Atlas of plant and animal histology

The common feature shared by the structures of the endocrine system is the ability to synthesize and release of molecules known as hormones. These hormones are released into the extracellular milieu and subsequently reach the bloodstream to be transported everywhere in the body. However, some hormones may perform their functions locally. Hormones convey information to target cells, influencing their activities. The effect depends on the specific hormone, the target cell type, and the physiological state of the organism. The response of the body to hormones is commonly slower than to other chemical signals, such as neurotransmitters in the nervous system or in motoneuron-muscular cell transmission.

Hormones can be defined as molecules that induce specific target cells to perform a function. More than 100 different hormones have been identified in humans. Chemically, they can be classified as steroids, proteins, and those derived from amino acids. Each hormone is detected by specific receptors found in the target cells. These receptors can be located in the plasma membrane, where they recognize peptidergic and catecholaminergic hormones, initiating intracellular signaling pathways that can lead to an increase in cAMP, for example. Other hormones, such as thyroid hormones and steroids, can easily cross the plasma membrane and be detected by intracellular receptors, which contain a molecular domain that allows them to bind to the DNA, therefore regulating gene expression.

Cells that synthesize and release hormones are known as endocrine cells. They can be found throughout the body as single cells in organs like the gonads, digestive ducts, and liver. Collectivelly, these endocrine cells are known as the diffuse endocrine system. Other endocrine cells associate to form glands, referred to as endocrine glands, such as the hypophysis, pineal gland, thyroid gland, parathyroid gland, and adrenal glands (Figure 1). The major center that regulates the endocrine system is the hypothalamus, found in the rostral encephalon (ventral position). The endocrine glands are heavily irrigated by blood capillaries. Within these glands, the endocrine cells are arranged into cords or clusters, and sometimes in follicles, as observed in the thyroid gland.

1. Hypothalamus

The hypothalamus is an intermediary structure between the central nervous system and the endocrine system. It releases hormones that either inhibit or promote the release of hormones by other endocrine cells and glands throughout the body. In this way, the hypothalamus promotes body homeostasis by regulating heartbeat, blood pressure, appetite, digestive secretions, the activity of other glands, among others. Located at the base of the encephalon, the hypothalamus is closely associated with the hypophysis.

The hypothalamus releases several hormones (or neurohormones). The antidiuretic hormone increases the reabsorption of water; the corticotropin-releasing hormone influences the hypophysis function, which in turn acts on the adrenal glands to regulate metabolism; the gonadotropin-releasing hormone induces the hypophysis to release the follicle-stimulating hormone that influences gonadal function; there are hormones that either inhibit or stimulate the release of growth hormone by the hypophysis; the oxytocin hormone is involved in many functions of the body; and prolactin-inhibiting or stimulating hormones modify the release of prolactin hormone by the hypophysis, therefore stimulating the production of milk by the mother after giving birth.

2. Hypophysis

The hypophysis is found in the basal part of the encephalon, being both functionally and physically connected to the hypothalamus. The hypothalamus and hypophysis are the main regulators of the endocrine system. The hypophysis is a mixed gland consisting of an anterior lobe, or adenohypophysis, and a posterior lobe, or neurohypophysis. The infundibulum is the stalk connecting the hypophysis to the hypothalamus. The adenohypophysis is glandular tissue, while the neurohypophysis is secretory nervous tissue.

The organization of the adenohypophysis is like any other endocrine gland; that is, its cells are arranged in clusters or cords around fenestrated capillaries. Within the adenohypophysis, a part distalis and a part tuberalis can be distinguished. The pars distalis contains various cell types that release hormones: somatotropic cells release the growth hormone (GH), lactotropic cells release prolactin (PRL), corticotropic cells release adenocorticotropic hormone (ACTH), gonadotropic cells release the follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and thyrotropic cells release the thyroid stimulating hormone (TSH). The pars intermedia is a region between the adenohypophysis and neurohypophysis that shows cells organized in follicles with no clearly delimited function yet.

