Itpr1 regulates the formation of anterior eye segment tissues derived from neural crest cells.

Mutations in ITPR1 cause ataxia and aniridia in individuals with Gillespie syndrome (GLSP). However, the pathogenic mechanisms underlying aniridia remain unclear. We identified a de novo GLSP mutation hotspot in the 3'-region of ITPR1 in five individuals with GLSP. Furthermore, RNA-sequencing and immunoblotting revealed an eye-specific transcript of Itpr1, encoding a 218amino acid isoform. This isoform is localized not only in the endoplasmic reticulum, but also in the nuclear and cytoplasmic membranes. Ocular-specific transcription was repressed by SOX9 and induced by MAF in the anterior eye segment (AES) tissues. Mice lacking seven base pairs of the last Itpr1 exon exhibited ataxia and aniridia, in which the iris lymphatic vessels, sphincter and dilator muscles, corneal endothelium and stroma were disrupted, but the neural crest cells persisted after completion of AES formation. Our analyses revealed that the 218-amino acid isoform regulated the directionality of actin fibers and the intensity of focal adhesion. The isoform might control the nuclear entry of transcriptional regulators, such as YAP. It is also possible that ITPR1 regulates both AES differentiation and muscle contraction in the iris.

Ciliary margin-derived BMP4 does not have a major role in ocular development.

Heterozygous Bmp4 mutations in humans and mice cause severe ocular anterior segment dysgenesis (ASD). Abnormalities include pupil displacement, corneal opacity, iridocorneal adhesions, and variable intraocular pressure, as well as some retinal and vascular defects. It is presently not known what source of BMP4 is responsible for these defects, as BMP4 is expressed in several developing ocular and surrounding tissues. In particular, BMP4 is expressed in the ciliary margins of the optic cup which give rise to anterior segment structures such as the ciliary body and iris, making it a good candidate for the required source of BMP4 for anterior segment development. Here, we test whether ciliary margin-derived BMP4 is required for ocular development using two different conditional knockout approaches. In addition, we compared the conditional deletion phenotypes with Bmp4 heterozygous null mice. Morphological, molecular, and functional assays were performed on adult mutant mice, including histology, immunohistochemistry, in vivo imaging, and intraocular pressure measurements. Surprisingly, in contrast to Bmp4 heterozygous mutants, our analyses revealed that the anterior and posterior segments of Bmp4 conditional knockouts developed normally. These results indicate that ciliary margin-derived BMP4 does not have a major role in ocular development, although subtle alterations could not be ruled out. Furthermore, we demonstrated that the anterior and posterior phenotypes observed in Bmp4 heterozygous animals showed a strong propensity to co-occur, suggesting a common, non-cell autonomous source for these defects.

Schlemm's canal is a unique vessel with a combination of blood vascular and lymphatic phenotypes that forms by a novel developmental process.

Schlemm's canal (SC) plays central roles in ocular physiology. These roles depend on the molecular phenotypes of SC endothelial cells (SECs). Both the specific phenotype of SECs and development of SC remain poorly defined. To allow a modern and extensive analysis of SC and its origins, we developed a new whole-mount procedure to visualize its development in the context of surrounding tissues. We then applied genetic lineage tracing, specific-fluorescent reporter genes, immunofluorescence, high-resolution confocal microscopy, and three-dimensional (3D) rendering to study SC. Using these techniques, we show that SECs have a unique phenotype that is a blend of both blood and lymphatic endothelial cell phenotypes. By analyzing whole mounts of postnatal mouse eyes progressively to adulthood, we show that SC develops from blood vessels through a newly discovered process that we name "canalogenesis." Functional inhibition of KDR (VEGFR2), a critical receptor in initiating angiogenesis, shows that this receptor is required during canalogenesis. Unlike angiogenesis and similar to stages of vasculogenesis, during canalogenesis tip cells divide and form branched chains prior to vessel formation. Differing from both angiogenesis and vasculogenesis, during canalogenesis SECs express Prox1, a master regulator of lymphangiogenesis and lymphatic phenotypes. Thus, SC development resembles a blend of vascular developmental programs. These advances define SC as a unique vessel with a combination of blood vascular and lymphatic phenotypes. They are important for dissecting its functions that are essential for ocular health and normal vision.

