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. 1999 Dec;80(6):357–367. doi: 10.1046/j.1365-2613.1999.00134.x

Experimental cerebral aneurysms in the female heterozygous Blotchy mouse

Michèle Coutard 1
PMCID: PMC2517836  PMID: 10632785

Abstract

The Blotchy mouse is characterized by an X-linked inherited disorder of connective tissue synthesis. The susceptibility to aneurysm formation in the cerebral arteries of the circle of Willis was compared in female heterozygous ‘Blotchy’ and control mice subjected to unilateral carotid artery ligation either alone or associated with hypertension. Cerebral aneurysms developed only in hypertensive Blotchy mice (6/31 vs. 0/30 in hypertensive controls). Aneurysms of the aorta and its major branches occurred in normotensive mice only in the Blotchy group in which hypertension increased the incidence of mesenteric and coeliac aneurysms. A light microscopic study of interruptions of the internal elastic lamina (IIEL) showed that they developed in arteries of both Blotchy and control mice but to a greater extent in the Blotchy group where hypertension further increased their incidence. The IIEL incidence in the aortic arch varied in parallel to the occurrence of aneurysms in all the different arterial sites. Thus, in an apparently normally viable animal, the presence of a mutated gene which indirectly leads to defective elastin and collagen fibre synthesis, favours the formation of both peripheral and cerebral aneurysms. However, the development of cerebral aneurysms requires the addition of an increase in haemodynamic stress.

Keywords: cerebral aneurysms, mouse

The Blotchy mouse presents one of a series of mutations at the X-linked ‘mottled locus’ in the C57BL6 strain (Lyon 1960; Rasberry & Cattanach 1993). In the male hemizygote, the different allelic variants display coat hypopigmentation and various phenotypes with more or less severity in connective tissue disorders and neurologic disturbance. The Blotchy type is one of the milder forms suffering mainly from connective tissue disorders (Rowe et al. 1974). A defective copper ion transport has been previously shown to be the primary defect underlying the ‘mottled’ phenotype (Hunt 1974). Recently, mutations in the Atp7a gene encoding for a copper-transporting ATPase have been demonstrated at the mottled locus (Levinson et al. 1994;; Mercer et al. 1994; Das et al. 1995; Cecchi et al. 1997). Mutations in the homologous ATP7A gene in human Menkes disease confirm the mottled mouse as a model for this human hereditary disease (Cecchi et al. 1997; Reed & Boyd 1997). In addition, similar splicing mutations in this gene occur in the Blotchy mouse and Occipital Horn Syndrome, a milder allelic form of Menkes disease (Das et al. 1995).

The general copper-ion deficiency resulting from the defective activity of the copper-transporting ATPase affects several copper-dependent enzymes. Among them, a reduced activity of lysyl oxidase, involved in the formation of cross-links in collagen and elastin fibers (Siegel et al. 1970), has been shown in skin and aorta in Menkes' disease (Royce & Steinmann 1990) and in the ‘mottled’ mouse (Rowe et al. 1977). However, abnormalities other than low activity of lysyl oxidase appear to underlie the connective tissue defects since lower levels of transcripts for both lysyl oxidase and tropoelastin have also been reported in cultured fibroblasts of the tortoiseshell mottled mouse and Menkes patients (Gacheru et al. 1993).

The male Blotchy mouse develops aneurysms mostly in the ascending aorta with an increased incidence with age (Brophy et al. 1988) and death frequently results from aortic rupture (Grahn et al. 1969; Brophy et al. 1988). However, cerebral aneurysms have not been described in this mouse model. The present study was performed to determine whether the genetically inherited defect in connective tissue which characterizes the Blotchy mouse may also predispose to aneurysms in the cerebral circulation.

