EMBROYOLOGY AND DEVELOPMENT OF LENS.pptx

description of embryology & development of lens

1. EMBRYOLOGY AND DEVELOPMENT OF LENS PRESENTER: Dr . LAXMI DHAWAL 1 ST YEAR MD RESIDENT ,LEIRC 1

2. NORMAL CRYSTALLINE LENS The Crystalline Lens Is:- Transparent Biconvex Located Posterior To Iris And Anterior To The Viterous Body in a saucer shaped depression , the patellar fossa. Supported By Zonules At Birth : 6.4 mm Equatorially & 3.5 mm Anteroposteriorly & Weighs 65 Mg. In Adult : 9-10 mm Equatorially & 5 mm Anteroposteriorly And Weighs Approx 260 Mg. 2

3. It has 2 surfaces :- anterior surface is less convex than posterior , posterior is more curved than anterior. Poles : centre of anterior and posterior surface is called anterior poles and posterior poles. Refractive index of lens :- 1.39 (nucleus 1.42, cortex 1.38).refractive power is 16-17 dioptres. Accomodative power of lens At birth : 14-16 D At 25 years of age : 7-8 D By the age of 40 years : 4.00-8.00 D At 50 years of age : 1-2 D 3

4. Divides eye into anterior and posterior cavity. Colour of lens Infants & young adults : colourless About 30 year : yellow tinge Old age : amber coloured Consists of 3 different parts : lens capsule , anterior lens epithelium , lens substance/lens fibers . 4

5. FUNCTIONS OF LENS To Maintain Clarity. To Refract Light. To Provide Accomodation , in Conjunction With Zonule And The Ciliary Body. 5

6. Embryology 6

7. (embryoblast) (embryoblast) 7

8. Formation of bilaminar disc 8

9. Formation of trilaminar disc (gastrulation) at 3 weeks. 9

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11. Cont.. More cells enter through groove forming 1 more layer between ectoderm & endoderm I.e mesoderm . 3 layer formation (gastrulation) 11

12. Formation of notochord Cells migrate through primitive pit----notochord process formation in the mesoderm. Notochord loses its canal---becomes definitive notochord. 12

13. Neurulation Process of folding in vertebrae embryo which includes transformation of neural plate into neural tube. Neural tube is derived from ectoderm. 13

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16. NORMAL DEVELOPMENT OF LENS Begins Very Early In Embryogenesis. At About 25 Days Of Gestation,2 Lateral Outpouching Is formed Called Optic Vesicles From Forebrain , Or Diencephalon. As Optic Vesicle Enlarge And Extend Laterally , They Become Closely Apposed And Adherent To The Surface Ectoderm . 16

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18. Lens placode At approx. 27 days of gestation ectoderm cells that overlie optic vesicles become columnar. Clinical correlation : failure of formation of lens placode from surface ectoderm in developing embryo---primary aphakia Lens Pit Appears at 29 days of gestation as an indentation (infolding) of the lens placode. Lens placode deepens and invaginates to form lens vesicle. 18

19. Lens vesicle As the lens pit continues to invaginate, the stalk of cells connecting it to the surface ectoderm degenerates by programmed cell death (apoptosis), separating the lens cells from the surface ectoderm(future corneal epithelium) approx. at 33 days of gestation. The resultant sphere, a single layer of cuboidal cells encased in a basement membrane (the lens capsule), is called the lens vesicle. 19

20. Fig: transverse section of forebrain showing optic grooves & lens placode 20

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22. Clinical correlation Absence of separation from surface ectoderm--- peters anomaly . Failure of formation of the optic vesicle---- anophthalmia {EOM(mesoderm) & lacrimal gland (ectoderm) are present}. Formation of the optic vesicle without proper subsequent development produces a rudimentary eye in orbit ----- nanophthalmia (or dwarf eye). Malformation of mesodermal tissue between the optic vesicle--- fusion of 2 eyes– synophthalmia . 22

23. Embryonic fissure 23

24. Closure of embryonic fissure Closure of the embryonic fissure occurs during the second month of embryonic development. Closure of the fissure begins on day 33 of gestation and allows for pressurization of the globe. The process begins at the equator and proceeds anteriorly and posteriorly. 24

