2.1.

Lensfree Video-Microscopy

Lensfree video-microscopy mentioned here implements in-line holographic imaging technique, which is explored extensively of late.^18–23 It consists of a 12-bit APTINA MT9P031 CMOS RGB imaging sensor with a pixel pitch of 2.2 μm, measuring $5.7\times 4.3\mathrm{mm}$, and light-emitting diode (LED) (dominating wavelength 525 nm) with a pinhole of 150 μm. In a typical experiment, the lensfree video-microscope is placed inside the incubator and the petri dish containing the cells is placed on lensfree video-microscope. Illumination is provided by the LED along with the pinhole from a distance of $\sim 5\mathrm{cm}$. The light scattered by the sample and the light passing directly from the source to the imaging sensor interfere to form a holographic pattern, which is recorded by the sensor. A USB cord connecting the laptop and the system passes through the provision at the rear of the incubator (Fig. 1).

Fig. 1

Schematic diagram (a) and photograph of lensfree video-microscope (b) that is used to obtain time-lapse images inside the incubator. Red arrows indicate lensfree video-microscopes placed inside a standard incubator (c).

2.2.

Cell Culture

Murine NIH3T3 fibroblasts were obtained from the American Type Culture Collection and were grown in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% newborn calf serum (PAN Biotech, Aidenbach, Germany) and 1% antibiotics (penicillin and streptomycin) (Gibco, Life Technologies, Saint-Aubin, France). Primary cultures of human fibroblasts prepared from skin biopsies performed on healthy donors (male, 30 to 39 years) were established in DMEM-Glutamax (Invitrogen, Life Technologies, Saint-Aubin, France) supplemented with 20% fetal calf serum (FCS) and used at passage 2. ${\mathrm{Nemo}}^{-/-}$ (seeding density: 30,000 cells) cells are SV40 immortalized human fetal female fibroblasts with a mutant Nemo gene (a gift from Dr. A. Smahi). Vero cells (a gift from F. Barre-Sinoussi’s lab) (seeding density: 26,000 cells) are kidney cells from Cercopithecus aethiops. BJ newborn foreskin fibroblasts are from the American Type Culture Collection. Cells (Nemo, Vero, 343, BJ) were grown in DMEM Glutamax plus 10% FCS.

2.3.

Comparison of Lensfree Video Proliferation Assay with Standard EdU Proliferation Assay

A standard 35-mm petri dish was filled with 2.5 ml of culture media at a cell density of $2.5\times {10}^4\text{cells}/\mathrm{ml}$. For cell incubation with EdU, cells were seeded on day 1 in DMEM supplemented culture media with 10% FCS. Cells were treated on day 2 with 10 μM EdU (Click iT® EdU Invitrogen) followed by 2.5-h incubation. The petri dish was imaged with lensfree video-microscope during EdU incubation period. Following the incubation period, EdU was removed through three washes in phosphate-buffered saline (PBS) and cells were transferred in tubes after trypsin (Gibco) treatment. According to manufacturer’s recommendation, cells were fixed in 1 ml 1% formaldehyde (Sigma-Aldricha, L'Isle d'Abeau, Chesnes, Saint-Quentin Fallavier, France). After 15 min, cells were washed once in PBS/1% bovine serum albumin (BSA). Then, cells were treated for 30 min with $1\times$ saponin solution for membrane permeabilization. After washing once in PBS containing 1% BSA, cells were centrifuged at 500 g for 5 min and supernatant was eliminated. Cells were incubated for 30 min at room temperature in the dark in presence of the click cocktail reaction buffer containing the fluorescent dye (Invitrogen). After washing and centrifugation at 500 g for 5 min, cells were harvested and stored at 4°C until fluorescence activated cell sorting (FACS) analysis.

2.4.

Inhibiting Cell Proliferation Using Actinomycin D

A standard 35-mm petri dish was filled with 2.5 ml of culture media at a cell density of $2.5\times {10}^4\text{cells}/\mathrm{ml}$. Cells were seeded in three petri dishes on day 1. On day 2, actinomycin D (ActD) was added at concentrations of $0\mu \mathrm{g}/\mathrm{ml}$ (untreated control), $5\mu \mathrm{g}/\mathrm{ml}$, and $10\mu \mathrm{g}/\mathrm{ml}$. The petri dishes were imaged simultaneously by three lensfree video-microscopes immediately following the administration of the drug for a period of 6.5 h. Similarly, three petri dishes were prepared in parallel adhering to the same protocol for EdU proliferation assay. EdU was added for the final 2.5 h without changing the concentration of ActD.

