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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 47  |  Issue : 3  |  Page : 107-117

Postnatal and aging-related changes of the retina in albino rats: a histological, immunohistochemical, and quantitative study


Department of Anatomy and Embryology, Faculty of Medicine, Tanta University, Tanta, Egypt

Date of Submission01-Jan-2018
Date of Acceptance01-Jan-2019
Date of Web Publication23-Jan-2021

Correspondence Address:
MBBCh Nancy N.A El-Hady Ibrahim
Department of Anatomy and Embryology, Faculty of Medicine, Tanta University, Tanta
Egypt
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DOI: 10.4103/tmj.tmj_5_19

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  Abstract 


Background Retina is the best structural model for histological study of the nervous system. Aging is a biological phenomenon that involves gradual degradation of the structure and subsequent deterioration of the retina.
Aim This study aimed to clarify the postnatal changes in the albino rat retina as well as the effect of aging on its structure.
Materials and methods Twenty albino rats were divided into four groups (five rats each) according to their ages into 1, 3, 18, and 24 months. The retinae of all groups were prepared for light microscopic examination (histological and immunohistochemical), electron microscopic examination, and quantitative study including statistical analysis of the number of retinal ganglion cells.
Results All layers of the retina were highly organized at 1 and 3 months with strong positive immunoreactivity of the ganglion cells to tubulin β III stain at 3 months. Ultrastructurally, oval light cones, irregular dark rods, and bipolar cells were noticed and the nuclei of ganglion cells layer exhibited finely dispersed chromatin at 1 month and at 3 months these cells were arranged in several layers of variable sizes and shapes. Aging caused degenerative changes to all layers of the retina at the age of 18 months which became marked with complete loss of the outer layers at 24 months. Ganglion cells showed weak immunoreactivity to tubulin β III stain and statistically, their number showed a highly significant increase from 1 to 3 months and then showed a highly significant reduction with aging.
Conclusion Retina became highly organized postnatally and aging caused its deterioration with significant reduction in the number of ganglion cells.

Keywords: aging, light microscopy, postnatal, quantitative study, retina, transmission electron microscopy


How to cite this article:
El-Hady Ibrahim NN, Radwan DA, Zoair MM, El-Kattan AK. Postnatal and aging-related changes of the retina in albino rats: a histological, immunohistochemical, and quantitative study. Tanta Med J 2019;47:107-17

How to cite this URL:
El-Hady Ibrahim NN, Radwan DA, Zoair MM, El-Kattan AK. Postnatal and aging-related changes of the retina in albino rats: a histological, immunohistochemical, and quantitative study. Tanta Med J [serial online] 2019 [cited 2021 May 17];47:107-17. Available from: http://www.tdj.eg.net/text.asp?2019/47/3/107/307634




  Introduction Top


The retina is the inner photosensitive layer of the eye formed of a delicate sheet of nervous tissue. Its outer surface lies close to the choroid and the inner surface is adjacent to the vitreous body. It consists of several layers of neurons interconnected by synapses [1].

The neural retina consists of 10 layers of cells [2]. The outermost layer contains the rods and cones (light sensitive photoreceptors). Visual information is conducted from them deeply to the bipolar and ganglion cells and laterally via horizontal cells into the outer retina and amacrine cells in the inner retina. Photoreceptors, and bipolar and horizontal cells communicate with each other in the outer plexiform layer while bipolar, amacrine, and ganglion cells synapse in the inner plexiform layer. The supporting cells of the retina are represented by Muller, astrocytes, and microglial cells [3],[4].

Photoreceptors, bipolar cells, and ganglion cells are the three major classes of neurons which include the primary visual pathway in the retina. Cones (a subclass of photoreceptors) and ganglion cells develop before birth while rods (the other subclass of photoreceptors) and bipolar cells are generated during the first week after birth [5]. Moreover, the morphological development of the fovea may be complete postnatally in the human retina by at about 17–18 months due to rapid elongation of the central photoreceptors which make interaction with the retinal pigment epithelium (RPE)cell layer [6].

