Ординатура / Офтальмология / Английские материалы / Age-Related Changes of the Human Eye_Cavallotti, Cerulli_2008
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J. Feher and Z. Olah |
are responsible for the development of malpositions of the lower eyelid known as senile entropion and ectropion.1 In the former case, the margin of the lower eyelid turns inward, eyelashes irritate the corneal surface continuously, and keratoconjunctivitis develops with potential corneal ulceration. In the latter case, the margin of the lower eyelid turns outward, tarsal conjunctiva and lower lachrymal punt are exposed, and chronic lachrymation (epiphora) and conjunctival inflammation develop. There are two main theories to explain the pathomechanism of senile entropion and ectropion—the spastic and the atonic theory. These malpositions of the lower eyelid are therefore often called spastic entropion or ectropion as well as atonic entropion or ectropion. Both theories ascribe essential importance to alterations of the orbicular muscle and neighboring connective tissues.1 Although several morphological and functional observations have been carried out to reveal the pathologic background of abnormal muscle activity, we are far from knowing the exact mechanisms that cause the senile involution of the orbicular muscle, and far from explaining the variable clinical picture. In both conditions, plastic surgery is the choice of treatment. Until now, however, more then 120 original and modified surgical procedures have been introduced, suggesting the poor effectiveness of any of them. In fact, recidivates are quite frequent after each procedure.
Early electron microscopy of eyelid aging and its relation to senile entropion and ectropion revealed significant ultrastructural abnormalities in the orbicular muscle fibers,2 but no differences related either to entropion or ectropion.3 Some abnormalities of the mitochondria4 and sarcoplasma5,6 were also described. Here we present a completed ultrastructural morphology of orbicular muscle aging and design putative correlations between these abnormalities.
Aging of the orbicular muscle may be generally related to a part of muscle aging known as sarcopenia. The second aim of this chapter is to reveal the earliest ultratstructural alterations related to this poorly explored and uncurable pathology. Sarcopenia is a slowly progressive and complex process that appears in aged muscle that is associated with a decrease in mass, strength, and velocity of contraction. This process is the result of many molecular, cellular, and functional alterations. With the advancement of age, type I muscle fibers decrease in number and increase in size.7 Type I fiber predominance seen in older subjects could be related to a selective decrease of type II fibers as the body ages. It also suggests a possible conversion of type II fibers to type I fibers.8 In the elderly, central nuclei, ring fibers, fiber splitting, scattered highly atrophic fibers, moth-eaten fibers, and vacuoles were also observed. Ring fibers were most easily identified with antidesmin labeling, and highly atrophic fibers exhibited a rough network of labeling. An increased content of actin and spectrin was also observed at the periphery of ring fibers. A qualitative ultrastructural analysis also showed obvious changes, including some myofilament loss, collections of lipofuscin that were also observed in satellite cells, proliferation of the sarcoplasmic reticulum, and increased wrinkling of nuclear membranes and sarcolemma.9 Interestingly, satellite cell populations were not significantly lower in healthy, sedentary older adults compared to young adult men and women.10 Over time, mitochondrial size and mitochondrial percentage per fiber area decrease, and the cristae of mitochondria became irregularly spaced, disrupted,
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and replaced by lamellar, myelin-like structures. Giant mitochondria are often visible. They contain lipid droplets and lipofuscin in the myofibrils, which are often in close relationship with the damaged mitochondria.11
It has been suggested that sarcopenia may be triggered by reactive oxygen species (ROS) that have accumulated throughout a person’s lifetime. In fact, a significant increase in the oxidation of DNA and lipids was found in elderly muscle— more evident in males—along with a reduction in catalase and glutathione transferase activities. Experiments on Ca2+ transport showed an abnormal functional response of aged muscle after exposure to caffeine, which increases the opening of Ca2+ channels, as well as reduced activity of the Ca2+ pump in elderly males. These results proved that oxidative stress plays an important role in muscle aging, and that oxidative damage is much more evident in elderly males, suggesting a gender difference that may be related to hormonal factors.
The progression of sarcopenia is directly related to a significant reduction of the regenerative potential of muscle normally due to a type of adult stem cells known as satellite cells. These cells lie outside the sarcolemma and remain quiescent until external stimuli trigger their re-entry into the cell cycle as growth factors. One possibility is that the anti-oxidative capacity of satellite cells could also be altered and this, in turn, can determine the decrease of their regenerative capacity. Data concerning this hypothesis are discussed.12
Changes in Anatomy and Kinematics
Topographic anatomy of the eyelids is affected by aging and sex. Normal aging processes may cause laxity of eyelid tissues (skin, muscle, connective tissue) and atrophy of the orbital fat. These changes are responsible for the well-known aesthetic changes, but they may also contribute to the aetiology of several eyelid disorders, such as ectropion, entropion, dermatochalasis, and blepharoptosis. Such aging changes may also affect the position of the eyelids, eyeball, and eyebrow.
