Small-caliber Blood Vessels Research Articles

Tissue Engineering of an Implantable Bioartificial Hemofilter

The first step in the tissue engineering of an implantable bioartificial kidney is the development of an implant that produces ultrafiltrate to replace glomerular function. A fabricated device containing synthetic hollow hemofiltration fibers was placed around the femoral vascular pedicle in rats, which initiated new tissue formation with a mature and durable neocapillary bed. The transudate fluid produced by this newly formed capillary bed accumulated through the hollow fibers into a subcutaneous port to allow evaluation of the fluid. In its first phase, this study evaluated various hollow fibers and tissue induction processes by the measurement of fluid volume, urea nitrogen, and total protein continuously for 6 weeks. New tissues formed within the implants surrounding the fibers, and the vascular density, vessel sizes, and percent cross-sectional vascular area were assessed by means of histomorphometric analysis after 6 weeks. The volume of fluid formation correlated with both vascular density and fiber membrane surface area. The implant fluid-to-serum ratios demonstrated a permselective filtrate. In a second phase, platelet-derived growth factor and vascular endothelial growth factor versus carrier alone were infused directly into the implants for the first 4 weeks in vivo through osmotic pumps and followed up to 9 weeks. Cumulative implant fluid volumes were significantly greater in the growth factor-treated group than in control animals and were associated with greater numbers of small-caliber blood vessels. These results provide the initial proof of concept in developing a tissue-engineered hemofilter prototype on a small scale in a rodent model.

Engineering of Blood Vessels from Acellular Collagen Matrices Coated with Human Endothelial Cells

Small-caliber synthetic grafts used for coronary bypass artery grafting are compromised by thrombogenicity and accelerated intimal thickening, resulting in early graft occlusion. Herein we describe the fabrication and physical properties of small-caliber blood vessels using decellularized porcine aortic segments. These vessels were further coated with human saphenous vein endothelial cells (HSVECs) for future clinical applications. Chemical staining of decellularized vessels showed that they preserved their native matrix architecture, including several collagen layers in between internal and external elastin layers. The burst pressure for the decellularized vessels was higher than 1,000 mmHg. HSVECs, seeded on the luminal side, adhered to the matrix and formed a uniform monolayer. HSVEC-seeded vessels produced prostaglandin I2 and released vasoactive agents in response to the calcium ionophore A23187. These results show that engineered blood vessels coated with the host endothelial cells possess morphologic and functional characteristics of human small-caliber vessels. Tissue-engineered vessels may potentially be useful clinically as vascular grafts.

A Histologic Evaluation of Various Stages of Palatal Healing Following Subepithelial Connective Tissue Grafting Procedures: A Comparison of Eight Cases

It is often necessary to procure tissue from the same area of the palate in cases where connective autogenous grafting procedures are warranted due to limitations caused by anatomical features. The purpose of this study was to determine whether the length of time between a first procurement and a second would have any bearing on the quality of tissue available for recipient sites. Eight patients requiring more than one grafting procedure underwent surgery at baseline and again at various intervals ranging from 6 weeks to 11 months. Specimens were taken from the palate and evaluated microscopically, and photographs were taken for purposes of visual comparison. The 6.9- to 7.7-week specimens exhibited complete reepithelialization. The lamina propriae were composed of a cellular proliferation of fibroblasts with loosely arranged collagen deposition and an occasional thin vascular channel. However, remodeling of the wound appeared complete in the specimens removed at the 9-week interval and beyond. The lamina propria was, in general, composed of thick, dense, interlacing bundles of collagen. Small-caliber blood vessels were interspersed throughout the fibrous element. Reharvesting of tissue performed earlier than at 9 weeks may result in poorer autogenous graft quality due to indications that remodeling of the connective tissue is still progressing and not as mature as specimens noted at weeks 9 to 47.

