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The fine structure of the epidermis of the external gills of the larval frog, Rana pipiens, was observed during the period of gill degeneration. Many of the epidermal cells underwent morbid alterations. In typical degenerating epidermal cells the nucleus became irregular in outline, chromatin formed electron-dense clumps and the nuclear envelope increased in width. Lysosomes became prevalent in the cytoplasm of these cells and progressively larger autophagic vacuoles appeared. As degeneration continued, apical epidermal cells were found embedded deeper within the epidermis. In some instances cell processes from two neighboring epidermal cells extended partially around the relocated degenerating apical epidermal cells probably contributing to their translocation; phagocytosis was completed by a single cell in other instances. The degree of participation of the phagocytic epidermal cells in the degenerative process was uncertain. The distribution of acid phosphatase activity was followed in specimens treated by the Gomori technique. Profiles of cisterna-like elements, positive for the enzyme, suggested that perhaps entire Golgi cisternae contributed to the formation of autophagic vacuoles in combination with small vesicles. Heterophils and macrophages observed in the degenerating gills were believed to participate in the removal of the dying cells.The development and degeneration of external gills of both anurans and urodeles were studied by several nineteenth century investigators (Whitney, 1867; Boas, 1882;Maurer, 1888;Naue, 1890; Clemens, 1895). Experimental embryologists of the early twentieth century found amphibian external gills useful organs for studying the interactions between the germ layers of embryos (Ekman, '13; Harrison, '21; Severinghaus, '30; Moser, '40). Physiologists have explored the relationship between the concentration of respiratory gases and the growth of external gills (Drastich, '25; Savage, '61; Boell et al., '63; Lprvtrup and Pigon, '68). However, cellular changes occurring during degeneration of the external gills of anurans have never been reported.The external gills of the larval frog, Rana pipiens, and many other species of anurans are transitory respiratory organs that arise from the third, fourth and fifth visceral arches, just prior to the heart-AM. J. ANAT., 132: 301-318.beat stage. The finger-like external gill filaments consist of an epidermis of two cell layers, connective tissue containing collagenous fibers and scattered mesenchymal cells, and one or more large capillary loops. The gills grow to a maximum length of about 1 mm. After only two or three days of function, the opercular fold begins to enclose them in the opercular chamber. Concurrently, the external gills start to degenerate resulting in their disappearance within five to seven days. The external gills are replaced by internal gills, that presumably take over the respiratory function (Rugh, '51).The external gills of the larval frog are an excellent system for investigation of developmental cell death ...

The fine structural changes accompanying the regeneration of adrenocortical transplants in the rat were studied with the electron microscope. One-half gland autotransplants were made to the dorsal musculature of male Wistar rats, weighing approximately 120 gm. Transplants were recovered after 2, 4, 7, 14, 21 and 60 days of regeneration. During the first week of regeneration there was a n increase in the granular endoplasmic reticulum at the expense of the agranular form. The internal structure of the numerous mitochondria was tranformed from the normal tubular and vesicular forms of the zonae glomerulosa and fasciculata, respectively, to a lamellar type. The quantities of free ribonucleoprotein particles also were reduced. During this period the viable cortical cells are considered to be more deeply involved in protein synthesis and growth rather than hormone biosynthesis. Following 14 days of regeneration and thereafter, the characteristics of the normal intact adrenal cortex became established. Highly osmiophilic "dark cells," present in the inner zones of the normal intact adrenal cortex, first appeared after 14 days of regeneration, and were widely scattered throughout the cortex after three weeks. After two months of regeneration, the "dark cells" were again concentrated in the inner cortical zones. During this latter period, hormone biosynthesis appears to be the major cell function. Possible structural-functional relationships are discussed.