Microvascular anatomy of the pig eye: scanning electron microscopy of vascular corrosion casts

The microvasculature of the eye of 10 pigs was investigated using scanning electron micrographs of corrosion casts. The ciliary body, iris and bulbar conjunctiva were supplied by the iridociliary ring artery via the long posterior ciliary artery. Capillaries of the ciliary process were of large diameter (23.2-27.5 microm) with an irregular bore, forming a thoroughfare channel draining blood in the ciliary arterioles into the pars plana venous vessels. Arterioles and venules in the iris exhibited a zigzag or spiral features. The third palpebra was supplied by the anterior ciliary artery. The capillary bed of the third palpebra was dense and was formed by many rows of fine hair-pin loops. Capillaries in the bulbar conjunctiva formed a sparse network disposing approximately parallel to the epithelium and formed a well-developed venous plexus, draining into the vortex veins. Retinal arterioles formed a slender and long course to capillaries. Retinal capillaries were extremely thin (3.0-4.0 microm in diameter). The choroid was supplied by the short posterior ciliary arteries. Choroidal arterioles exhibited a thick and short course to the choriocapillaris. The choriocapillaris was flat and sinusoid-like (8.9-13.9 microm in diameter), forming a dense sheet-like network. Blood from the choroid emptied into the episcleral vein via the vortex vein. Blood from the retina was drained by the posterior ciliary veins. The functional significance of this vascular architecture was discussed.     

Microvasculature of the rat eye: scanning electron microscopy of vascular corrosion casts

Methylmethacrylate castings of the eye microvasculature were prepared from 10 Spargue-Dawley rats and studied by electron microscopy. The choroidal arterioles are larger in diameter than retinal arterioles, and have a shorter course to choroidal capillaries than retinal arterioles to retinal capillaries. Retinal capillaries are extremely thin in diameter and form a sparse retinal capillary network. The choriocapillaris is large and sinusoid-like, forming a compactly arranged network in the choroid. These differences in the microvasculature between the choroid and retina help explain the differences in ocular hemodynamics; that is, the blood flow in the choroid is faster than that in the retina. Capillaries of the iris show a zigzag configuration, which may be an accommodation for dilation and constriction of the pupil. Capillaries of the ciliary body are of large diameter forming leaf-like configurations, presumably to contribute to the secretion of the aqueous humor. Capillaries of the conjunctiva exhibit a somewhat coiled configuration, the arrangement of which reduce tension of the conjunctiva vessels caused by eyeball movement. Intra-arterial cushions, which control blood flow at the branching sites, are found in both choroidal or retinal arterioles.     

Microvascular architecture of the rabbit eye: a scanning electron microscopic study of vascular corrosion casts

The microvasculature of the eyes of 5 rabbits was investigated using scanning electron microscopy on corrosion casts. The study revealed that the pars plana vessels draining blood from the iris and ciliary body coursed directly into the anterior vortex venous system constituting the scleral venous plexus (the venous circle of Hovius). The episcleral vasculature was found to possess a specialized morphology, with channels draining the aqueous humor. The capillaries of the third palpebral, bulbar and palpebral conjunctiva formed a single-layered capillary network approximately parallel to the epithelium and formed a well-developed venous plexus in the stroma. The retina was found to be merangiotic, meaning that vessels were present only in a small part of the retina, extending in a horizontal direction to form bands on either side of the optic disc. Channels representing the aqueous veins that drained blood mixed with aqueous humor were found to derive directly from the suprachoroidal space and communicate with the scleral venous plexus via the anterior vortex veins. The functional significance of the microvasculature of the iris, cilia, retina and choroid is discussed in this report as well. The elaborate microvasculature of the conjunctiva may be a prerequisite for the exchange of nutrients and gasses between the cornea and the vessels across the conjunctival epithelium when the eyelids are shut during sleep, and possibly for the dynamics of eye drop delivery. The scleral venous plexus in rabbits may be analogous to the scleral venous sinus (Schlemm's canal) in rats, primates and humans.     