Proliferation and differentiation into endothelial cells of human bone marrow mesenchymal stem cells (MSCs) on poly DL-lactic-co-glycolic acid (PLGA) films

The functional realization is the most important problem in vascular tissue engineering. The small-caliber blood vessel substitutes are prone to thrombi, which results in functional loss of blood vessels. However, this is probably due to the imperfection of endothelial layer in the substitutes. In this study, MSCs were seeded on a series of porous PLGA films with various porosity and pore size made by sodium chloride (NaCl) particulate leaching, and cell proliferation on each film was inspected. The film made of the 75% (w/w) particulate proportion and 30–50 μm pore size maximized the proliferation rate and was chosen as the scaffolds for the differentiation of MSCs into endothelial cells. The induced cells expressed endothelial cells specific Flk-1, VIII factor and CD34, possessed endothelial cells specific Weible-palade (W-P) body, and had the abilities of ingesting low density lipoprotein and secreting prostacyclin (PGI2). The results show that MSCs not only have the ideal biological compatibility with the porous PLGA films, but also have the potency of differentiating into functional endothelial cells, which should facilitate the endothelialization in vascular tissue engineering.

Engineering of fibrin-based functional and implantable small-diameter blood vessels

Conclusion: Fibrin can serve as an appropriate scaffold for engineering small caliber blood vessels that have both vasoreactivity and considerable mechanical strength. Summary: Currently, autogenous tissue, such as saphenous vein or radial artery, is the gold standard for an arterial substitute for a small caliber vessel. The authors engineered implantable small diameter blood vessels using a fibrin gel with embedded ovin smooth muscle and endothelial cells. Fibrin gels can achieve high seeding of smooth muscle and endothelial cells with uniform cellular distribution. Fibrin might thus be an alternative to collagen as a scaffold for an engineered small caliber vessel. The fibrin matrix based tissue engineered vessels developed in this study exhibited considerable reactivity in response to both receptor and non-receptor mediated dilators and vasoconstrictors. Apoprotinin when added to fibrin gel preparation at moderate concentrations increased mechanical strength of the engineered blood vessel but decreased vascular activity. Mechanical strength and reactivity of the blood vessels were comparable to those of native veins. The mechanical strength of the fibrin-based blood vessels was sufficiently strong to withstand implantation into the jugular veins of lambs. Late microscopic analysis indicated tissue engineered vessels integrated well to native tissue while demonstrating similar flow rates and patency as native vessels. At fifteen weeks after implantation the fibrin-based vessels demonstrated matrix remodeling with production of both elastin and collagen fibers and orientation of smooth muscle cells perpendicular to the direction of blood flow. Comment: One of the “Holy Grails” of vascular surgery is the development of a commercially available, off the shelf, highly effective arterial substitute for a small caliber vessel. This study indicates that use of a fibrin scaffold for development of such an arterial substitute is a potentially fertile line of inquiry. These engineered vessels appear to remain patent when implanted, seamlessly integrate into native vessel, and display vaso-reactivity similar to normal veins. Obviously, development of a commercial product is many years away. Nevertheless, it is good to know that, even after the failure of endothelial seeding of prosthetic grafts, investigators are still pursuing the development of a small caliber arterial substitute.

The potential for neurovascular intravenous angiography using K-edge digital subtraction angiography

Background: Catheterization of small-caliber blood vessels in the central nervous system can be extremely challenging. Alternatively, intravenous (i.v.) administration of contrast agent is minimally invasive and therefore carries a much lower risk for the patient. With conventional X-ray equipment, volumes of contrast agent that could be safely administered to the patient do not allow acquisition of high-quality images after i.v. injection, because the contrast bolus is extremely diluted by passage through the heart. However, synchrotron-based digital K-edge subtraction angiography does allow acquisition of high-quality images after i.v. administration of relatively small doses of contrast agent. Materials and methods: Eight adult male New Zealand rabbits were used for our experiments. Animals were submitted to both angiography with conventional X-ray equipment and synchrotron-based digital subtraction angiography. Results: With conventional X-ray equipment, no contrast was seen in either cerebral or spinal blood vessels after i.v. injection of iodinated contrast agent. However, using K-edge digital subtraction angiography, as little as 1 ml iodinated contrast agent, when administered as i.v. bolus, yielded images of small-caliber blood vessels in the central nervous system (both brain and spinal cord). Conclusions: If it would be possible to image blood vessels of the same diameter in the central nervous system of human patients, the synchrotron-based technique could yield high-quality images at a significantly lower risk for the patient than conventional X-ray imaging. Images could be acquired where catheterization of feeding blood vessels has proven impossible.

