(Stroke. 1996;27:706-708.)
© 1996 American Heart Association, Inc.
Articles |
`Medial Defects' in the Prenatal Human Cerebral Arteries
An Electron Microscopic Study
From the Department of Anatomy, Kawasaki Medical School, Okayama, Japan.
Correspondence to Katsukuni Fujimoto, MD, Department of Anatomy, Kawasaki Medical School, 577 Matsushima, Kurashiki, 701-01 Japan.
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Abstract |
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Background and Purpose Fine structural studies were performed to investigate the histogenesis of human intracranial arteries. Special attention was paid to whether "medial defects" exist in these arteries.
Methods Segments of the intracranial extracerebral arteries of normal human embryos (n=6) were examined with transmission electron microscopy.
Results Focal defects of the medial smooth muscle cells were disclosed at every bifurcation of the developing arteries. This configuration persisted until the arteries obtained enough muscle coat. These areas, in which an absence of medial smooth muscle cells (ie, a medial defect) existed, were occupied by fibrous connective tissues of elastin and collagen.
Conclusions The medial defect observed at the arterial bifurcation of the embryos seems to be a development process that accompanies human ontogenesis rather than a congenital anomaly, supporting a possible pathogenesis for intracranial saccular aneurysms.
Key Words: angiogenesis • cerebral arteries • histology • microscopy, electron • muscle, smooth
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Introduction |
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In 1930, Forbus1 documented a focal defect of medial smooth muscle cells at the bifurcation of arteries ("medial defect") and concluded that this medial defect was responsible for the pathogenesis of intracranial saccular aneurysms. Medial defects at the bifurcation of the intracranial arteries have been thoroughly investigated because intracranial saccular aneurysms generally have been assumed to arise at the branching of these arteries. In a fine structural study of human intracranial aneurysms, Stehbens2 noted that the medial defect is acquired postnatally and is often secondarily involved in early aneurysmal changes rather than being the cause. However, because the pathogenesis of intracranial aneurysms is still unknown, a congenital hypothesis3 in which the medial defect is involved in the pathogenesis cannot be neglected.
Although the pathological consequences of medial defects have been
disputed, no electron microscopic study of the intracranial arteries of
human embryos has been undertaken. The present investigation was
carried out to assess whether medial defects exist at the bifurcation
of the intracranial extracerebral arteries of human embryos at the fine
structural level. Consequently, medial defects were found in the
intracranial arteries of the human embryos examined (Fig 1
).
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The present study was carried out entirely on materials from the collection of human embryos in the Department of Anatomy, Shimane Medical University (Izumo, Japan). The specimens were collected in accordance with the regulations of the ethics committee of Shimane Medical University. The specimens were obtained from elective terminations of pregnancy within the guidelines of the Ministry of Public Welfare of Japan, and informed consent for the use of fetal tissue was obtained from the parents. Specimens with external abnormalities were excluded. Characteristics of the specimens are shown in the Table
.4 From the aspect of
organogenesis, a differentiation between embryos and fetuses should be
made. However, in this study, both embryos and fetuses are collectively
described as embryos to avoid confusion.
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The specimens were immersed and kept in toto in a mixture of 5% glutaraldehyde, 4% paraformaldehyde, and 0.2% picric acid (0.1 mol/L phosphate buffer, pH 7.4) at 4°C. Major arteries of the circulus arteriosus cerebri and their branches were dissected out and immersed again in a fresh fixative. Then they were post-fixed in phosphate-buffered 1% osmium tetroxide (pH 7.4) for 2 hours at 4°C. After dehydration with a series of graded ethanol, specimens were embedded in Epon 812 epoxy resin. Quasi-serial sections were prepared perpendicular or parallel to the long axis of the arteries to avoid artifact due to sampling errors. Sections were stained with uranyl acetate and lead nitrate and examined with a JEOL-200CX or Hitachi H-7100 transmission electron microscope.
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Results |
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Neither a specific structure nor cytoplasmic organelles were found in the intracranial arteries of the embryos when compared with those of adults. The arterial walls of a 7-week-old embryo were composed of a single layer of endothelial cells. These endothelial cells were devoid of coverage by smooth muscle cells and scleroprotein on the abluminal surface and were surrounded by cells of the subarachnoid space. In a 9-week-old embryo, the walls of the cerebral arteries consisted of an inner tunica intima, a central tunica media, and an outer tunica adventitia.
In the main stems of the major arteries comprising the circle of Willis, the endothelial cells were rather plump and contained a large nucleus of irregular shape during the early embryonic stage. Endothelial cells of the intima were tightly held together by the junctional complexes at their margin. Disposition of medial smooth muscle cells started to appear in embryos at around 8 weeks of gestation. The tunica media of the major arteries was composed of only a single layer of smooth muscle cells in the embryos younger than 9 weeks old. At 12 weeks of gestation, two or three layers of smooth muscle cells had been added to the muscle coat of the media of the major arteries. There were close membranous appositions between the endothelial and medial smooth muscle cells and among medial smooth muscle cells, since ground substances are not fully produced until 20 weeks of gestation. Fibroblasts, their cytoplasmic processes, and some ground substances were scattered in the adventitia. The ground substances (scleroprotein) consisted mainly of elastic and collagen fibers. The cerebral arteries of the embryos of more than 20 weeks old exhibited configurations similar to those of adult ones.
