Development of the adrenal cortex in the fetal foal: an ultrastructural study.

The adrenal cortex from twelve fetal foals (gestational ages from 61 to 300 days) was examined by light and electron microscopy. Adrenal glands from three newborn foals were also examined by light microscopy. Between 61 and 100 days of gestation the adrenal cortex became organised into two distinct regions, the zona glomerulosa and zona fasciculata, which grew steadily in thickness until the 300th day. Between 300 days and birth there was a dramatic increase in the width of the zona fasciculata. From 200 days a narrow band of compact cells marked the cortico-medullary border. Though these cells were ultrastructurally indistinguishable from those of the overlying zona fasciculata it seems possible that they may later give rise to the zona reticularis of the adult gland. The cells of the zona glomerulosa were columnar in shape and were tightly packed together into cell clusters. They contained smooth and rough endoplasmic reticulum and mitochondria with lamellar cristae. The ultrastructural characteristics of the cells of the zona glomerulosa changed little throughout the course of gestation. In the zona fasciculata the predominant cell type was angular in profile, and its cytoplasm contained smooth and rough endoplasmic reticulum and mitochondria with lamellar cristae. The second and larger cell type was situated nearer the cortico-medullary border. It contained an abundance of smooth endoplasmic reticulum, mitochondria with lamellar and/or vesicular cristae, and large perinuclear Golgi complexes. These cells, which appeared to be derived from the smaller more angular cells of the zona fasciculata, showed signs of a high level of metabolic and synthetic activity. As gestation progressed the number of functionally differentiated cells increased slowly in a radial direction towards the zona glomerulosa. Indirect evidence from two newborn foals suggests a rapid increase in the functional differentiation of zona fasciculata cells between 300 days and birth.