EFFECT OF X-RAYS ON THE MORPHOGENESIS OF THE EMBRYONIC CHICK TIBIOTARSUS

A reevaluation of X-irradiation-induced phocomelia and proximodistal limb patterning

Phocomelia is a devastating, rare congenital limb malformation in which the long bones are shorter than normal, with the upper portion of the limb being most severely affected. In extreme cases, the hands or fingers are attached directly to the shoulder and the most proximal elements (those closest to the shoulder) are entirely missing. This disorder, previously known in both autosomal recessive and sporadic forms, showed a marked increase in incidence in the early 1960s due to the tragic toxicological effects of the drug thalidomide, which had been prescribed as a mild sedative. This human birth defect is mimicked in developing chick limb buds exposed to X-irradiation. Both X-irradiation and thalidomide-induced phocomelia have been interpreted as patterning defects in the context of the progress zone model, which states that a cell's proximodistal identity is determined by the length of time spent in a distal limb region termed the 'progress zone'. Indeed, studies of X-irradiation-induced phocomelia have served as one of the two major experimental lines of evidence supporting the validity of the progress zone model. Here, using a combination of molecular analysis and lineage tracing in chick, we show that X-irradiation-induced phocomelia is fundamentally not a patterning defect, but rather results from a time-dependent loss of skeletal progenitors. Because skeletal condensation proceeds from the shoulder to fingers (in a proximal to distal direction), the proximal elements are differentially affected in limb buds exposed to radiation at early stages. This conclusion changes the framework for considering the effect of thalidomide and other forms of phocomelia, suggesting the possibility that the aetiology lies not in a defect in the patterning process, but rather in progenitor cell survival and differentiation. Moreover, molecular evidence that proximodistal patterning is unaffected after X-irradiation does not support the predictions of the progress zone model.

Computer model of endochondral growth and ossification in long bones: biological and mechanobiological influences

Endochondral growth and ossification, the processes by which cartilage increases in size and is replaced by bone, are affected by biological factors such as intrinsic genetic makeup and systemic chemical agents. In addition, these processes are affected by epigenetic mechanical factors: they may be accelerated in regions of intermittent high shear stress and decelerated in regions of intermittent high hydrostatic pressure. Previous models of bone development have not incorporated both biological and mechanobiological influences on endochondral growth and ossification. We have implemented a finite element analysis to model a developing bone rudiment from 8 weeks of gestational development to approximately 2 years after birth. As a function of time, we calculated a maturity index that reflects the progression of a region of cartilage through the endochondral ossification sequence of proliferation, hypertrophy, mineralization, and replacement by bone. We calculated a specific growth rate for each region of cartilage and estimated overall longitudinal growth of the rudiment. Regions of cartilage replaced by bone were remodeled. The results from the maturity index can be compared with distributions of proliferative, hypertrophic, and mineralized cartilage seen on histology at various stages in development. The results of the simulation predicted prenatal and postnatal developmental events, including formation of a secondary ossific nucleus, a layer of articular cartilage, and a growth plate. Our results demonstrate the necessity to include biological and mechanobiological influences when endochondral growth and ossification are considered.

Kinetic studies on epiphyseal growth cartilage in the normal mouse

The synthesis of DNA was studied in the proximal tibial growth plate of 25-day-old healthy NMRI mice by using the thymidine analog bromodeoxyuridine (BrdUrd), which is incorporated into cells in the S-phase. Such cells were found only in the upper three fifths of the morphologically defined proliferating zone. This zone was therefore subdivided into a functional proliferating zone (the S-phase zone) where most, if not all, chondrocytes proliferate, and a remaining maturation zone. The BrdUrd containing immunoreactive cells could then be followed at different intervals and they were found at the chondro-osseous junction after only 36 h. By using double-labeling with BrdUrd and iododeoxyuridine (IdUrd) the duration of cell cycle components could be estimated; that is, the time for DNA synthesis (S-phase), second gap and mitosis (G2 + M-phase), and remaining first gap (G1). We determined an S-phase time of 7.1 h and an average cell-cycle duration of 36 h. The G2 + M-phase was estimated as 3.5-4 h, leaving an average G1-phase time of 25 h, which probably varies considerably between chondrocytes. By combining these data with morphometrical data regarding distances between cells, we calculated a total growth rate of 9.0 microm/h. Of this rate, 80% was entirely related to the process of hypertrophy--that is, longitudinal expansion without any corresponding increase in cell number--and 75 % was the result of processes outside the S-phase zone. Five percent of the growth was due to the expansion of cell distances within the S-phase zone. In this way longitudinal expansion can be studied at different levels in the growth plate and the data permit calculation of changes in volumes of the extracellular matrix. The largest increases in matrix volume occurred in the hypertrophic zone. These data may serve as a basis for further studies on matrix turnover in relation to growth.

