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210825 Endocrine System 5 – the Function of the Pineal and Thymus

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210825 Endocrine System 5 – the Function of the Pineal and Thymus Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Keywords Endocrine system/ Hormones/Pineal gland/Thymus gland Systems of life This article has been Endocrine system double-blind peer reviewed In this article... ● Endocrine functions of the pineal and thymus glands ● Hormones secreted by the pineal and thymus glands ● Role of the pineal and thymus glands in mediating essential physiological processes Endocrine system 5: the function of the pineal and thymus glands Key points Authors John Knight is associate professor in biomedical science; Zubeyde Bayram- The pineal gland is Weston is senior lecturer in biomedical science; Maria Andrade is honorary associate a neuroendocrine professor in biomedical science; all at College of Human and Health Sciences, gland in the brain, Swansea University. which produces the hormone melatonin Abstract The endocrine system consists of glands and tissues that produce and secrete hormones to regulate and coordinate vital bodily functions. This article, the Melatonin is key to fifth in an eight-part series on the endocrine system, explores the anatomy and synchronising the physiology of the pineal and thymus glands, and describes how they regulate and body’s biological coordinate vital physiological processes in the body through hormonal action. rhythms and has an influence on Citation Knight J et al (2021) Endocrine system 5: pineal and thymus glands. immunity Nursing Times [online]; 117: 9, 54-58. The thymus has both immune and he endocrine system comprises with a rich blood flow of around 4ml/ endocrine functions, glands and tissues that produce min/g, second only to that of the kidneys. and produces hormones to regulate and coor- Unlike most other brain components, the hormones that are Tdinate vital bodily functions. pineal gland is found outside of the neuro- essential to normal This fifth article in an eight-part series protective blood brain barrier (Chlubek immune function examines the anatomy, physiology and and Sikora, 2020). It increases in size function of the pineal and thymus glands. throughout early childhood and becomes An association Two of the smallest endocrine glands, fully developed at around age 5 to 7 years between the these are known to undergo significant (Patel et al, 2020). activities of the age-associated changes that influence Histologically, 95% of the cells of the thymus and pineal human physiology. pineal gland are the pinealocytes, the rest glands has led to the are supporting cells (Ibañez Rodriguez et postulation of a Pineal gland: location and histology al, 2016). The primary role of pinealocytes thymus–pineal axis The pineal gland, also known as the pineal is to synthesise and secrete the chronobi- body, is a neuroendocrine gland found otic hormone melatonin, which plays a key The pineal and towards the centre of the brain. It is located role in synchronising the body’s biological thymus glands both medially between the two cerebral hemi- rhythms. Structurally, pinealocytes are undergo significant spheres (Fig 1) and is attached to the rear of believed to be related to the photosensitive age-associated the third ventricle by a small pineal stalk. rods and cones of the retina; although, in changes thought The term ‘pineal’ refers to the structure humans, they only have an indirect to be associated of the gland, which often resembles a pine response to light, as detected at the retina, with declining cone. However, there is much variation in through a series of complex neural path- physiological shape, with many human pineal glands ways (Zhao et al, 2019). function and disease being pea-shaped or fusiform (tapering at Pinealocytes take up the essential both ends) (Afroz et al, 2014). The average amino acid tryptophan, which is adult pineal gland is a tiny brown struc- enzymatically converted into the neuro- ture, typically 5-8mm long, that weighs transmitter serotonin and then into mela- around 150mg. It is highly vascularised, tonin (Fig 2). Nursing Times [online] September 2021 / Vol 117 Issue 9 54 www.nursingtimes.net Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. Location of the pineal gland Cerebral hemisphere Corpus callosum Thalamus (enclosus third ventiricle) Pineal gland Hypothalamus (part of epithalamus) Optic chiasma Cerebral aqueduct Fourth ventricle Pituitary gland Cerebellum Pons Medulla oblongata Spinal cord The suprachiasmatic nucleus as The major trigger for melatonin release is usually corresponds to the period of circadian clock reduced light, with neural output from the deepest sleep. As light gradually returns, Melatonin