Pituitary Gland
The pituitary gland is located directly below the hypothalamus in the sella turcica (“Turkish saddle”), a depression in the sphenoid bone. It is protected on there sides ny the bones of the skull and on the top by a tough membrane called the diaphragm sellae.
The pituitary gland is about the size of a plump lima bean, measuring about .39” (1.0cm) long, to .39” (1.0 -1.5cm) to .59” wide and .12 inches (.5cm) thick.
A major organ of the endocrine system, the anterior pituitary, also called the adenohypophysis, is the glandular, anterior lobe of the pituitary gland. The anterior pituitary regulates several physiological processes including stress, growth, and reproduction.
Its regulatory functions are achieved through the secretion of various peptide hormones that act on target organs including the adrenal gland, liver, bone, thyroid gland, and gonads. The anterior pituitary itself is regulated by the hypothalamus and by negative feedback from these target organs.
Disorders of the anterior pituitary are generally classified by the presence of over- or underproduction of pituitary hormones. For example, aprolactinoma is a pituitary adenoma that overproduces prolactin. In Sheehan's syndrome of postpartum hypopituitarism, the anterior pituitary uniformly malfunctions and underproduces all hormones. Proper function of the anterior pituitary and of the organs it regulates can often be ascertained via blood tests that measure hormone levels.
Anatomy
The pituitary gland is a pea-sized gland that sits in a protective bony enclosure called the sella turcica. It is composed of three lobes: anterior, intermediate, and posterior. In many animals, these three lobes are distinct. However, in humans, the intermediate lobe is but a few cell layers thick and indistinct; as a result, it is often considered part of the anterior pituitary. The pituitary gland is divided into an anterior lobe and a posterior lobe. The anterior lobe is the larger section of the pituitary, accounting for 75 percent of the total weight of the gland. The anterior lobe contains endoctrine secretory cells, which produce and secrete hormones directly into the circulatory system via extensive capillary network that surrounds the region. The posterior lobe (or neuro-hypophysis) does not manufacture any hormones. It containts the axons from two different groups of hypothalamic neurons. Hormones produced in the hypothalamus are transported from the hypothalamus to the posterior pituitary within the axons. In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary.
Hormone secretion
The posterior pituitary as a down growth of the brain, is a neurosecretory organ. The secretion of hormones from the posterior pituitary is controlled directly by neurons in the hypothalamus. The connecting stalk between the hypothalamus and the lobes of the pituitary gland, the infundibulum, carries the hormones of the posterior pituitary from nuclei in the hypothalamus. The hypothalmic supraoptic nuclei manufacture anti-diuretic hormone and the hypothalmic paraventricular nuclei manufacture oxytocin. These hormones are then stored in pituitary axons until their release is triggered.
The anterior pituitary is a glandular secretory organ. The secretion of hormones from the anterior pituitary is controlled by inhibiting and releasing factors secreted by neurons in the hypothalamus. These inhibiting and releasing factors are released into a primary capillary plexus where they travel, via portal veins, to a secondary capillary plexus where they stimulate the glandular tissue of the anterior pituitary to release its hormones.
The anterior pituitary arises from an invagination of the oral ectoderm and forms Rathke's pouch. This contrasts with the posterior pituitary, which originates fromneuroectoderm.
Major hormones secreted
| Hormone | Other names | BKA | Structure | Secretory cells | Staining | Target | Effect |
| Corticotropin | ACTH | Polypeptide | Secretion ofglucocorticoids | ||||
| Polypeptide | Inhibit perception of pain | ||||||
| Thyrotropin | TSH | Secretion of thyroid hormones | |||||
| - | FSH | Glycoprotein | Growth of reproductive system | ||||
| Lutropin | LH, ICSH | Glycoprotein | Sex hormone production | ||||
| Somatotropin | GH, STH | Polypeptide | Promotes growth; lipidandcarbohydratemetabolism | ||||
| Lactogenic hormone | PRL | Polypeptide | Lactotrophs and | Secretion ofestrogens/progesterone;milkproduction |
The acidophilic cells (GH and PRL) have extensive rough-endoplasmic reticulum and produce single chain polypeptides without any glycosylation or phosphorylation. Basophilic staining results from lysosome action modifying the hormones (or prohormones in the case of corticotrophs) by glycosylation.
