and Hirohiko Kakizaki2
(1)
Dacryology Service, L.V. Prasad Eye Institute, Banjara Hills, Hyderabad, 500034, India
(2)
Department of Ophthalmology, Aichi Medical University, Aichi, Japan
Introduction
The understanding of lacrimal embryology is very crucial to the understanding of lacrimal anatomy and its subsequent surgical applications. In addition, numerous congenital anomalies of the lacrimal system and their appropriate management largely depend on a sound knowledge of evolution. A thorough insight of lacrimal embryology is essential for advancing this science in terms of fundamental reasoning and developing minimally invasive interventions.
The human embryonic period generally covers the first 8 weeks postovulation, after which the embryo is called the “fetus” [1]. The moment when an embryo transforms into a fetus is not clearly determined, though [2] main parts of the human body are formed simultaneously during the embryonic period, and the lacrimal system is roughly completed by the first 10 weeks postovulation [2]. The structure itself does not change largely after that. The lacrimal drainage system can be broadly divided into embryonic and fetal developments for a lucid understanding.
Lacrimal Drainage System Development During Embryogenesis
The lacrimal passages develop along the line of cleft between the maxillary process and the lateral nasal process. From its inception, the maxillary process grows much rapidly in comparison with the lateral nasal process and subsequently overlaps the paraxial region around the eye, leading to formation of a fold of ectoderm between the processes (Fig. 2.1) [1, 2].
Fig. 2.1
Schematic diagram showing the development of lacrimal system between the maxillary and fronto-nasal process (Photo Courtesy: Dr Himika Gupta)
Embryonic development is estimated with the help of Carnegie stages [3]. Carnegie stages have been named after Carnegie Institute of Washington, which began collecting and classifying embryos in the early 1900s. The Carnegie stages divide the human embryonic period to 23 stages [3]. Criteria beyond morphological features include range of age in days, number of somites present, and embryonic crown rump lengths (CRL) [3].
The development of the lacrimal system begins at Carnegie Stage 16 (CRL: 11 mm), when an epithelial thickening of the lacrimal groove forms the lacrimal lamina [4]. At Carnegie Stage 19 (CRL: 17 mm), the lacrimal lamina separates from the surface ectoderm and forms the lacrimal cord [4]. The lateral extreme of the cord closest to the surface ectoderm bifurcates, thus giving rise to the canaliculi (Fig. 2.2) [4]. At Carnegie Stage 20 (CRL: 19–21 mm), the lacrimal cord is arranged lateral to the nasal capsule and finally lateral and inferior to the inferior meatal lamina [4]. At Carnegie Stage 22 (CRL: 26 mm), the proximal portion of the lacrimal system is perfectly differentiated, although it does not have a lumen as yet. The surrounding mesenchyme starts condensing [4]. The cells of the lacrimal cord condense at its periphery but are more loosely organized as we explore the interiors toward the future lumen [4]. At the end of the embryonic period (Carnegie Stage 23, CRL: 27–28 mm), morphology of the lacrimal system is well developed [4]. The lateral portion of the lacrimal system is clearly differentiated into the superior and inferior lacrimal canaliculus proximally and the lacrimal sac distally [4]. The canaliculi are close to the conjunctiva [5]. The medial portion of the lacrimal cord continues caudal and lateral to the inferior meatal lamina although the epithelia have not yet joined [4].
Fig. 2.2
Schematic diagram showing the outbudding of solid canaliculi from the lacrimal cord (Photo Courtesy: Dr Himika Gupta)
Lacrimal System Development During Fetal Period
From the 10th week (CRL: 48–55 mm), various significant changes occur such as canalization of the lacrimal cord and development of the surrounding tissues (Fig. 2.3) [4, 5]. Canalization occurs at the same time throughout the nasolacrimal apparatus [5]. The canalicular epithelium comes in contact with the palpebral conjunctival epithelium and both epithelia form a continuous epithelial lamina [4]. The caudal extreme of the lacrimal duct and the inferior meatal lamina makes contact and the latter begins to cavitate [1, 2, 4]. Central cells toward the lumen possibly undergo apoptosis and subsequently degenerate and shed off leaving a clear lumen behind. Muscular fibers of the Horner’s muscle are observed to surround the lacrimal canaliculi and mesenchymal tissue is interposed between the canaliculi and the muscle fibers [1, 2, 4, 5]. During the 12th week of development, reabsorption of the inferior meatal lamina is clearly visible (CRL: 74 mm). After the 13th week of development (CRL: around 85–90 mm), the surrounding tissues of the lacrimal apparatus such as ligament and tendon are clearly formed [4].
Fig. 2.3
Schematic diagram showing the process of canalization (Photo Courtesy: Dr Himika Gupta)
Although the canalicular lumina become patent by the 4th month after gestation, the lacrimal puncta do not open onto the eyelid margins until the eyelids separate during the 7th month [1, 2]. However, the lower end of the duct is often separated from the inferior meatus at birth by a membrane constituted by the apposed mucosal linings of the lower ductal end and the nasal fossa. Only in 30 % is the lowermost end patent at birth [1, 2]. An obstruction at this site balloons out later into the inferior meatus and its opening mostly occurs after birth [1, 2].
Clinco-Embryological Correlations
Position of the Puncta
The inferior punctum lies 0.5–1.0 mm more temporally than the superior one, so that they do not superimpose during eyelid closure [1, 2]. This anatomy has embryological explanations and results because of a relative rapid growth of the maxilla compared to that of the frontal bone [1, 2]. The lacrimal caruncle has been shown to be in close relation to the lower eyelid developmentally and its supero-temporal margin smoothly continues in level with the lower eyelid margin [6], and hence is a reasonable guide to lead to and judge a normal punctal position.
Ectopic Canaliculus and Caruncle
Punctal Agenesis
The basic etiopathogenesis of punctal agenesis is likely to be failure of canaliculi outbudding from the upper end of the solid lacrimal cord in an embryo of 18–24 mm (Fig. 2.5) [1, 2]. Punctal agenesis has important associated ocular and systemic associations. Lyons et al. [8] found 23 % of their cases (n = 57) to have ocular abnormalities like lacrimal fistula, blepharitis, distichiasis, eyelid tags, absence of caruncle, and divergent strabismus. Punctal agenesis has well-known association with systemic syndromes like ectodermal dysplasia [1, 9], Hay-Wells [9], and Levy-Hollister syndromes [10].