Aging is a multifactorial process that results in a decreased visual acuity and impaired dark adaptation. This functional decline might be due to structural changes in the optical components of the eye such as aqueous humor, lens, and vitreous body and the loss of retinal cells in aged persons [7],[8].

The number of ganglion cells in the fovea and peripheral retina decreases during aging. Also, the interneurons between photoreceptors and ganglion cells are subjected to age-related cell loss [9]. Rods of photoreceptor cells are more affected by aging than cones. Approximately 15% of all rods are lost between the second and fourth decades and one-third by the ninth decade. On the other hand, only 6% of cones are lost by the fourth decade and 23% by the ninth decade [10].

This study aimed to study the postnatal changes of the albino rat retina as well as the effect of aging on its structure histologically, immunohistochemically with quantitative measurement of its ganglion cell number.


  Materials and methods Top


This study was approved by the protocol of Research Ethics Committee of Tanta Faculty of Medicine. Twenty male and female albino rats at different ages were used. The animals were housed in clean properly ventilated cages with steel wire tops under similar environmental condition and fed the same laboratory diet. They were divided according to their ages into four groups (five rats in each group) at ages 1 month (young rats), 3 months (adult rats), 18 and 24 months (aging rats).

The rats of the previous groups were sacrificed with a suitable dose of ether and the eyeballs of the two sides were removed. Those of the right side were fixed in 10% buffered formalin and prepared for histological study (by hematoxylin and eosin stain) and immunohistochemical study (by tubulin β III stain) [11],[12]. The anterior segments of the left eyeballs were removed with release of the major nonretinal components of the eye. The neural retinae were fixed in 2.5% glutaraldehyde solution and prepared for transmission electron microscopic examination [13],[14].

Immunohistochemical staining of tubulin β III

Tissue sections were deparaffinized and put in PBS (pH 7.4) and then they were kept in cold methanol at −20°C for 4 min and washed in PBS for several times. Tissues were treated for 30 min with 3% (dilution from 30%) H2O2 and washed with PBS. Then, the primary antibody mouse anti-tubulin βIII diluted 1 : 500 in PBS was added and incubated for 1 h. The secondary antibody biotin anti-mouse which was diluted to 1 : 500 in PBS containing 1% bovine serum albumin was added for 30 min The slides were washed three times in PBS before adding the extra avidin–peroxidase and diluted to 1 : 500 in PBS containing 1% BSA for 30 min They were washed three times in PBS and treated with diaminobenzidene tetrahydrochloride for 30 s and then washed in distilled water, dried and then placed in xylene for 5 min and mounted with a mixture of di-styrene, a plasticizer and xylene to preserve stain. The immunopositive cells appeared in the retinal ganglion cell (RGC) bodies as brown coloration [12].

Quantitative analysis of retinal ganglion cells

RGCs were identified among tubulin β-III positive cells by their morphological characteristics. Ten consecutive microscopic fields (0.3×0.3 mm2 each) were recorded in each retina of different subgroups. Images were captured using a video camera coupled to a binocular microscope at 400 times magnification. The cells with overlapping nuclei were excluded from the analysis. Morphometric measurements were performed at the Central Laboratory in Faculty of Medicine in Tanta University, using Leica Qwin 500 (Wincom Company, China) Image Analyzer computer system [12],[15].

Statistical analysis

The RGCs number in all groups were expressed as the mean±SD. For multiple comparisons, the statistical difference among all groups was assessed by one-way analysis of variance followed by t-test to compare pairs of groups. The difference was considered significant when probability of differences P value is up to 0.05, highly significant if the P value is less than 0.001, and nonsignificant if the P value is more than 0.05 [16].


