The biological action of thyroxine on embryonic bones grown in tissue culture

Thyroid hormone and the growth plate

Thyroid hormone was first identified as a potent regulator of skeletal maturation at the growth plate more than forty years ago. Since that time, many in vitro and in vivo studies have confirmed that thyroid hormone regulates the critical transition between cell proliferation and terminal differentiation in the growth plate, specifically the maturation of growth plate chondrocytes into hypertrophic cells. However these studies have neither identified the molecular mechanisms involved in the regulation of skeletal maturation by thyroid hormone, nor demonstrated how the systemic actions of thyroid hormone interface with the local regulatory milieu of the growth plate. This article will review our current understanding of the role of thyroid hormone in regulating the process of endochondral ossification at the growth plate, as well as what is currently known about the molecular mechanisms involved in this regulation.

Expression of cyclin-dependent kinase inhibitors in epiphyseal chondrocytes induced to terminally differentiate with thyroid hormone

A growing body of evidence suggests that systemic hormones and peptide growth factors may exert their effects on cell growth and differentiation in part through regulation of the cell division cycle. We hypothesized that thyroid hormone regulates terminal differentiation of growth plate chondrocytes in part through controlling cell cycle progression at the G1/S restriction point. Our results support this hypothesis by demonstrating that treatment of epiphyseal chondrocytes with thyroid hormone under chemically defined conditions results in the arrest of DNA synthesis and the onset of terminal differentiation, indicating that thyroid hormone is one factor capable of regulating the transition between cell growth and differentiation in these cells. This terminal differentiation process is associated with induction of the cyclin/cyclin-dependent kinase inhibitors p21(cip-1 waf-1) and p27kip1, suggesting that thyroid hormone may regulate terminal differentiation in part by arresting cell cycle progression through induction of cyclin-dependent kinase inhibitors.

Expression of thyroid hormone receptor isoforms in rat growth plate cartilage in vivo

Although thyroid hormone has been known for many years to be a potent regulator of skeletal maturation in vivo, it has not definitively been determined whether this effect is a result of a direct or indirect action of the hormone. Previous in vivo studies have suggested that thyroid hormone may stimulate longitudinal bone growth by increasing the secretion of growth hormone; however, growth hormone alone is unable to stimulate cartilage maturation. There are also indications that thyroid hormone is able to act directly on growth plate chondrocytes through growth hormone-independent mechanisms. In this study, we demonstrate that rat growth plate chondrocytes in vivo express genes encoding three of the four isoforms of the thyroid hormone receptors described to date, but the corresponding protein can only be detected for the TRalpha1 and TRbeta1 isoforms of the receptor. As has been noted in other tissues, there is generally poor correlation between the mRNA levels for each isoform and the relative amount of corresponding protein as measured by immunoblotting, suggesting the possibility that receptor expression may be regulated by post-transcriptional mechanisms.

Comparison of the effects of papain n vitamin A on cartilage. I. The effects in rabbits

The administration of large amounts of vitamin A to rabbits has been shown to result in depletion of cartilage matrix. The normal basophilic, metachromatic, and Alcian blue staining properties of the matrix are lost, especially in articular and epiphyseal cartilage. The cartilage cells remain intact, but are reduced in size. These changes sometimes appeared as early as 48 hours after the initiation of daily injection of 1 million units of vitamin A, and were usually well established by 5 days. Some rabbits failed to show changes in cartilage, even after 5 daily injections. Increased amounts of material presumed to be chondroitin sulfate were present in the sera of vitamin A-treated rabbits, usually by 72 hours after the first injection. This was demonstrated by a turbidimetric procedure using hexamminecobaltic chloride. In rabbits given sulfur-35 (Na(2)S(35)O(4)) 5 days before the initiation of vitamin A treatment, it was shown that sulfur-35 was lost from articular and epiphyseal cartilage. This was associated with an increase in the non-dialyzable sulfur-35 in both serum and in the cobalt-precipitable material. These rabbits also excreted more sulfur-35 than rabbits not given vitamin A. There was a reduction in sulfur-35 activity in chondromucoprotein extracted from the ear cartilage of vitamin A-treated rabbits. The changes are interpreted as indicating that the administration of large amounts of vitamin A to rabbits results in removal of chondroitin sulfate from cartilage matrix. The administration of small amounts of crude papain causes histologic changes in cartilage that are remarkably similar to those seen in vitamin A-treated rabbits. The possibility is suggested that the changes in cartilage produced by administration of vitamin A to rabbits may be the result of activation of a proteolytic enzyme or enzymes, with properties similar to those of papain.