Comparative Doppler imaging of the ophthalmic vasculature in normal Beagles and Beagles with inherited primary open-angle glaucoma

The objective of this study was to compare orbital and ocular vasculature velocity, measured by Doppler imaging, in normal Beagles and Beagles with inherited primary open-angle glaucoma. Eight normal Beagles and 13 Beagles with different stages of primary open-angle glaucoma were evaluated twice with a 2–4-week period between measurements. Doppler imaging was performed with the dogs anesthetized, and the Doppler transducer applied directly on the corneal surface. The majority of the orbital vasculature (external ethmoidal artery; internal ophthalmic artery and vein; and external ophthalmic artery and vein) and ocular blood vessels (anterior ciliary artery and veins; long posterior ciliary arteries; short posterior ciliary arteries; primary retinal arteries; and the vortex veins) were identified and Doppler blood velocity parameters were determined. The glaucomatous dogs demonstrated significant differences in the Doppler velocity parameters of several orbital vessels (external ethmoidal, external ophthalmic, and internal ophthalmic arteries), and several ocular vessels (anterior ciliary, short posterior ciliary, and long posterior ciliary arteries). These differences included decreased blood velocities, and increased pulsatility and resistive indexes. The Doppler blood flow velocities of the primary retinal arteries were unchanged between the normal and glaucomatous dogs. In the glaucomatous dogs, the Doppler imaging suggests increased vascular resistance downstream in both the orbital and ocular vasculature. These blood velocity parameter changes may be primary or secondary, and may offer therapeutic opportunities to increase perfusion, prolong the retina and optic nerve head function, and maintain vision in the canine glaucomas.     

Vasculature of the orbital rete in the Japanese deer (Cervus nippon)

The vasculature of the orbital rete (rete mirabile ophthalmicum) in Japanese deer ( Cervus nippon ) was studied using corrosion casting, scanning electron microscopy, and histology. The orbital rete is a flat, triangular- or leaf-shaped arterial network, which consists of a complex of small arterioles, that intermixes with a similar complex of the supraorbital vein at the base of the orbital cavity. Blood to the retina passes through the orbital rete. The orbital retial arterioles leave the parent external ophthalmic artery at right angles forming T-shaped bifurcations, and follow a tortuous, undulating course. Each retial arteriole is connected by side branches and forms a rope-ladder-like network. Some of the side branches are surrounded by a groove representing the intra-arterial cushion that regulates blood flow at branching sites. The central retinal artery supplying the retina originates from the orbital rete. The ciliary arteries supplying the choroid arise from the external ophthalmic artery proximal to the orbital rete. The anatomical specializations of the orbital rete may involve buffering the blood pressure and flow to the retina and regulating ocular tissue temperature as in the carotid rete. In addition, the orbital rete may help dampen the tension that the vessel exerts on the retina, by stretching in response to eyeball movement.     

Doppler imaging of the ophthalmic vasculature of the normal dog: blood velocity measurements and reproducibility

The purpose of this study was to assess the feasibility and reproducibility of color Doppler imaging (CDI) of the vasculature of the normal canine orbit and eye. Eight normal Beagles were evaluated by Doppler imaging. The goals of the study were to determine the location, spectral waveform morphology, specific blood velocity parameters, and reproducibility for the ophthalmic and orbital vessels most frequently identified in the normal dog. Vessels identified a majority of the time (> 50%) included: external ophthalmic artery, dorsal external ophthalmic vein, ventral external ophthalmic vein, internal ophthalmic artery, anterior ciliary artery and vein, short and long posterior ciliary arteries, primary retinal arteries, and vortex veins. Other vessels imaged less frequently included: external ethmoidal artery (50%), and primary retinal veins (25%). For each blood vessel the time averaged velocity, peak systolic velocity, minimum diastolic velocity, pulsatility index, and resistive index were determined. The ophthalmic and orbital vessels have unique spectral waveforms and velocities which serve as a basis for identification. Reproducibility of the most commonly imaged vessels of the canine eye and orbit with Doppler imaging was high (< 10% variation). Doppler imaging has the potential for determining noninvasively and consecutively the blood velocity parameters found in orbital and ocular diseases, including orbital inflammations and neoplasms; intraocular inflammations and neoplasms; vascular diseases including systemic vascular disease (hypertension), vasculopathies, and anemia; the glaucomas; and documentable follow-up after medical and/or surgical treatment of these diseases.     

