Distributions of the cardiac plexuses and ganglia in the Beijing duck

Distributions of the cardiac plexuses and cardiac ganglia were gross-anatomically and histologically studied in eight Beijing ducks. The cardiac plexuses consisted of two components, the cardiac nerve arising from the sympathetic trunk and the cranial and caudal cardiac nerves arising from the vagus. Branches of these nerves made the cardiac plexuses on the epicardium. The cardiac plexuses could be divided into the six plexuses, that is, the right and left coronary plexuses, pericardiac transverse sinus plexus, caudal cardiac plexus, and right and left superior cardiac plexuses. There were small ganglia in the caudal cardiac plexus and the right and left coronary plexuses. These ganglia containing multipolar neurons were found like a linking chain in a single nerve.

Use of body measurements and serum metabolites to estimate the nutritional status of mallards wintering in the Mississippi Alluvial Valley, USA

We collected mallards (Anas platyrhynchos) from bottomland hardwood habitats on the Bayou Meto Wildlife Management Area and the White River National Wildlife Refuge, Arkansas County, Arkansas during the winter of 1990 to 1991 to determine if measures of physiological condition could be predicted from structural size, serum metabolite levels, or from direct measures of carcass composition. Serum triglyceride levels were correlated (r = 0.57, P = 0.007) with total body fat in males and slightly increased the value (from R2 = 0.64 to 0.76) of intact body mass alone for predicting total body fat in males. Overall, however, serum metabolites appeared to be poor indicators of the magnitude of nutrient masses in mallards. Three potential indices of nutritional status were developed from carcass composition data: protein/total ash, fat/ total ash, and fat/fat-free body mass. Protein masses of male mallards changed over winter (P = 0.02). Consequently, fat-free masses are not constant and represent poor indicators of structural size for mallards wintering in the Mississippi Alluvial Valley.

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.

Angiotensin II-binding sites in chicken brain and pituitary: autoradiographic localization

The distribution of binding sites for 125I-Val5-angiotensin II in the brain and the pituitary of control and dehydrated adult chickens of both sexes was determined by receptor autoradiography. Specific 125I-Val5-angiotensin II binding was mainly located in sensory areas, the limbic system and the hypothalamus including circumventricular organs and the nucleus paraventricularis. Since both the central angiotensin II system and the avian antidiuretic arginine-vasotocin system interact in osmoregulation, possible changes in angiotensin II binding intensities after dehydration were investigated in hypothalamic areas known to be involved in body-fluid homeostasis. There was an obvious increase in binding sites neither in areas where arginine-vasotocin synthesizing neurons were located, e.g. the nucleus paraventricularis and nucleus supraopticus, nor in other relevant areas for osmoregulation, e.g. the circumventricular organs of the hypothalamus. Furthermore, no sex difference in angiotensin II binding was observed.

Surface morphology of basilar papilla of the tufted duck Aythya fuligula, and domestic chicken Gallus gallus domesticus

Quantitative details of the surface morphology of the hearing organ, the Papilla basilaris, as seen in the scanning electron microscope are described for the tufted duck Aythya fuligula and for comparison for the domestic chicken Gallus gallus domesticus, for which some published information is already available. As in the other avian species investigated to date, each papilla shows a unique constellation of features. The papilla of the tufted duck is 3.5 mm long in the unfixed state and contains 8,200 sensory hair cells. It shows systematic changes in its surface features along the length and across the width of the sensory epithelium. In general, its features and those of the chicken Papilla basilaris can be described as relatively primitive in comparison with other species. The tufted duck papilla does, however, show one feature that has so far been found to be well developed only in advanced papillae; the number of stereovilli per hair cell bundle is generally much higher on hair cells of the neural than those on the abneural side. This difference is only weakly developed in the chicken. It is clear that features considered to be evolutionarily advanced were acquired independently of one another during evolution and that each bird species can show a mosaic of primitive and advanced features.

