The Pituitary Gland: Structural Organization, Hormone Secretion & Hypothalamic Control
A self-contained interactive learning module on the master endocrine gland and its regulation by the hypothalamus.
1 Learning Objectives
2 Introduction & Location
The pituitary gland (hypophysis cerebri) is a pea-sized endocrine gland, roughly 1 cm in diameter and weighing about 0.5 g, situated in the hypophyseal fossa of the sella turcica of the sphenoid bone at the base of the brain. It is suspended from the floor of the third ventricle by a stalk, the infundibulum, and lies just below the hypothalamus, to which it is anatomically and functionally connected.
Because it regulates the secretory activity of most other endocrine glands (thyroid, adrenal cortex, gonads), the pituitary is traditionally called the "master gland" of the endocrine system — although it is itself under the direct control of the hypothalamus.
3 Structural Organization
Developmentally and structurally the pituitary gland has two distinct parts of different embryonic origin:
3.1 Adenohypophysis (Anterior Pituitary)
Derived from an upgrowth of oral ectoderm called Rathke's pouch, the adenohypophysis is glandular in nature and constitutes about 80% of the gland. It is subdivided into:
- Pars distalis — the largest part; contains chromophil (acidophil & basophil) and chromophobe cells that secrete the six major anterior pituitary hormones.
- Pars intermedia — a thin, often rudimentary zone between pars distalis and pars nervosa; secretes melanocyte-stimulating hormone (MSH).
- Pars tuberalis — a thin collar of cells wrapping around the infundibular stalk; rich in blood vessels of the portal system.
3.2 Neurohypophysis (Posterior Pituitary)
Derived from a downgrowth of neural tissue (diencephalon), the neurohypophysis is neural in nature and does not synthesize hormones itself. It consists of:
- Pars nervosa — composed of unmyelinated axon terminals of hypothalamic neurons and specialized glial cells called pituicytes; stores and releases neurohormones.
- Infundibulum (neural stalk) — connects the hypothalamus to the neurohypophysis and carries the hypothalamo-hypophyseal nerve tract.
- Median eminence — the region of the hypothalamus where releasing/inhibiting hormones are secreted into the primary capillary plexus of the portal system.
Fig. 1 — Schematic organization showing the two developmentally distinct lobes and their hypothalamic connections.
4 Hormone Secretion & Functions — Interactive Explorer
Click a tab below to explore each pituitary hormone: its source cell, chemical nature, target organ and physiological function.
5 Hypothalamic Control of the Pituitary
5.1 Control of the Adenohypophysis — The Hypophyseal Portal System
Unlike most endocrine glands, the anterior pituitary receives no direct nerve supply. Instead, it is regulated by neurohormones secreted by neurosecretory cells in the hypothalamus (arcuate nucleus and surrounding areas) into the primary capillary plexus at the median eminence. These hypophysiotropic hormones travel via the hypothalamo-hypophyseal portal veins to a secondary capillary plexus in the pars distalis, where they either stimulate or inhibit hormone release from specific target cells.
| Hypothalamic Hormone | Effect on Anterior Pituitary |
|---|---|
| TRH (Thyrotropin-Releasing Hormone) | Stimulates TSH & Prolactin release |
| CRH (Corticotropin-Releasing Hormone) | Stimulates ACTH release |
| GnRH (Gonadotropin-Releasing Hormone) | Stimulates FSH & LH release |
| GHRH (Growth Hormone-Releasing Hormone) | Stimulates GH release |
| Somatostatin (GH Inhibiting Hormone) | Inhibits GH (and TSH) release |
| Dopamine (Prolactin Inhibiting Factor) | Inhibits Prolactin release |
5.2 Control of the Neurohypophysis — Neurosecretion
The posterior pituitary does not synthesize its own hormones. Oxytocin and antidiuretic hormone (ADH/vasopressin) are actually synthesized by neurosecretory cell bodies located in the supraoptic and paraventricular nuclei of the hypothalamus. These hormones travel down the axons of the hypothalamo-hypophyseal tract bound to a carrier protein (neurophysin), and are stored in axon terminals within the pars nervosa until a nerve impulse triggers their release directly into the bloodstream.
5.3 Feedback Regulation
Target gland hormones (e.g., thyroxine, cortisol, sex steroids) exert negative feedback on both the hypothalamus and anterior pituitary, suppressing further release of releasing hormones and tropic hormones once adequate circulating levels are reached — maintaining hormonal homeostasis.
6 Key Terms — Flip to Reveal
1 Interactive Simulation: Hypothalamo-Hypophyseal Axis
Click "Trigger Signal" to visualize how a hypothalamic releasing hormone travels through the portal system to stimulate anterior pituitary hormone release, and compare it with the direct neural pathway to the posterior pituitary.
Observe how the same hypothalamic stimulus is transmitted differently: chemically via portal blood to the anterior lobe, and electrically/axonally to the posterior lobe.
2 Suggested Video Resources
For richer audio-visual reinforcement, learners are encouraged to view the following curated video lectures (external links — open in a new tab):
- Pituitary Gland Structure & Hormones — search results
- Hypothalamo-Hypophyseal Portal System — search results
- Neurosecretion and the Posterior Pituitary — search results
3 Animation: Negative Feedback Loop
Fig. 2 — Negative feedback regulation maintains stable circulating hormone levels.
1 Discussion / Reflection Prompts
Post your responses on the class discussion forum or beneath this module on zoologys.co.in.
1 Recommended Textbooks
- Hadley, M.E. & Levine, J.E. — Endocrinology, 6th Edition, Pearson.
- Turner, C.D. & Bagnara, J.T. — General Endocrinology, Saunders College Publishing.
- Norris, D.O. & Carr, J.A. — Vertebrate Endocrinology, 5th Edition, Academic Press.
- Guyton, A.C. & Hall, J.E. — Textbook of Medical Physiology, Elsevier (Endocrine chapters).
- Jameson, J.L. et al. — Endocrinology: Adult and Pediatric, Elsevier.
2 Journal Articles & Web Resources
- Le Tissier, P. et al. (2017) "Anterior pituitary cell networks," Frontiers in Neuroendocrinology.
- Ludwig, M. & Leng, G. (2006) "Dendritic peptide release and peptide-dependent behaviours," Nature Reviews Neuroscience.
- National Institute of Diabetes and Digestive and Kidney Diseases — Pituitary Gland fact sheet.
- Khan Academy — Endocrine System module (open access).
3 About the Author
Dr. Chandralekha Deka is an Assistant Professor in the Department of Zoology, Pandit Deendayal Upadhyaya Adarsha Mahavidyalaya (PDUAM), Amjonga, Goalpara, Assam. This module was created on 05/12/2023 as part of a UGC Four Quadrant-based e-content initiative for B.Sc. Zoology students.
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