The arterial supply of the eye of the yak (Bos Grunniens)

The heads and necks of 10 yaks were dissected to study the arterial supply to the eye of the yak in the Qinghai-Tibetan Plateau. The supply came from the internal ophthalmic, external eight ophthalmic, superficial temporal and malar arteries. The internal ophthalmic was one of sources of the posterior long ciliary artery. The external ophthalmic artery gave rise to branches to supply the dorsal oblique muscle and otherwise, and to take part in the formation of the ophthalmic rete mirabile. The ophthalmic rete mirabile gave off many branches to supply the rectus muscles of the eye and otherwise. The malar artery was one of the branches derived from the infraorbital artery, and its branches supplied the inferior, superior and third eyelids and otherwise. The superficial temporal artery detached off some branches to supply the lateral angle of the eye and otherwise, and anastomosed with the lacrimal artery of the ophthalmic rete mirabile.     

The Vascular Architecture of the Porcine Small Intestine

The vascular anatomy of the porcine small intestine was studied by injection of intestinal vessels with India Ink. Examination of transverse and longitudinal serial sections of the injected intestine facilitated a three-dimensional interpretation of the vascular pattern. An artery from the mesentery penetrated the tunica muscularis, supplied muscular branches and passed on to the submucosa where it formed an arterial rete. From the submucosal arteries, arterioles arose and followed a direct axial course to the tips of villi where they ramified into a subepithelial capillary plexus. Some of the capillaries, at the midpoint of the villus, fused into paraxial venules, which emptied into a “transverse venule” at the base of the villus. Other villus capillaries were continuous with those of the crypts. The pericryptal capillary plexus received a few arterial branches from the submucosal arteries. The transverse venule and the pericryptal capillary plexus emptied into large, segmentally dilated veins in the submucosa. The submucosal veins formed an extensive anastomosing network drained by large venous trunks which passed through the muscle layers to the mesentery.

The observations suggest possible relationships between the vascular pattern and intestinal fluid movement.

     

Microvascularization of the Third Eyelid in Dogs

The third eyelid protects the eye, contribute to the aqueous portion of the pre‐ocular film and its distribution over the corneal surface, as well as removal of foreign material from the anterior surface of the eye. It is rather vulnerable during fights and clinicians often have to perform surgery for third eyelid neoplasia. In dogs, this lid has a triangular shape, being widest at its free margin. It is vascularized by branches of the malar artery. Being of such importance as an adnexa of the eye, the authors concentrated their attention on its vascularization, which is not well defined. Methods: Scanning Electronic Microscopy of vascular corrosion casts and intact tissue. Observations: It was quite interesting to notice the presence of two patterns of vascularization inside the third eyelid. One can say that there are two segmental levels of vessels. In one of these there is a predominance of arteries and veins, while in the other a meshwork of third order arteries, capillaries and post‐capillary venules are present. In fact, the branch of the malar artery that reaches the base of the third eyelid divides into smaller branches that cross almost the complete length of the lid giving only a few collateral vessels, in a monopodical way, and only near the free margin of the lid is possible to observe dicothomical ramifications that became rather anastomotic (being anastomoses of type I and II are present) and contribute to the microvascular meshwork. This microvascular net is drained by post‐capillary venules and veins that empty as a brush into bigger veins that occupy the same segmental level as the third eyelid artery. Remarks: This uncommon angioarchitecture is probably an adaptation to the fact that in the medial canthus of the eye this lid has a smaller space and the way of obtaining an efficient vascularization is by going straight to the top of the lid.     