Evaluation of anterior segment parameters and possible influencing factors in normal subjects using a dual Scheimpflug analyzer.

PURPOSE: To investigate normal anterior segment parameters and analyze the possible influencing factors using a dual Scheimpflug system. SETTING: Department of Ophthalmology, Affiliated Sixth People's Hospital Shanghai Jiao Tong University, Shanghai, China. DESIGN: A prospective observational case series. METHODS: A total of 153 normal subjects (153 eyes) were studied. The anterior segment parameters, including the central corneal thickness (CCT), anterior chamber depth (ACD), pupil diameter (PD), keratoconus prediction index (KPI), simulated keratometry (SimK) values, anterior instantaneous curvature (AIC), posterior axial curvature (PAC), corneal eccentricity, total corneal power (TCP), axial curvature (AC), total corneal wavefront (TCW), high order aberration (HOA), and spherical aberration (SA), were determined using a dual Scheimpflug analyzer. RESULTS: The CCT and ACD were both negatively correlated with age (r = -0.203, p = 0.012; r = -0.589, p<0.001). There was no significant difference in the refractive indices of AIC and SimK. Compared with the negative correlation of HOA and SA (r = -0.358, p<0.001), a positive correlation was found between TCW and HOA (r = 0.561, p<0.001). Unlike the decreased tendency of AC, the TCP increased gradually from the center to the periphery in the central 8 mm diameter. TCP showed a significant correlation with AC in the analyzed area. CONCLUSIONS: AIC and SimK provide different information in clinic, but the refractive indices of them showed no difference in this healthy study population, and age should be considered when using CCT and ACD values.

Developmental anatomy in the zonular connection with lens capsule in macaque eye.

Morphological analyses of zonule conjugated with lens capsule were performed on the developmental change in eyes from the age of fetus to 7 years old of the rhesus macaques (Macaca fuscata). The zonule was filamentous network in late fetus. After birth, the zonular microfibrils originated from the nonpigmented epithelium of the ciliary process. On the extending path toward the lens capsule through the chamber, microfibril assembled with neighbor fibril and also cohered with one another forming bundle. With growth, these bundles bifurcated into anterior and posterior groups on the equatorial region of capsule. The developmental distribution of bundles in the capsule was characteristic on anterior group, that is, bifurcation into radial and circumferential extension. On the other hand, the posterior bundle undivided but radially extended within short distance from the equator. In the process of fixating with capsule, bundles untangled into fibrils and penetrated circumferentially into the superficial layer and radially into deep apical layer of the capsule. Zonule was composed fibrillin 1 microfibrils and on the extending path toward the lens capsule through the chamber, microfibril self-assembled with neighbored fibril in composition of fascicle and also cohered with one another forming bundle. Each bundle had alternating pale and dense horizontal bands in the intracapsular extension and the stripe pattern changed in flaccid or extensive tension of zonule between capsule and process. Zonular fibril intermingled with collagen fibril of capsule with interlacing molecule of laminin. At the base of ciliary muscle, elastin-positive connective tissue intercalated circumferentially between ciliary processes. The developmental changes of the intralamellar distribution and extension of zonule with striped pattern informed the functional role upon the elasticity in coordination with the lens capsule micromolecules.

[Zebrafish-a new animal models of anterior segment embryonic development and diseases basic research].

In recent years, zebrafish has become ideal animal models of human disease with its unique characteristics, such as small body, fecundity, fast development and growth, embryo transparency, and so on. Furthermore, the structure and gene of zebrafish eye are highly conservative with human eye, which make ophthalmologists to pay close attention to zebrafish. This review focus on the studies and applications on zebrafish embryonic development of anterior segment, including the morphogenesis of cornea, lens and anterior chamber, and diseases of anterior segment (corneal diseases, cataract, glaucoma).

