the microvasculature of te ovary a review by sem of corrrosion

8/12/2019 The Microvasculature of Te Ovary a Review by Sem of Corrrosion

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208 MACCHIARELLI et al.

of the ovary. These modifications, both functionaland structural in nature, mainly involve the ovarianmicrovasculature supplying the luteo-follicularcomplex (LFC) [15]. In addition, the vessels

forming the ovarian pedicle and running in thehilus have structural characteristics of relevantfunctional significance during the ovarian cycle [68].

Historical Background

The above considerations suggest that a fineevaluation of the morphology and distribution ofovarian vessels, especially if studied by a three-dimensional (3D) approach, gains deep significance

to the understanding of whole ovarian function.In the past, 3D studies of ovarian vascularizationwere performed by light microscopic observationsof serial sections of dye-injected samples [912],followed by graphic reconstruction and drawings.Angiography [13, 14], diaphanoscopy [15, 16] andmicroangiography of sectioned ovaries [15, 17] havealso been usefully applied to the study of theanatomy and topography of the ovarian vessels.In addition, direct observation of corroded orclarified vascular casts, after injection of coloredlatex [18, 19], Micropaque[20], vinylite [21, 24] or

other plastic compounds [14] into the ovarian arteryof various mammals including humans revealedfine details of vasculature. However, theoccurrence of peripheral filling defects, thedifficulty of adequately casting the venous tree,and/or the limitations related to the naked eyeobservation of these casts have certainly left severalproblems unexplored.

At present, scanning electron microscopy (SEM)of vascular corrosion casts allows the bestmorphological 3D reconstruction of the vascularsupply, including its finest ramifications, in both

normal and experimental conditions, as observedin ovary of rodents [2541], horses [42], cows [43,44] and sheep [45, 46]. This technique allowedsignificant information on the ovarian cycle, andbetter c lari f ied the role of vascularmorphodynamics in the development of the LFC[37, 4749]. In addition [8, 50, 51], the ovarianvascular pedicle and its proximal ramificationshave been investigated by applying this technique.

In this review the most salient finding on theblood vessels changes during stimulated cycles and

pregnancy as seen by vascular corrosion cast inrabbit will be described.

Methods

Experimental protocol

Adult female New Zealand white rabbits(Oryctolagus cuniculus), weighing 45 Kg, wereused. The animals were caged separately for 3weeks and kept under controlled conditions (14hours light/10 hours dark, 2022 C) with free accessto food and water. Control rabbits were in estrous.Pseudopregnancy was induced by an i.v. injectionof 70100 I.U. of human chorionic gonadotropin(hCG), through the external ear vein. Rabbits were

then sacrificed 1012 hours, 1271417 days afterhCG injection. Ovulation was assumed to occurabout 1012 hours after hCG administration [52].Reflex ovulation was induced in other rabbits bymating. The day one of pregnancy was taken tobe the following day after mating. These animalswere sacrificed at day 10 of pregnancy [8, 37].

Sampling and electron microscopy

The animals were anaesthetized with an i.v.injection of Nembutal (Abbot U.S.A.) (50 mg/kgbody weight). The chest was opened and the

thoracic aorta was cannulated. After washing outthe blood with physiological saline (roomtemperature), a Mercox (Okenshoji Co. Ltd.,Tokyo, Japan) resin (50 cc) was slowly injected untilpolymerization started [53]. The resin-injectedovaries were placed for 34 hours in a warm waterbath to complete polymerization, corroded in a 10%NaOH solution for 2448 hours at 60 C, and gentlywashed for a few hours under tap water. Then,they were immersed in distilled water for 23 daysat 60 C to completely remove macerated tissues,and washed again under running tap water.

Ovarian vascular casts were exposed understereomicroscopic magnifications. Half of thesamples were frozen at 18 C and cut with acooled razor blade, to allow the visualization ofthe internal structures of the casts. All sampleswere air dried, mounted on aluminum stubs andcoated with gold or platinum [31, 3435]. SEMobservations were performed at low acceleratingvoltage (37 kV), in Hitachi FE S-4000 and S2R orCambridge Stereoscan 150 scanning electronmicroscopes.