CT and Cross-sectional Anatomy of the Normal Head of the Loggerhead Sea Turtle (Caretta caretta)

The third eyelid protects the eye, contribute to the aqueous portion of the pre-ocular film and its distribution over the corneal surface, as well as removal of foreign material from the anterior surface of the eye. It is rather vulnerable during fights and clinicians often have to perform surgery for third eyelid neoplasia. In dogs, this lid has a triangular shape, being widest at its free margin. It is vascularized by branches of the malar artery. Being of such importance as an adnexa of the eye, the authors concentrated their attention on its vascularization, which is not well defined.
Methods:  Scanning Electronic Microscopy of vascular corrosion casts and intact tissue.
Observations:  It was quite interesting to notice the presence of two patterns of vascularization inside the third eyelid. One can say that there are two segmental levels of vessels. In one of these there is a predominance of arteries and veins, while in the other a meshwork of third order arteries, capillaries and post-capillary venules are present. In fact, the branch of the malar artery that reaches the base of the third eyelid divides into smaller branches that cross almost the complete length of the lid giving only a few collateral vessels, in a monopodical way, and only near the free margin of the lid is possible to observe dicothomical ramifications that became rather anastomotic (being anastomoses of type I and II are present) and contribute to the microvascular meshwork. This microvascular net is drained by post-capillary venules and veins that empty as a brush into bigger veins that occupy the same segmental level as the third eyelid artery.
Remarks:  This uncommon angioarchitecture is probably an adaptation to the fact that in the medial canthus of the eye this lid has a smaller space and the way of obtaining an efficient vascularization is by going straight to the top of the lid.     

Vasculature of the ophthalmic rete in night herons (Nycticorax nycticorax): scanning electron microscopy of corrosion casts

Vasculature of the ophthalmic rete (rete ophthalmicum) in the night heron (Nycticorax nycticorax) was studied using scanning electron microscopy of vascular corrosion casts and light microscopy on tissue sections. Most blood to the eyeball and a lesser volume of blood to the brain passed through the ophthalmic rete via the external ophthalmic artery. The collateral retial arterioles originated from the external ophthalmic artery forming a flat and fusiform-shaped arterial network at the ventrotemporal region of the eyeball. The arterial network was intermixed with a similar complex of the veins from the eye. The ophthalmotemporal artery, which supplied the eyeball posteriorly, and supraorbital and infraorbital arteries, which supplied the eyeball anteriorly, originated from the rete. Blood from the eye, which is a site of potential heat loss, drained into the ophthalmic rete via the ophthalmotemporal vein. On the casts of retial arterioles, slit-like cleavages at branching sites representing flap valves, which might play a role as sluice valves, were seen. In addition, marks of circularly running grooves, which might represent tufts of smooth muscle cells and might contribute to a sphincter activity, were observed. These anatomical specializations of the avian ophthalmic rete, involving parallel arrangement of arteries and veins, may function to facilitate counter-current heat exchange and to regulate blood pressure and volume to the eye and the brain.     

Management of intraocular inflammatory disease

The uvea of the eye is divided into the anterior uvea (iris and ciliary body) and posterior uvea (choroid). Clinical signs of anterior uveitis include conjunctival hyperemia, corneal edema, aqueous flare, miosis, and a decrease in intraocular pressure. Inflammation of the posterior uvea often involves the retina as well resulting in a chorioretinitis. Clinical signs of chorioretinitis may include multifocal lesions in tapetal or nontapetal fundus, retinal detachment, and a decrease in vision. The etiology of uveitis is complex and includes numerous infectious, neoplastic, immune-mediated, and other diseases. Treatment is directed at the underlying systemic disease, if present, as well as symptomatic treatment for the eye. The diagnostic and therapeutic approach to a patient with uveitis is discussed.     

