- Primary research
- Open Access
Nucleic acid distribution pattern as a possible biomarker for metabolic activities of neoplastic cells: a digitally-aided fluorescence microscopy study on normal and neoplastic lymphocytes of acute and chronic canine lymphocytic leukemia
© Isitor et al; licensee BioMed Central Ltd. 2009
- Received: 31 August 2008
- Accepted: 11 May 2009
- Published: 11 May 2009
Metabolic states of neoplastic cells are increasingly being relied upon for diagnostic and prognostic assessment of neoplastic conditions. The nucleic acid distribution pattern of cells in general, in terms of degree of condensation of the nuclear chromatin and overall spread of the nucleic acid within the nuclear and cytoplasmic compartments, can reflect the metabolic state of the cell. This simple but logical concept appears not be put into consideration to date as numerous attempts are being made towards formulating reliable biomarkers for rapid diagnosis, prognosis and subsequent therapeutic interventions for neoplastic conditions. We comparatively evaluated nucleic acid distribution patterns of normal lymphocytes and neoplastic cells of lymphocytic lineage, employing light and fluorescence microscopy procedures, as well as digital imaging analytical methods.
The results demonstrate distinctiveness in the pattern of nucleic acid distribution for the normal lymphocytes and three lymphocytic neoplastic cell-types of canine lymphocytic leukemia that are categorized as small, intermediate and large neoplastic lymphocytes. Variably-shaped cytoplasmic processes laden with single-stranded nucleic acids (SSNA) were observed for the small and intermediate-sized neoplastic lymphocytes, compared with large neoplastic lymphocytes and the normal lymphocytes; the latter two categories of cells being virtually devoid of similar processes. Prominent cytoplasmic and nuclear clumps of SSNA, indicative of a higher rate of metabolic activity, were also observed within the neoplastic cells compared with fewer and narrower SSNA of the normal cells.
The comparative relative increases of SSNA in cytoplasmic processes and other cellular areas of small and intermediate-sized neoplastic lymphocytes is reflective of greater metabolic activity in neoplastic cells in general compared with their normal cellular counterparts.
- Acute Lymphocytic Leukemia
- Chronic Lymphocytic Leukemia
- Neoplastic Cell
- Lymphocytic Leukemia
- Pixel Intensity
Nucleic acids, mostly in the form of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), constitute a major structural framework of the eukaryotic cell; the DNA being mainly distributed within the nucleus as component of the chromatin, while RNA abound within nuclear and cytoplasmic compartments. Depending on the metabolic state of the cell, the chromatin morphology varies between a more condensed form (heterochromasia) to a loosely organized or euchromatic state. In either state, the basic chromatin organization is depicted by the "beads-on-a-string" morphology, with the DNA molecules wound around the nucleosome core of 8 histone molecules . The degree of tightness of the DNA molecules around the histone core determines the level of heterochromasia or euchromasia, and in turn chromatin accessibility or permissivity. A non-neoplastic cell with normal metabolic machinery can be expected to express lower DNA transcriptional activity relative to its neoplastic counterpart with a higher degree of divisional activities, which in turn translates to variability of chromatin morphology and the cellular nucleic acid architecture in general. This variability is the basis for consideration of the nucleic acid morphological state as a possible biomarker in the present investigation, utilizing canine lymphocytic leukemia (CLL) cells as a model.
CLL is of general interest, not only from the standpoint of canine health management, but also because of their striking resemblance to human lymphocytic leukemia [2, 3]. CLL is relatively quite prevalent in the canine population worldwide, and the cell types appear not to have been properly elucidated to date. Existing literature commonly described two types of canine leukemia cells as small and large lymphoid types. Some authors acknowledged uncertainties with respect to terminologies and classifications of the cell types [4, 5]. Given present day strides in imaging modalities for improved histological and cytological tissue processing, there is a need for a closer evaluation of the cell mix of CLL. In the past, pathologists and cytologists have relied heavily on nuclear morphologic attributes of cells in classifications of neoplasms in general. While distinctiveness of the nuclear chromatin pattern of certain cells cannot be doubted, the limitations of the use of such morphologic features under routine light microscopy with common stains, such as Wright's, Giemsa or combination of both, are enormous.
