151
not only stabilizes a thin fi lm of metastable vaterite, but exhibits strong antimicrobial
activity against two pathogenic gram-negative bacteria, Pseudomonas aeruginosa
and Proteus vulgaris ” (Lakshminarayanan et al. 2008 ).
Regrettably, there are only few reports (Jenkins 1975 ; Ferguson 2010 ) on egg shells
of crocodiles, and no detailed information regarding salt water species C. porosum
except a single paper by Griggs and Beard ( 1985 ). Roberts and Sharp ( 1985 )
investigated the orientation of calcium carbonate phases in their egg shells by using
X-ray diffractometry. In all Crocodylia species the calcitic microarchitecture that
develops is one in which the basal plane of the unit cell in each of the two crystal
systems tends to lie parallel to the shell surface. Preferred orientation reaches a
maximum at the exterior surface, and its development through the shell is normally
uninterrupted. While many reptiles have parchment-shelled eggs, crocodilian
eggs have a calcareous shell, like birds, which is pierced by many pores. Unlike
avian eggs, however, the shell membrane in crocodile eggs is quite thick (Grigg and
Beard 1985 ).
The structure of the crocodylian egg shell seems to be very complex. Here, the
detailed description made by Ferguson ( 1982 ):
“The alligator ( Alligator mississippiensis ) egg consists of the following layers
from outside in: (1) an outer densely calcifi ed layer (100–200 μm thick) consisting
of small vertically stacked calcite crystals orientated with their crystallographic c
axes at right angles to the shell surface; (2) a honeycomb layer (300–400 μm thick)
consisting of horizontally stacked calcite crystals with their crystallographic c
axes parallel to the shell surface; (3) an organic layer (approximately 10 μm thick)
containing a higher percentage of organic matrix to calcite crystals, and through
which the shell cleaves and falls away from the egg shortly before hatching; (4) a
mammillary layer (20–30 μm thick) which is more pronounced in the central opaque
region of the shell and which attaches the latter to the eggshell membrane; and (5) an
eggshell membrane (150–250 μm thick) consisting of an interwoven mesh of fi bres,
separated from the albumen by an amorphous limiting membrane, in which there
are numerous pores,” (Ferguson 1982 ).
However, in spite of complex, multilayered mineral-containing structure of the
eggshell, are there some naturally occurring processes, which can lead to demineralization of these constructs? Indeed, two factors like organic acids produced by
nest micro-organisms and presence of hydrated carbon dioxide can infl uence the
integrity of the eggshell. Ferguson ( 1982 ) proposed terms like “erosion craters and
cratered pore orifi ces,” (Ferguson 1982 ) and described this phenomenon as follow:
“These erosion craters expose at their broad bases the underlying honeycomb
layer which contains large numbers of vesicular holes, interconnecting with
other cavities throughout the shell. Thus, development of erosion craters renders the
entire egg more porous as incubation proceeds. In addition, these craters, together
with the mobilization of calcite crystals out of the organic layer for use in embryonic
mineralization, progressively weaken the shell causing it to crack and cleave off
the eggshell membrane and mammillary layer, thus facilitating hatching. No air
space is present in the alligator egg, neither are there any chalazae. The embryo has
the usual amniote arrangement of extra-embryonic membranes which are fi rmly
3.4 Egg Shells of Marine Vertebrates
not only stabilizes a thin fi lm of metastable vaterite, but exhibits strong antimicrobial
activity against two pathogenic gram-negative bacteria, Pseudomonas aeruginosa
and Proteus vulgaris ” (Lakshminarayanan et al. 2008 ).
Regrettably, there are only few reports (Jenkins 1975 ; Ferguson 2010 ) on egg shells
of crocodiles, and no detailed information regarding salt water species C. porosum
except a single paper by Griggs and Beard ( 1985 ). Roberts and Sharp ( 1985 )
investigated the orientation of calcium carbonate phases in their egg shells by using
X-ray diffractometry. In all Crocodylia species the calcitic microarchitecture that
develops is one in which the basal plane of the unit cell in each of the two crystal
systems tends to lie parallel to the shell surface. Preferred orientation reaches a
maximum at the exterior surface, and its development through the shell is normally
uninterrupted. While many reptiles have parchment-shelled eggs, crocodilian
eggs have a calcareous shell, like birds, which is pierced by many pores. Unlike
avian eggs, however, the shell membrane in crocodile eggs is quite thick (Grigg and
Beard 1985 ).
The structure of the crocodylian egg shell seems to be very complex. Here, the
detailed description made by Ferguson ( 1982 ):
“The alligator ( Alligator mississippiensis ) egg consists of the following layers
from outside in: (1) an outer densely calcifi ed layer (100–200 μm thick) consisting
of small vertically stacked calcite crystals orientated with their crystallographic c
axes at right angles to the shell surface; (2) a honeycomb layer (300–400 μm thick)
consisting of horizontally stacked calcite crystals with their crystallographic c
axes parallel to the shell surface; (3) an organic layer (approximately 10 μm thick)
containing a higher percentage of organic matrix to calcite crystals, and through
which the shell cleaves and falls away from the egg shortly before hatching; (4) a
mammillary layer (20–30 μm thick) which is more pronounced in the central opaque
region of the shell and which attaches the latter to the eggshell membrane; and (5) an
eggshell membrane (150–250 μm thick) consisting of an interwoven mesh of fi bres,
separated from the albumen by an amorphous limiting membrane, in which there
are numerous pores,” (Ferguson 1982 ).
However, in spite of complex, multilayered mineral-containing structure of the
eggshell, are there some naturally occurring processes, which can lead to demineralization of these constructs? Indeed, two factors like organic acids produced by
nest micro-organisms and presence of hydrated carbon dioxide can infl uence the
integrity of the eggshell. Ferguson ( 1982 ) proposed terms like “erosion craters and
cratered pore orifi ces,” (Ferguson 1982 ) and described this phenomenon as follow:
“These erosion craters expose at their broad bases the underlying honeycomb
layer which contains large numbers of vesicular holes, interconnecting with
other cavities throughout the shell. Thus, development of erosion craters renders the
entire egg more porous as incubation proceeds. In addition, these craters, together
with the mobilization of calcite crystals out of the organic layer for use in embryonic
mineralization, progressively weaken the shell causing it to crack and cleave off
the eggshell membrane and mammillary layer, thus facilitating hatching. No air
space is present in the alligator egg, neither are there any chalazae. The embryo has
the usual amniote arrangement of extra-embryonic membranes which are fi rmly
3.4 Egg Shells of Marine Vertebrates