The valva portalis renalis in the duck (Anas platyrhynchos)

The morphology of the valva portalis renalis of the duck was investigated, using histological, SEM and TEM techniques. The wall thickness of the vena iliaca externa, the vena portalis renalis caudalis, the vena iliaca communis and the vena renalis caudalis was morphometrically evaluated. The blood pressure in these veins was measured using a three-way H2O manometer. The valva portalis renalis was composed primarily of epithelioid cells and lined with endothelium. Throughout the entire valva there was a dense complex of nerve structures made up of fibers and fiber bundles which also extended beneath the endothelium of the valva and around the subendothelial epithelioid cells. The wall thicknesses of the veins supplying the renal portal system (vena iliaca externa and vena portalis renalis caudalis) were greater than those of the vessels collecting the renal refluent venous blood (vena iliaca communis and vena renalis caudalis). In addition, the blood pressure values taken in the vena iliaca externa and the vena portalis renalis caudalis were much higher than those in the vena iliaca communis and the vena renalis caudalis. The above observations suggest that the renal portal system works at higher blood pressure levels than the general venous system and that the valva portalis renalis regulates its aperture in order to maintain a constant blood pressure and a continuous blood flow in the renal portal system vessels, hence avoiding damage to the renal parenchyma caused by pressure overloads.

Bone growth in ducks through mathematical models with special reference to the Janoschek growth curve

The Janoschek growth curve was reviewed and a reparameterized form was presented which includes 4 biologically interpretable parameters: the weight at hatching/birth (W0), asymptotic (adult) weight (A), the time to grow to half of the final weight (t50) and a shape parameter (p) adjusting the degree of maturity when growth rate peaks. The model obtained was compared to the Richards, Bertalanffy, Gompertz and Logistic growth curve using bone weight data in White Pekin ducks. The Janoschek yielded least residual sums of squares for humerus mass and the Richards model for femur mass with only minor differences between both equations. All 3-parameter models failed to give reasonable estimations of the weight at hatching; in some cases further growth curve characteristics were unrealistic. The femur showed a higher degree of maturity at hatching, an earlier growth rate peak and its development preceded that of the humerus. This differential growth is an evolutionary feature first seen in birds.

Domestication and plasticity of brain organization in mallards (Anas platyrhynchos)

The sizes and histological differentiation of structures in the central nervous system of wild and domestic ducks were compared using allometric methods. Whole brain volume is 14.3% less in domestic ducks than in wild birds, and the size of certain brain structures is more variable in domestic ducks than in the wild birds. These findings are consistent with results of studies in other domestic birds and in domestic mammals. Although the brain subdivision volumes are not reduced to the same extent, the relative composition of the brain is maintained in domestic ducks. The overall reduction in rhombencephalic parts of the brain (16.3%) is greater than that in the prosencephalon (13.5%). Within sensory centers the nuclei of the trigeminal system undergo the most marked decrease (21.3%). Optic (13.3%) and limbic (11.1%) structures are less reduced, and the olfactory regions do not show any differences in volume compared to these regions in wild mallards. Reductions in the volumes of brain structures in domesticated animals have been thought to be processes of functional adaptation controlled by artificial selection. Our observations are discussed with regard to hypotheses about trends in domestication and evolutionary plasticity.

Ultrastructure of cerebrospinal fluid-contacting neurons immunoreactive to vasoactive intestinal peptide and properties of the blood-brain barrier in the lateral septal organ of the duck

Immuno-electron-microscopic investigations of cerebrospinal fluid (CSF)-contacting neurons immunoreactive to vasoactive intestinal peptide in the duck lateral septum have revealed that this cell type gives rise to an adventricular dendrite terminating with a bulbous swelling in the lateral ventricle. The swelling bears a cilium and contains mitochondria and immunolabeled dense-core vesicles. Two types of processes emerge from the basal part of the perikaryon. The first has a large diameter, contains diffusely distributed immunoreaction, and receives synaptic input, indicating that this process is a basal dendrite. The other type is of a beaded appearance, displays immunolabeled dense-core vesicles, and represents the axon of the CSF-contacting neuron. VIP-immunoreactive terminal formations are located within the neuropil of the lateral septum and the nucleus accumbens. Some of them form synaptic contacts with immunonegative profiles. No VIP-immunoreactive terminal formations are seen in the perivascular spaces of the lateral septum. Tracer experiments with horseradish peroxidase have revealed that the blood-brain barrier is lacking in the lateral septal organ and nucleus accumbens of the duck. Capillaries, arterioles, and venoles of this region are coated by nonfenestrated endothelial cells connected by "leaky" junctions, allowing the tracer to penetrate from the lumen into the perivascular space and further into the intercellular clefts of the neuropil. Our immuno-electron-microscopic investigations show that VIP-immunoreactive CSF-contacting neurons of the lateral septum closely resemble CSF-contacting neurons occurring in other brain regions, e.g., the hypothalamus. The arrangement of VIP-immunoreactive terminal formations suggests that, in the lateral septum, the VIP-like neuropeptide serves as a neurotransmitter (-modulator). The lack of a blood-brain barrier in the lateral septal organ and the nucleus accumbens raises the possibility that this region is a window in the avian brain allowing exchange of information between the central nervous system and the bloodstream; it thus resembles a circumventricular organ.