Genetic deletion of the adenosine A2A receptor confers postnatal development of relative myopia in mice.

PURPOSE: To critically evaluate whether the adenosine A2A receptor (A2AR) plays a role in postnatal refractive development in mice. METHODS: Custom-built biometric systems specifically designed for mice were used to assess the development of relative myopia by examining refraction and biometrics in A2AR knockout (KO) mice and wild-type (WT) littermates between postnatal days (P)28 and P56. Ocular dimensions were measured by customized optical coherence tomography (OCT), refractive state by eccentric infrared photorefraction (EIR), and corneal radius of curvature by modified keratometry. Scleral collagen diameter and density were examined by electron microscopy on P35. The effect of A2AR activation on collagen mRNA expression and on soluble collagen production was examined in cultured human scleral fibroblasts by real-time RT-PCR and a collagen assay kit. RESULTS: Compared with WT littermates, the A2AR KO mice displayed relative myopia (average difference, 5.1 D between P28 and P35) and associated increases in VC depth and axial length from P28 to P56. Furthermore, the myopic shift in A2AR KO mice was associated with ultrastructural changes in the sclera: Electron microscopy revealed denser collagen fibrils with reduced diameter in A2AR KO compared with WT. Last, A2AR activation induced expression of mRNAs for collagens I, III, and V and increased production of soluble collagen in cultured human scleral fibroblasts. CONCLUSIONS: Genetic deletion of the A2AR promotes development of relative myopia with increased axial length and altered scleral collagen fiber structure during postnatal development in mice. Thus, the A2AR may be important in normal refractive development.

Deletion of G protein-coupled receptor 48 leads to ocular anterior segment dysgenesis (ASD) through down-regulation of Pitx2.

The development of the anterior segment of the mammalian eye is critical for normal ocular function, whereas abnormal development can cause glaucoma, a leading cause of blindness in the world. We report that orphan G protein-coupled receptor 48 (Gpr48/LGR4) plays an important role in the development of the anterior segment structure. Disruption of Gpr48 causes a wide spectrum of anterior segment dysgenesis (ASD), including microphthalmia, iris hypoplasia, irdiocorneal angle malformation, cornea dysgenesis, and cataract. Detailed analyses reveal that defective iris myogenesis and ocular extracellular matrix homeostasis are detected at early postnatal stages of eye development, whereas ganglion cell loss, inner nuclear layer thinness, and early onset of glaucoma were detected in 6-month-old Gpr48(-/-) mice. To determine the molecular mechanism of ASD caused by the deletion of Gpr48, we performed gene expression analyses and revealed dramatic down-regulation of Pitx2 in homozygous knockout mice. In vitro studies with the constitutively active Gpr48 mutant receptor demonstrate that Pitx2 is a direct target of the Gpr48-mediated cAMP-CREB signaling pathway in regulating anterior segment development, suggesting a role of Gpr48 as a potential therapeutic target of ASD.

Anterior segment growth and peripheral neural pathways in chick.

PURPOSE: To learn if peripheral nerve pathways are necessary for corneal expansion and anterior segment growth under a 12-hr light:dark cycle or for the inhibition of corneal expansion under constant light rearing. METHODS: Recently hatched White Leghorn chicks under anesthesia received unilateral ciliary ganglionectomy (CGx), cranial cervical ganglionectomy (Sx), or section of the ophthalmic nerve (TGx), along with sham-operated and/or never-operated control cohorts. Chicks were reared postoperatively under either a 12-hr light:dark cycle or under constant light. After 2 weeks and with the chicks under anesthesia, corneal radii of curvature and diameters were obtained with a photokeratoscope, refractometry and A-scan ultrasonography were performed, and the axial and equatorial dimensions of enucleated eyes were measured with digital calipers. Corneal areas were calculated from corneal curvatures and diameters. RESULTS: Despite the rich peripheral innervation to the eye, the selective denervations performed here exerted remarkably limited effects on corneal expansion and anterior segment development in chicks reared under either lighting condition. Ophthalmic nerve section did reverse in large part the inhibition of equatorial expansion of the vitreous chamber occurring under constant light rearing. CONCLUSIONS: The ciliary, sympathetic, or ophthalmic peripheral nerve pathways to the eye are not required either for corneal expansion and anterior segment development under a 12-hr light:dark cycle or for the inhibition of corneal expansion under constant light rearing. The ocular sensory innervation may be a means for regulating vitreous cavity shape.

