Animal model
An animal model (short for animal disease model) is a living, non-human, often genetic-engineered animal used during the research and investigation of human disease, for the purpose of better understanding the disease process without the risk of ha ...
s of ischemic stroke are procedures inducing
cerebral ischemia
Brain ischemia is a condition in which there is insufficient bloodflow to the brain to meet metabolic demand. This leads to poor oxygen supply or cerebral hypoxia and thus leads to the death of brain tissue or cerebral infarction/ischemic stroke. ...
. The aim is the study of basic processes or potential therapeutic interventions in this disease, and the extension of the pathophysiological knowledge on and/or the improvement of medical treatment of human ischemic stroke.
Ischemic stroke has a complex pathophysiology involving the interplay of many different cells and tissues such as
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
s,
glia
Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form myel ...
,
endothelium
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vesse ...
, and the
immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...
. These events cannot be mimicked satisfactorily
in vitro
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...
yet. Thus a large portion of stroke research is conducted on animals.
Overview
Several models in different species are currently known to produce cerebral ischemia. Global ischemia models, both complete and incomplete, tend to be easier to perform. However, they are less immediately relevant to human stroke than the focal stroke models, because global ischemia is not a common feature of human stroke. However, in various settings global ischemia is also relevant, e.g. in global anoxic brain damage due to cardiac arrest. Different species also vary in their susceptibility to the various types of ischemic insults. An example is
gerbils
Gerbillinae is one of the subfamilies of the rodent family Muridae and includes the gerbils, jirds, and sand rats. Once known as desert rats, the subfamily includes about 110 species of African, Indian, and Asian rodents, including sand rats an ...
. They do not have a
Circle of Willis
The circle of Willis (also called Willis' circle, loop of Willis, cerebral arterial circle, and Willis polygon) is a circulatory anastomosis that supplies blood to the brain and surrounding structures in reptiles, birds and mammals, including huma ...
and stroke can be induced by common carotid artery occlusion alone.
Mechanisms of inducing ischemic stroke
Some of the mechanisms which have been used are:
* Complete global ischemia
** Decapitation
** Aorta/vena cava occlusion
** External neck tourniquet or cuff
** Cardiac arrest
* Incomplete global ischemia
** Hemorrhage or hypotension
** Hypoxic ischemia
** Intracranial hypertension and common carotid artery occlusion
** Two-vessel occlusion and hypotension
** Four-vessel occlusion
** Unilateral common carotid artery occlusion (in some species only)
* Focal cerebral ischemia
** Endothelin-1-induced constriction of arteries and veins
** Middle cerebral artery occlusion
** Spontaneous brain infarction (in spontaneously hypertensive rats)
** Macrosphere embolization
* Multifocal cerebral ischemia
** Blood clot embolization
** Microsphere embolization
** Photothrombosis
Hypoxic Ischemia models
One of the most commonly used animal models of hypoxic ischemia was originally described by Levine in 1960 and later refined by Rice et al., in 1981. This approach is useful to study hypoxic ischemia in the developing brain, since newborn rat pups are utilized in this model. Briefly, 7 day old rat pups undergo a permanent unilateral carotid artery ligation with a subsequent 3 hour exposure to a hypoxic environment (8% oxygen). This model creates a unilateral infarct in the hemisphere ipsilateral to the ligation, since the hypoxia alone is subthreshold for injury at this age. The area of injury is typically concentrated in periventricular regions of the brain, especially cortical and hippocampal areas.
Focal ischemia models
They are divided into techniques including reperfusion of the ischemic tissue (transient focal cerebral ischemia) and those without reperfusion (permanent focal cerebral ischemia). The following models are established :
* Endothelin-1 -induced constriction of arteries and veins
*
Middle cerebral artery
The middle cerebral artery (MCA) is one of the three major paired cerebral arteries that supply blood to the cerebrum. The MCA arises from the internal carotid artery and continues into the lateral sulcus where it then branches and projects to ma ...
occlusion (MCAO)
** MCAO avoiding
craniotomy
A craniotomy is a surgical operation in which a bone flap is temporarily removed from the skull to access the brain. Craniotomies are often critical operations, performed on patients who are suffering from brain lesions, such as tumors, blood clot ...
Endothelin-1 -induced constriction of arteries and veins
Endothelin-1 is a potent vasoconstrictor which is produced endogenously during ischemic stroke and which contributes to overall loss of cells and disability. Exogenous endothelin-1 can also be used to induce stroke and cell death after sustained vasoconstriction with reperfusion. It can be microinjected to induce focal stroke in small tissue volumes (e.g., cortical grey matter, white matter or subcortical tissue) or after injection near the
Middle cerebral artery
The middle cerebral artery (MCA) is one of the three major paired cerebral arteries that supply blood to the cerebrum. The MCA arises from the internal carotid artery and continues into the lateral sulcus where it then branches and projects to ma ...