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209THE MICROVASCULATURE OF THE OVARY

Results and Discussion

Vascolarization of the hilus and medulla

The supply of blood to the ovary in the rabbit isdifferent to that of other species such as the rat,guinea pig and hamster [18, 19], primates and cattle[3]. In these species the uterus and ovary sharecommon arteries and veins. A branch of the uterineartery anastomoses with the ovarian artery to formthe ovarian branch of the uterine artery. In therabbit, the ovarian blood supply is largelyindependent of the uterine artery [18]. The utero-ovarian anastomosis in the rabbit is long and thinand it is therefore not likely to be a significantsource of blood. In rabbit, the ovarian artery, after

branching from the aorta below the origin of therenal artery divides into two branches. The caudalbranch serves the uterine tube and the uterine horn,and the cranial branch mainly supplies the ovary[8, 10, 18]. The venous drainage follows the samepathway of the arterial flow.

In all samples, we observed the proper ovarianbranch of the ovarian artery (ramus ovaricus) enteredthe ovarian hilus near the caudal pole of the organand run parallel to the major axis of the hilus. Theextraovarian venous drainage was formed by

several vessels emptying into a distal large vein(Figs. 1, 2). The ramus ovaricus showed variousdegrees of coiling and branched in the medulla(Fig. 3). The coiling of the ramus ovaricusand its

ramifications was maintained in all samples. Avenous meshwork and/or flat vein branches closelyenveloped the arterial coils found in the hilus andouter medulla (Fig. 4). At this level numerousarterio-venous contacts were demonstrated in allsamples [8, 54]. The venous drainage followed themodifications of the arterial supply.

The ramus ovaricusand its ramifications

We observed that the ramus ovaricus, e.g. theproper ovarian branch of the ovarian artery, usuallyentered the ovarian hilus as a single vessel near

the caudal pole of the organ. This seems to be themore common situation in the rabbit [21]. Thecourse of ovarian arteries appears quite differentfrom that of veins; in fact, several collecting veins,draining different portions of the ovary, arenormally present just outside the organ (Figs. 12)[35].

In the specimens studied, the ramus ovaricus,when approaching the hilus, characteristicallyshowed various degrees of coiling. The coilingwas also present in the hilar portion (and its

Figs. 1, 2. View of the surface of a vascular corrosion cast of the rabbit ovary. The proper ovarian branch of the ovarianartery (ramus ovaricus) (OA) and several veins (vv) are seen in the hilus. Numerous vascular plexuses of folliclesof varying size are present. Fp= pre-antral follicles, Fa= antral follicle, OV= ovarian vein. SEM: 24 Fig. 2:From Macchiarelli et al., 1993 (Fig. 1).


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acting by intermittent pulsatility; b.they can adaptthe blood supply to the changes in size of the ovary;c.they can reduce the high blood pressure presentin the ovarian artery (3050% of abdominal aortic

pressure) and prevent the turbulence of the bloodflow increasing the diameter of the coils; d. theycan equalize the arterial flow in the ovarian hilus.In the rabbit, the helical spirals of the ramus ovaricusundergo a re-orientation [21], e.g. an extensionof the loops, when the size of the ovary is increasedby injection of hCG, providing the enlarging gonadwith an adequate blood supply [21, 23]. A completerestoring of the spiral configuration of the ovarianartery was described by Reynolds [21] to occur noearlier than the sixth day of pseudopregnancy. Inour study, we did not observe significant structural

changes in the spiral arrangement of the ovarianartery in the hilus and its juxtamedullary branches,in both estrous and hCG-stimulated animals. Inparticular, we observed that arterial segments orbranches in juxtamedullary position were spiraledin both groups (Figs. 3 and 5). Presumably, thechanges in the spiraling degree reported byReynolds [21] involve the vessel running in themesovarium and not systematically studied in ourspecimens, trimmed for the SEM observation,whereas the arterial segments close to the medullaand their first ramifications appear substantially

unaffected by hCG, being capable of a sort ofsectorial regulation of the blood flow [8, 54].

In pregnant ovaries we originally observed thatthe ramus ovaricus, very long and thick, showed atight, although irregular, spiraling (Fig. 4) [8]. Thiswell correlates with the general overwhelmingvascularization shown by the whole rabbit ovarythat houses several, voluminous corpora luteawhen pregnancy is established [37].