Aging primarily affects the size of the horizontal eyelid fissure. In adolescence, between the ages of 12 and 25 years, the horizontal eyelid fissure lengthens 3 mm, while the position of other eyelid structures remains virtually unchanged. Between the average ages of 35 and 85 years, the horizontal eyelid fissure gradually shortens again by about 2.5 mm. While the lengthening of the horizontal eyelid fissure between the ages of 12 and 25 years probably reflects growth of the facial structures, the shortening from the age of 35 years onwards is likely due to progressive laxity of the medial and lateral canthal structures. With aging, the distance between the lateral canthal angle and the anterior corneal surface decreases almost 1.5 mm. which means that the shortening of the horizontal eyelid fissure can at least partially be attributed to medial displacement of the lateral canthus. The positions of the lateral canthus and the center of the pupil are identical in men and women, and remain fairly stable throughout life. Aging causes the sagging of the lower eyelid,
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especially in men, and a higher skin fold and eyebrow position in both sexes. Laxity of the lower eyelid causes an increase of the distance between the pupil center and the lower eyelid of about 1 mm in men, and 0.5 mm in women. In a prospective consecutive observational case series, 32 normal adult subjects—comprised of 12 younger (aged 29+/-5 years) and 20 older subjects (aged 74+/-6 years)—underwent lower eyelid tensometry. Younger males had higher eyelid tension than females, and there was no significant reduction in tension with age.13 Aging also causes raised eyebrows and increased skin creases in men and women, but in the study it did not affect the position of the pupil center and the lateral canthus. Men showed a 0.7 mm larger horizontal eyelid fissure than women. However, the eyebrows in women were situated about 2.5 mm higher than in men. Aging does not affect the position of the eyeball proper, or of the lateral canthus.14
Cutis laxa is an uncommon entity characterized by laxity of the skin, which hangs in loose folds, producing the appearance of premature aging. Histological analysis and ultrastructural examination of skin biopsy revealed reduction and fragmentation of elastic fibers.
Dermatochalasis is a severe degree of the aging process in the eyelid and orbital soft tissue complex. It can lead to extreme weakness or even dehiscence of the supporting fascia and other surrounding soft tissue—rarely leading to free mobility of the orbital fat pads and hence postural herniation into the eyelids, as seen in this unusual case.
Recent studies on the changes in the kinematics of blinking over time demonstrated that disorders of blink systems typically seen in persons 50 years of age or older occur against a backdrop of normal age-dependent changes in eyelid kinematics. Alterations in main sequence slope imply that the operation of central adaptive systems during aging. Reduction in main sequence slope is interpreted as a reduction in aggregate orbicular muscle motoneuron activity. Such a central neurologic adjustment in the motor output of blink systems may serve to compensate for an age-related increase in blink reflex excitability. Compensatory reduction in the main sequence relationship may offset a potentially hyperexcitable blink reflex, thereby reducing the likelihood of disorders such as blepharospasm.
These authors described passive and active changes in the kinematics of blinking with age. Passive changes in blink amplitude-peak velocity reflect age-related changes in static eyelid position that can be attributed largely to either weakness of the Muller’s muscle and superior levator muscle, or to the laxity of the connective tissue in the superior transverse ligament, palpebral ligaments, or dehiscence of the levator aponeurosis. Active changes in main sequence relationships demonstrate that blink plasticity interacts with aging processes. The active changes observed in the neural control of blink kinematics do not, by themselves, represent a trend toward the development of blink disorders in an aging population. If the aging of blink systems actively contributed to an age-related trend toward diseases of elevated blink excitability, an increased main sequence slope would have been expected. By contrast, the opposite result suggests that the active, central changes in eyelid kinematics may represent an adaptive response to the established, age-associated increase in blink reflex excitability.15
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Epidermal innervation according to age and anatomical site was evaluated in 82 biopsy samples from surgical procedures. Eyelid epidermis showed the highest ratio of nerve fiber surface to epidermal surface. A trend exhibiting age-associated decreased epidermal innervation of facial skin was found. Epidermal innervation of abdominal skin did not change with age, and an age-associated increased innervation was observed in mammary and palpebral skin.16
In another study, trigeminal blinks in normal human subjects between 20 and 80 years of age were characterized. In normal humans over 60 years of age, lid-closing duration, and the excitability and latency of the trigeminal reflex blink, increase significantly relative to younger subjects. Reflex blink amplitude, however, does not change consistently with age. For subjects less than 70 years of age, a unilateral trigeminal stimulus evokes a 37 percent larger blink in the eyelid ipsilateral to the stimulus than in the contralateral eyelid. Subjects who are 70 years old, however, exhibit blinks of equal amplitude. In all cases, blink duration is identical for both eyelids.17
Myofiber Abnormalities
We have performed electron microscopic and histochemical studies on surgical specimens of 86 patients, aged 42-88 years, and affected by various pathologies (tumors, enctropin, ectropin, trauma). Orbicular muscle contains mainly type I fibers. Age-related myofiber abnormalities comprise the decrease of the filamentary structure and disorganization of the normal banding structure, particularly the Z line. The decrease of filaments were the most common aging changes in the orbicular muscle. In some cases, simple quantitative changes were observed without any qualitative alterations (see Fig. 2.1).
