Anatomy of the Endocrine System PDF

ANATOMY OF THE ENDOCRINE SYSTEM Authors: Drs. Davalos, Rivera, Guzmán, Ezquerra Objectives We are going to review the anatomy of four glands of the endocrine system: Hypophysis Thyroid Parathyroid Adrenals For each of these glands the learning objectives are to know: General aspects Arterial blood s...

ANATOMY OF THE ENDOCRINE SYSTEM Authors: Drs. Davalos, Rivera, Guzmán, Ezquerra Objectives We are going to review the anatomy of four glands of the endocrine system: Hypophysis Thyroid Parathyroid Adrenals For each of these glands the learning objectives are to know: General aspects Arterial blood supply Venous and lymphatic drainage Innervation 2 Hypophysis Generalities o Called pituitary gland or hypophysis cerebri. o Reddish-grey, ovoid body, about 12 mm in transverse and 8 mm in anteroposterior diameter. o Average weight of 500 mg. o Located in the sella turcica. Connected to the hypothalamus via the pituitary stalk (tube like structure between median eminence of the hypothalamus and the posterior pituitary). Surrounding anatomic structures Laterally to the pituitary: Cavernous sinuses Internal carotid arteries Venous plexuses Cranial nerves III-VI Superiorly to the pituitary: Optic chiasma Optic tracts Third ventricle Inferiorly: Sphenoid bone and sphenoidal sinuses 5 Division Adenohypophysis Has two major parts Which differ in their origin, structure and function. Neurohypophysis Is an ectodermal derivative of the stomatodeum, and includes: Is a diencephalic downgrowth connected with the hypothalamus, includes: Pars anterior (pars distalis or glandularis) Pars intermedia Pars tuberalis. Pars posterior (pars nervosa, posterior or neural lobe) Infundibular stem Median eminence. 6 Channel formed by the dura mater and the lateral wall of the body of the sphenoid. The internal carotid, CN I, II, III, IV, VI, walk through its interior. Arteries Cavernus Sinus Hypophysial arteries are branches of the intercavernous segment of the internal carotid artery The inferior branch supplies the posterior lobe of the pituitary gland The superior branch leads into the median eminence to start the hypophysial portal system to the anterior lobe 7 Arteries o Single superior hypophysial artery divides into: Medial and lateral branches -> anastomose across the midline and form an arterial ring around the infundibulum. Arteries Several superior hypophysial arteries supply the median eminence, upper infundibulum Via the artery of the trabecula, the lower infundibulum. A confluent capillary net, extending through the neurohypophysis, is supplied by both sets of hypophysial vessels. Most of the anterior lobe of the pituitary gland has no direct arterial supply. Venous drainage Most of the blood from the pituitary gland drains into the cavernous sinus and then into systemic circulation. o Neurohypophysis to: Dural venous sinuses, via the large inferior hypophysial veins. o Hypothalamus, via capillaries passing to the median eminence. o Adenohypophysis drainage appears to be restricted: few vessels connect it directly to the systemic veins. Thyroid gland Generalities o Is anterior in the neck below and lateral to the thyroid cartilage. o It consists of two lateral lobes with an isthmus. o The thyroid gland arises as a median outgrowth from the floor of the pharynx near the base of the tongue. Arterial supply Superior thyroid artery is the 1st branch of the external carotid artery, and divides into: Anterior glandular branch: anastomoses with its twin from the opposite side Posterior glandular branch: passes to the posterior side of the gland and may anastomose with the inferior thyroid artery Arterial supply Inferior thyroid artery: branch of thyrocervical trunk (arises from subclavian artery), divides into: Inferior branch: supplies the lower part and anastomoses with the posterior branch of the superior thyroid artery. Ascending branch: supplies the PTH glands. Venous drainage 3 veins: Superior thyroid vein. Middle thyroid vein. Inferior thyroid vein. Drain into the internal jugular vein. Drain into the right and left brachiocephalic veins. Lymphatic drainage Nodes beside the trachea (paratracheal nodes). Deep cervical nodes inferior to the omohyoid muscle along the internal jugular vein. Laryngeal nerves After branching for the vagus and looping around the subclavian artery on the right and the arch of the aorta on the left, the recurrent laryngeal nerves ascend in a groove