The spiraling of the juxtamedullary portion ofthe ramus ovaricusand its principal ramificationsappeared in our samples independent from hCG

stimulation and well maintained also duringpregnancy. This suggests that an adaptation ofthe blood supply to the needs of the ovary mayoccur in two ways: 1. cyclically (distal arterialsegments), in relation to the periovulatory events,as previously reported [6, 21, 23]; 2.continuously(proximal arterial segments), according to the focal,districtual ovarian activities that involve thefollicles at various developmental stages, thecorpora lutea during growth and regression as wellas the stromal compartment.

We observed [8] small arterioles directlystemming from the coiled arteries found in the hilusand supplying outer cortical small follicular basketswithout running in the medullary and inner cortical

areas, as usually occurs. The same follicular basketsshowed a satellite venous drainage (Figs. 6, 7).These follicles, for their dimensions, location andpattern of vascularization seem actually excludedfrom a full functional development [56]. On theother hand, these follicles showed an individualand well recognizable, although peculiar,vascularization. Presumably they underwentrecruitment from the resting primordial pool [31],but were stopped in their further growth anddestined to form a reservoirof steroid-synthetic cellsfor the interstitial gland [35], being in any case

active in modulating a districtual control of ovarianactivities.

In fractured samples, we observed that thearteries running in the medulla still showedpronounced coiling (Fig. 8). The arteries branchedprofusely as they course through the medulla but,in the rabbit, we could not demonstrate thepresence of a true plexus in the cortico-medullaryregion reported in other mammals [50]. Smallcortical arteries, radially oriented, arose from thecoiled and branching inner medullary arteries,giving off arterioles which penetrate the thecae of

follicles (Fig. 9). Rich follicular capillary plexuseswere formed from these cortical arteriolar twigsand drained by small collecting venules (Figs. 10,11) [8, 34, 35].

The Countercurrent-Exchange Mechanism andIts Morphological Bases Inside the Ovary

The ovarian vasculature in the rabbit receives amodest supply from the uterine vasculature,lacking both a prominent utero-ovarian arterial

anastomosis and a common utero-ovarian vein inclose contact with the ovarian artery, as instead itis found in other mammals [3, 20, 57]. In theseanimals, such a vascular organization permits thelocal utero-ovarian transfer of a luteolytic factor,identified as the prostaglandin (PG) F2, that maydirectly pass into the arterial supply of the ovaryby a countercurrent veno-arterial mechanism,thanks to the close anatomical contacts shown bythe utero-ovarian vein and the ovarian artery [7].This local transfer, and the related ipsilateral, short-


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circuiting control of the luteolysis uterus-dependent[6, 7], was not demonstrated in the rabbit [3]. Thus,according to both anatomical characteristics andfunctional behavior shown by the rabbit ovarianvascularization, only a general, systemic regulationof the luteolysis through the utero-pituitary-ovarianroute was considered efficient in this species [18].

Our observations confirm that, in the ovarianvascular pedicle, although the ramus ovaricusandthe main venous vessel draining the ovary show acommon course, nevertheless close associations

between them are only rarely observed (Fig. 12)[18]. This means that for the rabbit there is a lackof a further general countercurrent-exchangemechanism involving an intrinsic ovarian control.Such a venous-arterial transfer, hypothesized (andin a few cases demonstrated) in other species, couldallow the recirculation in the ovary of substances(steroids, aminoacids, peptides with low molecularwieght) discharged by the ovary itself into thevenous blood [7, 58]. Although the diffusiondistance in the walls of the ovarian vessels seems

Fig. 7. Vascular corrosion cast of rabbit ovary 12 h after hCG stimulation. Artero-venous contacts are seen between hilarand outer medullary vessels. Note the presence of a relatively thin venous plexus (V) enmeshing the arterial coils(A). Two small peripheral follicular baskets (F) are directly drained by venules (v) emptying into the hilar plexiformveins. SEM: 60. Bar = 400 m. From Nottola et al., 1997 (Fig. 6).

Figs. 8, 9. Vascular corrosion casts of rabbit estrous ovary; fractured samples. The arteries (ma) maintain a spiral configurationin the medulla, often becoming straight when they enter the cortex (ca). Capillary nets (arrowheads) can be seenaround these arteries. cv= venous ramifications in the cortex; F= follicular wall. SEM: 55. Bar = 450 m. FromNottola et al., 1997 (Figs 11, 12) .

