This work was supported by National Medical Research Council (NMRC)/Cooperative Basic Research Grant (CBRG)/0078/2014.

This study is in accordance with the National University of Singapore Institutional Animal Care and Use Committee (approved protocol no. R15-0225).
1. Surgical Preparation of the Animal Model
<ol>
	<li>Vessel cannulations
	<ol>
		<li>Anesthetize a male Sprague-Dawley rats (6 - 7 weeks old) weighing (203 &#177; 20) g with ketamine (37.5 mg/ml) and xylazine (5 mg/ml ) cocktail through intraperitoneal (i.p.) injection (2 ml/kg). Do not recap the needle or remove it from the syr...

<ol>
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M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integration+of+cardiovascular+regulation+by+the+blood/endothelium+cell-free+layer.">Integration of cardiovascular regulation by the blood/endothelium cell-free layer.</a> Wiley Interdiscip Rev Syst Biol Med. 3 (4), 458-470 (2011).</li><li>Ng, Y. C., Namgung, B., Leo, H. L., Kim, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Erythrocyte+aggregation+may+promote+uneven+spatial+distribution+of+NO/O+in+the+downstream+vessel+of+arteriolar+bifurcations.">Erythrocyte aggregation may promote uneven spatial distribution of NO/O in the downstream vessel of arteriolar bifurcations.</a> J Biomech. , (2015).</li><li>Ong, P. 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L., Popel, A. S., Intaglietta, M., Johnson, P. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+computer-based+method+for+determination+of+the+cell-free+layer+width+in+microcirculation.">A computer-based method for determination of the cell-free layer width in microcirculation.</a> Microcirculation. 13 (3), 199-207 (2006).</li><li>Soutani, M., Suzuki, Y., Tateishi, N., Maeda, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+Evaluation+of+Flow+Dynamics+of+Erythrocytes+in+Microvessels.">Quantitative Evaluation of Flow Dynamics of Erythrocytes in Microvessels.</a> Influence of Erythrocyte Aggregation. 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The measurement of CFL width is essential for a better understanding of the hemodynamics in the microcirculation. In particular, the measurement of CFL widths has been performed in mesenteric6, spinotrapezius24 and cerebral25 microcirculations. Conventional measurement of in vivo CFL widths was restricted to estimations by manual inspection of the recorded video frames. The manual measurements required the averaging of several successive video frames before visually identifying t...

In vivo animal studies are instrumental to basic science for understanding human physiology and pathology. In particular, in vivo microhemodynamic studies can elucidate the potential impairment of microcirculatory functions altered by abnormal rheological conditions of blood. A number of previous microhemodynamic studies1 have used the rat cremaster muscle model for visualizing microvascular blood flow. The cremaster muscle is a thin layer of striated muscle surrounding the testes. Thus, the blood flow in the muscle can be visualized with a trans-illumination microscope by means of surgical exposure. This enables us to acquire the in vivo blood flow images without the use of any fluorescence or contrast agents. In addition, the entire blood perfusion of the muscle network can be controlled by reducing the upstream blood flow with abdominal aorta occlusion2. Owing to these advantages, the cremaster muscle model has been widely used to investigate the formation of cell-free layer (CFL) in microvessels1,3. 
The CFL width is a prominent hemodynamic parameter in microcirculation, which has been of great interest for its important roles in regulating microcirculatory functions. The CFL is formed by the shear-induced transverse inward migration of red blood cells (RBCs) towards the flow center4. Consequently, this migration leads to the depletion of RBCs near the vessel walls, eventually resulting in a cell-free plasma layer. Accordingly, the parietal CFL naturally becomes a diffusion barrier to oxygen (O2) delivery from the RBC core to the tissues, and to the scavenging of nitric oxide (NO) by the RBCs5,6. In addition, the production of NO can also be modulated by the dynamic variations of the CFL width7,8. Therefore, the roles of the CFL in both gas transport and the regulation of homeostasis in microcirculation need to be fully ascertained to better understand blood flow in microcirculation. Recent studies have focused on bridging the hemodynamics and gas transport functions of the CFL in the microcirculation9-12. Furthermore, a separate set of studies has also investigated how the pathological elevation in RBC aggregation modulates CFL formation and its effect on O2 and NO bioavailability in tissues13,14. 
The roles of the CFL become more significant in microcirculation where the relative size of the CFL width to the vessel diameter is prominent. This necessitates an effective approach of quantifying the CFL in in vivo blood flow. Particularly, image acquisition and image analysis are the two key components determining the accuracy of CFL width measurement. Successful visualization of tissue blood flow should be preceded by an appropriate surgical preparation of the animal model. Additionally, a proper image analysis technique is needed to overcome the limitations of conventional manual measurements that are mostly induced by human errors15,16. With advancements in optical instrumentation and computing power for digital image processing, it is now possible to achieve a more accurate and consistent measurement of the CFL width17-19. Nonetheless, the accuracy of these measurements, being image-based, still ultimately depends on the quality of the images.
Therefore, this study explores the factors influencing the measurement of the in vivo CFL width. We focused particularly on demonstrating the surgical preparation and digital image analysis for measurements of the CFL width in arterioles of the rat cremaster muscle.

