Proliferation of pituitary corticotrophs following adrenalectomy as revealed by immunohistochemistry combined with bromodeoxyuridine-labeling

Adult pituitary cell maintenance: lineage-specific contribution of self-duplication

The identification of a stable pool of progenitor/stem cells in the adult pituitary has renewed the interest of identifying mechanisms for maintenance of pituitary cells throughout life. Whereas developmental studies have shown that progenitor expansion is the major source of new differentiated cells during pituitary organogenesis, the contribution of these progenitors for maintenance of the adult tissue is not clear although progenitors were clearly involved in cell expansion following end-organ ablation, notably after adrenalectomy and/or gonadectomy. We have used a genetic trick that eliminates dividing cells by apoptosis in order to assess the contribution of differentiated corticotropes and melanotropes for maintenance of their population in the adult pituitary. The system relies on chromosome instability created by the action of the Cre recombinase on inverted loxP sites. Expression of Cre recombinase in corticotropes and melanotropes led to progressive loss of corticotropes whereas melanotropes were unaffected. Because the Cre transgene is not expressed in progenitors, the data indicate that maintenance of the adult corticotrope pool is primarily due to self-duplication of differentiated cells. In contrast, melanotropes do not divide. Maintenance of corticotropes by self-duplication contrasts with the reported proliferative response of undifferentiated cells observed after adrenalectomy. If corticotrope reentry into cell cycle constitutes a normal mechanism to maintain the adult corticotrope pool, this same mechanism may also be perturbed during corticotrope adenoma development in Cushing's disease.

Expression of IGFBP7 mRNA in corticotrophs in the anterior pituitary of adrenalectomized rats

The number of corticotrophs increases in the anterior pituitary (AP) gland in adrenalectomized (AdX) rats. In this study, aimed at identifying the growth factor implicated in this proliferation, we analyzed proteins secreted from a cDNA library of the AP of AdX rats, using the signal sequence trap method. A PCR analysis of several cDNAs that coded for insulin-like growth factor binding protein (IGFBP) 5, IGFBP7, and vacuolar H+-ATPase accessory subunit Ac45 revealed an increased and decreased expression level of IGFBP7 mRNA in the AP of AdX rats and AdX rats injected with dexamethasone, respectively. IGFBP7 mRNA was predominately expressed in the corticotrophs of the APs of both sham-operated and AdX rats. The AP of AdX rats contained an increased number of IGFBP7 mRNA-expressing cells and corticotrophs compared with that of sham-operated rats, but the ratio of IGFBP7 mRNA-positive corticotrophs per total number of corticotrophs did not significantly change in either group. Histochemical analysis of labeled proliferating cell nuclear antigen (PCNA) and sex-determining region Y box-2 (SOX2) revealed the presence of several PCNA-positive signals and the absence of SOX2 cells among the corticotrophs, suggesting that IGFBP7 mRNA-expressing corticotrophs are derived from in situ corticotrophs and that they increase in number as corticotrophs increase. The possible roles of IGFBP7 in the corticotrophs are also discussed.

The parvocellular vasopressinergic system and responsiveness of the hypothalamic pituitary adrenal axis during chronic stress

Vasopressin (VP) secreted from parvocellular neurons of the hypothalamic paraventricular nucleus (PVN) stimulates pituitary adrenocorticotropic hormone (ACTH) secretion, through interaction with receptors of the V1b subtype (V1bR) in the pituitary corticotroph, mainly by potentiating the stimulatory effects of corticotrophin-releasing hormone (CRH). Chronic stress paradigms associated with corticotroph hyperresponsiveness lead to preferential expression of hypothalamic VP over CRH and upregulation of pituitary V1bR, suggesting that VP has a primary role during adaptation of the hypothalamic pituitary adrenal (HPA) axis to long-term stimulation. However, studies using pharmacological or genetic ablation of V1bR have shown that VP is required for full ACTH responses to some stressors, but not for the sensitization of ACTH responses to a novel stress observed during chronic stress. Studies using minipump infusion of a peptide V1 antagonist in long-term adrenalectomized rats have revealed that VP mediates proliferative responses in the pituitary. Nevertheless, only a minor proportion of cells undergoing mitogenesis co-express markers for differentiated corticotrophs or precursors, suggesting that new corticotrophs are recruited from yet undifferentiated cells. The overall evidence supports a limited role of VP regulating acute ACTH responses to some acute stressors and points to cell proliferation and pituitary remodelling as alternative roles for the marked increases in parvocellular vasopressinergic activity during prolonged activation of the HPA axis.