Morphogenesis of the anterior segment in the zebrafish eye.

BACKGROUND: The ocular anterior segment is critical for focusing incoming light onto the neural retina and for regulating intraocular pressure. It is comprised of the cornea, lens, iris, ciliary body, and highly specialized tissue at the iridocorneal angle. During development, cells from diverse embryonic lineages interact to form the anterior segment. Abnormal migration, proliferation, differentiation, or survival of these cells contribute to diseases of the anterior segment such as corneal dystrophy, lens cataract, and glaucoma. Zebrafish represent a powerful model organism for investigating the genetics and cell biology of development and disease. To lay the foundation for genetic studies of anterior segment development, we have described the morphogenesis of this structure in zebrafish. RESULTS: As in other vertebrates, the zebrafish anterior segment derives from diverse origins including surface ectoderm, periocular mesenchyme, and neuroepithelium. Similarly, the relative timing of tissue differentiation in the anterior segment is also conserved with other vertebrates. However, several morphogenic features of the zebrafish anterior segment differ with those of higher vertebrates. These include lens delamination as opposed to invagination, lack of iris muscles and ciliary folds, and altered organization in the iridocorneal angle. In addition, substantial dorsal-ventral differences exist within the zebrafish anterior segment. CONCLUSION: Cumulatively, our anatomical findings provide a reference point to utilize zebrafish for genetic studies into the mechanisms of development and maintenance of the anterior segment.

Haploinsufficient Bmp4 ocular phenotypes include anterior segment dysgenesis with elevated intraocular pressure.

BACKGROUND: Glaucoma is a blinding disease usually associated with high intraocular pressure (IOP). In some families, abnormal anterior segment development contributes to glaucoma. The genes causing anterior segment dysgenesis and glaucoma in most of these families are not identified and the affected developmental processes are poorly understood. Bone morphogenetic proteins (BMPs) participate in various developmental processes. We tested the importance of Bmp4 gene dosage for ocular development and developmental glaucoma. RESULTS: Bmp4+/- mice have anterior segment abnormalities including malformed, absent or blocked trabecular meshwork and Schlemm's canal drainage structures. Mice with severe drainage structure abnormalities, over 80% or more of their angle's extent, have elevated IOP. The penetrance and severity of abnormalities is strongly influenced by genetic background, being most severe on the C57BL/6J background and absent on some other backgrounds. On the C57BL/6J background there is also persistence of the hyaloid vasculature, diminished numbers of inner retinal cells, and absence of the optic nerve. CONCLUSIONS: We demonstrate that heterozygous deficiency of BMP4 results in anterior segment dysgenesis and elevated IOP. The abnormalities are similar to those in human patients with developmental glaucoma. Thus, BMP4 is a strong candidate to contribute to Axenfeld-Rieger anomaly and other developmental conditions associated with human glaucoma. BMP4 also participates in posterior segment development and wild-type levels are usually critical for optic nerve development on the C57BL/6J background. Bmp4+/- mice are useful for studying various components of ocular development, and may allow identification of strain specific modifiers affecting a variety of ocular phenotypes.

Corneal surface area: an index of anterior segment growth.