Fig. 10. Vascular corrosion cast of rabbit estrous ovary; fractured sample. Note the presence of a straight arteriole (a)supplying a follicular basket (F). SEM: 125. Bar = 200 m. From Nottola et al., 1997 (Fig . 13) .


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segments of coiled arteries and veins located inthe hilus and outer medulla (Fig. 7, 1315). Thesehave the morphological requisites for theestablishment, at this level, of a countercurrent-

like system allowing a veno-arterial exchange ofsmall molecules through the wall of the apposedvessels, lacking between the extraovarian largervessels [8].

A countercurrent exchange may occur to someextent between any closely apposed pair of arteriesand veins, and this efficiency mainly depends onfour factors: 1. area of contact, 2. difference inconcentrations between the two tubes; 3.flow ratein the tubes; 4. resistance against the transfer [7,58]. Thus, the equalization of blood flow suggestedby Reynolds [55] may be tuned not only by sectorial

adaptation of the vascular tree but also by theestablishment of different degrees of concentrationof solutes. The latter factor may modulateredistribution, recirculation, and sectorialconcentration of substances involved in ovarianphysiology [14]. For example, ovarian steroidhormones reabsorbed by branches of the ovarianartery can locally regulate the formation of theirprecursors, controlling their final production by anegative feed-back system. However, the degreeof exchange of endogenous products may varythroughout the cycle and/or pregnancy [7] and

their transfer needs to be evaluated under variousdifferent and specific conditions.

It has been recently suggested that acountercurrent exchange mechanism in the ovarymay permit a temperature gradient betweenfollicles and stroma. In fact, pre-ovulatory folliclesare cooler than stroma in humans and other species,thus creating an adequate environment for meiosisresumption [62]. On the basis of this results, itshould not be excluded that the existence of such atemperature gradient may affect not only oocytematuration, but also ovulation, atresia and

ultimately fertility.Therefore, as it results by our observations, a

countercurrent exchange may occur not onlybetween the utero-ovarian vein and the ovarianartery (uterus-to-ovary control) and between theovarian vein and the ramus ovaricus (ovary-to-ovary control), as above discussed, but evenbetween the hilar juxtamedullary and medullaryvessels. In this way, an additional vascularmorphodynamic regulation of the numerous andcomplex ovarian functions is suggested,

Fig. 11. Vascular corrosion cast of rabbit estrous ovary. Anundulated arteriole (a) supplying a follicular

basket (F) is seen. v= venules. SEM: 125. Bar =140 m. From Nottola et al., 1997 (Fig . 14) .

Fig. 12. Vascular corrosion cast of rabbit ovary 12 h afterhCG stimulation. Several veins are seen drainingthe ovary and emptying into a large vein (OV).The ramus ovaricus (OA) is relatively distant fromthis vein in the ovarian pedicle. aa= coiled artery

branching close to the ovarian hilus , F= capillaryplexuses of the follicles. SEM: 30. Bar = 1 mm.From Nottola et al., 1997 (Fig . 2).

to be considerable [59], nevertheless a local thinningof the apposed walls of the vessels has beenreported in some mammals [60, 61].

We observed close structural relations between


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emphasizing a vascular role in the local, districtualcontrol of some paracrine activities of the ovary.The location of a countercurrent system in the hilusand outer medulla seems even more suitable thanthe others, thanks to the reduced caliber of thevessels involved [54].

Vascularization of the cortex

The rabbit cortical microvasculature is arranged

in differently sized vascular plexuses (VP)supplying follicles or corpora lutea, andsurrounded to by a fine network of thin interstitialcapillaries (Fig. 16)

Intersitial stromal tissue capillaries were diffuselydistributed in the cortex among the follicularplexuses. They were often arranged in a largerounded-meshed network, likely supplyingprimordial or primary follicles: e.g. the follicleswhich are not provided with a proper techalvascularization. The interstitial capillary

Figs. 13, 14. Vascular corrosion casts of rabbit ovary 12 hafter hCG stimulation. Note the presence of flatvenous processes covering the arterial spirals in the

juxtamedullary area. A= artery, V= vein. Fig 13,SEM: 35; Bar = 750 m. Fig. 14, SEM: 200. Bar= 140 m. From Nottola et al., 1997 (Figs 8, 9).

