Osmosis: Pituitary Gland Anatomy and Histology – Anterior vs Posterior

Overview

The Osmosis video from Elsevier explains the pituitary gland, its location in the sella turcica, and the two main parts: the anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis). It highlights their distinct embryological origins, tissue types, and staining characteristics, and it notes the role of the pars intermedia as a remnant zone between the two lobes.

• Location and dual-lobe organization with different embryological origins
• Histology and staining differences between adenohypophysis and neurohypophysis
• Key cell types and hormones produced, including oxytocin and ADH

Pituitary Gland Anatomy and Location

The video begins by situating the pituitary gland within the sella turcica, a saddle-shaped bone cavity at the base of the brain, with an approximate diameter of 1 cm. It then introduces the two main components of the gland: the anterior pituitary, or adenohypophysis, and the posterior pituitary, or neurohypophysis, noting that each arises from distinct embryological sources that shape their structure and function.

"The pituitary gland is a small gland with a diameter of approximately 1 cm" - Osmosis

Embryology and Structural Distinctions

The adenohypophysis originates from Rathke's pouch, an ectodermal outpocketing of the oral cavity, which contributes to its glandular epithelial tissue composition. In contrast, the neurohypophysis develops from neural tissue that descends from the hypothalamus, resulting in tissue that is predominantly neural secretory in character. These embryonic differences are reflected in the two lobes' histological appearance and functional roles in hormone secretion.

Adenohypophysis: Histology and Cell Types

Under Mason's trichrome stain, the anterior pituitary reveals two main cell categories: chromophils, which stain deeply (darkly), and chromophobes, which stain weakly. The chromophils further divide into acidophils and basophils. Acidophils include somatotrophs and lactotrophs that secrete growth hormone and prolactin, respectively. Basophils include corticotrophs, gonadotrophs, and thyrotrophs, responsible for hormones like ACTH, LH, FSH, and TSH, as well as MSH in some contexts. The anterior lobe also shows a richer vascular supply and diverse basophilic and acidophilic cell populations that support hormonal output.

"The chromophils can then be divided into acidophils that stain dark red and basophils that stain purplish blue due to their affinity for acidic or basic dyes, respectively" - Osmosis

Posterior Pituitary: Neural Tissue and Herring Bodies

The posterior pituitary consists mainly of unmyelinated axons with neurosecretory activity. These axons extend through the tissue, and their nerve terminals dilate to form herring bodies, which store and release oxytocin or antidiuretic hormone (ADH) into nearby capillaries. The posterior lobe thus functions as a conduit for hypothalamic hormones produced in neural tissue rather than a classic endocrine gland with glandular epithelium.

"The herring bodies release either oxytocin or ADH, which is carried away by nearby capillaries" - Osmosis

Pars Intermedia and Blood Supply

Between the anterior and posterior lobes lies the pars intermedia, a thin strip of cystic tissue that is considered part of the anterior pituitary. These cysts are remnants of Rathke's pouch, and higher magnification reveals colloid-filled cysts and irregular basophilic cell clusters that occasionally invade into the neurohypophysis. The adenohypophysis shows a notably richer blood supply than the neurohypophysis, with numerous small vessels throughout the pars intermedia and anterior lobe, supporting robust hormonal release into the circulation.

Recap: Core Features and Clinical Relevance

In summary, the pituitary gland has two distinct components with separate embryological origins and cellular compositions: the anterior adenohypophysis, a glandular tissue region rich in chromophils and chromophobes, and the posterior neurohypophysis, which is neural tissue with unmyelinated axons and neurosecretory terminals. Mason's trichrome staining helps differentiate nuclei and basophilic structures, while the pars intermedia marks the transitional zone derived from Rathke's pouch remnants. The posterior lobe's Herring bodies are key structures for hormone storage and release into the capillaries, enabling the transport of peptide hormones such as oxytocin and ADH to the systemic circulation.