Microvasculature of the nasal salt gland of the duckling, Anas platyrhynchos: quantitative responses to osmotic adaptation and deadaptation studied with vascular corrosion casting

From Corrosion Casting to Virtual Dissection: Contrast-Enhanced Vascular Imaging using Hafnium Oxide Nanocrystals

Vascular corrosion casting is a method used to visualize the three dimensional (3D) anatomy and branching pattern of blood vessels. A polymer resin is injected in the vascular system and, after curing, the surrounding tissue is removed. The latter often deforms or even fractures the fragile cast. Here, a method is proposed that does not require corrosion, and is based on in situ micro computed tomography (micro-CT) scans. To overcome the lack of CT contrast between the polymer cast and the animals' surrounding soft tissue, hafnium oxide nanocrystals (HfO2 NCs) are introduced as CT contrast agents into the resin. The NCs dramatically improve the overall CT contrast of the cast and allow for straightforward segmentation in the CT scans. Careful design of the NC surface chemistry ensures the colloidal stability of the NCs in the casting resin. Using only 5 m% of HfO2 NCs, high-quality cardiovascular casts of both zebrafish and mice can be automatically segmented using CT imaging software. This allows to differentiate even μ $\umu$ m-scale details without having to alter the current resin injection methods. This new method of virtual dissection by visualizing casts in situ using contrast-enhanced CT imaging greatly expands the application potential of the technique.

3-dimensional resin casting and imaging of mouse portal vein or intrahepatic bile duct system

In organs, the correct architecture of vascular and ductal structures is indispensable for proper physiological function, and the formation and maintenance of these structures is a highly regulated process. The analysis of these complex, 3-dimensional structures has greatly depended on either 2-dimensional examination in section or on dye injection studies. These techniques, however, are not able to provide a complete and quantifiable representation of the ductal or vascular structures they are intended to elucidate. Alternatively, the nature of 3-dimensional plastic resin casts generates a permanent snapshot of the system and is a novel and widely useful technique for visualizing and quantifying 3-dimensional structures and networks. A crucial advantage of the resin casting system is the ability to determine the intact and connected, or communicating, structure of a blood vessel or duct. The structure of vascular and ductal networks are crucial for organ function, and this technique has the potential to aid study of vascular and ductal networks in several ways. Resin casting may be used to analyze normal morphology and functional architecture of a luminal structure, identify developmental morphogenetic changes, and uncover morphological differences in tissue architecture between normal and disease states. Previous work has utilized resin casting to study, for example, architectural and functional defects within the mouse intrahepatic bile duct system that were not reflected in 2-dimensional analysis of the structure(1,2), alterations in brain vasculature of a Alzheimer's disease mouse model(3), portal vein abnormalities in portal hypertensive and cirrhotic mice(4), developmental steps in rat lymphatic maturation between immature and adult lungs(5), immediate microvascular changes in the rat liver, pancreas, and kidney in response in to chemical injury(6). Here we present a method of generating a 3-dimensional resin cast of a mouse vascular or ductal network, focusing specifically on the portal vein and intrahepatic bile duct. These casts can be visualized by clearing or macerating the tissue and can then be analyzed. This technique can be applied to virtually any vascular or ductal system and would be directly applicable to any study inquiring into the development, function, maintenance, or injury of a 3-dimensional ductal or vascular structure.

Valves in small veins and venules

It is commonly believed that valves are absent in veins smaller than two millimetres in diameter. Consequently, current investigations on the pathophysiology of chronic venous disease (CVD) consider and evaluate only the valvular competence of large veins. The authors review literature from their own collections as well as from medical database searches to assess the functional relevance of these valves. Microscopic venous valves (MVVs) were first described in 1934 in the human digits and have subsequently been demonstrated in other parts of the human body as well as in many tissues and organs of animals. Their location and arrangement suggests that MVVs prevent blood reflux in small sized veins and restrict flow from postcapillary venules back into the capillary bed. This haemodynamic role of MVVs is strongly supported by the clinical finding that grafting skin rich in MVVs results in long-lasting healing leg ulcers attributable to CVD. The huge body of knowledge available concerning MVVs urges us to correct textbooks of anatomy. Studies on the pathophysiology of CVI should acknowledge that the valvular "chain" is not limited to large veins, but extends down to the venular level where MVVs play an important role in venous haemodynamics.

Vertebrate salt glands: short- and long-term regulation of function

Excess salt loads in most non-mammalian vertebrates are dealt with by a variety of extra-renal salt-secreting structures collectively described as salt glands. The best studied of these are the supra-orbital nasal salt glands of birds. Two distinct types of response to osmoregulatory disturbances are shown by this structure: a progressive adaptive response on initial exposure to a salt load that results in the induction and enhancement of the secretory performance or capabilities of the gland; and the rapid activation of existing osmoregulatory mechanisms in the adapted gland in response to immediate osmoregulatory imbalance. Not only is the time-frame of these two types of response very different, but the responses usually involve fundamentally different processes: e.g., the growth and differentiation of osmoregulatory structures and their components in the former case, compared with the rapid activation of ion channels, pumps etc. in the latter. Despite marked differences in the nature and time-frame of these responses, they both are apparently triggered by neuronally released acetylcholine, which acts at muscarinic receptors on the secretory cells to induce an inositol phosphate-dependent increase in cytosolic-free calcium concentrations ([Ca2+]i). Therefore, the question arises as to how the cells produce the appropriate distinct response using a single common signal (i.e., an increase in [Ca2+]i). Examination of the features of this signaling pathway in the two conditions described, reveals that they each are uniquely tuned to generate a response with the characteristics appropriate for the cells' requirements. This tuning of the signal involves often rather subtle changes in the overall signaling pathway that are part of the adaptive differentiation process.

Partial uncoupling of salt gland blood flow and secretion in the Pekin duck (Anas platyrhynchos)

1. The aim of this study was to investigate the relationship between the blood flow through and the secretion by the salt glands of conscious, salt-water-adapted Pekin ducks. 2. Intravenous loading with hypertonic saline induced a steady-state secretion from the salt glands with a concomitant increase in whole-organ blood flow. The distribution of elevated local glandular blood flow was, however, uneven and in addition demonstrated vasomotor patterns that ranged from constant to rhythmic. 3. During on-going salt gland secretion, the infusion of three vasoactive agents, 5Val-angiotensin II (ANG II), 8Arg-vasotocin (AVT) and noradrenaline, via the carotid artery had differential effects on salt gland blood flow and secretion. 4. ANG II (80 pmol min-1 (kg body wt)-1) had no effect on mean arterial blood pressure (MABP), produced a transient 30% decrease in glandular blood flow and strongly diminished salt gland secretion (retention of 6.4 mosmol NaCl). 5. AVT (20 pmol min-1 (kg body wt)-1) had no effect on MABP and did not alter salt gland secretion despite a 35% reduction in blood flow. 6. Noradrenaline (20 nmol min-1 (kg body wt)-1) elevated MABP by 15 mmHg, reduced salt gland blood flow by more than 50%, but diminished salt gland secretion only slightly (retention of 2.7 mosmol NaCl). 7. Using ANG II, AVT and noradrenaline as hormonal tools, integrated changes in blood flow rate did not correspond with integrated changes in salt gland excretion. The partial dissociation between both parameters shows that control of secretion by the salt gland is more complex than simply being linearly dependent upon blood flow through it.