Pituitary Gland Functions and Structure

Pituitary Gland Objectives

• After studying lectures 2&3, students should be able to describe the pituitary gland's structure and its relationship to function.
• Define pituitary cell types, understand how their numbers are regulated by physiological demands, and describe the effects of growth hormone on growth and metabolism.
• Define the role of Insulin-like Growth Factor I in mediating growth hormone actions.
• List stimuli that regulate growth hormone secretion and explain the underlying mechanisms.
• Understand abnormal pituitary function and growth hormone secretion, and how these conditions are treated.

Pituitary Gland Introduction

• The pituitary gland, also called the hypophysis, is a small endocrine gland (1 cm in diameter, weighing 0.5-1 g).
• It's located in the sella turcica of the sphenoid bone at the base of the skull.
• It's connected to the hypothalamus by the pituitary stalk (hypophyseal stalk).
• The pituitary gland is divided into two divisions: anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).
• These divisions, though close, arise from different embryonic origins (ectoderm vs. neuroectoderm).
• They have distinct structures and functions.
• A small, relatively avascular structure, called the pars intermedia, lies between the two divisions.

Pituitary Gland Anatomy and Development

• Both anterior and posterior pituitary divisions develop embryologically from different sources.
• The anterior pituitary arises from the pharyngeal epithelium (upward growth—Rathke's pouch).
• The posterior pituitary originates from the hypothalamus as a downward diverticulum.
• These two parts grow towards each other and fuse in the median region between the mouth cavity and brain.

Anterior Pituitary (Adenohypophysis) - Histology

• The anterior pituitary has two major cell types: chromophobe cells and chromophil cells.
• Chromophobe cells (50%): do not contain granules and stain poorly. They are thought to be precursor cells for chromophil cells.
• Chromophil cells (50%): contain granules and stain darkly. These are further classified based on staining properties or secretory nature.

Anterior Pituitary Cell Types (Chromophil Cells)

• Classification by staining:
• Acidophilic (alpha cells, ~35%): stain acidophilically.
• Basophilic (beta cells, ~15%): stain basophilically.
• Classification by secretory nature:
• Somatotropes: secrete growth hormone (GH).
• Corticotropes: secrete adrenocorticotropic hormone (ACTH).
• Thyrotropes: secrete thyroid-stimulating hormone (TSH).
• Gonadotropes: secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Lactotropes: secrete prolactin.

Regulation of Anterior Pituitary Secretion

• The hypothalamus regulates anterior pituitary function by releasing and inhibiting hormones.
• These hormones are transported to the anterior pituitary through hypothalamo-hypophyseal portal vessels.

Hypothalamic Releasing and Inhibiting Hormones

• Specific hypothalamic hormones regulate the release of different anterior pituitary hormones.
• Growth hormone-releasing hormone (GHRH) stimulates GH release.
• Growth hormone-releasing polypeptide (GHRP) stimulates GHRH and GH release.
• Growth hormone-inhibitory hormone (GHIH, or somatostatin) inhibits GH release.
• Thyrotropin-releasing hormone (TRH) stimulates TSH release.
• Corticotropin-releasing hormone (CRH) stimulates ACTH release.
• Gonadotropin-releasing hormone (GnRH) stimulates FSH and LH release.
• Prolactin-inhibitory hormone (PIH), primarily dopamine, inhibits prolactin release.

Anterior Pituitary Hormones (Tropic Hormones)

• Some anterior pituitary hormones (e.g., ACTH, TSH, FSH, LH) are tropic hormones, meaning they stimulate other endocrine glands. Prolactin acts in a direct manner on target tissues (e.g., mammary glands).

Gonadotropic Hormones

• Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are collectively called gonadotropins as they affect the gonads.
• LH and FSH are glycoproteins with specific subunit structures (alpha and beta). Their half-lives differ (e.g., LH ≈ 60 minutes).

Prolactin

• Prolactin is a single-chain polypeptide (protein) hormone, with a specific molecular structure and a short half-life (e.g., about 20 minutes).
• Prolactin is essential for mammary gland development and milk production/secretion in females.

This quiz covers lectures 2 and 3 about the pituitary gland, focusing on its structure, function, and role in growth hormone regulation. Students will explore pituitary cell types, their physiological regulation, and the effects of insulin-like growth factor I. Additionally, the quiz addresses abnormal functions and treatment of growth hormone-related conditions.