The development of smooth‐surfaced endoplasmic reticulum in adrenal cortical cells of fetal guinea pigs

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Abstract
In most steroid‐secreting cells smooth‐surfaced endoplasmic reticulum is prominent. This organelle is a site of major enzymes involved in steroid biosynthesis. In fetal guinea pig adrenal cortical cells the development of tubular smooth‐surfaced reticulum precedes all other apparent morphological changes at the cellular level, serving as the earliest indicator of structural differentiation. Its appearance coincides with the earliest evidence of steroid production by the adrenals (Ortiz et al., 1966). Further development and differentiation of this organelle in inner cortical cells gives rise to complex paracrystalline arrays. Subsequent segregation of cells containing these arrays to the deeper cortical regions, the deep zona fasiculata and zona reticularis, heralds the histological zonation of the cortex into three definitive regions. In the outer cortical cells of the developing zona glomerulosa, cisternal reticulum becomes prominent and in late fetal life large areas of the cisternal membranes become smooth‐surfaced. Combined with information from bioassay experiments of Price and coworkers (Ortiz et al., 1966) the early appearance of the smooth reticulum supports the thesis that the adrenal is capable of steroid biosynthesis early in fetal development, before zonation of the cortex occurs.As the endoplasmic reticulum develops, polyribosomes become widely spaced on its surface. They may be involved in the synthesis and maintenance of the expanding smooth‐surfaced reticulum and its associated steroidogenic enzymes, analogous to the development of smooth endoplasmic reticulum and associated enzymes in rat hepatocytes (Dallner et al., 1966a,b).The gradual confinement of cells containing paracrystalline arrays of smooth reticulum to the deep cortical zones implies functional significance for this compartmentalized form of endoplasmic reticulum.