Location of premotor neurons of the motor nuclei innervating craniocervical muscles in the mallard (Anas platyrhynchos L.)

Reticular premotor neurons of craniocervical muscles in the duck were localized with the retrograde tracer HRP and the anterograde tracer WGA-HRP. In the reticular formation neck premotor neurons were found in the gigantocellular reticular nucleus and in the ventromedial part of the parvocellular reticular nucleus rostral to the obex, and caudal to the obex in the nucleus centralis ventralis of the medulla. Results were compared with premotor areas of jaw muscles. The ventromedial part of the parvocellular reticular formation contains neck as well as jaw premotor neurons. This area may serve as the neural substrate for the coordination of neck and jaw movements.

Crossbreeding effects for body weight and carcase traits in Pekin duck

1. Three sire lines of Pekin duck, three dam lines and 9 crosses between these two groups were tested in replicated tests from 1987 to 1991. Body weight at the age of 7 weeks (n = 5189), carcase percentage, percentage of abdominal fat and thickness of breast muscles were recorded (n = 1432 for carcase traits). 2. The data were analysed by linear models. All models contained the factors test and sex. When analysing only crosses, general and specific combining abilities (GCA and SCA respectively) were included as well as their interactions with the test. Using the complete data set (lines and crosses), direct genetic and heterotic effects were considered. 3. The factors test and sex were significant in both data sets for all traits. 4. For body weight, all genetic factors were significant. The average heterosis was 65 g +/- 9 g (2.2%); the highest observed heterosis was 7.7%. 5. For carcase percentage, the heterosis was always negative (up to -2.1%), whereas in abdominal fat, a desirable negative heterosis was observed only in one cross. No significant heterotic or combining ability effects were observed in the thickness of breast muscles. 6. Significant interactions between at least one genetic factor and test were found in all traits. 7. The relative importance of GCA and SCA, possible sources of confounding estimates of genetic effects, the importance of heterotic effects and genotype x environment interactions are discussed.

Are Merkel and Grandry cells two varieties of the same cell in birds?

For over a century it has been held that the Grandry corpuscle and the Merkel corpuscle are unique sensory organs in aquatic and nonaquatic birds, respectively. In other words, the Grandry and the Merkel cells are two varieties of the same receptor cell and never coexist together. Contrary to what has been believed, we found unmistakable Merkel cells in addition to Grandry cells in the lingual mucosa of the duck, Anas platyrhynchos var. domestica, an unprecedented observation in an aquatic bird. This study concerns the fine structure of these cells in the tongue of the duck. Although some ultrastructural features are shared by the Grandry and the Merkel cells, i.e., the presence of numerous dense-cored granules and microvillous projections at the cell surface, the morphology of these two types of cells clearly differ, especially in their size. Cells suggestive of stages intermediate between these two cells have never been recognizable. The present study indicates that the Grandry and the Merkel cells are not two varieties of the same cell in birds. It is suggested that the Merkel cell may exist in every vertebrate species, while the Grandry cell is unique to aquatic birds.

[The innervation of the syrinx of the duck (Anas platyrhynchos)]

The innervation of the Syrinx was studied in 15 female Peking Ducks of 10 to 12 months old by using L.M. and E.M. In this organ a nervous plexiform net is present located in the connective below the epithelium along the membrana tympaniformis medialis. This net-work is made of the nervous trunks which repeatedly anastomise among themselves and display during their course acapsulate ganglions. However, no specialized contact device were encountered at the periphery.