The application of digitally-aided fluorescence microscopy in diagnostic cytology appears to be gaining momentum in recent times. The use of incident light or epifluorescence microscopes, coupled with versatile fluorochromes, such as acridine orange, appears to offer great promise towards resolving issues of specific cell identity in cytological preparations; issues mostly based on both nuclear and cytoplasmic nucleic acid distribution patterns. Of particular interest is the ability to relate metabolic states of neoplastic cells to prognosis for the condition, based on the degree of associated nucleic acid fluorescence . In the light of the present day paradigm shift in identification of biomarkers in cells and tissues that can aid in diagnostic, prognostic and therapeutic interventions for neoplastic conditions, there is a need to explore options other than prevailing biochemical and molecular based biomarkers. Some of the proven biomarker technologies, such as microarray profiling for DNA methylation status, involve the use of very expensive real time PCR equipment, far beyond the budget of most diagnostic setups in developing countries. Our present report represents one such investigation in which comparative digital analysis of image pixel distribution pattern of cell types of canine acute and chronic leukemia, as well as normal canine lymphocytes, can relate to their variable metabolic states and possibly serve as a reliable biomarker for diagnostic and prognostic purposes. It is also hoped that insight gleaned from such investigation would encourage greater application of digitally-aided fluorescence microscopy by pathologists in routine diagnostic and prognostic cytology.
Morphologic Attributes based on Light and Fluorescence Microscopy
Normal-appearing lymphocytes were observed in close association with some of the intermediate and large neoplastic cells (Figure 5, Figure 7A, B, D). These normal appearing cells were significantly smaller than the neoplastic cells, and were generally characterized by nuclear diffuse yellow-green DSNA fluorescence and punctate red interconnected clumps of SSNA fluorescence. Some of the normal-appearing cells seem to be in direct contact with the neoplastic cells through large processes of the latter, in a manner suggestive of induction of the normal-appearing cells by the neoplastic ones (Figure 7B). These large processes were of variable sizes, and exhibited intense red SSNA fluorescence.
There were, generally, no observed fluorescence attributable to the presence of mitotic figures, such as chromosomal clumping at the poles or equatorial regions of the cells in the blood smears of any of the leukemia cases.
Digital Analytical Attributes and Statistical Features
The curves for the normal lymphocytes were of variable outline, but generally tended to be either mountain-like with characteristic 17 to 23 spikes at their mountain-tops (Figure 10 and Figure 11), or with 5 to 6 spikes mostly located at the wide peak regions of the curves, while numerous minute spikes occurred at the left shoulders of the curves (Figure 12 and Figure 13). The positions of the highest pixel intensity reference point of 100 tended to correspond for the two images with mountain-like curves within their nuclei, and also for the other two images with extended curve left shoulders within their cytoplasmic areas. There was variability of the mean pixel intensity values (P < 0.05) among the images of the normal cell-type.
The curves for the small neoplastic cells (Figure 14, Figure 15 and Figure 16) were also mountain-like, but generally had narrower peak region and an extended left shoulder. Tall spikes, approximately 9 in number, projected from the tops and sides of the peak region, while numerous minute spikes radiated from the shoulder of the curves. There was a high degree of resemblance between the cytoplasmic positions of the highest pixel intensity reference points for the cells of this category. The mean pixel intensity values for the images were also variable (P < 0.05).
