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

Abstract

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.

Ligand-specific induction of endocytosis in taste receptor cells

“…These electron microscopic examinations indicate that the taste cells contain elements such as vesicles and microtubules that could indicate endocytic processes (Ezeasor, 1982;Fujimoto et al, 1993). Also different types of filaments or filament-associated structures have been found in taste cells, such as actin, espin and keratin (Hofer and Drenckhahn, 1999;Sekerkova et al, 2004;Takeda et al, 1990).…”

Expression ofSonic hedgehog,Patched, andGli1 in developing taste papillae of the mouse

“…Taste papilla, feather, and tooth development are all alike in that these structures begin as placodal thickenings and then evaginate or invaginate to form a raised structure with a mesenchymal core (Fristrom, 1988;Lumsden, 1988;Mistretta, 1991;Chuong, 1993;Fujimoto et al, 1993). In addition, Shh is expressed in tooth and feather primordia as well as in taste papilla precursors (Bitgood and McMahon, 1995;Nohno et al, 1995; Fig 1A).…”

“…Papillary development requires coordination between epithelium and mesenchyme, which grows and differentiates to form the supportive core of the papilla (Farbman and Mbiene, 1991;Mistretta, 1991;Fujimoto et al, 1993). Expression of Ptc in the mesenchyme underlying developing taste papillae indicates that this tissue is responding to the epithelial Shh signal.…”

“…Taste papillae begin as placodal thickenings in the lingual epithelium (Paulson et al, 1985;Farbman and Mbiene, 1991;Mistretta, 1991;Fujimoto et al, 1993). The placodal epithelium then begins to grow into the underlying mesenchyme and evaginates into a raised structure.…”

Parallel processing in mammalian taste buds?

“…It has been known for over a hundred years that there are at least three types of taste cells, as distinguished by histological staining—so-called dark cells, light cells, and basal cells (reviewed by [6,7]). These morphological distinctions were refined with electron microscopy and ultimately 4 major categories of taste cells have been distinguished—Types I, II, III, and IV [8,9]. Moreover, ultrastructural features of the Type III cells indicated that only these cells form synapses with axons [8,10–14].…”