MORPHOGENESIS OF BAARSCHEIBEN IN RATS

In the newborn rat, innervated Merkel cells were observed in the epidermis of the back and belly next to the tylotrich follicles. Small Haarscheiben could be seen on the belly at birth but were not visible on the back until 10 to 13 days postnatally. The Haarscheiben primordia on the back of the newborn where Merkel cells were found were 95 μm in diameter on the average and were surrounded by epidermal rete pegs. By 13 days, the diameters of the Haarscheiben on the back averaged 208 μm, which is similar to the adult. Haarscheiben on the belly also increased in diameter during this period, but throughout were larger than those on the back. Special cells sharing the morphologic features of both keratinocytes and Merkel cells were found in the epidermis of newborn and 13-day-old Haarscheiben. Similar cells in adult Haarscheiben, which have been called transitional cells, have clumps of tonofibrils in their cytoplasm like keratinocytes, as well as dense-cored granules like those in Merkel cells. From 0 to 13 days after birth, the nerve endings on the Merkel cells and transitional cells were filled with clear-cored, synaptic-sized vesicles. Similar vesicles found in adult nerve terminals were distinctly less concentrated. The neuronal membranes in adult and newborn rats had thickened regions apposed to the Merkel cells. However, the clear-cored vesicles were not directly associated with these areas