Forebrain specialization and the olfactory system in anseriform birds. An architectonic and tracing study

In anseriform birds the mediodorsal part of the rostral forebrain is covered by a corticoid (= layered) structure, establishing a unique feature of this avian group since in other birds the non-cortical accessory or dorsal hyperstriatum occupies the corresponding surface area of the hemisphere. The efferents of the olfactory bulb are shown to reach this region, which thus can be identified as a heavily enlarged retrobulbar area. The large expansion of this olfactory representation may indicate an important biological function. In comparison to the mammalian olfactory system the three stratified olfactory projection centers of birds should be regarded as retrobulbar, prepiriform and periamygdalar regions.

A percutaneous liver biopsy technique in ducks (Anas platyrhynchos) experimentally infected with duck hepatitis B virus

Aylesbury ducks (Anas platyrhynchos) chronically infected with the duck hepatitis B virus provide a useful model for studying hepadna-virus infection, replication and the effects of antiviral therapy. In these studies, it is necessary to have an effective method for obtaining repeat liver specimens for histological and molecular analyses. We have therefore developed a percutaneous liver biopsy technique which has a low rate of complications, can be performed at repeated intervals, and provides sufficient quantities of liver tissue for histological and nucleic acid hybridization analysis.

[The structure of the clavicular air sac of the duck (Anas platyrhinchos)]

The structural organization of the epithelium lining the clavicular air sac in Duck was studied by light and electron microscopic. This epithelium consists of two types of pavement like cells. The first flat types of cells are numerous with large mitochondria containing cytoplasm and elongated nuclei. The second short and dumpy cells are few in number and are after seen near the ostium. They could have long microvilli like pseudopod on their apical surface ready to engulf inhaled foreign particles. Desmosomal bridges too are found between contiguous cells.

Neuronal lysosome accumulation in degrading synapses of sensory-motor and limbic subsystems in the duck Anas platyrhynchos: indication of rearrangements during avian brain development?

Brains of developing duck embryos and ducklings were analysed daily by light microscopy after complete serial brain sectioning and lysosome staining (Gallyas technique), providing insight into synaptic degradation and degeneration during short periods of synaptogenesis. Both regional and temporal shifting pattern of lysosome accumulation (LA) in degraded synapses were detected in sensory-motor pathways during the course of development. LA occurred in sensory projections of embryos, and proceeded from forebrain sensory fields toward limbic regions and finally motor projections. LA disappeared from these structures at the age of 4-6 weeks. LA was analysed ultrastructurally in selected sensory, limbic and motor regions indicating that lysosomes selectively accumulated in transient synapses, leaving parts of either pre- or postsynaptic elements untouched. In no case did denervated neurons exhibit any signs of cell death. Apparently, the LA phenomenon seems critical in terms of both irreversible elimination and remodelling of persisting synapses. Thus, neuronal rearrangement mediated by lysosomal degradation, i.e. degeneration of synaptic components, is supposed to be an integral constituent of synaptogenesis during adaptive processes in sensory-motor systems. These results are discussed with regard to developing brain functions during behavioural adaptation of this precocious bird.

Analysis of the sternotrachealis muscle fibers in some Anseriformes: histochemistry and sex differences

Histochemical characteristics and sizes of the fibers of the sternotrachealis (ST) muscle have been investigated in some Anseriformes (mallard, Pekin duck, Muscovy duck, and goose) of both sexes. A sexual dimorphism has been shown in the muscle of the species examined. In the mallard and Pekin duck, the male ST muscle shows type IIIA fibers in addition to the type I, IIA, and IIB fibers observed also in the female. In the Muscovy duck, the male muscle has only type I and IIA fibers, whereas the female muscle presents type I fibers and both types IIA and IIB fibers. Moreover, the mean frequencies for each fiber type were significantly different between males and females. In the goose, both male and female muscles present only type I and IIA fibers. In all the species examined, the mean areas of each fiber type are significantly different between male and female, being always larger in the male muscles. The anatomical sexual dimorphism observed in the ST muscle is discussed in relation to function.

