ACKNOWLEDGEMENT AND DISCLAIMER: This is a draft chapter that has not been peer reviewed. While its production and figures were supported by the National Toxicology Program/National Institute of Environmental Health Sciences, it solely represents the opinion of the author, R. R. Maronpot.
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Introduction
The pituitary is functionally a master endocrine tissue involved in homeostasis and regulation of growth, reproduction, sexual maturation, pregnancy, metabolism, lactation, and stress responses. The adenohypophysis includes the anterior and intermediate lobes. Cellular components of anterior lobe produce growth hormone, prolactin, thyroid stimulating hormone, leutotrophic hormone, and follicle stimulating hormone that function as trophic factors for peripheral target tissues. The neurohypophysis (posterior pituitary) consisting of the pars nervosa and infundibular stalk produces oxytocin and vasopressin (antidiuretic hormone). The two parts of the pituitary are functionally controlled by the hypothalamus (hypothalamic-pituitary axis) and feedback mechanisms from hormonal target tissues. Studies during the past decade using genetically modified mice have provided significant understanding of human pituitary dysfunction and provided valuable insight into the importance of pituitary progenitor/stem cells and the plasticity of pituitary responses in maintaining homeostasis.
Embryology
The mouse pituitary gland (hypophysis) consists of an anterior and posterior lobe. As with other endocrine organs, there is separate embryological origin of the two components. The anterior pituitary (adenohypophysis) originates from Rathke’s pouch (an oropharyngeal diverticulum) while the posterior pituitary (neurohypophysis) forms from neuroectoderm on the floor of the 3rd ventricle (diencephalon) (Kerr 1946; Kaufman 1994; Kaufman et al., 2010). In the mouse, the placode that forms Rathke’s pouch (RP) is present at 7.5 days post coitum (dpc) with formation of a rudimentary RP at approximately 8.5 to 9 dpc (Kelberman et al., 2009; Dubois & El Amraoui 1995). A day later an infundibulum recess develops from neuroectoderm on the floor of the 3rd ventricle (ventral diencephalon) to ultimately become the posterior pituitary and pituitary stalk. This neuroectoderm forms in juxtaposition with RP.
The juxtaposition of the neural and oral ectoderm is maintained throughout subsequent development of the pituitary and allows for inductive interaction during development and formation of the anterior and posterior pituitary (Takuma et al., 1998; Kelberman et al., 2007). Multiple transcription factors and genes are involved in the highly coordinated sequential steps in cellular commitment and positioning of pituitary cell lineages, all responsive to intrinsic and extrinsic signaling gradients (Cushman et al. 2001; Dasen & Rodenfeld 2001; Dasen & Rosenfeld 1999; Asa & Ezzat 1999; Kelberman et al. 2009; Rizzoti 2015). Corticotrope, thyrotrope, somatotrope, lactotrope and finally gonadotrope cells develop in association with specific gene expression between 12.5 and 16.5 dpc (Kelberman et al., 2009). Starting at about 14 dpc, the internal carotid artery provides branches to the hypothalamic area to ultimately become the hypophyseal portal system, the main blood supply to the anterior pituitary (Kaufman et al., 2010). The hypophysis and pituitary stalk receive blood from superior hypophyseal arteries. Starting at about 15 dpc, progressive rapid proliferation of anterior and posterior pituitary and the vascular supply commences, along with obliteration of the lumens of PR and posterior pituitary and early ossification of the sella turcica. Published histologic features showing progressive pituitary gland development up to approximately 18 dpc are clearly shown in stained sections (Kaufman et al., 2010). At this stage of development there is merging of anterior and posterior pituitary with compression of RP and further maturation of the pituitary gland. Ultrastructural demonstration of granular cells producing thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH), leutinizing hormone (LH), and somatotrophic hormone (STH) is present at gestation day 16 in SMA mice following by a proportional increase in STH producing cells (Sano & Sasaki 1969). Throughout the process of pituitary organogenesis there is active migration of progenitor cells, cell patterning and cell differentiation under the influence of multiple growth and transcription factors (Zhu et al., 2007; Dasen et al., 1999; Dasen et al., 2001; Rizzoti & Lovell-Badge 2005). After birth, there is continued proliferation and differentiation of hormone-producing cells as well as establishment of stem/progenitor cells (Levy 2002; Levy 2008; Nolan et al., 1998).
Anatomy
The mouse pituitary is located in a midline shallow depression, the sella turcica, on the dorsal surface of the basiphenoid skull bone. A dual tissue origin results in an anterior (adenohypophysis) and posterior (neurohypophysis) pituitary gland that fuse during embryogenesis with the point of fusion becoming the pars intermedia. The adenohypophysis represents the major portion of the pituitary gland and consists of the pars distalis (pars anterior), the pars intermedia, and the pars tuberalis. A cleft remnant of Rathke’s pouch is located between the pars distalis and pars intermedia. The neurohypophysis (pars posterior) includes the median eminence of the tuber cinereum, infundibular stalk, and pars nervosa. The neurohypophysis connects to the hypothalamus by the infundibular stalk. The small pars tuberalis partially wraps around the infundibular stalk.
