Vascular
Basis
for Neural
Tube
Defects:
A
Hypothesis
Roger
E. Stevenson,
MD, JoAnn
C. Kelly,
PhD,
Arthur
S. Aylsworth,
MD, and
Mary
C. Phelan,
PhD
From the Greenwood Genetic Center and Self Memorial Hospital, Greenwood, South Carolina, and the Biosciences Research Center, Department of Pediatrics, University of North Carolina, Chapel Hill
ABSTRACT. A hypothesis is set forth that neural tube
defects are produced by inadequate nutrient supply to the rapidly growing neural folds. According to this hypothe-sis, a delay in establishing blood flow or an aberration of blood supply to neural tissue may interfere with nutrition
and prevent neural tube closure. The hypothesis was
tested by examining the vasculature of fetuses with spinal
neural tube defects. In each case, the arterial supply to
the region of the neural tube defect was disturbed.
Be-cause development of arterial supply to the neural folds predates neural tube closure, these vascular abnormalities
are considered to be primary malformations that lead to
neural tube defects rather than secondary morphologic disturbances resulting from neural tube defects. Pediat-rics 1987;80:102-106, spina bifida, neural tube defect, vas-culature, dysmorphology, embryogenesis.
The neural system, the tissue with the most
prolific growth during the prevascular stage, is the
first to outgrow its nutrient supply. Closure of the
neural tube, we propose, is dependent upon
vascu-larization of the neural system and its supporting
tissues. According to this hypothesis, a disturbance
in the timely development of vasculature may limit
nutrients available to the neural tissue and thereby prevent closure. To test the hypothesis, we infused
and dissected the vasculature of six fetuses with
neural tube defects. The studies demonstrate
dis-turbances in arterial supply to the spine in the
region of the spine defect in each case.
MATERIALS
AND
METHODS
A constant and adequate supply of nutrients is
necessary for the normal development of embryonic
tissues. During the initial weeks of development,
the conceptus receives nourishment from the
cyto-plasm of the ovum, the yolk sac, and, possibly,
uterine secretions. Transition from an avascular to
a vascular organism is a critical benchmark in early
embryonic development. This transition, normally
accomplished during the fourth postconceptional week, is essential for normal development as the
cell mass exceeds its ability to grow, divide, and
differentiate with the nutrients available by
diffu-sion alone. Continued growth and development
re-quire vascular channels to deliver nutrients into the
enlarging cell mass and to retrieve metabolic waste
products.
Received for publication Aug 11, 1986; accepted Sept 26, 1986. Reprint requests to (R.E.S.) Greenwood Genetic Center, 1 Gre-gor Mendel Circle, Greenwood, SC 29646.
PEDIATRICS (ISSN 0031 4005). Copyright © 1987 by the
American Academy of Pediatrics.
Six fetuses demonstrating the spectrum of spinal defects, from the base of the cranium to the lumbar region, were included in this study. All fetuses were detected through maternal serum a-fetoprotein
screening or ultrasound examination during the
midtrimester. Only in case 2 was a positive family
history known.
The arterial system of each fetus was infused through the umbilical artery using a mixture of barium and gelatin. The infusion mixture was pre-pared by suspending 30 g of barium sulfate in 50 mL of warm normal saline and 4 g of gelatin in 50 mL of hot water. While still hot, the gelatin solution
was added to the barium suspension, with stirring
or blending at high speed for five minutes.
Infusion was performed during a one-minute
pe-nod with the barium/gelatin mixture heated to
55#{176}C.Five milliliters or less were adequate to fill
the central arterial system in these fetuses, each
weighing less than 1 kg. Adequate perfusion was
presumed when contrast medium was noted in the
cutaneous arterioles of the distal extremities, trunk,
NORMAL CASE 1 CASE 2
tal arteries in control fetus and in six fetuses included in study.
removed in block, taking care to leave the aorta in
place from the arch distally. Once the viscera were
removed, the contrast-filled dorsal intersegmental
arteries and their intercostal branches were
imme-diately evident beneath the pleura of the posterior
thoracic wall. In the lumbar area, the dorsal
inter-segmental arteries were embedded in the psoas
muscle and were exposed by the removal of the
anterior layers of muscle.
The size of the dorsal intersegmental arteries,
their position of egress from the aorta, and their
course was noted in each case and compared to the
vascular system of two fetuses without neural tube
defects.
