- Be able to identify the parts of the pituitary gland, and describe their embryological origin.
- Name the cell types that produce the various hormones of the anterior pituitary, and know whether they are acidophils or basophils.
- Describe the role of hypothalamic neurosecretion in the function of the posterior pituitary, and in the regulation of the anterior pituitary, and recognize the histological structures involved in these functions.
I. PITUITARY GLAND
These slides show mostly pars distalis, but also have a small area of neurohypophysis (either pars nervosa or infundibular stalk) on one side (see slide orientation diagrams) and are stained in alternate sets with H&E or with Masson trichrome. The two classes of anterior pituitary cells (acidophils and basophils) are most easily distinguished with Masson trichrome staining, but you should also see how they look with routine H&E staining.
A. Pars distalis (aka anterior pituitary or adenohypophysis)
Slide 226 Pituitary gland Masson cross Webscope Imagescope orientation
Slide 226 Pituitary gland H&E cross Webscope Imagescope
Slide 226 Pituitary gland H&E sagittal Webscope Imagescope orientation
Slide 227 Pituitary gland Masson sagittal Webscope Imagescope orientation
Study the pars distalis (anterior lobe) in the slide stained with Masson trichrome #226 pars distalis Webscope ImageScope . The cells are arranged in irregular clusters or cords and are distinguishable by their staining as either acidophils, basophils, or chromophobes. The acidophils stain red or orange-red, while the basophils stain various shades of blue or blue-gray. Remember that the acidophils include two different cell types, somatotropes (growth hormone) and mammotropes (prolactin), while the basophils include gonadotropes (FSH and LH), thyrotropes (TSH) and corticotropes (ACTH). ACTH is actually a cleavage product of pro-opiomelanocortin (POMC), which is made by corticotropes and processed primarily into ACTH in these cells. To a lesser extent, corticotropes also produce other signaling factors derived from POMC such as lipotropins (involved in lipid metabolism), endorphins (endogenous opioids that reduce pain perception), and melanocyte stimulating hormone (MSH). Occasional cells in the anterior pituitary show no distinctive staining and are called “chromophobes”. You will only be required to distinguish acidophils from basophils. Your best strategy is first to identify acidophils, which are more distinctively stained, and then the remaining cells are almost entirely basophils. The cells are not uniformly distributed throughout the pars distalis, but instead there are areas where acidophils predominate, other areas where basophils are more numerous, while still other regions may show a more even mixture of acidophils and basophils.
What would be the most pronounced histological difference between the pituitaries of castrated and non-castrated males (assuming no hormone replacement)? answerFirst consider the products of the cells in the anterior pituitary. Acidophils include somatotropes (produce growth hormone) and mammotropes (produce prolactin). Basophils are either thyrotropes (produce TSH), gonadotropes (produce LH and FSH) or corticotropes (produce ACTH). Basophilic cells are all regulated by feedback inhibition, where the products of the target organs they stimulate can act directly on the anterior pituitary to downregulate the production of the initial hormone (LH, FSH, TSH, and ACTH). In castration, the cells in the testis that are normally stimulated by LH and FSH are gone and can’t produce hormones to downregulate their production. Instead, the absence of negative feedback causes an increase in LH and FSH produced by the basophilic gonadotropes of the anterior pituitary. These cells subsequently hypertrophy in a vain attempt to induce some type of response in the testis (which are no longer there). Histologically, this is seen as hypertrophy and increased numbers of basophils in the anterior pituitary. Now see if you can figure out what would happen to the pituitary if other glands were removed (for example the thyroid).
Note the abundant sinusoidal capillaries (often filled with red blood cells) #226 sinusoidal capillaries Webscope ImageScope that lie between the cell cords or clusters. You can appreciate how readily the hormones secreted from the cells can reach the blood. Since collagen stains bright blue with the Masson trichrome method, you can see the delicate connective tissue partitions between cords and around blood vessels.
In the routine H&E-stained sections, you can also identify acidophils and basophils, although the difference is not as obvious as it is with Masson trichrome staining. Here again, you should first identify acidophils, which stain various shades of reddish pink, and then the remaining cells are almost entirely basophils, which vary generally from bluish/grayish-pink to blue.
