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Understanding Early Eye Development

A primer on ocular embryology.

In this review of eye embryology, we explain in detail the stages of development of various ocular structures. Understanding the development of the human eye in utero helps us as eye care providers to better understand the multitude of ocular pathologies we treat and diagnose every day in our clinics.

AT A GLANCE

  • Understanding ocular anatomy and embryology provides important insights into the complex ocular pathologies eye care providers diagnose and treat every day.
  • The earliest stages of eye development begin around week 4, when the optic grooves begin to appear at the cranial pole of the embryo.
  • The macula develops at a different pace compared with the rest of the retina, initially growing rapidly and then slowing to achieve full maturity postnatally.

EARLY DEVELOPMENT

A comprehensive understanding of eye embryology begins with the initial formation of structures. The earliest stages of ocular development become evident around the beginning of week 4 of gestation with the appearance of optic grooves, or sulci, at the cranial pole of the embryo (Figure, A-C). These grooves are superficial indents within the cranial fold, which soon transform as the neural tube forms from the closing fold of the forebrain (Figure, D). To visualize this, imagine pushing into a balloon. The weight creates indents (ie, the optic grooves) in the balloon, which, as they are pressed deeper, eventually cause the balloon to form outward bulges, known as the optic vesicles.

Figure. The earliest stages of ocular development from optic grooves to the lens, optic nerve, and optic stalk formation.

Initially crude and bud-like in shape, the optic vesicles extend laterally into the connective tissue (ie, the mesenchyme) and toward the surface ectoderm, where they adopt a mushroom-like shape with an everted head, termed the optic cup. Concurrently, a feature of the surface ectoderm known as the lens pit deepens and invaginates, eventually forming the lens vesicle (Figure, E-G). To revisit the balloon analogy, the optic cup’s expansion along with the lens vesicle’s invagination is akin to a person grabbing and manipulating a balloon, causing these important developmental structures to take form.

COMPLEX STRUCTURES

Optic Nerve and Retina

The optic nerve, formed from the optic stalk, connects the forebrain to the developing optic vesicle. By month 3 of gestation, the mesodermal tissue, including the capillaries, enters the optic nerve, with its outer coverings (dura mater, arachnoid, and pia) developing between months 3 and 7. Myelination of the optic nerve begins at around 7 months and extends toward the lamina cribrosa.

The retina originates from the optic cup, forming a double layer. The outer layer becomes the retinal pigment epithelium, while the inner layer develops into the neural retina. These layers fuse during fetal development, with the inner layer differentiating into several cell types that organize into distinct layers. The macula, a specialized region of the retina, develops at a different pace compared with the rest of the retina; initially growing rapidly, its development slows and ultimately achieves full maturity postnatally.

Choroid and Sclera

The choroid and sclera, derived from tissue adjacent to the optic cup, are analogous to the pia and dura mater of the brain, as they are the inner and outer layers of the eye, respectively. The choroid is pigmented and vascular, while the sclera is tough and fibrous, surrounding the optic apparatus and connecting with the cornea.

Lens

Lens development begins around day 35 of gestation from an outpouching of the surface ectoderm. It is surrounded by mesenchyme and located within the optic cup. The posterior side of the lens forms transparent epithelial cells called primary lens fibers, which meet secondary lens fibers from the anterior surface at the equatorial zone. Initially nourished by the hyaloid artery and tunica vasculosa lentis, the lens eventually relies on diffusion from the aqueous humor.

Vitreous Body

The vitreous body is a transparent, avascular, gel-like substance derived from the mesenchyme. The vitreous body forms initially as the primary vitreous humor, surrounded by the secondary vitreous humor, which is composed of hyalocytes, hyaluronic acid, and collagen.

Cornea

The cornea is derived from three sources: the surface ectoderm (corneal epithelium), the mesoderm (corneal stroma), and the neural crest cells (corneal endothelium). The cornea is multi-layered, avascular, and transparent. The anterior and posterior chambers of the eye develop as potential spaces between the cornea and lens, with communication occurring via the scleral venous sinus.

Ciliary Body

The ciliary body, a specialized part of the choroid, contains projections that become the vascular cores of its processes. The pigmented part is continuous with the outer layer of the optic cup, while the smooth ciliary muscle, responsible for lens focusing, derives from mesenchyme at the rim of the optic cup. The iris develops from the rim, partially covering the lens and connecting with the ciliary body, pigment epithelium, and neural retina. The dilator and sphincter pupillae muscles of the iris originate from the neuroectoderm of the optic cup.

FOUNDATIONAL KNOWLEDGE OF EYE DEVELOPMENT MATTERS

Understanding the anatomy and embryology of the eye helps us in diagnosing and managing developmental ocular diseases and highlights how these early processes shape the eye’s structure and function throughout your patients’ lifetime.

Srinivas S. Kondapalli, MD
  • Retina Specialist, Everett & Hurite Eyecare Specialists, Pennsylvania and Ohio
  • skondap@gmail.com
  • Financial disclosure: None
Ada Noh, OD
  • Owner and Optometrist, Noh Eyes, Little Rock, Arkansas
  • dr.adanoh@gmail.com
  • Financial disclosure: None