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Biomedical subjects

B Willekens

Publications and source records attributed to B Willekens.

10 recordsLinked to original sources
PubMed1992

Membrane architecture as a function of lens fibre maturation: a freeze fracture and scanning electron microscopic study in the human lens.

The ultrastructure of fibre membranes in human lenses, varying in age from premature to 40 years, was investigated using a strict protocol regarding their localization within the lens. The ultrastructural approaches used were scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of ultrathin sections and freeze-fracture replicas. Irrespective of the age of the lens, superficial fibre membranes are characterized by a high density of intramembrane particles (IMPs) and numerous gap junctions (GJs). In contrast deep cortical fibres, at the SEM-level characterized by grooves and ridges, are largely free of IMPs but still contain numerous GJs. In between these regions a transitional zone was observed. At the SEM-level the transitional fibres are characterized by wrinkled membranes and formation of grooves and ridges. In freeze-fracture replicas the presence of numerous square arrays (SAs) associated with GJs is most remarkable. It is concluded that at all ages studied, the maturation and compaction of lens fibres results in a transformation of membrane architecture leading to clear-cut ultrastructural differences between superficial and deep cortical membranes. It is argued that this ultrastructural heterogeneity parallels the gradients observed biochemically for intrinsic membrane proteins and cholesterol:phospholipid ratios. The observations confirm the electrophysiological view that superficial membranes have an 'average' permeability and that deep cortical membranes are 'degenerate' or 'non-leaky'.

Adolescent↗
PubMed1990

Square arrays in early cortical lens opacities.

A combined freeze-fracture and scanning electron microscopic study of early opaque spots in the aging human lens showed the absence of gap junctions and the presence of square arrays in the membranes of disturbed fibers and neighboring unaffected fibers. Square arrays, with membrane particles of 6-7 nm, are considered as rearranged gap junctions and/or intramembranous particles, with particle sizes between 8.5-9.5 nm; they are a sign of electric and metabolic uncoupling. These ultrastructural observations lend support to the idea of an uncoupling mechanism in the aging human lens, conserving the transparency of unaffected parts of the lens, as postulated previously.

Aging↗
PubMed1990

Morphology of the aging human lens. II. Ultrastructure of clear lenses.

Clear human post mortem lenses, varying in age between 23 and 82 years, were investigated using scanning and transmission electron microscopy. The lenses were obtained from the Corneabank Amsterdam. The post mortem delay ranged from 8-24 hours. Apart from some minor mitochondrial changes no hypoxic post mortem disturbances in epithelial cells and lens fibers were observed. In the lens fibers three main age-related fine structural alterations were found: i) membrane ruptures, ii) watervacuoles and iii) multilamellar bodies. The frequency of these alterations increased with age and they remained restricted to the superficial equatorial cortex. They were absent in the anterior and posterior cortex, supranuclear equatorial cortex and nucleus. The relevance of these observations in relation to the etiology and early pathogenesis of senile cataract is discussed. The membrane ruptures and watervacuoles are in morphological support of the view, based on biochemical evidence, that oxidative stress leads to destabilization and desintegration of membranes and consequently disturbs the waterbalance of fibers. It is postulated that the lamellar bodies are involved in the repair of ruptured membranes and breakdown of affected proteins thus explaining the late onset of senile cataractous changes.

Adult↗
PubMed1990

Biomicroscopy and scanning electron microscopy of early opacities in the aging human lens.

A large sample of lenses obtained from donor eyes spended for cornea transplantation were biomicroscopically screened and a representative subsample of lenses (about 30) exhibiting early opacities were studied at the scanning electron microscopic (SEM) level. Two types of early cortical opacities (Lens Opacities Classification System cortical grade 1a, 1b) could be distinguished biomicroscopically: (1) radial shades, and (2) circular shades. The prevalence of both types is age related, with an earlier occurrence of radial shades than of circular shades. In addition, radial shades proved to occur as single entities in isolation in lenses from people younger than the age of 40, whereas circular shades are only occasionally found in isolation in older lenses. SEM analysis substantiated the existence of the two early forms of opacity. Radial shades were found to consist of restricted parts of a small group of fibers in the deep cortex. These affected parts had membranes with a fine globular aspect and in cross-section proved to be filled with medium to large globular elements. Neighboring fibers and the nonaffected parts of the fibers involved in the radial shades had a normal SEM ultrastructure. Circular shades proved to originate as fractures of a large cohort of fibers in the deep equatorial cortex perpendicular to the course of the fibers. The fractured faces were slightly swollen, but the membranes on both sides of the fracture had a normal ultrastructure. In cross-sections of more advanced circular shades it proved that the broken fibers are filled with globular elements and are opaque. This opacification proved to extend only in one direction: either anteriorly or posteriorly. The results strongly suggest that two distinct early cataractogenic cellular processes are operating in the aging human lens. The possible induction of circular shades by radial shades, as can tentatively be concluded on account of the difference in appearance during aging of circular and radial shades, needs further verification by SEM analysis of more severely affected lenses. The observations corroborate the suggestion by Bron and Brown of the existence of mechanisms within the lens that separate damaged from undamaged fibers (segregation) or separate affected parts from unaffected parts of individual fibers (sealing).