What would happen to the various endocrine glands in the absence of hormone production by the anterior pituitary (for example hypophysectomy)? answerThe anterior pituitary produces hormones that directly stimulate the mammary glands, the adrenal glands, the thyroid, and the ovaries or testis. Removal of GH, PRL, ACTH, TSH, FSH and LH from the system would have varying effects. Loss of ACTH would lead to the collapse of the adrenal cortex and a nearly complete loss of production of glucocorticoids and some weak androgens. The lack of prolactin would lead to decreased milk production in mammary glands. Without TSH, the follicular cells of the thyroid would shrink, leading to hypothyroidism. In the gonads, FSH and LH are essential in producing androgens and estrogens necessary for reproduction. The loss of these two hormones would prevent the maturation of eggs or sperm in the gonads as well as hinder the development of secondary sex characteristics.
B. Pars nervosa (aka posterior pituitary or neurohypophysis)
Slide 227 Pituitary gland monkey Masson cross Webscope Imagescope orientation
Slide 227 Pituitary gland monkey H&E cross Webscope Imagescope orientation
Slide 227 Pituitary gland Masson sagittal Webscope Imagescope
Although the pars nervosa can be found on the human pituitary slides in our collection, the monkey pituitary specimens (H&E and trichrome-stained) contain a significant portion of the pars nervosa (posterior lobe) and are probably better for studying this tissue. The pars nervosa looks like brain tissue, which it is. It is an extension of the brain, composed primarily of nerve fibers (axons) which originate from nerve cell bodies in the hypothalamus and pass to the pars nervosa by way of the hypothalamo-hypophyseal tract and the infundibular stalk. These nerve fibers carry oxytocin and antidiuretic hormone (ADH, vasopressin) to nerve endings, from which they are released into nearby capillaries upon neural stimulation from the hypothalamus. There is not much to see in the posterior lobe in these histological sections. Since there are no neuron cell bodies in this structure, most of the prominent nuclei belong to pituicytes , which are the characteristic glial cells of the pars nervosa. You will also see the nuclei of blood vessel endothelial cells , and fibroblasts which are in the connective tissue around these vessels.
C. Pars intermedia
Slide 227 Pituitary gland monkey Masson cross Webscope Imagescope
Slide 227 Pituitary gland monkey H&E cross Webscope Imagescope
Slide 226 Pituitary gland H&E sagittal Webscope Imagescope
Slide 227 Pituitary gland Masson sagittal Webscope Imagescope
The pars intermedia is very poorly developed in the human pituitary, but is prominent in the pituitaries of most other mammals. For example, in the monkey pituitary, you will see the pars intermedia as a prominent layer several cells thick, lying between the pars distalis and pars nervosa. In some places you may also see a long cleft between the pars distalis and the pars intermedia, which is a substantial remnant of the lumen of the embryonic Rathke’s pouch, an ectodermal outpocketing of the oral cavity which gave rise to both the pars distalis and the pars intermedia. Cells of the pars intermedia also produce POMC (pro-opiomelanocortin), which in these cells is processed primarily into endorphins and MSH (melanocyte stimulating hormone).
In contrast to its substantial presence in other mammals, the pars intermedia of the human pituitary is usually represented merely by a thin layer of basophilic cells that can be seen in both the human trichrome and H&E-stained human sections lying against the pars nervosa, and is probably of little functional importance. Between the pars intermedia and pars distalis are occasional fluid-filled cysts (again visible in both trichrome #227 Webscope ImageScope and H&E-stained #226 Webscope ImageScope sections), which are the only vestiges of the lumen of Rathke’s pouch. Although most of the human axial sections in your sets do not show the human pars intermedia very well, the sagittal sections in some of the sets show some indication of it.
D. Infundibular stalk and hypophyseal portal vessels
Slide 229B Infundibular stalk, and hypophyseal portal vessels H&E Webscope Imagescope orientation
Slide 229 Infundibular stalk, hypophyseal portal vessels H&E Webscope Imagescope
This slide shows the hypophyseal portal blood vessels #229 Webscope ImageScope that carry releasing hormones from the hypothalamus to regulate the release of hormones from the pars distalis of the pituitary. Look for these prominent blood vessels in portions of the pars distalis that extend up around the infundibular stalk. In many of these slides the sinusoidal capillaries #229 Webscope ImageScope lying between cords of pituitary cells throughout the anterior pituitary are shown particularly well. Scan over the slide, reviewing other features you saw in slides 226 and 227 (e.g. acidophils vs. basophils; neurohypophysis vs. adenohypophysis).