JK SCIENCE
ORIGINAL ARTICLE
Light Microscopic Study of The Lumbar Intervertebral
Disc Showing Inherent Difference Between Anterior &
Posterior Annulus Fibrosus - A Risk Factor For
Posterior Disc Herniation
Shahnaz Choudhary, Ashwani K Sharma, Harbans Singh, Richa, Neelu Sharma
Abstract
The present study is based upon the light microscopic observations made on the foetuses to determine the
structural discrepancy in anterior and posterior annulus fibrosus in the development of human foetal lumbar
intervertebral disc. A morphological comparison of the structure of anterior and posterior annulus fibrosus
was made to consider the arrangement of lamellar bundles. As the foetus advances in age, the anterior
annulus fibrosus becomes thicker than the posterior annulus. A very intricate structure was observed in
posterior annulus with large number of loose and discontinuous lamellar bundles and more fibre-interlacing
angles. Loose connection of the lamellar bundles in posterior annulus of almost all the foetuses indicates an
inherent weakness which may lead to posterior rupture of the disc in later life.
Key Words
Annulus fibrosus, Nucleus Pulposus, Intervertebral Disc, Lamellar Bundles
Introduction
The intervertebral discs are the major structural links
between adjacent vertebrae. They serve to allow greater
motion between vertebral bodies than would occur if
vertebrae were in direct opposition. Each intervertebral
disc consists of central gelatinous nucleus pulposus,
contained within the more collagenous annulus fibrosus
peripherally and cartilage end plates inferiorly and
superiorly. The annulus fibrosus forms the outer boundary
of each disc. It is composed of fibrocartilagenous tissue
and fibrous proteins, which are arranged in concentric
layers or lamellae, with fibres in the outer lamellae
continuing into the longitudinal ligaments and vertebral
bodies (1). Disc composition changes significantly during
development, growth, aging and degeneration and this in
turn alters how discs respond to mechanical stress (2).
The matrix of intervertebral disc comprises mainly a
fibrillar collagen network that offers tensile strength. The
clinical picture of backache and lumbar herniated disc
has often been an enigma in the medical practice. Most
of the herniation occurs in the posterior region of the
intervertebral disc at the lumbosacral joint. This region
carries most of the strain of spinal column and half of the
motion between the lower thoracic region and sacrum
takes place here. Major age related changes in the
intervertebral disc in humans are reported to occur at the
end of first decade of life. Even before this, as early as
two years of age, mild microscopic degenerative changes
are seen (3). It has been observed that the disc undergoes
degenerative changes earlier in life than other tissues do
(4). This indicates that the disc begins to show signs of
degeneration at a very early age. Although studies have
been conducted on the structural variation of the anterior
From the Department ofAnatomy, Govt Medical College Jammu (J&K) - India
Correspondence to : Dr Shahnaz Choudhary, Demonstrator, Postgraduate Department of Anatomy, Government Medical College, Jammu
Vol. 13 No. 2, April-June 2011
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and posterior annulus fibrosus in adults but there is paucity
of the literature depicting structural difference in the
annulus fibrosus in foetal life which would be indicative
of the posterior disc herniation in later life. This study
has been directed at comparing the structure of anterior
and posterior annulus fibrosus in foetal life and to analyse
the extent of variation between these two parts.
Material And Methods
The 35 foetuses ranging from 53 to 300 mm CR Length
were collected from operation theatres and labour rooms
of the Government Medical College, Jammu and various
nursing homes and preserved in 10% formalin followed
by measurement of CR length. The lumbar region of
vertebral column was dissected out and subjected to
histological processing (5). Serial sections of 7 µm were
cut in transverse and longitudinal manner and stained with
H&E and Masson's trichrome stains and then analysed
under a binocular research light microscope.