Effects of radiation on chondrocytes in culture

Using an in vitro model, we studied the direct effects of radiation on both DNA and glycosaminoglycan (GAG) synthesis of chondrocytes. Chondrocytes from articular cartilage of 21-day-old rabbits were cultured in monolayer or in pellets with matrix as a three-dimensional tube culture. These cells were exposed to a single dose of X-rays at either 2 or 10 Gy. Following irradiation at 2 Gy, DNA synthesis in the chondrocytes was temporarily suppressed but rapidly returned to the control level, while at 10 Gy, DNA synthesis was markedly suppressed and there was no increase in cell number. On the other hand, GAG synthesis was not affected by a single dose of X-rays at either 2 or 10 Gy, nor did GAG content decrease in the three-dimensional tube cultures. These results show that radiation affects the proliferation and differentiation of chondrocytes differently, and that the synthesis of the components of cartilaginous matrix such as GAG is relatively radioresistant in contrast to DNA synthesis.

Radiation effects on the matrix synthesis in non-ossifying embryonic cartilage in vitro: a functional and morphological study

Radiation effects on collagen and glycosaminoglycan (GAG) synthesis were studied in non-ossifying embryonic chick cartilage, in vitro. Fragments of the non-calcifying region of 15-day-old chick sterna were irradiated with 20, 50 and 100 Gy. The counterparts served as controls. Fragments were cultured up to 14 days. Pulse labeling experiments with 3H-proline and 3H-glucosamine started immediately after irradiation and on day 3, 6 and 13 post-irradiation. Fragments for morphological evaluation were fixed on day 1, 4, 7 and 14, simultaneously with the end of the labeling period. The same dose- and time-dependent inhibition was observed for the collagen and GAG synthesis. Radiation doses of 20, 50 and 100 Gy cause respectively and inhibition of about 20, 35 and 60% at the end of the culture period. Morphological alterations were observed after radiation doses of 50 and 100 Gy. Dose- and time-dependent necrosis all over the sternum was observed. A restricted number of cells showed an increased GAG synthesis after irradiation (50 Gy). This could explain the levelling off of the radiation effect on the incorporation of the labeled precursors towards the end of the culture period for radiation doses up to 50 Gy.

Effects of ionizing radiation on the metabolism and longitudinal growth of cartilaginous embryonic chick tibiae in vitro

The effect of ionizing radiation on the metabolism and longitudinal growth of cartilaginous tibiae of 6.5-day-old chick embryos was studied in vitro over a 3-day period. Before being cultured, tibiae received absorbed doses of 2 to 200 Gy. Of each pair, the counterpart served as control. Compared to the strong inhibition of [3H]thymidine incorporation, already 50 percent at 10 Gy, the effects of ionizing radiation on [3H]uridine and [3H]proline incorporation were limited: 20 and 40 percent respectively at 150 Gy. Metabolism of the cartilage cells in our organ culture was almost completely arrested at 200 Gy. Light and electron microscopy showed no morphological differences between irradiated and sham-irradiated tibiae until 150 Gy. At 200 Gy necrosis of most of the cells was observed. No differences in form and arrangement of extracellular fibers were noticed. The results of the metabolic studies and the morphological observations were correlated with the effects of ionizing radiation on the longitudinal growth. In contrast to DNA synthesis, RNA transcription and synthesis of collagen fibres were radioresistant processes.