Oligohydramnios (OLIGO) causes fetal pulmonary hypoplasia and decreases lung weight, DNA content, and protein content. Architecturally, lungs that are subjected to OLIGO appear developmentally delayed with thicker septa. We believe that apoptosis is involved in the architectural maturation of the lung by aiding in the interstitial thinning characteristic of lung development in the last several days of rat gestation. We hypothesized that OLIGO would decrease apoptosis and contribute to the difference in the architecture of the lung parenchyma in developing rat lungs.

We produced OLIGO in fetal rats by puncturing the uterine wall and fetal membranes of one uterine horn on day 16 of gestation (term = 22 days) using a 22 Ga. needle. Position matched fetuses in the opposite uterine horn served as controls (CONTROL). On days 17 through 22, under general anesthesia, the fetuses were delivered by hysterotomy; lungs were removed and fixed immediately in paraformaldehyde. We studied 17 pairs of fetuses delivered in the last six days of gestation.

Fetal and lung weights confirmed that pulmonary hypoplasia (decreased lung weight as a percentage of total body weight) was produced with OLIGO compared to CONTROL (p = 0.005). Using in situ end labeling of DNA with fluorescein tagged biotinylated ATP (TUNEL reaction) and a fluorescence microscope at high power, we identified and counted apoptotic cells. We counted over 220,000 total cells, determined an apoptotic index (AI, AI = number of apoptotic cells / 1000 cells) for each of the fetuses, and compared the ratio of AI (OLIGO/CONTROL) for each pair of fetuses.

The AI in the CONTROL fetuses did not vary by gestation, and ranged from 1.86 to 4.01. The AI for OLIGO fetuses ranged from 1.01 to 4.13 and did change with gestation. The AI ratio OLIGO/CONTROL was <1 on days 17 and 18, and>1 on days 19-22, and the ratios on days 17 and 18 were statistically significantly different from the ratios on days 19 through 22 (p<.05). The effect was most apparent on day 17 when the AI ratio ranged from 0.31 to 0.84(mean = 0.57).

We conclude that oligohydramnios produced in the rat on day 16 of gestation acutely decreases apoptosis in fetal lungs for 48 hours, leading to delayed morphologic maturation, and that the subsequent trend towards increased apoptosis over the last four days of gestation may be a developmental“catch up” mechanism in these fetuses.