Endocrine System

Both the nervous system and endocrine system are involved in controlling important body functions and maintaining homeostasis. While the nervous system uses electrical impulses and chemicals called neurotransmitters to send messages, the endocrine system uses chemicals called hormones that carry messages through the blood to the body’s organs and tissues. The endocrine system (Figure 1) is a complex network of glands/organs which include the pituitary gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, and gonads (ovary and testis). For the purpose of simplicity, this chapter will discuss the cellular composition of each endocrine organ as well as the hormones produced by each, but not hormone functions.

Figure 1: Endocrine glands of the body

Pituitary Gland

The pituitary gland is divided into the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). As seen in Figure 2 and 3, the anterior pituitary consists of glandular tissue, while the posterior pituitary consists of mostly nervous tissue.

Figure 2: Anterior and posterior pituitary gland under scanning power (version A)

The anterior pituitary contains two major cell types that secrete hormones, acidophils and basophils, distinguished by how they stain with Hematoxylin and Eosin (H&E) staining (Figure 4, Table 1). The acidophils stain pink/red and produce the growth hormone (GH) and prolactin (PRL). The basophils stain blue/purple and produce tropic hormones: follicle stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), and thyroid stimulating hormone (TSH). A tropic hormone is a hormone that regulates hormone secretion by another endocrine gland. A third cell type may be visible, the chromophobes. These cells do not stain well (have clear cytoplasm) and may play a local role in regulation of pituitary hormones.

The nervous tissue of the posterior pituitary is composed of many axons with their axon terminals (Herring bodies) and surrounding glial cells (Figure 3, Table 2). These axons are part of neurons that originate in the hypothalamus. The glial cells are called pituicytes, they play a key role in the maintenance of blood vessels. The hormones, oxytocin and antidiuretic hormone (ADH, also known as vasopressin), are synthesized in cell bodies of neurons in the hypothalamus but stored and released at axon terminals in the posterior pituitary.

Figure 3: Pituitary gland with and without illustration overlay (version B)

Figure 4: Anterior pituitary with and without illustration overlay (version A)

Table 1: Cells of the anterior pituitary and their hormones

HINT: To remember the hormones that are produced and released by the basophils of the anterior pituitary, remember B-FLAT (first letter of hormones produced).

Table 2: Cells of the posterior pituitary and their hormones

Thyroid and parathyroid gland

The thyroid gland and parathyroid glands are both located in the neck just below the larynx. The parathyroid glands are approximately four (up to eight) tiny glands located on the posterior surface of the thyroid gland.

The thyroid gland is made up of many thyroid follicles, which carry out the synthesis of thyroid hormones (T3 and T4). The follicles consist of a single layer of cuboidal cells called follicular cells. The lumen (space within the follicle) is filled with a substance called colloid. Between the thyroid follicles are slightly larger cells called parafollicular cells. These cells are important for the synthesis and release of calcitonin (Figure 5, Table 3).

Figure 5: Thyroid gland with and without illustration overlay

Table 3: Cells of the thyroid gland and their hormones

The parathyroid gland consists mostly of chief cells. These densely-packed rounded cells are responsible for the synthesis and secretion of  parathyroid hormone. While the cells in this micrograph are stained very darkly, this is not always the case (Figure 6, Table 4). Additionally, you will often see larger cells that do not stain well. These are oxyphil cells, whose exact function is currently unknown.

Figure 6: Parathyroid gland with and without illustration overlay

Table 4: Cells of the parathyroid gland and their hormones

adrenal gland

The adrenal glands (or suprarenal glands) sit on top of the kidneys. The adrenal gland consists of two main regions, the adrenal cortex and adrenal medulla. These glands are surrounded by a layer of connective tissue that acts as a protective capsule.

The adrenal cortex, the outer region of the adrenal gland, is glandular and divided into three layers or zonas (singular, zona) (Figure 7, Table 5). The superficial layer, the zona glomerulosa produces the mineralocorticoid hormones, such as aldosterone. The cells of zona glomerulosa tend to be small and arranged in small clusters. The next layer, the zona fasciculata produces the glucocorticoid hormones, such as cortisol. The zona fasciculata is composed of long mostly straight cords of large, light-staining cells. The deepest layer of the adrenal cortex, the zona reticularis produces androgens (sex hormone precursors).

The adrenal medulla is the innermost region of the adrenal gland and is a mixture of glandular and nervous tissue (Figure 7, Table 5). The secretory cells of the adrenal medulla (chromaffin cells) synthesize and release the catecholamine hormones, epinephrine and norepinephrine, into the blood in response to the activation of the sympathetic nervous system.

Figure 7: Adrenal gland with and without illustration overlay

Table 5: Regions of the adrenal gland and their hormones

Pancreas

The pancreas is a gland posterior to the stomach that has both exocrine and endocrine functions. The exocrine tissue contain acinar cells that produce digestive enzymes and duct cells that produce bicarbonate to be secreted into the small intestine.  The endocrine tissue of the pancreas, called the islets of Langerhans (pancreatic islets), appear as clusters of lightly-stained cells. The islets of Langerhans contain cells important in blood glucose regulation; alpha cells produce glucagon, while beta cells produce insulin (Figure 8, Table 6). It is difficult to differentiate the cells of the islets with the staining technique (H&E) used to produce these slides.

Figure 8: Pancreas with and without illustration overlay

Table 6: Cells of the pancreas and their hormones

Gonads (ovary and testis)

The gonads, the primary reproductive organs, are the testes in males and ovaries in females. These organs are responsible for producing the germinal/sex cells (sperm and ovum/egg). However, they also secrete hormones and are therefore part of the endocrine system.

The ovary is comprised of many ovarian follicles, each containing an oocyte (immature egg) (Figure 9, Table 7). The granulosa cells of the follicles surround the oocyte. They assist in development of the oocyte and produce the steroid hormones estrogen and progesterone. Theca cells, which surround the granulosa cells, produce androgens (sex hormone precursors) important in the production of estrogen. You will be learning more about the different stages of follicle development in the female reproductive system chapter.

Figure 9: Ovary with and without illustration overlay

Table 7: Cells of the ovary and their hormones

The testes are the male gonads containing seminiferous tubules that produce the male germ/sex cells – spermatozoa/sperm (Figure 10, Table 8). You will be learning more about the cells of the seminiferous tubules later in the male reproductive system chapter. Between the seminiferous tubules are interstitial (Leydig) cells that produce and secrete the steroid hormone testosterone.

Figure 10: Testis with and without illustration overlay

Table 8: Cells of the testes and their hormones

Chapter Illustrations By:

Aislin Sparrow, B.A.

Juan Manuel Ramiro Diaz, PhD

Georgios Kallifatidis, PhD