  Results Top


Light microscopic examination

Histological results (hematoxylin and eosin stain)

Sections in the eyeballs of rats aged 1 month showed well-differentiated retina. The layers of the retina were highly organized. They are arranged from outwards to inwards as follows: the pigment epithelial cell layer, photoreceptor processes, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, and nerve fiber layer. The pigment epithelium appeared as a single layer of flat cells separating the retina from the choroid ([Figure 1]a). The photoreceptor processes consisted of two segments: an outer lightly stained segment and an inner darkly stained segment. The outer nuclear layer showed densely packed cells with darkly stained rounded nuclei ([Figure 2]a1).The inner nuclear layer consisted of polygonal cells with lightly stained nuclei. They were separated from the outer nuclear layer by the outer plexiform layer. The ganglion cell layer was formed of one layer of cells with large vesicular nuclei and was separated from the inner nuclear layer by the inner plexiform layer. The optic nerve axons emerged from the ganglion cells ([Figure 2]a2).
Figure 1 H&E ×400. Sections of albino rat retina: (a) At 1 month, the pigment epithelium (PE) is arranged as a single layer of cells separating retina from choroid (Ch) and photoreceptor processes (PR) are clearly differentiated. The outer nuclear layer (ONL) and inner nuclear layer (INL) are well defined and separated from each other by outer plexiform layer (OPL). One layer of ganglion cells (GC) is separated from inner nuclear layer by inner plexiform layer (IPL). The optic nerve axons arise from ganglion cells forming a nerve fiber (NF) layer. (b) At 3 months, the same organization retina is noticed with increased density of cells. (c) At 18 months, the inner layers of the retina become greatly separated from the PE which exhibits flattened nuclei (N) in some areas and absent in others. The PR become degenerated and vacuolated (V). The OPL appears very thin or even absent in some areas (arrows). (d) At 24 months, flat nuclei of the PE cells present in some areas and absent in many areas (arrow heads). The PR show marked vacuolation (V) or become completely lost (*). The OPL is absent in many areas (arrows).

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Figure 2 H&E×1000. Sections of albino rat retina: (a1, a2) at 1 months, flat nuclei (N) of the pigment epithelial (PE) cells are noticed. The photoreceptor processes (PR) consist of two segments: an outer lightly stained segment (OS) and an inner darkly stained segment (IS). The outer nuclear layer (ONL) consists of densely packed cells with darkly stained rounded nuclei. It is separated from the inner nuclear layer (INL) by the outer plexiform layer (OPL). The INL consists of polygonal cells with lightly stained nuclei (N). It is separated from the ganglion cells by the inner plexiform layer (IPL). The ganglion cells (GC) appear with large vesicular nuclei (n). The axons (Ax) of the optic nerve arise from the ganglion cells. (b1, b2) At 3 months, a single layer of flat cells of the PE is noticed. The segments of the photoreceptors are demarcated into an outer lightly stained segment (OS) and an inner darkly stained segment (IS). The ONL consists mainly of darkly stained nuclei of photoreceptors. The GCs arrange in two layers of cells with large vesicular nuclei (N). They are separated from the INL by the inner IPL. The axons of the optic nerve (Ax) emerge from the ganglion cells. (c) At 18 months, decreased cell densities of the ONL and INL are observed which become continuous with each other. The inner IPL appears vacuolated. The nuclei of GCs appear pyknotic with vacuolation of the surrounding nerve fibers (NF). (d) At 24 months, degeneration of the cells in the ONL is seen as well as lost photoreceptor processes (*) leaving only an apparent outer limiting membrane (OLM). The INL shows scanty eccentric darkly stained nuclei with a vacuolated cytoplasm. The inner IPL appears spongiform with atrophic nerve plexuses. The GCs show pyknotic nuclei with marked vacuolation in the NF layer.

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At 3 months, the retina appeared with the same arrangement ([Figure 1]b). The outer and inner segments of photoreceptor processes were demarcated. The pigment epithelial layer was arranged as a single layer of flat cells. The outer nuclear layer consisted mainly of darkly stained nuclei of photoreceptors ([Figure 2]b1).Two layers of ganglion cells with large vesicular nuclei were observed ([Figure 2]b2).

At 18 months, degenerative changes were noticed in all layers of the retina. The flat cells of the pigment epithelium were observed in some areas and were absent in others. The photoreceptor processes were greatly degenerated and vacuolated ([Figure 1]c). The outer and inner nuclear layers showed decreased cell densities with absence of the outer plexiform layer between them in many areas. The inner plexiform layer exhibited atrophic nerve plexuses. Ganglion cells showed shrunken, pyknotic nuclei with decreased cell density and greatly vacuolated nerve fibers ([Figure 2]c).

