Page 293 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Urinary system (organa urinaria) 275

vesicles (crusta). The lamina propria consists of a connec- The mucosa is lined by transitional epithelium,
VetBooks.ir tive tissue lattice containing delicate elastic fibres. Together becoming pseudostratified towards the external urethral
with the submucosa, the lamina propria forms a mobile, orifice. Isolated areas of cuboidal or columnar cells may be
displaceable layer that facilitates the alternate folding and observed. The epithelial cells may contain mucus.
stretching of the bladder wall. The lamina propria and
submucosa are typically separated by a lamina muscula- Species variation
ris mucosae. Bundles of elastic fibres provide mechanical Pig and horse: The epithelium contains goblet cells.
reinforcement at the neck of the bladder. Lymphoid folli- Sheep and horse: Near the external urethral orifice, the
cles may be present in the lamina propria. A dense network epithelium changes to stratified squamous.
of capillaries, concentrated in the subepithelial region, is
particularly extensive in ruminants. The epithelium rests upon a dense connective tissue
The tunica muscularis consists of relatively thick layers layer. This is extensively vascularised, particularly in the
of smooth muscle (longitudinal – circular – longitudinal) ox. The lamina propria contains cavernous spaces, the
comprising interwoven spirally oriented fibres (Figure extent of which varies with species.
12.30).
Species variation
Species variation Cat and sheep: Cavernous spaces are lacking in the
Horse and pig: The outer longitudinal layer may be initial portion of the urethra. In other species, cavern-
lacking. ous tissue is present throughout the urethra, becoming
more prominent near the opening into the vestibule.
The boundary between the muscle layers is sometimes
indicated by a vascular plexus. The spiral arrangement of Occasional smooth muscle cells are present in the
muscle fibres facilitates sustained, uniform contraction of subepithelial connective tissue. The tunica muscularis
the bladder wall. Near the vertex and neck of the bladder, consists of inner circular and outer longitudinal layers. In
the muscle fibres from tight loops, though a functional the horse, the oblique orientation of muscle fibres may
sphincter is not formed. give the appearance of a third layer. At the opening into
The bladder wall is innervated by an autonomic plexus the genital tract, the smooth muscle cells intermingle with
with ganglion cells located between muscle layers (intra- skeletal muscle fibres (m. sphincter urethrae).
mural ganglia). Sympathetic and parasympathetic nerve
fibres regulate bladder motility and contraction of vessel Male urethra
walls. The mucosa also contains sensory fibres. The male urethra is closely connected, both structurally
and functionally, with the genital organs and is discussed
Urethra in Chapter 13, ‘Male reproductive system’.

Female urethra
The female urethra extends from the neck of the bladder to
the external urethral orifice. It consists of a tunica mucosa,
submucosa, tunica muscularis and a tunica adventitia.

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