Page 51 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 33

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1.44 Apical cell surface with cilia (schematic). 1.45 Apical cell surface with cilia and basal bodies.
Epithelial cell of oviduct, sheep (x24,000).

their number and appearance are usually correlated with The basal body consists of a ring of nine microtubule
the absorptive capacity of the cell. In cell types exhibiting triplets. Two microtubules of each triplet are continuous
little transcellular transport they may be short and irregu- with the nine peripheral microtubule pairs of the axo-
lar. In others, they are more uniform. On the surface of neme. The two central microtubules of the axoneme arise
cells that function primarily in trans-epithelial fluid trans- from the top of the basal body. Protein complexes inter-
port (e.g. in the intestine and renal tubules), microvilli connect the microtubules of the axoneme, contributing to
form a tightly packed brush border that is visible with the the stability and motility of the cilium.
light microscope. The microtubule doublets of the axoneme consist of
Microvilli contain actin filaments that are stabilised two components, A and B microtubules. The A micro-
and interconnected by fimbrin, fascin, espin and myosin tubule is composed of 13 tubulin subunits, arranged
I. At the tip of the microvillus, the filaments are secured side-by-side and a B microtubule containing ten tubulin
to the actin-bundling protein villin. The base of the actin subunits. The A and B microtubules are connected, at
filament bundle is linked to a horizontal network of fila- intervals of approximately 90 nm, by a passive elastic com-
ments (terminal web) that is connected by spectrin to the ponent referred to as nexin.
plasmalemma. Contraction of the terminal web (enabled At intervals of 24 nm, each doublet is associated with
by the presence of myosin II and tropomyosin) causes the a pair of ‘arms’ composed of ciliary dynein. The radial
microvilli to diverge. spokes connecting the nine doublets with the two central
microtubules are located at 29 nm intervals.
Cilia (kinocilia) Cilia exhibit a synchronously undulating motion, dur-
The term cilium is generally used to refer to a motile cell ing which the dynein arms of the microtubule doublet
process, though non-motile forms also exist. Cilia measure form temporary cross-links with the B microtubule of an
up to 10 μm in length. In the tracheobronchial tree, they adjacent doublet. Using energy derived from the hydrolysis
mobilise mucus and transport individual foreign particles of ATP, these dynein bridges move smoothly along the B
towards the pharynx. The cilia of the oviduct transport flu- microtubule. During this process, the passive elastic con-
ids and ova in the direction of the uterus (Figures 1.28 to nections involving nexin and the radial spokes accumulate
1.30, 1.44 and 1.45) (see also ‘Cell contractility and motility’). energy that is used for the subsequent reverse movement.
Cilia contain an axoneme composed of two central micro- Cilia are found on many cells of the body. Their prin-
tubules and nine peripheral microtubule doublets (9×2+2) cipal function is to transport fluids and solid materials,
connected by radial protein spokes (Figures 1.30 and 1.44). including mucus, dust and dead cells, along the cell sur-
At the base of the cilium, the nine microtubule doublets are face. Accordingly, cilia are abundant in the epithelium of
connected to a modified centriole, or basal body (kineto- the respiratory tract and the oviduct. A modified cilium
some). Similar in structure to a centriole, the basal body (flagellum) found on spermatozoa is responsible for the
coordinates the uniform, rhythmic movement of the cilia. forward movement of the cell.

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