Comparison of the effects of papain and vitamin A on cartilage. II. The effects on organ cultures of embryonic skeletal tissue

The effects of papain protease and of vitamin A on explanted limb bone rudiments from 7- and 13-day chick embryos and fetal mice have been studied and compared. The incubation of cartilaginous rudiments from 7-day chick embryos in a solution containing papain and cysteine resulted in complete loss of the metachromasia of the cartilage matrix within 1 hour; explants treated in this fashion recovered normal metachromatic staining properties when grown in normal medium for 4 days. The incubation of 7-day chick cartilage rudiments in a solution containing papain without cysteine resulted in partial loss of metachromasia from cartilage within 1 hour; the addition of vitamin A to the solution did not enhance the effect of papain during this period. The addition of papain to the culture medium in which 7-day chick embryo cartilage rudiments were grown resulted in uniform loss of the metachromasia of the cartilage matrix; similar explants grown in the presence of excess vitamin A also showed loss of the metachromasia of cartilage, but certain regions of the cartilage were affected earlier and more severely than others. Changes in cartilage cells, including loss of glycogen, occurred when the rudiment was grown in medium containing excess vitamin A, but not when it was grown in the presence of papain. Bone rudiments from 13-day chick embryos showed changes in cartilage similar to those seen in 7-day chick embryo rudiments when grown in the presence of papain or of excess vitamin A; the existing bone was not affected under these conditions. When grown in the presence of papain or excess vitamin A, the cartilage of late fetal mouse bone underwent changes similar to those already described in chick embryo rudiments. In contrast to the chick embryo rudiments, those from the fetal mouse showed rapid resorption of bone when grown in the presence of excess vitamin A. Papain had no effect on bone from either source. The changes seen in cartilage of explants grown in the presence of vitamin A and papain together were greater than those seen with either agent alone. The changes seen in fetal mouse bone grown in the presence of vitamin A were not enhanced by the additional presence of papain. On the basis of these observations, it is suggested that the changes in cartilage seen in experimental hypervitaminosis A may be the result of activation of a proteolytic enzyme or enzymes with properties similar to papain.

Edward Mellanby, 1884-1955

Death came suddenly, and without recognized warning, to Sir Edward Mellanby on 30 January 1955, before he had completed his 71st year. Nearly six years earlier he had retired, on reaching the official age-limit, from the Secretaryship of the Medical Research Council, which he had held for 16 years. During this period the range and the influence of the Council’s activities for the promotion and support of medical research had undergone a most notable expansion, primarily in the United Kingdom, but widely beyond it also in the British Commonwealth; and Mellanby’s enlightened and resourceful enterprise in proposal and planning, and his vigorous drive in administrative action, had undoubtedly been dominant factors in this remarkable development. Like his only predecessor in the appointment, the late Walter Morley Fletcher, Mellanby had been the Council’s chief executive officer in much more than a merely official sense. Yet, during all the years in which he had carried that heavy load of official responsibility, he had succeeded in maintaining a direct and personal activity in the researches which had so long provided for him, and also for his devoted wife and scientific partner, the central aim and interest of their joint working lives.

Skeletal morphogenesis in the urodele skull: II. Effect of developmental stage in thyroid hormone-induced remodeling