A Multiaxial Computer-Controlled Organ Culture and Biomechanical Device for Mouse Carotid Arteries

Much of our understanding of vascular mechanotransduction has come from studies using either cell culture or in vivo animal models, but the recent success of organ culture systems offers an exciting alternative. In studying cell-mediated vascular adaptations to altered loading, organ culture allows one to impose well-controlled mechanical loads and to perform multiaxial mechanical tests on the same vessel throughout the culture period, and thereby to observe cell-mediated vascular adaptations independent of neural and hormonal effects. Here, we present a computer-controlled perfused organ culture and biomechanical testing device designed for small caliber (50-5000 micron) blood vessels. This device can control precisely the pulsatile pressure, luminal flow, and axial load (or stretch) and perform intermittent biaxial (pressure-diameter and axial load-length) and functional tests to quantify adaptations in mechanical behavior and cellular function, respectively. Device capabilities are demonstrated by culturing mouse carotid arteries for 4 days.

Correlation Between Angiographic and Pathological Findings in Experimental Choroidal Neovascular Membranes After Transpupillary Thermotherapy

To evaluate the correlation between angiographic and pathological findings in experimental choroidal neovascular membranes after transpupillary thermotherapy (TTT). Experimental subretinal choroidal neovascular membranes were produced in the monkey eye macula by intense photocoagulation. TTT was delivered, targeting the choroidal neovascular membranes, using a diode laser at 810 nm. One week and 4 weeks after TTT, indocyanine green angiography (IA), fluorescein angiography (FA), optical coherence tomography (OCT), and histological examinations were performed. One week after TTT, indocyanine green dye leakage from choroidal neovascular membranes was reduced on IA, and FA showed staining in the late phase. Light microscopic examination showed numerous small-caliber blood vessels and persistent subretinal exudation. Four weeks after TTT, there was no indocyanine green dye leakage on IA, and FA showed staining. OCT showed a decrease in retinal detachment. Light microscopic examination showed fewer blood vessels and subretinal exudation had disappeared. Choroidal neovascular membranes were composed of fibrous tissue. Changes in angiographic and OCT findings observed after TTT were inferred to be the result of accelerated fibrosis due to TTT and the disappearance of exudation.

In vivo tissue-engineered small-caliber arterial graft prosthesis consisting of autologous tissue (biotube).

In this study, vascular-like tubular tissues called biotubes, consisting of autologous tissues, were prepared using in vivo tissue engineering. Their mechanical properties were evaluated for application as a small-caliber artificial vascular prosthesis. The biotubes were prepared by embedding six kinds of polymeric rods [poly(ethylene) (PE), poly(fluoroacetate) (PFA), poly(methyl methacrylate) (PMMA), segmented poly(urethane) (PU), poly(vinyl chloride) (PVC), and silicone (Si)] as a mold in six subcutaneous pouches in the dorsal skin of New Zealand White rabbits. For rods apart from PFA, biotubes were constructed after 1 month of implantation by encapsulation around the polymeric implants. The wall thickness of the biotubes ranged from about 50 to 200 microm depending on the implant material and were in the order PFA < PVC < PMMA < PU < PE. As for PE, PMMA, and PVC, the thickness increased after 3 months of implantation and ranged from 1.5-to 2-fold. None of the biotubes were ruptured when a hydrostatic pressure was gradually applied to their lumen up to 200 mmHg. The relationship between the intraluminal pressure and the external diameter, which was highly reproducible, showed a "J"-shaped curve similar to the native artery. The tissue mostly consisted of collagen-rich extracellular matrices and fibroblasts. Generally, the tissue was relatively firm and inelastic for Si and soft for PMMA. For PMMA, PE, and PVC the stiffness parameter (beta value; one of the indexes for compliance) of the biotubes obtained was similar to those of the human coronary, femoral, and carotid arteries, respectively. Biotubes, which possess the ability for wide adjustments in their matrices, mechanics, shape, and luminal surface design, can be applied for use as small-caliber blood vessels and are an ideal implant because they avoid immunological rejection.

