TRANSFORMATION OF SKIN FROM LARVAL TO ADULT TYPES IN NORMALLY METAMORPHOSING AND METAMORPHOSIS-ARRESTED SALAMANDER, HYNOBIUS-RETARDATUS

Transformation of skin from larval to adult types in a salamander, Hyn obius retardatus, which had been reported to show neotenic reproductio n in a specific environment, was examined morphologically in normally metamorphosing, precociously metamorphosing and metamorphosis-arrested larvae. Typical larval skin was composed of an epidermis constituted by three types of cells such as apical, Leydig, and basal cells. The L eydig cells were larval specific, and thus disappeared and were replac ed by adult epidermal cells during the metamorphosis. Disappearance of the Leydig cells was accomplished by apoptosis as confirmed by the te rminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end-labe ling method and electron microscopy during the normal metamorphosis an d precocious metamorphosis induced by exogenously applied triiodothyro nine. Typical adult skin was composed of epidermis constituted by stra tified squamous cells and of dermis mainly occupied with two types of dermal glands, mucous and serous glands. When the metamorphosis was ar rested by different procedures (thyroidectomy, hypophysectomy, goitrog en treatment, and rearing at low temperature), the larval-specific Ley dig cells fully remained in the epidermis, suggesting that the disappe arance of these depended on the thyroid activity. Contrary to this, de rmal glands behaved differently from the Leydig cells, though they dev eloped and differentiated from epidermal basal cells and constituted t he same skin. Those in the metamorphosis-arrested (thyroidectomized, h ypophysectomized, or goitrogen-treated) larvae, except in the larvae r eared at 4 degrees C, appeared a little later than in the controls. Th us, the aged, metamorphosis-ar rested larvae had skin which consisted of larval type epidermis (Leydig cells) and adult type dermis (mucous and serous glands).