Clinical genetics
C. Simon Herrington in Muir's Textbook of Pathology, 2020
During meiosis, one diploid parent cell gives rise to four haploid daughter cells. In humans, it occurs only during gamete formation. It takes place in two stages: meiosis I and meiosis II (Figure 5.5). One of the key features of meiosis is the formation of chiasmata between homologous chromosomes during meiosis I. This allows the exchange of material or recombination between homologous chromosomes. Such recombination ensures that, although one of each chromosome is passed into the gamete, the chromosome is a mixture of parts of both parental chromosomes. Some additional variation also occurs because of new mutations incorporated at DNA replication. A child will have around 70 new mutations not present in either parent. The majority of these mutations are harmless. The occurrence of a de novo mutation in a developmentally important gene is a major cause of developmental disorder in children, with a de novo causative mutation identified in at least 27% of cases in the UK.
Regulation of Reproduction by Dopamine
Nira Ben-Jonathan in Dopamine, 2020
Oogenesis can be divided into five stages. The first three occur prenatally, stage 4 occurs during ovulation, and stage 5 is at the time of fertilization. In stage 1, each primordial germ cell undergoes regular mitosis to produce two diploid oogonia. In stage 2, each oogonium undergoes mitosis to produce two diploid primary oocytes. In stage 3, each primary oocyte, enclosed in a primordial follicle consisting of a single layer of squamous follicular cells, starts to undergo the first meiotic division. This involves alignment of the homologous chromosomes, pairing and formation of chiasmata, crossing over, and exchange of genetic material. The oocyte then becomes arrested at prophase of the first meiotic division and remains in a genetically dormant state for many years to come.
Chromosome Pairing and Fertility in Plant Hybrids
Christopher B. Gillies in 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.
On the origin of the term decussatio pyramidum
Published in Journal of the History of the Neurosciences, 2018
František Šimon
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.
Contralateral field defect in traumatic globe luxation with optic nerve injury
Published in Clinical and Experimental Optometry, 2022
Sahil Agrawal, Deepsekhar Das, Sujeeth Modaboyina, Pallavi Singh, Asha Samdani, Neelam Pushker
In the chiasma, the upper nasal fibres of the contralateral eye decussate superiorly and posteriorly, while the lower nasal fibres cross over inferiorly and anteriorly (Figure 2A– D). This arrangement of infero-nasal nerve fibres at the inferior and anterior part of chiasma makes it prone to trauma due to any shearing force on the optic nerve, hence the development of a supero-temporal visual field defect. The authors hypothesise that, in scenarios where globe luxation is associated with no transection of optic nerve, a sudden and continuous forceful stretch or pull of optic nerve transmits forces to the chiasma which can result in neuronal oedema, vascular compromise and/or severing of the decussating nerve fibres. This wold relate more so to the fibres situated anteriorly and inferiorly, resulting in a contralateral supero-temporal field defect, as found in cases 1–3. This also explains the probable reason for no visual field defect in case 4, whereby there was complete transection of the optic nerve. Furthermore, the longer the duration of stretch on the optic nerve, the greater the chance that the field defect might not recover completely. All four patients in the present study had delayed presentation (more than 12 hours after injury).
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
Toshiyuki Oshitari
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].