Introduction
The avian eye’s greater capabilities than the human eyes are attributed
to the birds’ unique lifestyle and environments, as well as to physical
activities like flight that require excellent vision. The
ecomorphological design of the vertebrate eye reflects the
characteristics of the species’ environment and lifestyle. Although the
morphological aspects of the eye are similar for all vertebrates, its
microstructural and molecular features vary among species from different
ecological niches. The accessory
organs of the eye (eye adnexa) are important for proper functioning of
the eye (Bayon et al. 2007). They include the eyelids (upper, lower, and
third eyelids), conjunctiva-associated lymphoid tissue (CALT), the
Harderian gland (HG), and the lacrimal gland (LG) (Klec´kowska-Nawrot et
al. 2015). These structures mainly moisturize the eye and supply the
superior and inferior conjunctival sacs with nutrients and antibodies to
maintain their health (Dartt, 2009; Maggs, 2013; Jochems & Phillips,
2015; Klećkowska-Nawrot et al. 2016).
Recently, it was recorded that
many viruses replicate in the feather pulp and/or the feather follicle
epithelium, such as those found on the eyelids (Takashi et al. 2021),
for example, Marek’s disease virus (MDV), avian leukosis virus, and
chicken infectious anemia virus. Each one of these diseases may cause a
partial loss of vision and have far-reaching effects, and the
compensatory use of other senses (such as auditory and olfactory sensory
perceptions) is typically insufficient (Ahmed et al. 2016)
Ornitho-ophthalmology has played a significant role in this field with
very few exceptions e.g., pathogens transmitted via the germline, the
animal pathogens enter the host by breaching the barrier between the
external and internal milieus. This barrier consists of specialized
tissues characterized by two main components: an externally located
epithelium and the underlying connective tissue (Korbel, 2000; Korbel &
Bohnet, 2007). So, to prevent the entry of pathogens through the mucosal
tissues, a wide variety of protective mechanisms have evolved. These
range from barrier functions (e.g., keratinized skin) to highly
specialized immune cells (e.g., Langerhans cells in the skin) and the
organized lymphoid structures in the mucous membrane. Obviously, the
epithelial cells of the mucosal surfaces are able to sense pathogens and
actively shape the response of the immune cells underneath (Alan &
Athena, 2019).
For many years, research in mucosal immunology has focused on the
question of how these cells become activated and interact to protect the
host from invasion and dissemination of pathogenic microorganisms
(Yasmine & Timothy, 2014). Under physiological conditions, the mucosal
immune system has to remain tolerant of the microbiome to avoid chronic
mucosal inflammation, while at the same time being able to respond
quickly and appropriately to pathogenic challenges. This explains the
great importance of the eyelids in protecting the eye from diseases and
that they are not only responsible for controlling the amount of light
and moisturizing the eye but also play an important role in protecting
the eye from diseases (Knop N & Knop E 2005 a &b).
The present authors selected a common wild bird species in my country,
the black winged kite to investigate the morphological features and
microstructure as well as demonstrate the lymphoid follicles and
lymphocytes to illustrate the importance of the characterization of the
eye adnexa in the protecting of the eye from any diseases.