The endocrine system is a system of cellular communication. The means of communication is via hormones. The hormones are secreted by ductless glands directly into the bloodstream. Generally, the response to hormones is non-localized.
If a gland secretes its product through a duct, it is an exocrine gland.
Hormones can have a variety of structures. Hormones which are proteins include insulin and prolactin. Peptide chains are short chains of amino acids. Examples of hormones which are peptide chains are antidiuretic hormone and oxytocin. Steroid hormones are cholesterol derivatives. Steroid hormones include testosterone and estradiol. Norepinephrine and epinephrine are catecholamines.
In addition to glands with the primary role of producing hormones, several other organs have an endocrine component.
The pituitary is nicknamed the master gland. The infundibulum connects the pituitary to the hypothalamus.
The pituitary can be subdivided into the neurohypophysis. and the adenohypophysis.
The neurohypophysis can be further sub-divided into the pars nervosa, infundibular stalk, and median eminence. The pars nervosa is the posterior lobe. The neurohypophysis secretes two hormones: oxytocin and antidiuretic hormone.
The adenohypophysis can be further sub-divided into the pars distalis, pars intermedia, and pars tuberalis. The pars distalis is the anterior lobe of the pituitary. The adenohypophysis (more specifically, the pars distalis of the adenohypophysis) secretes tropic hormones. Tropic hormones affect cellular activity in their target organ.
There are several cell types in the pars distalis (anterior pituitary). Lactotropic cells secrete prolactin. Thyrotropic cells secrete thyroid stimulating hormone (TSH). Somatotropic cells secrete growth hormone. Corticotropic cells secrete adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH). Gonadotropic cells secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH).
The cells of the pars distalis (anterior pituitary) can be classified as acidophils or basophils depending on their affinity for acid or basic dyes, respectively. The acidophils are the somatotropic cells and the lactotropic cells. Thus, growth hormone and prolactin are secreted by acidophilic cells. The basophils are the gonadotropic cells, corticotropic cells and thyrotropic cells. Thus, the basophils secrete FSH, LH, ACTH, and TSH.
Histology hint Sarah Bellham: There are two mnemonics to use when thinking of acidophils and basophils of the anterior pituitary.
The pineal gland secretes melatonin. The cells of the pineal gland called pinealocytes. There are also glial cells in the pineal gland. Corpora arenacea is nicknamed "brain sand". Corpora arenacea is calcification seen in the pineal gland.
The thyroid gland secretes the thyroid hormones (T3 and T4) and calcitonin. When looking at the histology it can be seen that there are two cell types in the thyroid: follicular cells and parafollicular cells. Follicular cells release and store thyroid hormone. Parafollicular cells of the thyroid gland are also called C cells. They secrete calcitonin.
Read more about the thyroid here.
When looking at the histology it can be seen that there are two cell types in the parathyroid gland: principal cells and oxyphil cells. Principal cells are also called chief cells. Principal cells are the more prevalent. Oxyphil cells are also found in the parathyroid gland; however they are not as prevalent as principal cells.
The pancreas is both an endocrine and exocrine organ.
The endocrine portion of the pancreas is housed in the islets of Langerhans. Islets of Langerhans are visible on a histology slide. Islets of Langerhans are most numerous in the tail of the pancreas. Within the islets, there are several cell types. The alpha cells secrete glucagon. The beta cells secrete insulin. The delta cells secrete somatostatin.
The acini is the exocrine portion of the pancreas.
Read more about the pancreas here.
The adrenal gland is composed of the adrenal cortex and the adrenal medulla.
The layers of the adrenal cortex, from outermost to innermost are: zona glomerulosa, zona fasciculata, and zona reticularis. The respective principle products are: mineralocorticods, glucocorticoids, and sex steroids.
The cells of the adrenal medulla are chromaffin cells. They secrete the catecholamines (epinephrine and norepinephrine).
Read more about the adrenal gland here.
Sertoli cells are directly involved in spermatogenesis. Sertoli cells produce inhibin and androgen-binding protein. They are also called "sustentacular cells" or "nurse cells".
The interstitial cells of Leydig produce testosterone. These are also called "interstitial cells" or"interstitial cells of Leydig".
Estrogen and progesterone are secreted by the ovary.
Read more about the ovary here.
The placenta secretes human chorionic gonadotropin (HCG).
The thymus produces thymosin. Thymosin plays a role in the development of the immune system.
The gastric mucosa produces a hormone called gastrin. Gastrin stimulates the production of hydrochloric acid and pepsin.
Secretin and cholecystokinin are produced and secreted by the small intestine. Secretin stimulates the pancreas to produce bicorbanate rich fluid. Cholecystokinin (CCK) stimulates the gallbladder to contract. and the pancreas to secrete digestive enzyme.
There is an endocrine component to the heart. This is in addition to its obvious function of the pump for blood. Specialized cells produce atrial natriiuretic hormone, or atriopeptin.
Histology: A Text and Atlas
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