The identification of the motor nuclei innervating the tongue muscles in the mallard (Anas platyrhynchos); an HRP study

Horseradish peroxidase (HRP) histochemistry was used to identify the motoneurons innervating the tongue muscles in the mallard. Four nuclei are involved: the intermediate motor nucleus of N.VII innervating the stylohyoid, serpihyoid and ceratohyoid muscles, the retrofacial nucleus of N.IX innervating the m. geniohyoideus and the n. intermedius or motor nucleus of N.XII that innervates the mm. ceratoglossus and hyoglossus anterior and obliquus. The m. intermandibularis is innervated by a trigeminal motor subnucleus. There is no clear intranuclear organization. The results are summarized in Table I and discussed in connection with the role of each of the muscles during movements of the tongue.

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

The three-dimensional microvasculature of the nasal salt gland of the duckling was studied by vascular corrosion casting and scanning electron microscopy. Changes in the vascular volume of the gland in response to osmotic stress were also determined using cast weights and densities. The richly vascularized gland is supplied on its medial surface by large branches of the supraorbital and ethmoidal arteries. Numerous arterial branches enter the gland and distribute to lobes via the interlobar connective tissue. Lobar arterioles penetrate to the periductal areas of the lobes before dividing into capillaries supplying the ductal epithelium and secretory tubules. Capillaries envelope the secretory tubules and run radially from the ducts toward the lobe periphery, so that blood flows counter to the tubular secretion. Blood is collected via venous plexuses seen as distinct drainage units on the periphery of each lobe. Veins exhibit large numbers of bicuspid valves. Following 1 day and 4 days of osmotic loading (feeding 1% NaCl), vascular volume of the gland increased fivefold and ninefold, respectively, a response that precedes and exceeds that of the gland weight or Na,K-ATPase activity. When salt water-adapted ducklings were fed fresh water for only 24 hr (deadaptation), vascular volume fell to 2.8 times the control level. Changes in blood flow to the gland during osmotic adaptation and deadaptation are rapid and dramatic and may represent the initial steps in the control of gland secretion.

[The structure of the syrinx in the duck (Anas platyrhinchos)]

The morphological and structural aspects of the Membrana tympaniformis medialis of the syrinx were studied with L.M. and E.M. in 32 female and male Peking ducks from 6 to 12 months old. The membrane is covered by pseudostratified prismatic epithelium which shows different regional features. While at level of the proximal and distal third of the tympanic membrane the morphological and structural aspects of the cells resemble that of the adjacent structures (pessulus and primary bronchi), the middle part of the epithelium layer becomes gradually flattered; the epithelium is made up of oblique or horizontal cells which touch each other by means of many lateral interdigitations. At level of the upper parts of the cells there are several types of cell junctions. This particular disposition seem to be well adapted to the contractions of the syringeal muscles.

A novel perspective: the occluding zonule encircles the apex of the Sertoli cell as observed in birds

The modulation of Sertoli cell junctions was studied in the non-seasonal rooster (Gallus domesticus) and in the seasonally breeding mallard duck (Anas platyrynchos anatidae) using thin sectioning, a junction permeability tracer, and freeze-fracture replication. During the active spermatogenic phase, the junctions of the duck appeared similar to those of the rooster, thereby establishing the duck as an avian model of seasonal modulation of Sertoli cell junctions. As with mammalian seasonal breeders, during the active phase, occluding, gap, and adhering junctions formed a junctional complex all along the long axis of the Sertoli cell. Unlike in mammals, however, no 7-nm filaments were associated with the occluding junctions. An occluding zonule encircled the Sertoli cell apico-lateral membrane domain situated above the young germ cells, and constituted a barrier to the entry of lanthanum in the basal third of the seminiferous epithelium. Toward the basal side, forming focal junctions were located on the lateral Sertoli cell membrane domain facing the young germ cells. Toward the apical side, dismantling focal junctions were located on the apical Sertoli cell membrane domain facing the older germ cells. During the duck's testicular regression, 7-nm filaments were associated with an occluding junction. In freeze-fracture replicas, each junction was formed by a continuous junctional strand that encircled the apex of the cell. The strands composed a delicate narrow meshwork: an occluding zonule. The blood-testis barrier was localized near the apex of the epithelium. The seasonal reduction in the number of the strands and the changes in their orientation did not coincide with a change in the permeability of the occluding zonule to lanthanum. In addition, the cyclic disappearance of junction-associated filaments was not correlated with a change in the permeability of the junctions but with a change in the affinity of junctional particles for one or the other fracture face. It is proposed that the Sertoli cell plasma membrane domains situated apical and basal with respect to the occluding zonule be considered apical and lateral, respectively. The remaining domain facing the basement membrane would therefore be called basal. In the duck, the occluding zonule is not seasonally shifted from the base to the apex of the Sertoli cell. Instead, it remains stationed above the younger germ cells throughout the year.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal photoreceptor fine structure in the mallard duck (Anas platyrhynchos)

