There are two main lobes of the pituitary, ananterior lobe, and aposterior lobe joined and separated by a smallintermediate lobe. The anterior lobe (adenohypophysis) is the glandular part that produces and secretes severalhormones. The posterior lobe (neurohypophysis) secretesneurohypophysial hormones produced in thehypothalamus. Both lobes have different origins and they are both controlled by the hypothalamus.
The pituitary gland is composed of theanterior pituitary, theposterior pituitary, and anintermediate lobe that joins them.[6] The intermediate lobe isavascular and almost absent in humans, but in many other animals, it is distinct. The intermediate lobe in rats and mice have been extensively studied for the development of the pituitary and its functions.[7] In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary, which is an extension of thehypothalamus.[7]
The height of the pituitary gland ranges from 5.3 to 7.0 mm. The volume of the pituitary gland ranges from 200 to 440 mm3.[8] Its most common shape, found in 46% of people is flat, it is convex in 31.2% and concave in 22.8%.[8]
Endocrine cells of the anterior pituitary are controlled by regulatory hormones released byparvocellular neurosecretory cells in the hypothalamic capillaries leading to infundibular blood vessels, which in turn lead to a second capillary bed in the anterior pituitary. This vascular relationship constitutes thehypophyseal portal system. Diffusing out of the second capillary bed, the hypothalamicreleasing hormones then bind to anterior pituitary endocrine cells, upregulating or downregulating their release of hormones.[9]
The anterior lobe of the pituitary can be divided into thepars tuberalis (pars infundibularis) andpars distalis (pars glandularis) that constitutes ~80% of the gland. Thepars intermedia (the intermediate lobe) lies between the pars distalis and the pars tuberalis, and is rudimentary in the human, although in other species it is more developed.[7] It develops from a depression in the dorsal wall of thepharynx (stomal part) known as Rathke's pouch.
Theanterior pituitary contains several different types of cells that synthesize and secrete hormones.[10] Usually there is one type of cell for each major hormone formed in the anterior pituitary. At least five different cell types can be differentiated using variousstains.
Theposterior pituitary consists of the posterior lobe and thepituitary stalk (infundibulum) that connects it to the hypothalamus. It develops as an extension of the hypothalamus, from the floor of thethird ventricle. The posterior pituitary hormones are synthesized by cell bodies in thehypothalamus. Themagnocellular neurosecretory cells, of the supraoptic and paraventricular nuclei located in the hypothalamus, project axons down the infundibulum to terminals in the posterior pituitary. This simple arrangement differs sharply from that of the adjacent anterior pituitary, which does not develop from the hypothalamus.
The release of pituitary hormones by both the anterior and posterior lobes is under the control of thehypothalamus, albeit in different ways.[9]
The anterior pituitary regulates several physiological processes by secreting hormones. This includes stress (by secretingACTH), growth (by secretingGH), reproduction (by secretingFSH andLH), metabolism rate (by secretingTSH) andlactation (by secretingprolactin). Theintermediate lobe synthesizes and secretesmelanocyte-stimulating hormone. Theposterior pituitary (or neurohypophysis) is a lobe of the gland that is functionally connected to thehypothalamus by themedian eminence via a small tube called thepituitary stalk (also called the infundibular stalk or the infundibulum). It regulates hydroelectrolytic stability (by secretingADH), uterine contraction during labor and human attachment (by secretingoxytocin).
Prolactin (PRL), whose release is inconsistently stimulated by hypothalamic TRH, oxytocin, vasopressin, vasoactive intestinal peptide, angiotensin II, neuropeptide Y, galanin, substance P, bombesin-like peptides (gastrin-releasing peptide, neuromedin B and C), and neurotensin, and inhibited by hypothalamic dopamine.[14]
These hormones are released from the anterior pituitary under the influence of thehypothalamus. Hypothalamic hormones are secreted to the anterior lobe by way of a specialcapillary system, called thehypothalamic-hypophysial portal system.
Oxytocin, most of which is released from theparaventricular nucleus in thehypothalamus. Oxytocin is one of the few hormones to create apositive feedback loop. For example, uterine contractions stimulate the release of oxytocin from the posterior pituitary, which, in turn, increases uterine contractions. This positive feedback loop continues throughout labour.
The development of the pituitary gland is a complex process that occurs early in theorganogenesis stage ofembryonic development. It begins as a thickening of cells in the embryonicectoderm that form aneurogenic cranial placode, thehypophyseal placode oradenohypophyseal placode, that in the fourth week ofgestational age, gives rise toRathke's pouch.[15] Rathke's pouch is the ectodermal outpocketing from the roof of the developing mouth, and gives rise to the anterior pituitary.[16]A downward extension from theneuroectoderm as the infundibulum, forms the posterior pituitary.