2.5.

Monitoring Cell Proliferation Kinetics

A standard 35-mm petri dish was filled with 2.5 ml of culture media at a cell density of $2.5\times {10}^4\text{cells}/\mathrm{ml}$. Cells were seeded on day 1. On day 2, cells were imaged using lensfree video-microscope from $\sim 4.5\mathrm{h}$ before the addition of ActD (at 2.5, 1, and $0.5\mu \mathrm{g}/\mathrm{ml}$) until $\sim 4.5\mathrm{h}$ following the addition of the drug.

2.6.

FACS Analysis

EdU-incorporated cells were analyzed using a BD LSR II two-laser flow cytometer (BD Biosciences, San Jose, California). The red laser (633 nm) is used for the detection of Alexa Fluor® 647. Sample measurements were performed with DIVA® software (BD Biosciences). Cell debris and aggregates were excluded from the analysis using an appropriate threshold ($\sim \mathrm{30,000}$).

2.7.

Computational Methods

For pattern recognition, a typical holographic pattern obtained from a dividing metaphase cell was chosen to act as a template. Using normalized cross-correlation function available in MATLAB® with a constant threshold, the template was matched with the full FOV image of $24{\mathrm{mm}}^2$, and the cells exhibiting similar holographic patterns were recognized and counted.

To estimate the total number of cells present in an image (dividing and nondividing), the obtained image was converted to its binary form. Using constant appropriate thresholding based on gray-level, area, major, and minor axis lengths, holographic patterns corresponding to cells were identified and counted.

Percentage of dividing cells is the ratio of total number of dividing cells to average total number of cells calculated from images obtained over a period of time (typically 2.5 h). A period of 2.5 h was chosen in order to be in concurrence with the EdU incubation period of 2.5 h.

In cases of highly confluent cultures, the typical holographic signature corresponding to metaphase might be masked and left undetected. In these cases, cell retraction was used as a way to detect dividing cells. Retracting cells exhibit a pattern where the center of the holographic pattern is bright, while the edges are dark (Fig. 2, images at $\mathrm{T}0+0\mathrm{h}$ 52 min and $\mathrm{T}0+0\mathrm{h}$ 54 min). By detecting these alternating bright and dark intensity regions in an image, retracting cells were identified (Video 1).

Fig. 2

Temporal images of interest from real-time monitoring of a BJ cell (Video 1) showing the change in cell shape and cell adhesion taking place during cell division. Cell rounding can be observed in images obtained at $\mathrm{T}0+0\mathrm{h}$ 52 min and $\mathrm{T}0+0\mathrm{h}$ 54 min. Reduced cell substrate adhesion leading to signature holographic pattern of a dividing cell can be seen in images $\mathrm{T}0+0\mathrm{h}$ 56 min and $\mathrm{T}0+1\mathrm{h}$ 18 min. Cytokinesis and separation of daughter cells can be seen from $\mathrm{T}0+1\mathrm{h}$ 56 min until $\mathrm{T}0+4\mathrm{h}$ 56 min. Diagonal (dotted) green line indicates the longest axis of the cell prior to cell division. Red line denotes the cell division axis during cytokinesis. Yellow line denotes the axis of separation of daughter cells. The cell in this case has divided along the longest axis prior to cell division. Scale bar: 100 μm.

Video 1

Changes in cell shape and cell adhesion during cell division from the perspective of lensfree video-microscope. Frames were obtained every 2 min for $\sim 5.5\mathrm{h}$. Cell rounding and reduced cell-substrate adhesion during cell division gives a typical holographic pattern, which is considered as the signature for mitotic cells. Cell retraction, cell rounding, reduction in cell-substrate adhesion, cytokinesis, and separation of daughter cells of a BJ cell is observed. Scale bar: 100 μm. (mov, 419 KB) [URL: http://dx.doi.org/10.1117/1.JBO.19.3.036004.1].

