Anatomy Relevant to the Endocrine System

by Adam Lawson BA, MSc and Terra Doucette Hiller BA, BSN, RN

The endocrine system consists of several organs which produce and distribute hormones through the bloodstream. Hormones are a form of chemical communication which control metabolism, transport substances across the cell membrane, and maintain fluid and electrolyte balance; they also have a major effect on growth, development, adaptation, and reproductivity.

Hypothalamus - links the central nervous system to the endocrine system. Provides sensors which self regulates homeostasis of the entire body.
- It lies inferior to the thalamus and superior to the pituitary gland in the base of the brain; it forms the lower portion of the third ventricle.
- Neural projections connect it to the basal nuclei, the temporal lobe, the occipital lobe, and the brainstem.
  Rotate to 90° before proceeding. You may want to remove the dissection window plane (Ctrl-P) to view the small endocrine structures.
- It releases neurohormones through the infundibular stalk (a.k.a., pituitary stalk, infundibulum) to the posterior pituitary gland.
- It releases hormones into the rich vascular network of the anterior pituitary.
Information about the entire body and the circulatory system received from the nervous system and the ventricular system (via the third ventricle) induce the hypothalamus to produce hormones that affect the function of the entire body.
Pituitary gland (hypophysis) - produces, stores, and releases hormones that influence the entire endocrine system.
- It sits below the hypothalamus recessed in a shallow depression of the sphenoid bone.
  Transsphenoidal Approach to the Pituitary
  
  The pituitary can be accessed through the sphenoid sinus to remove tumors or masses. These tumors may induce blindness by compressing the optic chiasm or optic nerve.
  - A physician will pass tools through the nasal cavity before accessing the pituitary a portion of the sphenoid sinus called the "sella turcica."
  - The sella turcica and dura are cut to expose the pituitary and excise the mass. Nicking of the carotid artery is a rare complication when removing the mass.
  - The surgical site is packed with adipose tissue grafted from the abdomen or thigh of the patient. Nasal splints, soft packing and/or nasal tampons are placed in the nasal cavity.
- It is subdivided into two main regions:
  1. Anterior pituitary (adenohypophysis) - receives neurohormones from the hypothalamus through a vascular network which cause the inhibition or release of hormones.
    - The adenohypophysis produces hormones that control hormone production affecting pain perception, body growth, metabolism, sexual development, milk production, stress, circadian rhythms, immune response, and inflammatory response.
  2. Posterior pituitary (neurohypophysis) - collects, stores, and releases hormones produced by hypothalamus.
    - The neurohypophysis secretes hormones that affect blood pressure, aggression (in males), water retention, milk release, uterine contractions, arousal, bonding, and maternal behavior.
  The blood supplies of the anterior pituitary gland are different from the blood supplies of the posterior pituitary gland.
Thyroid gland - concentrates iodine from the bloodstream and produces hormones that directly affect metabolism and calcium absorption.
- It wraps around the anterior and lateral trachea at the second and fourth tracheal rings on level with the C6 and C7 vertebrae. This area of connection between the left and right thyroid is known as the isthmus of the thyroid.
  When a physician places a non-emergent tracheostomy, a incision is made around the location of the 2nd and 3rd tracheal ring. In some instances, the isthmus of the thyroid gland will have to be carefully cut in order to place the tracheostomy into the trachea.
- It sits below the thyroid cartilage and adjacent to the cricoid cartilage.
- The sternohyoid and sternothyroid muscles cover it.
- It produces hormones with several effects, including:
  - increased metabolism (e.g., increased oxygen consumption, energy consumption, body temperature, and heart rate)
  - growth (e.g., increased fetal development, brain development, protein synthesis)
  - increased hormone sensitivity (e.g., catecholamines)
  The thyroid may have evolved from pharyngeal tissue that "trapped" iodine directly from food particles in order to produce iodine-based hormones. Presumably, this tissue evolved over time to concentrate iodine from the bloodstream instead.
  
  The richly vascularised thyroid weighs 15-20 grams and receives 5ml of blood per minute per gram of thyroid tissue. It is important to observe a patient for signs of bleeding if the thyroid has been surgically removed. Thyroid tissue may also be killed using radiation when surgical removal is not indicated (e.g., hyperthyroidism).
Parathyroid glands - regulates calcitonin in the bloodstream.
- Parathyroid glands are typically located in the thyroid gland. Usually, two glands in each thyroid lobe can be found; one in the superior pole, and one in the inferior pole. Number and variation of parathyroid glands differ from individual to individual.
- Although typically located in the thyroid gland, some glands have been found outside the thyroid in the mediastinum.
Narrow ranges of calcium concentrations in the blood (9-10.5mg/dL) are maintained by the parathyroid hormones primarily to maintain proper neurological and muscular function.