When the brain is removed at necropsy, the pituitary stalk attachment to the brain is severed. Since the mouse pituitary is small and fragile, it is often fixed in situ for subsequent histologic examination. The histological preparation is typically a coronal section showing the pars distalis, pars intermedia and pars nervosa without the infundibular stalk and pars tuberalis. The pituitary is larger and heavier in females with weight differences among mouse strains (Liebelt 1994).
Histology
Adenohypophysis
The bilobed pars distalis, the largest component of the adenohypophysis, surrounds the pars intermedia and pars nervosa and is comprised of nests and cords of round to polygonal cells interspersed within a rich fibrovascular plexus. In hematoxylin and eosin-stained sections, pars distalis cells are chromophilic being tinctorially acidophilic or basophilic or alternatively are chromophobic (Figures 1 & 2). Chromophilic cells contain secretory cytoplasmic granules that account for their tintorial staining. Acidophilic chromophils are characterized by small round or oval cells with a central nucleus while basophilic chromophils are more polyhedral with an eccentric nucleus (Leibert 1994). Chromophobic cells have a large nucleus with 1 or 2 nucleoli, lack obvious secretory granules, and may serve as resting reserve cells or simply be in an early active synthesis phase. Immunohistochemistry has been used to more definitively identify specific cell types in the adenohypophysis (Sasaki & Iwama 1988; Sano & Sasaki 1969; Capen et al., 1991; Liebelt 1994). Several distinctive cell types of the mouse adenohypophysis have been identified by electron microscopy (Liebelt 1994). Histomorphology as well as ultrastructural cytologic features may be influenced by sex and the physiological state of the pituitary. Sex differences in the proportions of lactotrophs and somatotrophs have been documented by immunohistochemistry and electron microscopy in SMA mice along with documentation of presence of a few mammosomatotrophs (Sasaki & Iwama 1988).
The proportion of each cell type varies dynamically depending upon physiological conditions. Pars distalis cells are responsible for secretion of major pituitary hormones and include somatotrophs that secrete growth hormone and mammotrophs that secrete prolactin in females. Somatotrophs and mammotrophs are typically acidophilic in routine H&E stains. Basophilic gonadotrophs produce follicle stimulating hormone and leutinizing hormone, are more numerous in males than in females, but their frequency in females varies depending upon the estrus cycle stage. Thyrotrophs are also basophilic and secret thyroid stimulating hormone. However, tinctorial features of pituitary cells, especially in the pars distalis, may not accurately reflect phenotype and cell type frequency. Using immunohistochemistry, Baker and Gross concluded that the frequency and distribution of hormone producing cells in the pars distalis of Swiss-Webster mice were similar to that in other species (Baker & Gross 1978).
The adult rodent pituitary has a low steady-state cell turnover under both hypothalamic neuroendocrine control and feedback control from peripheral target tissues (Fauquier et al. 2008; Nolan et al. 1998). Dynamically changing pituitary hormonal responses to changing physiology are controlled by a complex constellation of molecular transcription factors and genes during development and in adulthood as well as during pituitary neoplasia (Zhu et al. 2007; Asa & Ezzat 1999; Sav 2014). The original notion that different chromophilic pituitary cells were each specialized to only produce a specific hormone is no longer valid. Pituitary cells are plurihormonal and proportions of trophic cells in mice vary under differing conditions as a reflection of pituitary plasticity. Examples include increases in lactotrophs in nursing mice, strain difference in high and low incidence mammary tumor strains (Sasaki et al. 1978); increased gonadotrophs following castration in mice (Levy 2008; Inoue & Kurosumi 1981); and increase in corticotrophs in cold stressed male and female mice (Senovilla et al. 2008). We now better appreciate the importance of pituitary flexibility in adjusting its hormone producing repertoire in response to changing physiological dynamics, including pregnancy, lactation, metabolism, stress, and age.
The plasticity of pituitary hormonal response comes from recruitment of uncommitted chromophobe cells (Levy 2002), transdifferentiation of committed cells (Nunez et al. 2003; Vidal et al. 2001), and/or cell division (Levy 2002) but for rapid responses to changing physiological conditions, cell proliferation is not likely to be a major contributor (Nolan et al 1998; Levy 2008). Also contributing to pituitary plasticity in response is recruitment of cells from the stem/progenitor cell compartment (Vankelecom 2010; Rizzoti et al. 2013). It is estimated that 1.5% of mouse anterior pituitary cells are multipotent progenitor/stem cells (Florio 2011). SOX2-expressing progenitor cells responding to hormonal demands are capable of generating all major cells types in the adult mouse pituitary. SOX2+/SOX9+ identifies transit-amplifying cells and SOX2+/SOX9- cells are multipotent progenitor/stem cells (Fauquier et al. 2008; Florio 2011).