Molecular and trophic mechanisms of tumorigenesis

A significant proportion of pituitary macroadenomas, and by definition all microadenomas, regain trophic stability after an initial period of deregulated growth. Classical proto-oncogene activation and tumor suppressor mutation are rarely responsible, and no histologic or molecular markers reliably predict behavior. GNAS1 activation and the mutations associated with multiple endocrine neoplasia type 1 and Carney complex, aryl hydrocarbon receptor interacting protein gene mutations, and a narrowing region of chromosome 11q13 in familial isolated acromegaly together account for such a small proportion of pituitary adenomas that the pituitary adenoma pathogenic epiphany is surely yet to come.

Non-hormonal cell types in the pituitary candidating for stem cell

Hormone balances in the body are primarily governed by the hypothalamus-pituitary system. For its pivotal role, the pituitary gland relies on an assortment of different hormone-producing cell types, the proportions of which dynamically change in response to fluctuating endocrine demands. Mechanisms of pituitary cellular plasticity are at present far from understood, and may include proliferation and transdifferentiation of hormonal cells. Whether new cells also originate by recruitment from stem cells is unsettled, although this idea has frequently been proposed. Here, I will review these data by focusing on the non-hormonal cell types that have been advanced as candidates for the pituitary stem cell position.

Vasopressin mediates mitogenic responses to adrenalectomy in the rat anterior pituitary

To determine whether increased vasopressinergic activity during chronic stress or adrenalectomy mediates trophic changes in the corticotroph, we examined the effect of peripheral V1 receptor blockade in rats, using the antagonist, dGly[Phaa1,D-tyr(et), Lys, Arg]vasopressin (VP), on the number of pituitary cells taking up bromodeoxyuridine (BrdU) and cells containing immunoreactive ACTH (irACTH). Adrenalectomy significantly increased the number of BrdU- and ACTH-labeled cells at 3 and 6 d, and a much larger increase was observed at 28 d. Minipump infusion of V1 antagonist for 28 d, at doses blocking the increases in ACTH and corticosterone induced by exogenous VP, prevented the increases in BrdU incorporation, but not irACTH cells observed 28 d after adrenalectomy. Unexpectedly, colocalization of BrdU with ACTH-positive cells was minor (about three cells per pituitary section), and this was unaffected by adrenalectomy or V1 antagonist infusion. In contrast, adrenalectomy for 6 or 14 d failed to increase BrdU incorporation or irACTH cells in V1b receptor knockout mice while inducing the expected increase in wild-type mice. The data show that VP is required for pituitary mitogenesis after adrenalectomy but, at least in rats, not for increasing the number of corticotrophs. The lack of colocalization of ACTH in mitotic cells suggests that recruitment of corticotrophs during adrenalectomy occurs from undifferentiated cells.

Stem cells in the postnatal pituitary?

Tissue-specific stem cells are uncovered in a growing number of organs by their molecular expression profile and their potential for self-renewal, multipotent differentiation and tissue regeneration. Whether the pituitary gland also contains a pool of versatile 'master' cells that drive homeostatic, plastic and regenerative cell ontogenesis is at present unknown. Here, I will give an overview of data that may lend support to the existence of stem cells in the postnatal pituitary. During the many decades of pituitary research, various approaches have been used to hunt for the pituitary stem cells. Transplantation and regeneration studies advanced chromophobes as possible source of new hormonal cells. Clonogenicity approaches identified pituitary cells that clonally expand to floating spheres, or to colonies in adherent cell cultures. Behavioural characteristics and changes of marginal, follicular and folliculostellate cells during defined developmental and (patho-)physiological conditions have been interpreted as indicative of a stem cell role. Expression of potential stem cell markers like nestin, as well as topographical localization in the marginal zone around the cleft has also been considered to designate pituitary stem cells. Finally, a 'side population' was recently identified in the postnatal pituitary which in many other tissues represents a stem cell-enriched fraction. Taken together, in the course of the long-standing study of the pituitary, several arguments have been presented to support the existence of stem cells, and multiple cell types have been placed in the spotlight as possible candidates. However, none of these cells has until now unequivocally been shown to meet all quintessential characteristics of stem cells.

