New estimates of blood flow rates in the vertebral artery of euarchontans and their implications for encephalic blood flow scaling: A response to Seymour and Snelling (2018)

Exploring the relationship between soft and hard tissues: The example of vertebral arteries and transverse foramina

Understanding how the brain is provided with glucose and oxygen is of particular interest in human evolutionary studies. In addition to the internal carotid arteries, vertebral arteries contribute significantly to the cerebral and cerebellar blood flow. The size of the transverse foramina has been suggested to represent a reliable proxy for assessing the size of the vertebral arteries in fossil specimens. To test this assumption, here, we statistically explore spatial relationships between the transverse foramina and the vertebral arteries in extant humans. Contrast computed tomography (CT) scans of the cervical regions of 16 living humans were collected. Cross-sectional areas of the right and left transverse foramina and the corresponding vertebral arteries were measured on each cervical vertebra from C1 to C6 within the same individuals. The cross-sectional areas of the foramina and corresponding arteries range between 13.40 and 71.25 mm² and between 4.53 and 29.40 mm² , respectively. The two variables are significantly correlated except in C1. Using regression analyses, we generate equations that can be subsequently used to estimate the size of the vertebral arteries in fossil specimens. By providing additional evidence of intra- and inter-individual size variation of the arteries and corresponding foramina in extant humans, our study introduces an essential database for a better understanding of the evolutionary story of soft tissues in the fossil record.

Morphology of the nutrient artery and its foramen in relation to femoral bone perfusion rates of laying and non-laying hens

If arteries penetrate bones through foramina, regional artery blood flow rates can be estimated from the foramen sizes. Femoral bone blood flow rates estimated from nutrient foramen sizes were previously not absolute, but only a relative blood flow index (Qi ), because the size relationship between the foramen and the occupying artery was unknown. The current study used vascular contrast and micro-computerized tomographic scanning to investigate femoral nutrient foramen and nutrient artery sizes in three groups of sub-adult chickens (non-laying hens, laying hens, and roosters) of similar ages. The results indicate that the cross-sectional area of the nutrient artery lumen occupies approximately 20.2 ± 4.1% of the foramen for femora with only one foramen. Artery lumen size is significantly correlated with foramen size. Vascular contrast imaging is capable of estimating blood flow rates through nutrient arteries, as blood flow rates estimated from artery lumen casts are similar to blood flow rates measured by infusion of fluorescent-labeled microspheres. Laying hens tend to have higher nutrient artery perfusion rates than non-laying hens, probably due to extra oxygen and calcium requirements for eggshell production, although the calculated blood flow difference was not statistically significant. Histological embedding and sectioning along with vascular contrast imaging reveal variable nutrient foramen morphology and nutrient artery location among femora with more than one nutrient foramen.

Comparative anatomy of the carotid canal in the Miocene small-bodied catarrhine Pliobates cataloniae

The small-bodied Miocene catarrhine Pliobates cataloniae (11.6 Ma, Spain) displays a mosaic of catarrhine symplesiomorphies and hominoid synapomorphies that hinders deciphering its phylogenetic relationships. Based on cladistic analyses, it has been interpreted as a stem hominoid or as a pliopithecoid. Intriguingly, the carotid canal orientation of Pliobates was originally described as hylobatid-like. The variation in carotid canal morphology among anthropoid clades shown in previous studies suggests that this structure might be phylogenetically informative. However, its potential for phylogenetic reconstruction among extinct catarrhines remains largely unexplored. Here we quantify the orientation, proportions, and course of the carotid canal in Pliobates, extant anthropoids and other Miocene catarrhines (Epipliopithecus, Victoriapithecus, and Ekembo) using three-dimensional morphometric techniques. We also compute phylogenetic signal and reconstruct the ancestral carotid canal course for main anthropoid clades. Our results reveal that carotid canal morphology embeds strong phylogenetic signal but mostly discriminates between platyrrhines and catarrhines, with an extensive overlap among extant catarrhine families. The analyzed extinct taxa display a quite similar carotid canal morphology more closely resembling that of extant catarrhines. Nevertheless, our results for Pliobates highlight some differences compared with the pliopithecid Epipliopithecus, which displays a somewhat more platyrrhine-like morphology. In contrast, Pliobates appears as derived toward the modern catarrhine condition as the stem cercopithecid Victoriapithecus and the stem hominoid Ekembo, which more closely resemble one another. Moreover, Pliobates appears somewhat derived toward the reconstructed ancestral hominoid morphotype, being more similar than other Miocene catarrhines to the condition of great apes and the hylobatid Symphalangus. Overall, our results rule out previously noted similarities in carotid canal morphology between Pliobates and hylobatids, but do not show particular similarities with pliopithecoids either-as opposed to extant and other extinct catarrhines. Additional analyses will be required to clarify the phylogenetic relationships of Pliobates, particularly given its dental similarities with dendropithecids.

