In semithin sagittal and serial frontal sections, the morphology of the TM and the aqueous humor outflow loops (equivalent of Schlemm's canal in the cow) of the 3-year-old Braford cows in principle resembled that of the 6-month-old calf eyes derived from the local abattoir.
15–17 In both groups the TM consisted of elastic and collagen fibers embedded in a ground substance. Bands of these fibers were incompletely separated from each other by trabecular cells, thereby forming a meshwork that was more reticular than lamellated. The subendothelial myofibroblast-like small elongated cells connecting the outflow loops with the ciliary muscle in the calf eyes (
Fig. 2A) were not seen in the Braford cow eyes. In addition, occasional accumulations of homogeneous plaques separating the endothelial cells from the first subendothelial layer (
Fig. 2B) were found at a few places in the Braford cow TM that were virtually absent in the calf eyes. These plaques were variable in amount interindividually and between the different loops of the same section.
At the electron microscopic level, the TM in both the calf eyes and the Braford cow eyes was connected to the outflow loops of the aqueous plexus by an elastic fiber net. From the sheath of the elastic fibers, fine fibrils spanned toward the endothelium of the loops (
Fig. 2C). In some areas, these fibrils were so numerous and densely packed that they had a plaque-like appearance (
Fig. 2C). At places of attachment, the endothelium formed dense band-like structures (
Fig. 2C). The connecting fibrils were more numerous in the Braford cow than in the calf eyes. The elastic fiber net was also continuous with that of the TM. The elastic fibers in the Braford cow eyes were thicker than in calf eyes and contained more electron light material within the osmiophilic core and a sheath of fine fibrillar material, nearly absent in the calf eyes (
Fig. 2D). Together with collagen fibers and ground substance, it formed the central core of the trabecular lamellae. The lamellae were incompletely covered by trabecular cells, and there was little basement membrane material adjacent to the cells (
Fig. 2D). Trabecular cells in the Braford cow eyes, as in the calf eyes, were connected to each other and contained rough endoplasmic reticulum, a well-developed Golgi apparatus, single mitochondria, and large amounts of granules that previously had been shown to be glycogen (
Fig. 2D).
17 In calf eyes the glycogen-rich TM cells were present up to the endothelium of the loop, whereas in Braford cow eyes the subendothelial TM cells rarely showed glycogen inclusions.