Corneal surface area and perimeter were assessed as novel indices to monitor anterior segment growth, using chicks reared under different photoperiods. We obtained central and mid-peripheral corneal curvatures using photokeratometry. Anatomical tracings of the anterior corneal surface also were made from freeze-dried non-fixed preparations of the anterior segments of the same eyes. Using either photokeratometry or anatomical data, the profile of the anterior corneal surface was fit to a general equation for conical sections; corneal surface area was estimated from surfaces of revolution. Optical techniques modeled the chick cornea as a circle or as an ellipse closely resembling a circle. The anatomical technique, in contrast, modeled the chick corneal profile as a hyperbola. Potential explanations of this discrepancy are discussed. Regardless of which model is evaluated, the corneal surface area and perimeter of two-week-old chicks are affected by the photoperiod of rearing. Corneal surface area in particular proved more sensitive than conventional measurements in identifying anterior segment effects of rearing under different photoperiods. Analysis of corneal area may prove useful in evaluating the mechanisms governing anterior segment growth.

LMX1B, a LIM homeodomain class transcription factor, is necessary for normal development of multiple tissues in the anterior segment of the murine eye.

Proper development of the anterior segment of the mammalian eye is critical for normal ocular function. Indeed, several congenital syndromes associated with anterior segment anomalies can lead to impaired vision and glaucoma. One such syndrome is nail patella syndrome (NPS), caused by haploinsufficiency for the LIM-homeodomain transcription factor LMX1B. Although mutations in LMX1B cosegregate with NPS, whether these mutations cause the glaucoma associated with NPS is not known. Here, we provide evidence that the LIM-homeodomain transcription factor lmx1b is an essential regulator of murine anterior segment development. Mice that are homozygous for a targeted mutation of lmx1b display iris and ciliary body hypoplasia, and cornea stromal defects. In addition, two cDNAs normally downregulated in presumptive cornea, mf1 and mfh1, exhibit persistent expression, while keratocan, a keratin sulfate proteoglycan expressed by keratocytes, is not detected in mutant corneas. Moreover, ultrastructural examination of homozygous mutants indicates that corneal collagen fibrillogenesis is perturbed. Taken together, our studies suggest a developmental etiology for glaucoma in NPS patients and highlight lmx1b as an essential regulator of anterior segment morphogenesis and patterning. genesis 26:15-25, 2000.

Alpha-B-crystallin expression in tissues derived from different species in different age groups.

alphaB-Crystallin is constitutively expressed in a variety of tissues including the nervous system, the eye, heart and striated muscles and the kidney. The functional significance of the protein in the different cell populations is not yet known. Experimental data indicate that mechanical stress to the cells might play a role but that there is also a close correlation with markers of oxidative activity. Increased expression of alphaB-crystallin is seen in a number of age-related degenerative diseases. Whether aging per se induces expression of the protein has not been investigated yet. In this study tissue samples of the anterior eye segment, optic nerve, heart muscle and thyroid gland from mouse, rat, pig, cow and human donors of different age groups were investigated with immunohistochemical methods. alphaB-Crystallin levels in heart muscle and optic nerve samples from different species and different age groups were investigated using protein immunoblotting (dot blot) and the mRNA levels using semiquantitative PCR methods. The results showed that neither in heart muscle known to show constitutively high amounts of the protein nor in nonlenticular eye tissues with variations in staining intensity of different cell populations or in glandular cells studied for the first time, there were significant age-related staining differences. Dot blot methods as a quantitative evaluation method gave similar results. There were, however, species differences. In the eye these differences could be due to functional differences related to the development of a fovea centralis and an accommodative system in primates. In addition, in all mouse tissues there was less protein expression than in the other species. Differences in the absolute life span might be a factor involved in alphaB-crystallin expression. In summary the findings show that an increase in alphaB-crystallin with age may occur but is not a general phenomenon in tissues constitutively expressing this protein.

Patterned expression of BDNF and NT-3 in the retina and anterior segment of the developing mammalian eye.

