Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 94

The TM is a complex, three‐dimensional structure comprising cells supported by an intricate ECM. The TM can be divided into three portions: uveal, the innermost portion; corneoscleral, the middle region; and the juxtacanalicular zone, the outermost section nearest the sclera (probably the site of greatest outflow resistance). The pore size of each TM zone decreases progressively from inward to outward, resulting in progressive increases in outflow resistance to the passage of AH. The juxtacanalicular zone consists of several endothelial cell layers that produce a matrix comprising GAGs, collagen, fibronectin, and other glycoproteins in which these cells are embedded. Thus, the juxtacanalicular zone, which is immediately adjacent to Schlemm's canal in primates or the angular aqueous plexus (AAP) in most domestic animals, is thought to be the major site of aqueous outflow resistance.

Figure 2.7 Chemical composition of the aqueous humor and lens. Water and protein are expressed as percentages of lens weight. Na⁺, Cl⁻, K⁺, and Ca²⁺ ions are expressed in microequivalent per milliliter of lens water. Other compounds are expressed in micromole per gram of lens weight or micromole per milliliter of aqueous humor. AA, amino acid; RNA, ribonucleic acid.

Table 2.6 Estimates of aqueous humor dynamics in selected species.

	Dog	Cat	Rabbit	Cow	Horse	Nonhuman primate
Estimated normal IOP (mmHg)	15–18	17–19	15–20	20–30	17–28	13–15
“C” outflow (μl/mmHg/min by tonography)	0.24–0.30	0.27–0.32	0.22–0.28	—	0.90	0.24–0.28
Uveoscleral outflow (μl/min)	15%	3%	13%	—	—	30–65%
Episcleral venous pressure (mmHg)	10–12	8	9	—	—	10–11
Aqueous formation (μl/min)	5.22	6.00–7.00	1.84	—	—	2.75

Other studies suggest that the main site of resistance to outflow is the endothelial lining of the AAP and its ECM. However, the site of filtration may be different from the site of flow resistance. AH transport through the endothelium of the AAP (or Schlemm's canal in nonhuman primates and domestic chickens) is thought to occur via transcellular pores, large vacuoles, or pinocytotic vesicles. However, paracellular routes between the endothelial cells of Schlemm's canal have also been proposed and may be pressure sensitive, particularly at higher IOPs.