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Chromatin degradation in differentiating fiber cells of the eye lens.

Bassnett S, Mataic D - J. Cell Biol. (1997)

Bottom Line: Dual labeling with TdT and an antibody against protein disulfide isomerase, an ER-resident protein, revealed a distinct spatial and temporal gap between the disappearance of ER and nuclear membranes and the onset of DNA degradation.Thus, fiber cell chromatin disassembly differs significantly from classical apoptosis, in both the sequence of events and the time course of the process.The fact that DNA degradation occurs only after the disappearance of mitochondrial, ER, and nuclear membranes suggests that damage to intracellular membranes may be an initiating event in nuclear breakdown.

View Article: PubMed Central - PubMed

Affiliation: Department of Ophthalmology and Visual Sciences, Washington University Medical School, St. Louis, Missouri 63110-1093, USA. Bassnetts@am.seer.wustl.edu

ABSTRACT
During development, the lens of the eye becomes transparent, in part because of the elimination of nuclei and other organelles from the central lens fiber cells by an apoptotic-like mechanism. Using confocal microscopy we showed that, at the border of the organelle-free zone (OFZ), fiber cell nuclei became suddenly irregular in shape, with marginalized chromatin. Subsequently, holes appeared in the nuclear envelope and underlying laminae, and the nuclei collapsed into condensed, spherical structures. Nuclear remnants, containing DNA, histones, lamin B2, and fragments of nuclear membrane, were detected deep in the OFZ. We used in situ electrophoresis to demonstrate that fragmented DNA was present only in cells bordering the OFZ. Confocal microscopy of terminal deoxynucleotidyl transferase (TdT)-labeled lens slices confirmed that DNA fragmentation was a relatively late event in fiber differentiation, occurring after the loss of the nuclear membrane. DNA fragments with 3'-OH or 3'-PO(4) ends were not observed elsewhere in the lens under normal conditions, although they could be produced by pretreatment with DNase I or micrococcal nuclease, respectively. Dual labeling with TdT and an antibody against protein disulfide isomerase, an ER-resident protein, revealed a distinct spatial and temporal gap between the disappearance of ER and nuclear membranes and the onset of DNA degradation. Thus, fiber cell chromatin disassembly differs significantly from classical apoptosis, in both the sequence of events and the time course of the process. The fact that DNA degradation occurs only after the disappearance of mitochondrial, ER, and nuclear membranes suggests that damage to intracellular membranes may be an initiating event in nuclear breakdown.

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Diagram of a midsagittal slice of a chicken lens  at E15. The lens is bounded  by an acellular collagenous  capsule. An epithelial monolayer covers the anterior surface of the lens and thickens  at the periphery to form the  annular pad. The bulk of the  lens consists of concentric  layers of highly elongated  lens fiber cells derived from  the edges of the epithelium.  Primary fiber cells, formed  early in development, are situated in the center of the  lens. The rest of the fiber  mass is composed of secondary  fiber cells, formed throughout life by the differentiation  of epithelial cells at the lens  equator. The tips of secondary fiber cells make contact  with fibers from the opposite  hemisphere of the lens at the  sutures. The outer fiber cells  contain a normal complement of organelles, including nuclei. However, organelles are absent from cells in the center of the lens, giving rise to the OFZ. The  OFZ increases steadily in size throughout embryonic development (small arrows).
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Figure 1: Diagram of a midsagittal slice of a chicken lens at E15. The lens is bounded by an acellular collagenous capsule. An epithelial monolayer covers the anterior surface of the lens and thickens at the periphery to form the annular pad. The bulk of the lens consists of concentric layers of highly elongated lens fiber cells derived from the edges of the epithelium. Primary fiber cells, formed early in development, are situated in the center of the lens. The rest of the fiber mass is composed of secondary fiber cells, formed throughout life by the differentiation of epithelial cells at the lens equator. The tips of secondary fiber cells make contact with fibers from the opposite hemisphere of the lens at the sutures. The outer fiber cells contain a normal complement of organelles, including nuclei. However, organelles are absent from cells in the center of the lens, giving rise to the OFZ. The OFZ increases steadily in size throughout embryonic development (small arrows).

Mentions: The lens of the eye is a transparent cellular structure that focuses light on the retina. The bulk of the lens consists of concentric layers of fiber cells that are formed throughout life by the differentiation of cells at the equatorial margin of the lens epithelium (see Fig. 1). Fiber cell differentiation is characterized by cellular elongation, the synthesis of certain crystallin proteins, and the degradation of all membrane-bound organelles, including nuclei (Piatigorsky, 1981). Because there is no cell turnover in the lens, all cells are retained within the tissue, those nearest the center being the oldest and those nearest the surface being the youngest.