3.1.

Real-Time Monitoring of Cell Culture and Automated Detection of Dividing Cells

Lensfree video-microscope works using the principle of in-line holography (Sec. 2). The absence of magnification lenses renders large FOV of $24{\mathrm{mm}}^2$ covering several thousand cells. Lensfree video-microscope is placed inside a standard incubator (as shown in Fig. 1) to monitor cell culture in real time.

The automated detection of dividing cells using lensfree video-microscopy is based on the changes in shape and adhesion that mitotic cells undergo. Before dividing into two daughter cells, almost all mammalian cells undergo dramatic shape transformation from being flat during interphase to being round during M phase, typifying a process termed “mitotic cell rounding.” This is usually accompanied by a reduction in cell-substrate adhesion. A schematic drawing illustrating changes in cell shape and substrate adhesion during division is presented in Fig. 3(a). Upon entering mitosis, the complex actin network is completely deconstructed and re-formed.²⁴ Hence, at metaphase, during mitosis, the cells adopt a round shape and a decreased adhesion to the substrate.^25–29 This helps in efficient and stable bipolar spindle formation and is, thus, vital to ensure a proper cell division.³⁰ Almost all proliferating animal cells undergo these changes before cytokinesis. While extensive research has aimed to understand the underlying mechanism of the driving force(s) leading to cell rounding,^31–36 these changes have not been exploited as a signature of mitotic cells. By contrast, lensfree video proliferation assay exploits the change in cell shape and cell adhesion as a natural marker to detect dividing cells. Jin et al. also mentioned the changes in the holographic pattern corresponding to the changes in cell morphology during cell division,²² but it was not exhaustively studied.

Fig. 3

(a) Schematic drawing showing the change in cell shape during cell division typifying a process called mitotic cell rounding. (b) Cell rounding and reduction in cell-substrate adhesion preceding the separation of daughter cells is clearly observed in the hologram obtained from the NIH 3T3 cell at $\mathrm{T}=\mathrm{T}0+20\min$. The two daughter cells are seen at $\mathrm{T}=\mathrm{T}0+100\min$. Scale bar: 50 μm.

It is to be noted that the holographic pattern obtained from a floating cell is different from the holographic pattern obtained from an adherent cell. When the cell is not adhered to the substrate, the holographic pattern obtained from the cell is similar to an airy pattern. The zero-order gray-level value is lower and interference rings are observed. In contrast, for an adherent cell, the zero-order gray level reaches larger values and the holographic pattern obtained is different. These changes can, hence, be observed during cell division as there is reduction in cell-substrate adhesion. To exemplify the approach, a montage of the recorded holographic pattern of a dividing NIH3T3 cell observed using the lensfree video-microscope is shown in Fig. 3(b). At $\mathrm{T}=\mathrm{T}0$, the cell is elongated and adhered to the substrate with larger zero-order gray-level value. There is a sharp change in the holographic pattern obtained from the cell at $\mathrm{T}=\mathrm{T}0+20\min$. The zero-order gray level reaches a lower value denoting reduction in cell-substrate adhesion. The daughter cells are observed at $\mathrm{T}=\mathrm{T}0+100\min$. All the cells that experience rounding and reduction in cell-substrate adhesion during division exhibit a similar holographic pattern [as in Fig. 3(b), $\mathrm{T}=\mathrm{T}0+20\min$]. Thereafter, cells that are in the process of division are identified among several thousand neighboring cells by pattern recognition (FOV of the image $24{\mathrm{mm}}^2$) [Fig. 4(a)]. In some cases, the pattern corresponding to cell division may appear in subsequent temporal images. These repetitive events, though not counted, are marked by yellow squares [Figs. 4(a), 4(b), and 4(c)]. We have verified the accuracy of the automated detection of patterns by manually detecting the patterns and by following it in the subsequent temporal images to ensure the occurrence of cell division. Since true negatives cannot be determined in this case, we calculated the F1 measure (harmonic mean of precision and recall with equal weightage) based on true positives, false positives, and false negatives. True positives constitute accurate automated detection of patterns from cells that further divide in the subsequent temporal images. False positives constitute erroneous detection of patterns, which either were not from cells or were from cells that did not divide in the subsequent temporal images. False negatives are the cell divisions that were missed. The F1 measure is close to 0.87 on a scale of 0 to 1, with 1 being the best score. The measurement was based on seven random sequences of nine images that are temporally separated by 20 min, of dimension $1900\times 1425\mu \mathrm{m}$, from different independent experiments (Table 1).

