**2. Pathological features of human diabetic retinopathy**

Although DR has long been considered as a hyperglycemia-mediated microangiopathy, it has been recognized as a neurodegenerative process in view of the presence of neurodegenerative abnormalities preceding clinically apparent microvascular changes. Numerous cellular and molecular changes reflective of the DR pathogenesis have been identified, though the multifactorial nature of DR makes it challenging to clearly identify clinically relevant pathogenic pathways implicated in each stage of retinopathy. The common clinical, cellular, molecular features and functional changes of human DR are summarized in **Table 1**.

> and inner nuclear layer (INL) in patients with minimal DR as compared with controls, while such a difference was not observed in diabetic patients without DR [16, 19]. Numerous studies have also documented evidence suggestive of increased retinal ganglion cell (RGC) loss

**Table 1.** Overview of common clinical, cellular, molecular features and functional changes of human DR.

**Proliferative diabetic retinopathy (PDR) (in** 

Animal Models of Diabetic Retinopathy (Part 1) http://dx.doi.org/10.5772/intechopen.70238 15

• Retinal blood flow may be increased [11] or equivalent to that of normal patients [9, 10, 13] • Infiltration of activated microglia into subretinal space (diabetic maculopathy) [22]

• Reduced b-wave amplitude [25, 27, 28]

**addition to features of NPDR)**

• Retinal or vitreous hemorrhage • Tractional retinal detachment (advanced) • Neovascular glaucoma (advanced) • Retinal edema (can occur at any stage of DR)

• Neovascularization

In addition to neural apoptosis, reactive gliosis is another prominent feature of DR. Expression of glial fibrillary acidic protein (GFAP), an intermediate filament protein expressed by astrocytes, is normally confined to the proximal retina in non-diabetic retinae. In DR, there is aberrant overexpression of GFAP by Müller cells spanning across the entirety of Müller cell processes [21]. Microglial cells are also activated in NPDR [22]. In PDR, the microglia surrounds the neovascularization area in the vitreous, with subsequent infiltration and migration of activated microglia into the subretinal space in cases with diabetic macular

Electroretinographic (ERG) alterations have long been documented in diabetic patients prior to the development of visible lesions of retinopathy. Delay in implicit times of oscillatory

in DR [20].

Functional changes (ERG)

degeneration [22].

**2.2. Electrophysiological alterations**

**Features Non-proliferative diabetic retinopathy** 

• Intraretinal hemorrhages • Microaneurysms • Cotton wool spots • Venous beading

loops, vessel dilatation)

• RGC loss [20]

• Leukostasis [14]

microaneurysms) [6] • BRB breakdown [9]

• IRMAs (e.g. vessel tortuosity, venous

• Reactive gliosis (overexpression of GFAP expression in Müller cells) [21] • Activated microglia [22]

• Decrease in retinal thickness (total, RNFL, GCL, INL, IPL) [15–19] • Pericyte loss [6] or apoptosis [8]

• Capillary non-perfusion and obliteration • Increased retinal blood flow [10–12] • Decreased arteriole-to-venule ratio (decreasing with increasing DR severity) [29]

• Capillary BM thickening [4, 5] • Acellular capillaries (associated with

• Increased OP peak latencies [25] • Reduced OP amplitudes [23, 25] • Delayed OP implicit times [23–25] • Increased b-wave implicit time [26] • (Reduced b-wave amplitude) [30]

**(NPDR)**

Clinical features [2]

Cellular and molecular features

#### **2.1. Cellular and molecular features**

The DR hallmark lesions of capillary basement membrane (BM) thickening [4, 5] and pericyte loss [6] or apoptosis [7, 8] have been well described in human patients. Other microvascular changes include blood-retinal barrier (BRB) disruption (as evidenced by fluorescein leakage) [9] and the presence of acellular capillaries [6]. In regards to hemodynamics, it has typically been reported that retinal blood flow is increased in NPDR [10–12]. Conversely, in PDR, the nature of retinal blood flow changes appears to be dependent on the degree of non-perfusion and the pathological features present, with no marked increases in blood flow in cases with arterial narrowing [9, 10, 13]. As persistent inflammation is also implicated in DR, studies have demonstrated increased leukostasis (increased leukocyte entrapment and leukocyte endothelial cell adhesion) in diabetic retinae, perhaps resulting from increased expression of adhesion molecules (e.g. ICAM-1) in human DR [14].

Histologically, retinal thinning, particularly thinning of the pericentral total retinal thickness and the retinal nerve fiber layer (RNFL), is present in both T1D and T2D patients with no DR, NPDR or pre-proliferative DR [15–19]. Studies analyzing individual intraretinal layer thicknesses showed thinning of the ganglion cell layer (GCL), RNFL, inner plexiform layer (IPL)


**Table 1.** Overview of common clinical, cellular, molecular features and functional changes of human DR.

and inner nuclear layer (INL) in patients with minimal DR as compared with controls, while such a difference was not observed in diabetic patients without DR [16, 19]. Numerous studies have also documented evidence suggestive of increased retinal ganglion cell (RGC) loss in DR [20].

