Remodeling of the fetal collecting duct epithelium

Abstract

Congenital urinary tract obstruction induces changes to the renal collecting duct epithelium, including alteration and depletion of intercalated cells. To study the effects of obstruction on the ontogeny of intercalated cell development, we examined normal and obstructed human fetal and postnatal kidneys. In the normal human fetal kidney, intercalated cells originated in the medullary collecting duct at 8 weeks gestation and remained most abundant in the inner medulla throughout gestation. In the cortex, intercalated cells were rare at 18 and 26 weeks gestation and observed at low abundance at 36 weeks gestation. Although early intercalated cells exhibit an immature phenotype, Type A intercalated cells predominated in the inner and outer medullae at 26 and 36 weeks gestation with other intercalated cell subtypes observed rarely. Postnatally, the collecting duct epithelium underwent a remodeling whereby intercalated cells become abundant in the cortex yet absent from the inner medulla. In 18-week obstructed kidneys with mild to moderate injury, the intercalated cells became more abundant and differentiated than the equivalent age-matched normal kidney. In contrast, more severely injured ducts of the late obstructed kidney exhibited a significant reduction in intercalated cells. These studies characterize the normal ontogeny of human intercalated cell development and suggest that obstruction induces premature remodeling and differentiation of the fetal collecting duct epithelium.

Figure 1

Classification of ICs. ICs and their functionally specific subtypes are identifiable by their expression of transport proteins, including the hydrogen ion transporter vATPase, ammonium transporter RhCG, and the chloride/bicarbonate exchangers pendrin and anion exchanger 1 (AE1).

Figure 2

ICs in the early human fetal kidney. ICs (arrowheads) in the 10-week gestation fetal kidney are restricted to the medullary ureteric bud (outline in A, magnified in B) and are not seen in the cortical ureteric ducts or in the ureteric tips of the nephrogenic zone (arrows). In the 18-week gestation fetal kidney, ICs are present in the medullary CD (C) and are seen rarely in the cortical CD (D). Stains: red, vATPase; green, RhCG; blue, DAPI. Scale Bars: A, 200 μm, B–D, 25 μm.

Figure 3

ICs abundance in the normal fetal and postnatal kidney. For each segment of the CD and for each gestational age, the abundance of ICs is expressed relative to the total number of CD cells. *The IC abundance of the inner medullary CD of the 1-year postnatal kidney was evaluated qualitatively.

Figure 4

ICs in the mid to late gestation human fetal kidney. In the 26-week kidney (A and B), type A ICs (arrowheads) predominate. Type B ICs (arrows) are first observed in the cortical CD at this age (B). In the 36-week gestation fetal kidney (C and D), a mix of type A (arrowheads), type B, and immature ICs is observed in both the outer medullary (C) and cortical (D) CD. Stains: red, vATPase; green, RhCG (A, C, and D) or pendrin (B); blue, DAPI. Scale Bars: A–D, 25 μm.

Figure 5

ICs in the postnatal human kidney. At 1 year postnatally, ICs were lost in the inner medullary CD (A) but persisted in the outer medullary CD with type A predominating (arrowheads in B). In the postnatal cortex (C and D), the abundance of ICs increased dramatically with type A (arrowheads), type B (arrows), and non-A non-B ICs (asterisk) present. Stains: red, vATPase (A–C) or pendrin (D); green, RhCG; blue, DAPI. Scale bars: A, 200 μm, B–D, 25 μm.

Figure 6

The connecting tubule. At 10 weeks gestation, rare nephron segments can be observed resembling the distal tubule, which expresses uniform apical vATPase expression, but which contained isolated cells with stronger vATPase expression characteristic of ICs (arrowheads in A). These ducts may represent the earliest formations of the distal nephron and specifically, of the connecting tubule. By 18 weeks, ICs (arrowheads in B) became more distinct. By 29 weeks (C), and through 1 year postnatal development (D), the connecting tubule (asterisk) can be clearly identified as a transition segment between the distal tubule (arrows) and the CD (outline). The connecting tubule appears to represent a progressive overlapping of the mesenchymally-derived distal tubule and ureteric bud-derived CD with ICs derived from the latter, as depicted in (E). Stains: red, vATPase; green, RhCG; blue, DAPI. Scale bars: A–B, 25 μm; C–D, 50 μm.

Figure 7

ICs in the early obstructed human fetal kidney. At 18 weeks gestation, an increase in IC fraction can be observed (A and B). Furthermore, unlike the normal 18-week gestation kidneys, where immature ICs predominate, differentiated type A (arrowheads) and type B (arrows) ICs can be observed in medullary CDs of the obstructed kidney. ICs are also observed in the cortical CD (asterisk) near the nephrogenic zone highlighting premature differentiation of ICs in the region (C). Stains: red, vATPase (A and C) or pendrin (B); green, RhCG; blue, DAPI. Scale bar: A, 100 μm; B, 25 μm; C, 50 μm.

Figure 8

ICs in the late gestation obstructed fetal kidney. As with the early gestation obstructed kidney, mildly affected CDs of the 36-week gestation obstructed fetal kidney (arrows in A, magnified in B) exhibit an increase in IC fraction. However, more severely dilated CDs displayed decreased IC abundance (asterisks in A, magnified in C). In the late gestation fetal kidney, injury and progressive dilation of the obstructed CD (measured as relative luminal surface area in pixels), correlated with a progressive decline in the relative number of intercalated cells in the CD (D; R2 = 0.255). Stains: red, vATPase; green, RhCG; blue, DAPI. Scale bars: A, 200 μm; B–C, 100 μm.

Figure 9

ICs in the postnatal obstructed fetal kidney. As with the 36-week kidney, some mildly affected CDs in the obstructed postnatal kidney displayed abundant ICs (arrows in A, magnified in B), whereas more severely affected, dilated CDs exhibited a depletion of ICs (asterisks in A, magnified in C). Correspondingly, dilated CDs exhibiting IC depletion (arrowheads) also demonstrated a significant decrease in aquaporin-2 expression compared with mildly affected CDs (arrow). Stains: red, vATPase; green, RhCG; blue, DAPI. Scale bars: A, 200 μm; B–D, 100 μm.