Molecular cloning and cellular expression of the cholesterol synthesizing enzymes during the prenatal development of the optic nerve in the dromedary camel (Camelus Dromedarius).

The Cholesterol-synthesizing proteins (HMGCS1 and HMGCS2) are mitochondrial enzymes that believed to catalyze the first reaction of ketogenesis, the process by which energy is provided from fats in the absence of carbohydrates. Typically, astrocytes developed from its progenitor cells in the embryonic optic nerve and enriched with HMGCS1 and 2. However, the detailed histomorphology of camel HMGCS1 and 2 remains to be clearly defined. Here, we investigated the changes that associate with astrocytes differentiation within the developing camel optic nerve. Firstly, we isolated cDNAs encoding HMGCS1 and 2 from the optic nerve. Then, we found that HMGCS1 shared high similarity to human, while HMGCS2 showed a lower similarity and was more diverse. Immunohistochemical studies revealed that distinct correlation of astrocytes differentiation with HMGCS1 and 2 expressions in the developing camel optic nerve. Both encoded proteins were localized throughout the cytoplasm, as well as the nuclei of the astrocytes. In addition, semi-quantitative PCR analysis and western analysis confirmed that both HMGCS1 and 2 were highly expressed in camel optic nerve as well as other tissue, but they were lower in both skeletal and heart muscles. Moreover, various stains such as Sudan black and florescence filipin stains were used to visualize the free cholesterol in the astrocytes, indicating the enzymatic activity of HMGCS1 and 2. Together, our study reported the first comprehensive investigation of the molecular cloning and cellular expression of HMGCS1 and 2 in the optic nerve of dromedary camel.

Anatomy of the stemmata in the Photuris firefly larva.

Fireflies (Coleoptera: Lampyridae) have distinct visual systems at different stages of development. Larvae have stemmata and adults have compound eyes. Adults use compound eyes to mediate photic communication during courtship. Larvae do not manifest this behavior, yet they are bioluminescent. We investigated the structure of stemmata in Photuris firefly larvae to identify anatomical substrates (i.e., rhabdomeres) conferring visual function. Stemmata were located bilaterally on the antero-lateral surfaces of the head. Beneath the ~ 130 µm diameter lens, we identified a pigmented eye-cup. At its widest point, the eye-cup was ~ 150 µm in diameter. The optic nerve exited the eye-cup opposite the lens. Two distinct regions, asymmetric in size and devoid of pigmentation, were characterized in stemmata cross-sections. We refer to these regions as lobes. Each lobe contained a rhabdom of a radial network of rhabdomeres. Pairs of rhabdomeres formed interdigitating microvilli contributed from neighboring photoreceptor cell bodies. The optic nerve contained 88 axons separable into two populations based on size. The number of axons in the optic nerve together with distinct rhabdoms suggests these structures were formed from 'fusion stemmata.' This structural specialization provides an anatomical substrate for future studies of visually mediated behaviors in Photuris larvae.

Loss of foxc1 in zebrafish reduces optic nerve size and cell number in the retinal ganglion cell layer.

Mutation of FOXC1 causes Axenfeld-Rieger Syndrome (ARS) with early onset or congenital glaucoma. We assessed retinal ganglion cell (RGC) number in zebrafish due to CRISPR-mediated mutation and antisense inhibition of two-forkhead box transcription factors, foxc1a and foxc1b. These genes represent duplicated homologues of human FOXC1. Using a CRISPR induced null mutation in foxc1b, in combination with antisense inhibition of foxc1a, we demonstrate reduced cell number in the retinal ganglion cell layer of developing zebrafish eyes. As early as 5 days post fertilization (dpf), fewer RGCs are found in foxc1b homozygous mutants injected with foxc1a morpholinos, and a thinner optic nerve results. Our data illustrates that foxc1 is required for the expression of atonal homolog 7 (atoh7), a gene that is necessary for RGC differentiation. As markers of differentiated RGCs (pou4f2) are downregulated in foxc1b-/- mutants injected with foxc1a morpholinos and no cell death is observed, our results are consistent with defects in the differentiation of RGCs leading to reduced cell number, as opposed to increased cell death of RGCs or off targets effects of morpholino injection. Our zebrafish model demonstrates that aberrant regulation of RGC number could act in concert with other known glaucoma risk factors to influence the development of congenital and early onset glaucoma due to FOXC1 mutation.

