2.1
Respiratory System (including
the operculum and pseudobranch)
Salmonids have four gill arches bilaterally placed on each
side of the head, each supporting a holobranch with its two
hemibranchs, the double vertical rows of gill filaments
(Fig. 2.2). A series of cartilaginous or bony projections, the
gill rakers, protrude forwards from the pharyngeal margin of
the gill arch. These are relatively sparse in most salmonids
but show a wide variation of morphologies in different species, and may form a fine grid that helps filter planktonic
organisms from the water, at the same time preventing food
particles from entering the gill chamber. Each hemibranch
comprises a row of posterior-laterally oriented filaments with
its respiratory epithelium covered lamella on each side
(Fig. 2.3). Anatomically the filaments looks like a ‘feather’,
supporting on each side a continuous symmetrically-spaced
individual lamella. The filaments are supported along their
proximal half by an interbranchial septum of connective
and muscle tissues, but the septum is reduced to about a
third of the filament length or even absent in more advanced
fish. Each lamella consists of a supportive scaffold of pillar
cells among which blood supply enters and leaves the
lamella, which are covered by a thin double-layer epithelium
separated by a space in which migrating inflammatory cells
may be seen. The inner layer of the epithelium sits on a
basement membrane that traverses the opposing face of
the lamella in grooves located within the pillar cells, and in
this way provides additional tensile support. However,
the bulk of the respiratory epithelium obvious through light
microscopy, is the outer squamous layer that provides a
large and intimate interface with the water for exchange of
gases, acid–base regulation, osmoregulation and excretion of
nitrogenous waste products. Chloride and mucous cells,
normally found near the base of the lamellae, may also be
found distally under pathological conditions, especially the
mucous cells (Fig. 2.4). Chloride cells are highly rich in
Fig. 2.1 Sections to illustrate different magnifications with representative bar scales. (a) Bar scale ¼ 100 μm, Â20. (b) Bar scale ¼ 50 μm, Â40.
(c) Bar scale ¼ 20 μm, Â60. (d) Bar scale ¼ 20 μm, Â100
6
2 Functional Anatomy
Respiratory System (including
the operculum and pseudobranch)
Salmonids have four gill arches bilaterally placed on each
side of the head, each supporting a holobranch with its two
hemibranchs, the double vertical rows of gill filaments
(Fig. 2.2). A series of cartilaginous or bony projections, the
gill rakers, protrude forwards from the pharyngeal margin of
the gill arch. These are relatively sparse in most salmonids
but show a wide variation of morphologies in different species, and may form a fine grid that helps filter planktonic
organisms from the water, at the same time preventing food
particles from entering the gill chamber. Each hemibranch
comprises a row of posterior-laterally oriented filaments with
its respiratory epithelium covered lamella on each side
(Fig. 2.3). Anatomically the filaments looks like a ‘feather’,
supporting on each side a continuous symmetrically-spaced
individual lamella. The filaments are supported along their
proximal half by an interbranchial septum of connective
and muscle tissues, but the septum is reduced to about a
third of the filament length or even absent in more advanced
fish. Each lamella consists of a supportive scaffold of pillar
cells among which blood supply enters and leaves the
lamella, which are covered by a thin double-layer epithelium
separated by a space in which migrating inflammatory cells
may be seen. The inner layer of the epithelium sits on a
basement membrane that traverses the opposing face of
the lamella in grooves located within the pillar cells, and in
this way provides additional tensile support. However,
the bulk of the respiratory epithelium obvious through light
microscopy, is the outer squamous layer that provides a
large and intimate interface with the water for exchange of
gases, acid–base regulation, osmoregulation and excretion of
nitrogenous waste products. Chloride and mucous cells,
normally found near the base of the lamellae, may also be
found distally under pathological conditions, especially the
mucous cells (Fig. 2.4). Chloride cells are highly rich in
Fig. 2.1 Sections to illustrate different magnifications with representative bar scales. (a) Bar scale ¼ 100 μm, Â20. (b) Bar scale ¼ 50 μm, Â40.
(c) Bar scale ¼ 20 μm, Â60. (d) Bar scale ¼ 20 μm, Â100
6
2 Functional Anatomy