RESULTS
Case
1
A 16-week-old male fetus had the following
mea-surements: weight 225 g, crown to heel length 19.6
cm, crown to rump length 11 cm, and head
circum-ference 12 cm. The fetus had anencephaly and
spinal rachischisis extending from the base of the
cranium to the level of the first lumbar vertebra
(Fig 1). The thoracic intersegmental arteries were
abnormal in number and spacing (Fig 1). Five of
these vessels bifurcated and were distributed to two
thoracic segments rather than the normal one.
Case 2
A 15-week-old female fetus had the following
Fig 1. Top, Location and size of spinal defects in fetuses
included in study; bottom, aorta and dorsal
intersegmen-measurements: weight 148 g, crown to rump length
10.2 cm, crown to heel length 16.5 cm, and head
circumference 13.5 cm. Spinal rachischisis
ex-tended from the fourth thoracic to the fifth lumbar
vertebrae (Fig 1). Craniofacial structures,
extremi-ties, and viscera appeared normal. A marked
dis-turbance of size and distribution of the thoracic
and lumbar dorsal intersegmental arteries was
dem-onstrated (Figs 1 and 2). Only three pairs of tho-racic arteries were present. Two of the three pairs
were hypoplastic, and one artery of the third pair,
arising at the thoracic-lumbar junction bifurcated
and its cephalad branch trifurcated. The three
lum-bar dorsal intersegmental arteries arose centrally from the aorta.
Case 3
A 17-week-old male fetus had the following
meas-urements: weight 260 g, crown to rump length of
14.5 cm, crown to heel length 23 cm, and head
circumference 15.5 cm. The gross appearance was normal except for spinal rachischisis extending
from the seventh thoracic to the fifth lumbar
ver-tebrae (Fig 1). Each of the lumbar intersegmental arteries arose from the aorta as a central trunk. In the thoracic area, the dorsal intersegmental arteries
were irregularly spaced and had numerous
bifurca-tions (Fig 1).
Case 4
Fig 2. Infusion of aorta with barium/gelatin mixture in control fetus without spinal defect (left) and in case 2 fetus (right). In control fetus, 14 dorsal intersegmental arteries arise in regular sequence to vascularize vertebral
measurements: weight 150 g, crown to heel length
19 cm, crown to rump length 12.5 cm, and head
circumference 14.5 cm. Spinal rachischisis
ex-tended from the tenth dorsal to the first sacral vertebrae (Fig 1). There was disturbed egress and course of the dorsal intersegmental arteries in the region of the spinal defect (Fig 1).
Case 5
A 16-week-old female fetus had the following
measurements: weight 157 g, crown to rump length
13.4 cm, and head circumference of 13.3 cm. The
lower extremities were hypoplastic and the spine
had a lumbar rachischisis (Fig 1). Arterial infusion
and dissection demonstrated normal number but
markedly disturbed egress of the dorsal
interseg-mental arteries from the lumbar aorta (Fig 1).
Case 6
An 18-week-old male fetus had the following
measurements: weight 255 g, crown to rump length
bodies, spinal cord, and body wall. In case 2 fetus, dorsal intersegmental arteries show disturbances of caliber, egress from aorta, and distribution (compare to line draw-ing in Fig 1).
16 cm, crown to heel length 26 cm, and head
cir-cumference of 14.5 cm. Spinal rachischisis extended
from the fourth lumbar vertebrae into the sacrum
(Fig 1). The lumbar dorsal intersegmental arteries
were abnormal with bifurcation of one artery,
hy-poplasia of one vessel and absence of one artery
(Fig 1).
DISCUSSION
These six fetuses with spina bifida demonstrate abnormalities of the arterial supply to the region of
the spine defect. Because embryologically these
ar-teries develop prior to closure of the neural tube,
we propose that the vascular disturbance limited
nutrition to the developing neural tissue and
sup-porting structures, preventing appropriate growth
and closure.
The cardiovascular system is the first to function in the embryo.’’ It is called upon early because of
the paucity of nutrients available in the ovum and
yolk sac and because of the limitations of nutrient
avascular cell mass becomes limited by the distance
nutrients can travel by ebb and flow and by cell to
cell transport. Only with development of the
vas-cular system can the embryonic fields develop into
eumorphic functional organs and systems.
