Now, I'll talk about the rest of the endocrine organs and glands that you need to know. The pineal gland is one that we saw in our brain dissection. It's a small cone-shaped body in the posterior epithalamus, near the third ventricle of the brain. The cells of the pineal gland secrete melatonin. It's a hormone that makes us drowsy, and it affects various tissues. Melatonin production increases at night to help set the circadian rhythm; that is, your sleep-wake cycles. The next gland to know is the thyroid gland. It's the largest gland that's entirely devoted to endocrine activities. It's located inferior to the thyroid cartilage and anterior to the trachea. It has a sort of butterfly shape, like a butterfly hugging the trachea, and has a left and a right lobe that are connected by an isthmus, a narrow connection there. The book goes into detail about thyroid hormones and lists a few thyroid hormones. You just need to know about thyroid hormone and calcitonin that are secreted by the thyroid gland. We'll take a look at the cells that are responsible for producing thyroid hormone and producing calcitonin. The thyroid is composed of spherical structures, these are hollow spheres that are called thyroid follicles, and they contain a fluid that's called colloid (spelled right there) in the lumen of the follicle. The cells that line the follicle are cuboidal, and they produce and secrete thyroid hormone. Thyroid hormone stimulates tissues throughout the body and increases cellular metabolism. In the spaces between the follicles, we'll find large endocrine cells that are parafollicular cells. These are also known as C cells. They secrete the hormone calcitonin in response to elevated blood calcium. This targets bone tissue. Calcitonin reduces blood calcium by inhibiting osteoclasts, those bone breaking cells, and increasing calcium loss in urine, so it's reducing calcium in the blood. On the posterior surface of the thyroid gland, which is illustrated with this sort of cottage cheese appearance here, we find the parathyroid glands. They're small glands on the posterior surface of the thyroid. There's usually four per person, and there are two types of cells in the parathyroid glands. There are chief cells. Chief cells secrete parathyroid hormone, which opposes calcitonin, it raises the blood calcium, and we're not going to talk about the other types of cells. We're just focused on these chief cells, the primary cells of the parathyroid gland. These cells, these chief cells, are secreting parathyroid hormone which opposes calcitonin and raises the blood calcium. And here's the mechanism for parathyroid hormone. Here's the parathyroid glands on the back of the thyroid gland, and if they detect low blood calcium in the circulating blood, then they secrete parathyroid hormone into the interstitial space, where it gets picked up by the blood, and then target organs respond to parathyroid hormone or to its effects to increase blood calcium levels by the following steps. There is increased activity of osteoclasts, they help to reabsorb bone connective tissue and release calcium into the blood. The kidney retains calcium and prevents calcium loss in urine, and the small intestine increases absorption of more calcium, so it promotes calcium uptake from the food, and all of this contributes to increased calcium in the bloodstream, and when that rising level of calcium is detected by the parathyroid gland, that inhibits the release of parathyroid hormone, so we don't get an infinite loop of just ramping up blood calcium forever. Remember that it's important to have a steady supply of calcium for the functioning of the muscles in particular, and the heart, which is a muscle. The next endocrine structure is the thymus. The thymus is a bilobed structure within the mediastinum superior to the heart and immediately posterior to the sternum. The size of the thymus varies between individuals, but it's always especially large in infants and children, and then with age it diminishes in size and diminishes in activity, especially after puberty. So in adults it's relatively small, in children relatively large. It functions in association with the lymphatic system to regulate and to maintain body immunity. It causes the maturation of white blood cells, so it's involved in the immune response. It's particularly important in individuals who are mounting some of their first immune defenses, so it's relatively large in children. The pancreas we've seen before, when we looked at the digestive system. It sits between the duodenum and the spleen. It's posterior to the stomach, and it has both exocrine and endocrine functions. It's composed mostly, as we talked about in the digestive system, of pancreatic acini that produce alkaline secretions that drain into ducts that lead into the duodenum to assist with digestion, but it also contains small clusters of endocrine cells that are called the pancreatic