Curves of the intermediate neoplastic cells were of two broad types. The first type (Figure 17 and Figure 18) had close resemblance with the curves of the small neoplastic cells, except for the longer left shoulder of the former. The positions of the highest pixel intensity points for this curve-type cells were similar at the cytoplasmic regions. The second curve-type (Figure 19) was characterized by two peak regions which flanked a trough-like region with numerous minute spikes. The mean pixel intensity values for the images of this category of cells were also variable (P < 0.05).
Curves of the large neoplastic cells varied between mountain-like type with narrow peak region and extended left shoulder (Figure 20 and Figure 21), to the type that was simply mountain-like with broader peak region (Figure 22). The curve in Figure 21 appears to be a slight variant of that in Figure 20, in that its left shoulder is sigmoid-like. The minute spikes present at the left shoulder of the curve in Figure 20 are replaced by taller spikes in the curve of Figure 21. The broader-peak curve-type (Figure 22) contains approximately 11 tall spikes at the top of the peak region, while smaller spikes radiate from the steep right shoulder. The mean pixel intensity values for the images of this category of cells were also variable (P < 0.05). The positions of the highest pixel intensities also corresponded quite well at cytoplasmic areas of the cells of the category.
The nucleic acid distribution patterns observed for the cells in the present investigation clearly supports the existence of three lymphocytic neoplastic cell types – small, intermediate and large cells, for the canine acute lymphocytic leukemia, contrary to previously held view on the existence of only small and the large types. Note that cells of the chronic lymphocytic leukemia closely resembled the small neoplastic cells of the acute lymphocytic leukemia, except for the clustered arrangement of the cells of the former. The slightly modified Von Bertalanffy's methodology  for the acridine orange-stained fluorescence microscopy procedure is a well accepted and reliable method for differentiation of nucleic acids within cells. The stain has been shown to have high affinity for nucleic acids by intercalating with DSNA molecules and by binding electrostatically with SSNA molecules, respectively emitting yellow-green and red color fluorescence [8–10]. The choice of the Von Bertalanffy's method over other methods, such as Jahanmehrs's , stems from the issue of specificity. The alcohol fixation process with Von Bertalanffy's method leaches-out cellular inclusions such as lysosomal and secretory contents that can stain positively with acridine orange, while Jahanmehrs's method which does not involve fixation step enables cellular inclusions to also fluoresce.
The three cell-type paradigm for the canine acute lymphocytic leukemia is supported by both morphologic staining attributes as well as digital analytical features of the cells. Small, intermediate and large lymphocytes have also been demonstrated in cytological preparations of normal tissue . With respect to morphologic attributes, the sizes of the respective small, intermediate and large neoplastic cells were 7 to 8 μm, 11 to 12 μm, and 18 to 20 μm. The small neoplastic cells, in addition to having high nuclear-cytoplasmic ratio, were characterized by presence of minute irregularly-shaped cytoplasmic processes, while the larger intermediate cells with more cytoplasm had more chunky, variably-sized, cytoplasmic processes (Figure 4 and Figure 6). Clumps of cytoplasmic and nuclear SSNA fluorescence were quite pronounced in both small and intermediate neoplastic cells; however, the presence of few non-fluorescing cisternal structures within the cytoplasm of the latter cells is a distinguishing feature. Nuclear DSNA fluorescence in the small and intermediate cells was similar, and characterized by diffuse areas of yellow-green fluorescence. The distinguishing morphologic features of the large neoplastic cells, apart from the larger size, bordered mostly on the more diffuse nature of the abundant cytoplasmic SSNA fluorescence, as well as presence of prominent cytoplasmic non-fluorescing cisternal structures (Figure 4 and Figure 5). The large neoplastic cells also had a smoother cytoplasmic outline virtually devoid of processes, while its nuclear fluorescence pattern was more of diffuse dull-green DSNA type with broad areas of clumpy interconnected SSNA red fluorescence. The observed non-fluorescence cisternal structures in the intermediate and large neoplastic cells appear similar to fibrillar inclusions reported for lymphoma and leukemia cells [13, 4], and also similar to filaments of plasma cells of human solitary myeloma . These earlier reports suggested possible secretory functions for the structures.
