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ASSESSMENT OF OCULAR ANATOMY OF THE MALE AFRICAN GRASSCUTTER (Thryonomys swinderianus)
ABSTRACT
Ophthalmology is the branch of science that deals with the study of the eye and its diseases. In recent times, the study of animal eyes has become necessary as they are frequently used in human ocular research and for therapeutic purposes in man. Knowledge of the ocular anatomy of a species is essential for understanding of its visual characteristics and in clinical management of its ocular pathologies. The African Grasscutter (Thryonomysswinderianus) is a herbivorous nocturnal rodent of the family thryonomidae whose generally acceptable and preferred meat has led to its vigorous hunting in the wild and consequent intensive domestication. The species has also generated a lot of scientific interest as seen in the numerous published scientific works elucidating its various features. However, literature regarding its ophthalmic characteristics is scarce. This study assessed the gross anatomic, histologic and morphometric ocular characteristics of the grasscutter using gross observations, morphometric measurements, and light microscopic methods. Gross observations and morphometry were made on the eye and orbit, and enucleated eyes were fixed and histologically processed for light microscopic studies. The results of the study showed that the external features of the grasscutter’s eye comprised a fibrous tunic and optic nerve surrounded by extraocular muscles, conjunctiva, upper and lower eyelids, with orbital fascia. They were located inside the bony orbit. Ocular features exhibited by the animal include laterally-facing orbit, large corneal size relative to globe size, high density of retinal rod photoreceptor cells and very low density of retinal ganglion cells. The orbit was shaped like a trapezoid-based pyramid with a complete bony orbital rim. Histology of the cornea comprised an anterior epithelium, middle stroma, and posterior epithelium, while uveal histology comprised richly pigmented and vascularized choroid, ciliary body, and iris. Tapetum lucidum appeared not to be present. Ocular morphometric parameters obtained include eye weight (0.47 ± 0.14 g), vertical corneal diameter (0.71 ± 0.05 cm), horizontal corneal diameter (0.74 ± 0.08 cm), vertical globe diameter (0.90 ± 0.09 cm), horizontal globe diameter (0.93 ± 0.07 cm), axial globe diameter (0.92 ± 0.08 cm), orbital volume (2.02 ± 0.94 cm3), orbital depth (1.84 ± 0.15 cm), oblique orbital diameter (1.87 ± 0.16 cm), orbital axes angle (202.30 ± 11.65º), and corneal thickness (257.5 ± 105.9 µm). The horizontal corneal diameter was found to be significantly greater (p < 0.05) than the vertical corneal diamete. The results showed that grasscutter vision is panoramic with wide monocular field and narrow binocular field necessary for survival in the wild against a wide range of predators. The large relative corneal size and preponderance of rod cells in the retina suggest adaptations for vision in the dark. Finally, the ocular morphometric parameters reported may aid in further research, diagnosis, and clinical management of ocular pathologies in this species and by extension, other species.
TABLE OF CONTENTS
TITLE PAGE ………………………………………………………………………………… i
