The arrow spans the entire vessel wall.
Adipose tissue surrounding the aorta is indicated.
More information on the vein embedded in the adipose tissue is presented below. Large vein next to aorta on slide 12. This image is a longitudinal section through a large vein . The vein is from the adipose tissue next to the aorta in the image above. Note that the thickness of the t. adventitia between the aqua arrows is much greater than the thickness of the t. media between the black arrows.