Body water conservation through selective brain cooling by the carotid rete: a physiological feature for surviving climate change?

Anatomical variations in the cerebral arterial circle of the Saimaa (Pusa hispida saimensis) and Baltic ringed seals (Pusa hispida botnica)

The intracranial arterial vascularization of the Saimaa ringed seals (Pusa hispida saimensis; Nordquist, 1899) and Baltic ringed seals (Pusa hispida botnica; Gmelin, 1788) disclosed patterns of anatomical architecture comparable to that of other pinniped species. Arterial silicone casts on skull scaffolds, and magnetic resonance imaging (MRI) showed that the besides joining the caudal communicating arteries upon entering the cerebral arterial circle, the bilateral internal carotid arteries bifurcated as laterally oriented rostral choroidal arteries and rostral cerebral arteries. The latter arteries almost immediately gave off the laterally oriented middle cerebral arteries. Numerous individual variations were evident in differences in the exact branching sites of bilateral vessels or the size or number of arterial branches. Two Saimaa ringed seals had only a tiny foramen for the left internal carotid artery to enter the intracranial space, and the intracranial part of this vessel was short. It did not reach the cerebral arterial circle. The intracranial part of the right internal carotid artery is bifurcated and also supplied the left side of the cerebral arterial circle. Both specimens had aplasia of the left rostral cerebral artery. The intracranial arterial arrangement of Saimaa and Baltic ringed seals reflects the arterial architecture of this body region in terrestrial mammals with little evidence for aquatic adaptations or changes related to thermoregulation.

Fascinating wonderful network: Rete mirabile of the maxillary artery in cats - minireview

Cats are one of the most common companion animals, and they differ from dogs in several important ways. Considering the central importance of anatomy in high-quality medicine, the treatment of the feline mandible, mostly during intraoral procedures requiring general anaesthesia, has many important features. In cats, the major artery of the brain is the maxillary artery that forms unique structure - the rete mirabile. The rete mirabile is a plexus like vascular structure that lies extracranially and communicates with brain arterial circle through the orbital fissure. The development of the brain vasculature is different in cats, and it includes obliteration mechanisms of the internal carotid artery. The course of the maxillary artery that forms the rete mirabile has a strong relationship to the angular process of the mandible. Emphasis should be placed on manipulation with the feline mandible, especially during open-mouth procedures, as mistakes can lead to blindness, deafness, and central neurological disorders due to compression of the maxillary artery by the angular process of the mandible. This paper focuses on the anatomy and function of the blood supply to the brain, which is very specific in domestic cats and other felids.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

Revised concept of rete-like collateral formation: Rete mirabile does not exist in humans

The carotid rete is a physiological network between the external and internal carotid arteries (ICA) in lower vertebrates. However, true carotid rete does not exist in humans. This review aimed to contrast the physiological function of human "rete-like collaterals" with that of lower vertebrate "rete mirabile". An explanation for the development of rete-like collaterals in human intracranial arteries was also discussed. The rete mirabile (carotid, vertebral, spinal, and thoracic) in lower vertebrates has a specific physiological role and does not form vasculature for the same purpose in humans. Therefore, the term "rete mirabile" should not be used for cases reported in humans. Instead, "rete-like collaterals" is preferred. In the literature, rete-like or arterial anastomosis was observed in the ICA cavernous portion and the intradural arteries. Based on the hypothesis of the segmental concept, it applies to the ICA and intracranial arteries. Whether in the ICA, middle cerebral artery, posterior cerebral artery, or posterior inferior cerebellar artery, the segmental concept is the same and should be considered to have formed secondary collaterals after segmental regress or dysgenesis of affected arteries. Summarily, the significance of this review lies in its reevaluation of vascular structures previously described as "carotid rete" in humans to a true and preferred term, "rete-like collaterals". It also provides insights into the historical context and potential genetic factors associated with the formation of arteries in humans, contributing to a better understanding of human vascular anatomy.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 1: Foundational principles and theories of regulation