A population of non-luteinising hormone/non-adrenocorticotrophic hormone-positive cells in the male rat anterior pituitary responds mitotically to both gonadectomy and adrenalectomy

The male rat anterior pituitary responds highly reproducibly to specific hormonal stimuli in terms of the extent and timing of mitotic and apoptotic (trophic) activity. The principal objective of the present study was to define the contribution of hormonally identifiable cells to the trophic responses to bilateral gonadectomy and bilateral adrenalectomy. The patterns of pituitary mitotic responses to adrenalectomy and gonadectomy are similar in amplitude and duration. When adrenalectomy and gonadectomy are combined, the amplitude of the pituitary mitotic response is unchanged. That is, the trophic stimuli are not additive. Dexamethasone-induced apoptosis in nascent cells is amplified not only by recent adrenalectomy, but also, and to an almost identical extent, by gonadectomy. Combining adrenalectomy and gonadectomy does not further enhance the size of the apoptotically-responsive cell population. Dual bromodeoxyuridine and adrenocorticotrophic hormone (ACTH) or luteinising hormone (LH) immunolabelling showed that more than 95% of all dividing cells are not and do not become positive for either of these hormones during the period of peak mitotic response. Following adrenalectomy, most newly-formed ACTH cells are derived from differentiation of pre-existing hormonally undifferentiated cells. Despite an overall increase in mitotic activity, there is no measurable increase in the number of LH immunopositive cells after gonadectomy. The nonadditive pituitary mitotic and apoptotic responses to adrenalectomy and gonadectomy strongly suggest that the same progenitor cell population responds mitotically to both. This weakens the prevailing view that hormonally identifiable cells with specific trophic profiles contribute significantly to pituitary cell subpopulation revision.

Differences in cell migration of cultured pituitary cells from infantile and adult rats: participation of the extracellular matrix and epidermal growth factor

During early postnatal development in the rat, the tissue architecture of the pituitary gland shows changes, revealing an intense migration process of the cells. The aim of this work was to examine anterior pituitary cell migration over type I and III collagen as well as type IV collagen, of cultured pituitary cells from infantile rats and adult rats, and the participation of the epidermal growth factor in this process. Differences in cell migration rate over these two types of collagen substrates were observed at both ages, and all in all, three times more cells migrated over type I/III collagen than over type IV collagen. These data show the migration-promoting role of type I/III collagen for pituitary cells. Furthermore, when infantile cells were challenged to migrate over bovine serum albumin, the migration rate diminished, and, on the contrary, adult cell migration was higher. However, over collagen, infantile cells increased their migration rate with epidermal growth factor stimulation and adult cells showed a decrease in migration when the growth factor was in the medium. During migration, pituitary cells associated and arranged in clusters. This behavior increased in the presence of epidermal growth factor in the infantile cultures. Moreover, epidermal growth-factor-stimulated infantile cells formed larger aggregates. Adult cells also showed associative behavior, but more cells were observed isolated than in cluster arrangements and the growth factor did not induce changes in this behavior. Results showed a difference in the response of cell migration and cell association capacity to epidermal growth factor after migration of infantile and adult pituitary cells. With these observations we propose that epidermal growth factor is a cell regulator of the pituitary tissue re-arrangement process during the infantile period.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

Temporally sensitive trophic responsiveness of the adrenalectomized rat anterior pituitary to dexamethasone challenge: relationship between mitotic activity and apoptotic sensitivity