25. What if choroidal fissure fails to close? If it fails to close by the 7 th week , it will lead to coloboma formation. A large spectrum of disease can result from defects in closure ranging from mild and asymptomatic to microphthalmia or even anophthalmia. Defects in closure at either pole have different results. Colobomas of the iris or ciliary body result from failures of complete anterior closure, while colobomas of the choroid, retina or optic nerve result from failures of posterior closure. 25

26. IRIS COLOBOMA 26

27. CHORIORETINAL COLOBOMA 27

28. OPTIC DISC COLOBOMA 28

29. LENS COLOBOMA Coloboma of the lens is due to defective or absent development of the zonules in any segment. There is a corresponding flattening of the equator of the lens due to a lack of tension on the lens capsule, which may lead to subsequent contraction and notching in that region. The term ‘lens coloboma’ is therefore a misnomer because the tissue lost in this defect is of the zonules rather than the lens. 29

30. CONT.. Anteriorly located coloboma often appears as a defect in the iris tissue, in the shape of a keyhole. They are classified as “typical” if found in the inferonasal quadrant of the affected structure and “atypical” if found elsewhere. Cornea, ciliary body and zonules may also be involved. Lens coloboma is seen as flattening of the equator of the lens in an area of absence of zonular fibers. This is best visualized in a dilated eye and may be incidentally diagnosed as they are almost always asymptomatic 30

31. Primary lens fibres and the embryonic nucleus The cells in the posterior layer of the lens vesicle stop dividing and begin to elongate between 33 and 35 days of gestation. As they elongate, they begin to fill the lumen of the lens vesicle. At approximately 40 days of gestation, the lumen of the lens vesicle is obliterated. The elongated cells are called the primary lens fibers. As the fiber cells mature, their nuclei and other membrane-bound organelles undergo degradation, a process that reduces light scattering. 31

32. The primary lens fibers make up the embryonic nucleus that will ultimately occupy the central area of the adult lens. The cells of the anterior lens vesicle give rise to the lens epithelium, a monolayer of cuboidal cells. Proliferation within the epithelium causes subsequent growth of the lens. The lens capsule develops as a basement membrane elaborated by the lens epithelium anteriorly and by lens fibers posteriorly. Clinical correlation : failure of tissue near the developing lens to induce lens fibers to elongate and pack together in an orderly way— cataract of primary lens fibers. 32

33. A, Lens vesicle. B, Anterior cells remain cuboidal, whereas the posterior cells elongate. C, The posterior cells eventually fill the lens vesicle, giving rise to the embryonic nucleus. D, The anterior cells give rise to the lens epithelium (LE). Note the lens bow region  (*)  extending from the epithelial cells, giving rise to the secondary lens fibers (SLF). ALE = anterior lens epithelium; BM = basement membrane; PLF = primary lens fibers . 33

34. Secondary lens fibres After they proliferate, the epithelial cells near the lens equator elongate to form secondary lens fibers. The anterior aspect of each developing lens fiber extends anteriorly beneath the lens epithelium, toward the anterior pole of the lens. The posterior aspect of each developing lens fiber extends posteriorly along the capsule, toward the posterior pole of the lens. 34

35. In this manner, new lens fibers are continually formed, layer upon layer. As each secondary fiber cell detaches from the capsule, it loses its nucleus and membrane-bound organelles. As the new fibers are laid down , the anterior shifted nucleus forms a line convex forward at the equator, known as lens or nuclear bow . The secondary lens fibers formed between 2 and 8 months of gestation make up the fetal nucleus. Clinical Correlation : faulty development of secondary lens fibers during embryogenesis---- microspherophakia (lens is small in diameter and spherical) 35

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37. Lens suture and the fetal nucleus Initial fibers formed surrounding the embryonic nucleus are arranged in a way that they terminate into 2 sutures. Upright (anterior) Y suture Inverted (posterior) Y suture Anterior suture is formed byjoining the apical aspect. Posterior suture is formed by joining basal aspect. 37

38. As growth continues lens become larger suture becomes asymmetric & dissimilar. Complexity of lens sutures contributes to lens opacity. Clinical correlation : opacification of the Y-suture of fetal nucleus. 38