The layers of the retina of rats aged 24 months showed marked degeneration. The nuclei of the pigment epithelial cells were flat or absent in some areas and were widely separated from the outer segment of photoreceptor processes ([Figure 1]d). The nuclei of the outer nuclear layer were lost while those of the inner nuclear layer were eccentric darkly stained with a vacuolated cytoplasm. The outer plexiform layer was hardly seen in the section while the inner plexiform layer appeared spongiform in shape with atrophic nerve plexuses. The ganglion cells were degenerated with pyknotic nuclei and marked vacuolation in the nerve fiber layer ([Figure 2]d).

Immunohistochemical results (tubulin β III stain)

Positive immunoreactivity of the ganglion cells to tubulin β III stain was noticed in 1-month aged rats ([Figure 3]a). Their immunoreactivity to the stain became strongly positive in 3 months aged rats ([Figure 3]b). At 18 months, the ganglion cells exhibited mild immunoreactivity to the stain ([Figure 3]c) while at 24 months, a weak immunoreactivity of these cells to the stain was observed ([Figure 3]d).
Figure 3 Tubulin β III immunostaining ×400. Sections of albino rat retina: (a) positive immunoreactivity is noticed in the cells of the ganglion cell (GC) layer of 1-month aged albino rat. (b) At 3 months, the GCs and the inner plexiform layer (IPL) show strong positive immunoreactivity to the stain. (c) At 18 months, a mild immunoreactivity to the stain in the GC layer and inner IPL is seen. (d) At 24 months, a weak immunoreactivity appears in the cells of the GC layer and the IPL.

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Transmission electron microscopic examination

From 1 to 3 months of albino rat age the flat pigment epithelial cells showed abundant euchromatin and numerous mitochondria. The outer segment of photoreceptor processes appeared closely packed and is formed of regularly arranged stacked membranes ([Figure 4]a at 1 month and 4b at 3 months). The outer nuclear layer is formed of two types of cells; few large oval cones with irregular shaped clumps of heterochromatin in their nuclei and several layers of small irregular darkly stained rods with condensed central clump of heterochromatin in their nuclei ([Figure 5]a at 1 month and 5b at 3 months). The inner nuclear layer showed nuclei of bipolar, amacrine, horizontal cells and glial Muller cells. The bipolar cells appeared polygonal in shape close to the outer plexiform layer and their nuclei revealed a heterochromatin pattern. The amacrine cells had rounded electron-lucent nuclei. The horizontal cells were large with euchromatic rounded nuclei and prominent nucleoli. They were situated toward the outer plexiform layer which are composed of many unmyelinated nerve axons. Muller cells separated bipolar from amacrine cells. They exhibited electron-dense irregular nuclei at 1 month and showed long processes extending toward the outer plexiform layer at 3 months ([Figure 6]a at 1 month and 6b at 3 months). The ganglion cells showed nuclei of different shapes and sizes and exhibited finely dispersed chromatin with prominent nucleoli at 1 month and became arranged in several layers at 3 months ([Figure 7]a at 1 month and 7b at 3 months).
Figure 4 TEM ×1500. Sections of albino rat retina: (a) At 1 month, the nucleus (N) of pigment epithelial cell (PE) exhibits abundant euchromatin. Its cytoplasm has numerous mitochondria (m) and its apex show microvilli (*) that interdigitate with the outer segment of photoreceptor processes (PR). (b) At 3 months, flat euchromatic nucleus (N) of the PE exhibits heterochromatin at the nuclear membrane with numerous mitochondria (m) in the cytoplasm. (c) At 18 months, the PE possesses an euchromatic nucleus (N) and large vacuolation (V) of its cytoplasm. Prominent separation (*) from the photoreceptor processes is noticed. The microvilli are disorganized (arrows) and lysosomes (Ly) are abundant in the cytoplasm. (d) At 24 months, shrunken nucleus (N) of PE cells with marked cytoplasmic vacuolation (V) presents with many lysosomes (Ly). Its microvilli are markedly irregular (arrows) and separated (*) from the degenerated membranes of the outer segment of the PR.