Depending on timing and dose, exogenous glucocorticoids induce a wave of apoptosis in the adult rat anterior pituitary, a response that is enhanced by adrenalectomy. In this study, we show that the size of the glucocorticoid-sensitive apoptotic population progressively increases during the week following surgical adrenalectomy, plateaus for a further week, then spontaneously declines to levels seen in intact animals by 4 wk. Mitotic activity, in contrast, rises rapidly post adrenalectomy but returns to baseline within 2 wk. Increased mitotic activity precedes the increase in the population of cells that undergo glucocorticoid-induced apoptosis and the subsequent decline in mitotic activity precedes the decline in apoptotic sensitivity despite persistent elevation of hypothalamic CRH and pituitary proopiomelanocortin transcripts. If glucocorticoid exposure is delayed until 4 wk post adrenalectomy when the apoptotic response has returned to baseline, glucocorticoid withdrawal, by transiently increasing mitotic activity, again primes the formation of an expanded glucocorticoid-sensitive apoptotic cell population. These data suggest that apoptotic sensitivity is largely confined to cells that have recently entered the cell cycle. This observation is further corroborated by demonstrating an abrupt glucocorticoid-induced step-down in the bromodeoxyuridine-labeling index to basal levels in rats given daily injections of bromodeoxyuridine during the week following adrenalectomy.

Increased proliferative activity and programmed cellular death in the turkey hen pituitary gland following interruption of incubation behavior

Incubation behavior or broodiness in turkey hens is characterized by ovarian regression, hyperprolactinemia, and persistent nesting. Nest-deprivation of incubating turkey hens results in disruption of broodiness accompanied by a precipitous decline in plasma prolactin (PRL) concentrations. The objective of the present study is to examine cellular changes in the pituitary gland associated with nest-deprivation for 0, 1, 2, 3, 4, or 7 days. Bromodeoxyuridine (BrdU) was administered prior to kill to study proliferative activity. Pituitary tissue sections were immunostained using turkey growth hormone (GH) antibody, and/or chicken PRL peptide antibody, and BrdU antibody. Plasma PRL concentrations declined significantly following nest-deprivation for 1 or more days. The midsagittal pituitary area immunoreactive (ir) to GH was significantly increased while that of PRL was significantly decreased following nest-deprivation for 2 or more days. Terminal deoxy-UTP nick end labeling and PRL-immunostaining revealed an abundance of apoptotic nuclei in both cephalic and caudal lobes of the anterior pituitary gland, suggestive of programmed cellular death of lactotrophs in the pituitary gland of hens nest-deprived for 2 or more days. Mammosomatotrophs were abundant in hens nest-deprived on Day 0 but were absent in hens nest-deprived for 1 or more days. Proliferating (BrdU-ir) cells were significantly abundant in the pituitary cephalic and caudal lobes following nest-deprivation for 1 or more days but were absent on Day 0 or in laying hens. Dual-labeling studies indicated that most of the BrdU-ir nuclei in the caudal lobe were not colocalized in somatotrophs in hens nest-deprived for 1-4 days but did colocalize with GH following 7 days of nest-deprivation. In conclusion, nest-deprivation of incubating turkey hens results in 1) a precipitous decline in plasma PRL concentration, 2) programmed cell death of lactotrophs, 3) disappearance of mammosomatotrophs, 4) increased proliferative activity of pituitary cells, and 5) recruitment of somatotrophs arising primarily from mitosis of nonsomatotrophic cells.

Treatment of pituitary-dependent Cushing's syndrome: long-term results of unilateral adrenalectomy followed by external pituitary irradiation compared to transsphenoidal pituitary surgery

BACKGROUND

The preferred treatment of Cushing's disease (CD) nowadays is transsphenoidal pituitary surgery (TPS). Prior to TPS, patients at the Leiden University Medical Centre were treated by unilateral adrenalectomy followed by external pituitary irradiation (UAPI). We report on long-term results of both UAPI and TPS and compare remission, relapse rates, and complications.

PATIENTS AND METHODS

A retrospective study was carried out on 130 patients with CD. Patients with pituitary macroadenoma were excluded. Eighty-six and 44 patients underwent UAPI and TPS, respectively. Of these patients, 85 and 41 were evaluable for long-term results.