Differentiation andmigration takes place in weeks 5 and 6. Rathke's pouch grows towards the developing brain. The upper part of the pouch eventually constricts and detaches from the oral cavity, and cells in Rathke's pouch differentiate to form three parts of the adenohypophysis: the pars distalis, pars intermedia, and pars tuberalis.
In weeks 4 to 8 the posterior pituitary is formed. The infundibulum from the diencephalon elongates downward, forming a stalk that connects with Rathke’s pouch. This stalk will develop into the posterior pituitary where specialized cells from the hypothalamus, known aspituicytes, migrate to help store and release hormones such asoxytocin andvasopressin.
From week 12 to week 16, the anterior pituitary starts to produce hormones, notablygrowth hormone and around the 12th to 16th week of gestation, the anterior pituitary begins producing hormones like growth hormone (GH), andadrenocorticotropic hormone (ACTH), essential for fetal development.
By the end of thefirst trimester the pituitary gland is completely formed remaining connected to the hypothalamus by the pituitary stalk (infundibulum), allowing the integration of signals from the brain and regulation of various endocrine functions.This dual-origin structure and function are what make the pituitary gland a unique and critical component of the endocrine system, acting as a bridge between the nervous and endocrine systems.
All of the functions of the pituitary gland can be adversely affected by an over- or under-production of associated hormones.
The pituitary gland is important for mediating the stress response, via thehypothalamic–pituitary–adrenal axis (HPA axis). Critically, pituitary gland growth during adolescence can be altered by early life stress such as childhood maltreatment or maternal dysphoric (depressive) behavior.[21]
It has been demonstrated that, after controlling for age, sex, and BMI, larger quantities ofDHEA andDHEA-S tended to be linked to larger pituitary volume.[22] Additionally, a correlation between pituitary gland volume andsocial anxiety subscale scores was identified which provided a basis for exploring mediation. Again controlling for age, sex, and BMI,DHEA andDHEA-S have been found to be predictive of larger pituitary gland volume, which was also associated with increased ratings of social anxiety.[22] This research provides evidence that pituitary gland volume mediates the link between higher DHEA(S) levels (associated with relatively early adrenarche) and traits associated with social anxiety.[22] Children who experience early adrenarcheal development tend to have larger pituitary gland volume compared to children with later adrenarcheal development.[22]
The GreekphysicianGalen referred to the pituitary gland by only using the (Ancient Greek) nameἀδήν,[23]gland.[24] He described the pituitary gland as part of a series of secretory organs for the excretion ofnasal mucus.[23] AnatomistAndreas Vesalius translatedἀδήν withglans, in quam pituita destillat, "gland in which slime (pituita[25]) drips".[23][26] Besides this 'descriptive' name, Vesalius usedglandula pituitaria, from which the English namepituitary gland[27] is ultimately derived.
The expressionglandula pituitaria is still used as official synonym besidehypophysis in the official Latin nomenclatureTerminologia Anatomica.[28] In the seventeenth century the supposed function of the pituitary gland to produce nasal mucus was debunked.[23] The expressionglandula pituitaria and its English equivalentpituitary gland can only be justified from a historical point of view.[29] The inclusion of this synonym is merely justified by noting that the main termhypophysis is a much less popular term.[30]
Note:hypophysial (orhypophyseal) means "related to the hypophysis (pituitary gland)".
The German anatomistSamuel Thomas von Sömmerring coined the namehypophysis.[23] This name consists[23][29] of ὑπό ('under')[24] and φύειν ('to grow').[24] In later Greek ὑπόφυσις is used differently by Greek physicians asoutgrowth.[23] Sömmering also used the equivalent expressionappendix cerebri,[23][26] withappendix asappendage.[25] In various languages,Hirnanhang[26] inGerman andhersenaanhangsel[31] inDutch, the terms are derived fromappendix cerebri.
The pituitary gland is found in all vertebrates, but its structure varies among different groups.
