Pre- and postnatal development of rabbit foliate papillae with special reference to foliate gutter formation and taste bud and serous gland differentiation.

The epithelial downgrowth arises in the presumptive foliate papilla region of the tongue approximately at prenatal day 22, and the distal portion of the primary epithelial cell cords transforms to the bifurcated serous gland and excretory duct between prenatal day 30 and postnatal day 2. The excretory ducts extend upward through the primary epithelial cell cords at rather random intervals and are open to the tongue surface between postnatal days 1 and 2. In this process, successive connections between the adjacent ascending excretory ducts occur mainly due to desquamation of the keratinized lining cells of the ducts, resulting in the formation of the foliate gutter. Taste bud primordia appear in the primary epithelial cell cords by nerve penetration through the basal lamina from prenatal day 30 and nearly fully formed taste buds facing the ascending excretory ducts have been already observed before the foliate gutter formation is completed. In the differentiation process of chemoreceptor (type III) cells from less-differentiated basal (type IV) cells, subsurface cisterns of endoplasmic reticulum are occasionally present where nerve endings make contact. Since subsurface cisterns have been considered to be involved in reciprocal or efferent synaptic transmission, it is reasonable to consider that these morphological specializations may take a role in neurotrophic signals for differentiation of the chemoreceptor cells.