In most cases, however, decrease of myofilaments were accompanied with disorganization of both fibrillary and banding structures. In some sites, there were only small focal changes, whereas at other places, the damage extended over many sarcomeres and even many fibers. From a morphological point of view, Z-line alterations serve as a reference and four subtypes of myofiber changes related to aging can be distinguished.
Nemaline bodies (rods) are by far the most common and most widely studied alteration of the skeletal muscle. This alteration is characterized by rod-form accumulation of Z-line material. They consist of a lattice-like arrangement of squares measuring about 1 nm on each side (see Fig. 2.2).
Nemaline bodies are clearly detectable at the light-microscopy level, and ultrastructurally, they originate from the Z-disks of the sarcomeres. These paracrystalline structures stained positively for alpha-actinin.18 In fact, one of the main components of rods is alpha-actinin—an actin-binding protein that localizes to the Z-disk.19,20 More than 70 mutations in the skeletal muscle alpha-actin (ACTA1) gene have now been identified. By and large, mutations are associated with three muscle diseases: a) nemaline myopathy, b) congenital actin myopathy, and c) intranuclear rod
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Fig. 2.1 Electron microscopy of aged orbicular muscle fibers. An apparently normal fiber in the center is surrounded with myofibers of highly variable diameter, but well preserved banding structure can be seen in each fiber (x32 K 68y)
Fig. 2.2 Nemaline bodies (rods). They show a typical paracrystalline structure. Several thin filaments are in connection with the rods. Next to these alterations, the myofibrillary structure is more or less irregular and the normal banding pattern is disrupted (87y years x 22 K)
myopathy. The majority of ACTA1 mutations are dominant, a small number are recessive, and most isolated cases with no previous family history have de novo dominant mutations.21 Nemaline myopathy is a rare autosomal dominant skeletal muscle myopathy characterized by severe muscle weakness and the subsequent
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appearance of nemaline rods within the muscle fibers. The intrafamilial variability suggests that the ACTA1 genotype is not the sole determinant of phenotype.22
Recently, a missense mutation in TPM3, which encodes the slow skeletal alphatropomyosin, was linked to nameline myopathy in a large kindred group with an autosomal-dominant, childhood-onset form of the disease.23 The primary defect, caused by expression of the mutant alpha-tropomyosin, was a decrease in the sensitivity of contraction to activating Ca2+, which could help explain the muscular hypotonia seen in this disease. Interestingly, this mutation did not directly result in nemaline rod formation, which suggests that rod formation is secondary to contractile dysfunction and that load-dependent processes are likely involved in nemaline rod formation in vivo.24 Although a number of genes have been identified in which mutations can cause nameline myopathy, the pathogenetic mechanisms leading to the phenotypes are poorly understood. All these together suggest a common process or mechanism operating in nemaline muscles independent of the variable degrees of pathology. More recently, electron micrographs showed elevated focal repair in nemaline muscles, suggesting that in nameline myopathy, a novel repair feature may operate.25 Sporadic late onset nemaline myopathy was also found to be associated with monoclonal gammapathy.26
Z-line streaming is a focal widening of the Z line, or sometimes widening of the whole Z line in one or more sarcomeres. These alterations were apparently common in both normal and abnormal orbicular muscle (see Fig 2.3).
In excessive Z-line streaming, a longitudinal fibrillary structure was displayed, which continued into the I band next to the abnormal Z line (see Fig. 2.4). The sarcomeres around Z-line streaming showed either a normal or an abnormal banding pattern.
Although both Z-line streaming and rods contain electrodense Z-line material, and that material enters into both fine filaments, the paracrystalline structure of the
Fig. 2.3 Z-line streaming in an apparently normal muscle. The normal banding pattern, sarcoplasmic reticulum, and mitochondria can be seen (x40 K 73y)
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Fig. 2.4 Excessive Z-line streaming: The longitudinal pattern is highly disturbed, and most Z lines are widened. Between the irregular mitochondria fibers, glycogen particles and tubules of sarcoplasmic reticulum can be see (x40 K 69y)
rods is clearly different from the filamentary structure of Z-line streaming. Z-line streaming may occur in normal aged muscle, but it is more common in muscle disease such as muscle dystrophies, denervation atrophy, collagen vascular disease, hypothyroid myopathy, and central core disease. In rats on a low-protein diet, Z-line streaming and disintegration of sarcomeric striation was associated with depletion of glutathion. The depletion of glutathione by protein malnutrition is responsible for inducing myofibrillary damage through the excess leaking of Ca2+ into the cytosol.27,28 Electron microscopy of biopsy specimens from the gastrocnemius muscles of volunteer human marathon runners showed evidence of inflammation and muscle fiber alterations, including Z-line streaming.29
Cytoplasmic bodies are curious structures that vary widely in size and shape, but have a characteristic appearance. It consists of a round or oval, amorphous electrodense central area surrounded by a halo of less electrodense amorphous material (see Fig. 2.5).