Fig. 15. Vascular corrosion cast at 10 days of pregnancy.Hilus of the ovary. Close artero-venous contactsare maintained during pregnancy (arrow). V= hilarvein, A= hilar spiral artery, v= small vein draininga peripheral cortical follicle (F) directly into the

large hilar veins. SEM:

25. Bar = 1 mm. FromNottola et al., 1997 (Fig . 10) .

morphology was poorly affected by hCGstimulation [34].

According to the various shape and size thefollowing eight different morphological types ofvascular plexuses were identified 1. Early antralfollicles 2. Mature follicles 3. Small atretic follicles,4. Large atretic folllicles 5. Peri-ovulatory follicles6. Growing pseudopregnant corpora lutea, 7.Regressing pseudopregnant corpora lutea 8.

Pregnant corpora lutea. The main fetaures of thesestructures are briefly summarized in Table 1.

Vascular supply of developing follicles

Early antral follicles (observed in estrous rabbit)were supplied by VP (Type 1) showing a diameterranging from 100 to 250 m [3436]. They wereformed by a network of thin capillaries continuouswith the surrounding stromal capillary plexus thatdelimited a small empty central cavity (Figs. 1619). The capillaries were not of sinusoidal type


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and formed a polygonal-mesh network similar tothat found in the interstitial stroma (Fig. 20) [34].

Mature follicles were supplied were vascularplexuses larger than 250 m (Type 2). These were

observed in estrous rabbits and showed a morecomplex organization of the their vascular bed(Figs. 16; 2125). Their wall, in fact, was made upof a multilayered plexus. The inner layer delimiteda large empty cavity and was formed by dilatedand tortuous typical sinusoid capillaries havingnumerous angiogenetic figures (Figs. 22 and 24).The sinusoidal capillary layer was surrounded bynumerous tortuous vessels of capillary, venular andarteriolar nature (Figs. 2123 and 25).

According to the previous SEM studies onovarian vascular casts [31, 48] and to electron

microscopic data on unstimulated rabbit ovaries[2], Types 1 and 2 VP corresponded to the thecamicrovasculature of growing follicles in theevolutive phase, from primary to preovulatorystages [2628, 31, 34, 3749]. This VP, in fact,displayed a gradual increase of the wall thickness,and a development of the inner vascular layer froma polygonal-meshed network of thin capillaries(Type 1 VP) to a rounded-meshed network of thicksinusoids (Type 2 VP). Therefore, the present dataclearly showed that both angiogenesis and dilationof existing capillaries characterize the development

of a proper microvascular bed in growing follicles.

Vascular supply of atretic follicles

Small atretic follicles were supplied byirregularly rounded or ovoid VP (Type 3) having adiameter of about 100 to 300 mm and formed by apolygonal-meshed network of thin capillaries thatinvaded the central cavity (Figs. 26, 27). Generally,the meshes of the network appeared larger thanthose seen in the evolutive follicle (types 12) and/or in the corpora lutea (types 67).

Large atretic follicles showed VP with a diameter

larger than 250 m (Type 4). They had aplurilayered capillary wall with large gaps(avascular areas) delimiting a central cavity whichwas partially occupied by newly formed vessel (Fig.28). The inner layer capillaries were notmorphologically homogeneous due to thecontemporary presence of thin straight capillaries(45 m in diameter) and tortuous-dilatedcapillaries (612 m in diameter) (Figs. 29, 30).

Types 3 and 4 VP showed significantmorphological differences from the VP of

Fig. 16. View of a freeze-fractured vascular cast of therabbit ovary. Medullar (M) and cortical (C) zonesare seen. Medullary vessels (arrows) and theircortical branches supplying the follicle vascularplexuses are exposed. Fp= pre-antral follicles, Fa=antral follicles. SEM: 20. From Macchiarelli et al.,1993 (Fig. 2).


