Growth and transformation of the mandibular joint in the rat. IV. The effects of thyroidectomy at birth

Fourth Annual

Dental Medicine

Number

(Concluded) GROWTH AND TRANSFORMATION THE RAT. IV. THE EFFECTS

OF THE MANIDIBULAR JOINT OF THYROIDECTOMY AT BIRTH

IN

H. BECKS, D.D.S., M.D., D. A. COLLINS, A.B., M.S., D.D.S., C. W. AXLIKG, PH.D., M.D., R. 0. Scow, M. E. SIMPSON, PH.D., M.D., AND H. M EVANS, M.D., SAN FRAKCISCO AND BERKELEY,

CALIF.

T

HYROID deficiency is known to lead to delayed tooth eruption and dentofacial anomalies. Our knowledge of the mechanism of these changes in the jaws and teeth is, however, incomplete. Recent investigations have disclosed the importance of the mandibular condyle in the growth and development of the lower jaw, and the responsiveness of this structure to hormonal influences.l, 2, 3 The purpose of this paper, the fourth in a series of studies on the mandibular joint, is to describe the histologic structure of the condyle in rats whose thyroid glands were removed at birth. Thyroidectomy in the newborn rat results in a severe slowing of general growth and differentiation.4> 5 On the basis of roentgenographic examination of the skeleton, Scow and Simpson found that thyroidectomized rats differed greatly from normal in that the rate of growth was reduceid and there was a delay in the appearance of secondary ossification centers. Histologic examinations of\ the tibia, metacarpal, and caudal vertebrae confirmed and extended these observations.6 The mandibular condyle of the rat at birth, the time of thyroidectomy, is composed entirely of hyaline cartilage. During growth, proliferation is active in this cartilage. No secondary center of ossification like that in the epiphyses of long bones is formed. Erosion of the cartilage and replacement, by bony tissue take place from the ramus of the mandible. The density of this bony tissue increases and reaches a maximum by approximately 250 days of age. The cartilage is eventually reduced to a thin layer. However, the zone of undifferentiated cells beneath the articular surface remained even in the oldest rat thus far studied (1,316 days of age). For a study of the effects of thyroidectomy on the mandibular condyle, there were available the heads of the rats which were thyroidectomized at birth* and which had been used in the roentgenographic and histololgic analyses previously reported.4l 6 Nine rats in which the -thyroidectomy had been established as complete were sacrificed, with normal litter mate controls, at 56, 72, 76, 84, 94, and 105 days of age. The skulls of these animals were divided into halves, roentFrom the DWsion of Dental Medicine, College of Dentistry, the George Williams Hooper Foundation for Medical Research, and the Institute of Experimental Bi,ology, University of California. Aided by grants from the American Foundation for Dental Science, the Research Board of the University of California, the Rockefeller Foundation, and the California State Dental Association. “The method of operation and criteria for completeness of operation are described (elsewhere.4 315

316

SECKS,

COLL11\‘s,

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genographed, and fixed 4n neutral formoi. The mandibular joints were dlssectecy, embedded in nitrocellulose, sectioned, and stained with hematoxylin and eosin. Following thyroidectomy, the heads of the rats mere dwarfed like the rest O$ the body. The skulls were rounded in shape, with a disproportionately great breadth in the plane through the glenoid fossae. The normal controls showed ?onger, relatively narrow heads. This same disproportion was seen on examination of the dissected mandibular condyles. Although actually smaller, the condyles of these thyroidectomized dwarfs showed broadening in t,heir mediolateral diameter (the plane through which the histologic sections were taken.).

Mediolateral sec.iona roidectomy performed at objective 0; Extension, 06 Fig. l.--Normal, 56 Fig. e.-Thyroidectornized, Fig. 3.-Normal, 72 B’ig. 4.-Thyroidectomized,

uf mnndibular condyle of normal and thyroidectomized birth.) Hematoxylin and eosin stain. Magnification cm.; X92. days old (Spec. 8061 Pl. 8121). 56 days old (Spec. 8068 PI. 8465). dars old (Spec. 8083 Pl. 8423). 72 days old (Spec. 8096 PI. 9631).

:

rats. (ThyElomal 1’1,

The histologic structure of the mandibular condyle of a normal control for the youngest rat sacrificed, 56 days of age, is shown in Fig. 1. The cartilage on the surface of the condylar head had al*eadp undergone marked reduction in thickness. Multiple capillary tuft,s impinged upon it at the zone of erosion. The neck of the condyle consisted of sturdy bony trabeculae. The marrow occurred only in small islands between these trabeculae. In the thyroidectomized animal of the same age the condyle was smaller

GROWTH

AND

TRANSFORMATION

OF MANDIBULAR

JOINT

IN

RAT.

IV.