Fig. 4

(a) Number of cells exhibiting the pattern corresponding to cell division is detected using pattern recognition in a full field of view image of $24{\mathrm{mm}}^2$, spanning across several thousand cells. New cell divisions are encircled in green, while repetitive cell divisions (cells that were round from the previous image) are marked in yellow squares. (b) Magnified region of interest showing cells that are encircled in green and marked in yellow squares among adherent (bright holographic pattern) neighboring cells. Scale bar: 50 μm. (c) Time-lapse images showing a cell exhibiting the pattern corresponding to cell division in subsequent temporal images (temporally separated by 20 min). Initially, the cell is encircled in green and later it is marked in yellow square. Scale bar: 50 μm.

Table 1

Validation of automated pattern recognition of dividing cells and automated cell count.

3.2.

Comparison of the Lensfree Video Proliferation Assay with Standard EdU Proliferation Assay

In order to validate the method, it was directly compared to the standard EdU proliferation assay. Cells were imaged using the lensfree video-microscope inside an incubator during the EdU incubation period of 2.5 h. The acquired images were subjected to pattern recognition. A total of nine images were acquired per experiment. The number of cells undergoing division was calculated for each image and was summed. In order to compare the results obtained with the two assays, the total number of dividing cells calculated with the lensfree video proliferation assay was divided by the average total number of cells that were present in the images to yield the percentage of dividing cells. Automated detection of the total number of cells (in dividing and nondividing cells) was based on binary conversion of the image and an appropriate threshold to remove noise. The number of cells from random images was manually calculated to verify the accuracy of automated detection (Table 1). Six independent experiments were performed. It was observed [Fig. 5(a)] that the proliferation rate (percentage of dividing cells) obtained using our method is $18\pm 5\%$ (s.d. $n=6$ experiments), whereas the proliferation rate obtained using EdU proliferation assay is $33.8\pm 6\%$ (for the EdU incubation period of 2.5 h). The difference in the results could be due to false positives from EdU proliferation assay or false negatives from lensfree video proliferation assay (or both). EdU proliferation assay detects the cells that are in the initial stage of cell division (S phase). False positives may occur in EdU proliferation assay if a cell that is marked during the S phase is stopped from dividing at the G2-M checkpoint [Fig. 5(b)] due to improper replication of DNA. Indeed the cells are allowed to pass through the checkpoints only after they have repaired DNA damages.^37–39 Lensfree video proliferation assay detects the cells that are in the final process of cell division (M phase) and is unbiased by the G2-M checkpoint. However, false negatives may occur if holographic signature corresponding to mitosis is not well detected. Also, the temporal resolution of the experiments performed was 20 min; this may have also limited the detection efficiency causing false negatives. Nevertheless, the standard deviation of the proliferation rate measurement over $N=6$ experiments is 5%, showing the consistency of the method comparable to that of EdU proliferation assay. Further, the impact of a cell proliferation inhibitor is clearly depicted in the results obtained by the method (Sec. 3.3). Though we cannot claim that the method is quantitative at this stage, it is qualitative and can monitor the differences in cell proliferation, which is essential in various studies, especially drug/siRNA screening.

Fig. 5

(a) Percentage proliferation rate measured by lensfree video proliferation assay and EdU proliferation assay. Data points obtained from six independent experiments are shown. The horizontal line in the plot marks the median. Error bar s.d., six independent experiments. (b) Cell cycle showing different checkpoints at various stages (marked by red arrow). EdU proliferation assay detects cells that are in the initial stage of division (S phase), whereas lensfree video proliferation assay detects cells that are in the final stage of cell division (M phase).

3.3.