39. Tunica vasculosa lentis Around 1 month of gestation, the hyaloid artery, which enters the eye at the optic nerve head (also called the optic disc), branches to form a network of capillaries, the tunica vasculosa lentis , on the posterior surface of the lens capsule . 39

40. These capillaries grow toward the equator of the lens, where they anastomose with a second network of capillaries, called the anterior pupillary membrane, which derives from the ciliary veins and covers the anterior surface of the lens. At approximately 9 weeks of gestation, the capillary network surrounding the lens is fully developed; it disappears by an orderly process of programmed cell death shortly before birth. 40

41. Cont … Clinical Correlation : Sometimes a remnant of the tunica vasculosa lentis persists as a small opacity or strand, called a Mittendorf dot , on the posterior aspect of the lens. In other eyes, remnants of the pupillary membrane are often visible as pupillary strands. Epicapsular Star, this anomaly is a star-shaped distribution of tiny brown or golden flecks on the central anterior lens capsule. Epicapsular star Mittendorf dot 41

42. The zonules of zinn The zonular fibers are secreted by the ciliary epithelium, although how these fibers insert into the lens capsule is not known. The zonular fibers begin to develop at the end of third month of gestation. 42

43. Summary of development of lens Lens fibres Age Embryonic lens 0-3 month Fetal nucleus 3-8 month Infantile nucleus Last week of fetal life to puberty adult (after puberty) DAY EVENTS DAY 25 Optic vesicle forms Day 27 Lens placode Day 29 Lens pit Day 33 Lens vesicle 7 weeks Secondary lens fibres 2-8 months Fetal nucleus 8 weeks Y-shaped nucleus 3 months Zonular fibers are secreted by the ciliary epithelium 43

44. Congenital anomalies of lens Ectopia lentis Displacement of lens from patellar fossa. It may be congenital , developmental or acquired. Trauma is most common cause of acquired lens displacement. 44

45. Ectopia lentis et pupillae Subluxated lens Luxated/dislocated lens Lens is partially displaced from its normal position but remains in pupillary area. Lens is completely displaced from pupil , implying separation of all zonular attachment. Bilateral & not symmetric Lens and pupil are displaced in opposite direction. Pupil :- irregular , usually slit shaped 7 displaced from normal position. Iris :- dilates poorly. Dislocated lens may bisect pupil or may be completely absent from pupillary space 45

46. Congenital aphakia Absence of crystalline lens. Very rare anomaly. Usually associated with other anomaly. Two types : Primary aphakia : lens placode fails to form in developing embryo Secondary aphakia : the developing lens is spontaneously absorbed . 46

47. Lenticonus and Lentiglobus Lenticonus : Cone shaped deformation of anterior or posterior lens surface. Lentiglobus : localized deformation of lens surface is spherical. Fig. posterior lenticonus Fig. Anterior lenticonus 47

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49. Marfan Syndrome A disease where a mutation in the FBN1 gene causes a defect in the creation of the protein fibrillin-1. The reduction in fibrillin-1 causes a decrease in the production of microfibrils, critical elastic connective tissues used throughout the body. As a result, the zonules of the eye stretch , and the lens in these patients will dislocate out of the visual axis, causing significant visual impairment  49

50. Weill- marchesani syndrome Rare systemic connective tissue disease , conceptually the converse of marfan syndrome. Autosomal recessive or dominant ,polymorphism in FBN1 gene as marfan syndrome. Ectopia lentis : Lens subluxation in anterior direction. Microspherophakia is common , so that pupillary block with angle closure glaucoma. 50

51. Microspherophakia Lens is smaller in size & spherical. Faulty development of secondary lens fibres. Microphakia Term used for a lens with a smaller than normal diameter. 51

52. Persistent fetal vasculature Also known as persistent hyperplastic primary vitreous (PHPV). During embryonic development of eye the compartment between the retina & crystalline lens contain vascular system (hyaloid artery) that provides nutrient are supposed to regress in the 3 rd trimester. Failure of regression of a component of fetal vessels within the eye. Congenital , non hereditary ocular malformation. Eyes with PHPV : smaller than normal Fig.PFV cataract showing a white spot in the pupil. 52

