The posterior pituitary or neurohypophysis is a neurosecretory gland containing axons of secretory neurons whose cell bodies reside in the supraoptic and paraventricular nuclei. These neurons produce oxytocin and antidiuretic hormone (ADH or vasopressin), which are released at the axon termini that are closely associated with capillaries. Occasionally, the axon termini enlarge due to accumulation of secretory vesicles and are visible as neurosecretory bodies (NB, Herring bodies). Oxytocin stimulates myoepithelial cell contraction during lactation and smooth muscle contraction in the myometrium of the uterus. Antidiuretic hormone acts on the kidney collecting ducts to increase the absorption of water.

• Examine the image at the right and these slides containing the posterior pituitary (sample 1, sample 2). Identify capillaries, non-myelinated axons of secretory neurons (A), and their supporting cells called pituicytes (P).

Clinical note: Diabetes insipidus is a disorder in which the neurohypophysis fails to secrete antidiuretic hormone in response to normal stimuli. The disorder involves excessive thirst, water intake, and urination and can be treated by injection of vasopressin or similar drugs.

The pineal gland or epiphysis cerebri is a small neurosecretory organ in the brain containing secretory neurons (pinealocytes), interstitial glial cells, and axons that communicate with other parts of the brain. The pineal gland functions to regulate the body’s circadian rhythms by relating light intensity and duration to hormone secretion. Melatonin, the major hormone secreted by the pinealocytes, regulates the activity of the hypothalamus, pituitary, and other endocrine organs. The interstitial glial cells support the pinealocytes and axons within the gland. Pia mater surrounds the gland, and connective tissue septa extend from the pia mater into the gland, dividing it into irregular lobules. A distinctive feature of the pineal gland is the presence of corpora aranacea (brain sand), concretions of mineralized extracellular proteins of unknown function. Brain sand has clinical relevance because radiologists find that it is a good marker of the brain’s midline.

Examine the pineal gland in the images below and on these two slides (sample 1, sample 2). Pineal tissue can be identified from surrounding brain tissue by the presence of brain sand. Identify the abundant vasculature, small clusters of pinealocytes (P), with scant, poorly stained cytoplasm and ovoid euchromatic nuclei. Distinguish the interstitial glial cells (I), which are smaller than the pinealocytes and typically have elongated, darker staining nuclei. Note the fibrous material surrounding the cells is primarily neuropil.
 

Next is the thyroid.