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As organisms become more complex, they require an effective way of communicating in order to function properly and maintain homeostasis. Thus, organisms utilize hormones to send messages across the body (Neal, 2016). More specifically, hormones are chemicals signals released into the bloodstream that affect the physiological activity of cells in the body (William & Larsen, 2003; Tortora & Nielsen, 2008). Typically, hormones are derived from peptides, steroids or amino acids (Morley, 2019). Hormones allow for the effective coordination and regulation of different organs (Williams & Larsen, 2003). They travel through the blood and be recognized by cells of the body. If the hormone is nonpolar, it can directly diffuse through the cell membrane and reach the nucleus to achieve its desired effect. The location of the receptors is important because hormones that interact with receptors inside the cell (nuclear receptors) will typically regulate gene function (Morley, 2019). If the hormone is polar, cells have the ability to recognize particular hormones and coordinate a response via specific hormone receptors (Tortora & Nielsen, 2008). Moreover, hormones perform their function by selectively binding to receptors located on their target cells and initiating a multiple-messenger cascade to reach its desired effect (Tortora & Nielsen, 2008; Williams & Larsen, 2003). Therefore, hormones that interact with receptors on the surface will often regulate enzyme activity or ion channels (Morley, 2019)
A gland is an organ that makes and releases hormones with specific roles/functions. In the body, glands are characterized as either exocrine or endocrine glands. Exocrine glands release their products via ducts that allow the secretions to reach their target cells. On the other hand, endocrine glands release the substances (hormones) they produce into the interstitial fluid and subsequently into the blood stream (Tortora & Nielsen, 2008). In the body, there are organs and tissues that are not exclusively endocrine glands but have secretory cells that release hormones when stimulated (Williams & Larsen, 2003). Altogether, the endocrine system is composed of glands and hormone secreting cells that produce and regulate the secretion of hormones. The glands of the endocrine system can be further divided. Typically, “classical” glands, such as pituitary, thyroid, parathyroid, adrenal, pancreas and gonads, have the primary function of producing and secreting hormones (Neal, 2016). Organs such as the heart, kidney, GI tract and others are referred to as “nonclassical” glands because they have other primary functions, but they also secrete important hormones or substances (Neal, 2016).
For a while, the pituitary gland was referred to as the “master” gland of the endocrine system because the hormones it secretes control other endocrine glands. However, the hypothalamus coordinates signals to the pituitary gland, thus forming the hypothalamic-pituitary unit. The hypothalamus is a highly evolutionarily conserved small region of the brain that links the nervous and endocrine systems together (Williams & Larsen, 2003). It receives signals from the limbic system, cerebral cortex, thalamus and other regions of the brains. The hypothalamus is also responsible for controlling the autonomic nervous system to regulate body temperature, thirst, hunger and other reactions (Tortora & Nielsen, 2008). The pituitary gland is located in a compartment in the skull known as the sella turcica and it is connected to the hypothalamus (Neal, 2016). The pituitary gland has two portions known as the anterior lobe and posterior lobe with different functions. The anterior pituitary (adenohypophysis) lobe is made up epithelial tissue and it develops from the hypophyseal Rathke’s pouch in the roof of the mouth (Tortora & Nielsen, 2008). In an adult, the anterior lobe is formed by the pars distalis and the pars tuberalis. The adenohypophysis primarily produces hormones that regulate growth and reproduction (Tortora & Nielsen, 2008). These hormones, such as ACTH, GH, TSH and others, are released when hypothalamic hormones travel to the anterior lobe through the hypophyseal portal system (Neal, 2016). Of the hormones produced by the anterior lobe, those that regulate the activity of other endocrine glands (TSH, ACTH, FSH and LH) are referred to as tropic hormones as seen in Figure 1. The posterior pituitary lobe (neurohypophysis) is composed of neural tissue and develops from the neural tube (Tortora & Nielsen, 2008). It consists of axons and axon terminals of the hypothalamus (Williams & Larsen, 2003). The main function of the posterior pituitary lobe is to store and release hormones, unlike the anterior lobe which functions to produce hormones (Tortora & Nielsen, 2008). Neurosecretory cells of the hypothalamus produce oxytocin (OT) and antidiuretic hormone (ADH), which are packaged and transported via vesicles to the axon terminals found in the posterior pituitary gland (Tortora & Nielsen, 2008; Neal, 2016). These hormones are then released when nerve impulses signal the posterior pituitary gland to release them. As seen in Figure 1, oxytocin primarily targets myoepithelial cells of the breast and smooth muscle cells of the uterus in females (Morley, 2019). The function of Vasopressin (ADH) is to maintain fluid equilibrium and vascular hydration in the body (Morley, 2019). According to Campbell, Satoh, & Degnan, (2004) the hypothalamic-pituitary system is highly conserved across vertebrates from fish to mammals.
The pineal gland is an endocrine gland that connects to the roof of the third ventricle of the brain. According to Tan, Xu, Zhou, & Reiter (2018), observations indicate that the size of the pituitary gland can varry depending on health status or environmental factors, but it is typically the size of a soybean. More research is currently being conducted to determine if these factors are the reason behind variations in size. Additionally, the pineal gland is known to have masses of neuroglia and pincalocytes (Tortora & Nielsen, 2008). The primary function of the pineal gland is to produce and release the hormone melatonin into the blood stream and the cerebrospinal fluid (CSF). The pineal gland is composed of pinealocytes, microglia and astrocytes; research suggest that pinealocytes are specialized to produce and secrete melatonin (Tan, Xu, Zhou, & Reiter, 2018). The hormone melatonin produced in the pineal gland has been found to exhibit a circadian rhythmn during circulation through the body (Tan, Xu, Zhou, & Reiter, 2018). Moreover, it functions as the body’s biolocal clock because levels of melatonin have been found to increase during sleep and decrease before awakening (Tortora & Nielsen, 2008). Melatonin acts as a chemical signal for vertebrates to synchronize their physiological activities such as feeding, metabolism, reproduction, sleep and more (Tan, Xu, Zhou, & Reiter, 2008).