Previous research using microdensitometric scanning and computer graphic image analysis showed that T-banded segments of human metaphase chromosomes usually exhibit an asymmetrical distribution of high density (HD) chromatin between sister chromatids. Here, we employed the same methods to analyze HD chromatin distribution at opposite ends of T-banded human lymphocyte chromosomes. This study revealed that in most chromosomes with an asymmetrical distribution of HD chromatin at both ends, the highest densities of each arm were located in opposite chromatids. The frequency of this configuration was 0.792 per chromosome, indicating that the highest chromatin densities of the terminal segments of T-banded human chromosomes were non-randomly distributed at opposite chromosome arms. The possible relationship of this observation to the mode of replication of the terminal chromosome region is briefly discussed.

We described spontaneous minute sister chromatid exchanges (SCE) in telomeric regions of human and Chinese hamster ovary (CHO) chromosomes more than 10 years ago. These structures, which we called t-SCE, were detected by means of highly precise quantitative microphotometrical scanning and computer graphic image analysis. Recently, several authors using the CO-FISH method also found small SCEs in telomeric regions and called them T-SCE. The use of different terms for designating the same phenomenon should be avoided. We propose ter SCE as a uniform nomenclature for minute telomeric SCEs.

Los recientes progresos tecnológicos ocurridos en citogenética molecular han permitido la detección de numerosas minúsculas aberraciones en la región cromosómica contigua al telómero o segmento subtelomérico, que han sido relacionadas con diversos cuadros de retardo mental, malformaciones congénitas y otros síndromes de interés médico. Investigaciones previas, llevadas a cabo mediante exploración microfotométrica y análisis gráfico computacional del subtelómero, revelaron diferentes patrones de distribución de las densidades de la cromatina y la existencia de intercambios sumamente pequeños entre cromátidas hermanas en dicho segmento cromosómico. La detección de aberraciones crípticas sindromáticas, el elevado número de intercambios cromosómicos y las observaciones microscópicas sobre la estructura subtelomérica sugieren que reflejan la gran actividad genómica y la complejidad estructural prevalente en la región. En la presente revisión se describen brevemente varios síndromes cromosómicos subteloméricos, así como la estructura molecular y citológica de la región subtelomérica y las principales funciones del segmento cromosómico terminal a fin de brindar un panorama general sobre esta área de investigaciones en rápido crecimiento de considerable significación biomédica

Résumé Les derniers progrès technologiques en cytogénétique moléculaire, ont permis de repérer de nombreuses et minuscules aberrations dans la région chromosomique du télomère ou segment subtélomérique liées au retard mental, des malformations congénitales et d’autres syndrômes d’intérèt médical. Des recherches antérieures, menées à bout au moyen d’exploration microphotométrique et d’analyse graphique par ordinateur du subtélomère, ont montré de différents paramètres de distribution des densités de la chromatine et l’existence de minuscules échanges entre chromatides soeurs dans ce segment chromosomique. La détection d’aberrations cryptiques syndromatiques, le grand nombre d’échanges chromoso-miques et les observations microscopiques de la structure subtélomérique suggèrent une grande activité génomique et la compléxité structurelle prévalente dans la région. On décrit ici brièvement plusieurs syndrômes chromoso-miques subtélomériques, ainsi que la structure moléculaire et cytologique de la région subtélomérique et les principales fonctions du segment chromosomique terminal, afin d’offrir un panorama général sur cette sujet de recherches qui s’accroit considérablement

Summary Recent technological advances occurred in molecular cytogenetics have allowed to detect numerous minute aberrations in the chromosome region contiguous to the telomere or subtelomeric segment which has been related to various conditions of mental retardation and congenital malformations and to other clinical syndromes of medical interest. Previous research on the subtelomere carried out by means of scanning microphotometry and computer graphic analysis, revealed different patterns of the distribution of chromatin densities and the existence of minute exchanges between sister chromatids in this chromosomic segment. The detection of cryptic syndromatic aberrations, the high number of chromosome crossovers and the microscopical observations on the subtelomeric structure suggest that they reflect the great genomic activity and the structural complexity prevailing in the region. In the present review, several subtelomeric chromosome syndromes, the cytological and molecular structure of the subtelomeric region as well as the main functions of the chromosome terminal segment are briefly described in order to give a general panorama on this fast growing research area of considerable significance in bio-medicine