S. Samiullah

The major pigment in eggshells of brownegg
laying hens is protoporphyrin IX, but traces of
biliverdin and its zinc chelates are also present. The
pigment appears to be synthesized in the shell gland.
The protoporphyrin IX synthetic pathway is well defined,
but precisely where and how it is synthesized
in the shell gland of the brown-egg laying hen is still
ambiguous. The pigment is deposited onto all shell
layers including the shell membranes, but most of it
is concentrated in the outermost layer of the calcareous
shell and in the cuticle. Recently, the genes that
are involved in pigment synthesis have been identified,
but the genetic control of synthesis and deposition of
brown pigment in the commercial laying hen is not
fully understood. The brown coloration of the shell is
an important shell quality parameter and has a positive
influence on consumer preference. The extent of
pigment deposition is influenced by the housing system,
hen age, hen strain, diet, stressors, and certain
diseases such as infectious bronchitis. In this article,
the physiological and biochemical characteristics of the
brown pigment in commercial brown-egg layers are reviewed
in relation to its various functions in the poultry
industry.