Transport of exogenous albumin across lymphatic endothelium--an ultrastructural study.

The pathways involved in protein transport across the lymphatic endothelium of the rat renal cortex after in vivo drip fixation were studied ultrastructurally. Both qualitative and quantitative analyses were made on tissues from physiological saline-injected rats and on tissues from rats injected intravenously with rat albumin (0.2 mg/g body wt., 0.4 mg/g body wt. or 0.8 mg/g body wt.). The volume density values of intracytoplasmic vesicles were as follows: (1) saline-injected rats: 0.024, (2) 0.2 mg/g body wt. albumin-injected rats: 0.029 (p less than 0.05), (3) 0.4 mg/g body wt. albumin-injected rats increased to 0.033 (p less than 0.01), (4) 0.8 mg/g body wt. albumin-injected rats had a value of 0.022. The numerical density of intracytoplasmic vesicles increased from 27/micrometers 3 after saline injection to 35/micrometers 3 (p less than 0.05) after 0.2 mg/g body wt. albumin injection. When rats were injected with 0.4 mg/g body wt. albumin, the numerical density was 44/micrometers 3 (p less than 0.01, in comparison with saline-injected rats) but this value decreased to 27/micrometers 3 in rats injected with 0.8 mg/g body wt. albumin. In the four groups, the range of vesicles was 28-38% opening into the lymphatic lumen and 7-16% of vesicles opening into the interstitial space. The remaining vesicles were free in the endothelial cytoplasm. There were more open junctions and wider cell junctions in the 0.8 mg/g body wt. albumin-injected group. It is concluded that normally albumin molecules are transported into the lymphatic capillaries by intracytoplasmic vesicles or through the normal interstitial space between endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)