For the purpose of this study, we used the experimental model of induction of aneurysms in the arteries of the circle of Willis described in the rat (Hashimoto et al. 1978; Handa et al. 1983; Coutard & Osborne-Pellegrin 1997) but without the additional use of β-aminopropionitrile (BAPN), which is a specific inhibitor of the enzyme lysyl-oxidase (Tang et al. 1983). Mice were subjected to a modified local cerebral blood flow in some arteries of the circle of Willis by ligation of one common carotid artery (Hashimoto et al. 1980), either alone or in association with experimental hypertension induced by renal ischaemia. Carotid ligation was performed in all mice since in this previous report cerebral aneurysms were never found in rats without carotid ligation even in the presence of hypertension and fed BAPN (Hashimoto et al. 1980).

In addition, a light microscopic study of different arteries from normotensive and hypertensive Blotchy and control mice was performed. Several aneurysms were also studied microscopically. We have determined and compared the incidence of interruptions of the arterial internal elastic lamina (IIEL) in the aortic arch, descending thoracic and abdominal aorta in several mice of the four groups. The IIEL incidence represents a good index of the general disruption of the aortic elastic lamellae which is prominent in the Blotchy mouse (Brophy et al. 1988). A possible link between IIEL incidence and the presence of aneurysms in different arteries was also studied.

Rather than using male Blotchy mice, this study was performed in female heterozygous Blotchy mice which provide the opportunity to study, in an animal with an apparently normal life span, the effect of a mutated gene, leading to defective elastin and collagen fibre synthesis, on the formation of cerebral aneurysms.

Methods

Animals and surgical procedure

For the animal care and the experimental protocol, this study was conducted with authorization (N°75594) from the Ministère de l'Agriculture, de la Technologie et de la Recherche.

Several female heterozygous Blotchy mice (C57BL/6 J moblo) were purchased from the Jackson Laboratory (Bar Harbor, ME, USA) and bred with male wild C57BL6 mouse. Then several heterozygous female offspring were in turn bred to obtain sufficient numbers of heterozygous and control female mice. When aged 3, 6 or 9 months, wild type (control) and heterozygous female Blotchy mice (distinguishable from black-coated wild-type females by their mosaic coat colour) were used for the surgical procedure which was performed over a period of 2 years until about 30 hypertensive controls, 30 hypertensive Blotchy, 10 normotensive controls and 10 normotensive Blotchy were available for observation. Mice were anaesthetized by an intraperitoneal injection of sodium pentobarbital (40 mg/kg). In all mice the left common carotid artery was ligated with a nylon thread (Ethicon 9/0) in two close locations and the artery cut between them. In addition, in about 2/3 of the Blotchy and control mice, a median abdominal incision of the skin and muscle was made and the renal arteries exposed. The main posterior branch of one renal artery was then ligated with a nylon thread (Ethicon 10/0) and the contralateral kidney was excised or, alternatively, the main posterior branches of both renal arteries were ligated. The former method, named ‘Loomis’, has been previously reported to induce a sustained increase in blood pressure, at about 150 mmHg, from the second week following surgery till the end of the experimental period (10 weeks) in adult mice of the Cox#x002F;Swiss strain (Ebihara & Martz 1971). The second method, consisting of bilateral ligation of the posterior branches of the renal arteries has been previously used to induce hypertension in other animal species (Handa et al. 1983; Hashimoto et al. 1987). The abdominal muscle and skin were sewn up separately. A considerable number of animals died during anaesthesia and very soon after this traumatic surgical procedure (30–40%). This large number of premature deaths cannot confound the results since aneurysm formation is a chronically developing pathology. Other mice died at different times after the intervention. When possible, they were autopsied and considered in the present data when the time elapsed from the surgical procedure was at least 6 months. 5 Blotchy and 3 control with only carotid ligation and 3 Blotchy and 4 control hypertensive mice died before 6 months and 12 of them did not show cerebral aneurysms. The state of the brain in 3 of them (1 hypertensive Blotchy and 1 hypertensive control and 1 normotensive control) did not allow the study of their cerebral arteries.

