Anatomy

PITUITARY GLAND

  • Overview

    • The pituitary gland is a small, oval structure attached to the undersurface of the brain by the stalk-like infundibulum.

    • Location:

    • The hypothalamus lies inferior and anterior to the thalamus.

    • It connects to the pituitary gland through the infundibulum.

  • Structure:

    • The pituitary gland consists of two lobes:

    1. Anterior lobe (Adenohypophysis):

      • Pars anterior (pars distalis)

      • Pars tuberalis:

      • A projection from the pars anterior extending up along the anterior and lateral surfaces of the pituitary stalk.

      • Pars intermedia:

      • May be separated by a cleft that is a remnant of the embryonic pouch.

    2. Posterior lobe (Neurohypophysis)

    • The pituitary gland is often referred to as the master of the endocrine gland due to the numerous hormones it produces and the various activities it influences.

  • Protection:

    • The pituitary gland is located in the hypophyseal fossa of the sella turcica of the sphenoid bone providing it protection.

    • This infundibulum pierces the diaphragmatic sellae to reach the pituitary gland.

ANATOMY II COURSE OUTLINE

  1. PITUITARY GLAND

    • A. Anatomical Relationship

    • B. Blood Supply and Venous Drainage

  2. PINEAL GLAND

    • A. Pineal Gland Functions

  3. THYROID GLAND

    • A. Anatomic Relationship

    • B. Blood Supply

    • C. Venous Drainage

    • D. Lymphatic Drainage

    • E. Nerve Supply

    • F. Thyroid Functions

  4. PARATHYROID GLAND

    • A. Blood Supply and Venous Drainage

    • B. Lymphatic Drainage and Nerve Supply

    • C. Parathyroid Gland Functions

  5. REFERENCES

PINEAL GLAND

  • The pineal gland is a small cone-shaped body projecting posteriorly from the posterior end of the roof of the 3rd ventricle of the brain.

  • Composition:

    • Composed of groups of cells known as pinealocytes, supported by glial cells.

  • Innervation and Blood Supply:

    • The gland has a rich blood supply and is innervated by postganglionic sympathetic nerve fibers.

  • Influences:

    • The pineal gland influences the activities of:

    • Pituitary gland

    • Islet of Langerhans in the pancreas

    • Parathyroid glands

    • Adrenals

    • Gonads

    • The primary effect of the pineal gland is inhibitory:

    • Direct inhibition of hormone production.

    • Indirect inhibition by affecting the releasing factors produced by the hypothalamus.

THYROID GLAND

  • Structure:

    • The thyroid gland consists of two lobes connected by a narrow isthmus.

    • These lobes are vascular organs surrounded by a sheath derived from the pretracheal layer of deep fascia, which attaches the gland to the voice box (larynx) and trachea.

    • Each lobe is pear-shaped, its apex directed upwards as far as the oblique line on the lamina of the thyroid cartilage; its base lies below at the level of the fourth or fifth tracheal ring.

    • The isthmus extends across the midline in front of the second, third, and fourth tracheal rings.

    • A pyramidal lobe may project upward from the isthmus, usually to the left of the midline.

    • It can be connected to the hyoid bone by a fibrous or muscular band, with the muscular variant known as levator glandulae thyroideae.

  • Anatomic Relationship:

    • Anterolateral:

    • Sternothyroid, superior belly of the omohyoid, sternohyoid, and the anterior border of sternocleidomastoid.

    • Posterolateral:

    • Carotid sheath, common carotid artery, internal jugular vein, and vagus nerve.

    • Posterior:

    • Superior and inferior parathyroid glands, anastomosis between superior and inferior thyroid arteries.

    • Medial:

    • Larynx, trachea, pharynx, and esophagus, with associated structures including cricothyroid muscle and external laryngeal nerve.

BLOOD SUPPLY AND VENOUS DRAINAGE

  • Blood Supply:

    • Superior and Inferior Hypophyseal Arteries:

    • Branches of Internal Carotid Artery (ICA).

  • Venous Drainage:

    • Venous blood is drained through intercavernous sinuses.

  • Thyroid Gland Blood Supply:

    • Comprised of superior thyroid artery,** inferior thyroid artery**, and occasionally *thyroidea ima*.

    • Superior Thyroid Artery:

      • A branch of the external carotid artery that descends to the upper pole of each lobe accompanied by the external laryngeal nerve.

    • Inferior Thyroid Artery:

      • A branch of the thyrocervical trunk that ascends behind the gland to the cricoid cartilage; it then turns medially and downward to reach the posterior border of the gland.

    • Thyroidea ima: (if present) may arise from the brachiocephalic artery or the arch of the aorta and ascends in front of the trachea to the isthmus.

  • Venous Drainage for Thyroid Gland:

    • Superior Thyroid Veins: Drain into the internal jugular vein.

    • Middle Thyroid Veins: Drain into the internal jugular vein.

    • Inferior Thyroid Veins: Anastomose with one another in front of the trachea and drain into the left brachiocephalic vein in the thorax.

LYMPHATIC DRAINAGE AND NERVE SUPPLY

  • Lymphatic drainage mainly occurs laterally to deep cervical lymph nodes, either upper or lower.

  • Some lymph vessels descend to paratracheal nodes.

  • Nerve Supply:

    • Sympathetic innervation comes from superior, middle, and inferior cervical sympathetic ganglia.

  • Thyroid Functions:

    • Thyroxine and Triiodothyronine: Increase metabolic activity of most body cells.

    • Parafollicular Cells: Produce thyrocalcitonin, which lowers blood calcium levels.

PARATHYROID GLAND

  • Structure:

    • Parathyroid glands are ovoid bodies measuring about 6 mm in greatest diameter.

    • Typically four in number, they lie closely related to the posterior border of the thyroid gland, within its fascial capsule.

  • Blood Supply:

    • Supplied by superior and inferior thyroid arteries.

  • Venous Drainage: Similar to thyroid gland - drains into the superior, middle, and inferior thyroid veins.

  • Lymphatic Drainage:

    • Drains to deep cervical and paratracheal lymph nodes.

  • Innervation: Same as thyroid, from cervical sympathetic ganglia.

  • Functions:

    • Chief Cells: Produce parathyroid hormone (PTH).

    • PTH stimulates osteoclastic activity in bones, mobilizing bone calcium and increasing blood calcium levels.

    • Stimulates absorption of dietary calcium from the small intestine and reabsorbs calcium in the kidneys.

    • Diminishes phosphate reabsorption in the proximal convoluted tubules of the kidneys.

    • Blood calcium levels control PTH secretion and it is termed Calcium-Phosphate hormones because it regulates these levels.

REFERENCES

  • Wineski, L. (2018). Snell’s Clinical Anatomy by Regions (10th ed.). Wolters Kluwer.

  • Bolislis, MD. (April 2024). The Endocrine System Anatomy [PPT].