Adolescent↗
PubMed1989

Lamellar separation in the human lens: the case for fibre folds. A combined in vivo and electron microscopy study.

Lamellar separation is seen as parallel lines in the lens cortex. It has been the subject of a joint study between Oxford and Amsterdam. The condition was studied in vivo by macro photography and in vitro by scanning electron microscopy. The lines are seen to run concentric with the lens equator, crossing the lens fibres at right angles. The lines are commonly limited by a spoke cataract, a water cleft, or a lens suture. The line may continue beyond these features with a change in direction. The lines may branch and rejoin. The lines extend in depth into the lens, polarised light showing that the lines are due to reflection. The name 'lamellar separation' implied that the appearance is due to separation between the lens lamellae. Lamellar separation is now shown by electron microscopy to be due to folds crossing the lens fibres. The clinical study showed the lines occurring with spoke cataract and the electron microscopy showed the association with the novel finding of peripheral breaks in the fibres. A new name 'Fibre Folds' is proposed.

Aged↗
PubMed1986

An ultrastructural study of Elschnig's pearls in the pseudophakic eye.

In two pseudophakic human eyes, obtained post mortem, Elschnig's pearls were visible biomicroscopically. One eye contained a medallion lens and the other an iridocapsular lens (implanted for 53 months and 39 months, respectively). The medallion lens was fixed to the iris but was not attached to the Soemmerring's ring. Elschnig's pearls and star-shaped cells were found on the posterior capsule in the pupillary space. One loop of the iridocapsular lens was encased in the Soemmerring's ring whereas the other was located between the iris and the lens remnants. The Elschnig's pearls were on the anterior side of the ring; only a few were in the pupillary space. Two other pseudophakic eyes with clear posterior capsules also contained small numbers of Elschnig's pearls on or just near the peripheral lens remnants.

Cataract Extraction↗
PubMed1985

Lens fiber organization in four avian species: a scanning electron microscopic study.

The three-dimensional organization of the eye lenses of the chicken, the canary, the song-thrush and the kestrel was studied using light and scanning electron microscopy. The lenses of birds are characterized by the presence of two distinct compartments: the annular pad and the main lens body, separated by a cavum lenticuli. The annular pad fibers had a hexagonal circumference all contained a round nucleus and except for the canary were smooth-surfaced and lacking anchoring devices. In the canary, however, the annular pad fibers were studded with edge protrusions and ball-and-socket junctions. The semicircular main lens body fibers of all four species were studded with ball-and-socket junctions and edge protrusions. In contrast with mammals these anchoring devices were present throughout the lens up to the embryonal nucleus. Superficially the main lens body fibers were extremely flat. Additionally membrane elevations and depressions and globular elements were found on these central fibers in three species, the kestrel being the exception. At the transition between annular pad and main lens body the fibers turned their course and the nuclei became oval and disappeared in the deeper aspect of the main lens body. The cavum lenticuli was filled with globules tied off from the annular pad fibers. It seems attractive to assume that the presence of a separated annular pad, a cavum lenticuli filled with globular elements, the extreme flatness of the superficial central fibers and the studding of these central fibers with anchoring devices up to the embryonal nucleus are morphological expressions of the mouldability of the bird's eye lenses and consequently would explain their efficient accommodative mechanism including formation of a lenticonus. The presence of nuclei in the annular pad fibers and their typical change at the transitional zone between annular pad and main lens body are suggestive for a two-phased differentiation in bird's lens fibers: differentiation of the germinative epithelial cells to annular pad fibers which migrate to the main lens body after which they differentiate further to main lens body fibers.

Animals↗