[Histological investigation of nail growth in human embryos and regrowth of nail after its removal in adult monkeys--with reference to clinical observation in injuries to the nail and its surrounding tissues].

As the basis to develop adequate treatments which can preserve the nail plate or assure its regrowth in frequently encountered injuries to the nail-surrounding tissues, a histological investigation was made of nail growth in human embryos and regrowth of nail after its removal in adult monkeys. The author's investigation in these two series indicated the following. Firstly the nail plate composed of parakeratotic layer is produced solely of "nail matrix", thus, supporting Zaias' and opposing Lewis' theory as to the nail production. Secondly though the normal, completely formed nail matrix and nail bed have no granulosa cell nor horny layer, the nail bed is temporarily covered with "false nail" which contains these two layers during the process of embryonal nail growth as well as during that of regrowth after its removal. This "false nail" well incorporates with the volar surface of growing or regrowing nail plate, and is gradually pushed distally as the nail grows and stretches distally . These findings raise a question why healthy nail does not grow over ordinary skin graft placed on the nail bed damaged by trauma. Is there any essential difference between "false nail" and ordinary skin graft, though both having these two layers in common? The author thinks there must be and presumes that it lies in the difference of turn-over time as exemplified with autoradiographical study by Zaias, i.e. turn-over time of germinative layers of healthy nail bed is much slower than in ordinary skin, therefore not so thick horny layer as in ordinary skin is formed by the time the growing or regrowing nail plate spread over it, which allows "incorporation" between palmar surface of the growing nail and germinative layer of the nail bed. On the contrary, too thick horny layer provided or formed in the grafted ordinary skin does not allow the "incorporation" between these tissues. Clinical observations coincide with the above result and presumption.