Loss of zebrafish Ataxin-7, a SAGA subunit responsible for SCA7 retinopathy, causes ocular coloboma and malformation of photoreceptors.

Polyglutamine (polyQ) expansion in Ataxin-7 (ATXN7) results in spinocerebellar ataxia type 7 (SCA7) and causes visual impairment. SCA7 photoreceptors progressively lose their outer segments (OSs), a structure essential for their visual function. ATXN7 is a subunit of the transcriptional coactivator Spt-Ada-Gcn5 Acetyltransferase complex, implicated in the development of the visual system in flies. To determine the function of ATXN7 in the vertebrate eye, we have inactivated ATXN7 in zebrafish. While ATXN7 depletion in flies led to gross retinal degeneration, in zebrafish, it primarily results in ocular coloboma, a structural malformation responsible for pediatric visual impairment in humans. ATXN7 inactivation leads to elevated Hedgehog signaling in the forebrain, causing an alteration of proximo-distal patterning of the optic vesicle during early eye development and coloboma. At later developmental stages, malformations of photoreceptors due to incomplete formation of their OSs are observed and correlate with altered expression of crx, a key transcription factor involved in the formation of photoreceptor OS. Therefore, we propose that a primary toxic effect of polyQ expansion is the alteration of ATXN7 function in the daily renewal of OS in SCA7. Together, our data indicate that ATXN7 plays an essential role in vertebrate eye morphogenesis and photoreceptor differentiation, and its loss of function may contribute to the development of human coloboma.

Eye-specific segregation and differential fasciculation of developing retinal ganglion cell axons in the mouse visual pathway.

Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for axon navigation through intermediate choice points, whether and how axons are organized within tracts is less clear. Here we analyze the organization of retinal ganglion cell (RGC) axons in the developing mouse retinogeniculate pathway. RGC axons are organized by both eye-specificity and topography in the optic nerve and tract: ipsilateral RGC axons are segregated from contralateral axons and are offset laterally in the tract relative to contralateral axon topographic position. To identify potential cell-autonomous factors contributing to the segregation of ipsilateral and contralateral RGC axons in the visual pathway, we assessed their fasciculation behavior in a retinal explant assay. Ipsilateral RGC neurites self-fasciculate more than contralateral neurites in vitro and maintain this difference in the presence of extrinsic chiasm cues. To further probe the role of axon self-association in circuit formation in vivo, we examined RGC axon organization and fasciculation in an EphB1-/- mutant, in which a subset of ipsilateral RGC axons aberrantly crosses the midline but targets the ipsilateral zone in the dorsal lateral geniculate nucleus on the opposite side. Aberrantly crossing axons retain their association with ipsilateral axons in the contralateral tract, indicating that cohort-specific axon affinity is maintained independently of guidance signals present at the midline. Our results provide a comprehensive assessment of RGC axon organization in the retinogeniculate pathway and suggest that axon self-association contributes to pre-target axon organization.

Prenatal ethanol exposure in mice phenocopies Cdon mutation by impeding Shh function in the etiology of optic nerve hypoplasia.

Septo-optic dysplasia (SOD) is a congenital disorder characterized by optic nerve, pituitary and midline brain malformations. The clinical presentation of SOD is highly variable with a poorly understood etiology. The majority of SOD cases are sporadic, but in rare instances inherited mutations have been identified in a small number of transcription factors, some of which regulate the expression of Sonic hedgehog (Shh) during mouse forebrain development. SOD is also associated with young maternal age, suggesting that environmental factors, including alcohol consumption at early stages of pregnancy, might increase the risk of developing this condition. Here, we address the hypothesis that SOD is a multifactorial disorder stemming from interactions between mutations in Shh pathway genes and prenatal ethanol exposure. Mouse embryos with mutations in the Shh co-receptor, Cdon, were treated in utero with ethanol or saline at embryonic day 8 (E8.0) and evaluated for optic nerve hypoplasia (ONH), a prominent feature of SOD. We show that both Cdon-/- mutation and prenatal ethanol exposure independently cause ONH through a similar pathogenic mechanism that involves selective inhibition of Shh signaling in retinal progenitor cells, resulting in their premature cell-cycle arrest, precocious differentiation and failure to properly extend axons to the optic nerve. The ONH phenotype was not exacerbated in Cdon-/- embryos treated with ethanol, suggesting that an intact Shh signaling pathway is required for ethanol to exert its teratogenic effects. These results support a model whereby mutations in Cdon and prenatal ethanol exposure increase SOD risk through spatiotemporal perturbations in Shh signaling activity.