Preparation of arteries

Mice were sacrificed under anaesthesia (pentobarbital i.p.) between 6 and 12 months after the surgical procedure. A solution of 3% glutaraldehyde in 150 mm cacodylate buffer was perfused via a needle placed in the left ventricule. The vena cava was cut for outflow of perfusates. Only a small volume of the fixative (1 ml) was perfused towards the brain since, if all arterial blood was removed it was very difficult to observe and dissect the cerebral arteries. Under a dissecting microscope, the brain was removed from the skull, the ventral part of the brain was exposed and the arteries of the circle of Willis were gently removed. The aorta and its major branches were exposed and observed for the presence of aneurysms. Thoracic and abdominal aorta with mesenteric and coeliac arteries were removed and together with cerebral arteries were placed in the glutaraldehyde solution. They were then rinsed in cacodylate buffer for 24 h and postfixed in 1% OsO4 for about one hour for cerebral and two hours for the other arteries. Then they were processed for routine electron microscopy. Just before being embedded in Epon, cerebral arteries were again observed under the dissecting microscope to check for the presence of aneurysmal structures which are more easily detectable when the arteries are slightly blackened with osmium. The presence of cerebral aneurysms was assessed by another observer blinded to the animal groups. Arteries were cut into pieces of about 5 mm length (or less for cerebral arteries) before being placed serially in small embedding moulds containing Epon.

Quantification of IIEL

To determine the incidence of interruptions of the arterial elastic lamina, longitudinal semithin sections of about 1.5 μm thickness were made from the entire length of the thoracic (6–7 blocks) and abdominal (2–4 blocks) aortas in normotensive control (n = 6), hypertensive control (n = 8), normotensive Blotchy (n = 7) and hypertensive Blotchy (n = 12) mice. Animals of similar ages (about 17–18 months) were selected for this comparative study. The number of IIEL per cm of aortic arch, descending thoracic aorta and abdominal aorta was calculated by adding values of the number of IIEL determined on each section divided by the total length of the artery studied, which was measured with the aid of a micrometer in the microscope eyepiece. In addition, sections from several mesenteric and cerebral aneurysms were made to observe the morphology of these structures. Sections were stained with toluidine blue.

Statistical analysis

The heart weight per 100 g body weight was compared between normotensive and hypertensive mice using a Mann–Whitney U-test (n = 84). The age and duration of carotid ligation were compared between paired groups using a Student's t test.

The occurrence of aneurysms between paired groups was studied by the χ2 test followed by the Fisher's exact test. (for small numbers of events).

Other statistical analyses were performed using either a multiple (3-way) anova or, in most cases, a 1-way anova with post hoc comparisons by a Fisher's test.

Results

There was no statistically significant difference between the age of animals in the four groups studied for the incidence of aneurysms in cerebral and noncerebral arteries. However, the time after carotid ligation was greater in control compared to Blotchy hypertensive mice (P ≤ 0.01).

The only evidence of a hypertensive state in mice operated to induce hypertension was the heart weight. The heart weight/100 g body weight was higher in the hypertensive than in the normotensive group: (0.69 ± 0.22 vs. 0.58 ± 0.21 (P ≤ 0.001). No differences were found between Blotchy and control mice either in the normotensive group (0.584 ± 0.22 vs. 0.581 ± 0.21) or in the hypertensive group (0.693 ± 0.22 vs. 0.686 ± 0.23).

