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Urinary system (organa urinaria) 273

glomerulus. These cells are in contact with the vascular The tunica muscularis comprises smooth muscle. Some
VetBooks.ir endothelium and the macula densa. Juxtaglomerular cells degree of layering is evident. A sparse inner longitudi-
are characterised by myosin filaments and dense granules nal layer is surrounded by a thin layer of circular fibres.
containing renin. Release of the contents of these granules Externally, occasional longitudinal fibres are seen.
initiates the renin–angiotensin system. The muscle of the renal pelvis is continuous with that
of the ureter. Specialised smooth muscle cells act as a pace-
Extraglomerular mesangial cells maker, resulting in peristaltic transport of urine in the pelvis
Extraglomerular mesangial cells (Goormaghtigh cells, lacis and ureter. Deep in the renal sinus, the tunica adventitia is
cells) lie between the macula densa and the afferent and thin and contains nerves, adipose tissue and blood vessels.
efferent arterioles. They are continuous with the intraglo- Towards the hilus it becomes considerably more substantial.
merular mesangium. The function of the extraglomerular
mesangial cells is unclear. It has been proposed that they Species variation
act as reserve cells for juxtaglomerular cells. Carnivores and ruminants: The tunica adventitia con-
tains smooth muscle fibres.
Interstitium
The renal interstitium fills the spaces between the renal Ureter
tubules and collecting ducts. It is composed of modified Aided by a well-developed layer of smooth muscle, the
fibrocytes, relatively few collagen fibrils and an extensive ureter conveys urine that has accumulated in the pelvis to
matrix containing proteoglycans. There is evidence that the bladder. The tunica mucosa is lined with transitional
interstitial fibroblasts produce erythropoietin, though the epithelium. In the non-distended ureter, longitudinal folds
significance of this phenomenon under normal physiologi- are visible in the mucosa (Figure 12.27). The lumen thus
cal conditions has been questioned. Prostaglandins and appears stellate in histological sections. The lamina propria
bradykinin are also released by cells of the interstitium. is loose and typically devoid of glands.
These substances regulate renal perfusion and thereby
exert an influence on the systemic circulation. Species variation
Interstitial cells also product thrombopoietin and renin. Equids: The mucosa contains elongated mucous glands
(glandulae uretericae) that extend up to 10 cm distal to
Urinary passages the renal pelvis (Figure 12.28). These are responsible
for the characteristic viscous, stringy consistency of
Renal pelvis (pelvis renalis) equine urine.
The renal pelvis can be considered as a proximal expansion
of the ureter that forms a funnel around the renal papilla As in the renal pelvis, the tunica muscularis has inner
(crest). The visceral, inner layer forms the epithelial lining and outer longitudinal layers separated by a circular layer.
of the renal papilla. It consists of a variable number of These are joined by obliquely oriented fibres to form a sin-
cell layers. gle functional unit.
Species variation
Species variation
Carnivores and small ruminants: Narrow recesses Cat: The inner muscle layer is absent.
(recessus pelvis) extend from the main portion of the
pelvis. The epithelium of these recesses, and that of the Near the ureteric orifices, the longitudinal muscle fibres
pseudopapillae, is simple cuboidal.
fan out into the bladder forming the muscular core of the
Equids: The terminal recesses (recessus terminales) of trigone. The ureters open into the bladder obliquely. This
the pelvis are lined with stratified epithelium. arrangement results in compression of the ureteric lumen
as the bladder fills. The ureters are considered to contain an
Peripherally, the epithelium of the medullary tissue autonomous impulse generation system.
transforms into the transitional epithelium (epithelium The external surface of the ureter is covered with a
transitionale) of the outer layer of the pelvis. Transitional tunica adventitia or a tunica serosa, depending on its
epithelium lines the urinary passages as far as the external topographic anatomical relationships.
urethral orifice. Due to its specialised structure, this type of
epithelium (see Chapter 2, ‘Epithelial tissue’) serves as a pro- Species variation
tective barrier against the hypertonic urine within the urinary Birds: The ureter emerges from the ventral aspect of
passages. The underlying lamina propria consists of loose the kidney in the region of the middle renal division.
connective tissue. In the horse, the lamina propria contains The ureter of the chicken is formed from the union of
mucous tubulo-acinar glands (glandulae pelvis renalis). approximately 17 primary branches, each of which drains

Vet Histology.indb 273 16/07/2019 15:04