Morphometric analyses of adrenal gland growth in fetal and neonatal sheep. III. Volumes of the major organelles within zona fasciculata steroidogenic cells.

We have undertaken a morphometric analysis of the distribution patterns of the major subcellular organelles of the zona fasciculata in fetal (53, 100, 130, 144 days) and neonatal (2 days) sheep as they may be expressed as volume densities, volumes per cell, volumes per 100 microns 3 of cytoplasm per cell, and volumes per mm3 of steroidogenic cells per gland. Data obtained indicate that certain of the volume densities and the 3 other volume indicators of the 6 organelles considered change with gestational age. While the volume of mitochondria per cell increased in late gestation because of cellular hypertrophy, the real time volumes of mitochondria per unit volume of cell or gland did not vary significantly during the period of development studied. The volume of the SER, however, changed markedly, being minimal at 100 days fetal development (0.68 gestation) and increasing, probably initially during the 123-130 day (0.84-0.88 gestation) period, until the end of the fetal period (term = 147 days). The SER exhibited the most marked variability in the three volume indicators, the volumes correlating closely with cell size and steroidogenic capability. The RER volumes altered little, decreasing per 100 microns 3 of cytoplasm from mid-gestation, probably as a reflection of the cell maturity state attained. Changes in the distribution of the Golgi apparatus paralleled those of the SER; but the lipid storage droplets were sparse and their volumes constant. In the fetal sheep, the Golgi apparatus appears to be a significant component of the steroidogenic organelle complex, while the lipid droplet compartment does not. Consideration of growth during the 2nd major growth phase of the adrenal cortex (from before 0.85 gestation), as expressed in mitotic events, showed that at 130 days (0.88 gestation) cellular replication was randomly spread through the zona fasciculata, whereas at 144 days (0.98 gestation) mitoses were twice as prevalent in the outer half of that zone as they were in the inner half. At 130 and 144 days, the cells of the outer half of the zona fasciculata contained significantly more smooth endoplasmic reticulum than did those of the inner half. The maturation of the zona fasciculata, which allows the maximal production of cortisol in late pregnancy, thus occurs in a centripetal direction. This may result from the fact that the outer cells of the zona fasciculata are first in contact with the rising levels of ACTH and cortisol, two primary determinants of its growth and differentiation during the last 3 weeks of gestation.