Cerebral aneurysms

Table 1 summarizes the incidence of aneurysmal structures in the circle of Willis in control and heterozygote Blotchy mice, either normotensive or hypertensive, all submitted to modified local arterial blood flow induced by carotid ligation. In control wild-type mice no aneurysmal structures were observed in the normotensive group whereas in the hypertensive group several mice showed small blebs (small blister-like protrusions of the arterial wall) in the anterior artery and one mouse showed a microaneurysm at the bifurcation of the anterior and olfactory arteries. One animal showed small corrugations on part of the surface of a slightly dilated artery. In the normotensive Blotchy group, only 2 animals showed a close succession of small dilatations in arteries of the complex of the anterior artery and anterior communicating artery. In the hypertensive Blotchy group, large aneurysms (dilatations exceeding several times the calibre of the involved artery) developed in the anterior part of the circle of Willis in several mice (6/31 mice all from different female progenitors) while no aneurysms formed in normotensive Blotchy (0/12) or in hypertensive control mice (0/30). One hypertensive Blotchy mouse showed a subarachnoid haemorrhage due to aneurysmal rupture. Microaneurysms (small dilatations not greater than twice the arterial calibre) were also observed in the anterior part and posterior part of the circle of Willis and 2 hypertensive Blotchy mice showed small corrugations of the anterior artery.

Table 1.

Incidence of cerebral aneurysmal structures

graphic file with name iep0080-0357-t1.jpg

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Results are expressed as the total number and (percent) of mice displaying aneurysmal structures.

*P ≤ 0.05 compared to control hypertensive mice using the χ2 test followed by the Fisher's exact test. There are no statistically significant differences between groups for age and duration of carotid ligation was lower in hypertensive Blotchy compared to hypertensive control mice

**P ≤ 0.01 using a Student's t test.

Figure 1a illustrates a cerebral aneurysm in the anterior part of the circle of Willis from a hypertensive Blotchy mouse. Semi-thin sections of 2 different cerebral aneurysms are shown in Figure 1b,c. The internal elastic lamina was absent and the aneurysmal wall was greatly thickened compared to the adjacent very thin normal arterial wall and inflammatory cells were dispersed within the fibrous wall. In addition, numerous leucocytes were also located below the endothelium and accumulated particularly at the borders of the aneurysm at the site of the rupture of the internal elastic lamina. Early stages in aneurysmal development are illustrated in Figure 1d,e. Figure 1d shows a small area lacking the internal elastic lamina with the presence of a dark infiltrate, probably corresponding to plasma constituents which had entered the wall during endothelial damage which had probably occurred earlier since bulging regenerating luminal cells were present. Inflammatory cells were already present at the borders of this area. A microaneurysm which only displayed a small evagination and the absence of both the internal elastic lamina and medial cells as abnormal structural features is illustrated in Figure 1e.

Figure 1.

Figure 1

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a, Cerebral aneurysm in the anterior part of the circle of Willis at the bifurcation of the olfactory and anterior artery of a hypertensive Blotchy mouse (→). mca, middle cerebral artery; oa, olfactory artery; aa, anterior artery. b-e, semithin sections, stained with toluidine blue, from cerebral arteries of hypertensive Blotchy mice; b,c: cerebral aneurysms, d,e: small early aneurysmal structures. IEL, internal elastic lamina; (→) inflammatory cells. Scale bars: a,1 mm; b and c,100 μm; d and e, 50 μm.

A particular feature was observed in only one Blotchy mouse consisting of a large haemorrhage within the cerebral cortex which was probably responsible for the animal's death (Figure 2) which occurred one week after the surgical procedure.

Figure 2.

Figure 2

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Cortical cerebral haemorrhage in the brain from a hypertensive Blotchy mouse. Scale bar: 2 mm.

Non-cerebral aneurysms

Table 2 summarizes the incidence of aneurysms of the aorta and its major branches in control and heterozygous Blotchy female mice, either normotensive or hypertensive. In control mice no aneurysms developed in the normotensive group and in 30 hypertensive mice only one animal developed several aneurysms. In the group of normotensive Blotchy mice, a few animals presented dilatation of the ascending aorta and aneurysms of the mesenteric artery. In the Blotchy mouse, with hypertension there was a marked increase in the incidence of mesenteric (P < 0.01) and coeliac (P < 0.01) aneurysms but no significant increase in the incidence of aneurysms in the ascending thoracic aorta

Table 2.