PURPOSE: The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are hypothesized to play an important role in vertebrate eye development because of their patterned expression in the developing and adult neuroretina, their regulated response to retinal and optic nerve injury, and the effects of altered neurotrophin signaling on retinal development. To further characterize the role of these neurotrophins in mammalian eye development and maintenance, the pattern of expression of BDNF and NT-3 was analyzed in the developing and mature mouse eye. METHODS: Using mouse strains in which the reporter gene lacZ, encoding the enzyme beta-galactosidase, was targeted to either the BDNF or NT-3 locus, the expression of BDNF and NT-3 in the eyes of mice heterozygous for these mutations was analyzed by enzyme histochemistry during embryogenesis, postnatal development, and adulthood. RESULTS: BDNF and NT-3 expression were first observed in the inner and outer segments of the developing optic cup at embryonic days 10.5 to 11.5. As the retina matured, BDNF expression was restricted to retinal ganglion cells and a subset of cells in the inner nuclear layer (INL), whereas NT-3 expression was confined to a small subset of cells in the INL and ganglion cell layer. Both neurotrophins were expressed within the developing retinal pigment epithelium. In the anterior segment, BDNF and NT-3 were expressed at high levels in the developing and mature ciliary epithelium. In the lens and cornea, however, these neurotrophins displayed distinct patterns of expression during development and adulthood. BDNF expression was found in the lens epithelium, immature trabecular meshwork, corneal endothelium, and corneal epithelium, whereas NT-3 expression was confined to the corneal epithelium. CONCLUSIONS: BDNF and NT-3 exhibit different, yet overlapping, patterns of expression during the development and differentiation of the mouse eye. In addition to the neuroretina, the spatiotemporal expression of BDNF and NT-3 may play an important role in the development and maintenance of the lens, ciliary body, trabecular meshwork, and cornea.

Normal development of refractive state and ocular component dimensions in the marmoset (Callithrix jacchus).

Refractive state and ocular dimensions were studied longitudinally in nine normal marmosets. Animals were anaesthetised and examined (with some exceptions) at 4, 6, 7, 8, 10, 15, 24 and 39 weeks of age. Cycloplegic retinoscopy showed that hyperopia early in life rapidly diminished. Refraction corrected for the artefact of retinoscopy stabilised by 8 weeks of age, but at a slightly myopic value, rather than at emmetropia. The ocular components continued to change throughout the period studied. Corneal radius, measured by photokeratometry, increased slightly during development. Anterior segment depth and vitreous chamber depth (VCD), measured by A-scan ultrasonography, increased throughout development while lens thickness initially increased and then decreased. Data from the eyes of these normal animals were compared with that from the contralateral eyes of animals which received short periods of monocular deprivation early in life (Troilo, D., & Judge S.J. (1993). Ocular development and visual deprivation myopia in the common marmoset (Callithrix jacchus jacchus). Vision Research, 33, 1311-24); eyes which viewed through no lens or a plano lens (Graham, B. & Judge, S.J. (1999)). The effects of spectacle wear in infancy on eye growth and refractive error in the marmoset (Callithrix jacchus). Vision Research, 39, 189-206), and eyes of normal animals in another colony. There were no significant differences between the first two groups and the normal animals in our colony while age-matched animals from the other colony were slightly but significantly less myopic than our animals.

Flicker parameters are different for suppression of myopia and hyperopia.

