In rats, a portion of Meckel's cartilage -that lying within the mandible but proximal to the rostral convergence of the bars -gives rise to no definitive structures. It offers especially favorable opportunity to study cartilage resorption. By the eighteenth fetal day it is established as a hyaline cartilage bar, and a thin perichondral bone shell starts to form on its lateral aspect, completing encirclement in the next two days. On day 19 cartilage within this bone shows chondrocyte hypertrophy, lacunar enlargement, and matrix calcification. Osteoclasts open a fenestra laterally in the bone and commence removal of calcified cartilage matrix. The erosion front expands rapidly, moving medially (preceded by cartilage hypertrophy and calcification) and extending proximally and distally along the segment. Chondroclasts (multinucleated cells identical with osteoclasts) dominate the erosion front. Capillaries and various mononucleated cells follow. Bone formation is much delayed except in the most rostral extremity, so that, contrary to the situation in endochondral osteogenesis, one is examining calcified cartilage resorption in uncomplicated form. This resorption, including the perichondral bone shell, is virtually complete by day 21, and intramembranous bony reorganization of the site is in progress at birth.Several features of chondroclasts, including some in dispute or not easily seen in vivo, are well displayed. These include ameboid form with pseudopodial extensions (sometimes filamentous), and fusion of some released chondrocytes with entering chondroclasts. Osteo/chondroclasts are often found in contact with perichondral bone at one extremity and calcified cartilage elsewhere on the same cell. There is evidence that matrix calcification is prerequisite to the chondroclastic activity.
SIX FIGURES
It was first shown by Collip, Selye and Thomson ('33)that growth of the very young rat is not completely under pituitary control. They hypophysectomized rats 18 days of age and older and found that those 18 to 21 days of age at operation grew for about a week post-operatively whereas those over one month of age ceased growing immediately after operation. In 1940 Van Eck succeeded in removing the pituitary from rats 9 days of age. He observed that these very young hypophysectomized rats doubled in body weight during the first three post-operative weeks.Recently we have focused attention on this problem. Though persistent efforts to hypophysectomize the new-born rat failed, hypophysectomy of the rat 6 days of age was sucessfully achieved. The object of this report is to give the first analysis of the effects upon growth and differentiation of such animals.EXPERIMENTAL PROCEDURE Hypophysectomy was performed on over 300 rats 6 days of age, 40 of which lived beyond the immediate post-operative interval (the first 10 days p.0.). All rats were of the LongEvans strain, and the majority were of the female sex.
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