In comparison with the normal lymphocytes, which are mostly characterized by a distinct pattern of nuclear diffuse yellow-green DSNA fluorescence, alongside punctate interconnected narrower clumps of red SSNA fluorescence, the overall nucleic acid distribution pattern of the neoplastic cells is highly indicative of aggressive metabolic activity. The existence of prominent cytoplasmic and nuclear clumps of SSNA fluorescence within the neoplastic cells compared with the fewer and narrower SSNA fluorescence of the normal cells suggest a greater degree of metabolic activity in the neoplastic cells. Since the SSNA fluorescence is most likely due to increased RNA content, which in turn reflects higher level of nuclear DNA transcriptional activity, it can be argued that the neoplastic cells are generally more transcriptionally active compared to the normal cells. Also of immense interest is the observation of SSNA-fluorescent cytoplasmic processes and chunks in the small and intermediate cells. These processes, which are more chunky in the intemerdiate cells, appear to be extruding cytoplasmic fragments laden with nucleic acid capable of bringing about induction of normal lymphocytes into neoplastic cells. This notion is supported by the fact that some earlier reports indicate increases of the neoplastic cells in chronic lymphocytic leukemia through accumulation rather than proliferation of the cells [3, 15–17]. These earlier reports, however, suggest increase life span and decreased apoptosis as probable reasons for the noticeable build-up of cells in terms of number rather than our observation which suggests an inductive process whereby normal appearing lymphocytes are converted to neoplastic cells through nucleic acid materials extruded from the neoplastic cells and possibly internalized through endocytosis by the normal cells. The lack of observation of fluorescing patterns attributable to mitotic divisional process also buttresses this view point. Although canine lymphocytic leukemia has been successfully transplanted into fetuses by means of intact cells in the past [2, 18], it may be possible to carryout such transplants through the use of cell-free extracts containing cytoplasmic chunks of nucleic acids. The non-fluorescing cisternal structures, observed in the cytoplasm of the intermediate and large neoplastic cells, appear to be secretory structures of non-nucleic acid substances. Such substances may also be of inductive relevance, since normal appearing lymphocytes tended to be in close contact with the large neoplastic cells, especially at the cytoplasmic regions containing the cisternal structures (Figure 5).
Digital analytical attributes for both normal and the neoplastic cells, in terms of the features of the generated curves of frequency distribution of pixel intensity values, further explain the uniqueness of the cell-types as well as their metabolic states. Within each cell-type, the curve outlines tended to have a fair degree of resemblance, even though their mean pixel intensity values obtained from 135 clusters of pixel population at the highest pixel intensity points manifested variability through the ANOVA test, α = 0.05. The variability of the mean pixel intensity values within cell-types simply reflects variable concentration of nucleic acids which in turn denotes variable states of metabolic activity of the cells, while semblance of their curves of frequency distribution of pixel intensity values reflect similarity in nucleic acid distribution pattern in each cell-type. Of particular interest is the fact that there was a high degree of correspondence of the positions of highest pixel intensity points among cells of the same class. The cells that manifested greater similarity with respect to the positioning of the highest pixel intensity points apparently showed greater curves similarities (Figure 10 and Figure 11, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18). Subsequently, curve-shape variability as well as variability in the positions of highest pixel intensity points were mostly recorded for the large neoplastic cells (Figure 20, Figure 21 and Figure 22).