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Figure 5 TEM ×1000. Sections of albino rat retina: (a) At 1 month, the outer nuclear layer is formed of two types of cells. The nuclei of the cones (N) show variable numbers of irregular shaped clumps of heterochromatin. The nuclei of rods (n) exhibit a highly condensed central clump of heterochromatin. Both types have a thin layer of cytoplasm (arrows). (b) At 3 months, the ONL possesses rods and cones nuclei surrounded by a thin cytoplasmic sheath. The rods nuclei (R) are polygonal in shape with a central condensed heterochromatin. The cones nuclei (C) are oval or triangular in shape and exhibit clumps of heterochromatin. (c) At 18 months, the ONL appears with degenerative changes. Some nuclei of the cones (C) are pyknotic and shrunken with condensed chromatin and others appear with an irregular nuclear membrane (arrow) and vacuolation of the cytoplasm (*). The nuclei of rods (R) exhibit an irregular outline with heterochromatin condensation. (d) At 24 months, the retina shows three layers: an ONL, outer plexiform layer (OPL), and an inner nuclear layer (INL). The cells of the outer nuclear layer exhibit shrunken pyknotic nuclei (N) with marked cytoplasmic vacuolation (*). The OPL is thin with atrophic axons (arrow heads) and lymphocytic migration (arrow).

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Figure 6 TEM ×800. Sections of albino rat retina: (a) at 1 month and (b) at 3 months the inner nuclear layer (INL) is formed of many types of cells separated from the outer nuclear layer (ONL) by the outer plexiform layer. Bipolar cells (BP) are polygonal in shape close to the outer plexiform layer (OPL) and their nuclei show heterochromatin. Amacrine cells (Am) have rounded electron-lucent nuclei. Horizontal cells (H) are large and rounded with fine dispersed chromatin in their nuclei. Muller cells (M) exhibit electron-dense dark irregular nuclei and long process (arrow) extending between the cells of the inner nuclear layer toward the outer plexiform layer. They separate bipolar cells from amacrine cells. The inner nuclear layer is separated from the ONL by the outer plexiform layer. (c) At 18 months, the retina shows degenerative changes in the cells of the INL. BPs appear with shrunken nuclei and large vacuolation of the cytoplasm (*). Amacrine (Am) and horizontal (H) cells exhibit an irregular nuclear membrane and vacuolation of the cytoplasm (arrows). (d) At 24 months, the INL and inner plexiform layer (IPL) are noticed. BPs appear with shrunken nuclei and a marked cytoplasmic vacuolation (*). Muller cell (M) is noticed with very small irregular pyknotic nucleus and vacuolated cytoplasm (arrow head).

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Figure 7 TEM ×600. Sections of albino rat retina: (a) At 1 month, the ganglion cell layer (GC) is separated from the inner nuclear layer (INL) by the inner plexiform layer (IPL). Their nuclei (N) are of different shapes and sizes exhibiting a fine dispersed chromatin with prominent nucleoli (nu). (b) At 3 months, the GCs are of variable shapes and sizes and are arranged in several layers. (c) At 18 months, the nucleus of the GC show an indented nuclear membrane (arrow) and vacuolation of the cytoplasm (*). Blood vessel (BV) can be seen as well as apparent vacuolation (V) of nerve axons in the nerve fiber layer (NFL) near the inner surface of the retina (arrow). (d) At 24 months, the ganglion cell layer appears with neovascularization (BV). The nucleus of the GC is shrunken, darkly stained with condensed euchromatin and irregular nuclear membrane (arrow head). Marked vacuolation (V) of the surrounding nerve axons in the NFL near the inner surface of the retina (arrow) can be observed.