Inhibiting Cell Proliferation Using Actinomycin D

The rate of cell proliferation may be altered by various stimulating or inhibiting conditions or agents. We further assessed our methods by measuring the influence of ActD, which is well known for inhibiting cell proliferation.^40,41 ActD was added to culture plates at predetermined concentrations of $0\mu \mathrm{g}/\mathrm{ml}$ (control), $5\mu \mathrm{g}/\mathrm{ml}$, and $10\mu \mathrm{g}/\mathrm{ml}$. Following the administration of the drug, the culture plates were imaged in parallel using three lensfree video-microscopes (as shown in Fig. 1) for 6.5 h. Images were acquired every 20 min, and the obtained images were subjected to pattern recognition to calculate the number of dividing cells. It is noteworthy that manipulation of the culture plates during the addition of the drug triggered the detachment of a few cells that also gave rise to a holographic pattern similar to the one corresponding to cell division. In order to avoid the interference of these floating cells in the calculation, the initial three images following the addition of the drug were not considered for measurement.

The number of dividing cells was calculated for a total of 324 images obtained from six independent experiments, with 108 images per condition (control, $5\mu \mathrm{g}/\mathrm{ml}$, and $10\mu \mathrm{g}/\mathrm{ml}$). As shown in the graphs (Fig. 6), the number of dividing cells was, on an average, between 30 and 40 per image for untreated cells, and a total of $625\pm 66$ (s.d. $n=6$ experiments) cells divided during the experiment time frame. On the contrary, the number of dividing cells was reduced to $<5$ per image for cell cultures treated with 5 and $10\mu \mathrm{g}/\mathrm{ml}$ of ActD, within the initial 3 h following the addition of the drug. Only $48\pm 17$ and $34\pm 15$ (s.d. $n=6$ experiments) cells divided in cell populations treated with the drug at concentrations of 5 and $10\mu \mathrm{g}/\mathrm{ml}$, respectively. Thus, the impact of the presence of the drug on the number of dividing cells could be demonstrated but with little or no variation of the effect at different concentrations of the drug.

Fig. 6

Number of dividing cells calculated for three different conditions from six independent experiments is plotted. Trendline of polynomial order 3 of mean is shown. The number of cell divisions in an image is between 30 and 40 in the case of control, whereas in the case of $10\mu \mathrm{g}/\mathrm{ml}$ actinomycin D (ActD) and $5\mu \mathrm{g}/\mathrm{ml}$ ActD, the number of cell divisions per image is reduced to $<5$ denoting a strong effect of the drug.

We verified the results obtained by lensfree video proliferation assay using EdU proliferation assay. The average percentage of dividing cells for the control was $45\pm 4\%$ (s.d. $n=6$ experiments) during the 2.5 h EdU incorporation time. On the complete contrary, average percentage population of dividing cells in the presence of the ActD (5 and $10\mu \mathrm{g}/\mathrm{ml}$) was close to zero (Table 2). Hence, similar to the results obtained by lensfree video proliferation assay, no difference could be observed between the two drug concentrations.

Table 2

EdU proliferation assay to verify the influence of actinomycin D (ActD).

3.4.

Monitoring Cell Proliferation Kinetics

The most expected contribution from any cell proliferation assay, and the least met with, is to give an accurate description of the kinetics of cell proliferation and its variations. By monitoring cell cultures before and after the addition of ActD, we are able to show how lensfree video proliferation assay meets this critical requirement. To give an accurate description of the kinetics of cell proliferation and its variations, influenced by different concentrations of ActD, cell cultures were monitored before (4.5 h) and immediately after the addition of ActD (for 4.5 h) at smaller concentrations (compared to previously used 5 and $10\mu \mathrm{g}/\mathrm{ml}$) of 2.5, 1, and $0.5\mu \mathrm{g}/\mathrm{ml}$ (Fig. 7). In the case of $2.5\mu \mathrm{g}/\mathrm{ml}$, the average total number of cell divisions (for a period of 4.5 h) before and after the addition of the drug were $433\pm 112$ and $63\pm 7$ (s.d. $n=4$), respectively. The average total number of dividing cells was reduced by $\sim 80\%$. As expected, when the concentration was further reduced to 1 and $0.5\mu \mathrm{g}/\mathrm{ml}$, the effect was less pronounced. The average total number of cell divisions before the addition for 1.0 and $0.5\mu \mathrm{g}/\mathrm{ml}$ were $310\pm 35$ and $327\pm 18$ (s.d. $n=4$), respectively. After the addition of the drug, the average total number of dividing cells was reduced by $\sim 50\%$ in both the cases ($128\pm 34$ and $131\pm 14$ for 1 and $0.5\mu \mathrm{g}/\mathrm{ml}$, respectively). These results demonstrate a rapid effect of ActD on cell proliferation at $2.5\mu \mathrm{g}/\mathrm{ml}$, compared to a more gradual effect at 1 and $0.5\mu \mathrm{g}/\mathrm{ml}$.