At approximately 18 days after fertilization,
is-lands of angioblasts along the ventrolateral aspects
of the neural tube coalesce to form the dorsal
aor-tas.2’4 Dorsal intersegmental arteries arise from the
aortas while they are yet in the primitive paired
states and intimately associate with the neural groove.5 As the aortas fuse to form a single midline
vessel, the dorsal intersegmental arteries give off
additional branches permitting growth and
devel-opment of the body wall. There are usually 14 pairs
of dorsal intersegmental arteries in continuous
se-quence from the level of the third dorsal to the
fourth lumbar vertebrae.’ The spinal branch of each
of these paired arteries serves the spinal medulla, its membranes, and the vertebrae, whereas other
branches vascularize the body wall. The two dorsal
segments cephalad to this series of arteries are
supplied by branches from the subclavian arteries,
and the segments caudal to the fourth lumbar seg-ment are supplied by a single midline sacral artery.
Because of the rapid growth of the neural folds
and adjacent supporting structures in the early
stages of embryonic development, we infer that
progress toward completion of neural tube
forma-tion would be hindered by diffusion constraints and
is
dependent upon timely vascularization of thesetissues.”6’7 The neural tube is vascularized by an
abundance of arteries, including the internal
ca-rotid artery to the brain and pairs of arteries arising at each of the approximately 30 somites to the
hindbrain and spinal cord. The cervical
interseg-mental arteries coalesce into the vertebral arteries, bilateral trunks that supply the cervical cord,
hind-brain, and anastamose with the internal carotid
arteries. The dorsal and lumbar intersegmental
ar-teries remain as individual, bilateral branches from
the aorta supplying the spinal cord, vertebrae, and
body wall.
Neural tube closure begins at the level of the fourth somite, near the future craniospinal junc-tion, and proceeds in rostral and caudal
direc-tions.68 At 20 days after fertilization, human
em-bryos showing the first evidence of closure have
seven somites.”68 The arterial system, although
still primitive, is represented by well-defined dorsal
aortas prior to this stage.2 Developmental progress of the dorsal intersegmental tributaries of the
aor-tas has not yet been defined in the few embryos
available from the early somite stage.
By conjecture, one might suggest that the initial
somite development and neural tube closure in the
midembryo region occurs because of their proximity
to the developing cardiac tube. Early pulsations
from the cardiac tube may move blood to the
mi-dembryo region better than to the more distant
caudal and cranial regions.
According to our hypothesis, neural tube closure
follows and is dependent upon vascularization of
the neural folds. The available human embryos
from this developmental stage support the concept
that at least the major arterial channels are present prior to neural tube closure. The dissections of fetuses with defects of neural tube closure presented
in this article demonstrate major disturbances of
the arterial supply to the region of the defects. We
have previously demonstrated the vascular
patho-genesis of sirenomelia, a condition with lumbar
spina bifida in 10% of cases.9 In sirenomelia, a
vascular steal exists that siphons blood from the
aorta at the level of the diaphragm, depriving
tis-sues vascularized by tributaries of the abdominal
aorta of adequate nutrients for normal embryonic
development.
Evidence for the vascular basis of anencephaly
has been reported by Vogel and McClenahan’#{176} and
Vogel.”2 They demonstrated anomalous vascula-ture in anencephalic specimens and suggested that the cerebral vessels never become connected to the
systemic circulation. The failed arterial
connec-tions prevent adequate nutrition to the cranial
neural tissue, resulting in anencephaly. Other
non-neural malformations that have been shown to have
a vascular pathogenesis include nonduodenal
intes-tinal atresias, gastroschisis, sirenomelia, acardia,
defects of branchial arch derivation, and limb
re-duction defects.9”’8
Our hypothesis and dissection findings support
the view that spina bifida occurs because of failure of neural tube closure rather than because of over-distension and rupture of a previously closed neural
tube, as suggested by Gardner and Breuer.’9
Our findings suggest that neural tube defects, one
of the most common serious birth defects, may
occur because of a disturbance of embryonic vas-culature. This expands the range of malformations attributed to faulty vasculature and gives reason to
redirect the search for the underlying heritable and
environmental forces that may contribute to the formation of neural tube defects.
ACKNOWLEDGMENTS
We thank Drs Harold A Taylor, Robert A. Saul, John
Koepke, Fred Dalldorf, Keith Nance, David Kaufman,
Brian Shiro, David Walker, David Slater, Roger Bley,
REFERENCES
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