Vasculitic renal disease*

SUMMARY: The kidney may be involved in several forms of systemic vasculitis, particularly those affecting small‐calibre blood vessels. Hence, the majority of cases involve patients with Wegener's granulomatosis, Churg‐Strauss syndrome or microscopic polyangiitis. Anti‐neutrophil cytoplasmic antibodies (ANCA) are found in the majority of these patients and their detection has increased the sensitivity and specificity of diagnosis. However, the role of ANCA in the pathogenesis of these lesions remains uncertain. Moreover, the approach to treatment, both in terms of induction and maintenance of remission, remains problematic. This brief review discusses aspects of ANCA and current approaches to management.

The Anatomy of the Ligaments of the Wrist and Distal Radioulnar Joints

The ligaments of the wrist are responsible for guiding and constraining the complex motion of the carpal bones relative to the forearm bones, the metacarpals, and contiguous carpal bones. The majority of wrist ligaments are found within the joint capsule as organized thickenings composed of parallel collagen fascicles, small caliber nerves and blood vessels, and lined on their deep surfaces by synoviocytes. The palmar radiocarpal ligament complex is composed of the radioscaphocapitate, long radiolunate, radioscapholunate and short radiolunate ligaments. The ulnocarpal ligaments include the ulnolunate, ulnotriquetral and ulnocapitate ligaments. Dorsally, the radiocarpal joint is spanned by the dorsal radiocarpal ligament. Palmar ligaments connecting the proximal and distal carpal rows include the scaphotrapeziotrapezoid, scaphocapitate, triquetrocapitate and triquetrohamate ligaments. Within each row are interosseous ligaments connecting adjacent carpal bones, each divisible into dorsal and palmar components. There are unique regions within some of the ligaments, such as a zone of fibrocartilage in the proximal regions of the scapholunate and lunotriquetral interosseous ligaments, and strong deep regions connecting the trapezoid, capitate, and hamate. The distal radioulnar joint is connected by the triangular fibrocartilage complex, composed of a fibrocartilaginous disc and the palmar and dorsal radioulnar ligaments. The ulnocarpal ligaments attach to the palmar radioulnar ligament rather than directly to the ulna, allowing increased independence between wrist and forearm motion.

Linfomatosis intravascular: una etiología infrecuente de isquemia cerebral recurrente

Intravascular lymphomatosis is a rarely seen clinicopathological condition. To review the literature on cerebral ischemia and intravascular lymphomatosis. Intravascular lymphomatosis is usually caused by an uncommon type of non-Hodgkin lymphoma, usually of B cells, characterized by the localization of predominantly neoplastic cells within the small calibre blood vessels (arterioles, venules and capillaries). Therefore, it tends to cause multifocal vascular occlusions, with symptoms generally limited to the central nervous system, in the form of recurrent, multifocal cerebral infarcts--one in every 5,000 consecutive cases is usually of this aetiology--or rapidly progressive encephalopathy. Cerebral or meningeal biopsy, or biopsy of peripheral nerve, muscle, adrenal or lymphoid tissue, prostate or lung is usually diagnostic. The differential diagnosis is with vasculitis, multi-infarct dementia, occult neoplasia or infections. In spite of starting suitable treatment with chemotherapy or radiotherapy, the average survival from onset of symptoms is usually four months. Intravascular lymphomatosis should be taken into account in the differential diagnosis of repeated cerebral ischaemia of unusual aetiology.

Rapid intraoperative construction of autologous small caliber blood vessels.