This contribution is the first of a four-part, historical series encompassing foundational principles, mechanistic hypotheses and supported facts concerning human thermoregulation during athletic and occupational pursuits, as understood 100 years ago and now. Herein, the emphasis is upon the physical and physiological principles underlying thermoregulation, the goal of which is thermal homeostasis (homeothermy). As one of many homeostatic processes affected by exercise, thermoregulation shares, and competes for, physiological resources. The impact of that sharing is revealed through the physiological measurements that we take (Part 2), in the physiological responses to the thermal stresses to which we are exposed (Part 3) and in the adaptations that increase our tolerance to those stresses (Part 4). Exercising muscles impose our most-powerful heat stress, and the physiological avenues for redistributing heat, and for balancing heat exchange with the environment, must adhere to the laws of physics. The first principles of internal and external heat exchange were established before 1900, yet their full significance is not always recognised. Those physiological processes are governed by a thermoregulatory centre, which employs feedback and feedforward control, and which functions as far more than a thermostat with a set-point, as once was thought. The hypothalamus, today established firmly as the neural seat of thermoregulation, does not regulate deep-body temperature alone, but an integrated temperature to which thermoreceptors from all over the body contribute, including the skin and probably the muscles. No work factor needs to be invoked to explain how body temperature is stabilised during exercise.

Arterial Blood Supply to the Cerebral Arterial Circle in the Selected Species of Carnivora Order from Poland

Carnivores are a wide, diverse group of mammals whose representatives live all over the world. The study presents the results of the analysis of the arterial vascularization of the blood supply to the cerebral arterial circle of selected species in the Caniformia suborder living in Poland. The selected group consists of wild and farm animals-105 animals in total. Three different methods were used-latex preparation, corrosion cast, and cone-beam computed tomography angiography. The main source of blood for encephalon in the described species is the internal carotid artery, and the second one is the vertebral artery. The results were discussed in relation to the current knowledge of this field of research. Information on the potential physiological meaning of such vascular pattern has been provided.

The rete mirabile: a possible control site for swimbladder function

In a recent study, a large number of transport proteins was detected in the transcriptome and proteome of saline perfused rete mirabile tissue of the European eel. In this study, the data set was reanalyzed for the presence of receptor proteins and proteins involved in intracellular signaling pathways. A large number of expressed receptor proteins and proteins involved in intracellular signal transduction was detected. Several G-protein-coupled receptor signal pathways were significantly enriched in their expression level, in particular receptors and signaling pathways involved in the control of blood flow. The enriched signaling pathways also include pathways involved in trafficking of crucial transport proteins like, monocarboxylate transporters, V-ATPase, and aquaporin. The data, therefore, suggest that the rete mirabile has the capacity to control swimbladder function by regulating blood flow and by modifying countercurrent multiplication.

Comparison of the Rostral Epidural Rete Mirabile and the Patterns of Its Blood Supply in Selected Suiformes and Hippopotamuses

The rostral epidural rete mirabile (rete mirabile epidurale rostrale) is built of considerable small arterial vessels that anastomose with each other. This structure is formed as a result of the division of a large arterial vessel into many small arteries. Furthermore, on the other side of the rete mirabile, these small arteries are joined together to form one large artery, through which blood flows out of the rete mirabile. This system participates in decreasing the temperature of the brain and thus protects the body from thermal stress. Moreover, it influences body-water balance and has a crucial role in the retrograde transfer of neuropeptides. The goal of this study was to describe the rostral epidural rete mirabile and pathways that provide blood to this structure as well as compare it in selected Suiformes and hippopotamuses. The study was performed on desert warthogs (Phacochoerus aethiopicus), Eurasian wild boars (Sus scrofa), collared peccaries (Pecari tajacu), pygmy hippopotamuses (Choeropsis liberiensis), and common hippopotamuses (Hippopotamus amphibius). Preparations were made using the latex method and corrosion cast. An elongated shape characterizes its anatomy with a much wider rostral part than caudal part in the Eurasian wild boars, desert warthogs, and collared peccaries. The main source of blood was the branch to the rostral epidural rete mirabile branched off from the internal carotid artery. Moreover, blood enters the rete by the caudal branch and rostral branch to the rostral epidural rete mirabile. In hippopotamuses, the major source of blood was the rostral branches to the rostral epidural rete mirabile.