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developing follicles. According to our observations

[35] the Type 3 VP belongs to follicles in thedegenerative type of atresia consisting in regressionand final obliteration of the follicle structures. Infact, Type 3 plexuses were devoid of a central cavitysince it was as invaded by thin capillaries relatedto the inflammatory process occurring during thefollicle regression [63]. We considered Type 4 VPas supplying follicles in the luteinizing orhypertrophic type of atresia according to theobservation of Guraya and Greenwald [64, 65],since these plexuses showed tortuous and thickcapillaries, resembling sinusoids, that

characteristically supply ovarian structures withendocrine activity [48, 56]. Therefore, Type 4 VPare involved in the formation of the "interstitialgland " that is known to be mainly sustained bytheca cells belonging to the so called luteinizedatretic follicles [6466]. In addition, the changeswe observed in Type 3, 4 VP were primarilycharacterized by an invasion of the follicular cavityas typically seen during inflammatory reaction. Ithas been suggested that the inflammatory stimuluscan be induced by alterations in the permeabilityof the follicular basal lamina [67]. Furthermore,

also the vascular changes seen in preovulatoryfollicles have been considered as part of aninflammatory event [4]. Therefore, we have reasonsto believe that the changes found in our study likelyrepresent a consequence of the degenerativeprocess, rather than an active cause of atresia [56].As a consequence, although our data confirmindeed the occurrence of significant vascularchanges in both luteinizing and obliterant atreticfollicles in rabbit; nevertheless, they do not fullysupport the ischemic theory.

Vascular changes induced by ovulation (ovulatoryfollicles and corpora lutea)

At the time of ovulation (12 hrs after hCGstimulation), large VP (Type 5) supplied the pre-ovulatory follicles. These VP were ovoid,plurilayered, with large avascular areas located onthe apical pole (Fig. 31). The inner capillarynetwork was clearly enlarged having a sinusoidalshape and showing numerous round resin blebs(leakages) projecting into the inner cavity (Fig. 32).

Pseudopregnant corpora lutea were supplied byirregular plexuses (Types 67) made of a network

of thick vessels (Figs. 33). The inner capillaryplexus was thicker, richly anastomosed andplurilayered. Capillaries were enlarged and, inearly developing phase (Type 6) showed enhancedsinusoidal formations with numerous resinleakages as well as blind protrusions invadingcentrum of the follicular cavity (Fig. 34).

Pregnant coprora lutea were supplied by veryrich VP (Type 810 days after matingpregnantcorpora lutea). These plexuses were large andovoid in shape. They presented a outer surfacemade up of several thick venules, draining a dense

capillary network by tightly spiralized arteriespenetrating the body of the gland (Fig. 35). Theglandular tissue was supplied by a continuousdense network of moderately dilated capillaries,mainly sinusoidal in shape which seemed toconverge towards a central areas occupied by twoor three large cut venules [37].

Our observations clearly showed that hCGstimulation induced a quick functional andstructural vascular reaction which appeared moreevident in Type 5 VP (belonging to periovulatory

Table 1. Classification of ovarian vascular plexuses

Type stage Experimental phase Size in m

1 Early antral follicles All phases 1002502 Mature follicles > 2503 Small atretic follicles 1003004 Large atretic follicles > 2505 Periovulatory follicle 1024 hrs after hCG > 8006 Developing 27 days after hCG > 1000

Pseudopregnant CL7 Regressing 17 days after hCG 4001200

Pseudopregnant CL8. Pregnant CL 10 day after mating 2000


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Fig. 17. Rabbit ovary. Vascular corrosion cast of pre-antral follicles. The capillaries forming the follicular vascular baskets(F) are thin. Interstitial tissue capillaries (i) form a rough reticule. SEM: 70 From Kikuta et al., 1991 (Fig . 6).

Fig. 18. Vascular corrosion cast of estrous rabbit ovary. Type 1 of vascular plexuses. View of the outer surface of themicrovasculature of a pre-antral follicle. Several thin capillaries are drained by thicker venules. SEM: 230. FromMacchiarelli et al., 1993 (Fig. 4B).

Fig. 19. Vascular corrosion cast of estrous rabbit ovary, fractured sample. Type 1 of vascular plexuses: pre-antral growingfollicle. Note the monolayered capillary wall and the central avascular cavity. SEM: 230. From Macchiarelli etal., 1993 (Fig. 4A).

Fig. 20. Vascular corrosion cast of rabbit ovary. Interstitial-stromal capillary plexus of the inner cortex. The vascular network

shows a square mesh (asterisks). SEM: 200 From Kikuta et al., 1991 (Fig . 15) ; From Macchiarell i et al., 1991,(Fig. 7).