RESULTS

Remission following UAPI and TPS was identical at 64% (54/85 and 27/41). Cumulative relapse was also comparable - 17% (9/54) and 22% (6/27), respectively, - for UAPI and TPS, although the mean follow-up periods were different - 21.4 years and 8.5 years, respectively. Cumulative disease-free survival curves after UAPI and TPS are identical until 5 years of follow-up, but diverge thereafter indicating more sustained remissions following UAPI (P = 0.17, Wilcoxon statistic). Pituitary dysfunction following UAPI (36%) and pituitary surgery (55%) likewise did not differ significantly. However, pituitary dysfunction was an immediate event after TPS, whereas it developed after a mean interval of 17.8 years following UAPI.Low-dose dexamethasone testing during follow-up had no value in predicting therapeutic outcome.

CONCLUSIONS

The results of unilateral adrenalectomy followed by external pituitary irradiation do not justify that this therapy is totally abandoned in favour of transsphenoidal pituitary surgery. Unilateral adrenalectomy followed by external pituitary irradiation is a valid therapeutic modality for the treatment of Cushing's disease, and could be considered as alternative to bilateral adrenalectomy and under some circumstances to transsphenoidal pituitary surgery.

Pituitary-adrenocortical responses to the chronic administration of granulocyte colony-stimulating factor in rats

Granulocyte colony-stimulating factor (G-CSF) is a hematopoietic growth factor, but it may play a role in the regulation of the neuroendocrine system activity. Only few data are available about its possible influence on the pituitary gland. We have recently reported an acute stimulatory effect of G-CSF (and of GM-CSF) on adrenocorticotropic hormone (ACTH) secretion in rats in vivo. The purpose of the present study was to evaluate whether chronic administration of G-CSF affects ACTH and corticosterone secretion and growth processes of the rat anterior pituitary gland and adrenal cortex in vivo. We have demonstrated that G-CSF (at a dose of 10.0 microg/kg body weight (BW)) injected s.c. once daily (for 7 days), stimulated both ACTH and corticosterone secretion. Simultaneously, G-CSF treatment did not change the total anterior pituitary cell proliferation as revealed by immunohistochemical staining of proliferating cell nuclear antigen (PCNA). On the other hand, proliferative activity of corticotrophs, detected in the sections of the anterior pituitary using double-labeling. was significantly increased after treatment with G-CSF. Moreover, this growth factor induced an increase in the proliferation ratio in the entire adrenal equatorial section. These findings suggest an involvement of G-CSF in the regulation of pituitary-adrenal axis and support the hypothesis of bidirectional associations between the immune system and the endocrine glands.

Influence of granulocyte-macrophage colony stimulating factor on pituitary-adrenal axis (PAA) in rats in vivo

We have studied the in vivo influence of granulocyte-macrophage colony stimulating factor (GM-CSF) on blood plasma concentration of adrenocorticotropic hormone (ACTH) and corticosterone in Wistar rats. The administration of 10 micrograms/kg b.w. GM-CSF at 45 (P < 0.01), 90 (P < 0.01) and at 45 (P < 0.001), 90 (P < 0.001) and 180 min (P < 0.001) increased the secretion of ACTH and corticosterone, respectively. Prolonged administration of 10 micrograms/kg b.w. of GM-CSF increased the ACTH (P < 0.001) and corticosterone (P < 0.001) concentration in blood plasma. We have also found that chronic treatment with 10 micrograms/kg b.w. of GM-CSF increased the proliferative activity of corticotrophs (P < 0.05), but it did not significantly change the total cell proliferation in the anterior pituitary gland. Moreover, this cytokine increased cell proliferation of the adrenal cortex (P < 0.001). These experiments suggest that GM-CSF activates the pituitary-adrenal axis and support the hypothesis of bidirectional associations between the immune and neuroendocrine systems.

Mitosis and apoptosis in the pituitary gland: tumour formation or hyperplasia?

Direct analyses of trophic activity in the pituitary have been hampered by the lack of normal human tissue to study and by the short duration of the histologically overt phases of mitosis and apoptosis, which renders significant trophic events difficult to quantify. In rats and dogs, pituitary cell turnover is rapid in youth, declines markedly with age and virtually ceases by 'late middle age'. Specific stimuli superimpose brief but dramatic trophic events on this active background. There is little convincing evidence, as yet, for plasticity, i.e. the persistence of cell population changes after transient stimuli have passed. In contrast to spontaneous pituitary adenomas in rats, human pituitary tumours show relatively modest increases in mitotic activity. In the light of these observations, this chapter examines the accepted models of pituitary adenoma formation and propagation, with special reference to trophic activity, clonality and tumour behaviour.