Incidence of noncerebral aneurysms

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Results for aneurysms are expressed as the total number and (percent) of mice affected in each group.

*P < 0.01 compared to normotensive Blotchy mice calculated using the χ2 test followed by the Fisher's exact test. There are no statistically significant differences between groups for age using a Student's t test.

Thoracic aortae from hypertensive control and Blotchy mice are illustrated in Figure 3a and aneurysms of the proximal coeliac and mesenteric arteries of the same Blotchy mouse in Figure 3b. The thoracic aorta from this Blotchy mouse showed particularly exacerbated alterations with the presence of a large aneurysm with a translucent, thin bleb in the elongated ascending part and ectasia of the descending part. Microscopically, the aneurysmal wall of the ascending aorta showed extensive fragmentation and absence of both medial elastic lamellae and of the internal elastic lamina along a great length (Figure 3c). Figure 3d illustrates the morphology of a mesenteric aneurysm showing the complete absence of normal arterial wall structure and an extensive intimal proliferation with neosynthesis of elastic fibers around cells on the luminal side. This difference in morphology between mesenteric and thoracic aortic aneurysms suggests that local factors (structural charac-teristics of the artery, local haemodynamics) may play a role in determining the structure of aneurysms.

Figure 3.

Figure 3

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a, Thoracic aortae from hypertensive control (C) and Blotchy (B) mice. Note in the Blotchy the large-sized aortic aneurysm in the ascending part and the ectasia of the descending part. b, Mesenteric (M) and proximal coeliac (C) aneurysms from a hypertensive Blotchy mouse. c-d, Semi-thin sections, stained with toluidine blue, of aneurysmal arteries. c, ascending thoracic aorta from hypertensive Blotchy, note extensive fragmentation of elastic lamellae. d, mesenteric aneurysm from hypertensive Blotchy, note the important intimal proliferation with elastin fibers neosynthesis (dark wavy lines). Scale bars: a,b, 2 mm; c,d, 100 μm.

In the group of hypertensive Blotchy mice, only 50% of mice displaying a cerebral aneurysm also presented a peripheral aneurysm. Thus, for any given animal, the propensity to develop an aneurysm within the cerebral circulation did not necessarily imply the formation of aneurysms in the periphery.

Incidence and morphology of IIEL in the aorta and its major branches

The incidence of IIEL in the aortic arch, descending thoracic and abdominal aorta from normotensive and hypertensive Blotchy and control mice is reported in Figure 4. The number of IIEL was significantly (P ≤ 0.01) higher in the aortic arch and abdominal aorta than in the descending thoracic aorta. In the descending thoracic aorta, hypertensive Blotchy mice showed significantly higher levels of IIEL than normotensive Blotchy mice and hypertensive control mice. In the abdominal aorta, normotensive Blotchy showed significantly higher levels of IIEL than normotensive control and hypertensive Blotchy mice showed higher IIEL levels than hypertensive control mice.

Figure 4.

Figure 4

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Incidence of interruptions of the internal elastic lamina (IIEL) in 3 different aortic sites in normotensive (□ Control; Inline graphic Blotchy) and hypertensive (Inline graphic Control; ▪ Blotchy) mice. Results are expressed as mean ± SEM. *P ≤ 0,05 vs. normotensive (NT) and hypertensve (HT) control mice.

Figure 5 illustrates the morphology of different arteries. In the hypertensive Blotchy mice, as in the ascending thoracic aorta (Figure 5a), the descending thoracic aorta (Figure 5c), although to a lesser extent, and more often the abdominal aorta (Figure 5e) displayed areas of absence of medial elastic lamellae and of the internal elastic lamina compared to hypertensive control mice which lacked or showed very few of these structural alterations (Figure 5 b,d). IIEL were also observed in other arteries studied such as the mesenteric and iliac (Figure 5f) arteries.

Figure 5.