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From 18 to 24 months, the albino rat retina showed degenerative changes which became marked with progressive damage of all layers at 24 months. The pigment epithelial cells showed large vacuolation and abundant lysosomes in the cytoplasm and disorganized microvilli with areas of separation from the degenerated membranes of the outer segment of the photoreceptor processes ([Figure 4]c at 18 months and 4d at 24 months). Degenerative changes were noticed in the outer nuclear layer as some nuclei of cones were pyknotic and shrunken with condensed chromatin and others showed irregular nuclear membrane while rods nuclei exhibited irregular outline with heterochromatin condensation. Vacuolation of their cytoblasm was observed at 18 months and became marked at 24 months. The outer plexiform layer appeared thin with atrophic axons and lymphocytic migration ([Figure 5]c at 18 months and 5d at 24 months). In the inner nuclear layer, bipolar cells showed shrunken nuclei with large vacuolation of the cytoplasm and degeneration of the membranes between many cells. The amacrine, horizontal, and Muller cells exhibited pyknotic nuclei with an irregular nuclear membrane and vacuolation of the cytoplasm. The inner plexiform layer was formed of many unmyelinated nerve axons ([Figure 6]c at 18 months and 6d at 24 months). Degenerated ganglion cells exhibited condensed euchromatin, irregular indented nuclear membranes, and large vacuolations in the cytoplasm with degenerated mitochondria. Vacuolation of nerve axons were obvious in the nerve fiber layer were observed at 18 months and became marked at 24 months ([Figure 7]c at 18 months and 7d at 24 months).

Statistical analysis of the average number of ganglion cells

The mean number of RGCs in group I (rats aged 1 month) was 8.1±1.6. There was a highly significant increase in the mean number of ganglion cells in group II (rats aged 3 months) (13±3.4) (P<0.001). This increase became highly significantly decreased again in the retinae of old albino rats. In group III (rats aged 18 months), the mean number of ganglion cells exhibited a highly significant decrease as compared with group II (number is 5±1.07) and in group IV (rats aged 24 months) the number was 2.2±0.9 ([Table 1] and Histogram 1).
Table 1 Quantitative measurements of the number of ganglion cells of the retinae and their statistical comparison in different groups

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  Discussion Top


The vertebrate retina has been widely used as a model to study the development of the central nervous system. Its accessibility and relatively simple organization allow better analysis of basic mechanisms of development such as cell proliferation, differentiation, and death [17]. Anatomical changes occur in the retina with aging include loss and attenuation of cells such as the RPE cells, ganglion cells, and photoreceptors [18]. Therefore, this study aimed to clarify the postnatal changes in the albino rat retina as well as the effect of aging on its structures.

From the age of 1 month till the age of 3 months, this study showed that the layers of the retina were highly organized including the pigment epithelial layer, photoreceptor processes, outer and inner nuclear layers with appearance of outer and inner plexiform layers as well as the ganglion cell and the nerve fiber layers. These findings were confirmed by the works of Sharma et al. [19] and Coombs et al. [20] who observed maturation of the synapses in the outer and inner plexiform layers in the first 2–3 weeks postnatally in rodents. The previous authors reported that peak synaptogenesis occurred in the outer plexiform layer within the first week after birth, while it was maximal in the inner plexiform layer in the second and third postnatal weeks. Moreover, Nadal-Nicolás et al. [21] stated that the rate of retinal growth was very similar between strains during the first 3 months of age as the retina reached 90% of its size, then grew steadily and slowly henceforward.

El-Sayyad et al. [22] reported that the young rats have a normal pattern arrangement of mitochondria, rough and smooth endoplasmic reticulum within the cytoplasm of RPE. They explained the numerical increase of mitochondria with development as the indicator of the high-energy requirements of these cells to fulfill their important functions. Moreover, their demand for excess oxygen is essential to perform their functions such as phagocytosis of the outer segment of the photoreceptors, storage of retinoid, absorption of scattered light, and fluid transport [23].

