170
It was observed, that salt glands of sea birds are 10–100 times larger in size than
in non-marine species. The salt concentration of corresponding fl uids “varies
between species and increases as the glands hypertrophy in response to elevated salt
intake” (Suepaul et al. 2010 ; see also Schreiber and Burger 2001 ). The anatomical
description of sea bird salt glands have been described as follow:
“They are compound tubular glands with a well-developed lobular structure, and
a variable countercurrent arrangement of capillary blood fl ow relative to the fl ow of
secretion within tubules. This allows controlled excretion of salt ions from the
bloodstream into collecting ducts, which open into the nasal cavity,” (Suepaul et al.
2010 ; see also by Gerstberger and Gray 1993 ).
Fine structure, innervation and functional control of marine vertebrate’s salt
glands are well described (see for review Gerstberger and Gray 1993 ). The fi rst
detailed morphologic description of the avian salt gland at the electron microscopic
level was by Doyle ( 1960 ) using specimens of the Great Black-Backed gull ( Larus
marinus ) and the petrel ( Oceanodroma leucorrhoa ).
Concerning the embryological origin of the salt glands, Marples ( 1932 ) stated
that the gland is eventually formed by branching of the two main ducts arising from
the rudiment of the nasal cavity, and then growing backward to the fi nal position.
Histological studies performed with the Adelie penguin ( Pygoscelis adeliae )
allowed the fi rst rudiments of supraorbital salt glands to be traced back to “solid
crescent structures on either side of the nasal cartilage,” (Stonehouse 1975 ) growing
posteriorly to develop dorsal to the eye (Herbert 1975 ). Thus, the glandular matrix
develops from the ducts as branched tubules radiating from a central canal.
The structure of the salt gland is essentially the same in all birds (Babonis et al.
2009 ). It is roughly triangular in cross section, and is divided into lobules by septa
of dense fi brous connective tissue. The salt glands are principally hierarchically
structured. Thus, “each of these lobules consists of many branched tubules which
join a central collecting canal. The secretory tubules are lined by wedge shaped
columnar cells,” (Kühnel 1972 ; see also Babonis et al. 2009 ). On the ultrastructural
level, characteristic infoldings of the plasma membrane are well visible in electron
microscope (see, for example, Kühnel 1972 ). The basal striations, which can be
observed using light microscopy, are associated with the mitochondria (Fig. 3.31 ).
Closely packed microtubules are located parallel to each mitochondria. As described
by Kühnel ( 1972 ) in detail:
“Usually there exist two or three capillaries, running from the central region of
the lobule towards the periphery. Unmyelinated nerve fi bers are observed regularly
outside the basement membrane of the tubule cells and the central duct cells in the
perivascular space, as well as independently from the blood vessels. Structurally,
these interstitial nerves consist of several axons (0.4-1.8 μm thick) surrounded by a
Schwann cell sheath,” (Kühnel 1972 ).
Salt glands as example of cephalic glands principally differ from similar structures because of high specialization of their secretory epithelium that is represented
exclusively by salt secreting principal cells. Some species of marine snakes are the
best examples for this (Dunson et al. 1971 ; Dunson and Dunson 1974 ; Babonis
et al. 2009 ).
3 Biocomposites and Mineralized Tissues
It was observed, that salt glands of sea birds are 10–100 times larger in size than
in non-marine species. The salt concentration of corresponding fl uids “varies
between species and increases as the glands hypertrophy in response to elevated salt
intake” (Suepaul et al. 2010 ; see also Schreiber and Burger 2001 ). The anatomical
description of sea bird salt glands have been described as follow:
“They are compound tubular glands with a well-developed lobular structure, and
a variable countercurrent arrangement of capillary blood fl ow relative to the fl ow of
secretion within tubules. This allows controlled excretion of salt ions from the
bloodstream into collecting ducts, which open into the nasal cavity,” (Suepaul et al.
2010 ; see also by Gerstberger and Gray 1993 ).
Fine structure, innervation and functional control of marine vertebrate’s salt
glands are well described (see for review Gerstberger and Gray 1993 ). The fi rst
detailed morphologic description of the avian salt gland at the electron microscopic
level was by Doyle ( 1960 ) using specimens of the Great Black-Backed gull ( Larus
marinus ) and the petrel ( Oceanodroma leucorrhoa ).
Concerning the embryological origin of the salt glands, Marples ( 1932 ) stated
that the gland is eventually formed by branching of the two main ducts arising from
the rudiment of the nasal cavity, and then growing backward to the fi nal position.
Histological studies performed with the Adelie penguin ( Pygoscelis adeliae )
allowed the fi rst rudiments of supraorbital salt glands to be traced back to “solid
crescent structures on either side of the nasal cartilage,” (Stonehouse 1975 ) growing
posteriorly to develop dorsal to the eye (Herbert 1975 ). Thus, the glandular matrix
develops from the ducts as branched tubules radiating from a central canal.
The structure of the salt gland is essentially the same in all birds (Babonis et al.
2009 ). It is roughly triangular in cross section, and is divided into lobules by septa
of dense fi brous connective tissue. The salt glands are principally hierarchically
structured. Thus, “each of these lobules consists of many branched tubules which
join a central collecting canal. The secretory tubules are lined by wedge shaped
columnar cells,” (Kühnel 1972 ; see also Babonis et al. 2009 ). On the ultrastructural
level, characteristic infoldings of the plasma membrane are well visible in electron
microscope (see, for example, Kühnel 1972 ). The basal striations, which can be
observed using light microscopy, are associated with the mitochondria (Fig. 3.31 ).
Closely packed microtubules are located parallel to each mitochondria. As described
by Kühnel ( 1972 ) in detail:
“Usually there exist two or three capillaries, running from the central region of
the lobule towards the periphery. Unmyelinated nerve fi bers are observed regularly
outside the basement membrane of the tubule cells and the central duct cells in the
perivascular space, as well as independently from the blood vessels. Structurally,
these interstitial nerves consist of several axons (0.4-1.8 μm thick) surrounded by a
Schwann cell sheath,” (Kühnel 1972 ).
Salt glands as example of cephalic glands principally differ from similar structures because of high specialization of their secretory epithelium that is represented
exclusively by salt secreting principal cells. Some species of marine snakes are the
best examples for this (Dunson et al. 1971 ; Dunson and Dunson 1974 ; Babonis
et al. 2009 ).
3 Biocomposites and Mineralized Tissues