Historical and comparative approaches
The comparative embryological method is based on the fact that the similarity of embryos of different species is manifested much more clearly than the similarity of adult individuals. For a deeper evolutionary analysis, the triple parallelism method is used. It combines data from three scientific disciplines at once:
- Comparative anatomy (helps differentiate homologous and analogous structures).
- Embryology (relies on K. Baer's law of embryonic resemblance and the phenomenon of recapitulation).
- Paleontology (studies phylogenetic series and transitional forms).
Classical experimental embryology was founded by Wilhelm Roux and Hans Driesch in experiments on sea urchins. They proved that the development of the embryo directly depends on the interaction of cells with each other. However, such direct manipulations (cell separation or transplantation) are impossible on human material due to strict ethical limitations.
Reproductive and cellular technologies
In modern medicine, the study of early human embryogenesis is implemented through assisted reproductive technologies. In vitro fertilization (IVF) makes it possible to observe embryo development in vitro at the cleavage and blastocyst stages. In male factor infertility, ICSI is used — the artificial injection of a single spermatozoon directly into the oocyte.
A special place is occupied by therapeutic cloning (somatic cell nuclear transfer technology). The process takes place in several stages:
- A patient's somatic cell (e.g., skin epithelium) and a donor egg from which the nucleus has been previously removed are taken.
- Cell fusion and activation are stimulated using an electrical impulse.
- The genome of the somatic cell is reprogrammed, returning to the zygote state.
- Over several days, a blastocyst consisting of 100–250 cells is formed.
- Germ cells are isolated from it and cultured on special media to obtain the desired tissues.
Unlike reproductive cloning, where the embryo is preserved and transferred to a surrogate mother, in therapeutic cloning, the embryo is destroyed at an early stage to isolate stem cells for autologous transplantation.
Prenatal diagnosis and genetic analysis
Comprehensive prenatal diagnosis is performed up to the 22nd week of pregnancy to detect congenital malformations and chromosomal diseases. It includes medical genetic counseling, ultrasound (from the 2nd trimester allows visualizing intrauterine respiratory movements), biochemical screening of serum markers, and invasive procedures.
Invasive methods allow obtaining fetal cells directly. These include amniocentesis (sampling of amniotic fluid), chorionic villus sampling, placentocentesis, and cordocentesis. In IVF programs, preimplantation diagnosis is available: at the 8-blastomere stage, cells are extracted using a micropipette for analysis (the polar body can also be examined). This is critically important, for example, to determine the embryo's Rh factor in severe maternal Rh sensitization.
The obtained cellular material is studied at different levels:
- Cytogenetic method: microscopic evaluation of the karyotype with the construction of an idiogram (a graph where chromosomes are arranged in descending order of size).
- FISH method (Fluorescence in Situ Hybridization): chromosomal analysis using fluorescent labels for automated study of a large array of cells.
- Chromosomal microarray analysis (CMA): high-resolution genome-wide study detecting microdeletions, microduplications, and mosaic forms of aneuploidy.
- Molecular genetic methods: PCR, sequencing (determining the nucleotide sequence of genes), and genome-wide hybridization on a microarray.
How to remember the difference between types of cloning: Therapeutic — Transplantation (cells are grown to treat the patient, the embryo itself is destroyed). Reproductive — Rroduction/Birth (the embryo is preserved and implanted into a surrogate mother to create a new organism).
Class and exam questions
Why are classical experiments, like those on sea urchins, not used in human embryology?
Direct manipulations with human germ cells (e.g., their artificial separation or transplantation) are inapplicable for ethical reasons. Research is implemented through cell culture and the use of 'surplus' embryos after IVF.
What is an idiogram?
This is a graphic representation of a karyotype obtained during a cytogenetic study, in which chromosomes are arranged strictly in descending order of their size.
What is the advantage of PCR in genetic analysis?
Polymerase chain reaction allows increasing the amount of the required DNA sequence millions of times in vitro within a few hours, which is why an extremely small amount of initial biological material is required for analysis.
What else to study on this topic
- Molecular Genetic and Biochemical Methods
- Karyotype and Classifications of Human Chromosomes
- Levels of genetic information realization
- Chromosomal theory of heredity
- Morphology and Chemical Composition of Chromosomes
- Structure and functions of the cell nucleus
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