Aspects of anterior pituitary growth, with special reference to corticotrophs

The spatial and proportional representation of the various cell groups in the anterior pituitary is fairly constant, although it may differ between sexes. Recognizable changes occur in a number of physiological and pathological situations. The relative roles of hormones and growth factors in these processes are not fully elucidated, nor are their kinetics. In this paper, published work on basal proliferation, growth factor expression and the growth of specific cell types is reviewed. In addition, we present new data to indicate that the maximum level of proliferation in the anterior pituitary of the male Sprague-Dawley rat occurs around 28 days. We have also demonstrated a circadian rhythm of mitosis in the adult male, with a peak around 1100 h. Cell kinetic analysis suggests a duration for G2 of about 2 hours, and for S phase of 10 1/2 to 11 hours. Finally, we provide data which confirm that the expansion of the corticotroph population after bilateral adrenalectomy is partly the result of an early proliferative response in both corticotrophs and other pituitary cells. Our data also suggest that a further expansion takes place which may reflect differentiation of a population other than committed corticotrophs.

Axons containing the growth associated protein GAP-43 specifically target rat corticotrophs following adrenalectomy

An extensive network of nerve fibers immunoreactive for the neuronal growth associated protein GAP-43 (GAP-43-IR) is present within the anterior pituitary (AP) of the rat, and the density of these fibers has been reported to increase 4 days after adrenalectomy (ADX). In the present study, we employed confocal dual-label immunofluorescence microscopy to determine whether GAP-43-IR fibers are specifically associated with corticotrophs at various intervals after ADX. A dramatic increase in the density of GAP-43-IR was apparent 4 days after ADX, and this increase was sustained at 7 and 14 days post-ADX. The percentage of corticotrophs in apparent contact with GAP-43-IR axons was 87% at 4 days after ADX and 92% at 14 days. In addition, fewer than 15% of GAP-43-IR terminals were associated with cells other than corticotrophs in either group. This highly specific targeting of corticotrophs during a period in which these cells are undergoing both hypertrophy and hyperplasia indicates that axonal sprouting is occurring in response to ADX. While the less intense GAP-43-IR in the AP of intact rats precluded precise quantitative analysis, the majority of corticotrophs also appeared to be selectively innervated in these animals. The observations that GAP-43-IR axons selectively contact corticotrophs, and that both the specificity and thoroughness of innervation are maintained by targeted growth of GAP-43-IR axons following ADX, strongly suggest that these fibers are of functional significance.

Anterior pituitary cell population control: basal cell turnover and the effects of adrenalectomy and dexamethasone treatment

We have used an extremely accurate, dedicated, real time computerized image analysis system to facilitate the manual quantification of changes in the prevalence of mitotic figures and apoptotic bodies in male rat pituitary following surgical adrenalectomy and, 14 days later, dexamethasone treatment. Under basal conditions, the prevalence of mitotic figures and apoptotic bodies was 0.066+/-0.016% and 0.030+/-0.012% (mean+/-SE) respectively. Dexamethasone treatment reduced the prevalence of mitotic figures and, in adrenalectomized animals, produced a highly significant and reproducible burst of apoptotic activity that peaked 48 h after the beginning of treatment (0.261+/-0.022%) before falling sharply to control levels within a further 8 h. Two weeks after the start of dexamethasone treatment, total pituitary cell numbers continued to decline. The rate of accumulation of mitotic figures in vivo after colchicine treatment indicates that mitosis is histologically overt in 2 microm thick hematoxylin and eosin stained sections under the light microscope for around 80 min; that apoptosis--identified as classical apoptotic bodies--is overt for 44 min and that, on average, a young, adult, male rat anterior pituitary cell either dies or divides as frequently as once every 60-70 days. These data show that transient and apparently trivial fluctuations in the prevalence of apoptotic and mitotic events have a profound effect on pituitary cell population dynamics, and demonstrate that dexamethasone treatment of adrenalectomized rats produces a decline in total anterior pituitary cell numbers that continues for at least 2 weeks after the start of glucocorticoid treatment.