Arterial circle of the brain in the common wildebeest (Connochaetes taurinus)

The main aim of this study was to analyze the arterial circle of the brain and rostral epidural rete mirabile in specimens of the common wildebeest (Connochaetes taurinus). The arterial circle of the brain is a circulatory anastomosis that supplies blood to particular lobes of the brain and surrounding structures. The study was conducted on male and female adults (n = 12) by bilateral injection of an acetone solution of vinyl superchloride or latex LBS3060, which resulted in a cast of arteries of the head and encephalic base. We describe the exact structure of the arterial circle of the brain and rostral epidural rete mirabile. The shape of the arterial circle of the brain in the common wildebeest is heart-shaped, as in other Bovidae. It is formed by bilateral rostral cerebral arteries, caudal communicating arteries, and the basilar artery, with a general pattern of vessels forming and branching off from the arterial circle of the brain, as described in other ruminants. The rostral cerebral and caudal communicating arteries emerge from an intracranial segment of the internal carotid artery, which in turn arises from vessels of the rostral epidural rete mirabile. This structure, well developed in the examined individuals, has mechanisms underlying selective cooling of the brain, protecting animals from hyperthermia, and affecting body water balance.

Selective Brain Cooling: A New Horizon of Neuroprotection

Therapeutic hypothermia (TH), which prevents irreversible neuronal necrosis and ischemic brain damage, has been proven effective for preventing ischemia-reperfusion injury in post-cardiac arrest syndrome and neonatal encephalopathy in both animal studies and clinical trials. However, lowering the whole-body temperature below 34°C can lead to severe systemic complications such as cardiac, hematologic, immunologic, and metabolic side effects. Although the brain accounts for only 2% of the total body weight, it consumes 20% of the body's total energy at rest and requires a continuous supply of glucose and oxygen to maintain function and structural integrity. As such, theoretically, temperature-controlled selective brain cooling (SBC) may be more beneficial for brain ischemia than systemic pan-ischemia. Various SBC methods have been introduced to selectively cool the brain while minimizing systemic TH-related complications. However, technical setbacks of conventional SBCs, such as insufficient cooling power and relatively expensive coolant and/or irritating effects on skin or mucosal interfaces, limit its application to various clinical settings. This review aimed to integrate current literature on SBC modalities with promising therapeutic potential. Further, future directions were discussed by exploring studies on interesting coping skills in response to environmental or stress-induced hyperthermia among wild animals, including mammals and birds.

The Remarkable Cardiovascular System of Giraffes

Gravity affects the physiology of many animals, and the effect is, for good reason, most pronounced in tall species. The physiology-in particular, cardiovascular function-of giraffes has therefore captivated the interest of physiologists for centuries. Several studies document high mean arterial blood pressure of giraffes of about 200 mm Hg. This appears necessary to establish a cerebral perfusion pressure on the order of 100 mm Hg at the cranial end of the carotid arteries. Here, we discuss the unique characteristics of blood vessels, the heart, and the kidney of giraffes and how these functional and structural adaptations are related to very high blood pressure. We also discuss how the cerebral circulation of giraffes is established and what we know about how the blood flow and arterial and venous pressures in giraffes change when they stop to drink and subsequently lift their heads 5-6 m in one sweeping movement.

Selection of preclinical models to evaluate intranasal brain cooling for acute ischemic stroke

Stroke accounts for a large proportion of global mortality and morbidity. Selective hypothermia, via intranasal cooling devices, is a promising intervention in acute ischemic stroke. However, prior to large clinical trials, preclinical studies in large animal models of ischemic stroke are needed to assess the efficacy, safety, and feasibility of intranasal cooling for selective hypothermia as a neuroprotective strategy. Here, we review the available scientific literature for evidence supporting selective hypothermia and make recommendations of a preclinical, large, animal-based, ischemic stroke model that has the greatest potential for evaluating intranasal cooling for selective hypothermia and neuroprotection. We conclude that among large animal models of focal ischemic stroke including pigs, sheep, dogs, and nonhuman primates (NHPs), cynomolgus macaques have nasal anatomy, nasal vasculature, neuroanatomy, and cerebrovasculature that are most similar to those of humans. Moreover, middle cerebral artery stroke in cynomolgus macaques produces functional and behavioral deficits that are quantifiable to a greater degree of precision and detail than those that can be revealed through available assessments for other large animals. These NHPs are also amenable to extensive neuroimaging studies as a means of monitoring stroke evolution and evaluating infarct size. Hence, we suggest that cynomolgus macaques are best suited to assess the safety and efficacy of intranasal selective hypothermia through an evaluation of hyperacute diffusion-weighted imaging and subsequent investigation of chronic functional recovery, prior to randomized clinical trials in humans.

