Figure 1 Figure 2

The hypophysis, or pituitary gland, is a pea-size gland located at the base of the brain (Figure 1). It is the smallest, but the most important endocrine gland.

The name pituitary originates from the Greek ptuo (to spit) and the Latin pituita (mucus). It was thought that mucus, produced by the brain, was excreted through the nose by the pituitary. The name hypophysis, short for hypophysis cerebri, also derives from the Greek hypo, for under, and physis, for growth. It was in fact considered a sort of appendix or attachment beneath the brain.

The hypophysis sits in a small cavity of the sphenoid bone called the sella turcica,Turkish saddle (Figure 2). The floor, the dorsum and the front of the sella are formed by the sphenoid bone: the anterior wall is called the tuberculum sellae and the posterior wall is called dorsum sellae. The lateral walls of the sella are formed by the cavernous sinuses, which contain the internal carotid arteries and the 3rd, 4th and 6th cranial nerves, as well as the first (ophthalmic) and second (maxillary) divisions of the 5th cranial nerve. The 6th nerve runs in the center of the cavernous sinus, whereas the other nerves are more lateral. The roof of the sella is formed by a reflection of the dura, called the diaphragma sellae. From one side it prevents the arachnoid and thus the cerebrospinal fluid from entering the sella; from the other side it separates the pituitary from the overlying optic chiasm. This diaphragm is perforated to allow the pituitary stalk to pass through it. The size and the functionality of the opening are important in protecting the pituitary from transmitted pulsations of the choroid plexus and in protecting the optic fibers against suprasellar extension of an expanding pituitary mass.
The hypophysis weighs approximately 100 mg at birth. It then grows rapidly during childhood to reach an average adult weight of 500 mg toward the end of your twenties. The adult pituitary measures approximately 12 mm in width (the transverse diameter, side to side) and 10 mm in length (the antero-posterior diameter, from front to back). The height is 5.7 mm ( +- 1.7mm) and should never exceed 10 mm. The pituitary increases in size, by 12% to 100%, during pregnancy and lactation due to hypertrophy and hyperplasia of the prolactin-secreting cells.

The pituitary consists of three sections: the anterior lobe, the intermediate lobe and the posterior lobe.

Figure 3

1. the pars lateralis (also called pars distalis) is the largest part and contains mainly cells that produce growth hormone (GH) and prolactin (PRL), as well as follicle stimulating hormone (FSH) and luteinizing hormone (LH)

2. the pars medialis contains mainly cells that produce adrenocorticotropic hormone (ACTH) and thyroid stimulating hormone (TSH), as well as FSH and LH

3. the pars tuberalis is the upward extension of the pars lateralis. It surrounds the infundibular stem and contains mainly cells that produce TSH, LH, and FSH

Figure 4

Name	Hormone Secreted	% of all cells	Stain
Somatotroph	Growth Hormone (GH)	~50%	Acidophilic
Lactotroph	Prolactin (PH)	~20%	Acidophilic
Corticotroph	ACTH	~15%	Basophilic
Gonadotroph	Follicular Stimulating Hormone (FSH) and Luteinizing Hormone (LH)	~10%	Basophilic
Thyrotroph	Thyroid Stimulating Hormone (TSH)	~5%	Basophilic

Somatotroph and lactotroph are acidophilic (stain red); corticotroph, gonadotroph and thyrotroph are basophilic (stain blue). Despite immunocytochemical staining with antibodies directly against all the known anterior pituitary hormones, some cells remain unstained. These are called chromophobe by conventional staining methods, but electron microscopy has identified secretory granules in many of them. It is not known whether these cells represent undifferentiated immature secretory cells, or whether they produce hormones that have not been identified yet.

In humans the intermediate lobe is rudimentary, representing less than 1% of the total mass of an adult pituitary gland. It is larger (about 3.5%) during fetal life and in lower vertebrates (such as mice) where it secretes the melanocyte-stimulating hormone (MSH), which brings about skin color changes. It has been traditionally considered a vestigial structure in the humans, meaning it lacks any function. The appendix is another example of a vestigial structure. However, recent findings challenge this notion. The intermediate lobe contains follicles filled with proteinaceous material (similar to those found in the thyroid gland (Figure 5). Their function is unknown.

The posterior lobe (or neurohypophysis) is basically a downward extension of the hypothalamus. The hypothalamus is a thin layer of tissue that forms the floor and the lateral walls of the third ventricle. It extends from the optic chiasm (OC) anteriorly (forward) to the mammillary bodies (MB) posteriorly. The hypothalamus contains large neurons (magnocellular) that aggregate to form the supraoptic and paraventricular nuclei and secrete anti-diuretic hormone (ADH) and oxytocin. It also contains small neruons (parvicellular) that synthesize peptides such as somatostatin, thryotropin-releasing hormone, corticotropin-releasing hormone, and gonadotropin-releasing hormone. The hypothalamic neurons send down unmyelinated fibers that exit from the inferior surface of the hypothalamus, forming a swelling called the median eminence, continue as the infundibular stem, penetrate the sellar diaphragm and terminate in as the neurohypophysis. Thus the neurohypophysis consists of dilated terminals of the neurons of the hypothalamic nuclei, nerve-supporting and nonsecretory cells (pituicytes), capillaries, and neurosecretory materials stored at the nerve endings in the form of granules, called Herring bodies (FIgure 6). The infundibular stem plus the pars tuberalis of the anterior hypophysis plus the blood vessels make up what is called the pituitary stalk.