Evaluation of the Developmental Morphology of the Eyeball of the Red Sokoto Goat (Capra Hircus)

Abstract

This study traced the developmental histology of the eyeball of the Red Sokoto goat during the prenatal and postnatal stages. Thirty Red Sokoto goat fetuses, 10 neonatal and 10 adult goat heads obtained from the abattoir were used for this study. The fetuses were aged using the standard formula while those of the adult goats were estimated by dentition. The eyes were dissected from the head and assigned to five groups (A-E). Eye samples were routinely processed for histological studies. Sections were examined with a light microscope and photographed with a Moticam digital camera attached to a microscope. Corneal thickness was measured in all groups and data obtained were analysed using analysis of variance. Variant means were separated using the least significant method (LSD) and significant difference was accepted at probability level (p < 0.05). The corneal primordium was present in the eyes of fetuses in group A. The anterior and posterior epithelia were of the simple cuboidal type. The corneal epithelium became bilayered while the primordium of the limbus and the sclera appeared. In group B, the Descemet’s membrane was visible. All the layers of the sclera except the lamina fusca were fully differentiated. The differentiating lamina cribrosa appeared at this stage. In group C, the Bowman’s layer was observed. The limbus, lamina cribrosa and sclera were well differentiated with the entire scleral layers present. In group D, the corneal epithelium had more cellular layers and pigments were observed in limbus. In group E, the corneal epithelium was of the non keratinized stratified squamous type. The Descemet’s membrane was thick. Apart from the cornea, all other regions of the fibrous coat had more melanocytes and pigments in group E. The central and peripheral corneal thicknesses of the adults (242.29 ± 23.95; 179.34 ± 8.37) were significantly higher (p < 0.05) than those of the fetuses of 2nd (68.50 ± 7.14; 71.20 ± 6.93), 3rd (90.47 ± 8.85; 68.35 ± 4.73) and 4th (81.71 ± 14.11; 88.87 ± 15.26) months of gestation and also that of the neonate (91.61 ± 15.81; 90.99 ± 9.08). In the eyes of group A, the iris was present as well as the iridopupilary membrane. The annular sinusoid was also present, choroidal differentiation was also initiated. Also pigment cells appeared in the stroma of the iris along with the dilator pupilary muscle myocytes. The ciliary body appeared as ciliary folds covered by a bilayered epithelium. The formation of the trabecular meshwork had been initiated while the iridocorneal angle was still closed. The angular aqueous plexus was seen at this stage. The choroid also started differentiating. In group B, the constrictor pupillary muscle had appeared while the iridopupillary membrane had started regressing. The granula iridica also had started forming and the pectinate ligament appeared. The formation of the tapetum fibrosum was initiated at this stage. In group C, the ciliary folds formed the ciliary processes. The iridocorneal angle opened at this stage with the dorsal angle opening before the ventral one. In group D, the iridopupilary membrane had broken down and the anterior iridal surface was thrown into folds. The ciliary cleft was also formed at this stage. In group E, the crypts of Fuschs were seen on the anterior iridal surface. Few radially and circularly oriented muscle fibers were also seen in the ciliary body and the suprachoroidal space also appeared. The retinal primordium was present in the eyes of group A as well as the commencement of differentiation into retinal layers and cell types. The retinal cell types and layers appeared in the eyes of group B. The physiologic cup appeared in the eyes of group C at the optic nerve head. In group E, the rows of cells in the outer nuclear layer and inner nuclear layer of the retina were fewer than those in group D. Glial cells were seen at the optic nerve head at this stage. In group A, the lens capsule was present. The lens epithelium was of the pseudostratified and simple columnar types centrally and equatorially respectively. This changed to simple cuboidal and simple columnar centrally and equatorially in groups C and D. In group E, the lens epithelium was made up of simple cuboidal type centrally and simple columnar type at the equator. Hyaloid vessels were present in group A, became smaller and sparse in group C, few were seen on the posterior surface of the lens and equator in group D and they were absent in group E. The development of the nervous and vascular layers progressed posterior-anteriorly while that of the fibrous layer progressed anterior-posteriorly. Also the pigmentation of the different parts of the eye increased with fetal growth and was highest in the adult. This study has shown that by late 3rd trimester, the RSG eyeball has acquired a full complement of the nervous, vascular and fibrous layers along with other structures necessary for vision. Therefore it essentially had the same morphology as in the adult and appeared capable of carrying out the function ascribed to the eyeball in the adult.