On strategies for comparing occupational exposure data to limits

Although research interest in the Harderian gland (HG) has increased during the last few years, only a small amount of information exists about its organogenesis. In mouse the HG appears in the posterior part of eye region, in the form of nonluminated tubules between the sixteenth and eighteenth days of gestation. At birth it is still not differentiated histologically. In birds the HG originates from the conjunctival epithelium at a late embryonic stage. In the English sparrow, Passer domesticus (incubation period of about 13 days), it appears between the seventh and the eighth days of incubation. In the chick embryo (incubation period of about 21 days) it originates between the eleventh and the twelfth days. Among reptiles the lizard Podarcis s. sicula has proved to be a useful model to clarify the embryological origin of the orbital glands since it possesses the anterior lacrimal gland contiguous to the HG in the medial corner of the orbit. The anlage of the orbital glands appears on about the twenty-second day of development (incubation period of about 43 days) in the form of a short tubule projecting from the conjunctival epithelium, at the time of development of the nictitating membrane. At this stage the mesenchymal cells surrounding the glandular blastema form a well-defined sac, later occupied by the orbital glands. From this stage until hatching the growth of the glandular blastema continues with the formation of acini which move posteriorly into the preformed mesenchymal sac. At the thirty-sixth day of development the more lateral acini differentiate into the HG. Only at the forty-first day do the more medial acini differentiate into the anterior lacrimal gland. At hatching the HG is fully differentiated. In anuran amphibia the primordium of the HG appears during the metamorphosis at the time of development of the nictitating membrane.