CERTIFICATION ……………………………….……………………………………………. ii
DEDICATION ………………………………..…………………………………………….. iii
ACKNOWLEDGEMENT ………………….………………………………………………. iv
LIST OF TABLES ………………………….………………………………….…………….. v
LIST OF FIGURES ……………………….…………………………………..…..………… vi
TABLE OF CONTENTS ………………………………………………………………….. viii
ABSTRACT ………………………………………………………………………………… xii
CHAPTER ONEi
INTRODUCTION.. 1
1.1 BACKGROUND OF THE RESEARCH.. 1
1.2 STATEMENT OF RESEARCH PROBLEMS. 3
1.3 OBJECTIVES OF THE STUDY.. 3
CHAPTER TWO4
LITERATURE REVIEW… 4
2.1 DEVELOPMENTAL ANATOMY OF THE EYE.. 4
2.1.1 FORMATION OF THE OPTIC CUP. 4
2.1.2 FORMATION OF THE RETINA.. 5
2.1.3 FORMATION OF THE OPTIC NERVE.. 6
2.1.4 FORMATION OF THE LENS. 6
2.1.5 FORMATION OF STRUCTURES THAT ARISE FROM NEURAL CREST-DERIVED MESODERM… 7
2.1.6 FORMATION OF THE EYELIDS AND ASSOCIATED STRUCTURES. 8
2.2 ANATOMY OF THE EYE.. 9
2.2.1 THE TUNICS. 10
2.2.1.1 THE FIBROUS TUNIC.. 10
2.2.1.1.1 THE SCLERA.. 10
2.2.1.1.2 THE CORNEA.. 11
2.2.1.1.2.1 ANTERIOR EPITHELIUM… 11
2.2.1.1.2.2 SUBEPITHELIAL BASEMENT MEMBRANE.. 12
2.2.1.1.2.3 STROMA.. 12
2.2.1.1.2.4 DESCEMET’S MEMBRANE.. 13
2.2.1.1.2.5 POSTERIOR EPITHELIUM… 13
2.2.1.1.3 THE LIMBUS. 13
2.2.1.2 THE VASCULAR TUNIC.. 14
2.2.1.2.1 CHOROID.. 14
2.2.1.2.2 CILIARY BODY.. 16
2.2.1.2.3 IRIS. 18
2.2.1.3 THE NEUROEPITHELIAL TUNIC.. 19
2.2.1.3.1 RETINAL PIGMENT EPITHELIUM… 19
2.2.1.3.2 NEURAL RETINA.. 20
2.2.1.3.2.1 ROD AND CONE LAYER.. 21
2.2.1.3.2.2 EXTERNAL LIMITING MEMBRANE.. 22
2.2.1.3.2.3 OUTER NUCLEAR LAYER.. 22
2.2.1.3.2.4 OUTER PLEXIFORM LAYER.. 22
2.2.1.3.2.5 INNER NUCLEAR LAYER.. 22
2.2.1.3.2.6 INNER PLEXIFORM LAYER.. 24
2.2.1.3.2.7 GANGLION CELL LAYER.. 24
2.2.1.3.2.8 NERVE FIBER LAYER.. 24
2.2.1.3.2.9 INTERNAL LIMITING MEMBRANE.. 24
2.2.2 OPTIC NERVE.. 24
2.2.3 THE LENS. 25
2.2.3.1 LENS CAPSULE.. 25
2.2.3.2 LENS EPITHELIUM… 25
2.2.3.3 LENS FIBERS. 25
2.2.4 ACCESSORY STRUCTURES OF THE EYE.. 26
2.2.4.1 CONJUNCTIVA.. 26
2.2.4.2 NICTITATING MEMBRANE.. 27
2.2.4.3 UPPER AND LOWER EYELIDS. 27
2.2.4.4 LACRIMAL SYSTEM… 28
2.2.5 THE ORBIT. 29
CHAPTER THREE
MATERIALS AND METHODS. 31
3.1 EXPERIMENTAL ANIMALS. 31
3.2 MACROSCOPIC STUDIES. 31
3.3 HISTOLOGIC STUDIES. 32
3.4 DATA ANALYSIS. 33
CHAPTER FOUR
RESULTS. 34
4.1 GROSS MORPHOLOGIC FINDINGS. 34
4.1.1 EYELIDS. 34
4.1.2 CONJUNCTIVA.. 35
4.1.3 HARDERIAN GLAND.. 35
4.1.4 EYEBALL.. 35
4.1.5 FIBROUS TUNIC.. 35
4.1.6 UVEA.. 36
4.1.7 RETINA AND OPTIC NERVE.. 36
4.1.8 LENS. 36
4.1.9 ORBIT. 36
4.1.10 GROSS MORPHOMETRY.. 37
4.2 HISTOLOGIC FINDINGS. 37
4.2.1 FIBROUS TUNIC.. 37
4.2.1.1 SCLERA.. 37
4.2.1.2 CORNEA.. 38
4.2.1.3 LIMBUS. 38
4.2.2 UVEA.. 39
4.2.2.1 CHOROID.. 39
4.2.2.2 CILIARY BODY.. 39
4.2.2.3 IRIS. 40
4.2.3 NEUROEPITHELIAL TUNIC.. 40
4.2.3.1 RETINAL PIGMENT EPITHELIUM… 40
4.2.3.2 NEURAL RETINA.. 40
4.2.4 OPTIC NERVE.. 41
4.2.5 LENS. 41
4.2.6 HARDERIAN GLAND.. 41
4.2.7 EYELID.. 42
4.2.8 LACRIMAL GLAND.. 42
4.2.9 NICTITATING MEMBRANE.. 42
4.2.10 CORNEAL MORPHOMETRY.. 42
4.3 FIGURES ………………………………………………………………………….43
4.4 TABLES …..……………………………………………………………………….65
CHAPTER FIVE
DISCUSSION.. 66
5.1 FIBROUS TUNIC.. 66
5.2 UVEA.. 68
5.3 RETINA.. 69
5.4 EYELID.. 70
5.5 LACRIMAL GLAND.. 70
5.6 HARDERIAN GLAND.. 70
5.7 VISUAL FIELD.. 71
5.8 CONCLUSION.. 71
REFERENCES. 72
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