Fig. 7

Cell cultures subjected to different concentrations of ActD were monitored for a period of $\sim 9\mathrm{h}$ before and after the addition of ActD. Time $\mathrm{T}=0\mathrm{h}$ marks the addition of the drug. Trend line of order 4 of the mean number of cell divisions per image plummets immediately following the addition of the drug in the case of $2.5\mu \mathrm{g}/\mathrm{ml}$, whereas a more gradual decrease is observed in the cases of 1.0 and $0.5\mu \mathrm{g}/\mathrm{ml}$. In the case of $2.5\mu \mathrm{g}/\mathrm{ml}$, the average total number of cell divisions (for a period of 4.5 h) before and after the addition of the drug were $433\pm 112$ and $63\pm 7$ (s.d. $n=4$), respectively. The average total number of dividing cells was reduced by $\sim 80\%$. However, when the concentration was further reduced to 1 and $0.5\mu \mathrm{g}/\mathrm{ml}$, the effect was less pronounced. The average total number of cell divisions before the addition of the drug for 1.0 and $0.5\mu \mathrm{g}/\mathrm{ml}$ were $310\pm 35$ and $327\pm 18$ (s.d. $n=4$), respectively. After the addition of the drug, the average total number of dividing cells was reduced by $\sim 50\%$ in both the cases ($128\pm 34$ and $131\pm 14$ for 1 and $0.5\mu \mathrm{g}/\mathrm{ml}$, respectively). These results demonstrate a rapid effect of ActD on cell proliferation at $2.5\mu \mathrm{g}/\mathrm{ml}$ compared to the less intense effect at 1 and $0.5\mu \mathrm{g}/\mathrm{ml}$.

3.5.

Application to Other Cell Types Including Primary Cells

To test the versatility of the lensfree video proliferation assay, we followed the proliferation kinetics of other cell types: primary human fibroblasts (343), immortalized human fetal fibroblasts (${\mathrm{Nemo}}^{-/-}$), and Vero cells. Cells were allowed to adhere to the substrate for 4 h following cell plating. We calculated the percentage of cell division or proliferation rate. Percentage of cell proliferation is the ratio of number of dividing cells to average total number of cells calculated from images acquired over a period of 2.5 h. We followed the kinetics for over 12 h. A moving average of the percentage of dividing cells (with a period of $\sim 2.5\mathrm{h}$) was plotted. To be in accordance with our previous calculation of the percentage of dividing cells for NIH3T3 cells (in Sec. 3.2), 2.5 h was chosen as a period for moving graph. It could be inferred (Fig. 8) that the percentage of dividing cells for immortalized cell line ${\mathrm{Nemo}}^{-/-}$ is higher compared to Vero and primary human fibroblasts (343).

Fig. 8

The kinetics of cell division of three cell types, ${\mathrm{Nemo}}^{-/-}$ (immortalized fetal fibroblasts), Vero (kidney cells from Cercopithecus aethiops), and 343 (primary human fibroblasts), followed for a period of 16 h. Moving average (of the percentage of dividing cells) with a period of 2.5 h is depicted in the graph. Note that the percentage of dividing cells in immortalized ${\mathrm{Nemo}}^{-/-}$ cells is higher compared to Vero and primary human fibroblasts (343) cells. In the case of 343 cells, only very few cell divisions ($<5$) were observed in an image, and hence, the percentage is very low.

4.1.

