Organogenesis of the orbital glands in the lizard Podarcis s. sicula: a histological, histochemical and ultrastructural study.

The orbital glands of the lizard Podarcis s. sicula are represented by the anterior and posterior lacrimal glands and the Harderian gland. The anlage of the Harderian gland appears on about the 22nd day of development in the form of a short tubule projecting from the conjunctival epithelium. This event is coincident with the appearance of the nictitating membrane. At this stage the mesenchymal cells surrounding the glandular blastema proliferate at a high rate and form a definite sac, later occupied by both the Harderian gland and the anterior lacrimal glands. At the 26th day of development, the glandular blastema forms acini at its distal end. The prospective glandular cells are not yet differentiated histologically. At the 36th day of development, differentiated serous glandular cells become visible. At the 41st day of development, the acini fill up the preformed mesenchymal sac. Only at this stage does the most medial part of the gland differentiate into mucous-secreting anterior lacrimal gland. At the same time, a small primordium of the posterior lacrimal gland can be seen in the posterior commissure of the eye. The appearance of junctional complexes between epithelial cells and mesenchymal cells in the early developmental stages supports the role of the mesenchyme in the differentiation of the glandular cells. Since the glandular anlage differentiates laterally into Harderian gland and medially into anterior lacrimal gland, spatial and temporal differences seem to exist in the inductive process. Furthermore, a concentration gradient of the inductive substance(s) may be envisaged, since an intermediate zone is present between the Harderian gland and the anterior lacrimal gland, consisting of mixed glandular cells containing both mucous and serous secretory granules.