Results
All the foetuses have been studied in the ascending order
of their crown rump lengths. In smaller foetus the disc is
composed of the central notochord which differentiates
into nucleus pulposus, surrounded by the mesenchymal
tissue which forms the annulus fibrosus. The annular
structure was identifiable from 65 mm CR length foetus
and the laminate structure rapidly became more obvious
with the advancing age. The thickness of anterior and
posterior annulus is almost the same in smaller foetus
and nucleus pulposus is in the centre but from 95 mm CR
length foetus, the dorsal migration of nucleus pulposus
begins thus it comes to lie relatively posteriorly.
As the foetus advances in age, the anterior annulus
gradually becomes thicker than the posterior and consisted
of complete bundles of collagen fibres (lamellae) arranged
in a concentric manner and are closely packed in a uniform
way (Fig.1a), whereas the posterior annulus was made
of very intricate structure with numerous incomplete and
discontinuous lamellae, due to splitting and jolting of fibres
of different lamellae and an interlocking of various
lamellae (Fig. 1b). The fibres are vertical rather than
arched in the posterior part. The collagen fibres of
consecutive bundles form a crisscross pattern forming
the fibre interlacing angles, which seemed to be greater
in the posterior than in the anterior annulus fibrosus. The
number of distinct bundles was significantly greater in
90
Fig 1a. Showing Complete Bundles of Collagen Fibres
Arranged in Concentric Manner in Anterior
Annulus Fibrosis in 115 mm CR Length
Foetus (X50)
Fig 1b. Showing Incomplete and Discontinuous Lamellae
in Posterior Annulus Fibrosis in 115 mm CR Length
Foetus (X50)
Fig 2a. Showing Distinct Lamellar Structures in Anterior
Annulus Fibrosis in 280 mm CR length Foetus (X50)
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Vol. 13 No.2, April-June 2011
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Fig 2b. Showing More Complex Lamellar Structures in the
Posterior Annulus Fibrosis in 280 mm CR Length
Foetus (X50)
the anterior outer annulus fibrosus than the posterior outer
annulus in all the foetuses ranging from 180 to 300 mm
CR length (Fig.2a). In the posterior annulus, a net like
separation of lamellae was observed forming a very
complex arrangement which is suggestive of loose
connection between the lamellae (Fig.2b).
Discussion
The histological sequence of intervertebral disc of foetuses
under consideration shows central notochord, which
differentiates into the nucleus pulposus and the peripheral
condensation of primitive embryonic mesenchyme, which
shows progressive differentiation into the annulus fibrosus
with well formed collagenous tissue.
The present study indicates that there is a gradual
increase in the thickness of annulus fibrosus due to
increase in collagen bundles, which are arranged in
lamellae lying parallel to one another. The fibres of
consecutive bundles form a crisscross pattern forming
the fibre interlacing angles which are more obvious in
the posterior annulus fibrosus corroborating the results
from previous reports (6, 7). Although the thickness of
the individual lamella was not measured but the total
thickness of the anterior annulus was greater than the
posterior.
The laminate structure in the current study indicates that
the structure is very complex in the posterior region of
the disc, which involves a great number of discontinuous
lamellar bundles. The splitting and jolting of the collagen
bundles which forms the fibre interlacing angles, showed
Vol. 13 No. 2, April-June 2011
a wide range of variation between anterior and posterior
annulus fibrosus. It is of interest that the netlike separation
of lamellar bundles in the posterior annulus fibrosus is
apparent from foetal life, which suggests an inherent
characteristic of the laminate structure of the disc. There
is loose connection of the posterior annulus with increased
fibre interlacing angles in all the foetuses studied in the
present series which suggests that the posterior portion
of the disc begins to give way first, may be because of
its inherent weakness.
Though mechanical factors (8) are also responsible for
intervertebral disc rupture and herniation in adults, but
the incidents of disc herniation in adolescents in some
cases suggest that these may not be the only etiological
factors. The embryological variation in the structure of
the disc may be a high risk factor in the etiopathogenesis
of disc prolapse. Whether this inherent difference in the
anterior and posterior part of the disc acts as precursor
for degenerative changes commonly seen in posterior
part of the disc in juvenile and adult life needs to be further
investigated.
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