UNASSIGNED: According to the classical ladder theory, the mesonephric arteries (MAs) have a segmental arrangement and persist after regression of the mesonephros, with some of these vessels becoming @entity8 . To avoid interruption of blood flow, such a @entity455 would require an intermediate stage in which two or more segmental MAs are connected to a @entity8 . To examine developmental changes, especially changes in the segmental distribution of MAs, we studied serial paraffin sections of 26 @entity1 embryos (approximately 5-7 weeks). At 5-6 weeks, 1-2 pairs of MAs ran anterolaterally or laterally within each of the lower thoracic vertebral segments, while 2-5 pairs of MAs were present in each of the lumbar vertebral segments, but they were usually asymmetrical. The initial metanephros, extending along the aorta from the first lumbar to first sacral vertebra, had no arterial supply despite the presence of multiple MAs running immediately anterior to it. Depending on increased sizes of the adrenal and metanephros, the MAs were reduced in number and restricted in levels from the twelfth thoracic to the second lumbar vertebra. The elimination of MAs first became evident at a level of the major, inferior parts of the metanephros. Therefore, a hypothetical arterial ladder was lost before development of glomeruli in the metanephros. At 7 weeks, after complete elimination of MAs, a pair of symmetrical renal arteries appeared near the superior end of the metanephros. In conclusion, the MAs appear not to persist to become a @entity8 .

Fetal development of the mesonephric artery in XXXX with reference to replacement by the adrenal and renal arteries.

@entity1 :: ('9606', 'Species') :: ['human'] @entity8 :: ('MESH:D007674', 'Disease') :: ['definitive renal arteries', 'definitive renal artery'] @entity455 :: ('MESH:D014652', 'Disease') :: ['vascular switching']
@entity1 :: (9606,Species) :: ['humans']