Autologous pericardium, briefly tanned in glutaraldehyde, is an excellent biomaterial when used in various cardiac procedures, including repair and replacement of heart valves. A generalized lack of calcification and an absence of inflammatory response in these combined experiences has led the authors to consider the use of treated autologous pericardium for the construction of substitute small caliber blood vessels. Rapid, intraoperative construction of a vascular graft from autologous pericardium, briefly treated in 0.62% glutaraldehyde, is accomplished through the use of two concentric, mating helical stents that support a rectangular pattern of tissue into a cylindrical, nonkinking, compliant conduit. A disposable automated assembly tool provides for precise assembly of the tissue and stents. The tools and methods for construction of vascular grafts from nonvascular tissue are known as the Rapidgraft (Ramus Medical Technologies, Santa Barbara, CA). The technique can be used with any suitable tissue. Accelerated fatigue test studies have confirmed that stent supported pericardial grafts are capable of withstanding physiologic pressures and flows beyond 7 equivalent years. Six autologous pericardial grafts measuring 5 mm in diameter by 5.5 cm in length showed 100% patency beyond 5 months in a growing calf carotid artery model. Pathological examination of explanted grafts confirm the presence of a continuous endothelial lined surface, infiltration of the tissue by fibrous connective tissue such that the individual layers of the pericardial vessel wall could not be identified, and there was no inflammatory response. Based on the encouraging results in animal studies, a small multicenter clinical trial has been initiated to evaluate the performance of the Rapidgraft as a replacement conduit for the radial artery in cases in which the radial artery has been harvested as a coronary artery bypass conduit. Results from the radial artery trial will be used as an indication to evaluate the Rapidgraft as a coronary artery substitute. We conclude that the material properties, including the biological origin of a vessel wall, may be significant determinants of graft patency, and that the Rapidgraft may be an answer to the need for small caliber arterial graft with the potential for long-term patency and durability.

Autonomic innervation of the human middle ear: An immunohistochemical study

Purpose: Although there have been numerous studies of autonomic innervation of the middle ear mucosa, and the mechanism of effusion into the middle ear cavity in animals, the autonomic innervation of the human middle ear has not received much attention. The purpose of this study is to show the presence of catecholaminergic nerve fibers in the human middle-ear mucus membrane that may play an important role in the pathogenesis of middle-ear effusion. Materials and Methods: A total of 126 celloidin-embedded temporal bone sections from the temporal bone bank at the House Ear Institute were used for immunohistochemical study. A polyclonal antibody to tyrosine hydroxylase enzyme was used to show the presence of catecholaminergic nerve fibers. Results: Tyrosine hydroxylase immunoreactive nerve fibers containing numerous fine varicosities along their course, characteristic of noradrenergic neurons, were observed throughout the middle-ear mucosa including the promontary, sinus tympani, mesotympanum, and hypotympanum. In addition, these nerve fibers were seen in close promixity to the small-caliber blood vessels. A striking variation in the intensity of staining as well as in the amount of nerve fibers was observed among the temporal bone sections. Conclusion: It is possible that the catecholaminergic nerve fibers, like elsewhere in the body, may exert a direct influence on the middle-ear mucosal blood vessels. We speculate that the effusion into the middle-ear space is an active, rather than a passive process. It is conceivable that cholinergic-sympathetic nerves might exist in the human middle-ear mucus membrane, and that these autonomic nerves, in conjunction with the neuropeptides, may play an active role in the pathogenesis of human middle-ear effusion.

Cutaneous and systemic necrotizing vasculitis in swine.

