Effect of hypophysectomy on the 24‐hour mitotic rhythm of corneal epithelium in urodele larvae

Abstract

It is a generally recognized fact that the mitotic rates of mitotically labile tissues of many plants and animals indicate a definite daily cycle or periodicity. However, the basic factors underlying or affecting this cyclic phenomenon of mitotic rates are still being investigated. In animals, the endocrine organs have been suspected as playing a role in controlling or affecting the various biological rhythms observed in the organism, whether these rhythms be manifested in mitotic rates of tissues, body temperatures, activity, number of circula,ting blood cells, etc. The pituitary gland and suprarenals have been the subjects of particular interest and investigation. Zander et al. ('54) have reported on the effects of hypophysectomy on the rhythmic character of mitotic activity in the rat retromolar mucosal epithelium; Ferguson et al. ('57) have explored the role of the pituitary and hypophysectomy in the cyclic nature of rectal temperature in mice. Halberg et al. ('59) and Halberg ('60), reviewing the work done on the effects of hypophysectomy on biological rhythms conclude that as a result of hypophysectomy, mitotic rhythms are altered only, but not obliterated. Instead of the pituitary, Halberg implicates the adrenal gland as the dominant pacemaker of mitotic rhythms and a number of other circadian rhythms.The presence of a definite rhythm over a 24-hour period in the rate of cell division in the epidermis and gastric mucosal epithelium of urodele larvae has been established (Scheving, Chiakulas and Abzug, '59; Chiakulas and Scheving, '61). As will be reported in this paper, a rhythm over a 24-hour period also exists in the mitotic rate of larval corneal epithelium. Since hypophysectomy in urodele larvae is a relatively simple procedure, the effects of hypophysectomy on this cyclic nature of the mitotic rate in corneal epithelium was investigated and will be discussed. Adrenalectomy was not feasible because of the diffuse, scattered nature of the adrenal tissue in urodele larvae. MATERIALS AND METHODSAmblystoma maculatum larvae, 20-25 mm in length, were used in this study.2 The larvae were reared in the laboratory from egg stages and were maintained under a regimen consisting of 12 hours of light alternating with 12 hours of darkness and a constant temperature of 22°C. The animals were kept in filtered tap water and were fed on a diet of enchytraeid worms.Hypophysectomy was accomplished in the following manner: Animals were anesthetized in MS-222 and strapped down individually in an operating dish. The mouth was pried open by means of a blunt forceps. Fine jeweler's forceps were used to penetrate the oral epithelium and chondrocranium of the roof of the mouth in the immediate region of the pituitary gland which was visible through these tissues. The forceps were then inserted around the base of the pituitary and the gland was gently removed.Complete and persistent blanching of the epidermis due to the contraction of the melanophores in the absence of the melanophore expanding hormone of the pituitary, was t...