. The process is accomplished through the division of cells. The phases of the mitotic cycle are: interphase and mitosis (M-phase).
Mitotic errors occurring de-novo are caused by mitotic chromosomal malsegregations during mitosis of embryonic/somatic cells. Abnormal cell cleavages resulting in chaotic complex aneuploidies are the major source of mitotic errors. Mitotic errors are responsible for embryo developmental arrests, implantation failure and early miscarriages. It is estimated that approx. 80% of early human embryos are chromosomally aneuploid/mosaic as a result of abnormal mitotic divisions.
The trophectoderm (TE) cells change their shape during mitosis (from flat to oval). More or less synchronous contractions of TE cells can lead to the occurrence of temporal (physiological) blastocoel collapses – mitotic pulses.
Mitotic sequences (MSeq) are series of cleavages occurring from the same generation of the cells. The product of each MSeq is a new generation of daughter cells. The more synchronous the timing of MSeq is (expressed as CI), the more uniform developmental competencies of the resulting daughter cells are achieved.
The markers combining the embryo’s morphology with its kinetics as revealed by continuous time‑lapse imaging.
Cells containing multiple nuclei. MN cells are usually aneuploid or polyploid (karyokinesis without cytokinesis frequently found in trophectoderm cells). However, some MN cells can be euploid e.g. 2-cell stage embryos are frequently MN even though they are euploid and multinucleation can be corrected during development to 4-cell stage.
Unlike apoptosis, which is a form of programmed cell death, necrosis is a form of traumatic cell death that results from acute cellular injury.
Cells undergoing necrosis
The best developing embryos can complete up to 8 mitotic cycles (MC) and the first cells can enter the 9th MC in approx. 120 hrs. This means the best embryos may have more than 128 (up to 256) TE cells.
Pronucleus, or PN, is the nucleus containing a paternal or maternal haploid set of chromosomes occurring after the sperm enters the oocyte. The appearance of two pronuclei (2PN-stage) is the first sign of successful fertilization.
During a picture analysis, the area of the embryo inside the zona pellucida is recognized and measured by the neural network (CATI) and the values are expressed in a Segmentation Graph.
During syngamy, the male and female pronuclei don’t fuse, however their membranes dissolve, leaving no barriers between the male and female chromosomes. Their chromosomes can then combine and become part of a single diploid nucleus in the resulting embryo.
Time‑Lapse System Time‑lapse imaging is a technique where the frequency at which film frames are captured (the frame rate) is much lower than that used to view the sequence. When played at normal speed, time appears to be moving faster and thus lapsing. A TLS can take images of embryos at specified time intervals, which enables their assessment without removing them from the incubator – undisturbed cultivation.
The blastocyst’s outer cells will become the first embryonic epithelium (the trophectoderm) which will form the placenta.
An oocyte showing no signs of fertilization (i.e. presence of pronuclei).
Viability of the embryo correlates with the embryo’s competence to implant (implantation potential, or IP) and is expressed by its ability to develop to blastocyst stage in 5-6 days of in-vitro culture and to start the process of hatching.