317

(Fig. 2). The process of erosion was less active, and the earNage had thus maintained the thickness and architecture characteristic for that of the 20-dayold normal rat.l On the other hand, the bone of th.e condylar head and neck was dense, corresponding to that of a normal animal. even older than 56 da,ys. Fig. 4 shows that seventy-two days after thyroidectomy, the condylar head had a mediolateral diameter approximately equal to that in the normal control of corresponding age (Fig. 3). However, its shape had been altered from that characteristic of the normal by flattening. The cartilage of the head no longer

Mediolateral sections of mandibular condyle of normal and (Thyroidectomy performed at birth.) Hematoxylin and eosin stain. VI, objective 6 : Extension. 66 cm. : x92. Fig. 5.-Normal, 84 days old (Spec. 8109 Pl. 8378). Fig. 6.-Thyroidectomized, 84 days old (Spec. 8117 PI. 8440). Fig. ‘i.--Normal, 105 days old (Spec. 8131 Pl. 842ti). Big. 8.--Thyroidectomized, 105 days old (Spec. 8133 PI. 8408).

thyroidectomized Magnification :

rats. Homal

showed a c.onspicuous zonal arrangement. Its chondrocytes were smaller and there was an increase in the relative amount of matrix. The thickness of the cartilage had been mainta.ined as in the condyle seen at the ea.rlier age. Caleification of this cartilage was not. as heavy or as well defined as in the norma& Gondyle. Many of the bony trabeculae spread out on the cartilage and fused with it. The mandibular eondyle of the g&day-old th~lroi~ectomized rat (Fig. 6) was broader than that of its control (Fig. 5) and showed by its flatness a marked

chstortion in shape. The basophilie cartilage next to the bone in the eondyiar neck was wider, more irregular, and less sharply defined than in the control. The cells were large and rounded; t,here were many more vacuolated cells. Large masses of cartilage extended into the sturdy bony trabeculae. Marrow eontact with the cartilage was more restricted. In its density the bony tissue resembled that found in normal rats of advanced age. In Fig. 6, the condy1.eof the 105day-old thyroidectomized rat continued to show a breadth greater than that of its normal control (Fig. 7). The thickness of she cartilage, however?now corresponded closely to that in the control. There were still more vacuolated cells. The area of increased basophilia of the cartilage was more clearly defined, but was still more irregular than in the control. Marrow contact with the cartilage was further reduced and occurred along an irregular boundary. The density of the bone in t,he .condylar head and neck was similar to that of th.e control. ummary The influence 01 :hy~oidectomy at birth on the growth and transformation of t,he mandibular condyle was studied histologically. The cartilage of the condylar head remained wide for long periods, as in young animals. However, the histologic characteristics of this condyle were not those of youth; the chondroeytes were not arranged in orderly columns. A distortion of the head of the condyle occurred, and eonsisted especially in a disproportionately great breadth and marked flat,tening. The bone of the condylar head and neck was denser than in normal animals of corresponding ages. Later, by 105 days of age, the reduction in width of the cartilage and the dense bony tissue resulted in a, histologic structure of the condylar head and neck which approached that of the control of the same age. Thus, at this time t,he mandibular condyle appeared slowly to be gaining the structure characteristic of the normal rat. The bone age in these anima.ls, as determined by roentgenographic aialysis of representative skeleta! structUtre& was 18 days. The long bones which were examined histologically (tibia,, metacarpal, and eaudal vertebrae) showed a development which confirmed t.his age. In sharp contrast, however, at none of the ages studied did the structure of the mandibular condyle correspond to that of a normal animal a,syoung as 1& days. References 1. Collins, 2. 3.

4. 5. 6.

D. A., Becks, FL., Simpson, 51. E., and Evans, H. X.: Growth and Transformation of the Mandibular Joint in the Rat. I. Normal Female Rats. Ax. J. ORTEIODONTICS AND ORAL SUKG. (Oral Surg. Sect.) 32: 431, 1946. Collins, D. A., Be&s: II., Simpson, M. E., and Evans, H. M.: Growth and Transformation of the Mandibular Joint in the Rat. II. Hypophysectomized Female Rats, AK J. ORTHODONTICS AND ORAL SURG. (Oral Burg. Sect.) 32: 443; 1946. Hecks, H., Collins, D. A., Simpson, M. E., and Evans, H. M.: Growth and Transformation of the Mandibular Joint in the Rat. III. The Effects of Growth Hormone and Thyroxin Injections in Hypophyseetomized Female Rats, Aal, J. ORTXODONTJCS AND OP,~L SURF. (Oral Surg. Sect.) 32: 447, 1946. Scow, R. O., and Simpson, M. E.: Tbyroidectomy in the Newborn R.at, Anat. Rec. 91: 209.226, 1945. in Newborn Rats, Proc. Sot. &per. Salmon T. N.: Effect of Thvro-Parathyroidectomy B)iol. & Med. 35: 489.49i, 1936. Scow, Robert: Be&s, H., Simpson, M. E., Asling, C. W.? and Evans? H. X: Skeletal (In preparation.) Changes in Rats Thyroidectomized on the Day of Birth.