Among the normal cell-type, the corresponding positions of highest pixel intensity values were at nuclear SSNA-fluorescent points (Figure 10, Figure 11, Figure 12 and Figure 13). Those of the small neoplastic cells corresponded at cytoplasmic SSNA-fluorescent points (Figure 14, Figure 15 and Figure 16), while those of the intermediate neoplastic cell-type corresponded at both cytoplasmic SSNA-fluorescent points (Figure 17 and Figure 19), and nuclear SSNA-fluorescent points (Figure 19); their cytoplasmic SSNA-fluorescent points being quite close to the non-fluorescing cisternal structures. Those of the large neoplastic cell-type fairly corresponded at cytoplasmic SSNA-fluorescent points, close to the non-fluorescing cisternal structures (Figure 20, Figure 21 and Figure 22). Since cytoplasmic SSNAs are most likely to be RNA molecules in the form of ribonucleoproteins of ribosomes or m-RNAs and t-RNAs, their abundance within the three neoplastic cell-types compared to the normal lymphocytes with mostly nuclear SSNA lends supports to the concept of higher metabolic activity of the neoplastic cells vis-à-vis the normal cells. Based on the level of similarity of the positions of the highest pixel intensity points, as well as semblance in the outline of the curves of frequency distribution of pixel intensities, it may be possible to classify cells in general through such attributes. Classification of cells, based upon the uniqueness of their curves of frequency distribution of pixel intensities of nucleic acid fluorescent images, as well as concordance of points of highest pixel intensity values, could offer a cost-effective means for determination of changes between normal cells and their neoplastic counterparts. The rationale for this proposition stems from the fact that certain recognized biomarkers, such as DNA methylation reactions, simply reflect epigenetic changes on the genomic apparatus of the cell . Since epigenetic hypermethylation and hypomethylation respectively results in a tighter (heterochromasia) and loosened (euchromasia) states of the chromatin, as reported in the literature [20–22], it seems likely that variation of the curves of pixel frequency distribution of the nucleic acid and concordance of highest points of pixel intensity values can be relied upon as an indicator or biomarker for the cellular changes. Such simple biomarker system can be applied towards cost-effective, routine diagnoses and prognostic monitoring of therapeutic interventions of neoplastic conditions in general. It is to be noted, however, that our findings at this stage are preliminary and still speculative; howbeit they could form reasonable basis for further work to be pursued for development of a reliable and cost-effective diagnostic and prognostic biomarker procedure for neoplastic cells based on their nucleic acid distribution pattern.
Blood samples were collected from a total of 7 dogs, 3 of which were clinically normal with no history of any neoplastic condition, while 2 of the dogs were clinically confirmed for acute canine lymphocytic leukemia, and the remaining 2 for chronic lymphocytic leukemia. The leukemia cases were independently presented to our Veterinary Teaching Hospital at separate occasions. Diagnosis was based on observed clinical signs, history, and hematological features of the lymphocytes in the smears stained with Wright-Giemsa stain. Thin blood smears (6 slides per dog) were prepared; of the 6 slides smears per animal, half was processed for staining according to the standard Wright-Giemsa method (Benjamin, 1978), while the remaining half was stained with acridine orange (AO) stain for fluorescence microscopy viewing. Prior to staining, smears were air-dried, at room temperature, for at least 24 hrs in a dust-free incubator compartment, and subsequently fixed in 95% ethanol for 2 minutes.
AO Staining Procedure
The AO staining procedure was performed according to a slightly modified Von-Bertallanfy et al. method (1958) for viewing with a Riechert epifluorescence microscope, interfaced with a high resolution megapixel Olympus DP12 digital image acquisition camera. The slightly modified Von-Bertallanfy et al. method involved hydration of the smears in distilled water for 5 seconds, brief rinsing in 1% acetic acid, brief rinsing in distilled water, and staining in 0.01% AO solution (pH 6.0, color index 788) for 3 mins. The stained smears were rinsed in 0.06 M phosphate buffer (pH 6.0) for 1 min, and air-dried in a dark dust-free compartment. Fluorescence microscopy viewing was accomplished with the aid of the epifluorescence microscope, with a mercury lamp source generating blue-spectrum excitation incidence beam.
