Introduction

 Long recognized as the “Master Endocrine Gland” in vertebrates. Historically, much work by early endocrinologists examined the regulatory role of the pituitary on other endocrine tissues. Several families and groups of hormones are synthesized and/or produced by this gland which regulate the activity of several major endocrine structures.

Anatomy

 The pituitary gland (aka hypophysis) is composed of two major lobes: the adenohypophysis and the neurohypophysis. The adenohypophysis derives from an invagination of oral ectoderm known as Rathke’s pouch while the neurohypophysis is formed as a downgrowth from the floor of the diencephalon of the brain (neural ectoderm). The portion of the neurohypophysis that connects to the brain (hypothalamus) is the infundibulum. Where the adenohypophysis and neurohypophysis meet, you may encounter the intermediate lobe (pars intermedia).

 Some definitions may be helpful at this point to understand some of the terms used to describe the anatomy of the pituitary:

  Pituitary Gland (Hypophysis)

       Adenohypophysis
                    Pars distalis
                    Pars tuberalis                   Anterior lobe
                    Pars intermedia                 Intermediate lobe
                                                            Neurointermediate lobe
       Neurohypophysis
                    Pars nervosa                   Posterior lobe
                    Infundibulum

 The size and morphology of the pituitary gland varies between vertebrate species.

 The pituitary in most mammals, including humans, is located within the sella turcica of the sphenoid bone. It is the most inaccessible endocrine gland in the body. The superior and inferior hypophysial arteries supply blood to the pituitary: anterior and posterior branches of the superior hypophysial artery supply blood to the infundibulum and hypothalamus. The adenohypophysis is supplied with blood via the hypothalamo-hypophysial portal system. The inferior hypophysial artery supplies the pars nervosa.

Pars Distalis and Pars Tuberalis

 Cells found in the anterior pituitary are classified based on their affinity for certain types of stains. In addition, they are also named for the hormones they secrete. Based on stain characteristics, there are acidophils (eosin), basophils (hematoxylin) and chromophobes (which stain very little. Basophils may also be classified as PAS + or – depending on the presence or absence of carbohydrate groups.

 Immunocytochemical analysis has allowed these cells to be classified based on the hormones produced: somatotrophs, lactotrophs, thyrotrophs, corticotrophs, and gonadotrophs. If we put staining characteristics together with immunocytochemical data we arrive at the folllowing characterization of cells in the adenohypophysis:

                     Somatotroph              acidophil                GH
                     Lactotroph                 acidophil                PRL
                     Thyrotroph                 PAS + basophil     TSH
                     Gonadotrophs             PAS+ basophil     FSH and LH
                     Corticotroph               PAS- basophil      ACTH

Families of Anterior Pituitary Hormones

Growth hormone (GH) and prolactin (PRL) are polypeptides which exert profound effects on growth, reproduction and metamorphosis. In  adults, somatotrophs may account for 4-10% of pituitary wet weight (5-10 mg/gland). GH circulates bound to one or more serum binding proteins and has marked anabolic effects that include amino acid incorporation into protein and synthesis of collagen. Prolactin regulates mammary gland growth and lactation in females but also has the most extensive and diverse list of functions of any peptide hormone found in vertebrates.
 
 Glycoprotein hormones include thyrotropin (TSH),  follicle stimulating hormone (follitropin, FSH) and luteinizing hormone (lutropin, LH). All are dimers consisting of  an ?- and ?-subunit. Specificity resides in the ?-subunit.

Pro-opiomelanocortin and Melanocortins include corticotropin (ACTH), ?-melanotropin (MSH), and ?-lipotropin (LPH). ACTH, MSH and LPH are part of a larger molecule, pro-opiomelanocortin (POMC). In the pars distalis, POMC is split to form ACTH while in the pars intermedia, POMC is cleaved into MSH. Clevage of LPH produces ?-endorphin and methionine enkephalin (met enkephalin) both of which act as endogenous analgesics.
 

Neurohypophysis

 The posterior lobe of the pituitary consists of axon terminals, capillaries, and supportive cells known as pituicytes. Hormones of the pars nervosa are made in the hypothalamus then transported to the pars nervosa where they are stored and released into capillaries

Neurohypophysial Hormones

 Oxytocin and arginine vasopressin  (AVP, ADH) are synthesized as the prohormones, oxyphysin and pressophysin in the hypothalamus. Both are 9 amino acid peptides (nonapeptides) but have vary different actions. Oxytocin stimulates milk release from the mammary gland and uterine contractions. Vasopressin stimulates water resorption by the collecting ducts of the kidneys (water retention and blood pressure).