The retinal photoreceptors of the mallard duck (Anas platyrhynchos) consist of rods, single cones and double (unequal) cones present in a ratio of about 1:2:1. The rods have relatively large cylindrical outer segments which in the light-adapted state reach to the retinal epithelial (RPE) cell bodies and are surrounded by the pigment-laden apical processes of these RPE cells. The inner segment displays an apically located ellipsoid of mitochondria and plentiful polysomes, some rough ER and Golgi zones. The rod nucleus is located within the outer nuclear layer and the synaptic pedicle displays both invaginated (ribbon) and superficial (conventional) synaptic sites. Single cones display a thin tapering outer segment, a large often heterogeneous oil droplet in the apical region of the inner segment, an ellipsoid of mitochondria and a prominent paraboloid of glycogen. Double cones consist of a larger chief member which shows a thin tapering outer segment, a large heterogeneous oil droplet and a prominent paraboloid. The small accessory cone shows a thin outer segment, a small granular oil droplet and a paraboloid. As in the single cone, polysomes, RER and Golgi zones are also present within the inner segment. Near the outer limiting membrane the two members of the double cone show a membrane specialization on their contiguous surfaces. Both single and double cones display several invaginated (ribbon) synapses as well as numerous superficial synaptic sites.

Fine structure of the retinal pigment epithelium of the mallard duck (Anas platyrhynchos)

As part of a comparative morphological study, the fine structure of the retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) has been investigated by light and electron microscopy in the mallard (Anas platyrhynchos). In this species the RPE consists of a single layer of cuboidal cells which display numerous very deep basal (scleral) infoldings and extensive apical (vitreal) processes which enclose photoreceptor outer segments. The RPE cells are joined laterally by prominent basally-located tight junctions. Internally smooth endoplasmic reticulum is the most abundant cell organelle with only small amounts of rough endoplasmic reticulum present. Polysomes are abundant as are basally-located mitochondria which often displayed a ring-shaped profile. The cell nucleus is large and vesicular. Melanosomes are plentiful only within the apical processes of the RPE cells in the light-adapted state. Myeloid bodies are large and numerous and very often have ribosomes on their outer surface. Bruch's membrane (complexus basalis) shows a pentalaminate structure but with only a poorly represented central elastic lamina. Profiles of the choriocapillaris are relatively small and the endothelium of these capillaries while extremely thin facing the retinal epithelium is but minimally fenestrated.

The vascularization of the Bursa cloacalis (of Fabricius) in the duck

The morphological and structural features of the vascular component of the bursa of Fabricius in the duck are described. By means of an intravasal perfusion of suitably colored neoprene Latex or Microscopaque, or a perfusion of Indian ink, it was possible to follow visually the routes of arteries, veins and the microcirculation; this last represents a major characteristic of the bursal follicle in the duck. The microcirculation has its origins in several roots arising from the follicular arterioles, which in the region of the cortex constitute an open vascular layer. From these pre-capillary arterioles, adjacent to the limiting layer between cortex and medulla, a dense capillary network is formed. With the aid of an electron microscope the presence of a cortico-medullary barrier of an epithelio-reticular nature was revealed; close to it there are the terminal branches of the intrafollicular network. The barrier appears to be structurally more compact than that of the cortical capillaries of the mammalian thymus.