Axial eye growth rates in the chicken are controlled by local retinal image-processing circuits. These circuits quantify the loss of contrast for different spatial frequencies and promote axial eye growth rates in correlation with the amount of retinal image degradation ("deprivation myopia"). They also distinguish whether the plane of focus lies in front of or behind the retina. How the sign of defocus is detected still remains unclear. Cues from chromatic aberration are not important. In an attempt to isolate retinal circuits controlling the development of myopia or hyperopia, young chickens were raised in flickering light of different frequencies (12 and 6 Hz) and duty cycles (4-75%) produced by rotating chopper disks. The effects of flickering light on refractive errors and change in axial growth rates induced by translucent occluders or defocusing lenses were measured by infrared retinoscopy and A-scan ultrasound, respectively. Retinal electrical activity was evaluated by flicker ERG after matching flicker parameters and stimulation brightness at retinal surface. Changes in retinal and vitreal dopamine content caused by flicker in occluded and normal eyes were determined by HPLC-ECD. Strikingly, suppression of myopia occurred for similar flicker parameters, whether induced by translucent occluders ("deprivation") or negative lenses ("defocus"). The degree to which myopia was suppressed was correlated with the duration of flicker dark phase and with the ERG amplitude. In contrast, suppression of hyperopia did not correlate with these parameters. We conclude that two different retinal circuits with different temporal characteristics are involved in the processing of hyperopic defocus/deprivation and of myopic defocus, the first one dependent on flicker ERG amplitude. However, we did not find any correlation between the rate of dopamine release and the degree of inhibition of deprivation myopia in flickering light.

Morphometric analysis of pars plana development in humans.

BACKGROUND: Quantitative features of pars plana development in humans are not well characterized. Knowledge of the dimensions of this region is important for our understanding of the anatomy of infant eyes for the purposes of surgery. The purpose of this study was to provide a morphometric description of pars plana growth in relation to 1) postconceptional age and 2) axial length. METHODS: We reviewed histologic specimens from 204 human eyes of postconceptional ages ranging from 10 weeks to 5 years. Axial length and temporal pars plana width were measured directly with a reticule system. The relationships among pars plana width, axial length, and postconceptional age were examined. RESULTS: The most rapid phase of pars plana growth occurred between 26 weeks and 35 weeks gestation. A linear relationship between pars plana width and axial length existed once the axial length reached 12 mm (correlation coefficient = 0.918; P = 0.0001). In infants of 38-42 weeks postconceptional age, mean pars plana width was 1.87 mm (range, 0.9-2.8 mm; standard direction = 0.48 mm), and mean axial length was 17.7 mm (range, 15-23 mm; standard direction = 1.9 mm). After the age of 62 weeks postconception, all eyes had a temporal pars plana width > or = 3 mm and an axial length > or = 19 mm. The estimated age at which there was a 95% chance of the pars plana width being > or = 3 mm was 64.4 weeks (95% fiducial confidence limits, 60.8-86.2 weeks). CONCLUSIONS: The pars plana first develops during the second trimester of gestation. A rapid growth phase occurs between 26 weeks and 35 weeks postconception. The dimensions of the pars plana are correlated closely with axial length and postconceptional age. Assuming that a pars plana width of 3 mm is required for surgery to be performed via a pars plana approach, we estimate that pars plana vitreous surgery can be performed when the patient is at least 62 weeks old (postconception), which is a 6-month-old full-term infant.

The effects of blockade of retinal cell action potentials on ocular growth, emmetropization and form deprivation myopia in young chicks.

To investigate the influence of brain mediated functions on control of ocular growth, young chicks were treated monocularly with intravitreally injected tetrodotoxin (TTX) to block retinal ganglion cell action potentials. TTX injections (0.7 micrograms in 7 microliters) were given on day 6 after hatching in both binocularly open and monocularly deprived chicks. Injections were repeated every 48 hr for a period of 8 days (TTX-open; TTX-MD). Control groups of animals received intravitreally injected phosphate buffered saline (PBS-open; PBS-MD) to one eye on the same schedule. There was a minimum of eight animals in each group. Recovery from form-deprivation myopia during blockade of retinal cell action potentials was also investigated. Results demonstrate that blockade of retinal cell action potentials by TTX produces reduced growth of the anterior segment of the eye and crystalline lens in both binocularly open and MD chicks. Blockade of retinal cell action potentials does not prevent form-deprivation induced vitreous chamber elongation and myopia. Form deprived myopic eyes were found to emmetropize despite blockade of retinal ganglion cell action potentials giving further evidence for local ocular control of emmetropization. Blockade of retinal ganglion cell action potentials did not prevent changes in choroidal thickness in eyes developing axial myopia or eyes recovering from induced myopia.