In addition to neural apoptosis, reactive gliosis is another prominent feature of DR. Expression of glial fibrillary acidic protein (GFAP), an intermediate filament protein expressed by astrocytes, is normally confined to the proximal retina in non-diabetic retinae. In DR, there is aberrant overexpression of GFAP by Müller cells spanning across the entirety of Müller cell processes [21]. Microglial cells are also activated in NPDR [22]. In PDR, the microglia surrounds the neovascularization area in the vitreous, with subsequent infiltration and migration of activated microglia into the subretinal space in cases with diabetic macular degeneration [22].

#### **2.2. Electrophysiological alterations**

working-age population worldwide. It can be broadly classified as non-proliferative diabetic retinopathy (NPDR) or proliferative diabetic retinopathy (PDR). According to the AAO International Clinic DR Disease Severity Scale, NPDR is further subdivided into mild, moderate or severe NPDR, depending on the extent of microaneurysm, intraretinal hemorrhage, venous beading and intraretinal microvascular abnormality (IRMA) formation [2]. With worsening retinal ischemia and increasing microvascular damage, NPDR may progress to PDR, which is characterized by the presence of neovascularization and/or vitreous or preretinal hemorrhage [2]. Severe cases of PDR may result in retinal edema, tractional retinal detachment and neovascular glaucoma. Diabetic maculopathy or macular edema, the most

DR-associated visual impairment results in large socioeconomic costs for both the society and individuals. This calls for effective screening methods and increased efforts to understand the pathophysiological progression and to look for effective treatment strategies using both

Although DR has long been considered as a hyperglycemia-mediated microangiopathy, it has been recognized as a neurodegenerative process in view of the presence of neurodegenerative abnormalities preceding clinically apparent microvascular changes. Numerous cellular and molecular changes reflective of the DR pathogenesis have been identified, though the multifactorial nature of DR makes it challenging to clearly identify clinically relevant pathogenic pathways implicated in each stage of retinopathy. The common clinical, cellular, molecular

The DR hallmark lesions of capillary basement membrane (BM) thickening [4, 5] and pericyte loss [6] or apoptosis [7, 8] have been well described in human patients. Other microvascular changes include blood-retinal barrier (BRB) disruption (as evidenced by fluorescein leakage) [9] and the presence of acellular capillaries [6]. In regards to hemodynamics, it has typically been reported that retinal blood flow is increased in NPDR [10–12]. Conversely, in PDR, the nature of retinal blood flow changes appears to be dependent on the degree of non-perfusion and the pathological features present, with no marked increases in blood flow in cases with arterial narrowing [9, 10, 13]. As persistent inflammation is also implicated in DR, studies have demonstrated increased leukostasis (increased leukocyte entrapment and leukocyte endothelial cell adhesion) in diabetic retinae, perhaps resulting from increased expression of

Histologically, retinal thinning, particularly thinning of the pericentral total retinal thickness and the retinal nerve fiber layer (RNFL), is present in both T1D and T2D patients with no DR, NPDR or pre-proliferative DR [15–19]. Studies analyzing individual intraretinal layer thicknesses showed thinning of the ganglion cell layer (GCL), RNFL, inner plexiform layer (IPL)

common cause of vision loss, may also arise at any stage of DR [3].

14 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

**2. Pathological features of human diabetic retinopathy**

features and functional changes of human DR are summarized in **Table 1**.

experimental animal models and clinical trials.

**2.1. Cellular and molecular features**

adhesion molecules (e.g. ICAM-1) in human DR [14].

Electroretinographic (ERG) alterations have long been documented in diabetic patients prior to the development of visible lesions of retinopathy. Delay in implicit times of oscillatory potential (OPs), particularly OP1, precede retinopathy development [23, 24]. The OPs are generated by inner retinal neurons and are often considered to be reflections of feedback circuits between amacrine and bipolar cells and/or circuits between amacrine and ganglion cells. Eyes with NPDR display a reduction in OP amplitudes [24, 25] and an increase in OP peak latencies [25]. There is some discrepancy regarding the onset of changes in b-wave responses, which are largely generated by depolarizing bipolar cells with some contribution from Müller cells. B-wave implicit times appear to be increased even in early stages of DR [26] while reductions in b-wave amplitudes have been suggested to be predominantly found in eyes with PDR [25, 27, 28]. Changes in OP amplitude and implicit times have also been suggested to be a reflection of the severity and prospective progression of DR [24, 25, 27].