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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Intravital microscope</td>
			<td style="width:167px;">Olympus</td>
			<td style="width:119px;">BX51WI</td>
			<td style="width:253px;">Equipment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">High speed camera</td>
			<td style="width:167px;">Photron</td>
			<td style="width:119px;">1024PCI</td>
			<td>Equipment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Blue filter</td>
			<td style="width:167px;">HOYA</td>
			<td style="width:119px;">B390</td>
			<td>Equipment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Pressure sensor &#38;biopac system</td>
			<td style="width:167px;">Biopac system</td>
			<td style="width:119px;">TSD104A, MP100</td>
			<td>Equipment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Temperature controller</td>
			<td style="width:167px;">Shimaden</td>
			<td style="width:119px;">SR 1</td>
			<td>Equipment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Plasma Lyte A</td>
			<td style="width:167px;">Baxter</td>
			<td style="width:119px;">NDC:0338-0221</td>
			<td>Warm in 37 &#176;C water bath before use</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Saline 0.9%</td>
			<td style="width:167px;">Braun</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Heparin (5000 IU/ml)</td>
			<td style="width:167px;">LEO</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">PE-10 polyethylene tube</td>
			<td style="width:167px;">Becton Dickinson</td>
			<td style="width:119px;">427400</td>
			<td>.024&#34; OD X .011&#34; ID&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">PE-50 polyethene tube</td>
			<td style="width:167px;">Becton Dickinson</td>
			<td style="width:119px;">427411</td>
			<td>.038&#34; OD X .023&#34; ID</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">PE-205 polyethene tube</td>
			<td style="width:167px;">Becton Dickinson</td>
			<td style="width:119px;">427446</td>
			<td>.082&#34; OD X .062&#34; ID</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">2-0 non-absorbable silk suture</td>
			<td style="width:167px;">Deknatel</td>
			<td style="width:119px;">113-S</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">5-0 non-absorbable silk suture</td>
			<td style="width:167px;">Deknatel</td>
			<td style="width:119px;">106-S</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Water circulating heating pad</td>
			<td style="width:167px;">Gaymar</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Water bath</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:119px;">Isotemp 205</td>
			<td>Equipment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Sterile Cotton Gauze&#160;</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:119px;">22-415-468</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Cotton-tipped applicators</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:119px;">23-400-124</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Dumont Forceps</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS14188</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Micro Dissecting forceps</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS15915</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Iris forceps 1x2 teeth</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS15917</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Vessel cannulation forceps</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS500377</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Micro scissor</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS14177</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Iris scissor</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS14225</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Vessel clip</td>
			<td style="width:167px;">Kent Scientific</td>
			<td style="width:119px;">INS14120</td>
			<td>Surgical instrument</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Gemini cautery system</td>
			<td style="width:167px;">Braintree Scientific</td>
			<td style="width:119px;">GEM 5917</td>
			<td>Surgical instrument</td>
		</tr>
	</tbody>
</table>

visualization and quantification of the cell-free layer in arterioles of the rat cremaster muscle

The cell-free layer is defined as the parietal plasma layer in the microvessel flow, which is devoid of red blood cells. The measurement of the in vivo cell-free layer width and its spatiotemporal variations can provide a comprehensive understanding of hemodynamics in microcirculation. In this study, we used an intravital microscopic system coupled with a high-speed video camera to quantify the cell-free layer widths in arterioles in vivo. The cremaster muscle of Sprague-Dawley rats was surgically exteriorized to visualize the blood flow. A custom-built imaging script was also developed to automate the image processing and analysis of the cell-free layer width. This approach enables the quantification of spatiotemporal variations more consistently than previous manual measurements. The accuracy of the measurement, however, partly depends on the use of a blue filter and the selection of an appropriate thresholding algorithm. Specifically, we evaluated the contrast and quality of images acquired with and without the use of a blue filter. In addition, we compared five different image histogram-based thresholding algorithms (Otsu, minimum, intermode, iterative selection, and fuzzy entropic thresholding) and illustrated the differences in their determination of the cell-free layer width.

The visualization of the CFL in vivo is largely dependent on the surgical preparations of the animal. Excessive blood loss or extended surgery duration may subject the animal to shock and blood flow aberrations. Maintenance of tissue temperature using a heating pad as well as a customized platform during the surgery and experiment is also crucial for maintaining the physiological conditions of the rat. By using a 100 W halogen lamp in the microscope system, no discernible tissue damage was observed even at the e...

