Kidney & Urinary bladder

The kidneys and urinary bladder are essential components of the urinary system, responsible for filtering blood, regulating fluid and electrolyte balance, and storing urine before elimination. Although these functions are conserved across mammals, differences in kidney anatomy and urine-concentrating capacity reflect species-specific physiological adaptations. Humans and pigs have multipapillary kidneys containing several renal pyramids, whereas macaques, rats and mice typically have unipapillary kidneys with a single main renal papilla.

Kidney and urinary bladder samples are available for all four mammalian species. To provide greater anatomical resolution, the kidney was separated into cortex and medulla during smapling in macaque, pig, rat and mouse. Comparable to the human RNA data from GTEx. The separate cortex and medulla samples can be explored at the individual sample level, but are combined into one main tissue type, kidney, for the tissue specificity analysis. The urinary bladder is analysed as a separate tissue type.

Figure 1. Tissue sample overview and the hierarchy of grouped tissues representing kidney and urinary bladder

Comparative transcriptomics of the mammalian kidney & urinary bladder

The transcriptomic data is used to identify genes with elevated expression in individual tissues or groups of tissues, classified into three categories:

Tissue enriched: Genes with elevated expression in a single tissue compared with all other tissues.
Group enriched: Genes with elevated expression in a limited group of tissues compared with the remaining tissues.
Tissue enhanced: Genes with elevated expression in one or more tissues compared with the average expression across tissues (gap ratio ≥2)

Tissue specificity is evaluated at two levels: first, within each species using all available tissue types, and second, using only tissues that can be directly compared between the species and human. This distinction makes it possible to explore both tissue-specific expression within each species and the conservation of expression patterns across evolution.

The results can be explored for individual genes on the gene summary pages or searched to identify genes with specific expression patterns. To assess evolutionary conservation, pairwise overlap analysis compares each species individually with human, identifying genes that share the same tissue specificity classification in corresponding tissues and quantifying the extent of overlap between species.

Tissue	Species	Elevated	Human elevated overlap
Tissue	Species	Elevated	Complete overlap	Partial overlap
Kidney	Macaque	340	95	95
	Mouse	325	68	78
	Rat	361	71	92
	Pig	327	80	89
Urinary bladder	Macaque	41	2	4
	Mouse	70	3	12
	Rat	65	3	7
	Pig	44	3	10

Kidney

Kidney cortex

The kidney cortex contains glomeruli responsible for blood filtration, together with proximal and distal tubular segments involved in reabsorption and secretion. The basic nephron organization is conserved across mammals, but nephron number, glomerular size and the relative abundance of different tubular structures vary between species, potentially affecting bulk tissue expression profiles.

Kidney medulla

The kidney medulla contains loops of Henle and collecting ducts that establish and maintain the osmotic gradients required for urine concentration. Kidney architecture differs between species, with humans and pigs having multipapillary kidneys containing several renal pyramids, whereas macaques, rats and mice typically have unipapillary kidneys with a single main renal papilla. Rodents also generally have a relatively prominent medulla and a strong ability to concentrate urine. These anatomical and physiological differences may influence tissue-level expression profiles, particularly for genes involved in water transport, ion regulation and urine concentration.

Species coverage

Urinary bladder

The urinary bladder is lined by a specialized urothelium that forms a highly impermeable barrier against urine and can expand during filling. Although urothelial organization is broadly conserved, differences in epithelial thickness, bladder capacity and the relative proportions of smooth muscle and connective tissue may influence gene expression across species.