FIGURE 7-13

Fovea of Macaca mulatta just before birth (159 days; term at 162 to 165 days). One interrupted row of ganglion cells

and one to two layers of bipolar cells still extend across foveal depression. Wide, well-developed horizontal outer plexiform layer of Henle (asterisks) and elongated cone inner and outer segments are present. Parafoveal area has large accumulation of cells in ganglion (G) and inner nuclear layer (N) characteristic of mature macula. Ch, Choroid; Sc, sclera. (×95.) (From Cook CS, Ozanics V, Jakobiec FA: Prenatal development of the eye and its adnexa. In Tasman W, Jaeger EA, editors: Duane’s foundations of clinical ophthalmology, vol 1, Philadelphia, 1994, Lippincott.)

the endothelium, coinciding with the beginning of aqueous formation.48 The material comprising Descemet’s membrane before birth has a banded appearance, whereas the tissue secreted by the endothelium after birth (which has a more posterior position) has a homogeneous, unbanded appearance.49,50

By week 8, a second wave of mesenchyme proliferates, migrates between the developing epithelium and endothelium, and gives rise to the fibroblasts, collagen, and ground substance of the stroma.3,51A third wave of mesenchyme migrates into the area between the developing endothelium and lens, giving rise to the pupillary membrane. These three waves of mesenchyme, as well as that giving rise to the sclera, are of neural crest origin (see Box 7-1).3

At 3 months all layers of the cornea are present ­(Figure 7-14) except Bowman’s layer, which appears during the fourth month45 and is presumably formed by fibroblasts of the anterior stroma and secretions of the epithelial cells;52,53 whatever the stage, Bowman’s layer is always acellular.49 Fibroblast arrangement and subsequent production of collagen fibrils begins in posterior corneal stroma and proceeds anteriorly. Rapid growth of the corneal stroma causes an increase in curvature relative to the rest of the globe.5 At birth the cornea is circular and steep (55D); the curvature decreases to 44D at age 6 months1.

SCLERA

The sclera first develops anteriorly from condensations in the mesenchyme near the limbus. Growth continues posteriorly until the sclera reaches the optic nerve, and by the end of the third month the sclera has surrounded the developing choroid.26 During the fourth month connective tissue fibers cross the posterior scleral foramen, running through the optic nerve fibers and producing the first connective tissue strands of the lamina cribrosa.3 By the fifth month, the sclera (including the scleral spur) is well differentiated.4

UVEA

Choroid

The mesenchyme that forms the choriocapillaris must be in contact with the developing pigment epithelium to differentiate.4,6 The vessels appear during the second month, and the diaphragm-covered fenestrations are evident by week 12.28 Bruch’s membrane develops during month 4. During the fifth month the layers of the large and medium vessels are evident, as are the vessels that will become vortex veins.54 The short posterior ciliary arteries also are evident and begin to anastomose to form the circle of Zinn.3

At midterm in fetal development the elastic sheet of Bruch’s membrane is present, the basement membrane of the RPE is developing, and the collagenous layers are thickening. The basement membrane of the choriocapillaris is the last component to appear.3 By term the choroidal stroma is pigmented.28

Ciliary Body

The region of the outer layer of the optic cup, which will become the outer pigmented epithelium of the ciliary body, begins to form ridges late in the third month.55

The inner nonpigmented epithelium, from the inner optic cup layer, grows and folds with it. These folds, almost 70 in number, become the ciliary processes. Zonula occludens are evident in the nonpigmented epithelium during the third month.1 Neural crest cells differentiate into stromal elements; the fenestrations in the capillaries in the processes are visible in week 14.3,55 During the fourth month the major arterial circle of the iris is formed by the anastomosing long ciliary arteries and replaces the annular vessel.56 Gap junctions and desmosomes appear, joining the apices of the two epithelial layers during the fourth month. The ciliary muscle begins to develop from neural crest44 during the fifth month. However, the annular muscle (of Müller) remains incomplete at birth.3,4 Aqueous humor production begins at 4 to 6 months of gestation.55