Materials for cytogenetic research
The primary goal of the cytogenetic method is the reliable study of the human karyotype. To perform this analysis, specialists require viable nucleated cells. In standard clinical practice, the main material for study is peripheral blood cells, specifically lymphocytes.
When it is necessary to evaluate the fetal karyotype, other sources of biological material are used. Primarily, these are cells obtained during amniocentesis (from amniotic fluid). An alternative option for obtaining fetal tissue is chorionic villus sampling.
Karyotyping and idiogram
Karyotyping refers to the method of precise determination of the total number and structural arrangement of chromosomes. The main purpose of this process is the diagnosis of various chromosomal diseases, which include genomic mutations (changes in chromosome number) and chromosomal aberrations (structural rearrangements).
The entire diagnostic process is based on a thorough analysis of the photokaryogram. To ensure standardized results, specialists compile an idiogram. This is a systematized karyotype in which all chromosomes are strictly distributed into corresponding groups and arranged in order of decreasing physical size.
Express diagnostics of X-sex chromatin
Express diagnostics of sex chromatin is used for the rapid determination of a patient's genetic sex, as well as to detect abnormalities associated with abnormal numbers of sex chromosomes. The most common variant is the study of X-sex chromatin, also known as Barr bodies.
The material for this rapid test is traditionally cells of the oral mucosa (buccal epithelium). The essence of the method is that one fully inactivated X chromosome forms a very dense heterochromatic clump located near the nuclear envelope.
- Female norm (XX karyotype): one such heterochromatic clump (Barr body) is present in the nucleus.
- Male norm (XY karyotype): Barr bodies are absent in cell nuclei.
Study of male Y-sex chromatin
To confirm the presence and number of male sex chromosomes, Y-sex chromatin is studied, which forms the so-called F-body.
Unlike the detection of Barr bodies, the method for detecting F-bodies requires special staining of preparations and subsequent fluorescence microscopy. With proper visualization, an F-body appears under the microscope as a brightly fluorescent dot located inside the nucleus. Interpretation of the results is based on a direct correspondence: the number of F-bodies detected in the interphase nucleus always exactly matches the number of Y chromosomes in the karyotype of the subject.
Briefly: answers to topical questions
What specific types of structural chromosomal aberrations can be detected by the cytogenetic method?
The cytogenetic method (karyotyping) diagnoses structural changes in chromosomes. The following types of chromosomal aberrations are detected:
- Deletion — loss or deletion of a chromosome segment.
- Duplication — addition (doubling) of a segment.
- Inversion — rotation of a segment by 180° (can be pericentric and paracentric).
- Translocation — exchange of segments or transfer of a fragment to another chromosome (includes reciprocal and Robertsonian translocations).
- Transposition — attachment of a detached fragment to its own chromosome, but in a different location.
- Insertion — insertion of a segment (e.g., from one chromosome into another).
What are the medical indications for a cytogenetic study?
Cytogenetic testing is prescribed in obstetrics, gynecology, genetics, and hematology. Main indications include:
- Reproductive system disorders — hypergonadotropic amenorrhea, absence of the uterus, sexual development disorders, infertility, gonadal dysgenesis, recurrent early pregnancy loss.
- Prenatal diagnostics — presence of echomarkers of chromosomal pathology, decreased PAPP-A levels, elevated $\beta$-hCG, individual risk greater than 1 in 250, medical indications on the fetal side.
- Suspected hereditary pathology — multiple somatic developmental anomalies and dysplasias, malformations, short stature, sex chromatin count abnormalities, family history of miscarriages.
- Hematological diseases — comprehensive primary diagnostics of paroxysmal nocturnal hemoglobinuria (PNH).
Rule for calculating Barr bodies: their number is always one less than the total number of X chromosomes (XX = 1 body, XY = 0 bodies).
Class and exam questions
What is the difference between a karyotype and an idiogram?
The karyotype reflects the actual number and structure of chromosomes in a cell. An idiogram, however, is an artificially systematized karyotype on a photokaryogram, where chromosomes are arranged in groups in strict order of decreasing size.
What biological material is used to search for Barr bodies?
For express diagnostics of X-sex chromatin (Barr bodies), a scraping of oral mucosa cells—buccal epithelium—is most commonly used.
How is the number of detected F-bodies interpreted?
The number of fluorescent F-bodies detected in the cell nucleus using fluorescence microscopy equals the number of Y chromosomes in the patient's karyotype.
What else to study on this topic
- Types of chromosomal aberrations and genomic mutations detected by karyotyping.
- Principles of chromosome classification when compiling an idiogram.
- Mechanism of X chromosome inactivation and heterochromatic clump formation.
- Indications for prenatal diagnostics (chorionic villus sampling, amniocentesis).
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