This study showed the two cells of the outer nuclear layer: rods and cones. The inner nuclear layer contained nuclei of many types of cells including bipolar, amacrine, and horizontal cells with the main glial Muller cells. These cells exhibited electron-dense nuclei with long processes extending between cells of the inner nuclear layer toward the outer plexiform layer. The ganglion cells were detected within the nerve fiber layer near the inner surface of the retina exhibiting many mitochondria in their cytoplasm. Horizontal cell functions are to integrate and regulate the input from multiple photoreceptor cells as they provide inhibitory feedback to rod and cone photoreceptors, whereas bipolar cells lie between the photoreceptors and ganglion cells and act directly or indirectly to transmit signals from the photoreceptors to the ganglion cells [24].

Wang et al. [25] stated that Muller cells are derived from multipotent precursors and are the last cell type to be generated in the developing retina. They confirmed the importance of these cells as they play an important trophic role in the regulation of synaptic activity in the inner retina, reuptake of neurotransmitters, and maintenance of a correct lamination of cell layers.

From the age of 18 to 24 months, there was degeneration of the layers of the retina. The pigment epithelial cells showed disorganized microvilli with abundant lysosomes and vacuolation of the cytoplasm. The outer segment of photoreceptor processes were disorganized with areas of separation, vacuolation, and degeneration of stacked membranes. Sparrow et al. [23] confirmed these findings and reported that the RPE cells are lost with age and the decline being ∼2.3% of total RPE per decade of life. Interestingly, the decrease in their cell density occurs in the peripheral retina but not observed in the fovea, probably because of inward migration of peripheral RPE so as to compensate for the loss of foveal RPE. The widespread damage of photoreceptor processes occur with the damage of the pigment epithelium clarifying the impairment of vision during old age [26].

The outer nuclear layer showed decreased cell density and thickness or completely lost with absence of the outer plexiform layer at 24 months. Their nuclei were pyknotic and shrunken with vacuolation of the cytoplasm. The cells of the inner nuclear layer showed shrunken pyknotic nuclei with marked cytoplasmic vacuolation. The inner plexiform layer appeared spongiform in shape. These results coincided with those of El-Sayyad et al. [22] who explained these degenerative changes in the outer and inner nuclear layers by the detection of striking aging phenomena in the nuclear layers with different patterns of nuclear cell death. The nuclear damage represents one of the steps of retinal degeneration that led to impaired vision [27]. Nadal-Nicolás et al. [21] explained reduction of the nuclear layers by the loss of rod photoreceptors in the outer nuclear layer and loss of bipolar, amacrine, or Muller cells in the inner nuclear layer.

In this study with the advancement of age from 18 to 24 months, the albino rat retina showed massive degenerative changes of the ganglion cell layer as it exhibited decreased cell density. The ganglion cells showed shrunken pyknotic nuclei with condensed euchromatin. There were neovascularization and marked vacuolation of the surrounding nerve fibers. These results agree with those of Lupi et al. [28] in wild-type mice. The previous authors suggested that this age-related loss in the nerve fiber layer might result from age-dependent reduction and degeneration of the ganglion cells [29]. Moreover, Klein et al. [27] explained the presence of neovascularization in the nerve fiber layer in this study by abnormal retinal angiogenesis in the retina with aging and reported that it is associated with the most common causes of vision loss.

Immunohistochemically using tubulin β III stain, the retina of albino rats in the postnatal and aging groups in this study showed a positive reaction in groups with different immunoreactivities. These results coincided with those of Chidlow et al. [30] and Jiang et al. [12] who reported that identification of a wide range of retinal cellular targets as well as retinal neuronal loss assessed easily by labeling for tubulin β III that demarcated the RGCs.

Quantitative measurements in this study, in rats aged 3 months, showed a highly significant increase in the mean number of ganglion cells in their retinae as compared with rats aged 1 month. This increase was significantly decreased again in the retinae of old albino rats aged 18 and 24 months. These results agree with those of Neufeld and Gachie [31] and Harwerth et al. [29]. Several authors reported RGC degeneration with aging [32],[33],[34], but some reported that the RGC population is not changed with age in rodents or other species [35],[36].


  Conclusion Top


This study has attempted to provide histological description of the albino rat retina at different postnatal ages. Age-related loss of RGCs in albino rats was associated with diminution of vision with aging.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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