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The pituitary is a tiny endocrine gland located at the base of the brain, where it is connected to the hypothalamus. The gland integrates peripheral and central (hypothalamic) inputs to generate a tuned and coordinated hormone release, thereby regulating downstream target endocrine organs (such as adrenal glands and gonads) for producing appropriate hormones at the proper time. The pituitary is the key regulator of the endocrine system and is therefore rightfully termed the master gland1.
The mouse pituitary consists of three lobes (Figure 1), i.e., the anterior lobe (AL), the intermediate lobe (IL), and the posterior lobe (PL). The major endocrine AL contains five hormonal cell types, including somatotropes that produce growth hormone (GH); lactotropes generating prolactin (PRL); corticotropes that secrete adrenocorticotropic hormone (ACTH); thyrotropes responsible for thyroid-stimulating hormone (TSH) production; and gonadotropes that make luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The PL consists of axonal projections from the hypothalamus in which the hormones oxytocin and vasopressin (antidiuretic hormone) are stored. The IL is located in-between the AL and PL and houses melanotropes that produce melanocyte-stimulating hormone (MSH). In the human pituitary, the IL regresses during development, and melanotropes are spread within the AL1. In addition to the endocrine cells, the pituitary gland also contains a pool of stem cells, essentially marked by the transcription factor SOX22,3,4,5,6. These SOX2+ cells are located in the marginal zone (MZ), the epithelial lining of the cleft (an embryonic remnant lumen between the AL and IL), or are spread as clusters throughout the parenchyma of the AL, thereby proposing two stem cell niches in the gland (Figure 1)2,3,4,5,6.
Given the indispensable nature of the pituitary, malfunctioning of the gland is associated with serious morbidity. Hyperpituitarism (characterized by over-secretion of one or more hormones) and hypopituitarism (defective or missing production of one or more hormones) can be caused by pituitary neuroendocrine tumors (PitNETs; e.g., ACTH-producing tumors leading to Cushing's disease) or by genetic defects (e.g., GH deficiency resulting in dwarfism)7. In addition, pituitary surgery (e.g., to remove tumors), infections (e.g., hypothalamic-pituitary tuberculosis, or infections following bacterial meningitis or encephalitis), Sheehan's syndrome (necrosis because of insufficient blood flow due to heavy blood loss at birth-giving), pituitary apoplexy and traumatic brain injury are other important causes of pituitary hypofunction8. It has been shown that the mouse pituitary possesses the regenerative capacity, being able to repair local damage introduced by transgenic ablation of endocrine cells9,10. The SOX2+ stem cells acutely react to the inflicted injury showing an activated phenotype, marked by enhanced proliferation (resulting in stem cell expansion) and increased expression of stemness-related factors and pathways (e.g., WNT/NOTCH). Moreover, the stem cells start to express the ablated hormone, finally resulting in substantial restoration of the depleted cell population over the following (5 to 6) months9,10. Also, during the neonatal maturation phase of the gland (the first 3 weeks after birth), the pituitary stem cells are thriving in an activated state6,11,12,13, whereas organismal aging is associated with declined in situ stem cell functionality, due to an increasing inflammatory (micro-) environment at aging (or 'inflammaging')10,14. In addition, tumorigenesis in the gland is also associated with stem cell activation7,15. Although stem cell activation has been detected in several situations of pituitary remodeling (reviewed in7,16), underlying mechanisms remain unclear. Since in vivo approaches (such as lineage tracing in transgenic mice) have not delivered a clear or comprehensive picture of pituitary stem cells, the development of reliable in vitro models to explore stem cell biology in normal and diseased pituitary is essential. Standard in vitro culture of primary pituitary stem cells remains inadequate because of very limited growth capacity and non-physiological (2D) conditions with rapid loss of phenotype (for a more detailed overview, see16). 3D sphere cultures (pituispheres) have been established from pituitary stem cells as identified by side population and SOX2+ phenotype2,3,4. The pituispheres clonally grow from the stem cells, express stemness markers and show differentiation capacity into the endocrine cell types. However, they do not considerably expand while showing only limited passageability (2-3 passages)3,4. Sphere-like structures were also obtained from non-dissociated pituitary stem cell clusters when cultured in 50% diluted Matrigel for 1 week, but expandability was not shown17. The pituisphere approach is mostly used as a read-out tool for stem cell numbers, but further applications are limited by inferior expansion capacity16.
To address and overcome these shortcomings, a new 3D model has recently been established, i.e., organoids, starting from the major endocrine AL of mice containing the MZ and parenchymal stem cells. It has been shown that the organoids are indeed derived from the pituitary's stem cells and faithfully recapitulate their phenotype18. Moreover, the organoids are long-term expandable, while robustly maintaining their stemness nature. Therefore, they provide a reliable method to expand primary pituitary stem cells for profound exploration. Such exploration is not achievable with the limited number of stem cells that can be isolated from a pituitary, which are also not expandable in 2D conditions16. It has been shown that the organoids are valuable and reliable tools to uncover new pituitary stem cell features (translatable to in vivo)14,18. Importantly, the organoid model faithfully mirrors the pituitary stem cell activation status as occurring during local tissue damage and neonatal maturation, showing enhanced formation efficiency and replicating upregulated molecular pathways14,18. Hence, the pituitary-derived organoid model is an innovative and powerful pituitary stem cell biology research model as well as a stem cell activation readout tool.
This protocol describes in detail the establishment of mouse pituitary-derived organoids. To this aim, the AL is isolated and dissociated into single cells, which are embedded in extracellular matrix-mimicking Matrigel (hereon referred to as ECM). The cell-ECM assembly is then cultured in a defined medium, essentially containing stem cell growth factors and pituitary embryonic regulators (further referred to as 'pituitary organoid medium' (PitOM)18; Table 1). Once the organoids are fully developed (after 10-14 days), they can be further expanded trough sequential passaging and subjected to extensive downstream exploration (e.g., immunofluorescence, RT-qPCR, and bulk or single-cell transcriptomics; Figure 1). In the longer run, it is expected that the pituitary stem cell organoids will pave the way to tissue repair approaches and regenerative medicine.