. It is often used as a model of focal stroke to evaluate candidate pro-regenerative therapies. One advantage of this model of stroke is that it causes highly reproducible infarcts. Another benefit is that it can be used in elderly rats with only very low resulting mortality.
Embolic middle cerebral artery occlusion
Middle cerebral artery
The middle cerebral artery (MCA) is one of the three major paired cerebral arteries that supply blood to the cerebrum. The MCA arises from the internal carotid artery and continues into the lateral sulcus where it then branches and projects to ma ...
(MCA) occlusion is achieved in this model by injecting particles like blood clots (thrombembolic MCAO) or artificial spheres into the carotid artery of animals as an animal model of ischemic stroke. Thrombembolic MCAO is achieved either by injecting clots that were formed in vitro or by endovascular instillation of
for in situ clotting . The thrombembolic model is closest to the pathophysiology of human cardioembolic stroke. When injecting spheres into the cerebral circulation, their size determines the pattern of brain infarction: Macrospheres (300–400 µm) induce infarcts similar to those achieved by occlusion of the proximal MCA , whereas microsphere (~ 50 µm) injection results in distal, diffuse embolism . However, the quality of MCAO – and thus the volume of brain
infarct
Infarction is tissue death (necrosis) due to inadequate blood supply to the affected area. It may be caused by artery blockages, rupture, mechanical compression, or vasoconstriction. The resulting lesion is referred to as an infarct
(from the ...
s – is very variable, a fact which is further aggravated by a certain rate of spontaneous lysis of injected blood clots.
The technique of endovascular filament (intraluminal suture) MCAO as an animal model of ischemic stroke was described first by Koizumi . It is applied to rats and mice. A piece of surgical filament is introduced into the internal carotid artery and forwarded until the tip occludes the origin of the
middle cerebral artery
The middle cerebral artery (MCA) is one of the three major paired cerebral arteries that supply blood to the cerebrum. The MCA arises from the internal carotid artery and continues into the lateral sulcus where it then branches and projects to ma ...
, resulting in a cessation of blood flow and subsequent brain
infarction
Infarction is tissue death ( necrosis) due to inadequate blood supply to the affected area. It may be caused by artery blockages, rupture, mechanical compression, or vasoconstriction. The resulting lesion is referred to as an infarct
(from th ...
in its area of supply. If the suture is removed after a certain interval, reperfusion is achieved (transient MCAO); if the filament is left in place the procedure is suitable as model of permanent MCAO, too. The most common modification is based on Longa (1989) who described filament introduction via the external carotid artery, allowing closure of the access point with preserved blood supply via the common and internal carotid artery to the brain after the removal of the filament. Known pitfalls of this method are insufficient occlusion, subarachnoid hemorrhage ,
hyperthermia
Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...
, and necrosis of the ipsilateral extracranial tissue . Filament MCAO is not applicable to all rat strains .
middle cerebral artery
The middle cerebral artery (MCA) is one of the three major paired cerebral arteries that supply blood to the cerebrum. The MCA arises from the internal carotid artery and continues into the lateral sulcus where it then branches and projects to ma ...
(MCA) is surgically dissected and subsequently permanently occluded, e.g. by
electrocautery
Cauterization (or cauterisation, or cautery) is a medical practice or technique of burning a part of a body to remove or close off a part of it. It destroys some tissue in an attempt to mitigate bleeding and damage, remove an undesired growth, o ...
or
ligation
Ligation may refer to:
* Ligation (molecular biology), the covalent linking of two ends of DNA or RNA molecules
* In medicine, the making of a ligature (tie)
* Chemical ligation, the production of peptides from amino acids
* Tubal ligation, a meth ...
. Occlusion can be performed on the proximal or distal part of the MCA. In the latter, ischemic damage is restricted to the
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
. MCAO can be combined with temporal or permanent
common carotid artery
In anatomy, the left and right common carotid arteries (carotids) (Entry "carotid" in craniotomy
A craniotomy is a surgical operation in which a bone flap is temporarily removed from the skull to access the brain. Craniotomies are often critical operations, performed on patients who are suffering from brain lesions, such as tumors, blood clot ...
ischemic stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop funct ...
by transient transcranial MCAO is similar to that of permanent transcranial MCAO, with the MCA being reperfused after a defined period of focal cerebral ischemia . Like permanent MCAO, craniotomy is required and
Photothrombotic models of ischemic stroke use local intravascular photocoagulation of circumscribed cortical areas. After intravenous injection of photosensitive dyes like rose-bengal, the brain is irradiated through the skull via a small hole or a thinned cranial window, leading to photochemical occlusion of the irradiated vessels with secondary tissue ischemia . This approach was initially proposed by Rosenblum and El-Sabban in 1977, and improved by Watson in 1985 in the rat brain. This method has also been adapted for use in mice.
Ischemic stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop funct ...