islets or the or the islets of Langerhans. We'll take a look at those pancreatic islets. When we look at the pancreatic tissue under the microscope, you see mostly pancreatic acini, you see that it's, you know, 99% of the pancreas is that pancreatic acini, making pancreatic juice. That's the exocrine function of the pancreas. The pancreatic islet cells, these big islands in the middle of the pancreas, that's just about 1% of the pancreas. These have endocrine functions: they secrete hormones that affect target tissues throughout the body, in particular the liver. Within the pancreatic islets are a few different types of cells. The alpha cells are responsible for secreting glucagon, which raises blood glucose. The beta cells secrete insulin to lower blood glucose, and the other types we're gonna skip because it's a lot of detail for the end of the semester. So, just knowing alpha cells glucagon, beta cells insulin. The adrenal glands are glands that sit on top of the kidneys, they sit on the superior borders of the kidneys, and like other endocrine glands they have this sort of cottage cheese appearance. Each adrenal gland has an outer cortex and an inner medulla, and each region secretes different hormones. We'll take a look at this structure. Here's a coronal section through the adrenal gland, and we can see the outer layer is the cortex, and the inner layer is the medulla. The adrenal cortex is organized into three different layers and each is producing a different corticosteroid hormone. You don't need to know the details of these. They target tissues throughout the body, they affect the stress response, they affect the immune response. You don't need to know the details of the three zones of the adrenal cortex. The adrenal medulla is the gland's inner core. The adrenal medulla is nervous tissue, like the posterior pituitary is nervous tissue. Endocrine tissues are typically epithelial, but the adrenal medulla and the posterior pituitary are nervous tissue. What they secrete is hormones that is a neurotransmitter, but it's traveling through the bloodstream instead of across a synapse. The adrenal medulla secretes norepinephrine and epinephrine, aka adrenaline. This targets basically all tissues, it is involved with the fight or flight response of the sympathetic nervous system. Remember from our discussion of the sympathetic nervous system that the adrenal medulla is the postganglionic neuron in some of the pathways of the sympathetic autonomic nervous system. Then, some organs that you've seen before! The testes... remember that the cells that are present inside of the seminiferous tubules are responsible for making the sperm, but those that are found in the interstitial spaces, those Leydig cells, are responsible for making androgens, they make masculinizing hormones, including testosterone. Testosterone affects the development of the reproductive organs and it also has some effects on behavior. In the ovaries...remember that the oocytes or eggs start their development inside structures that are called follicles. Cells that make up the wall of the follicle are called the follicular cells, and they make estrogen, which affects the development of the reproductive organs and also has some effects on behavior. Once the egg is ovulated, that is, it's left the follicle by rupturing the wall of the follicle, then the follicle is left behind as the corpus luteum, the white body, and the corpus luteum, these remaining follicular cells, produce progesterone which targets the uterus and maintains the uterine lining. Other organs also have endocrine function. The heart and the kidneys are both intimately connected to the circulating blood, and some of their endocrine function is to regulate blood pressure and to regulate blood volume. Kidney hormones help to regulate electrolytes, erythrocyte production, blood volume, and blood pressure. Calcitriol is an active vitamin D, it stimulates calcium uptake in the intestine, that's secreted by the kidney. Erythropoietin raises the rate of erythrocyte production, that is, red blood cell production, and renin is an enzyme that helps to form angiotensin II. Angiotensin, oh sorry, angiotensin II. Angiotensin II is involved in water conservation, which helps to maintain blood pressure. Atrial cells in the heart secrete atrial natriuretic peptide (ANP) that increases sodium and water excretion and lowers blood pressure. There are some gastrointestinal tract organs that secrete hormones that regulate digestive activities. For instance, arrival of high acidity food at one part of the tract might stimulate a later part of the gastrointestinal tract to produce a basic alkaline substance to neutralize the acid. So for these organs, just generally knowing that the kidney, the heart, and the gastrointestinal tract have some endocrine function as well should be just fine; don't stress the details of the hormones listed on this slide.