Among the chromophobic cells of the pars distalis is a small population of non-secretory folliculo-stellate cells that have properties of dendritic cells and are variably immunoreactivity for S-100 protein. Their phenotype suggests a lymphoid/mesenchymal anlage in pituitary development in addition to the known neuroectoderm and oral ectoderm tissue anlage (Allaerts et al. 1991; Allaerts et al. 1993; Vankelecom et al. 1993).
Cells of the pars intermedia are larger than those of the pars distalis, are polygonal with densely stained nuclei, and are arranged in groups separated by connective tissue and vascular channels (Liebelt 1994)(Figures 1 & 2). Using fluorescent immunohistochemistry, a-melanocyte stimulating hormone (MSH), b-MSH and adrenocorticotrophic hormone (ACTH) producing cells have been identified in the pars intermedia using fluorescent immunohistochemistry with enhanced excretion by these cells after dehydration and adrenalectomy (Roux & Dubois 1976). The pars intermedia and pars nervous are separated by a capillary plexus with contact zones where the pars intermedia and pars nervosa have direct communication (Jarskar 1977).
There is increasing evidence that a subpopulation of non-hormonal cells function as stem/progenitor cells in the embryonic and adult mouse pituitary gland. These stem cells express high-mobility group (HMG) SOX2 and SOX9 transcription factors and are located around the pituitary cleft (remnant of Rathke’s pouch) as well as scattered in the anterior lobe (Rizzoti 2015; Rizzoti et al., 2013; Andoniadou et al., 2013; Vekelecom 2010; Fauquier et al., 2008). They actively migrate during embryonic and postnatal pituitary organogenesis and in the adult mouse reside in niches where they maintain their stemness (Yoshida et al., 2016).
Neurohypophysis
The neurohypophysis consists of the pars nervosa, the infundibular stalk, and the median eminence of the tuber cinerum of the hypothalamus. The neurohypophysis is hypocellular and mostly occupied by unmyelinated axons of neurosecretory magnocellular neurons originating in the supraoptic and paraventricular nuclei of the hypothalamus (Kelberman et al., 2009) (Figures 1 & 2). The axons terminate in association with a rich capillary network that is supported by modified glial cells (pituicytes). Precursors for the hormones oxytocin and vasopressin are synthesized in the hypothalamic supraoptic and paraventricular nuclei, transported along the axons, stored in axon terminals for subsequent cleavage into active hormone with ultimate release into the vascular capillary network.
Vascular supply
Capillary networks develop in the mouse embryo pituitary as early as gestation day 16 and continue to extend into the parenchyma throughout embryogenesis (Hashimoto et al., 1998). Blood from branches of the internal carotid arteries and the circle of Willis form narrow capillaries in the posterior lobe with the pars intermedia being relatively avascular. A rich blood supply penetrates the pars tuberalis that provides a scaffold for a capillary plexus starting at the median eminence with ultimate drainage into the hypophyseal portal plexus that supplies the pars distalis (Kaufman et al., 2010; Capen 1996).
Physiology
Sometimes referred to as the master gland, the pituitary is controlled by the neurosecretory neurons in the hypothalamus that produce small peptides called releasing hormones and releasing-inhibitory hormones that enter the hypophyseal portal system to act on specific cell types in the adenohypophysis. The releasing hormones bind to specific receptors to initiate Ca2+ signals and subsequent secretion of the appropriate trophic hormone that is specific for a given target tissue. The whole endocrine system is kept in balance by a feedback mechanism whereby the blood concentration of a specific target tissue hormone acts on the hypothalamus to up- or down-regulate production of releasing or releasing-inhibitory signals from the hypothalamus. As examples, TSH is inhibited by T3/T4 from the thyroid; FSH and LH by androgens and estrogens; ACTH by adrenal corticosteroids; and growth hormone (GH), prolactin and MSH by their corresponding release inhibiting hormones.
Anterior Pituitary
Pars Distalis
The anterior pituitary secretes seven endocrine hormones controlled by releasing and inhibitory factors (Table 1): prolactin from mammotrophs, luteinizing hormone and follicle-stimulating hormone from gonadotrophs, growth hormone from somatotrophs, thyroid stimulating hormone from thyrotrophs, adrenocorticotrophic hormone from corticotrophs, and melanocyte-stimulating hormone from melanotrophs. The proportion of the various hormone producing trophic cells fluctuates as a dynamic reflection of changing physiology (Levy 2002). For example, during lactation numerous acidophils and chromophobe cells may be produced with the acidophils generated primarily from transition of chromophobe cells and to a much smaller extent from cell division. Other types of cells show differences associated with sexual maturity (Yamada et al., 1957). Strain differences in number and size of prolactin secreting cells along with difference