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follicles) and Type 6 VP (belonging to growingpseudopregnant corpora lutea). This reaction,stabilized after a few days as seen in type 7 VP.

The present data morphologically confirm theassumption that the follicular growth requires anincrease of the intraovarian blood flow likelymainly related to two factors: 1) intraovarian

growth of new vessels (angiogenesis) and 2)vasodilatation of existing capillaries [6, 48, 56].

The corpus luetum is indeed an endocrine glandw h i c h h o w e v e r p r e se n t a p e c u l i a rmorphodynamics. In our studies the postovulatoryfollicles and PPCL appeared vascularized bystraight or slightly undulated vessels. This aspect

Fig. 21. Vascular corrosion cast of rabbit ovary: small antral follicle. The follicular vascular basket is multilayered andis formed by an inner capillary plexus surrounded by an outer layer of capillaries, arterioles and venules. SEM 90 From Macchiarelli et al., 1992a (Fig . 3A).

Fig. 22. Vascular corrosion cast of rabbit ovary. Inner aspect of the capillary plexus of an antral follicle. Capillaries runsinusoidally (s), forming typical round meshes (asterisks). Blind ends, related to angiogenetic sprouts, are present(arrows). SEM: 275 From Macchiarelli et al., 1992a (Fig. 3B).

Fig. 23. Vascular corrosion cast of rabbit ovary. Inner aspect of the capillary plexus of a large antral follicle. The outervascular layer, formed by large vessels (arrowheads), is also seen. SEM: 40 From Macchiarelli et al., 1992a (Fig.3C).

Fig. 24. Vascular corrosion cast of rabbit ovary. Inner aspect of the capillary plexus of a large antral follicle. Note capillarythickening and the capillary sprouts. SEM: 345 From Macchiarelli et al., 1992a (Fig. 3D).


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is similar to the vascular architecture found incorpora lutea of late diestrous rats [31] and not-pregnant pigs and monkeys [50]. Small, flatvenules were seen forming on the surface of these

PPCL through the confluence of several sinusoidalcapillaries [36]. Numerous small cortical veinsoriginated from the venous drainage of folliclesand PPCL and converged in large medullary veins.These veins proceed toward the ovarian hilus andbecome extraovarian, as above described.

During pregnancy, the morphology of the vesselssupplying and draining the corpora lutea appearedgreatly affected by the functional load to whichthe ovary is subjected. In fact, as also reported forthe large vessels, and according to the generalenlargement of the organ, the arteries destined to

the PCL plexuses appeared as voluminous, longvessels still showing a high degree of spiraling.The presence of a rich venous drainage of PCLand of the pregnant ovary itself is functionallyrelevant. It is a common knowledge that there is alarge increase in total ovarian blood flow duringpregnancy in the rabbit, and that this blood flow ismainly distributed to the highly steroid-syntheticcorpus luteum tissue [68]. Recently, to explain thishigh rate of flow, it has been shown that lutealblood capillaries are maximally dilated andincapable of autoregulation, offering minimal

resistance to flow [69]. Therefore, according to thisview, the corpus luteum is subjected to acuteregulation of perfusion only through changes inextra-luteal vessels. Thus, the high degree ofspiraling shown during pregnancy by the arteriessupplying the corpora lutea may be considered,according to Reynolds models [55], amorphodynamic protective device capable ofreducing the otherwise dramatically high pressureof the blood flow destined to the corpus luteum [8,37].

On the basis of all these data, it should be

emphasized that an adaptation of the nurseartery (accompanied by an adequate adaptation ofthe venous drainage) to the cyc l icmorphofunctional changes of the LFC does occur,in both physiological and pharmacologicallyinduced phases of the ovarian cycle.

Concluding Remarks

Indeed, vascular corrosion cast clearly showed

Fig. 25. Vascular corrosion cast of rabbit ovary. Type 2vascular plexuses. Outer surface of the vascularplexus of a large (antral) follicle. Outermost thincapillaries (arrows), larger median venule orarterial vessels (asterisk), and innermost sinusoids(s) can be recognized. SEM: 120 From Kikutae tal. 1991 (Fig. 12).

