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Meiotic Abnormalities in Infertile Males
Published in Carlos Simón, Carmen Rubio, Handbook of Genetic Diagnostic Technologies in Reproductive Medicine, 2022
Mireia Solé, Francesca Vidal, Joan Blanco, Zaida Sarrate
The relationship between male infertility and low chiasmata count has been described by different authors [22,34–40], affecting the reproductive fitness of these individuals [41–44]. The use of M-FISH techniques has established that bivalents formed by medium and large chromosomes are the most susceptible to this phenomenon [28]. Despite this, since these chromosomes have a basal number of chiasmata higher than two [35], the reduction rarely leads to the presence of univalents. Accordingly, sperm FISH studies do not show preferred increased values of chromosomal abnormalities in medium and large chromosomes [40]. Therefore, the presence of at least one chiasma seems to guarantee the correct segregation of chromosomes during meiosis in most cases.
Chromosome Pairing and Fertility in Plant Hybrids
Published in Christopher B. Gillies, Fertility and Chromosome Pairing: Recent Studies in Plants and Animals, 2020
In addition to their widespread use in breeding programs, hybrids have provided and continue to provide the cytologist with a tool for unravelling the mysteries of the meiotic process. The disruption of chromosome pairing due to hybridity and its effect on chiasma frequency and fertility has shed much light on the nature of synapsis and recombination in nonhybrid plants too and has elucidated some of the constraints upon the evolution of the chromosomes themselves. Homoeologous chromosomes of hybrids can show a remarkable capacity to pair from end to end and to form chiasmata, despite considerable structural divergence. Sufficient homology is preserved within these bivalents to permit genetic ex change, but the homoeologous DNA molecules present large enough differences for Ph-like factors to discriminate between strictly homologous and homoeologous paired regions and to control accordingly recombination between particular chromosomes. In view of the greater understanding of the biochemistry of meiosis, hopefully such investigations will prompt research into the exact molecular function of these diploidizing factors. This could have important implications for the development and exploitation of new and reliable hybrid plant species.
Preimplantation Genetic Testing for Structural Rearrangements
Published in Darren K. Griffin, Gary L. Harton, Preimplantation Genetic Testing, 2020
Recombination within the homologous regions of the interstitial segments is another factor that decreases the proportion of balanced/normal gametes [26]. The frequency of recombination depends on the length of the segments and the efficiency of the synapsis established between the chromosomes. Although segment size and the presence of heterochromatin regions play important roles, recombination behavior within a quadrivalent of a given chromosome is not predictable. The same chromosome can display different chiasmata distributions depending on the translocation and the segment size. Therefore, each carrier has a unique risk associated with their rearrangement and reproductive outcome. In the context of PGT-SR, patients need to be properly genetically counseled as to the range of outcomes that could arise as a result of their balanced rearrangement.
Amplitude of Accommodation in Patients with Multiple Sclerosis
Published in Current Eye Research, 2019
Bekir Küçük, Mehmet Hamamcı, Seray Aslan Bayhan, Hasan Ali Bayhan, Levent Ertuğrul Inan
When the target distance is changed, the lens power must be altered to clearly view it; this is known as ocular accommodation.25 The accommodation reflex starts in the retinal ganglion cells with the light reflex. These impulses are sent through the optic nerve, the optic chiasma, and the optic tract. Most optic tract fibers go to the pretectal area, although some fibers synapse with the second-order neurons in the lateral geniculate nucleus of the thalamus. Then, second-order neurons carry the impulses through the optic radiation to the visual cortex. Impulses pass from the visual cortex to the prefrontal cortex, and fibers pass through the internal capsule to reach the midbrain. Then, the fibers in the midbrain synapse with the oculomotor nucleus and the Edinger-Westphal nucleus.26 The motor fibers are carried by the oculomotor nerve from the oculomotor nucleus to the medial rectus muscle, where both eyes converge. The efferent fibers of the Edinger-Westphal nucleus (which is the parasympathetic autonomic nucleus) accompany the oculomotor nerve and synapse in the ciliary ganglion. Then, the postganglionic fibers of the ciliary ganglion pass the short ciliary nerves to supply the sphincter pupillae muscle and the ciliary muscle, which allows the lens to thicken27 (Figure 1). This mechanism is controlled by the autonomic nervous system.23
On the origin of the term decussatio pyramidum
Published in Journal of the History of the Neurosciences, 2018
Most fibers of the pyramidal tract of the medulla oblongata cross to the contralateral side and are called the pyramidal decussation (Hirsch, 2000, p. 122). The Latin term decussatio is derived from decussis and means intersection of two lines to form the Roman numeral for 10 (= decem) (Oxford Latin Dictionary, 1968, p. 495). Decussation is a term used in anatomical nomenclature for various other structures as well. Its Greek equivalent is chiasma, derived from Greek character χ (chí). The best-known decussation is the chiasma opticum, but the Latin equivalent decussatio was also used for this in the past. This contribution deals with the history of the term decussatio pyramidum from the point of view of terminology in particular, although of course its technical content cannot be avoided.
Understanding intrinsic survival and regenerative pathways through in vivo and in vitro studies: implications for optic nerve regeneration
Published in Expert Review of Ophthalmology, 2021
The optic chiasma is one of the impregnable barriers for optic nerve regeneration [2]. Even after the optic chiasma, regenerating axons can travel down the opposite optic nerve [55,57]. A recent study demonstrated that several axon guidance cues are expressed in the optic chiasma, e.g. radial glial cell marker 2 (RC2), brain lipid binding protein (BLBP), slit guidance ligand 1 (Slit1), chondroitin sulfate proteoglycans (CSPGs), and Pax2 [66]. These guidance expression markers change during development, in adulthood, and after injury. Thus, these changes may contribute to the misguidance of regenerating axons in the adult optic chiasma [66].