Ultradian oscillations in brain temperature in sheep: implications for thermoregulatory control?

We compared body temperature patterns and selective brain cooling (SBC) in eight adult female sheep in an indoor (22-25 °C) and outdoor (mean ~ 21 °C) environment, by measuring brain, carotid arterial, and jugular venous blood temperatures at 5-min intervals using implanted data loggers. To investigate whether ultradian oscillations in brain temperature had thermoregulatory consequences for the sheep, we determined the cranial arterio-venous (AV) temperature difference as an indicator of respiratory evaporative heat loss (REHL). The 24-h pattern of SBC was similar in both environments, despite carotid blood temperature fluctuating 0.4 °C more outdoors compared to indoors. The sheep employed SBC more often during the night than during the day, but SBC was abolished at intervals of 1-3 h throughout the 24-h period. The suppression of SBC appeared to be associated with events that increased sympathetic nervous system activity, including shifts between stages of sleep. Short-term changes (over 5-min) in brain temperature were positively correlated with changes in the AV temperature difference 5 min later, and negatively correlated with changes in carotid temperature 10 min later. These data support the idea that increases in brain temperature modulate thermoregulation by increasing REHL, which leads to a decrease in carotid blood temperature. Ultradian oscillations in core temperature of sheep, therefore, appear to arise as a consequence of frequent brain temperature changes invoked by non-thermal inputs, in animals housed both in indoor and outdoor environments.

High‐Resolution Computed Tomography Imaging of the Cranial Arterial System and Rete Mirabile of the Cat ( Felis catus )

The objective of this study was to investigate the possibility of obtaining high‐resolution multiplanar computed tomography (CT) imaging of the cranial arterial circulation of the cat (Felis catus), the rete mirabile, and components of the skull, utilizing preserved cat specimens with an arterial system that was injected with a radiopaque contrast compound in the early 1970s. Review of the literature shows no high‐resolution CT studies of the cat's cranial circulation, with only few plain radiographic studies, all with limited cranial vascular visualization. In view of the inability of the radiographic techniques available from 1970s to mid‐2000s to provide high‐resolution imaging of the arterial circulation within the intact skull and brain of the cat, without dissection and histologic sectioning and disruption of tissues, no further imaging was performed for many years. In 2010, a high‐resolution micro CT scanner became available, large enough to scan the entire nondissected head of the arterially injected cats. All the obtained CT images were processed with a software program that provided 3D volume rendering and multiplanar reconstruction with the ability to change the plane angulation and slab thickness. These technical features permitted more precise identification of specific arterial and bony anatomy. The obtained images demonstrated, with a nondestructive method, high‐resolution vascular anatomy of the cerebral, orbital, facial arterial system, the rete mirabile, and skull bone components of the cat, with details not previously described in the literature. Anat Rec, 302:1958–1967, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Large herbivore assemblages in a changing climate: incorporating water dependence and thermoregulation

The coexistence of different species of large herbivores (ungulates) in grasslands and savannas has fascinated ecologists for decades. However, changes in climate, land‐use and trophic structure of ecosystems increasingly jeopardise the persistence of such diverse assemblages. Body size has been used successfully to explain ungulate niche differentiation with regard to food requirements and predation sensitivity. But this single trait axis insufficiently captures interspecific differences in water requirements and thermoregulatory capacity and thus sensitivity to climate change. Here, we develop a two‐dimensional trait space of body size and minimum dung moisture content that characterises the combined food and water requirements of large herbivores. From this, we predict that increased spatial homogeneity in water availability in drylands reduces the number of ungulate species that will coexist. But we also predict that extreme droughts will cause the larger, water‐dependent grazers as wildebeest, zebra and buffalo–dominant species in savanna ecosystems – to be replaced by smaller, less water‐dependent species. Subsequently, we explore how other constraints such as predation risk and thermoregulation are connected to this two‐dimensional framework. Our novel framework integrates multiple simultaneous stressors for herbivores and yields an extensive set of testable hypotheses about the expected changes in large herbivore community composition following climate change.