Applicability of the Approach to Various Other Cell Types

We have tested the approach with various other cell types, such as Vero, U87, RPE1, RWPE1, primary human fibroblasts, mesenchymal stem cells, PC3, HUVEC, and MCF10A cells in both two-dimensional (2-D) and three-dimensional (3-D) cell culture conditions (Fig. 9) and shown that the method is applicable to numerous cell types in 2-D cell culture. Since the cells in 3-D cell culture do not show enhanced substrate adherence as in 2-D, the change in the holographic pattern is less pronounced. The approach is not applicable to floating cells, such as hematopoietic cells, but it is to be noted that majority of the cell types are adherent to the substrate in nature.

Fig. 9

Other cell types exhibiting similar holographic pattern during cell division: (a) MCF10A, (b) RPE1, (c) U87 cells, (d) RWPE1, (e) mesenchymal stem cells, (f) PC3, (g) HUVEC, (h) RWPE1 in three-dimensional cell culture. Scale bar: 50 μm.

4.2.

Implementing the Performance of the System

In the first version of the lensfree video-microscope, images could only be acquired at 20-min intervals. Because of overheating of the sensor, the microscope had to be switched off in between acquisitions to keep the cells from the exposure to heating of the sensor. During image acquisitions (which takes only a few seconds), a small fan installed was switched on to reduce the heating. In the more recent version of the lensfree video-microscope, a Peltier device was added to maintain ambient temperature (37°C) at the sensor surface, thus permitting to perform image acquisitions at will. As with the lensfree video-microscope, not any label or fluorescent reporter is needed, and because the system does not use lasers, there are no special concerns about phototoxicity for the cells or photobleaching of reporters. Using this device, we performed 1-min-interval image acquisitions. This permitted us to image various stages of cell division, such as cell retraction, cell rounding, reduction in cell-substrate adhesion, cytokinesis, and separation of daughter cells of a BJ cell (Fig. 2, Video 1). Hence apart from detecting dividing cells, analysis on cell division could be performed through label-free tracking of thousands of single cells from cell retraction until separation of daughter cells, providing an entirely new perspective on cell division that includes determination of time taken at each of the above-mentioned steps to investigate any delay in cell division, determination of cell division axis, etc. The cell division axis determines the fate of the daughter cells, and several parameters controlling the cell division axis, such as cell geometry⁴² and environment,⁴³ are being investigated.^44–46 Lensfree video-microscope can be used as a tool to determine the cell division axis in thousands of single cell divisions (Fig. 2). It can be observed that in the particular example in Fig. 2, the cell divides along the longest axis prior to cell division.

4.3.

Detection of Cell Division in Confluent Cell Cultures

In cases of highly confluent cell cultures, the pattern corresponding to cell division is not efficiently identified since the pattern, in some cases, is masked by the neighboring cells. However, by using temperature-controlled lensfree video-microscope to acquire images at 4-min intervals, we show that the identification of dividing cells becomes possible in highly confluent cell cultures by detecting cell retraction. Retracting cells exhibit a pattern where the center of the holographic pattern is bright, while the edges are dark (similar to Fig. 2, images at $\mathrm{T}0+0\mathrm{h}$ 52 min and $\mathrm{T}0+0\mathrm{h}$ 54 min). By detecting these alternating bright and dark intensity regions in an image, retracting cells were identified (Video 2). However, further criteria are to be added to minimize the false positives and false negatives, which would be a future perspective.

Video 2

Video of highly confluent RPE1 cells showing automated detection of dividing cells. Since the images were acquired at 4-min intervals, cell retraction was clearly observed in dividing cells. Hence cell retraction was detected (Materials and Methods) to detect dividing cells. The detected cells are boxed in white square. This method needs further validation, which would be a future perspective. Frames were acquired every 4 min for $\sim 2\mathrm{h}$. Scale bar: 50 μm. (mov, 516 KB) [URL: http://dx.doi.org/10.1117/1.JBO.19.3.036004.2].

For the work presented here, we could monitor in parallel up to three culture plates (35 mm diameter) with three lensfree video-microscopes having imaging sensor of dimensions $5.7\times 4.2\mathrm{mm}$. The throughput can be dramatically increased by multiplexing 96 smaller sensors to read 96-well plates,⁴⁷ which would be an ideal setting for drug screening applications. The method could also be exploited to monitor cell behavior in a bioreactor.