A systemic vasculitis involving particularly the skin and kidneys has been recently described in swine under the name dermatitis/nephropathy syndrome. Twelve pigs with gross cutaneous lesions typical of this condition were necropsied, and morphologic, immunohistochemical, microbiologic, and epidemiologic characteristics were studied. The pigs were divided into three groups comprising eight pigs with acute lesions, two with chronic lesions, and two with acute lesions kept for sequential skin biopsies. Acute skin lesions consisted of round to irregular, red to purple macules and papules that often coalesced to form large, irregular patches and plaques. With time, the lesions became covered by crusts and faded gradually, sometimes leaving scars. Characteristic distribution included the perineal area of the hindquarters, limbs, dependent parts of the abdomen and thorax, and margins of the ears. In the acute phase of the disease, necrotizing and leucocytoclastic vasculitis of small-caliber blood vessels were observed within the dermis and panniculus and in various extracutaneous locations such as the renal pelvis and synovial membranes. All pigs had macroscopic evidence of pneumonia and generalized lymphadenopathy. Microscopically, they had interstitial pneumonia and perivascular cuffing of mononuclear cells in various tissues including skin. The presence of immunoglobulins and complement was demonstrated by immunofluorescence in and around necrotic vessels of the skin in the early stages. Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) antigens were detected by immunohistochemistry in macrophages located around vessels of the tissues examined (skin and kidneys) in acute and chronic cases. PRRSV RNA was demonstrated by reverse transcription-polymerase chain reaction in lung and spleen homogenates from all pigs. The PRRSV was isolated in cell culture from 11 of the pigs. These findings suggest that PRRSV infection may play a role in the pathogenesis of this systemic vascular disease of swine.

Nitrergic and VIPergic neurons in the choroid and ciliary ganglion of the duck Anis carina.

Immunohistochemistry for neuronal nitric oxide synthase (nNOS) and vasoactive intestinal peptide (VIP), and NADPH diaphorase histochemistry, were applied to investigate neurons in the choroid and the ciliary ganglion of the muscovy duck Anis carina. Up to 1000 neurons in the choroid stained for NADPH diaphorase and showed virtually complete colocalization for nNOS immunoreactivity. Almost all of them co-stained for VIP, while about 90% of VIP immunoreactive cell bodies showed colocalization for nNOS. Two-thirds of the neurons were located, mostly singly, at nodes of a wide-meshed nerve plexus in the suprachoroid and were only rarely grouped in ganglia of up to 3 neurons. Numerous varicose nNOS/NADPH-diaphorase-positive nerve fibers were seen around large arterial blood vessels. These fibers derived mainly from paravascular cell bodies that represented about one-third of all choroidal neurons and also displayed costaining for nitrergic markers and VIP. Colocalization of nNOS/NADPH-d and VIP could be demonstrated in most of the perivascular fibers, while slightly more VIP-positive axons in the suprachoroid plexus did not costain for nNOS/NADPH-d. Small-caliber blood vessels and those localized in the choriocapillaris were not endowed with VIP/nNOS/NADPH-diaphorase-positive fibers. A few reactive neuronal cell bodies were also found in ciliary nerves, while most ciliary axons were unstained. In the ciliary ganglion a small subpopulation of neurons showed VIP/nNOS/NADPH-diaphorase colocalization. There were also nNOS/NADPH-d-positive cap-like terminals on ciliary ganglion cells. The presence of VIP/nNOS/NADPH-diaphorase positive neurons and nerve fibers in both the choroid and ciliary ganglion, and in the choroidal perivascular plexus, indicates peripheral nitrergic and VIPergic control of blood flow in the choroid of the duck.

Effects of Acute Exposure to Hyperbaric Oxygen on the Rheology and Morphology of the Red Blood Cells in the Rat

