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ENDOCRINE ANATOMY TUTORIAL The endocrine system is comprised of a variety of glands that are involved in hormone secretion. These hormones travel to target organs via the bloodstream and play a role in regulating metabolism, growth, temperature regulation, and reproductive functions. The system is controlled via a feedback mechanism, which will be discussed in more detail in your physiology classes. Organs - Hypothalamus - Pituitary gland - Thyroid gland - Parathyroid glands - Endocrine pancreas - Adrenal glands - Gonads This tutorial focuses on the thyroid gland, parathyroid glands, endocrine pancreas, and adrenal glands, as the anatomy of the hypothalamus and pituitary gland will be covered under the nervous system and the gonads will be covered under the reproductive system. THYROID GLAND The thyroid gland is found in the anterior portion of the neck at the C5-T1 vertebrae. It is found deep to the sternothyroid and sternohyoid muscles. There are two lobes of the thyroid gland: right and left lobe. The lobes are connected by the isthmus of the thyroid gland. The gland is enclosed by a fibrous capsule, which is attached to the cricoid cartilage and tracheal rings. The fibrous capsule itself is surrounded by fascia. Arterial Supply The thyroid gland is supplied by the superior and inferior thyroid arteries. These arteries are found between the fibrous capsule and the surrounding fascia. 1. Superior Thyroid artery a. First branch of the external carotid artery b. Found in close proximity to the external branch of the superior laryngeal nerve 2. Inferior Thyroid artery a. Arises from the thyrocervical trunk, which is a branch of the subclavian artery b. Found in close proximity to the recurrent laryngeal nerve Clinical Correlate - Thyroid surgery can accidentally damage the recurrent laryngeal nerve and can result in hoarseness of the voice. Venous Drainage 1. Superior thyroid vein 2. Middle thyroid vein 3. Inferior thyroid vein These veins form the thyroid plexus of veins. The superior and middle thyroid veins drain to the internal jugular vein. The inferior thyroid vein drains to the brachiocephalic vein. Innervation Sympathetic - The thyroid gland’s sympathetic supply is from the cervical sympathetic ganglia / trunk Parasympathetic - The thyroid gland’s parasympathetic supply is from the vagus nerve. Lymphatics - Drain to the paratracheal and deep cervical nodes Function The thyroid gland produces 2 hormones that regulate metabolism: T4 (thyroxine) and T3 (triiodothyronine). These hormones have a variety of functions: - Growth and development - Fetal brain development - Stimulates heart rate and contractility - Stimulates protein synthesis - Degrades cholesterol and triglycerides - Role in thermal regulation - Increases basal metabolic rate The thyroid gland also produces the hormone calcitonin. - Calcitonin functions to lower blood calcium levels and opposes the actions of parathyroid hormone Clinical correlate There are pathologies of the thyroid gland where the gland can produce too much thyroid hormone (ie hyperthyroidism) or too little thyroid hormone (ie hypothyroidism). Thyroid pathologies may present with a goitre ie swelling of the thyroid gland, which may or may not be painful. Symptoms of hyperthyroidism include: - Anxiety, tremor, irritability - Sensitivity to heat - Irregular or fast heart rate (may present with palpitations) - Weight loss - More specific to Graves Disease: exophthalmos/proptosis (ie protrusion of the eyeballs), pretibial myxedema Symptoms of hypothyroidism include: - Sensitivity to cold - Weight gain - Constipation - Tiredness - Dry, scaly skin. Brittle hair and nails PARATHYROID GLANDSThere are typically 4 parathyroid glands: 2 superior parathyroid glands and 2 inferior parathyroid glands, however this can be variable. Superior parathyroid glands - Found on the posterior aspect of each thyroid lobe - Approximately 1 cm superior to the entry of the inferior thyroid artery into the thyroid gland - Found at the level of the inferior border of the cricoid cartilage Inferior parathyroid glands - Have an inconsistent location - Usually found near the inferior poles of the thyroid gland, posteriorly All parathyroid glands are found between the fibrous capsule of the thyroid gland and the surrounding fascia. Arterial Supply - From inferior thyroid artery (this artery supplies the posterior aspect of the thyroid gland, where the parathyroids are found) - Collateral arterial supply from superior thyroid artery Venous drainage - Via parathyroid veins - These veins drain to the thyroid venous plexus Innervation - Cervical sympathetic chain Lymphatics - Paratracheal and deep cervical nodes Function - Secretes parathyroid hormone (PTH) - PTH maintains calcium homeostasis in the body PTH is released when low blood calcium levels are detected and acts to raise calcium levels in the blood via: - Increased calcium reabsorption in the kidney tubules - Stimulates osteoclasts to increase bone resorption, which increases blood calcium levels - Both PTH (from parathyroids) and calcitonin (from thyroid gland) regulate blood calcium levels in the body Clinical correlate The parathyroid glands are found in close proximity to the thyroid gland and are at risk of accidental removal during thyroid surgery. Removal of the parathyroid glands will present with hypocalcemia ie symptoms of tetany, muscle cramps, paraesthesia/numbness in the extremities. ENDOCRINE