FIGURE 21.25 • Photomicrographs of a human fetal adrenal gland. a. Low-power micrograph of an H&E–strained section of a fetal adrenal gland. The permanent cortex (PC) is indicated in the upper portion of the micrograph. Below is the fetal zone (FZ ) in which the cells are arranged in anastomosing linear cords. Some of the capillaries (C) are engorged with red blood cells, thereby making them more apparent. 100. b. Higher-power micrograph of the same specimen showing the capsule (Cap) and the underlying permanent cortex. The cells are arranged in arched groups that extend into short cords. Note the close proximity of the nuclei and the small amount of cytoplasm in these cells. 200. c. This micrograph shows the cells of the fetal zone at the same magnification as in b. Note the slightly larger size of the nuclei and the considerable amount of cytoplasm in each of the fetal zone cells. Also note the eosinophilia of the cytoplasm, compared with the more basophilic cytoplasm of the cells of the permanent cortex. 200. (Original specimen courtesy of Dr. William H. Donnelly.)
The histologic appearance of the fetal adrenal gland is superficially similar to that of the adult adrenal gland. During late fetal life, most of the gland consists of cords of large eosinophilic cells that constitute approximately 80% of its mass. This portion of the gland, referred to as the fetal cortex (also called fetal zone), arises from the initial mesodermal cell migration. The remainder of the gland is composed of the peripheral layer of small cells with scanty cytoplasm. This portion, referred to as the permanent cortex, arises from the secondary mesodermal cell migration. The narrow permanent cortex, when fully established in the embryo, appears similar to the adult zona glomerulosa. The cells are arranged in arched groups that extend into short cords. They, in turn, become continuous with the cords of the underlying fetal zone (Fig. 21.25). In H&E preparations, the cytoplasm of the cells in the permanent cortex exhibits some basophilia; in combination with the closely packed nuclei, this gives this part of the gland a blue appearance, in contrast to the eosinophilic staining of the fetal zone.
With the TEM, the cells of the permanent cortex exhibit small mitochondria with shelf-like cristae, abundant ribosomes, and small Golgi profiles. The cells of the fetal zone, in contrast, are considerably larger and are arranged in irregular cords of varying width. With the TEM, these cells exhibit spherical mitochondria with tubular cristae, small lipid droplets, an extensive sER that accounts for the eosinophilia of the cytoplasm, and multiple Golgi profiles. Collectively, these features are characteristic of steroid-secreting cells.
The development of the fetal adrenal gland is a part of a complex process of maturation and preparation of the fetus for extrauterine life.
The fetal adrenal lacks a definitive medulla. Chromaffin cells are present but are scattered among the cells of the fetal zone and are difficult to recognize in H&E preparations. The chromaffin cells originate from the neural crest (see Fig. 21.19a) and invade the fetal zone at the time of its formation (see Fig. 21.19b). They remain in this location in small, scattered cell clusters during fetal life (see Fig. 21.19c).
The blood supply to both the permanent cortex and the fetal zone is through sinusoidal capillaries that course between the cords and join to form larger venous channels in the center of the gland. Unlike the postnatal adrenal, arterioles are absent in the parenchyma of the fetal adrenal gland.