The neurohypophysis consists of pars nervosa and the infundibulum, which connects the hypophysis to the hypothalamus. Many terminal axons are found in the pars nervosa coming from neurons found in the supraoptic and paraventricular hypothalamic nuclei.

3. Pineal gland (epiphysis)

The pineal gland, or epiphysis, is a component of the epithalamus (dorsal part of the thalamus). It is found in the midline of the encephalon, between the two brain hemispheres. It is a pinecone-like structure, which is why it is called "pineal". The gland is connected to the encephalon by a peduncle known as the epiphyseal stalk. The primary cells within the pineal gland are referred to as pinealocytes, although other cell types are also present, like interstitial cells and neurons. Altogether, these cells are delimited by a covering of glial cell extensions. Melatonin is the hormone released by the pineal gland. This hormone is involved in the regulation of the circadian rhythms (day and night). Melatonin is synthesized and released during the night, whereas its production is inhibited during the day.

4. Thyroid

The thyroid is a gland located at the front of the neck, anterior to the trachea. It consists of two lobes linked by a medial zone. A third pyramidal lobe may be occasionally observed. The thyroid is wrapped in a connective tissue capsule that can be divided into an inner and an outer layer. The inner layer extends lamellas of tissue that enter the gland and form walls that divide it into lobes and lobules.

The structural unit of the thyroid is the follicle. Follicles are round structures surrounded by connective tissue containing fenestrated capillaries. Each follicle is composed of a layer of simple cuboidal epithelium that encloses an acellular space filled with a gelatinous-like substance known as colloid.

The follicular cells are the epithelial cells of the follicle. They synthesized colloid, which is enriched in proteins, including thyroglobulin and some enzymes. The thyroid is the sole endocrine gland that stores its products externally. Thyroglobulin is an iodinated protein synthesized and released by the follicular cells into the follicle to form the colloid. Once released, this protein binds iodine. When needed by the body, the iodinated thyroglobulin is endocytosed and transformed into the hormones T3 (triiodothyronine) and T4 (tetraiodothyronine or thyroxine), which are exocytosed through the basolateral membranes into the surrounding connective tissue. From there, they enter the fenestrated capillaries and therefore get into the bloodstream.

Parafollicular cells are found in the interfollicular space. These cells do not release their products into the follicle but into the surrounding connective tissue where they are found. Calcitonin is the hormone released by parafollicular cells.

5. Parathyroid gland

Several small glands associated with the thyroid are known as parathyroid glands. They are classified into the superior and inferior parathyroid glands. Each of these glands is wrapped by a capsule of connective tissue that extends inward as sheets of tissue that divide the gland into lobules. Parathyroid glands contain two types of cells: principal and oxylic. Principal cells, which arise during the embryonic period, are the most abundant and release the parathyroid hormone (PTH). This hormone is involved in the metabolism of calcium and phosphate in the bloodstream. PTH and calcitonin have opposite effects on calcium metabolism. Oxylic cells develop during puberty, and they do not appear to have a secretory function.

6. Adrenal glands

The adrenal glands are cone-like structures found on top of the kidneys. They are enclosed by a capsule of connective tissue. Extensions of connective tissue from the capsule enter the gland, carrying blood vessels and nerves. Anatomically, each adrenal gland is divided into a cortical zone and a medullary zone.

The medullary region is the central inner region of the adrenal gland. It is composed of chromaffin cells surrounded by blood vessels, nerve fibers, and sinusoidal spaces. The nervous fibers are preganglionic sympathetic fibers that make direct contact with chromaffin cells, leading to the release of adrenaline, noradrenaline, and a protein known as chromogranin. Adrenaline and noradrenaline (catecholamines) reinforce the use of energy for stronger physical activity by stimulating glycogenolysis and the mobilization of fatty acids.

The cortical region is the outermost one. It is divided into three zones that can be distinguished using a light microscope: the glomerular, fascicular, and reticular zones. The glomerular zone is the outermost part, composed of round cells forming clusters that release aldosterone, a hormone that regulates blood pressure. The fascicular zone is the intermediate and largest part of the cortex. It contains cells organized in radially oriented tubules that release glucocorticoids, such as corticosterone and cortisol. The reticular zone is the innermost layer. It is formed of scattered cells that release androgens.