How does homeostasis happen? Integrative physiological, systems biological, and evolutionary perspectives

Homeostasis is a founding principle of integrative physiology. In current systems biology, however, homeostasis seems almost invisible. Is homeostasis a key goal driving body processes, or is it an emergent mechanistic fact? In this perspective piece, I propose that the integrative physiological and systems biological viewpoints about homeostasis reflect different epistemologies, different philosophies of knowledge. Integrative physiology is concept driven. It attempts to explain biological phenomena by continuous formation of theories that experimentation or observation can test. In integrative physiology, "function" refers to goals or purposes. Systems biology is data driven. It explains biological phenomena in terms of "omics"-i.e., genomics, gene expression, epigenomics, proteomics, and metabolomics-it depicts the data in computer models of complex cascades or networks, and it makes predictions from the models. In systems biology, "function" refers more to mechanisms than to goals. The integrative physiologist emphasizes homeostasis of internal variables such as Pco₂ and blood pressure. The systems biologist views these emphases as teleological and unparsimonious in that the "regulated variable" (e.g., arterial Pco₂ and blood pressure) and the "regulator" (e.g., the "carbistat" and "barostat") are unobservable constructs. The integrative physiologist views systems biological explanations as not really explanations but descriptions that cannot account for phenomena we humans believe exist, although they cannot be observed directly, such as feelings and, ultimately, the conscious mind. This essay reviews the history of the two epistemologies, emphasizing autonomic neuroscience. I predict rapprochement of integrative physiology with systems biology. The resolution will avoid teleological purposiveness, transcend pure mechanism, and incorporate adaptiveness in evolution, i.e., "Darwinian medicine."

Integrating Competing Demands of Osmoregulatory and Thermoregulatory Homeostasis

Mammals are characterized by a stable core body temperature. When maintenance of core temperature is challenged by ambient or internal heat loads, mammals increase blood flow to the skin, sweat and/or pant, or salivate. These thermoregulatory responses enable evaporative cooling at moist surfaces to dissipate body heat. If water losses incurred during evaporative cooling are not replaced, body fluid homeostasis is challenged. This article reviews the way mammals balance thermoregulation and osmoregulation.

From Anomalous Arteries to Selective Brain Cooling: Parallel Evolution of the Artiodactyl Carotid Rete

Terrestrial artiodactyls (even-toed ungulates) inhabit some of the world's most extreme environments, including arid deserts and high elevations. As medium-to-large-bodied mammals, artiodactyls have a suite of specialized physiologies to facilitate occupation of regions unavailable to other large mammals. One such physiology is selective brain cooling, wherein reduction of brain temperature below core body temperature has been demonstrated to reduce evaporative water loss. This physiology is enabled by an arterial heat-exchanger called the carotid rete. The ubiquity of the carotid rete throughout the clade, as well as its evolutionary history, is currently uninvestigated. Here, I use osteological correlates to survey clade-wide presence and morphology of the carotid rete, prior to conducting a preliminary evolutionary analysis. Nearly all living artiodactyls possess a carotid rete and are capable of selective brain cooling; however, major arteries supplying the rete are derived from different embryonic aortic arches on a suborder-specific basis. Ancestral character estimation infers this pattern of variation to be the result of independent evolutionary processes, suggesting carotid rete homoplasy arising via parallelism. This is a surprising finding given the role this structure plays in driving a physiology that has been implicated in mitigating artiodactylan responses to extreme environmental conditions. Future studies should incorporate extinct species represented in the fossil record to better parse between parallel and convergent mechanisms, as well as to better understand the relationship between the carotid rete, selective brain cooling, and survivorship of climate perturbation. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 303:308-317, 2020. © 2018 American Association for Anatomy.