C

FIGURE 1-2A-C. (A) Drawing of a 17-day-old embryo in gastrulation stage (dorsal view) with the amnion removed. (B) Cross section of a 17-day-old embryo through the primitive streak. The primitive streak represents invagination of epiblast cells between the epiblast and hypoblast layers. Note the epiblast cells filling the middle area to form the mesodermal layer. (C) Cross section of the embryo at the end of the third week shows the three definitive germ layers: ectoderm, mesoderm, and endoderm.

Surface ectoderm

Surface ectoderm

FIGURE 1-3. Drawing of an 18-day-old embryo sectioned through the neural plate. Note that the ectoderm in the area of the neural groove (shaded cells) has differentiated into neural ectoderm whereas the ectoderm on each side of the neural groove remains as surface ectoderm

(clear white cells).

FIGURE 1-3. Drawing of an 18-day-old embryo sectioned through the neural plate. Note that the ectoderm in the area of the neural groove (shaded cells) has differentiated into neural ectoderm whereas the ectoderm on each side of the neural groove remains as surface ectoderm

(clear white cells).

Toward the end of gastrulation, the ectoderm anterior to the primitive streak differentiates into columnar neural ectoderm; this expands, forming the neural plate from which the brain develops (Figs. 1-3, 1-4). Neural ectoderm on each side of the central neural groove expands to form bilateral elevations called the neural folds (Fig. 1-5). A central valley in the enlarging neural plate is called the neural groove. Ectoderm at the lateral margins of the neural plate has the flat, hexagonal morphology typical of surface ectoderm (Figs. 1-5, 1-6). By 21 days of human

FIGURE 1-4. Drawing of dorsal aspect of embryo at 18 days gestation showing neural groove and enlarging neural plate.

Neural groov (neural ectodei

Neural groov (neural ectodei

FIGURE 1-4. Drawing of dorsal aspect of embryo at 18 days gestation showing neural groove and enlarging neural plate.

ij ^A Surface

Node of Hensen

Neural folds

Neural folds

FIGURE 1-5. Dorsal view of a human embryo at 20 days gestation. The neural plate transforms into two neural folds on each side of the neural groove. The neural groove in the middle of the embryo is shaded to represent neural ectoderm; the unshaded surface of the embryo is surface ectoderm.

Neural crest cells

Pericardial^*^ area W

Optic sulci

Otic placode

Optic sulci

FIGURE 1-6. Drawing of 21-day-old embryo (dorsal view) showing the enlarging cephalic neural folds, which are separate and have not yet fused. The central neural folds have fused to form the neural tube. The neural tube, groove, and facing surfaces of the large neural folds are made up of neural ectoderm; surface ectoderm covers the rest of the embryo. Neural crest cells (X) are found at the junction of the neural ectoderm and surface ectoderm. Neural crest cells migrate beneath the surface ectoderm spreading throughout the embryo and specifically to the area of the optic sulci. Neural ectoderm, dark shading; surface ectoderm, white; neural crest cells, cross-hatched area. The neural groove is still open at this point, and somites have formed along the lateral aspect of the neural tube.

Neural crest cells Neural groove

Neural ectoderm

Surface ectoderm

Roof of neural tube

Somite

FIGURE 1-6. Drawing of 21-day-old embryo (dorsal view) showing the enlarging cephalic neural folds, which are separate and have not yet fused. The central neural folds have fused to form the neural tube. The neural tube, groove, and facing surfaces of the large neural folds are made up of neural ectoderm; surface ectoderm covers the rest of the embryo. Neural crest cells (X) are found at the junction of the neural ectoderm and surface ectoderm. Neural crest cells migrate beneath the surface ectoderm spreading throughout the embryo and specifically to the area of the optic sulci. Neural ectoderm, dark shading; surface ectoderm, white; neural crest cells, cross-hatched area. The neural groove is still open at this point, and somites have formed along the lateral aspect of the neural tube.

FIGURE 1-7. Drawing of approximately 23-day-old embryo, dorsal view, showing partial fusion of the neural folds. Brain vesicles have divided into three regions: forebrain, midbrain, and hindbrain. Note that the facing surfaces of the forebrain neural folds are lined with neural ectoderm (shaded cells) but the majority of the embryo is covered by surface ectoderm (clear white). On the inside of both forebrain vesicles is the site of the optic sulci (optic pits). The neural crest cells that will populate the region around the developing optic vesicles originate from the midbrain region.