Image Digitalization and Analysis
All acquired 24 bits BMP fluorescent images were pre-processed by conversion into 8-bit grayscale images, with the aid of Corel Photo Paint® standard graphic processing software. These pre-processed images were saved as uncompressed tagged image format (TIF). Further processing of the 8 bits grayscale TIF images was accomplished with the aid of a standard digital imaging software called TimWin®. The algorithms of the software collectively implement functions ranging from cellular logic bit plane pixel manipulations, to lookup table (LUT) color mapping operations of the pixels.
Digital imaging analytical procedure involved determination of spot pixel intensity values of the 8-bit gray tone images, through direct placement of the cursor over the image, and subsequent subtraction of background pixel intensity values (noise). An addition of a constant pixel intensity value equal to the difference between the maximum image pixel intensity value, following the background pixel value subtraction, and a set reference pixel intensity value of 100 was performed. Thus all processed images, free of background pixels or noise, had a maximum pixel intensity value of 100. The segmented images were captured by means of the Corel Capture utility®. Curves of frequency distribution of pixel intensity values of representative neoplastic and normal lymphocytes were generated with the aid of the TimWin® software, and comparatively evaluated. Details on the logistics of the digital analytical method are available in our earlier reports [23, 24].
Statistical analysis of the mean pixel intensity values of pixel population around the highest pixel reference value of 100, for the processed images, was performed by one-way analysis of variance (ANOVA) test, employing standard computer software statistical module. The mean pixel values of an equal count (n = 135) of pixel intensity values of 4 (normal lymphocytes) and 3 images for each neoplastic cell-type were used in the statistical analysis.
The authors are grateful to The University of the West Indies, St. Augustine Campus, for provision of grants for that made this investigation possible. We also acknowledge Mr. Ekkers and his team of Software Developers in the Netherlands for support with the digital imaging software modules.
- Kornberg RD, Lorch Y: Twenty-five years of the nucleosome, fundamental particle of the eukaryote chromosome. Cell. 1999, 98: 285-294. 10.1016/S0092-8674(00)81958-3.View ArticlePubMedGoogle Scholar
- Owen LN: Serial transplantation of canine lymphocytic leukemia. Eur J Cancer. 1971, 7: 525-528.View ArticlePubMedGoogle Scholar
- Workman HC, Vernau W: Chronic lymphocytic leukemia in dogs and cats: the veterinary perspective. Vet Clin North Am Small Anim Pract. 2003, 33: 1379-99. 10.1016/S0195-5616(03)00120-7.View ArticlePubMedGoogle Scholar
- Chapman AL, Bopp WJ, Torres J, Cohen H: An electron microscopic study of the cell types in canine lymphoma and leukemia. J Comp Pathol. 1981, 91: 331-40. 10.1016/0021-9975(81)90002-5.View ArticlePubMedGoogle Scholar
- Cohen H, Chapman AL, Ebert JW, Bopp WJ, Gravelle CR: Cellular transmission of canine lymphoma and leukemia in beagles. J Natl Cancer Inst. 1970, 45: 1013-23.PubMedGoogle Scholar
- Urashima M, Hoshi Y, Akatsuka J: Analysis of leukemic cell activity with simple acridine orange staining method. Int J Hematol. 1992, 56: 197-204.PubMedGoogle Scholar
- Von Bertalanffy L, Masin M, Masin F: A new and rapid method for diagnosis of vaginal and cervical cancer by fluorescence microscopy. Cancer. 1958, 11: 873-87. 10.1002/1097-0142(195809/10)11:5<873::AID-CNCR2820110503>3.0.CO;2-B.View ArticlePubMedGoogle Scholar