between the trachea and esophagus and provides innervation to the thyroid gland. Question A 46-year-old woman is evaluated for voice changes after thyroid surgery. The patient was diagnosed with papillary thyroid cancer and subsequently underwent thyroidectomy. Immediately after the surgery, she noticed persistent hoarseness and dysphagia. Which nerve was most likely injured during the surgery? Answer The recurrent laryngeal nerve (RLN) branches off the vagus nerve (cranial nerve X) and has an indirect course through the neck. It supplies innervation to all the intrinsic muscles of the larynx, except for the cricothyroid muscles, as well as sensation to the larynx below the level of the vocal cords. Parathyroid gland Generalities Four oval-shaped glands in the posterior surface of the thyroid gland: o 2 superior: at the junction of the thyroid and cricoid cartilages. o 2 inferior: between the inferior poles of the thyroid lobes and the superior mediastinum. Vasculature, drainage and innervation Arterial supply: inferior thyroid arteries. Venous drainage: thyroid plexus of veins. Lymphatic drainage: deep cervical nodes, paratracheal nodes. Innervation: Thyroid branches of the cervical ganglia. Adrenals gland ▪ 2 ADRENAL GLANDS ▪ 4g EACH ▪ RIGHT (Pyramidal) ▪ LEFT (Semilunar) ▪ Larger in women than in men ▪ Structured in 2 sections (Cortex & Medulla) Adrenal Glands are Retro-Peritoneal Organs. The right gland rests on the diaphragm and the medial anterior portion is crossed by the inferior vena cava. The left gland rests on the left crus of the diaphragm and the upper anterior surface is covered by the peritoneum. On both sides, the suprarenal glands are enclosed in the renal fascia of Gerota, with a sheath of fascia separating it from its kidney. Cortex Medulla The cortex is yellowish in colour. It secretes two cholesterol derived hormones – corticosteroids and androgens. Divided into three regions (superficial to deep): Lies in the center of the gland Dark brown in colour. It contains chromaffin cells, which secrete catecholamines (such as adrenaline) into the bloodstream in response to stress. Zona glomerulosa – produces and secretes mineralocorticoids These hormones produce a ‘flight-orfight‘ response. Chromaffin cells also secrete enkephalins which function in pain control. (aldosterone) Zona fasciculata – produces and secretes corticosteroids (cortisol). Zona reticularis – produces and secretes androgens Arterial Supply Three sources: The SUPERIOR ADRENAL ARTERIES arise from the Inferior Phrenic Arteries. The MIDDLE ADRENAL ARTERY arises from the Aorta just proximal to the origin of the Renal Artery. (It can be single, multiple, or absent) One or more INFERIOR ADRENAL ARTERIES arise from the Renal Artery. Venous Supply The adrenal venous drainage does not accompany the arterial supply and is much simpler. A single vein drains the adrenal gland, emerging at the hilum. The LEFT ADRENAL VEIN passes downward over the anterior surface of the left adrenal gland, this vein joins the inferior phrenic vein before entering the left renal vein. The RIGHT ADRENAL VEIN passes obliquely to drain into the Inferior Vena Cava posteriorly. Lymphatic Drainage Drainage is to the renal hilar nodes, lateral aortic nodes, and nodes of the posterior mediastinum above the diaphragm by way of the diaphragmatic orifices for the splanchnic nerves. Lymphatics from the upper pole of the right adrenal gland may enter the liver. Innervation The action of the suprarenal gland is regulated both by neuronal and hormonal stimulation The adrenal glands are innervated by the celiac plexus and greater splanchnic nerves. Sympathetic innervation to the adrenal medulla is via myelinated pre-synaptic fibres, mainly from the T10 to L1 spinal cord segments The nerves from the Celiac Plexus are distributed throughout the gland, even around blood vessels, where they act to regulate blood flow. In the medulla, where they stimulate the release of catecholamines from chromaffin cells. In the cortex, where they can influence steroid hormone production. Bibliography Drake, R. L., Vogl, W., Mitchell. Gray’s Anatomy for Students (3rd edition). Philadelphia: Elsevier/Churchill Livingstone. Pages 10171020. Standring, S., Borley (2016) 14th edition. Gray’s Anatomy: the anatomical basis of clinical practice. Pages 359-361. The Suprarrenal Glands Susan Standring MBE, PhD, DSc, FKC, Hon FAS, Hon FRCS Gray's Anatomy, Chapter 70, 1239-1243.e2