53. CONGENITAL CATARACT Occurs 1 in every 2000 live births. Lamellar cataract . Bilateral and symmetric. May be inherited as an autosomal dominant trait. Effect on vision varies with the size & density of the opacity. Affect a particular lamella of the lens both anteriorly & posteriorly & may be associated with radial extensions (‘riders’). 53

54. Polar cataract Involve subcapsular cortex and capsule of anterior or posterior pole of lens. 54

55. Anterior polar cataract Posterior polar cataract Small , bilateral , symmetric, non progressive opacities Bilateral , larger(sporadic posterior polar cataract are often unilateral) Do not impair vision More profound decrease in vision than anterior polar cataract. Non progressive progressive May be flat or project into the AC May be associated with posterior lenticonus or fetal vascular remnants. Have lamellar or bull’s eye appearance Associated with increased risk of intraoperative complication during cataract surgery 55

56. Sutural cataract opacification of the Y-suture of fetal nucleus. Bilateral & symmetric & are frequently inherited in an autosomal dominant pattern. Does not impair vision. Coronary cataract named so because they consist of a group of club shaped cortical opacities that are arranged around the equator of the lens like a crown, or coronary. Seen on dilated pupil & do not affect visual acquity . 56

57. Cerulean cataract Also known as blue-dot cataract. Small bluish opacities located in lens cortex. Non progressive & do not cause visual symptoms. Nuclear cataract Opacities of embryonic nucleus alone or both embryonic and fetal nucleus. Capsular cataract Opacification of lens epithelium and anterior lens capsule that spare cortex. Do not adversely affect vision. 57

58. Complete cataract All of the lens fibres are opacified. Cause profound visual impairment. Membranous cataract Lens protein are resorbed from either an intact or a traumatised lens , allowing the anterior & posterior lens capsules to fuse into a dense white membrane. Cause significant visual disability. 58

59. Congenital Rubella Syndrome Maternal infection with rubella virus , an RNA togavirus , can cause fetal damage , especially in first trimester. Cataract resulting from congenital rubella syndrome is characterised by pearly nuclear opacification . Live virus may be recovered from lens as late as 3 years. Cardiac defect (most common defect) E.g. PDA , Pulmonary artery stenosis Cochlear defect Bilateral sensorineural hearing loss Cataract Salt & pepper retinopathy ,glaucoma , transient or permanent corneal clouding c³ 59

60. Pathophysiology Maternal infection by respiratory route Maternal viremia abortion/still birth Placental infection 60

61. Panembryonic infection persistently establishes Mitotic inhibition apoptosis Slow the proliferation & mitosis of epithelial cells in lens Damage to vascular endothelium Destruction of ocular lens Growth retardation Bone lesion Lens fibres degenerate opaque Encephalitis Mental retardation Central & cochlear defect 61

62. Homocystinuria Autosomal recessive pattern. Inborn error of methionine metabolism. Serum level of homocysteine and methionine are elevated. Lens dislocation appears in infancy in approx. 30% of affected individuals, & 80% by the age of 15 years. Ectopia lentis , typically inferonasal , is almost universal by the age of 25 years in untreated case. 62

63. Pathophysiology Dietary amino acids Methionine Homocysteine Cystathione Cysteine Deficiency of cysteine Disruption of zonular development Affected fibers tend to be brittle & easily disrupted Lens dislocation Folate/B12 dependent Cystathione b-synthase (CBS) enzyme (pyridoxine-dependent) 63

64. Hyperlysenemia Inborn error of metabolism of the amino acid lysine. Associated with ectopia lentis . 64

65. Differences: Marfan syndrome Homocystinuria Autosomal dominant FB1 mutation on chromosome 15 Autosomal recessive CBS mutation on chromosome 21 Normal intelligence Intellectual disability Cardiac events : aortic dissection Thromboembolic events Lens displaced supero -temporally Bilateral , symmetric Lens displaced infero -nasally Bilateral , symmetric Zonules : intact but become stretched & elongated Zonules : disrupted Accomodation is intact Loss of accomodation 65

66. Bibliography : AAO-BSCS Lens and Cataract , 2023-2024 Kanski’s clinical ophthalmology Ninth edition Langman’s medical embryology 66

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