The parathyroid may have evolved from cells in gill structures that extracted calcium from the ocean environment directly; this may also explain why there are a number of separate parathyroid glands.
Thymus gland - matures T cells (especially during fetal development) that allow the body to identify antigens.
The thymus is technically a part of the immune system, but it is often included in the endocrine system because of its proximity and similarity to the thyroid gland.
- It is located between the sternum and heart pericardium; it covers the anterior surface of the aortic arch.
- It receives immature lymphocytes produced in the bone marrow. The immature cells are killed if they pose a risk of attacking the body's own cells; they are matured if they are likely to attack antigens.
- It becomes fat with age (this process is called "organ involution").
There is no definitive explanation for the location of the thymus; it has been suggested that it first evolved from pharyngeal tissues to protect the pharynx from new antigens.
Pancreas - produces digestive enzymes (exocrine) and hormones (endocrine) which affect the functioning of the digestive system and the nutrient (esp. glucose) metabolism.
- It has three regions:
  1. Head - lies above the duodenum.
  2. Body - lies behind and below the stomach.
  3. Tail - lies and points toward the spleen.
- The pancreas is made up of lobules, which can be visualized best in the cross-sections. Interdispersed in the lobules are the islets of langerhans, which secrete, insulin, glucagon, somatostatin and pancreatic polypeptides.
- The head of the pancreas, which sits close to the digestive duodenum, consists primarily of cells which produce digestive enzymes. The tail of the pancreas contains cells that primarily produce hormones involved in the endocrine function of the pancreas; it also is in close proximity of the splenic vein, which will then drain into portal vein and go directly to the liver.
  Click here to highlight the hepatic portal vein (with branches), the superior mesenteric vein, and the splenic vein. Rotate the man incrementally and examine the relationship between the pancreas and its adjacent vascularized organs.
Islets of Langerhans are predominantly in the tail of the pancreas. Individuals who have acute attacks of pancreatitis usually suffer attacks in the head of the pancreas where digestive enzymes are released; the islets of Langerhans are usually spared. Chronic pancreatitis, however, will affect the whole pancreas; the subsequent destruction of the islets of Langerhans may result in diabetes mellitus.
Adrenal gland - produces hormones that interact with kidney hormones to primarily affect blood pressure and water/electrolyte/sugar concentrations in the blood.
- They are located immediately above the kidneys at about the level of the T12 vertebra.
- They have two primary regions:
  1. Medulla - the neuroendocrine core of the adrenal gland; it releases hormones that induce parasympathetic "fight-or-flight" changes in the body (e.g., tachycardia, vasoconstriction, glycogenolysis).
    The adrenal medulla and the peripheral nervous system develop from the same group of embryological cells.
  2. Cortex - surrounds the medulla; it releases steroids that, with kidney hormones, maintain blood pressure and blood mineral/water/glucose concentrations.
    In the cross-section window:
    1. Zoom in to a scale of about 1.88x
    2. Click on a region of empty area in the dissection window to de-highlight the adrenal glands.
    3. Examine the adrenal glands and note the color difference between the cortices and the medulla.
    Cortical hormones (e.g., cortisol) have a wide variety of additional effects on the nervous, immune, and reproductive systems.
    
    The adrenal cortices develop from the same group of embryological cells as the kidneys and gonads (i.e., the intermediate mesoderm).
Gonads - endocrine organs which produce sex cells (i.e., sperm, ova) and primarily affect the development of the reproductive system; they have different locations, structure, and hormonal production dependent on sex:
- Ovary (female) - produces hormones which primarily induce the growth and development of secondary sex characteristics at puberty and changes in the uterus during menstruation or pregnancy.
  Note in this female one ovary points inward from the fallopian tube, while the other points outward. This is just this females variation, typically both ovaries will point outward from the fallopian tubes.
- Testis (male) - produces hormones which primarily induce the growth and development of secondary sex characteristics. They hang within the scrotum behind the penis.
  The testicles descend during fetal development. The vas deferens connects the testicles to the urethra and is homologous to the uterine tubes (fallopian tubes). The uterus is homologous to the prostate.
  
  Note: The visible human male only has one testicle. It was surgically removed.