This study investigates the effect of developmental stage on thyroid hormone (TH)-mediated remodeling in the skeletal tissues of hemidactyliine plethodontid urodeles. Rate of morphogenesis was quantified in 17 metamorphic tissues for three different size-age classes of Eurycea bislineata larvae immersed in a metamorphic dosage of T₄ . Extent of morphogenesis after a 3-week immersion was also quantified in these tissues plus four larval ones for the full size range of E. bislineata larvae and for less complete size ranges of E. wilderae, E. longicauda guttolineata, Gyrinophilus porphyriticus, and Pseudotriton ruber larvae. Although all tissues respond more slowly with decreasing size/age, two tissue-specific effects are evident in all species. Larval ossifications are less inducible than metamorphic ossifications, and progressive metamorphic events are more retarded and, in some cases, more prone to abnormal morphogenesis than regressive ones. The first effect agrees with the prediction that tissues that naturally remodel at metamorphosis are more responsive to a metamorphic dosage of TH than those that respond at a larval stage and lower TH. The second effect agrees with the prediction that progressive morphogenesis is more likely to be impaired at small size than regressive morphogenesis, although the frequent discrepancies between individuals of similar size implicate developmental age more than size in this effect. Collectively, these two effects provide only equivocal support for the hypothesis that direct development in plethodontids evolved via precocious TH activity. However, the unexpected transition from ceratobranchial replacement to ceratobranchial shortening in medium-sized larvae suggests that the former pathway requires a longer period of cell specification at low TH. Since ancestral plethodontids appear to have been distinguished by an exceptionally long larval period with exceptionally low TH activity, this developmental prerequisite may in turn be partly responsible for their singular evolution of ceratobranchial replacement. © 1995 Wiley-Liss, Inc.

Insulin and thyroid hormones stimulate matrix metabolism in primary cultures of articular chondrocytes from young rabbits independently and in combination

These studies examined the effects of heat-inactivated horse serum, insulin, triiodothyronine (T3), and thyroxine (T4), individually and in combination, on collagen and proteoglycan synthesis by primary cell cultures of articular chondrocytes from immature male rabbits. Insulin concentrations of 25 to 100 ng/ml (4.4 to 17.4 x 10(-9) M) increasingly stimulated collagen and proteoglycan synthesis in the absence of serum. The effects of 25 ng/ml (4.4 x 10(-9) M) insulin or 15% heat-inactivated horse serum on collagen synthesis were similar. Triiodothyronine (10(-10) to 10(-6) M) and T4 (10(-8) to 10(-4) M) also stimulated collagen synthesis in the absence of serum, with peak effects at 10(-8) and 10(-6) M, respectively. Biphasic stimulation of proteoglycan synthesis was obtained with 10(-11) to 10(-7) MT3 (maximum at 10(-8) M) and 10(-8) to 10(-5) M T4 (maximum at 10(-7) M). In these experiments, triiodothyronine was 10 to 100 times more potent than T4 in stimulating cartilage matrix production. The cells retained their chondrocytic phenotype under hormonal stimulation, secreting almost exclusively Type II collagen and large, chondroitin sulfate-rich proteoglycans. The addition of insulin to maximally-stimulating concentrations of either T3 or T4 in serum-free medium further stimulated matrix synthesis, suggesting that these hormones modulate chondrocyte metabolism via multiple biosynthetic/receptor pathways.

Oxidative influence on development and differentiation: an overview of a free radical theory of development

Metabolic gradients exist in developing organisms and are believed to influence development. It has been postulated that the effects of these gradients on development result from differential oxygen supplies to tissues. Oxygen has been found to influence the course of development. Cells and tissues in various stages of differentiation exhibit discrete changes in their antioxidant defenses and in parameters of oxidation. Metabolically generated oxidants have been implicated as one factor that directs the initiation of certain developmental events. Also implicated as factors that modulate developmental processes are the cellular distribution of ions and the cytoskeleton both of which can be influenced by oxidants. The interaction of oxidants with ion balance and cytoskeleton is discussed.

Skull development during anuran metamorphosis. II. Role of thyroid hormone in osteogenesis

We examined the role of thyroid hormone (TH) in mediating cranial ossification during metamorphosis in the Oriental fire-bellied toad, Bombina orientalis. Exogenous T3 (3,3',5-triiodo-L-thyronine) was administered in three treatment dosages (0.025, 0.25, and 2.5 micrograms) plus a control dosage via plastic micropellets implanted within the dermis of tadpoles of three Gosner developmental stages: 28/29, 30/31, 32/33. Tadpoles were recovered after 2, 4, 6, and 8 d, and scored for the presence of three bones - median parasphenoid and paired frontoparietals and exoccipitals--as seen in cleared-and-stained, whole-mount preparations. T3 induced precocious ossification in both a stage-dependent and a dosage-dependent manner; stage dependence corresponded precisely with the degree of osteogenic differentiation at the time of hormone administration. Precocious ossification thus was due to the T3-promoted growth and calcified matrix deposition of these centers. Differential TH sensitivity among osteogenic sites may underlie both the temporal cranial ossification sequences characteristic of metamorphosing amphibians as well as sequence differences commonly observed among taxa.

