Biomedical Engineering Reference
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maps at three different cross-sections: 0.6
μ
m above the focused plane (top), at the focused
plane (middle), and 0.6
m below the focused plane (bottom). Although healthy RBCs
show homogeneous distribution of refractive index,
Pf
-RBCs are not optically
homogeneous. Many factors contribute to refractive index change: the vacuole of parasite
occupies a fraction of volume in cytoplasm of RBC; Hb is metabolized and converted into
hemozoin crystal in the parasite membrane; and various parasite proteins are exported from
parasite into cytoplasm of
Pf
-RBCs. Regions of low refractive index indicate the vacuole of
P. falciparum
(
Figure 12.14B
μ
D
, black arrows) and regions of high refractive index
suggest the position of hemozoin (
Figure 12.14C and D
, gray arrows).
Refractive index maps measured by TPM can be used to quantitatively investigate the Hb
content in
Pf
-RBCs. The refractive index is averaged over the cytoplasmic volume of
Pf
-RBC for 15 cells per group (
Figure 12.15A
). Their mean values are 1.399
6
0.006,
1.395
6
0.005, 1.383
6
0.005, and 1.373
6
0.006 for healthy RBCs, ring, trophozoite,
(A)
(B)
40
1.41
1.40
30
1.39
20
1.38
1.37
10
1.36
0
Healthy
Ring Trophozoite Schizont
Healthy
Ring Trophozoite Schizont
(C)
120
(D)
40
100
30
80
60
20
40
10
20
0
0
Healthy
Ring
Trophozoite Schizont
Healthy
Ring
Trophozoite Schizont
Figure 12.15
Host RBC Hb concentrations decrease as P. falciparum matures. (A) Refractive index of healthy
RBC and Pf-RBCs at the indicated stages. (B) Mean corpuscular Hb concentration (MCHC) of
healthy and Pf-RBCs at the indicated stages. (C) Cytoplasm volume of Pf-RBCs at the indicated
stages. (D) Hb content in healthy and Pf-RBCs at the indicated stages. Each point in
(A) represents average refractive index for one cell. Graphs in (B
D) show the median (central
horizontal line), standard deviations (box), and minimum and maximum values (vertical lines).
For each condition, 15 samples were tested
[26]
.
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