Fine filaments from the adjacent muscle fibers pass through this halo and enter into the electrodense central area. Between the fine filaments, a few T tubules, glycogen particles, and a rich network of sarcoplasmic reticulum appear. The origin, the chemical composition, and significance of cytoplasmic bodies are unknown, but in our material they appear to contain Z-line material as well as myofilaments. Cytoplasmic bodies were found in muscle dystrophies, periodic paralysis, and collagen vascular diseases.
Double Z lines or duplication of the Z line rarely occurred and it appeared in several neighboring fibers (see Fig. 2.6). Although similar alterations were reported in regenerating muscle and in hypothyreosis, neither the cause nor significance of double Z lines are known.
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Fig. 2.5 Cytoplasmic body formation in the sarcoplasma. The electrodense oval area is surrounded by a halo of fine filaments—tubules of sarcoplasmic reticulum and T tubules (x22K 82yrs)
Fig. 2.6 Duplication of the Z line in some neighboring fibers. On both sides of each pair of Z lines, fine fibrils can be seen. No other ultrastructural alterations are observed in association with double Z line (x26 K 63 yr)
Mitochondrial Alterations
In normal conditions, mitochondria are located between myofibers and in the subsarcolemmal area. Their number is higher in the type I fibers. Numerous morphometric studies on skeletal muscle describe an age-related decline in the number of mitochondria
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and the loss of cristae. We found similar changes in our material, but it was not a generalized phenomenon. In some specimens, we found a significant increase in the number of mitochondria. In both these cases, however, the surrounding myofibers and mitochondria showed apparently normal structure (see Fig. 2.7).
Proliferation of inner mitochondrial membrane/cristae, usually accompanied with the enlargement of mitochondria, was the most characteristic change in aged orbicular muscle. These alterations were found exclusively in the subsarcolemmal area and almost all mitochondria showed more or less abnormalities—i.e., the proliferation of mitochondrial cristae may be accompanied with loss of cristae, even in the same mitochondria, and a decrease of matrix density. (see Fig. 2.8).
Mitochondrial inclusions of crystalline structure are very common ageand diseaserelated alterations of mitochondria. In our material, they were mostly observed in the subsarcolemmal area, but rarely in the interfibrillary mitochondria (see Fig. 2.9).
Two distinct types of crystals can be distinguished on the basis of shape, size, pattern, unit cell dimension, specific location of the crystals in the mitochondrial intermembrane space, and occurrence in different muscle fiber types. Type I crystals (see Fig. 2.10) are usually present in the intracrystal space, and they occur in type I muscle fibers, whereas the type II crystals (see Fig. 2.11) are preferentially located in the intermembrane space between outer and inner mitochondrial membranes, and occur in type II muscle fibers.
In humans, type I crystals are on average 200nm wide and 2mm long, while the type II crystals are more cubic in their dimensions—usually 100-300nm in all three dimensions.30 Immunoelectron microscopy revealed that these inclusions react heavily
Fig. 2.7 Accumulation of mitochondria between myofibers. Both mitochondria and myofibers show well-preserved structure, although there is a light difference between the upper-left part of the picture, where normal number and normal myofiber structure can be seen, and the lower-right part, where an accumulation of mitochondria is associated with compromised myofibers (x22K 66yrs)
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Fig. 2.8 Numerous large mitochondria in the subsarcolemmal area. Some of them are extremely large (mega or giant mitochondria) containing enormous cristae arranged either regularly or randomly. Some of the cristae are highly electrodense, some of them have lost membrane structure, and some contain electrodense granules—presumably unsaturated lipids. The diameter of myofibers apparently decreases, but their banding structure is well-preserved and shows a state of contraction. Interfibrillary mitochondria are slightly enlarged but show normal structure (x25K 70yrs)
Fig. 2.9 Crystalline mitochondrial inclusion. Almost all mitochondria in the subsarcolemmal area show electrodense inclusions—in some of them the crystalline structure is clearly visible even in this magnification, but some of them contain electrodense amorphous or granular inclusions. In the neighboring myofibers, only Z line can be distinguished (x19 K 79y)