secretion is governed by light SCN stimulating the pineal gland via sym- levels decrease again, with secretion usu- levels perceived by the retina. Action poten- pathetic nerves originating in the superior ally ceasing around 8am. tials (nerve impulses) generated from ret- cervical ganglion in the neck (Fig 3). The inal rods and cones are relayed to the visual SCN establishes the basic circadian Role of melatonin cortex of the brain via the optic nerves. rhythms that influence many physiological Initially, melatonin is released into the These cross over each other at the optic processes, including the sleep/wake cycle, densely arranged local blood vessels of the chiasm (Fig 3), above which is a specialised body temperature, blood pressure and pineal gland and the cerebrospinal fluid of collection of around 10,000 neurons called appetite (Douma and Gumz, 2018). the third ventricle; it then enters the gen- the suprachiasmatic nucleus (SCN). Melatonin secretion tends to begin at eral circulation and is distributed systemi- The SCN is a vital part of the hypothal- night, around two hours before sleep (typi- cally around the body (Zisapel, 2018). It is amus and functions as the body’s biolog- cally around 10pm); it maximises in the also synthesised and released from a wide ical clock (circadian master clock). Neurons early hours of the morning (between 2am range of other human organs, tissues and in the SCN have an inherent 24-hour and 4am), with plasma concentrations cells, including the retina, bone marrow, rhythm synchronised to cues in light inten- typically reaching 80-150 picograms (pg)/ lymphocytes, platelets, skin and gastroin- sity detected by the retina (Challet, 2015). ml (Chlubek and Sikora, 2020). This testinal tract (Tordjman et al, 2017). Fig 2. Biosynthesis of melatonin L-tryptophan 5-hydroxytryptophan Serotonin Melatonin N-acetylserotonin JENNIFER N.R. SMITH Nursing Times [online] September 2021 / Vol 117 Issue 9 55 www.nursingtimes.net Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 3. Melatonin release in response to reduced light Pineal gland Melatonin O HN O HN Inhibition Day (Light period) Stimulation Night (Dark period) Right retina Left retina Suprachiasmatic nucleus (SCN) Suprachiasmatic Superior cervical Optic chiasm nucleus ganglion Melatonin circulates in the plasma and, around one in three people in the UK (Bit. the mitochondria, where cellular respira- like most hormones, exerts its effects by ly/NHSInformInsomnia). Melatonin has tion can generate large quantities of free binding to specific receptors. These are been shown to be a non-addictive sub- radicals capable of damaging mitochon- widespread throughout the human body; stance of therapeutic value in treating drial DNA and potentially causing genetic few tissues are thought to be devoid of patients experiencing insomnia. Exoge- mutation (Cipolla-Neto and Amaral, 2018). melatonin receptors (Emet et al, 2016; nous melatonin prolongs sleep, improves Omar and Saba, 2010). sleep efficiency, enhances sleep quality Age-associated changes of the and subsequently, when awake, improves pineal gland Sleep/wake cycle functional performance and mental acuity The absence of a blood–brain barrier It has long been established that melatonin (Grima et al 2017). allows the pineal gland to freely accumu- is an important regulator of sleep in The blue light emitted from the screens late minerals, such as calcium, and trace diurnal species such as humans. Sleep, of electronic devices (such as TVs, com- elements, including zinc, cobalt, fluoride which accounts for around a third of our puters and mobile phones) is known to sup- and selenium (Chlubek and Sikora, 2020). lives, is recognised as a highly orchestrated press melatonin biosynthesis and secretion. The progressive accumulation of minerals neurochemical process, involving both Exposure to such displays for at least leads to the formation of corpora arenacea arousal and sleep-promoting regions of 30 minutes before sleep is associated with (‘brain sand’). Eventually, these mineral the brain that are influenced by a variety of increased sleep latency, poor sleep quality, aggregates enlarge to form pineal concre- chemical signals and cues (Zisapel, 2018). sleep disruption and increased daytime tions up to 1mm in size; this usually leads It is essential to both physical and psycho- sleepiness (Rafique et al, 2020). Reducing to a gradual hardening as the pineal gland logical health, and facilitates a significant blue-light exposure, particularly in the progressively becomes calcified with age reduction in external sensory input while evening before going to bed, has been dem- (Sergina et al 2018). Pineal calcification is simultaneously allowing unconscious: onstrated to
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