Watch this Scientific Journal Video about Visualization and Quantification of the Cell-free Layer in Arterioles of the Rat Cremaster Muscle at JoVE.com

Visualization and Quantification of the Cell-free Layer in Arterioles of the Rat Cremaster Muscle

This study demonstrates the surgical preparation of the rat cremaster muscle for the visualization of the in vivo cell-free layer. Considerable factors affecting the accuracy of the cell-free layer width measurement are discussed in this study.

The cell-free layer is defined as the parietal plasma layer in the microvessel flow, which is devoid of red blood cells. The measurement of the in vivo ...

The overall goal of this procedure is to quantify the spatiotemporal variations in the in vivo cell-free layer width. This method can help to answer key questions in the field of micro hemodynamics, to better understand the role of cell-philia in micro circulation. The main advantage of this method is that in vivo cell-philia width can be quantified more consistently and conveniently than previous manual measurement techniques, which were very time consuming.Before beginning the cell-free layer width measurement, run the cell-free layer pre-MatLab script file. Next click open file to select the video file for analysis and adjust the rotation slide to vertically align the walls of the single vessel using the zoom slide to adjust the zoom level as necessary. When the vessel is in the appropriate position, click confirm editing, then click set ROI to crop to define the region of interest.The aligned image will be displayed in a pop-up window. Adjust the rectangular objective on the image as necessary and double-click the objective to confirm the region of interest. Then click Extract Images to extract all of the edited video frames into consecutive bitmap images, which will be found in the folder with the same name as the selected video file.To measure the cell-free layer width, click Select Folder and click on the folder of images. The first image frame will appear in the grayscale image panel along with the corresponding gray intensity histogram in the image histogram panel. Select the desired image frame from the list box to perform the analysis and click Find Vessel Walls to identify the inner vessel wall in the image determined as the location where the light intensity profile peak transits from dark to light over two pixels.Check median filter to apply a median filter to the image to reduce the salt and pepper noise. Check auto-contrast for digital adjustment of the image intensities to enhance the image contrast. Then select a thresholding algorithm in the list box to automatically determine the thresholding value that divides the gray levels into two classes, white pixels with gray levels above the thresholding value, and black pixels with gray levels below the thresholding value.To measure the spatial variation of the cell-free layer widths, enter the pixel resolution in the pixel resolution box. Then click Calculate to obtain the spatial variation of the cell-free layer widths and click Export. csv to export the cell-free layer width data in a tabulated format.To measure the temporal variation of the cell-free layer widths at a specific analysis line along the vessel, click Temporal Variation and enter the frame-rate information. Enter the first and last frames of the images for the analysis in the start frame and last frame boxes respectively. Adjust the analysis line slide-bar to select the position of the analysis line along the vessel and confirm the position of the analysis line as illustrated on both the grayscale and binary images.Then click Calculate to obtain the temporal variation of the cell-free layer widths and click Export. csv to export the cell-free layer width data in a tabulated format. Here, a typical red blood cell flow through an unbranched arterial in the rat cremaster muscle where the cell-free layer can be observed between the RBC core and the inner vessel wall is shown.A good contrast between these components is critical for ensuring the accuracy of the cell-free layer width measurements. The initial phase of the image analysis involved the detection of the inner vessel wall. By acquiring the light intensity profile along the analysis line perpendicular to the vessel, the location is approximated at the peak that transits from dark to light over two pixels.Subsequently, the RBC core boundaries are detected using the image thresholding algorithm and the CFL widths can then be calculated. As red blood cells in the cell-free layer possess different light transmittances the difference in gray levels can be subdivided into two classes. However, the identification of an accurate threshold value between the two peaks in image histogram maybe restricted by poor image quality and contrast.To improve the contrast between the red blood cells and the cell-free layer, a blue filter can be used. This is even more evident in these images where the boundaries of the red blood cell cores were more accurately identified with a blue filter. The thresholding algorithm can also influence the cell-free layer width measurement as apparent in these images in which the different thresholding algorithms resulted in the identification of different red blood cell core boundaries and therefore different cell-free layer widths.The measurement of the in vivo cell-free layer width is very sensitive to the quality of images. Therefore, be sure to preform the surgery carefully and to use an appropriate optical assistant to obtain a good quality of image. Moreover, it is essential to select the appropriate image thresholding algorithm to ensure an accurate and consistent cell-free layer width measurement.

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Research

JoVE Journal

Biology

6.3K visualizações.  National University of Singapore. The overall goal of this procedure is to quantify the spatiotemporal variations in the in vivo cell-free layer width. This method can help to answer key questions in the field of micro hemodynamics, to better understand the role of cell-philia in micro circulation. The main advantage of this method is that in vivo cell-philia width can be quantified more consistently and conveniently than previous manual measurement techniques, which were very time consuming.Before beginning the cell-f...