Expression of the lymphatic marker podoplanin (D2-40) in human fetal eyes.

During human ocular development, expression of proteins varies in different maturation stages. This study aims to characterize structures in human fetal eyes stained by the lymphatic marker podoplanin (D2-40) with emphasis on the stage of maturation and the presence of intraocular lymphatic structures. Formalin-fixed paraffin-embedded eyes from 40 human fetuses between 10 and 38 weeks of gestation (WoG) were investigated. Immunohistochemical stains were performed for D2-40, LYVE-1 as a secondary lymphatic marker, and CD34 as a control for endothelial reactivity. A semiquantitative analysis of antigen expression in different segments of the eye was performed by light microscopy. The intensity of antigen expression was graded with a score ranging from 0 to 3. Podoplanin expression was found with a variable intensity in 97.5% of the eyes, in particular in lymphatic vessels of the conjunctiva (n = 26), conjunctival and corneal epithelium (n = 33), corneal endothelium (n = 4), trabecular meshwork (n = 28), and optic nerve sheaths (n = 23). A slight, equivocal staining reaction was noted in the choroid (n = 14). There was a correlation of antigen reactivity and the gestational age for corneal endothelial reactivity in earlier gestational stages (p = 0.003) and trabecular meshwork in older eyes (p = 0.031). D2-40 positive Müller cells were detected in two eyes ≥32 WoG. Thus, aside from conjunctival lymphatic vessels, podoplanin was expressed in several structures of the human fetal eye and the ocular adnexae at different gestational stages. Podoplanin positive structures were also found in the choroid and the chamber angle. However, lymphatic vessels or its progenitors could not be unequivocally identified in intraocular structures during 10-38 weeks of gestation. There is no evidence from our data that transient intraocular lymphactics develop in the fetal eye between 10 and 38 weeks of gestation.

Dynamics of optic canal and orbital cavity development revealed by microCT.

PURPOSE: Numerous studies have attempted to clarify the exact anatomy and variations of the optic canal with non-conclusive results due to its close proximity to many vulnerable structures. We sought to determine the dynamics of growth and development of these structures on fetal skulls, which will help us to better understand of gender and age-dependent variations, as well as fatal malformations. METHODS: Fifteen previously macerated fetal frontal and sphenoid bones were analyzed and the diameters of optic canal, and distance of orbit from frontomaxillary suture to frontozygomatic suture were measured using 3D reconstruction images obtained by micro-CT. RESULTS: Average diameter of the optic canal in 300 mm fetus was measured to be 1,546 ± 36 µm, in 400 mm fetus 2,470 ± 123 µm and in 500 mm fetus 3,757 ± 203 µm. This trend indicates a linear enlargement of optic canal during the fetal period. During the same time period, diameter of the orbit enlarges from 12,319 ± 559 µm in 300 mm fetus to 19,788 ± 736 µm in 500 mm fetus. Growth curve is significantly lower in comparison with the same curve in optic canal data. We also calculated the ratio of orbit diameter and optic canal diameter between those groups which decreased from a value of 7.9 ± 0.4 for 300 mm fetus to 5.3 ± 0.2 for 500 mm fetus. CONCLUSION: Dynamics of optic canal and orbital cavity development is different in early and late fetal period. Diameters of those structures are in better correlation with the fetal length.

Neurogenesis, gliogenesis and the developing chicken optic tectum: an immunohistochemical and ultrastructural analysis.