Chromatin degradation in differentiating fiber cells of the eye lens.

Bassnett S, Mataic D - J. Cell Biol. (1997)

Diagram of a midsagittal slice of a chicken lens  at E15. The lens is bounded  by an acellular collagenous  capsule. An epithelial monolayer covers the anterior surface of the lens and thickens  at the periphery to form the  annular pad. The bulk of the  lens consists of concentric  layers of highly elongated  lens fiber cells derived from  the edges of the epithelium.  Primary fiber cells, formed  early in development, are situated in the center of the  lens. The rest of the fiber  mass is composed of secondary  fiber cells, formed throughout life by the differentiation  of epithelial cells at the lens  equator. The tips of secondary fiber cells make contact  with fibers from the opposite  hemisphere of the lens at the  sutures. The outer fiber cells  contain a normal complement of organelles, including nuclei. However, organelles are absent from cells in the center of the lens, giving rise to the OFZ. The  OFZ increases steadily in size throughout embryonic development (small arrows).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2139849&req=5

Figure 1: Diagram of a midsagittal slice of a chicken lens at E15. The lens is bounded by an acellular collagenous capsule. An epithelial monolayer covers the anterior surface of the lens and thickens at the periphery to form the annular pad. The bulk of the lens consists of concentric layers of highly elongated lens fiber cells derived from the edges of the epithelium. Primary fiber cells, formed early in development, are situated in the center of the lens. The rest of the fiber mass is composed of secondary fiber cells, formed throughout life by the differentiation of epithelial cells at the lens equator. The tips of secondary fiber cells make contact with fibers from the opposite hemisphere of the lens at the sutures. The outer fiber cells contain a normal complement of organelles, including nuclei. However, organelles are absent from cells in the center of the lens, giving rise to the OFZ. The OFZ increases steadily in size throughout embryonic development (small arrows).
Mentions: The lens of the eye is a transparent cellular structure that focuses light on the retina. The bulk of the lens consists of concentric layers of fiber cells that are formed throughout life by the differentiation of cells at the equatorial margin of the lens epithelium (see Fig. 1). Fiber cell differentiation is characterized by cellular elongation, the synthesis of certain crystallin proteins, and the degradation of all membrane-bound organelles, including nuclei (Piatigorsky, 1981). Because there is no cell turnover in the lens, all cells are retained within the tissue, those nearest the center being the oldest and those nearest the surface being the youngest.

Bottom Line: Dual labeling with TdT and an antibody against protein disulfide isomerase, an ER-resident protein, revealed a distinct spatial and temporal gap between the disappearance of ER and nuclear membranes and the onset of DNA degradation.Thus, fiber cell chromatin disassembly differs significantly from classical apoptosis, in both the sequence of events and the time course of the process.The fact that DNA degradation occurs only after the disappearance of mitochondrial, ER, and nuclear membranes suggests that damage to intracellular membranes may be an initiating event in nuclear breakdown.

View Article: PubMed Central - PubMed

Affiliation: Department of Ophthalmology and Visual Sciences, Washington University Medical School, St. Louis, Missouri 63110-1093, USA. Bassnetts@am.seer.wustl.edu

ABSTRACT
During development, the lens of the eye becomes transparent, in part because of the elimination of nuclei and other organelles from the central lens fiber cells by an apoptotic-like mechanism. Using confocal microscopy we showed that, at the border of the organelle-free zone (OFZ), fiber cell nuclei became suddenly irregular in shape, with marginalized chromatin. Subsequently, holes appeared in the nuclear envelope and underlying laminae, and the nuclei collapsed into condensed, spherical structures. Nuclear remnants, containing DNA, histones, lamin B2, and fragments of nuclear membrane, were detected deep in the OFZ. We used in situ electrophoresis to demonstrate that fragmented DNA was present only in cells bordering the OFZ. Confocal microscopy of terminal deoxynucleotidyl transferase (TdT)-labeled lens slices confirmed that DNA fragmentation was a relatively late event in fiber differentiation, occurring after the loss of the nuclear membrane. DNA fragments with 3'-OH or 3'-PO(4) ends were not observed elsewhere in the lens under normal conditions, although they could be produced by pretreatment with DNase I or micrococcal nuclease, respectively. Dual labeling with TdT and an antibody against protein disulfide isomerase, an ER-resident protein, revealed a distinct spatial and temporal gap between the disappearance of ER and nuclear membranes and the onset of DNA degradation. Thus, fiber cell chromatin disassembly differs significantly from classical apoptosis, in both the sequence of events and the time course of the process. The fact that DNA degradation occurs only after the disappearance of mitochondrial, ER, and nuclear membranes suggests that damage to intracellular membranes may be an initiating event in nuclear breakdown.

Show MeSH
Related in: MedlinePlus