In the embryos more than 11 weeks old, the tunica media of the main
stems of the major arteries comprising the circle of Willis had three
to four layers of smooth muscle cells. At the lateral angle of the
arterial bifurcation, an abrupt absence of medial smooth
muscle cells was observed where the abluminal plasmalemma
of the endothelial cells directly faced the adventitia
(Fig 2). The defect was occupied by ground substances.
This configuration was designated as a medial defect in the developing
cerebral arteries. The space resulting from the medial defect ranged
from 3 to 12 µm in width, which corresponded to the thickness of the
tunica media of the main stems; in other words, the width of the defect
becomes narrower as the embryo develops. In the embryos of more than 18
weeks old, when the tunica media of the main stems of the major
arteries had six to eight layers of smooth muscle cells, the medial
defect was not observed at the lateral angle of their branches.
However, in such arteries, defects were observed at the lateral angle
of the distal segments of their branches. No statistical
analysis was performed, but in light of the extent of
examination, it was concluded that medial defects existed at the
lateral angles of every branch in the young specimens, and no regional
differences in the appearance of the defects between the anterior and
posterior halves of the circle of Willis were noted. Numerous collagen
fibers had accumulated in the space and were arranged perpendicularly
to the long axis of the branching artery at its orifice in embryos of
12 weeks of gestation (Fig 3). In the present study,
no pathological configurations (such as irregularity of cell shape,
vacuolated residues, dark bodies of unknown origin, multilamellar basal
lamina, or other necrotic changes of smooth muscle cells) were observed
in the specimens examined.
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Discussion |
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Many neuropathologists believe that intracranial saccular aneurysms are the result of congenital defects in the media at the bifurcation of arteries1 5 6 7 8 because, in most cases, they are located in the vicinity of the arterial forks at the base of the brain. However, it has also been reported that aneurysms are acquired secondary to a focal degeneration of the internal elastic lamina at the apexes of arterial bifurcations.3 9 Hassler6 and Stehbens10 have noted many examples of intimal and medial lesions in major cerebral arteries and their pathological consequences. In these accounts, Hassler reported that 31% of 157 autopsies had a medial defect in at least one cerebral arterial bifurcation. This defect is characterized by a lack of smooth muscle cells in a patch near the apex. The locations of aneurysms do not precisely correspond to those of medial defects. Whereas saccular aneurysms occur almost exclusively at the apexes of bifurcations, defects occur frequently at the lateral angles.6 11 12 13 14 These defects occur much more frequently than aneurysmal changes, and some investigators have proposed that the muscular layer plays a minimal role in maintaining the strength of a vessel wall. This notion has been supported by experiments in which the arteries of the tunica intima alone, with most of the media scraped away, could withstand pressure much higher than that of hypertension.15 16
As the cause of medial defects, Forbus1 suggested that arterial branches may form their own coats independently, possible resulting in a failure of the two muscular systems to unite, although he added that this theory does not adequately explain why the defects should always be located at an acute angle. To the best of our knowledge, only one electron microscopic study has been performed on medial defects at the apex of the neonatal human cerebral arterial bifurcations.17 The authors suggested that the medial defects might have resulted from necrosis of medial smooth muscle cells. To the extent that we examined the human embryos, no pathological or regenerative changes were identified in or around the region of the medial defects.
In the present study, medial defects were observed in every bifurcation of the arteries on the basal surface of the brain until the arteries obtained enough muscle coat. In a brief review of the current literature on angiogenesis, it should be noted that the endothelial cells form tubes that define the vascular pattern during embryogenesis. The endothelial cells at lateral angles of arterial bifurcations play an important role in forthcoming vascular proliferation. Autoradiographic studies have shown [3H]thymidine-labeled endothelial cells to be prominent at the bifurcations.18 19
In situ observations have revealed that endothelial proliferation is influenced by accompanying periendothelial cells.20 The periendothelial cells are pericytes in capillaries and smooth muscle cells in arteries. The functional interactions between endothelial cells and medial smooth muscle cells have been thoroughly documented in in vitro studies.21 A general consensus regarding the possible role of endothelial cells, whether as a promoter or inhibitor of smooth muscle cell proliferation, has not been fully achieved yet.22 23 24 However, several inhibitory roles of smooth muscle cells in endothelial proliferation have been proposed. In a coculture study of endothelial and smooth muscle cells, Orlidge and D'Amore20 found that endothelial cell growth ceased when endothelial cells made contact with the cytoplasmic processes of smooth muscle cells.
In the prenatal human cerebral arteries, the free-surfaced endothelial cells at the segments of medial defects might participate in defining the pattern of vascular organization. Therefore, these medial defects cannot be directly connected with a congenital anomaly associated with the pathogenesis of intracranial saccular aneurysms.
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Acknowledgments |
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Part of this study was supported by a research project grant from Kawasaki Medical School. The author is indebted to Dr Hiroki Ohtani and Fumio Satow (Department of Anatomy, Shimane Medical University, Izumo, Japan) for providing essential information regarding the critical staging of human embryos and fetuses.
Received October 5, 1995; revision received January 15, 1996; accepted January 15, 1996.
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References |
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- Forbus WD. On the origin of miliary aneurysms of the superficial cerebral arteries. Bull Johns Hopkins Hosp. 1930;47:239-284.
- Stehbens WE. Ultrastructure of aneurysms. Arch Neurol. 1975;32:798-807. [Abstract]
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