The Impact of ageing on fertility is a very well-known concept, but researchers are trying to establish the fact that reproductive health consequently affects ageing. Research done on a tiny nematode called C. Elegans showed that disrupting the process of meiosis in reproductive cells led to a visible decline in the worms’ health and accelerated the function of the ageing gene. It is very similar to that of humans, according to the new study, led by the University of Pittsburgh and UPMC researchers and published in Ageing Cell. This study has shown exciting results that established the first direct evidence that disturbing the health of reproductive cells leads to premature ageing and a decline in healthspan. The findings imply that the status of the reproductive system is crucial not just for reproduction but also for overall health, which is profound.
Meiosis is a method of cell division that divides parent cell chromosomes in half to produce four gamete cells. This is the first process of sexual reproduction that results in the production of egg and sperm cells. During reproduction, the sperm and egg fuse together, giving rise to a single cell with a complete set of chromosomes, half of which came from the male and the other half from the female.
Every parent cell is diploid, which means it contains a pair of each chromosomes. In the process of meiosis, each parent cell initially undergoes one round of DNA replication followed by two cycles of nuclear division, giving rise to four haploid daughter cells, meaning they only contain half the parent cell chromosomes. After cell division in the parent cell, meiosis can be divided into two phases, meiosis I and meiosis II, which have multiple phases. Meiosis I is unique to germ cells and different from mitosis, a cell division that produces replicas of the parent cell, while meiosis II is similar.
It is the first meiotic division, which begins with prophase I. During prophase I, the DNA and protein complex called chromatin condenses, forming chromosomes. The sister chromatids (replicated pairs of chromosomes) are joined at a central point called the centromere. Another structure that attaches itself to the sister chromatids is the meiotic spindle is initiated from long proteins called microtubules on the polar ends of the cell. Between prophase I and metaphase I, the pairs of homologous chromosomes arrange themselves into tetrads. Crossing-over or recombination occurs within the tetrad, in which any pair of chromatid arms can overlap and fuse in. Recombination is a process that leads to the production of new combinations of genes. The next phase is metaphase I, wherein the homologous pairs of chromosomes align across the equatorial plate. Then in anaphase I, the meiotic spindle fibres attached to homologous pairs pull them away from each other towards the opposite poles by condensing themselves. During the last phase, telophase I, the chromosomes are enclosed in nuclei as the cell undergoes a cytoplasm division called cytokinesis of the original cell and forms two haploid daughter cells.
Meiosis II is, in truth, a mitotic division of each daughter cell created during meiosis I. During prophase II, metaphase II, anaphase II, and telophase II are similar to the phases in meiosis I. The only difference is that during metaphase II, recombination does not happen. After meiosis II, four haploid daughter cells develop, forming either sperm or egg cells.
The ageing studies were done on tiny nematode worms called Caenorhabditis Elegans as it has a short lifespan of 3 weeks and has many genetic pathways similar to humans, thus making it an ideal system.
The researchers observed meiosis, a type of cell division seen in all higher organisms that results in the production of sperm or egg cells. Results showed that animals with mutations in meiosis genes had shorter lifespans than their standard counterparts. The mutations also worsened general health measures, including premature mobility, muscle function, and memory decline in the subjects. They also showed signs of disrupted protein homeostasis, which is one of the leading causes related to neurodegenerative diseases like Alzheimer’s disease.
Next, the team of researchers observed the changes in the gene expression of C. elegans. On day 1 of adulthood, mutants with changes in meiosis expressed genes highly similar to those normal worms express until after day 10. Which in human years is like a 20-year-old showing physiological functions and gene signatures of a 70-year-old.
Many genetic pathways are similar in C. elegans and humans. Therefore scientists were keen to find if the ageing genes related to protein mutation of meiosis are identical even in humans. They found that it is indeed very similar, which opens greater possibilities to discover more complex human systems and understand how reproductive health affects ageing.