Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 26 Cerebrospinal Fluid
(Thomson et al. , 1989, 1990 ). Lastly, the CSF of animals with
neurological disease is not always abnormal ( Tipold et al. ,
1995 ). Only occasionally does CSF analysis provide a specific
diagnosis ( Kjeldsberg and Knight, 1993 )—for example,
if infectious agents (bacteria or fungi) or neoplastic cells are
observed. In most situations, the chief utility of CSF analysis
is to assist in the diagnostic process by excluding the likelihood
of certain disease processes being present. As is the case
with all tests of relatively low specificity, examination of CSF
is most useful when the results are correlated with the history,
clinical findings, imaging studies, and ancillary laboratory
studies. As stated by Fankhauser (1962) , “ It is futile to make
a diagnosis based solely on the CSF findings and particularly
on single alterations of it. Only the entire picture of all findings
linked with the other clinical symptoms is of value in
reaching a diagnosis . ”
II . FUNCTIONS OF CEREBROSPINAL FLUID
Cerebrospinal fluid has four major functions: (1) physical
support of neural structures, (2) excretion and “ sink ” action,
(3) intracerebral transport, and (4) control of the chemical
environment of the central nervous system. Cerebrospinal
fluid provides a “ water jacket ” of physical support and buoyancy.
When suspended in CSF, a 1500-gm brain weighs only
about 50 gm. The CSF is also protective because its volume
changes reciprocally with changes in the volume of intracranial
contents, particularly blood. Thus, the CSF protects the
brain from changes in arterial and central venous pressure
associated with posture, respiration, and exertion. Acute or
chronic pathological changes in intracranial contents can
also be accommodated, to a point, by changes in the CSF
volume ( Fishman, 1992 ; Milhorat, 1987 ; Rosenberg, 1990 ).
The direct transfer of brain metabolites into the CSF provides
excretory function. This capacity is particularly important
because the brain lacks a lymphatic system. The lymphatic
function of the CSF is also manifested in the removal of large
proteins and cells, such as bacteria or blood cells, by bulk
CSF absorption (see Section II.D). The “ sink ” action of the
CSF arises from the restricted access of water-soluble substances
to the CSF and the low concentration of these solutes
in the CSF. Therefore, solutes entering the brain, as well as
those synthesized by the brain, diffuse freely from the brain
interstitial fluid into the CSF. Removal may then occur by
bulk CSF absorption or, in some cases, by transport across the
choroid plexus into the capillaries ( Davson and Segal, 1996 ;
Fishman, 1992 ; Milhorat, 1987 ; Rosenberg, 1990 ).
Because CSF bathes and irrigates the brain, including
those regions known to participate in endocrine functions,
the suggestion has been made that CSF may serve
as a vehicle for intracerebral transport of biologically
active substances. For example, hormone releasing factors,
formed in the hypothalamus and discharged into the
CSF of the third ventricle, may be carried in the CSF to
their effective sites in the median eminence. The CSF may
TABLE 26-1 Composition of the Brain–Fluid
Interfaces
Interface Cell Type Junction Type
Blood-brain
Blood-CSF
Blood-CSF
CSF-blood
CSF-brain
also be the vehicle for intracerebral transport of opiates
and other neuroactive substances ( Davson and Segal, 1996 ;
Fishman, 1992 ; Milhorat, 1987 ; Rosenberg, 1990 ).
An essential function of CSF is the provision and maintenance
of an appropriate chemical environment for neural
tissue. Anatomically, the interstitial fluid of the central nervous
system and the CSF are in continuity (see Section II.A);
therefore, the chemical composition of the CSF reflects and
affects the cellular environment. The composition of the
CSF (and the interstitial fluid) is controlled by cells forming
the interfaces, or barriers, between the “ body ” and the neural
tissue. These semipermeable interfaces, the blood-brain
barrier, the blood-CSF barrier, and the CSF-brain barrier,
control the production and absorption of CSF and provide
a fluid environment that is relatively stable despite changes
in the composition of blood ( Davson and Segal, 1996 ;
Fishman, 1992 ; Milhorat, 1987 ; Rosenberg, 1990 ).
III . CSF FORMATION, CIRCULATION, AND
ABSORPTION
The brain (and the spinal cord) as an organ is isolated in
many ways from the body and the systemic circulation. This
isolation is accomplished anatomically by several interfaces
between brain tissue and systemic fluids ( Table 26-1 ). At
these interfaces, selective carriers and ion pumps transport
electrolytes and essential nutrients and thereby control the
brain’s microenvironment. A substantial portion of this control
is achieved through the formation, circulation, and absorption
of CSF at these brain-fluid interfaces ( Davson and Segal,
1996 ; Fishman, 1992 ; Milhorat, 1987 ; Rosenberg, 1990 ).
A . Anatomy of Brain-Fluid Interfaces
1 . Blood-Brain Barrier
Brain capillary
endothelium
Choroid plexus
epithelium
Arachnoid cells
Arachnoid villi
Ependyma
Pia mater
Modifi ed from Rosenberg (1990) .
Tight junction
Apical tight junction
Tight junction
Valve
Gap junction
Gap junction
The important blood-brain (and blood-spinal cord) interface
is formed by the endothelial cells of the intraparenchymal
capillaries. In most areas of the brain and spinal cord,