Download Pituitary Gland

Pituitary Gland
Normal
The pituitary is a small bean-shaped organ that measures about 1 cm in greatest diameter and
weighs about 0.5 gm, although it enlarges during pregnancy. Its small size belies its great
functional significance. It is located at the base of the brain, where it lies nestled within the confines
of the sella turcica in close proximity to the optic chiasm and the cavernous sinuses. The pituitary is
attached to the hypothalamus by the pituitary stalk, which passes out of the sella through an
opening in the dura mater surrounding the brain. Along with the hypothalamus, the pituitary gland
plays a critical role in the regulation of most of the other endocrine glands. The pituitary is
composed of two morphologically and functionally distinct components: The anterior lobe
(adenohypophysis) and the posterior lobe (neurohypophysis).
The anterior pituitary, or adenohypophysis, constitutes about 80% of the gland. It is derived
embryologically from Rathke pouch, which is an extension of the developing oral cavity. It is
eventually cut off from its origins by the growth of the sphenoid bone, which creates a saddle-like
depression, the sella turcica. The anterior pituitary has a portal vascular system that is the conduit
for the transport of hypothalamic releasing hormones from the hypothalamus to the pituitary.
Hypothalamic neurons have terminals in the median eminence where the hormones are released
into the portal system, from where they traverse the pituitary stalk and enter the anterior pituitary
gland. The production of most pituitary hormones is controlled predominantly by positive-acting
releasing factors from the hypothalamus ( Fig. 24-1 ). Prolactin is the major exception, since its
primary hypothalamic control is inhibitory, through the action of dopamine, while pituitary growth
hormone receives both stimulatory and inhibitory influences via the hypothalamus. In routine
histologic sections of the anterior pituitary, a colorful array of cells is present that contain
eosinophilic cytoplasm (acidophil), basophilic cytoplasm (basophil), or poorly staining cytoplasm
(chromophobe) cells ( Fig. 24-2 ). Specific antibodies against the pituitary hormones identify five
cell types:
1.
Somatotrophs, producing growth hormone (GH): These acidophilic cells constitute half of all
the hormone-producing cells in the anterior pituitary.
2.
Lactotrophs (mammotrophs), producing prolactin: These acidophilic cells secrete prolactin,
which is essential for lactation.
3.
Corticotrophs: These basophilic cells produce adrenocorticotropic hormone (ACTH),
pro-opiomelanocortin (POMC), melanocyte-stimulating hormone (MSH), endorphins, and
lipotropin.
4.
Thyrotrophs: These pale basophilic cells produce thyroid-stimulating hormone (TSH).
5.
Gonadotrophs: These basophilic cells produce both follicle-stimulating hormone (FSH) and
luteinizing hormone (LH). FSH stimulates the formation of graafian follicles in the ovary, and
LH induces ovulation and the formation of corpora lutea in the ovary.
Figure 24-1 Hormones released by the anterior pituitary. The adenohypophysis (anterior
pituitary) releases five hormones that are in turn under the control of various stimulatory and
inhibitory hypothalamic releasing factors. TSH, thyroid-stimulating hormone (thyrotropin); PRL,
prolactin; ACTH, adrenocorticotrophic hormone (corticotropin); GH, growth hormone
(somatotropin); FSH, follicle-stimulating hormone; LH, luteinizing hormone. The stimulatory
releasing factors are TRH (thyrotropin-releasing factor), CRH (corticotropin-releasing factor),
GHRH (growth hormone-releasing factor), GnRH (gonadotropin-releasing factor). The
inhibitory hypothalamic influences are comprised of PIF (prolactin inhibitory factor or
dopamine) and growth hormone inhibitory factor (GIH or somatostatin).
Figure 24-2 A, Photomicrograph of normal pituitary. The gland is populated by several
distinct cell populations containing a variety of stimulating (trophic) hormones. B, Each of the
hormones has different staining characteristics, resulting in a mixture of cell types in routine
histologic preparations. Immunostain for human growth hormone.
The posterior pituitary, or neurohypophysis, consists of modified glial cells (termed pituicytes) and
axonal processes extending from nerve cell bodies in the supraoptic and paraventricular nuclei of
the hypothalamus, through the pituitary stalk to the posterior lobe. These neurons produce two
peptide hormones, anti-diuretic hormone (ADH, also called vasopressin) and oxytocin. The
hormones are stored in axon terminals in the posterior pituitary and are released into the circulation
in response to appropriate stimuli. Oxytocin stimulates contraction of the smooth muscle cells in
the gravid uterus and cells surrounding the lactiferous ducts of the mammary glands. ADH is a
nonapeptide hormone synthesized predominantly in the supraoptic nucleus. In response to a
number of different stimuli, including increased plasma osmotic pressure, left atrial distention,
exercise, and certain emotional states, ADH is released from the axon terminals in the
neurohypophysis into the general circulation. The posterior pituitary is derived embryologically from
an outpouching of the floor of the third ventricle, which grows downward alongside the anterior lobe.
In contrast to the anterior lobe, the posterior lobe of the pituitary is supplied by an artery and drains
into a vein, where its hormones are released directly into the systemic circulation. Thus, the
pituitary has a dual circulation, composed of arteries and veins and a portal venous system linking
the hypothalamus and the anterior lobe.
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