FIGURE 1-7. Drawing of approximately 23-day-old embryo, dorsal view, showing partial fusion of the neural folds. Brain vesicles have divided into three regions: forebrain, midbrain, and hindbrain. Note that the facing surfaces of the forebrain neural folds are lined with neural ectoderm (shaded cells) but the majority of the embryo is covered by surface ectoderm (clear white). On the inside of both forebrain vesicles is the site of the optic sulci (optic pits). The neural crest cells that will populate the region around the developing optic vesicles originate from the midbrain region.

gestation, while the neural tube is still open, the first sign of the developing eye is seen. The optic sulci or optic pits develop as invaginations on the inner surface of the anterior neural folds (Figs. 1-6, 1-7, 1-8). During expansion of the optic sulci, the central aspect of the neural folds approach each other and fuse, creating the longitudinal neural tube. Fusion of the neural folds

Anterior

FIGURE 1-8. Drawing of anterior view of embryo at similar stage to Figure 1-7 (23 days) shows the optic sulci on the inside of the forebrain vesicles. Shaded area, neural ectoderm. The optic sulci evaginate and expand toward the surface ectoderm as the neural tube closes anteriorly. (From Webster WS, Lipson AH, Sulik KK. Am J Med Genet 1988;31:505-512, with permission.)

FIGURE 1-8. Drawing of anterior view of embryo at similar stage to Figure 1-7 (23 days) shows the optic sulci on the inside of the forebrain vesicles. Shaded area, neural ectoderm. The optic sulci evaginate and expand toward the surface ectoderm as the neural tube closes anteriorly. (From Webster WS, Lipson AH, Sulik KK. Am J Med Genet 1988;31:505-512, with permission.)

is initiated in the region of the future neck and proceeds along the midline in both caudal and cranial directions. Following closure of the neural tube, the neural ectoderm and optic sulci are internalized, and the embryo is then covered by surface ectoderm (Fig. 1-7).

Neural Crest Cell Development

As the neural folds elevate and approach each other, a specialized population of mesenchymal cells, the neural crest cells, emigrate from the junction of the neural and surface ectoderm (see Fig. 1-6). Progenitor cells in the neural folds are multipotent, with potential to form multiple ectodermal derivatives, including epidermal, neural crest, and neural tube cells. These cells are induced by interactions between the neural plate and epidermis. The competence of the neural plate to respond to inductive interactions changes as a function of embryonic age.92 These stellate cells migrate peripherally beneath the surface ectoderm to spread throughout the embryo and surround the area of the developing optic sulci. Neural crest cells play an important role in eye development, as they are the precursors (anlage) to major structures, including cornea stroma, iris stroma, ciliary muscle, choroid, sclera, and orbital cartilage and bone (Table 1-1).55,64 The patterns of neural crest emergence and emigration correlate with the segmental disposition of the developing brain.72 Migration and differentiation of the neural crest cells are influenced by the hyaluronic acid-rich extracellular matrix and the optic vesicle basement membrane.17 This acel-lular matrix is secreted by a surface epithelium as well as the crest cells and forms a space through which the crest cells migrate. Fibronectin secreted by the noncrest cells forms the limits of this mesenchymal migration.65 Interactions between the migrating neural crest and the associated mesoderm appear to be essential for normal crest differentiation.76,77

TABLE 1-1. Embryonic Origins of Ocular Tissues.

Neural ectoderm (optic cup)

Surface ectoderm (epithelium)

Neural retina

Corneal and conjunctival

Retinal pigment epithelium

epithelium

Pupillary sphincter and dilator muscles

Lens

Posterior iris epithelium

Lacrimal gland

Ciliary body epithelium

Eyelid epidermis

Optic nerve

Eyelid cilia

Neural crest (connective tissue)

Epithelium of adnexa glands

Corneal endothelium

Epithelium of nasolacrimal duct

Trabecular meshwork

Mesoderm (muscle and vascular

Stroma of cornea, iris, and ciliary body

endothelium)

Ciliary muscle

Extraocular muscle cells

Choroid and sclera

Vascular endothelia

Perivascular connective tissue and

Schlemm's canal endothelium

smooth muscle cells

Blood

Meninges of optic nerve

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