We present the first comprehensive analysis of avian optic tectum development, including proliferation, migration and maturation of both neuronal and glial cells. The distribution of doublecortin, Tuj-1, vimentin and GFAP was characterized by immunohistochemistry between E3 and E20, and correlated with the electron microscopic structure in the chicken optic tectum. The immunohistological markers used in our study are known to be critical for distinct steps of neurogenesis and gliogenesis. We demonstrate that neurogenesis within the optic tectum starts at E3 with prominent doublecortin and moderate Tuj-1 expression. With the aid of electron microscopy, we also show that most of the cells are still undifferentiated at E4. Starting from E6, all postmitotic Tuj-1-positive neurons have left the ventricular zone and concurrently, with the end of proliferation around E12, doublecortin disappears from this region. Before hatching, doublecortin expression totally ceases, indicating that now all neurons have matured, this was also confirmed by ultrastructural investigations. Furthermore, vimentin expression starts around E4, prior to the appearance of the first radial glial cells at E6. Astrocytes can be detected by GFAP expression at E12. As radial glial cells (RGC) transform into astrocytes between E12 and E20, the vimentin signal is progressively replaced by the GFAP signal. We could also show that vimentin-positive RGCs do express doublecortin between E4 and E6, the time-point of prominent neurogenesis, reflecting their bipotent character.

Is the term "fasciculus opticus cerebralis" more justifiable than the term "optic nerve"?

The terminology of the optic nerve had already been changed three times, since 1895 until 1955 when the term "nervus opticus" was introduced in the "Terminologia Anatomica". Following our study we claim that, from the aspect of phylogenetic evolution of binocular vision development as well as optical embryogenesis where opticus is evidently presented as a product of diencephalic structures, the addition of the term "nervus" to opticus is not adequate and justified. From the clinical aspect the term "nervus opticus" is also inadequate, both as a "nerve" that has no functional regenerative properties, unlike other cranial nerves, as well as from a pedagogical and didactical aspect of educating future physicians. We suggest that the term "Fasciculus Opticus Cerebralis" should be used as it much better explains the origin as well as its affiliation to the central nervous system.

Suppression of the p75 receptor signal attenuates the effect of ephrin-B3 and promotes axonal regeneration of the injured optic nerve.

The p75 neurotrophin receptor (p75NTR) is known to transduce the signal from some myelin-associated axon growth inhibitors, including Nogo and myelin-associated glycoprotein. As ephrin-B3, a member of the ephrin family, is also expressed in myelin and inhibits axon growth, the purpose of this study was to assess the possible involvement of p75NTR in ephrin-B3 signaling. Here, we report that p75NTR is required for the inhibitory effect of ephrin-B3 on neurite growth in vitro. While ephrin-B3 inhibited neurite elongation of embryonic cortical neurons, the neurons with p75NTR knockdown or with EphA4 knockdown were less sensitive to ephrin-B3. Although no direct interaction of p75NTR with ephrin-B3 was observed, Pep5, a peptide that specifically inhibits RhoA activation mediated by p75NTR, reduced the effect of ephrin-B3. Therefore, p75NTR functions as a signal transducer for ephrin-B3. Moreover, axonal regeneration in vivo was induced by Pep5 application after optic nerve crush injury in mice. Thus, Pep5 is a promising agent that contributes to axonal regeneration in the central nervous system.

Olfactomedin 1 interacts with the Nogo A receptor complex to regulate axon growth.

Olfm1, a secreted highly conserved glycoprotein, is detected in peripheral and central nervous tissues and participates in neural progenitor maintenance, cell death in brain, and optic nerve arborization. In this study, we identified Olfm1 as a molecule promoting axon growth through interaction with the Nogo A receptor (NgR1) complex. Olfm1 is coexpressed with NgR1 in dorsal root ganglia and retinal ganglion cells in embryonic and postnatal mice. Olfm1 specifically binds to NgR1, as judged by alkaline phosphatase assay and coimmunoprecipitation. The addition of Olfm1 inhibited the growth cone collapse of dorsal root ganglia neurons induced by myelin-associated inhibitors, indicating that Olfm1 attenuates the NgR1 receptor functions. Olfm1 caused the inhibition of NgR1 signaling by interfering with interaction between NgR1 and its coreceptors p75NTR or LINGO-1. In zebrafish, inhibition of optic nerve extension by olfm1 morpholino oligonucleotides was partially rescued by dominant negative ngr1 or lingo-1. These data introduce Olfm1 as a novel NgR1 ligand that may modulate the functions of the NgR1 complex in axonal growth.

Neuregulin-1/ErbB4 signaling controls the migration of oligodendrocyte precursor cells during development.