The effects of acute exposure to hyperbaric oxygen (HBO) on the rheology and morphology of red blood cells (RBC) were studied in three groups of Sprague-Dawley rats: a control (CON) group comprised rats not exposed to HBO, a second group was exposed to HBO at 2.8 atm for 6 hr and studied immediately after the exposure (HBO), and a third group was examined after being allowed to recover in room air for 24 hr after exposure to HBO (REC). RBC deformability was assessed by two different techniques, the ektacytometer and the micropore filters. While the ektacytometer did not detect a significant difference among the three groups, the filtration method showed that acute exposure to HBO causes a significant increase in the filtration index (FI), denoting a significant reduction in RBC deformability. However, this reduction returned to the control Level after 24 hr of recovery in air (FI, 7.1 ± 1.2 in CON and 9.5 ± 2.6 in REC compared to 36.2 ± 3.7 in HBO, P < 0.0001). The morphology of RBC was studied by scanning electron microscopy. Immediately after exposure to HBO there was a marked increase in the echinocytes (41.6 ± 5.9%) compared to a control level of 16.7 ± 4.8%, P < 0.05, The increase in the echinocytes became less pronounced after 24 hr of recovery (23.5 < 9.3%). In conclusion, acute exposure to HBO causes significant alterations in RBC rheology and morphology. Due to the sensitivity of the micropore filters, we hypothesize that the decrease in RBC deformability might affect their flow through small-caliber blood vessels. These alterations, however, are reversible after a relatively short period of recovery in air.

Hybrid Concept on the Mechanical Test Method of Small Caliber Blood Vessel

A mechanical test on small caliber blood vessel is difficult because the strength is very low and the dimension is very small. In the present paper a multifunctional axial test apparatus design is proposed. This has the characteristics such that the load cell and the environmental container for the specimen are built up in terms of cassette, respectively, and thus both are easy to equip and take off, that is, easy to replace. Also, the test methodology by internal pressure for small caliber blood vessels has been proposed. By using both of these methodologies, the characteristics of the mechanical behavior of small caliber blood vessels, such as compliance, stress relaxation, and viscoelastic property have been clarified. An attempt has been made on the application of this method to a clinical case.

1 Functional cytology of the human testis

The seminiferous tubular compartment of the human testis occupies about two thirds of the volume of the organ and is supported by loose connective tissue containing the Leydig cells. Sertoli cells extend from the basal lamina to the lumen of the seminiferous tubule and provide structural and functional support to the germ cells which proliferate and mature through the complex process of spermatogenesis, lasting for approximately 70 days. Histological examination of the human seminiferous epithelium gives the impression of a very irregular pattern of germ cell development but recent cytological, ultrastructural and computer-modelling studies have shown that the arrangement of germ cells within the seminiferous tubules is in fact highly organized into a helical pattern based upon the geometry of concentric spirals. Thus the human spermatogenic cycle is precisely regulated in accordance with the more familiar ordered arrangements of germ cells seen in most non-human primates and other mammalian species. The population of Leydig cells in the adult human testis represents the third and final phase of their developmental history, preceded by distinct neonatal and, earlier, fetal Leydig cell generations. Human Leydig cells are irregular in outline, deeply staining with basophilic dyes and exhibit a characteristic circular or ovoid nucleus. The density of their cytoplasm is attributed to extensive domains of smooth endoplasmic reticulum, characteristic of steroidogenic tissue. Mitochondria are numerous and lipid droplets and lipfuscin inclusions accumulate with advancing age. Crystals of Reinke up to 30 microns in length are often noted in the Leydig cell cytoplasm although their function remains unknown. The intertubular tissue contains small calibre lymphatic vessels, fibroblasts, collagen, blood vessels and occasional macrophages. With increasing age, declining sperm production and lowered androgen levels are correlated with decreases in the numbers of Sertoli cells and Leydig cells although the mechanisms responsible for this attrition have not been defined. Continued collaboration between morphologists and physiologists is necessary if we are to understand fully the endocrine regulation of spermatogenesis and the factors contributing to disturbances of spermatogenic function.

Formation of a functional endothelium on vascular grafts

The lack of a functional endothelial cell lining on artificial polymeric vascular grafts severely reduces their effectiveness in replacing small caliber (less than 6 mm) blood vessels. Techniques have now been developed to transplant autologous endothelial cells from one site in the body onto the surface of grafts prior to implantation. Pre-clinical animal trials provide evidence that grafts sodded with autologous, fat-derived, microvessel endothelial cells exhibit a stable, antithrombogenic lining of endothelium. The new endothelial cell lining exhibits morphologies identical with endothelium on native blood vessels. The effectiveness of endothelial cell sodding techniques in pre-clinical animal trials provides support for expanded clinical trials.