A scanning electron microscopical study of the dermal microcirculation of the equine foot

The microcirculation of the dermal laminae and papillae of the equine foot from seven clinically normal Australian ponies was studied using an improved microvascular casting corrosion technique and scanning electron microscopy. Casts of veins, arteries, capillaries and arteriovenous anastomoses (AVAs) were readily identified by their characteristic surface morphology. Arteries entered the laminar circulation axially, between pairs of axial veins, and were connected to each other by smaller calibre interconnecting arteries. Short abaxial branches of the axial interconnecting arteries gave rise to tufts of predominantly, proximodistally orientated, capillaries arranged abaxially in rows. The laminar veins anastomosed with each other extensively (the axial venous plexus) and formed most of the vascular skeleton of casts of the dermal laminae. AVAs were found throughout the laminar circulation but the largest and longest (40 mu diameter) were found clustered close to the origin of the axial arteries. The density of the laminar AVAs was estimated to be 500 AVAs/cm2. Blood vessels of the dermal papillae of the periople, coronary band, distal laminae, sole and frog shared a basic structural organisation. The cast of each papillary unit consisted of a central artery and vein enmeshed in a sheath of fine capillaries. At intervals along the length of the central artery were short branches which gave rise to tufts of capillaries. The capillaries formed a tortuous anastomosing plexus which encircled the papillary unit and drained into the central vein at intervals along its length. AVAs were always present at the base of the papillary units and anastomoses connected the central artery and vein. AVAs are important components of the dermal microcirculation of the equine foot and their distribution and density is compatible with their proposed role in the pathophysiology of equine laminitis.     

Study of the distribution of retinal blood vessels in buffaloes (Bos bubalis)

The distribution of retinal blood vessels in the eye of buffalo was studied macroscopically and microscopically in twenty-two eyes of healthy animals. After macroscopic observation, 12 of 22 eyes were used for histological study. Ten eyes were stained with hematoxylin and eosin and two eyes with PAS stain. The present findings revealed that the eye of the buffalo was characterized by the complex network of retinal blood vessels (holangiotic or euangiotic). The central retinal artery and vein pierced the eye through the optic disc and gave off several branches. There were four pairs of primary vessels that were named dorsal, ventral, nasal and temporal retinal arteries and veins. The veins anastmosed with each other at the optic disc to form a somewhat circle. Three patterns of the distribution of blood vessels were described. The distribution of the arteries and veins was the same.     

Unilateral ocular subalbinism in a laboratory Beagle dog

Blue discoloration of the iris was found in the left eye of a male laboratory Beagle dog, which had a normal tricolor coat and clinically showed no visual impairment. Ophthalmoscopically, the affected eye revealed red-colored tigroid fundus, in which no tapetum was present. The retinal vasculature and the optic disc showed no noticeable changes. Histopathologically, in the left eye melanocytes had extremely few melanin granules in the anterior segment, including the anterior border layer, in the stroma and sphincter muscle of the iris and in the stroma of the ciliary body and choroid. However, the posterior pigment epithelium of the iris, the pigment epithelium of the ciliary body and the retinal pigment epithelium showed normal pigmentation. The tapetal elements were completely absent. Number and distribution of the S-100 protein-positive melanocytes with or without melanin granules in the iris, ciliary body and choroid of the left eye were similar to those of the normal right eye. Ultrastructurally, melanocytes in the anterior segment of the affected iris possessed no or few melanosomes which were incompletely melanized. In the right eye, no abnormal features were observed. Based on these results, the present case was diagnosed as unilateral ocular subalbinism with tapetal aplasia in a Beagle dog.     

Scanning electron microscopy of corrosion casts of the optic nerve microcirculation in dogs

Scanning electron microscopy of vascular corrosion casts of the optic nerve region in normal and glaucomatous Beagles demonstrated that the blood supply to the laminar optic nerve is derived from short posterior ciliary arteries, cilioretinal arteries, and longitudinal pial vessels. The short posterior ciliary arteries formed a ring of striated pillars around the scleral canal. The central retinal artery was not present in the dog. Differences between the casts in normal and glaucomatous dogs were not detected.     