During embryonic development, the oligodendrocyte precursors (OPCs) are generated in specific oligodendrogliogenic sites within the neural tube and migrate to colonize the entire CNS. Different factors have been shown to influence the OPC migration and differentiation, including morphogens, growth factors, chemotropic molecules, and extracellular matrix proteins. Neuregulins have been shown to influence the migration of neuronal precursors as well as the movement and differentiation of Schwann cells for peripheral myelination, but their role in the motility of OPCs has not been explored. In the present study, we have used the optic nerve as an experimental model to examine the function of Nrg1 and its ErbB4 receptor in the migration of OPCs in the developing embryo. In vitro experiments revealed that Nrg1 is a potent chemoattractant for the first wave of OPCs, and that this effect is mediated via ErbB4 receptor. In contrast, OPCs colonizing the optic nerve at postnatal stages (PDGFRα(+)-OPCs) does not respond to Nrg1-chemoattraction. We also found that mouse embryos lacking ErbB4 display deficits in early OPC migration away from different oligodendrogliogenic regions in vivo. The present findings reveal a new role for Nrg1/ErbB4 signaling in regulating OPC migration selectively during early stages of CNS development.

Dynamics of angiogenesis during murine retinal development: a coupled in vivo and in silico study.

The manner in which the superficial retinal vascular plexus (RVP) develops in neonatal wild-type mice is relatively well documented and poses an interesting challenge to the mathematical modelling community. Prior to birth, astrocyte sprouting and proliferation begin around the edge of the optic nerve head, and subsequent astrocyte migration in response to a chemotactic gradient of platelet-derived growth factor (PDGF)-A results in the formation of a dense scaffold on the surface of the inner retina. Astrocytes express a variety of chemotactic and haptotactic proteins that subsequently induce endothelial cell sprouting and modulate growth of the RVP. An experimentally informed, two-dimensional hybrid partial differential equation-discrete model is derived to track the outward migration of individual astrocyte and endothelial tip cells in response to the appropriate biochemical cues. Blood perfusion is included throughout the development of the plexus, and the evolving retinal trees are allowed to adapt and remodel by means of several biological stimuli. The resulting wild-type in silico RVP structures are compared with corresponding experimental whole mounts taken at various stages of development, and agreement between the respective vascular morphologies is found to be excellent. Subsequent numerical predictions help elucidate some of the key biological processes underlying retinal development and demonstrate the potential of the virtual retina for the investigation of various vascular-related diseases of the eye.

Dual-mode modulation of Smad signaling by Smad-interacting protein Sip1 is required for myelination in the central nervous system.

Myelination by oligodendrocytes in the central nervous system (CNS) is essential for proper brain function, yet the molecular determinants that control this process remain poorly understood. The basic helix-loop-helix transcription factors Olig1 and Olig2 promote myelination, whereas bone morphogenetic protein (BMP) and Wnt/ß-catenin signaling inhibit myelination. Here we show that these opposing regulators of myelination are functionally linked by the Olig1/2 common target Smad-interacting protein-1 (Sip1). We demonstrate that Sip1 is an essential modulator of CNS myelination. Sip1 represses differentiation inhibitory signals by antagonizing BMP receptor-activated Smad activity while activating crucial oligodendrocyte-promoting factors. Importantly, a key Sip1-activated target, Smad7, is required for oligodendrocyte differentiation and partially rescues differentiation defects caused by Sip1 loss. Smad7 promotes myelination by blocking the BMP- and ß-catenin-negative regulatory pathways. Thus, our findings reveal that Sip1-mediated antagonism of inhibitory signaling is critical for promoting CNS myelination and point to new mediators for myelin repair.

Megalin mediates the influence of sonic hedgehog on oligodendrocyte precursor cell migration and proliferation during development.

Oligodendrocyte precursor cells (OPCs) of the optic nerve are generated in the preoptic area, from where they migrate to colonize it entirely. Sonic hedgehog (Shh) induces the proliferation of these cells as well as influencing their migration, acting through its canonical receptor (Ptc-1). However, the multiligand receptor megalin (or LRP-2) is also involved in Shh-induced OPC proliferation and migration, and thus, we have evaluated the relevance of this interaction. During the stages at which Shh influences OPC development, we found megalin to be selectively expressed by optic nerve astrocytes, whereas Ptc-1 and Gli1 were found in OPCs. Indeed, this pattern of expression paralleled the rostral-caudal expression of the three Shh-related molecules during the time course of plp-dm20(+) -OPC colonization. The blockage of megalin partially abolished OPC chemoattraction and fully impaired Shh-induced proliferation. Using in vitro co-cultures of dissociated optic nerve cells, we demonstrated that Shh was internalized by astrocytes via megalin, and sufficient Shh was subsequently released to produce the biological effects on OPCs observed in the nerve. Together, these data indicate that at least part of the influence of Shh on OPCs is mediated by megalin during optic nerve development, and that astrocytes expressing megalin transiently capture Shh to present it to OPCs and/or to control the gradient of this molecule during development.