The division of the pituitary described above is typical ofmammals, and is also true, to varying degrees, of alltetrapods. However, only in mammals does the posterior pituitary have a compact shape. Inlungfish, it is a relatively flat sheet of tissue lying above the anterior pituitary, but inamphibians,reptiles, andbirds, it becomes increasingly well developed. The intermediate lobe is, in general, not well developed in any species and is entirely absent in birds.[32]
The structure of the pituitary in fish, apart from the lungfish, is generally different from that in other animals. In general, the intermediate lobe tends to be well developed, and may equal the remainder of the anterior pituitary in size. The posterior lobe typically forms a sheet of tissue at the base of the pituitary stalk, and in most cases sends irregular finger-like projection into the tissue of the anterior pituitary, which lies directly beneath it. The anterior pituitary is typically divided into two regions, a more anteriorrostral portion and a posteriorproximal portion, but the boundary between the two is often not clearly marked. Inelasmobranchs, there is an additional,ventral lobe beneath the anterior pituitary proper.[32]
The arrangement inlampreys, which are among the most primitive of all fish, may indicate how the pituitary originally evolved in ancestral vertebrates. Here, the posterior pituitary is a simple flat sheet of tissue at the base of the brain, and there is no pituitary stalk. Rathke's pouch remains open to the outside, close to the nasal openings. Closely associated with the pouch are three distinct clusters of glandular tissue, corresponding to the intermediate lobe, and the rostral and proximal portions of the anterior pituitary. These various parts are separated bymeningial membranes, suggesting that the pituitary of other vertebrates may have formed from the fusion of a pair of separate, but associated, glands.[32]
Mostarmadillos also possess a neural secretory gland very similar in form to the posterior pituitary, but located in the tail and associated with thespinal cord. This may have a function inosmoregulation.[32]
Although rudimentary in humans (and often considered part of theanterior pituitary), theintermediate lobe located between the anterior and posterior pituitary is important to many animals. For instance, in fish, it is believed to control physiological color change. In adult humans, it is just a thin layer of cells between the anterior and posterior pituitary. The intermediate lobe producesmelanocyte-stimulating hormone (MSH), although this function is often (imprecisely) attributed to the anterior pituitary.[citation needed]
The intermediate lobe is, in general, not well developed in tetrapods, and is entirely absent in birds.[32]
^Gibo H, Hokama M, Kyoshima K, Kobayashi S (1993). "[Arteries to the pituitary]".Nippon Rinsho.51 (10):2550–4.PMID8254920.
^Saladin, Kenneth S. (2012).Anatomy & physiology: the unity of form and function (6th ed.). New York, NY: McGraw-Hill. pp. 499–503.ISBN9780073378251.
^Hall, John E.; Guyton, Arthur C. (2011).Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, PA: Saunders/Elsevier. p. 895.ISBN9781416045748.
^Standring, Susan (2016).Gray's anatomy: the anatomical basis of clinical practice (41st ed.). Philadelphia, PA: Elsevier. p. 499.ISBN9780702052309. Digital version.
^abBoron, Walter F.; Boulpaep, Emile L. (2009).Medical Physiology (2nd ed.). Philadelphia: Saunders Elsevier. pp. 1016–1017.ISBN978-1-4160-3115-4.
^Textbook of Medical Physiology. Elsevier Saunders.
^Malendowicz, L.K; Rucinski, M; Belloni, A.S; Ziolkowska, A; and Nussdorfer, G.C. (2007) Leptin and the regulation of the hypothalamic-pituitary-adrenal axis. Int Rev Cytol. 263: 63-102.
^Sone, M. and Osamura, R.Y. (2001) Leptin and the pituitary. Pituitary. Jan-Apr; 4(1-2): 15-23.
^abcdMurray, CR; Simmons, JG; Allen, NB; Byrne, ML; Mundy, LK; Seal, ML; Patton, GC; Olsson, CA; Whittle, S (February 2016). "Associations between dehydroepiandrosterone (DHEA) levels, pituitary volume, and social anxiety in children".Psychoneuroendocrinology.64:31–9.doi:10.1016/j.psyneuen.2015.11.004.PMID26600008.S2CID22520320.
^abcdefghHyrtl, J. (1880).Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart. Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler.
^abcLiddell, H.G. & Scott, R. (1940).A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford: Clarendon Press.
^abLewis, C.T. & Short, C. (1879).A Latin dictionary founded on Andrews' edition of Freund's Latin dictionary. Oxford: Clarendon Press.
^abcSchreger, C.H.Th.(1805).Synonymia anatomica. Synonymik der anatomischen Nomenclatur. Fürth: im Bureau für Literatur.
^Pinkhof, H. (1923).Vertalend en verklarend woordenboek van uitheemsche geneeskundige termen. Haarlem: De Erven F. Bohn.
^abcdeRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 549–550.ISBN0-03-910284-X.
