2 Our model of long-term hypoxia (LTH) uses pregnant ewes maintained at high altitude (3,820 m, Barcroft Labora- tory White Mountain Research Station, Bishop, CA) from approximately 40 days’ gestational age onward resulting in a maintained fetal PO2 of ∼18 mmHg (normal ∼23– 25 mmHg) thus resulting in a moderate state of continuous hypoxia [14–16]. Under these conditions, fetal hypoxia is achieved without accompanying acidosis, the pregnancies are of normal duration, and the fetuses do not exhibit growth restriction [15, 17]. Considering that hypoxia is a potent fetal stressor even at the moderate level attained in our model of LTH, the ability of the LTH fetus to mount an adaptive response circumventing preterm birth and growth restriction is remarkable. The focus of our studies has been the adaptive responses of the fetal hypothalamo-pituitary adrenocortical (HPA) and adrenomedullary axis. During our studies on these components of the stress response, we have also initiated studies into how LTH impacts development of the fetal perirenal adipose tissue and interactions between this tissue and the HPA axis. 2. Effects of LTH on the Fetal HPA Axis Since hypoxia is a potent activator of the fetal HPA axis, we initially explored the eﬀect of LTH on the systemic endocrine HPA response by quantifying basal immunoreactive (IR) ACTH and cortisol concentrations in fetal plasma as well as the response of LTH fetal sheep to an acute secondary stressor (severe hypoxia via hypotension or umbilical cord occlusion: UCO) during the later stages of gestation [14, 16, 18]. Surprisingly, despite the continued stress of LTH, fetal plasma cortisol concentrations were not elevated in these fetuses compared to normoxic control fetuses at 136–141 days gestation (term is ∼145 days). Thus, the LTH fetus maintains a seemingly normal late gestation maturation of the fetal adrenal cortex and thus preserves the late gestation exponential rise in cortisol that precipitates organ matu- ration and, in this species, birth. Similar to cortisol, fetal plasma concentrations of IR-ACTH were also similar in the LTH fetus compared to normoxic controls. Thus, by initial observations, these fetuses apparently adapt to LTH such that this chronic hypoxic perturbation is no longer perceived by the fetus as stressful. Paradoxically, the HPA axis response to an acute sec- ondary stressor such as hypotension or UCO [14, 16] is decidedly diﬀerent than predicted based on the basal circu- lating levels of IR-ACTH and cortisol. Indeed, we found in response to these acute stressors that fetal plasma cortisol was elevated in the LTH fetuses compared to normoxic controls while concentrations of fetal plasma IR-ACTH achieved were similar in LTH and normoxic fetuses. These ﬁndings implicated a potential intra-adrenal mechanism(s) that allow the adrenal cortex of the LTH fetus to respond to an acute stress with increased cortisol production while maintaining the normal basal ontogenic maturation. Neural input has been shown to play a physiological role in regulating cortisol production. For instance, bilateral section of the splanchnic nerve reduces cortisol production Journal of Pregnancy in response to endogenous or exogenous ACTH [19, 20] while stimulation of the splanchnic nerves enhances cortisol output [21, 22]. In near term normoxic fetal sheep, it has been reported that sinoaortic denervation [23] or splanch- nic nerve section [24] decreased the cortisol response to acute stress without aﬀecting stress-induced ACTH release. However, in the LTH fetus, the enhanced cortisol response to a secondary stressor does not appear to be mediated by neural input to the adrenal cortex since adrenal denervated LTH fetuses responded in a manner similar to the intact LTH fetuses [25]. Thus, the increased response of the adrenal cortex to an acute secondary stress in the LTH fetus does not emanate from a neurally mediated mechanism. Based on these initial observations, we further explored the origin(s) of the observed adaptive responses of the HPA axis to LTH at the level of the hypothalamus and anterior pituitary as well as the adrenal cortex. 3. Effect of LTH on the Hypothalamus and Anterior Pituitary Increases in the bioactivity of plasma IR-ACTH have been noted in late gestation fetal sheep, both in terms of ontogeny and in response to an acute stressor [26–29]. In adults, the majority of circulating IR-ACTH consists of fully processed ACTH1−39. However in the ovine fetus, a considerable proportion of IR-ACTH is the unprocessed ACTH precursor, proopiomelanocortin (POMC), or the so-called 22 kDa proACTH, which is the partially processed N-terminal region of POMC through the carboxyl terminus of ACTH [30, 31]. In addition, a few other POMC-derived products containing the ACTH peptide, likely produced via incomplete and/or improper processing at the anterior pituitary or lung [32, 33], are also found in the fetal circulation. Both POMC and 22 kDa proACTH have been noted to inhibit ACTH- stimulated cortisol production or exhibit week ACTH bioac- tivity [34]. These precursors exhibit varying degrees of cross- reactivity in classical one-site ACTH radioimmunoassays or ELISAs thus giving rise to the concept of bioactivity of IR-ACTH. Therefore, during late gestation the processing of POMC to ACTH in the anterior pituitary corticotrope matures and this maturation is largely governed by the hypothalamic paraventricular nucleus. This was conﬁrmed by our observation that stereotaxic lesion of the fetal PVN, the source of the two major corticotrophin releasing factors, CRH and AVP, prevents the late gestation increase in POMC processing to ACTH [35]. Thus, we hypothesized that LTH, acting as a chronic stressor, may have enhanced POMC processing to ACTH in the fetal anterior pituitary. To evaluate this possibility, we analyzed both basal and stress-induced ACTH1−39and ACTH precursor concentra- tions in fetal plasma of LTH compared to control fetuses using a two-site IRMA speciﬁc for ACTH1−39 and an ACTH precursor IRMA [36]. In response to an acute secondary stressor, UCO, fetal plasma concentrations of ACTH1−39 were higher in the plasma of LTH fetuses compared to sim- ilarly stressed normoxic fetuses. Although ACTH precursors were also elevated, it was to a lesser extent than ACTH