Regional femoral bone blood flow rates in laying and non-laying chickens estimated with fluorescent microspheres

The metabolic rate of vertebrate bone tissue is related to bone growth, repair and homeostasis, which are all dependent on life stage. Bone metabolic rate is difficult to measure directly, but absolute blood flow rate () should reflect local tissue oxygen requirements. A recent ‘foramen technique’ has derived an index of blood flow rate () by measuring nutrient foramen sizes of long bones. is assumed to be proportional to ; however, the assumption has never been tested. This study used fluorescent microsphere infusion to measure femoral bone in anaesthetized non-laying hens, laying hens and roosters. Mean mass-specific cardiac output was 338±38 ml min⁻¹ kg⁻¹, and the two femora received 0.63±0.10% of this. Laying hens had higher wet bone mass-specific to femora (0.23±0.09 ml min⁻¹ g⁻¹) than the non-laying hens (0.12±0.06 ml min⁻¹ g⁻¹) and roosters (0.14±0.04 ml min⁻¹ g⁻¹), presumably associated with higher bone calcium mobilization during eggshell production. Estimated metabolic rate of femoral bone was 0.019 ml O₂ min⁻¹ g⁻¹. Femoral increased significantly with body mass, but was not correlated with nutrient foramen radius (r), probably because of a narrow range in foramen radius. Over all 18 chickens, femoral shaft was 1.07±0.30 ml min⁻¹ mm⁻¹. Mean in chickens was significantly higher than predicted by an allometric relationship for adult cursorial bird species, possibly because the birds were still growing.

Summary: Femoral bone blood flow, measured using fluorescent microspheres, is approximately two times higher in laying hens than in non-laying hens and roosters. Blood flow values were related to foramen sizes.

Cerebral blood flow rates in recent great apes are greater than in Australopithecus species that had equal or larger brains

Brain metabolic rate (MR) is linked mainly to the cost of synaptic activity, so may be a better correlate of cognitive ability than brain size alone. Among primates, the sizes of arterial foramina in recent and fossil skulls can be used to evaluate brain blood flow rate, which is proportional to brain MR. We use this approach to calculate flow rate in the internal carotid arteries (Q˙ICA), which supply most of the primate cerebrum. Q˙ICA is up to two times higher in recent gorillas, chimpanzees and orangutans compared with 3-million-year-old australopithecine human relatives, which had equal or larger brains. The scaling relationships between Q˙ICA and brain volume (V_br) show exponents of 1.03 across 44 species of living haplorhine primates and 1.41 across 12 species of fossil hominins. Thus, the evolutionary trajectory for brain perfusion is much steeper among ancestral hominins than would be predicted from living primates. Between 4.4-million-year-old Ardipithecus and Homo sapiens, V_br increased 4.7-fold, but Q˙ICA increased 9.3-fold, indicating an approximate doubling of metabolic intensity of brain tissue. By contrast, Q˙ICA is proportional to V_br among haplorhine primates, suggesting a constant volume-specific brain MR.

Bone foramen dimensions and blood flow calculation: best practices

Some blood vessels enter bones through foramina, leaving the size of the foramen as a gauge for estimating the rate of blood flow and hence the metabolic rate of the supplied tissues. Foramen dimensions have been measured using varied methods in previous foramen studies, to relate regional blood flows with associated physiological processes. With the increasing interests in this 'foramen technique', standard methods with minimized measurement errors are therefore required. This study provides details of microphotographic and micro-computerized tomographic methods, and introduces a new alternative method, which uses impression material to measure foramen dimensions. The three methods are compared and the results indicate that all of them are capable of obtaining precise and accurate foramen dimension values, although they all have limitations. A microphotograph of the external opening is suggested to be the standard method because of its ease of use, but the alternative methods provide more detailed information on foramen shape. If the foramen is mainly occupied by one artery, blood flow rates can be calculated from foramen size and artery wall-lumen ratio, which is evaluated from the literature survey in this study. If veins or nerves also penetrate the foramen, a relative index of blood flow rate is nevertheless possible for comparative purposes.

Interspecific scaling of blood flow rates and arterial sizes in mammals

This meta-study investigated the relationships between blood flow rate (Q̇; cm³ s^-1), wall shear stress (τ_w; dyn cm^-2) and lumen radius (r _i; cm) in 20 named systemic arteries of nine species of mammals, ranging in mass from 23 g mice to 652 kg cows, at rest. In the dataset, derived from 50 studies, lumen radius varied between 3.7 µm in a cremaster artery of a rat and 11.2 mm in the aorta of a human. The 92 logged data points of [Formula: see text] and r _i are described by a single second-order polynomial curve with the equation: [Formula: see text] The slope of the curve increased from approximately 2 in the largest arteries to approximately 3 in the smallest ones. Thus, da Vinci's rule ([Formula: see text]) applies to the main arteries and Murray's law ([Formula: see text]) applies to the microcirculation. A subset of the data, comprising only cephalic arteries in which [Formula: see text] is fairly constant, yielded the allometric power equation: [Formula: see text] These empirical equations allow calculation of resting perfusion rates from arterial lumen size alone, without reliance on theoretical models or assumptions on the scaling of wall shear stress in relation to body mass. As expected, [Formula: see text] of individual named arteries is strongly affected by body mass; however, [Formula: see text] of the common carotid artery from six species (mouse to horse) is also sensitive to differences in whole-body basal metabolic rate, independent of the effect of body mass.