The development of the lips and the jaws

Figure 2:4: 10^th week embryo (seen from below the roof of the mouth) [Re-drawn from Scheuer and Black 2000]

The neural crest cells derived from the midbrain and forebrain give rise to the mesenchyme in the frontonasal prominence whereas those from midbrain and hindbrain contribute to the mesenchyme of the maxillary and mandibular prominences. The failure of some of these facial prominences to either grow or fuse correctly results in the relatively common congenital facial anomalies (facial clefts) that include the cleft lip and palate (Schoenwolf et al., 2009).

2.2.3 The development of the lips and the jaws

The lower lip and the lower jaw are formed from the mandibular processes (from the 1^st branchial arch) of the left and right of the developing face, which fuse in the midline forming the lower boundary of the stomodeum (the primitive mouth).

The chin projects forward from the midline of the fused mandibular processes.

Fused palatine processes

Hard palate Hard palate

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The triangular elevation, which projects from the cranial aspect of the dorsal region of each mandibular process, is the maxillary process (also from the first branchial arch), which grows ventromedially to fuse with the lateral nasal process of its side. Each of the maxillary processes then passes below the nasal sac to fuse with the medial nasal processes (from the frontonasal process) to form the upper lip and the upper jaw. The upper lip and the upper jaw (now forming the upper boundary of the stomodeum) are therefore derived from both the maxillary and the frontonasal processes. The mesodermal portion of the middle part of the upper lip (the philtrum) and the upper jaw are from the frontonasal process, while the mesodermal component of the lateral part of the upper lip and the upper jaw are from the maxillary process. The ectoderm of the maxillary process overgrows the midline frontonasal mesoderm so that the skin of the entire upper lip is supplied by the maxillary nerve (Singh and Pal, 2006).

2.2.4 Cheeks

The formation of the upper and lower lips makes the stomodeum (now the mouth) very wide, which is bounded by the maxillary process in its lateral part and the mandibular process below. These two processes fuse progressively with each other to form the cheeks. The maxillary process fuses with the lateral nasal process not only in the lip region but also extends to the medial angle of the eye.

This is marked by a groove known as the nasolacrimal sulcus lined by a strip of ectoderm that later gives rise to the nasolacrimal duct (Singh and Pal, 2006).

44 2.2.5 Eyes

The development of the eyes begins early in the 4th week from two (right-left) lateral grooves on the neuroectoderm (ectoderm populated by the migrated neural crest cells) of the forebrain neural groove. These are the optic sulci. The optic sulci evaginate to form the optic vesicles, which continue to grow until they reach the surface ectoderm where the tip of each vesicle invaginates, transforming from a vesicle to a goblet-shaped optic cup attached to the forebrain by a narrow and hollow optic stalk. The surface ectoderm overlying the optic cup then thickens to form a lens placode, which invaginates and pinches off, becoming a hollow lens vesicle. The cells in the posterior part of the lens vesicle form long, slender, anteroposteriorly oriented primary lens fibres. However, the secondary lens fibers form most of the mature lens and these fibers originate from the cells in the anterior part of the vesicle, which develop into a simple epithelium on the face of the lens (Schoenwolf et al., 2009).

Now that the optic cup has two walls, the inner wall (the former optic disc) forms the neural retina, which fully differentiates between the 6^th week and 8^th month, while the outer wall of the cup forms the thin, melanin-containing pigmented epithelium.

In the neural retina (inner wall of the cup), six neuronal cells and one glial cell are formed: the rods and cone photoreceptors are the outermost regarded as the outer nuclear layer; the middle layer contains ganglions; and the innermost layer contains the amacrine, horizontal, and bipolar cells termed as the inner nuclear layer. The axons from these cells convert the optic stalk to the optic nerve, which then passes to the brain as the 2^nd cranial nerve.

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The sheath of mesenchyme derived from the neural crest cells and cranial mesoderm encloses the developing optic vesicle. Two coverings are formed over the optic cup from the differentiation of the sheath presenting as: the outer fibrous sclera and the thin inner vascular choroid. Again, the mesenchyme lying over the developing lens divides into two layers enclosing a space known as the anterior chamber. The anterior chamber therefore has two walls: the inner wall covering the lens now called the pupillary membrane and the outer wall deep to the surface ectoderm forming the cornea. The deep part of the pupillary membrane undergoes vacuolization, creating a new space called the posterior chamber, between the lens and the thin remnant of the pupillary membrane. This membrane later breaks down completely to form the pupil. The rim of the optic cup differentiates to form the iris and ciliary body. The extrinsic ocular muscles are formed from the mesoderm adjacent to the optic cup, which differentiates between the 5th and 6th weeks. The connective tissue components of the extrinsic ocular muscles are derived from neural crest cells. The surface ectoderm folds to give rise to the eyelids which are fused together in the 8th week to about the 5th month (Schoenwolf et al., 2009).

2.2.6 Eyelids

Formation of the eyelids begins in the 6th week with small folds of surface ectoderm projecting together with a core of mesenchyme above and below the developing cornea. The upper eyelid therefore originates from the frontonasal process and the lower one from the maxillary process. The eyelid primordia grow rapidly to meet and fuse with each other in the 8^th week enclosing a space between them known as the conjunctival sac. During the 5^th and 7^th months, the eyelids separate again and, therefore, the conjunctival sac communicates freely

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with the amniotic fluid. The mesoderm enclosed by the folds of ectoderm that gave rise to the eyelids gives rise to the eyelid muscles (orbicularis and levator).

During the development of the eyelids, the deep ectodermal layer of the upper ectodermal fold invaginates at the superolateral angles of the conjunctival sacs to form the lacrimal glands, which mature at about 6 weeks after birth. The lubrication of the cornea and the conjunctival sac is done by the tear fluid produced by these lacrimal glands and any extra tear fluid passes to the nasal cavity through the nasolacrimal duct (Schoenwolf et al., 2009).