Figure 5

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Semi-thin sections, stained with toluidine blue, of arteries from hypertensive mice. a-b, ascending thoracic aorta (a) Blotchy, note the disruption of both the internal and medial elastic lamellae compared to (b) control. c-d, descending thoracic aorta from (c) a Blotchy and (d) a control mouse. e-f, abdominal aorta (e) and iliac artery (f) of Blotchy mice. Scale bars: a-b, 200 μm; c-d, 100 μm; e-f, 200 μm.

When the 4 groups of mice are considered together, the IIEL incidence in the aortic arch was related to the occurrence of aneurysms in the different arterial sites: cerebral (P ≤ 0.01), ascending thoracic aorta (P ≤ 0.02) and abdominal aorta (P ≤ 0.02) whereas the IIEL incidence in the descending thoracic aorta was only related to aneurysms of the ascending thoracic (P ≤ 0.001) and abdominal (P ≤ 0.01) aorta. When the group of hypertensive Blotchy mice is considered separately, the occurence of cerebral aneurysms varied in parallel to IIEL incidence in the aortic arch and that of aneurysms in the ascending thoracic aorta was related to ILEI incidence in the descending thoracic aorta.

Discussion

The present study showed, in a genetic model, that an abnormality in the synthesis of extracellular matrix components may favour cerebral aneurysm development Since the female Blotchy has a decreased lysyl oxidase activity (Rowe et al. 1977) our data agrees with the reported increased incidence of experimental cerebral aneurysms in the rat induced by BAPN (Hashimoto et al. 1978) which is a specific inhibitor of lysyl-oxidase. Previously, BAPN treatment. has also been shown to increase the functional and structural distensibilities of the rat aorta (Berry et al. 1981).

In the adult rat, BAPN was still able to increase the formation of cerebral aneurysms (Hashimoto et al. 1978; Handa et al. 1983) although the bulk of elastin synthesis had already occurred, suggesting that BAPN had mainly affected collagen synthesis. The role of abnormalities in connective tissue, particularly a deficiency in collagen type III, has been suggested in aneurysmal formation (Pope et al. 1981; Neil-Dwyer et al. 1983; Ostergaard & Oxlund 1987; Schievink et al. 1990; Majamaa & Myllylä 1993; Brega et al. 1996; Gaetani et al. 1996) although still debated (Leblanc et al. 1989; Kuivaniemi et al. 1993; Adamson et al. 1994).

In Menkes disease in man, tortuous cerebral arteries have been observed but no cerebral aneurysms described (Ichihashi et al. 1990). However, death occurs in early childhood (3 years of age). In our study, the Blotchy mice displaying cerebral aneurysms were at least 10-month-old and had been subjected for at least 7 months to increased haemodynamic stress. In the Occipital Horn Syndrome (or X-linked cutis laxa), dilatation of the aortic root, the ascending aorta and the major branches of the aortic arch is observed and the vertebral arteries are elongated and tortuous (Subramanian et al. 1992). No cerebral aneurysms have been described, but they have not been specifically looked for.

In the Blotchy mouse, conditions of increased haemodynamic stress in part of the circle of Willis were necessary to induce aneurysm formation in cerebral arteries in contrast to other arteries, suggesting that the cerebral arteries are more protected from aneurysmal formation. Increased blood flow in some cerebral arteries by carotid ligation led only to mild alterations of an aneurysmal nature and hypertension was essential to induce true cerebral aneurysms, at least over the duration of this study. These data are in agreement with those obtained previously in the rat (Nagata et al. 1980) and confirm the importance of increased blood pressure in aneurysm formation in rodents which may also be essential in man. Although debated, higher levels of arterial pressure have been reported in patients presenting cerebral aneurysms (Andrews & Spiegel 1979; Taylor et al. 1995). Thus, haemodynamic factors are of importance in the formation of cerebral aneurysms as pointed out by Stehbens (1989) and their role may be particularly determinant in the presence of structural arterial defects as suggested by data of the present study.

