15.3: Pituitary Gland and Hypothalamus
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- 63460
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- Identify the two hormones released from the posterior pituitary, their target cells, and their principal actions
- Identify the six hormones produced by the anterior lobe of the pituitary gland
The hypothalamus–pituitary complex can be thought of as the “command center” of the endocrine system. This complex secretes several hormones that directly produce responses in target tissues, as well as hormones that regulate the synthesis and secretion of hormones of other glands. In addition, the hypothalamus–pituitary complex coordinates the messages of the endocrine and nervous systems. In many cases, a stimulus received by the nervous system must pass through the hypothalamus–pituitary complex to be translated into hormones that can initiate a response.
The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to the thalamus (Figure \(\PageIndex{1}\)). It has both neural and endocrine functions, producing and secreting many hormones. In addition, the hypothalamus is anatomically and functionally related to the pituitary gland (or hypophysis), a bean-sized organ suspended from it by a stem called the infundibulum (or pituitary stalk). The pituitary gland is cradled within the sella turcica of the sphenoid bone of the skull. It consists of two lobes that arise from distinct parts of embryonic tissue: the posterior pituitary (neurohypophysis) is neural tissue, whereas the anterior pituitary (also known as the adenohypophysis) is glandular tissue that develops from the primitive digestive tract. The hormones secreted by the posterior and anterior pituitary, and the intermediate zone between the lobes are summarized in Table \(\PageIndex{1}\).
Table \(\PageIndex{1}\): Pituitary Hormones
| Pituitary lobe | Associated hormones | Effect |
|---|---|---|
| Anterior | Growth hormone (GH) | Promotes growth of body tissues |
| Anterior | Prolactin (PRL) | Promotes milk production from mammary glands |
| Anterior | Thyroid-stimulating hormone (TSH) | Stimulates thyroid hormone release from thyroid |
| Anterior | Adrenocorticotropic hormone (ACTH) | Stimulates hormone release by adrenal cortex |
| Anterior | Follicle-stimulating hormone (FSH) | Stimulates gamete production in gonads |
| Anterior | Luteinizing hormone (LH) | Stimulates androgen production by gonads |
| Anterior (intermediate) | Melanocyte stimulating hormone (MSH) | Stimulates melanin formation in melanocytes |
| Posterior | Antidiuretic hormone (ADH) | Stimulates water reabsorption by kidneys |
| Posterior | Oxytocin | Stimulates smooth muscle contractions in male and female reproductive tracts and uterine contractions during childbirth |
Posterior Pituitary
The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic–hypophyseal tract within the infundibulum and end in axon terminals that comprise the posterior pituitary (Figure \(\PageIndex{2}\)).
The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nucleus produces the hormone oxytocin, whereas the supraoptic nucleus produces antidiuretic hormone (ADH). These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream.
Anterior Pituitary
The anterior pituitary originates from the digestive tract in the embryo and migrates toward the brain during fetal development. There are three regions: the pars distalis is the most anterior, the pars intermedia is adjacent to the posterior pituitary, and the pars tuberalis is a slender “tube” that wraps the infundibulum.
Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast, the anterior pituitary does manufacture hormones. However, the secretion of hormones from the anterior pituitary is regulated by two classes of hormones. These hormones—secreted by the hypothalamus—are the releasing hormones that stimulate the secretion of hormones from the anterior pituitary and the inhibiting hormones that inhibit secretion.
Hypothalamic hormones are secreted by neurons, but enter the anterior pituitary through blood vessels (Figure \(\PageIndex{3}\)). Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the anterior pituitary. This network, called the hypophyseal portal system, allows hypothalamic hormones to be transported to the anterior pituitary without first entering the systemic circulation. The system originates from the superior hypophyseal artery, which branches off the carotid arteries and transports blood to the hypothalamus. The branches of the superior hypophyseal artery form the hypophyseal portal system (see Figure \(\PageIndex{3}\)). Hypothalamic releasing and inhibiting hormones travel through a primary capillary plexus to the portal veins, which carry them into the anterior pituitary. Hormones produced by the anterior pituitary (in response to releasing hormones) enter a secondary capillary plexus, and from there drain into the circulation.
Six hormones are synthesized in the anterior pituitary. These are the growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Of the hormones of the anterior pituitary, TSH, ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- = “turning”) because they turn on or off the function of other endocrine glands.
The cells in the zone between the pituitary lobes (in humans considered part of the anterior pituitary) secrete a hormone known as melanocyte-stimulating hormone (MSH). MSH is capable of darkening of the skin by inducing melanin production in the skin’s melanocytes. People also show increased MSH production during pregnancy; in combination with estrogens, it can lead to darker skin pigmentation, especially the skin of the areolas and labia minora.
Figure \(\PageIndex{4}\): The Pituitary Gland. The pituitary gland consist of the anterior and posterior lobe. The anterior lobe is responsible for manufacturing hormones while the posterior lobe is where hormones from the hypothalamus are stored. (Image Credit: "Pituitary Gland" by Yancy Aquino is licensed under CC BY-NC-SA 4.0, slide provided by the Regents of the University of Michigan Medical School © 2022.)
Concept Review
The hypothalamus–pituitary complex is located in the diencephalon of the brain. The hypothalamus and the pituitary gland are connected by a structure called the infundibulum, which contains vasculature and nerve axons. The pituitary gland is divided into two distinct structures with different embryonic origins. The posterior lobe houses the axon terminals of hypothalamic neurons. It stores and releases into the bloodstream two hypothalamic hormones: oxytocin (OT) and antidiuretic hormone (ADH). The anterior lobe is connected to the hypothalamus by the vasculature of the hypophyseal portal system in the infundibulum and produces and secretes six hormones. Their secretion is regulated, however, by releasing and inhibiting hormones from the hypothalamus. The six anterior pituitary hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL). Figure \(\PageIndex{4}\) is a summary of the pituitary hormones and their principal effects.
Review Questions
Query \(\PageIndex{1}\)
Critical Thinking Questions
Query \(\PageIndex{2}\)
Glossary
Query \(\PageIndex{3}\)
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology