Page 65 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Epithelial tissue (textus epithelialis) 47

in which the orientation of filaments reflects the prevail- TRANSITIONAL EPITHELIUM (EPITHELIUM
VetBooks.ir ing forces of pressure and tension. This contributes to the Transitional epithelium is a specialised form of stratified
TRANSITIONALE)
structural integrity of the epithelium.
By the time cells reach the stratum granulosum they epithelium. Its deepest (basal) layer consists of cuboidal
are considerably flattened. Three to five layers of cells are to columnar cells that are connected to the basal lamina.
present in this layer, depending on the degree of kerati- These cells are overlaid by irregular ‘mushroom-shaped’
nisation. The cells are characterised by the accumulation cells that become rounder towards the surface, forming
of strongly basophilic granules containing keratohyalin (a caps over the deeper cell layers. The cytoplasmic processes
histidine-rich protein), the appearance of lamellar bodies, of these more superficial cells are not necessarily in contact
a high density of tonofilaments and early degradation of with the basal lamina. Transitional epithelium is found in
the nucleus and other organelles. These morphological the excretory passages of the urinary system (renal pelvis,
features represent the commencement of the keratinisa- ureter, bladder, urethra). As these structures are subjected
tion process (see Chapter 15, ‘Common integument’). to constant changes in pressure and distension, the epithe-
The stratum lucidum is a highly refractive layer in lium can undergo rapid changes in height (Figures 2–18
which the organelles barely take up any stain. The cyto- to 2.21).
plasm is comprised of compacted bundles of filaments in
a homogeneous eosinophilic matrix. A stratum lucidum is Glandular epithelium (epithelium glandulare)
found primarily in areas where the epidermis is thickened. Epithelial tissue involved in the synthesis and secretion
It serves to shield the deeper epithelial layers from exter- of specific substances (secreta) is termed glandular epi-
nal influences (e.g. noxious agents) and to protect against thelium. It is composed of a group of individual secretory
dehydration of the epidermis and loss of body fluid. cells. The secretory product is usually created in the form
The process of keratinisation is completed in the stra- of granules that are released from the cell by various
tum corneum. Features associated with this layer include mechanisms. When a number of secretory cells combine
a thickening of the cell membrane, the dissolution of to form an organ, the resulting tissue is referred to as a
the nucleus and other organelles and the formation of gland (e.g. salivary gland, lacrimal gland).
keratin (a type of scleroprotein) from keratohyalin and Glands can be classified according to various criteria,
tonofilaments. including their location, the way in which their secretory
The deeper layers of the epidermis (stratum basale, spi- product is distributed, the number of cells, their morphol-
nosum and granulosum) are nourished by diffusion and ogy, the mechanism by which their secretory product is
are pervaded by free nerve endings. released and the composition of the secretion (Figures 2.22
and 2.23). Based on the distribution of secretory product,
there are two main types of glands (Figure 2.24):

• endocrine glands and
• exocrine glands.

2.18 Bladder (dog). Transitional epithelium lines the excretory passages of the urinary system, including the
renal pelvis (peripheral portion), the ureters, the bladder and the urethra. Azan stain (x100).

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