Structure of Skeletal Muscle:
Role in the development of the nervous system[ edit ] Dying cells in the proliferate zone The initial expansion of the developing nervous system is counterbalanced by the removal of neurons and their processes. In humans, PCD in progenitor cells starts at gestational week 7 and remains until the first trimester.
It postulates that in order to ensure optimal innervation of targets, a surplus of neurons is first produced which then compete for limited quantities of protective neurotrophic factors and only a fraction survive while others die by programmed cell death.
This developed the concept of target derived regulation which became the main tenet in the neurotrophic theory.
Programmed cell death in the CNS is not dependent on external growth factors but instead relies on intrinsically derived cues. In the neocortexa 4: Regardless of the size of the transplant, the fraction of cells undergoing apoptosis remained constant. Furthermore, disruption of TrkBa receptor for brain derived neurotrophic factor Bdnfdid not affect cell death.
It has also been shown that in mice null for the proapoptotic factor Bax Bclassociated X protein a larger percentage of interneurons survived compared to wild type mice.
Apoptotic mechanisms in the CNS are still not well understood, yet it is thought that apoptosis of interneurons is a self-autonomous process. Nervous system development in its absence[ edit ] Programmed cell death can be reduced or eliminated in the developing nervous system by the targeted deletion of pro-apoptotic genes or by the overexpression of anti-apoptotic genes.
The absence or reduction of PCD can cause serious anatomical malformations but can also result in minimal consequences depending on the gene targeted, neuronal population, and stage of development.
For example, mice overexpressing Bcl-2 have generally normal motor skills and vision and only show impairment in complex behaviors such as learning and anxiety. During the development of the invertebrate nervous system, PCD plays different roles in different species.
The similarity of the asymmetric cell death mechanism in the nematode and the leech indicates that PCD may have an evolutionary significance in the development of the nervous system.
In contrast to invertebrates, the mechanism of programmed cell death is found to be more conserved in vertebrates. Extensive studies performed on various vertebrates show that PCD of neurons and glia occurs in most parts of the nervous system during development.
It has been observed before and during synaptogenesis in the central nervous system as well as the peripheral nervous system. For example, mammals exhibit extensive arborization followed by PCD in the retina while birds do not.
Instead of an immune response, the dying cell synthesizes substances to break itself down and places them in a vacuole that ruptures as the cell dies. Basic morphological and biochemical features of PCD have been conserved in both plant and animal kingdoms.
These have common features with animal apoptosis—for instance, nuclear DNA degradation—but they also have their own peculiarities, such as nuclear degradation triggered by the collapse of the vacuole in tracheary elements of the xylem.
Leaver, of the Department of Plant SciencesUniversity of Oxfordcarried out research on mutations in the mitochondrial genome of sun-flower cells.View Notes - Cell Biology Lecture 14 initiativeblog.com from BIOL at University of Manitoba.
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