Relaxation-expansion model for self-driven retinal morphogenesis: a hypothesis from the perspective of biosystems dynamics at the multi-cellular level.

The generation of complex organ structures such as the eye requires the intricate orchestration of multiple cellular interactions. In this paper, early retinal development is discussed with respect to the structure formation of the optic cup. Although recent studies have elucidated molecular mechanisms of retinal differentiation, little is known about how the unique shape of the optic cup is determined. A recent report has demonstrated that optic-cup morphogenesis spontaneously occurs in three-dimensional stem-cell culture without external forces, indicating a latent intrinsic order to generate the structure. Based on this self-organizing phenomenon, we introduce the "relaxation-expansion" model to mechanically interpret the tissue dynamics that enable the spontaneous invagination of the neural retina. This model involves three consecutive local rules (relaxation, apical constriction, and expansion), and its computer simulation recapitulates the optic-cup morphogenesis in silico.

Development of astroglia heterogeneously expressing Pax2, vimentin and GFAP during the ontogeny of the optic pathway of the lizard (Gallotia galloti): an immunohistochemical and ultrastructural study.

The successful regrowth of retinal ganglion cell (RGC) axons after optic nerve (ON) axotomy in Gallotia galloti indicates a permissive role of the glial environment. We have characterised the astroglial lineage of the lizard optic pathway throughout its ontogeny (embryonic stage 30 [E30] to adults) by using electron microscopy and immunohistochemistry to detect the proliferation marker PCNA (proliferating cell nuclear antigen), the transcription factor Pax2 and the gliofilament proteins vimentin (Vim) and GFAP (glial fibrillary acidic protein). PCNA(+) cells were abundant until E39, with GFAP(+)/PCNA(+) astrocytes being observed between E37 and hatching. Proliferation diminished markedly afterwards, being undetectable in the adult optic pathway. Müller glia of the central retina expressed Pax2 from E37 and their endfeet accumulated Vim from E33 and GFAP from E37 onwards. Astrocytes were absent in the avascular lizard retina, whereas abundant Pax2(+) astrocytes were observed in the ON from E30. A major subpopulation of these astrocytes coexpressed Vim from E35 and also GFAP from E37 onwards; thus the majority of mature astrocytes coexpressed Pax2/Vim/GFAP. The astrocytes were ultrastructurally identified by their gliofilaments, microtubules, dense bodies, desmosomes and glycogen granules, which preferentially accumulated in cell processes. Astrocytes in the adult ON coexpressed both gliofilaments and presented desmosomes indicating a reinforcement of the ON structure; this is physiologically necessary for local adaptation to mechanical forces linked to eye movement. We suggest that astrocytes forming this structural scaffold facilitate the regrowth of RGCs after ON transection.

Effects of smoking during pregnancy on the optic nerve neurodevelopment.

BACKGROUND: Tobacco smoking during pregnancy alters neurodevelopment. Optical coherence tomography (OCT) provides precise measurements of the retinal nerve fiber layer (RNFL), which forms part of the central nervous system. AIMS: To assess using the OCT how smoking during pregnancy would affect optic nerve development as detected in human offspring. STUDY DESIGN: Visual examination and OCT were performed on a group of children (n=70; 4.15-13.50 years of age), classified as being exposed or not to maternal smoking during gestational period. The association between smoking during pregnancy and RNFL thickness was assessed by a linear regression analysis adjusted for possible confounding factors. RESULTS: Although visual outcomes did not differ between groups, a significant decrease in the RNFL thickness was found in the group of infants exposed to smoke (105.3 vs 95.6; p=0.002), even when adjusting for gestational age, birth weight or gender. CONCLUSIONS: OCT measurements show that intrautero exposure to tobacco smoke interferes with the development of the optic nerve.