Fig. 26. Vascular corrosion cast of rabbit estrous ovary.Type 3 vascular plexuses: degenerative type ofatresia. The follicle is irregularly rounded, wellisolated from the surrounding structures andpresenting a polygonal-meshed network of thincapillaries. SEM: 250 From Kikuta et al., 1991 (Fig.16).


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Fig. 27. Vascular corrosion cast of rabbit ovary. Type 3 vascular plexuses. Outer surface of an ovoid vascularplexus made of thin capillaries. SEM: 120 From Macchiarelli et al., 1993 (Fig . 6B).

Fig. 28. Vascular corrosion cast of rabbit ovary. Type 4 of vascular plexuses. Large, round fractured vascular plexusshowing an inner wall made of differently arranged capillaries. Note avascular areas in the inner wall(asterisks). SEM: 230 From Macchiarelli et al., 1993 (Fig. 7A).

Fig. 29. Vascular corrosion cast of rabbit ovary. Type 4 of vascular plexuses. Higher magnification of Fig. 28; notethe occurrence of sinusoids (s) and thinner capillaries (c). SEM: 1400 From Macchiarelli et al., 1993 (Fig.7B).

Fig. 30. Vascular corrosion casts of rabbit ovary. Type 4 of vascular plexuses. A round fractured vascular plexusshowing very dilated inner vessels. SEM: 180 From Macchiarelli et al., 1993 (Fig. 7C).


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Figs. 31, 32. Vascular corrosion cast of rabbit ovary. Ovulatory follicles 12 h after hCG stimulation. The apex of afollicle (Fig. 31) and the inner aspect of the capillary plexus of another follicle (Fig. 32) are shown. Resinleakages (arrows) are also seen. SEM: 65 From Macchiarelli et al., 1992a (Figs. 4B and 4C).

Fig. 33. Vascular corrosion cast of rabbit ovary 48 h after hCG stimulation, growing pseudopregnant corpus luteum(PPCL). Type 6 vascular plexus. SEM: 540 From Macchiarelli et al., 1995 (Fig . 7).


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the occurrence of changes related to the adaptationof the microvaculature to the functional needs ofdeveloping follicles. In fact follicularmicrovasculature first becomes independent from

the interstitial tissue vascolarization with theformation of capillary plexuses arranged incharacteristics baskets [31]. Following this, peculiarchanges in such capillary plexuses gradually takeplace. These changes consist of the developmentof many sinusoids, angiogenesis (proved by thepresence of numerous capillary blind ends orangiogenetic sprouts), the formation of an avasculararea in the apex of the preovulatory follicles (thestigma), capillary functional changes, characterizedby increased vascular permeability. As a matter offact, the capillary gradually adapt their structure

and distribution not only to the incoming ovulation(stigma formation and capillary permeabilization)but also to the developing thecal steroidogenicfunction (capillary dilation and sinusoid formation).These morphodynamic changes reflect thetransformation of a capillary net originallysupplying a simple epithelium (those of primaryfollicles) in a typical sinusoidal network supplyingan endocrine gland (thecal gland of secretingfollicle and then corpora lutea).

Finally, it may be concluded that SEM of vascularcorrosion cast allowed a detailed reconstruction of

the dynamic vascular changes occuring duringovarian cycle. The functional and structuralchanges we have demonstrated, however deservea deep insight through correlated morpho-fuctionalstudies. In particular, we believe that relevantinformation may be gained through the correlationof biochemical data on the several growth factorswhich are expressed at the time of angiogenesiswith the angiogenetic figures we have depicted.Indeed, functional evaluation of the paracrine andvein-artery exchanges should also be furtherapproached. The aim of this review would be fully

realized if our morphological approach will be ofstimulus for further correlated studies in this vein.

Fig. 34. Vascular corrosion cast of rabbit ovary 12 h after

hCG stimulation. Inner surface of a postovulatoryfollicle transforming into a pseudopregnant corpusluteum (PPCL). A slightly coiled artery (a) can beseen supplying this structure. b, Resin blebs. SEM:45. Bar = 600 m. From Nottola et al.1997 (Fig.15).

Fig. 35. Vascular corrosion cast of rabbit ovary at 10 daysof pregnancy. Note the complex vascularization ofthe pregnant corpus luteum (PCL). a, long, tightlycoiled artery; v, venous drainage. SEM: 25. Bar= 1 mm. From Nottola et al. 1997 (Fig. 18).

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