Skull development during anuran metamorphosis: III. Role of thyroid hormone in chondrogenesis

Metamorphosis of cranial cartilages in anuran amphibians constitutes one of the most dramatic and extensive ontogenetic transformations in vertebrates. We quantitatively examined the role of thyroid hormone (3,3',5-triiodo-L-thyronine; T3) in mediating gross aspects of this morphological repatterning in the skull of the Oriental fire-bellied toad, Bombina orientalis. T3 was administered via plastic (Elvax) micropellets in three treatment dosages (2.5, 0.25, and 0.025 microgram) and one control dosage (0 microgram) to tadpoles of three Gosner developmental stages--28/29, 30/31, and 32/33; tadpoles were recoved up to 8 d (treatment and control dosages) or 14 d (control dosage) later. Response of larval cartilages to exogenous T3 was dosage dependent but not implant-stage dependent; chondrogenic tissues that participate in metamorphic transformation are competent to respond to T3 well before they normally do. Metamorphic effects of T3 were visible within 2 d; in most treatment groups, the normal metamorphic sequence was two-thirds complete after 8 d. While T3 also induced precocious ossification, the normal temporal relation between bone formation and cartilage transformation was dissociated in experimental groups. Morphological integration between cartilage and bone during cranial metamorphosis is at least partly the result of each tissue responding independently to endocrine factors.

activity and catch-up growth in hypothyroid rats

Hypothyroidism was induced in 38-day old male rats by feeding the animals a chow diet supplemented with propylthiouracil (PTU, 0.1% by weight) for 43 days. Wheel activity of PTU animals was not significantly different from that of euthyroid, ad lib feeding controls it was significantly lower when compared to pairfed controls. Body weight was significantly lower than that of euthyroid ad lib controls. After 75 days of PTU discontinuation catch-up growth of PTU animals was not complete: body weight and tibia length were significantly lower in the PTU condition in comparison to euthyroid, ad lib feeding condition. However, no difference existed between the catch-up growth of PTU and pairfed animals. It was suggested that growth arrest observed in early hypothyroidism may be partly due to factors nonspecific to thyroxine absence, such as hypophagia.

Endocrine dysfunction and slipped captial femoral epiphysis

Five patients with concomitant endocrinopathy and slipped capital femoral epiphysis were studied in detail. One had diabetes and hypothyroidism, one had hypothyroidism, one had hypergonadotropic hypogonadism and two had a craniopharyngioma (one of whom had severe panhypopituitarism post-operatively). An additional seven patients with cranio-pharyngioma revealed marked delay in closure of epiphyses and an additional undiagnosed case of slipped capital femoral epiphysis. Of the six patients with slipped capital femoral epiphysis, three had bilateral and three unilateral involvement. Of the five patients undergoing surgical stabilization, there was significant delay of epiphyseodesis, prompting us to recommend concomitant bone grafting. Histological examination of the femoral head from a three year old child with panhypopituitarism showed marked irregularity of the growth plate and loss of columnar integrity, which may be a predisposing factor to slipping in older children with endocrinopathies. The effects of various hormones on the physis are specifically discussed, especially as they relate to the possible etiology of slipped capital femoral epiphysis.

Growth of the rat femur in diffusion chambers

Growth in the length of 9-day-old rat femurs, isolated and cultivated in diffusion chambers, was studied for a limited period of 16 days. An increment of 40 per cent of the in vivo length was observed. It could be established that the elongation was due to chondral growth, of which 40 per cent occurred at the proximal and 60 per cent at the distal end of the bone during the 16 days of observation. When the trochanteric or the head epiphyses with their physeal plates were resected, no statistically significant loss in total length occurred. A small but statistically highly significant biphasic growth due to enchondral ossification was found. Resection of the trochanter produced a widening of the cervico-diaphyseal angle (valgisation) of an average of 11.2 degrees. The findings in this series of isolated femurs are --in a general way--similar to those observed in the living animal.

The effect of L-triiodothyronine on the growth and development of embryonic chick limb-bones in tissue culture

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