PITUITARY GLAND (Hypophysis)

Slide #198 - Mason's trichrome stain permits the ready distinction between connective tissue (blue) and other portions of the gland.   You should be able to distinguish the connective tissue capsule surrounding the gland and the septa which traverse the gland from capillaries containing rbcs (bright red) and from the various secretory components of the pituitary.

Anterior lobe (pars distalis) -The pars distalis is composed of two general cell types: chromophils (50%) and chromophobes (50%).    Chromophobes are smaller and paler than chromophils. The chromophils can be further subdivided into acidophils (40%) and basophils (10%). The acidophils (bright red) secrete GH (somatotropes) and prolactin (mammotropes). Basophils (bluish purple) secrete TSH (thyrotropes), LH (gonadotropes), FSH (gonadotropes), and ACTH (corticotropes). The different acidophils and basophils cannot be distinguished in the light microscope. Chromophobes are undifferentiated or resting chromophils that appear weakly stained with smaller nuclei and less distinct borders. Observe the numerous blood vessels , the delicate connective tissue framework, and the connective tissue capsule . Recall that the hypophyseal portal circulation carries releasing hormones from the hypothalamus to the adenohypophysis targeting the acidophils and basophils and causing release of hormones into the blood stream.

Note that capillaries are enlarged in the anterior lobe and may be referred to as sinusoidal capillaries. These capillaries are fenestrated. Locate the pars intermedia.   It appears somewhat basophilic and is believed to produce melanocyte stimulating hormone (MSH).   Look for cysts and the intraglandular cleft, remnant of Rathke's pouch.   You may also see the pars tuberalis.   It too is basophilic and believed to produce LH and FSH

Posterior lobe (pars nervosa) - note the pale-staining neural tissue. Nerve fibers fill most of the pars nervosa but they are not easily identifiable without special stains. Note that the main cell type here is a glial or supporting cell called a pituicyte.   Abundant dark-pink nuclei belong mostly to pituicytes (pituitary gland glial cells). Unmyelinated nerve cell axons and pituicyte cytoplasmic processes provide the pars nervosa with its fibrous appearance. The bulk of the pars nervosa consists of axons from neurons in the supraoptic and paraventricular nuclei of the hypothalamus.   Look for occasional small capillaries and blood vessels throughout the posterior lobe of the pituitary.

Identify the occasional Herring body .   They appear pale blue and are somewhat larger than the dark-pink pituicyte nuclei.   Herring bodies represent enlarged axon termini containing hormone-laden granules. They may appear to be extracellular but EM has shown them to be the neurosecretory material traveling down the axon to be released into the blood vessels.   Understand which hormones are stored in the pars nervosa, where they are synthesized, and which structures comprise the hypothalamohypophyseal tract.

Pars Intermedia . This structure (rudimentary in humans) lies between the pars distalis and pars nervosa. It consists mainly of colloid filled cysts lined by cuboidal epithelium and appears somewhat basophilic. Pars intermedia is believed to produce melanocyte stimulating hormone (MSH).   Look for cysts and the intraglandular cleft, remnant of Rathke's pouch.   You may also see the pars tuberalis.   It too is basophilic and believed to produce LH and FSH.