Podocyte number and density changes during early human life

Abstract

Background: Podocyte depletion, which drives progressive glomerulosclerosis in glomerular diseases, is caused by a reduction in podocyte number, size or function in the context of increasing glomerular volume.

Methods: Kidneys obtained at autopsy from premature and mature infants who died in the first year of life (n = 24) were used to measure podometric parameters for comparison with previously reported data from older kidneys.

Results: Glomerular volume increased 4.6-fold from 0.13 ± 0.07 μm³ x10⁶ in the pre-capillary loop stage, through 0.35 μm³ x10⁶ at the capillary loop, to 0.60 μm³ x10⁶ at the mature glomerular stage. Podocyte number per glomerulus increased from 326 ± 154 per glomerulus at the pre-capillary loop stage to 584 ± 131 per glomerulus at the capillary loop stage of glomerular development to reach a value of 589 ± 166 per glomerulus in mature glomeruli. Thus, the major podocyte number increase occurs in the early stages of glomerular development, in contradistinction to glomerular volume increase, which continues after birth in association with body growth.

Conclusions: As glomeruli continue to enlarge, podocyte density (number per volume) rapidly decreases, requiring a parallel rapid increase in podocyte size that allows podocyte foot processes to maintain complete coverage of the filtration surface area. Hypertrophic stresses on the glomerulus and podocyte during development and early rapid growth periods of life are therefore likely to play significant roles in determining how and when defects in podocyte structure and function due to genetic variants become clinically manifest. Therapeutic strategies aimed at minimizing mismatch between these factors may prove clinically useful.

Keywords: Glomerular maturation; Glomerular volume; Glomerulosclerosis; Podocyte; Podocyte density.

Fig. 1

Relationship of body weight to gestational and post-gestational ages. a Birth weight in relation to gestational age. The solid lines show the 10th and 90th percentiles for normal intrauterine growth. The cohort conformed to expected values for intrauterine growth. Those that survived <1 week are shown as open diamonds. Those that survived >1 week are shown by closed squares. There was no differences in the rate of intrauterine growth between those who died soon after birth and those who survived longer. b Body weight at death in relation to post-conceptional age. The 5th and 95th percentile range for the average value for boys and girls as shown by CDC growth charts is shown by the solid lines (assuming that normal gestation lasts 280 days). The cohort growth rates conformed approximately to the expected ranges. c Relationship between body weight and kidney weight at autopsy. Kidney weight was linearly related to body weight over the time period. d Faster growers versus slower growers. The plot is the same as in Figure 1b above. The solid line shows the 50th percentile growth curve for average boys and girls. The gray long-dashed line shows the correlation line between body weight and post-conceptional age for the cohort as a whole (R² = 0.83). These lines overlap, suggesting that on average the cohort represents an approximation of normal growth. To extend this analysis to determine to what effect slower versus faster growth would have on podometric measurements we compared a group that fell above the 50th percentile (“faster growers” [n = 6]) with a group that fell below the 50th percentile (“slower growers” [n = 8]) as shown by the box delineated by the dashed line. The comparison data are shown in Table 2. No statistically significant differences in podometric parameters were observed between the “faster” and “slower” growing groups, suggesting that differences in the growth rate within the range observed would not have a large impact on podometric parameters, although this would affect kidney weight

Fig. 2

Relationship of glomerular maturity to gestational age. Upper panel: GLEPP1 immuno-peroxidase was used to map the stages of glomerular development. All pre-capillary loop stage glomeruli were designated as “immature” development. All capillary loop stage and mature glomeruli were designated as “mature” development. Lower panel: the relationship of stage of development (as defined in the text) to gestational age. By 38 weeks’ gestation <10% of glomeruli were scored as at an “immature” stage of development

Fig. 3

Podometric estimations. Representative glomeruli at an “Immature stage,” mature stage (<38 weeks’ gestation) and mature stage (≥38 weeks’ gestation) are shown by GLEPP1 peroxidase staining (upper panels) and TLE4 red immunofluorescence (lower panels). Podocytes were identified by their having both GLEPP1-positive cytoplasm and TLE4-positive nuclei. Nonspecific TLE4 positive staining was observed in the interstitial compartment in some cases. Podocyte nuclear number, podocyte nuclear mean caliper diameter, GLEPP1-positive area and total glomerular tuft area were measured using software, as outlined in Materials and methods

Fig. 4

Changes in podometric parameters occurring with glomerular development and post-conceptional age. Left panels show a comparison of three stages of glomerular development including immature development, mature <38 weeks’ gestation stage and mature ≥38 weeks’ gestation stages. The right panels show individual data points for all mature stage glomeruli in relation to post-conceptional age. These data emphasize how glomerular volume increases as post-conceptional age increases and as glomerular development proceeds. In contrast, podocyte number increases from the immature stage to the capillary loop stage to reach a maximum averaging about 600 podocytes per tuft, but does not increase thereafter in association with continuing glomerular enlargement. Podocyte density, therefore, decreases with development and post-conceptional age requiring podocyte size to increase 1.8-fold between immature and mature (<38 weeks) and 2.6-fold by mature (≥38 weeks’ gestation). Values are mean ± SD. *p<0.05, **p < 0.01

Fig. 5

Changing podometric parameters throughout life. Podometric parameters for more mature glomeruli at <38 weeks and >38 weeks post-conception are shown in relation to previously published podometric parameters at 12.6, 35.5, 52.2, and 72.2 years of age. Glomerular volume can be seen to rapidly increase during the early growing stage of life and to continue to increase at a slower rate at later stages of adulthood. Podocyte number per tuft starts at about 600 and steadily decreases throughout life at a rate of 0.5% per year. Because of the very rapid increase in glomerular volume occurring during periods of rapid growth in childhood and adolescence, podocyte density decreases very rapidly, requiring a proportionate rapid increase in podocyte cell size. The continued glomerular volume increase and podocyte number decrease throughout adulthood mean that podocyte density decreases and requires podocyte volume to increase proportionately to cover the filtration surface area [7]