7. Sexual glands: gonads.

The sexual glands are found in male and female gonads.

The female gonads are the ovaries. They contain the ovarian follicles, which release two types of hormones: estrogen and progesterone. Estrogens are released by the somatic cells of the granulose layer of the developing secondary follicles. These hormones promote the growth and development of female secondary sexual characteristics during puberty. For example, they influence the development of the mammary glands and adipose tissue.

Progesterone is the main progestagen released by the corpus luteum, which is the somatic part of the follicle once the egg cell has been released. If the embryo successfully implants into the uterine wall, the placenta also becomes a source of progesterone. This hormone favors the transformation of the organs involved in reproduction, largely the uterus and the mammary glands, during each ovulation cycle. It is produced from puberty to menopause. Progesterone is also released by the adrenal glands, although in a much lower amount.

The ovarian cells produce additional hormones, including inhibin, activin, and folliclestatin, which play a role in regulating the secretion of FSH (follicle-stimulating hormone) by the hypothalamus.

The male gonads, or testis, also contain cells that release hormones. Leydig cells are found in the interstitial tissue among the seminiferous tubules. At puberty, these cells release the androgen hormone testosterone, which promotes the development of secondary sexual characteristics and the production of sperm cells. Sertoli cells, found in the seminiferous tubules, also release testosterone, which mostly participates in the production and maturation of sperm cells. Testosterone needs to bind to the ABC protein to perform these effects. The ABC protein is also released by Sertoli cells and increases sperm cell production.

Sertoli cells release the hormone inhibin, which plays a role in the hypothalamus by inhibiting the release of the FSH (follicle-stimulating hormone). In addition, Sertoli cells have a primary role during embryonic development. The Müllerian ducts are tubules developed in both male and female embryos, but only in females do they develop into mature components of the female reproductive system. In male embryos, Sertoli cells release the anti-Müllerian hormone, which inhibits the development of the Müllerian ducts.

8. Thymus

The thymus plays a significant role in immune functions. It is located near the heart and releases a set of hormones known as humoral hormones, which influence the development of puberty. The main function of these hormones is to develop the immune system by facilitating the maturation of T lymphocytes. This maturation process mostly happens before puberty.

9. Pancreas

The pancreas consists of both exocrine and endocrine components. The exocrine component releases enzymes into the digestive duct, whereas the endocrine cells release hormones. Endocrine cells are organized in clusters known as the Langerhans islands, which are innervated by the autonomous system and account for 1 % of the total weight of the pancreas. Langerhans islands contain various cell types. Alpha cells release glucagon, which increases the glucose levels in the bloodstream. Beta cells produce insulin to lower the glucose concentration in the bloodstream. Delta cells release somatostatin, which inhibits the contraction of the digestive muscles, thereby halting the digestive process. F cells produce the pancreatic peptide that regulates the secretion of the exocrine pancreatic cells. B cells are the most abundant type, accounting for about 40 to 60 % of the pancreatic endocrine cells, whereas alpha cells range from 20 to 30 %. It is clear that pancreatic endocrine cells are involved in the regulation of glycemy (levels of glucose in the blood), and any disfunction in this process may lead to pathologies like diabetes.

10. Others

There are endocrine cells that can be found scattered in other parts of the organism. Thus, there are cells that synthesize and release hormones along the digestive epithelium, collectivelly known as the enteric endocrine system. These cells produce hormones involved in digestion, including gastrin, cholecystokinin, and secretin. The liver is a regulator of the hormone concentration in the blood because it is a degradation center. At the same time, the liver releases some hormones, such as angiotensinogen, a precursor of angiotensin, which regulates blood pressure, and thrombopoietin, which promotes hematopoiesis in the bone marrow. The kidney releases erythropoietin and renin to stimulate the production of erythrocytes and regulate blood pressure, respectively. The placenta releases hormones like the chorionic hormone and progesterone, both related to pregnancy.