The parenchyma of the pars distalis consists of two general cell types: chromophobes and chromophils. Chromophobes stain poorly; chromophils stain well. Chromophils are further subdivided into acidophils and basophils. Basophils stain with basic dyes or hematoxylin, whereas the cytoplasm of the acidophil stains with acid dyes such as eosin. The cytoplasm of basophils also stains with the periodic acid-Schiff (PAS) reaction because of the glycoprotein in its secretory granules.

Acidophils can be further subdivided into two groups on the basis of special cytochemical and ultrastructural features. One group, called somatotropes, produces the growth hormone, somatotropic hormone (STH); the other group of acidophils, called mammotropes or luteotropes, produces the lactogenic hormone, luteotropic hormone (LTH). The groups of basophils can also be distinguished with the electron microscope and with special cytochemical procedures. One group produces the thyroid-stimulating hormone (TSH); another produces the gonadotropic hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH); and a third group produces adrenocorticotropic hormone (ACTH) and lipotropic hormone (LPH). Chromophobes are also a heterogeneous group of cells. Many are considered to be depleted acidophils or basophils; one group of chromophobes, however, may produce ACTH.

Figure 1, pituitary, human, Mallory x360; inset X1200.

This photomicrograph of the pars distalis is from an area where there is an almost equal distribution of acidophils (A) and basophils (B). The clumps and cords of cells are delineated by strands of connective tissue (stained blue) that surround them. A number of engorged capillaries (Cap) containing red blood cells (stained yellow) are also seen. The acidophil cytoplasm in this preparation stains a reddish or rust color. The basophils stain a reddish blue to deep blue, and the chromophobes (C) exhibit a pale-blue color. The inset shows the three general cell types at higher magnification. The secretory granules of the acidophils (A) and basophils (B) are just discernable. It is the granules that stain and provide the overall coloration to the two cell types. In contrast, the chromophobe (C) lacks granules and simply reveals a pale-blue background color.

Figure 2, pituitary, human, H&E x325.

The neurohypophysis seen here contains cells called pi-tuicytes, and unmyelinated nerve fibers form the supraoptic and paraventricular nuclei of the hypothalamus. The pitu-icytes are comparable with neuroglial cells of the central nervous system. The nuclei are round to oval; the cytoplasm extends from the nuclear region of the cell as long processes. In H&E preparations such as this, the cytoplasm of the pituicyte cannot be distinguished from the unmyelinated nerve fibers. The hormones of the neurohypophysis, oxytocin and antidiuretic hormone (ADH) (also called vasopressin), are formed in the hypothalamic nuclei and pass via the fibers of the hypothalamohypophyseal tract to the neurohypophysis, where they are stored in the expanded nerve terminal portion of the nerve fibers. The stored neurosecretory material appears as Herring bodies (HB). In H&E preparations, the Herring bodies simply appear as small islands of eosin-stained substance. Interspersed among the nerve fibers are capillaries (Cap).

Figure 3, pituitary, human, PAS/aniline blue-black x250; inset x700.

In this specimen, the aniline blue has stained the nuclei of the pituicytes; the nerve fibers have taken up some of the stain to give a light-blue background. With this staining technique, the Herring bodies (HB) appear as the dark black islands. The inset shows the Herring body near the bottom of the micrograph at high magnification. The granular texture of the Herring body as seen here is a reflection of the accumulated secretory granules in the nerve terminals. Also of note in this specimen are the capillaries (Cap), which are prominent as a result of the contrasting red staining of the red blood cells within them.

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