A few normotensive female Blotchy mice displayed dilatations of the ascending aorta and also mesenteric aneurysms whereas in the wild-type no aneurysms developed even at 20 months of age. In the ‘tortoise’ mouse, which is the least viable of the different ‘mottled’ mutants, many heterozygous females die of aortic rupture (Rowe et al. 1974), whereas in the female Blotchy we observed no aneurysmal rupture. With hypertension, the incidence of mesenteric and coeliac aneurysms was greatly increased whereas the incidence of ascending aortic aneurysms was not modified in the female Blotchy. In addition, some aneurysms developed in the abdominal aorta and in its proximal branches. This suggests that, in this mouse model, hypertension had a more important effect on the formation of aneurysms in arteries located in the abdominal part of the body than in the thoracic aorta. In the male Blotchy mouse, a reduction of heart rate by β-adrenergic blockade reduces the incidence of aneurysms in the ascending aorta (Moursi et al. 1995). It is possible that the level of heart rate is of greater importance than that of blood pressure in the formation of aneurysms in the thoracic aorta. However, in one hypertensive Blotchy mouse, a large aneurysm of the thoracic aorta had developed and ruptured causing death. Thus, hypertension may play a role in determining the size and the rupture of aortic aneurysms.

IIEL were observed in all arteries studied in both control and Blotchy mice but were fewer in number in control mice. It is possible that the IIEL had developed with age since the mice were sacrificed at least at 10 months of age. Hypertension increased the level of IIEL in Blotchy mice (but only significantly in the descending thoracic aorta) whereas in control mice IIEL incidence was not affected. These data suggest that in the Blotchy mouse, bearing defects in arterial wall extracellular fibrous protein structure, hypertension was able to accentuate structural alterations of the arterial wall. The formation of aneurysms in the Blotchy mouse appears to be related to the degree of IIEL formation, suggesting that these 2 arterial phenomena are related.

In addition to the development in several Blotchy mice of cerebral aneurysms in the circle of Willis in conditions of increased haemodynamic stress, one hypertensive Blotchy mouse displayed a haemorrhagic event within the cerebral cortex. It has been reported that when rats were rendered hypertensive early in life, several rats developed cerebral haemorrhages (Berry & Greenwald 1976) and microaneurysms in the cerebral hemisphere (Lee & Berry 1977). We have previously shown that the Brown Norway rat which, like the Blotchy mouse, presents a high susceptibility to IIEL formation (Capdeville et al. 1989) and a decreased activity of lysyl oxidase (Osborne-Pellegrin et al. 1990) is also susceptible to cortical haemorrhagic events when rendered hypertensive (Capdeville et al. 1989). In addition, we have previously shown that the Stroke-prone spontaneously hypertensive rat is more susceptible to develop arterial IEL breaks than the Stroke-resistant spontaneously hypertensive rat (Coutard & Osborne-Pellegrin 1991). This suggets that a susceptibility to spontaneous IEL rupture may be linked to this cerebrovascular pathology.

The present data showed that the experimental model of induction of cerebral aneurysms used in the rat (Handa et al. 1983) may also be applied to the mouse. Given the increasing use of this species for the obtention of genetically modified animals, application of this model to various ‘Knock-Out’ mice may be helpful in the understanding of cerebral aneurysm pathogenesis.

In conclusion, the present study in the female heterozygous Blotchy mouse supports the concept that the presence of one allele of a defective gene, which indirectly alters the synthesis of elastin and collagen and does not interfere with life expectancy (in normotensive conditions), could favour the formation of both cerebral and noncerebral aneurysms although at the cerebral level an increase in haemodynamic stress is an absolute requirement for aneurysm development.

Acknowledgments

The author would like to thank Catherine Chollet for her excellent technical assistance, Liliane Louedec for her expert help in animal husbandry, Sylvain Roger for photography and Dr Mary Osborne-Pellegrin for her helpful comments on the manuscript.

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