PANCREAS The pancreas is also a part of the GI system. It has both endocrine and exocrine functions. The exocrine functions will be covered under GI system. It is found anterior to the stomach, between the duodenum on the right, and the spleen on the left.It has 4 parts: (1) head, (2) neck, (3) body, (4) tail. The head of the pancreas is attached to the descending and horizontal parts of the duodenum. - The uncinate process is an inferior projection from the head and extends posterior to the superior mesenteric artery The neck of the pancreas is found adjacent to the pylorus of the stomach. The splenic vein and superior mesenteric vein unite to form the portal vein behind the neck of the pancreas. Also, the superior mesenteric artery lies behind the neck of the pancreas, and anterior to the uncinate process. The body of the pancreas is found anterior to the aorta and L2 vertebrae, and posterior to the omental bursa. The tail of the pancreas is the only portion that is intraperitoneal. Arterial Supply - Supplied by pancreatic branches of the splenic artery - The head of the pancreas has additional blood supply from: o Superior pancreaticoduodenal artery (branch of gastroduodenal artery, from the celiac trunk) o Inferior pancreaticoduodenal artery (branch of superior mesenteric artery) Venous drainage - Head of pancreas o Drains to the superior mesenteric branches of the portal vein - Pancreatic veins drain the rest of the pancreas o These veins empty into the splenic veins Innervation - Via vagus and abdominopelvic splanchnic nerves Lymphatics - The vessels follow the arterial supply - Empty into the pancreaticoduodenal nodes and pyloric nodes. From there, they drain into the superior mesenteric and celiac lymph nodes Function - Secretes hormones that regulate blood glucose Insulin: acts during the fed state - Secreted by the beta cells of the pancreas - Acts to lower blood glucose levels and increases glucose storage and utilization by the body - Increases lipogenesis (fat production) and decreases lipolysis - Muscle o Increases glycogen synthesis and glucose metabolism o Increases muscle mass / protein synthesis - Liver o Increases glycogen synthesis, decreases gluconeogenesis Glucagon: acts during the fasting state - Secreted by alpha cells of the pancreas - Acts to increase blood glucose levels - Increases lipolysis ie fat breakdown - Liver o Increases gluconeogenesis (ie glucose production) o Increases lipid breakdown and decreases lipogenesis (lipid production) o Increases amino acid breakdown and ketone body production Clinical Correlate The most common pathology related to the endocrine pancreas is diabetes mellitus. There are two types of diabetes: Type 1 and Type 2 diabetes mellitus. Type 1 diabetes: failure of the pancreas to produce insulin. Type 2 diabetes: the body is insensitive to insulin and target organs are unable to utilise glucose for energy. Both types result in high blood glucose levels and can present with excessive thirstiness, increased urination, tiredness, wounds that heal slowly, and blurred vision. However, both type 1 and type 2 diabetes are distinct diseases and have different pathophysiologies and management protocols. ADRENAL GLANDS The adrenal glands are 2 organs found on the superior pole of the kidneys, specifically superomedially. They are surrounded by renal fascia. The glands are comprised of an outer cortex and inner medulla. The cortex has 3 layers, from superficial to deep: - Zona glomerulosa - Zona fasciculata - Zona reticularis Interestingly, the cortex and medulla have 2 different embryological origins. - The cortex is derived from intermediate mesoderm - The medulla is derived from neural crest origin Arterial Supply - Superior suprarenal artery o From inferior phrenic artery - Middle suprarenal artery o From abdominal aorta - Inferior suprarenal artery o From renal artery Venous Drainage - Right suprarenal vein o Drains to the inferior vena cava directly - Left suprarenal vein o Drains to left renal vein first and then drains to the inferior vena cava Innervation - Celiac plexus and greater splanchnic nerves - The adrenal medulla has direct sympathetic innervation from pre synaptic fibres from T10-L1 spinal cord segmentsLymphatics - Adrenal lymphatic vessels drain to lumbar lymph nodes Function Cortex 1. Zona glomerulosa a. Produces mineralocorticoids ie aldosterone b. Functions in blood pressure control via salt and water retention in the kidneys. Aldosterone is a part of the RAAS system 2. Zona fasciculata a. Produces glucocorticoids ie cortisol b. Functions in the stress response 3. Zona reticularis a. Produces androgens ie DHEA (dehydroepiandrosterone) b. DHEA helps produces other hormones such as testosterone and estrogen Medulla - Compromised of chromaffin cells The adrenal medulla produces: 1. Catecholamines: epinephrine, norepinephrine a. Epinephrine (adrenaline): fight/flight response. Acts to increase heart rate and blood glucose levels and increases blood flow to the brain b. Norepinephrine: constricts blood vessels and increases blood pressure during the stress response 2. Enkephalins a. Functions in pain control Clinical correlate There are a variety of pathologies related to the adrenal glands. One of which is adrenal insufficiency, where the adrenal glands produce insufficient amounts of cortisol +/- aldosterone ie Addison’s disease. Symptoms of Addison’s disease include: - Fatigue, muscle weakness - Weight loss - Dizziness - Darkened skin