An easy mnemonic to remember the hormones produced and secreted by the anterior pituitary is "FLATPEG".
The pituitary gland secretes nine different hormones. Seven of the hormones are produced by the anterior pituitary gland and two are secreted by the posterior pituitary gland. The hormones of the anteriror pituitary gland include:
- Thyroid-stimulating hormnone (TSH)
- Adrenocorticotropic hormone (ACTH)
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)
- Prolactin (PRL)
- Growth hormone (GH)
- And melanocyte-stimulating hormone (MSH)
There are three type of cells found in the anterior lobe:
Chromophobes 50%; acidophils 40% and basophils 10%
Chromphobes are considered to be resting cells.
Acidophils secrete the following hormones: STH and LTH
Basophils (beta cells) secrete: TSH, ACTH and MSH.
Basophils (delta cells) secrete: LH and FSH
The hormones of the posterior pituitary gland include:
- Antidiuretic hormone (ADH)
- Oxytocin
Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by the hypothalamus. Neuro-endocrine neurons in the hypothalamus project axons to the median eminence, at the base of the brain. At this site, these neurons can release substances into small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophysial portal vessels).
Etymology
The anterior pituitary is also known as the adenohypophysis, meaning "glandular undergrowth", from the Greek adeno ("gland"), hypo ("under"), and physis ("growth").
The term pituitary which means “mucus in latin, was given to this particular gland because of the erroneous belief that the gland transferred mucus from the brain into the nose through the palate.
The anterior pituitary gland, also called the hypophysis , is located in the center of the brain. It produces two types of secretions: a growth hormone and hormones that stimulate certain other glands.
The anterior pituitary gland is subject to neurochemical stimulation by the hypothalamus , a nearby part of the brain. This stimulation results in the production of the hormones that promote testicular and ovarian functioning, and does not occur normally until around 12 years of age in girls and 14 in boys. The beginning of this glandular activity is known as the onset of puberty .
Puberty is sometimes delayed for no apparent reason until age 16 or 17. Because the hypothalamus is affected by emotional factors, all of the endocrine glands governed by the anterior pituitary can also be affected by feelings. Psychological factors can therefore upset the relationships in the glandular system and produce the physical symptoms of endocrine disorders.
It is extremely rare for the anterior pituitary to produce too much or too little of its hormones, but sometimes hypofunction may follow pregnancy because of thrombosis or changes in the blood vessels.
A truly hypofunctioning anterior pituitary gland can cause many serious disturbances: extreme thinness, growth failure, sexual aberration, and intolerance for normal variations in temperature. When appropriate diagnostic tests determine the deficiency, the patient is given the missing hormones in pill form.
Absence of the growth hormone alone is unknown. Most cases of dwarfism result from other causes. However, excess production of the growth hormone alone does occur, but only rarely. If it begins before puberty when the long bones are still growing, the child with the disorder will grow into a well-proportioned giant. When it begins after puberty, the head, hands, feet, and most body organs except the brain slowly enlarge. This condition is called acromegaly . The cause of both disorders is usually a tumor, and radiation is the usual treatment.
The thyroid, adrenal cortex, testicles, ovaries, and pancreatic glands are target glands for the anterior pituitary's stimulating hormones, which are specific for the functioning of each of these glands. Therefore, a disorder of any of the target organs could be caused either by an excess or a deficiency of a stimulating hormone, creating a so-called secondary disease . There are various tests that can be given to differentiate primary from secondary disorders.
Several pituitary gland hormones are trophic hormones. trophic (from the greek trophikos, meaning "turning toward" or "to change") hormones are hormones that regulate the produciotn of other hormones by different endocrine glands. these hormones "turn on" or activate the target endocrine glands. The trophic hormones are thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
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