Microvasculature of the retina, ciliary processes and choroid in the North American raccoon (Procyon lotor) eye: a scanning electron microscopic study of corrosion casts

We have studied the vasculature of the retina, ciliary processes and choroid in the North American raccoon (Procyon lotor), a nocturnal mammal, using light and scanning electron microscopic examination of corrosion casts. We carried out an identical study in the crab-eating macaque (Macaca fascicularis), which forages only during the daytime, in order to compare the ocular vasculature with that of nocturnal mammals. Our observations in raccoons demonstrated a photoreceptor layer associated with rich lymph and a poorly vascularized retina. The meridian region of the eye, which lies in the horizontal plane and pass around the optic disc, had a markedly sparse capillary network. This horizontal sparse vascular band may correspond to a visual streak. Ciliary process capillaries were delicate, and formed a well-developed and compact network. Choriocapillaries were quite thin and formed a coarse capillary network. This contrasted with the dense retinal and well-extended choroidal capillary networks noted in the macaques. Our findings suggest that the sparse retinal capillary network in raccoons is extremely beneficial for photon capture, thereby allowing the raccoon to see well at night, as the retinal vessels restrict the inflow of photons toward the photoreceptors. The well-developed lymph probably compensates for the sparse retinal capillaries and choriocapillaries and nourishes the retina in the nocturnal raccoon.     

The major blood vessels of the wing of the ostrich (Struthio camelus)

The major blood vessels of the ostrich wing were studied by dissection of the wings of 8 ostriches. The axillary artery entered the caudo-ventral aspect of the wing and divided into the brachial and deep brachial arteries. The deep brachial artery gave rise to the radial and ulnar collateral arteries which supplied the caudodorsal aspect of the brachium. The brachial artery divided in the cubital fossa into the smaller ulnar and large radial arteries. The ulnar artery supplied the cranio-ventral aspect of the antebrachium and manus, and the radial artery the caudo-ventral and dorsal aspects of the antebrachium and manus, including the feathers. With few exceptions the veins corresponded with the arteries that supplied the region they drained. The basilic vein passed along the cranial margin of the wing, unaccompanied by arteries or nerves, to drain the major part of the wing. The brachial artery and the basilic veins were the vessels most accessible for arterio- and venipuncture, respectively.     

The central retinal artery and regression of the hyaloid artery in perinatal cattle

The arterial supply to the retina and lens of 10 fetal, 10 neonatal and four adult Zavot-bred cattle of both sexes was studied macroscopically and by stereoscopic microscopy by means of vascular perfusion with latex, giving special emphasis on the hyaloid artery. The central retinal artery ramified in four major retinal arterioles, which formed a compact network throughout the retina (holangiotic or euangiotic pattern). The hyaloid artery was patent in all fetal stages and extended through the vitreous cavity of the eye to the caudal surface of the capsule of the lens. Atrophy of the hyaloid artery began immediately after birth and was completed on day 17 after parturition. No remnant of the hyaloid artery in the vitreous cavity was observed in the adult cattle examined at stereoscopic microscopic level.     

Fine Structure of the Choriocapillaris, Bruch''s Membrane and Retinal Epithelium of the Cow

The fine structure of the choriocapillaris, Bruch's membrane and retinal epithelium was investigated in both the tapetal and non-tapetal fundus of the bovine eye. In ail locations the retinal epithelium consists of a single layer of cuboidal cells. The epithelial cells are joined laterally by apically located tight junctions and throughout the retina display numerous basal infoldings and fine apical processes which enclose rod outer segments. All retinal epithelial cells are rich in smooth endoplasmic reticulum and mitochondria and contain phagosomes. Although not as abundant, profiles of rough endoplasmic reticulum and polysomes are also noted in all locations. In non-tapetal areas, melano-somes are numerous whereas over the central tapetum fibrosum they are absent. The absence of melanosomes over a functional tapetum is to be expected. While lysosomes are present throughout the epithelial layer, over the tapetal region they appear to be more numerous. The apparent increase in lysosomal numbers in this location may indicate an enhanced shedding of outer segment material over the tapetum. Although some retinal epithelial cells are modified to accomodate a tapetum lucidum their morphology is basically similar throughout the retina and probably indicates that ail regions of the retinal epithelium are capable of the normal functions of this indispensible retinal layer. The choriocapillaris is heavily fenestrated on the border facing the retina and overlying the tapetum also displays fenestrae on its choroidal edge. Bruch's membrane ( complexus basalis ) is pentalaminate throughout the retina and is slightly thicker in the posterior fundus.