- Kapuscinski J, Darzynkiewicz Z, Melamed MR: Luminescence of solid complexes of acridine orange with RNA. Cytometry. 1982, 2: 201-211.View ArticlePubMedGoogle Scholar
- Kapuscinski J: Interaction of nucleic acid with fluorescent dyes: spectral properties of condensed complexes. J Histochem Cytochem. 1990, 38: 1323-1329.View ArticlePubMedGoogle Scholar
- Zelenin AV: Acridine orange as a probe for molecular and cell biology. In Fluorescent and Luminescent Probes for Biological Activity. A Practical Guide to Technology for Quantitative Real-Time Analysis. Edited by: Mason WT. 1993, Academic Press, London, 83-99.Google Scholar
- Jahanmehr SAH, Hyde K, Geary CG, Cinkotai KI, Maciver JE: Simple technique for fluorescence staining of blood cells with acridine orange. J Clin Pathol. 1987, 12: 926-9. 10.1136/jcp.40.8.926.View ArticleGoogle Scholar
- Banks WJ: Introduction to exfoliative cytology. Applied Veterinary Histology. Edited by: Stamathis G. 1986, Williams & Wilkins, Baltimore, London, Los Angeles, Sydney, 556-Google Scholar
- Regan SRS, Calvert RC, Vitols K: Fibrillar bundles in canine lymphoma: An ultrastructural study. J Ultrastruct Res. 1971, 36 (3): 425-436. 10.1016/S0022-5320(71)80115-6.Google Scholar
- Udoji WC, Frigg AF: Cytoplasmic fibrils in plasma cells of solitary myeloma. Am J Clin Pathol. 1978, 70: 836-40.PubMedGoogle Scholar
- Kipps TJ: Chronic lymphocytic leukemia and related diseases. Williams' Hematology. Edited by: Beuler E, Lichtman Ma, Coller BS, Kipps TJ. 1995, New York: McGraw-Hill, 1017-39.Google Scholar
- Benjamin MM: Outline of Veterinary Clinical Pathology. 1978, Iowa State University Press, Ames, Iowa, USA, 27-37. 3Google Scholar
- Schriever F, Huhn D: New directions in the diagnosis and treatment of chronic lymphocytic leukemia. Drugs. 2003, 63: 953-969. 10.2165/00003495-200363100-00003.View ArticlePubMedGoogle Scholar
- Owen LN, Nielsen SW: Transplantation of canine lymphosarcoma. European Journal of Cancer. 1968, 4: 391-3.View ArticlePubMedGoogle Scholar
- Lippman Z, Gendrel AV, Black M, Vaughn MW, Dedhia N, McCombie WR, Lavine K, Mittal V, May B, Kasschau KD, Carrington JC, Doerge RW, Colot V, Martienssen R: Role of transposable elements in heterochromatin and epigenetic control. Nature. 2004, 430: 471-6. 10.1038/nature02651.View ArticlePubMedGoogle Scholar
- Jenuwein T, Allis CD: Translating the histone code. Science. 2001, 293: 1074-1080. 10.1126/science.1063127.View ArticlePubMedGoogle Scholar
- Vermaak D, Ahmad K, Hernikoff S: Maintenance of chromatin states: an open-and-shut case. Curr Opin Cell Biol. 2003, 15: 266-74. 10.1016/S0955-0674(03)00043-7.View ArticlePubMedGoogle Scholar
- Martienssen RA, Colot V: DNA methylation and epigenetic inheritance in plants and filamentous fungi. Science. 2001, 293: 1070-1074. 10.1126/science.293.5532.1070.View ArticlePubMedGoogle Scholar
- Isitor GN, Thorne R: Comparison between nuclear chromatin patterns of digitalized images of cells of the mammalian testicular and renal tissues: An imaging segmentation study. Comput Med Imaging Graph. 2007, 31: 63-70. 10.1016/j.compmedimag.2006.10.005.View ArticlePubMedGoogle Scholar
- Isitor GN, Asgarali Z, Pouching K: Nucleic acid distribution pattern in avian erythrocytes and mammalian lymphocytes: Comparative studies by fluorescence microscopy and digital imaging analytical techniques. Res Vet Sci. 2008, 85: 418-432. 10.1016/j.rvsc.2008.03.004.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.