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The cell kinetics of the cartilage growth plate are outlined and discussed in terms of the probable levels of control on the system. Possible mechanisms of growth control at the cellular level are examined for (i) the rate of cell division in the proliferation zone, (ii) the command to differentiate that limits the size of the proliferation zone and (iii) the ageing process in the cartilage plate. the evidence of cell kinetics does not point unequivocally to any particular mechanism.

This paper describes a study in the human femur of the relationship between cell division in growth cartilage and overall bone growth. Growth rates for the distal femur from birth to eighteen years were determined from serial radiographs available from the Harpenden Growth Study; An average of 1-4 cm/year was found for the ages of five to eight years. The development of the growth plate is illustrated in a series of photomicrographs of femur sections. These sections were also used for quantitative histology; The length of the proliferation zone was estimated from cell counts to be twenty-four cells per column. On the basis of this value and the measured growth rate, an approximate mean cycle time of twenty days was found for the proliferating cells of the human growth plate. Since the corresponding cycle time is two days for rodent growth plates, which also have a different structure, it is unwise to extrapolate the findings in this tissue from mouse to man.

1. The pattern of tritiated thymidine labelling in the cells of the epiphysial cartilage and metaphysis of the tibia in the rat is described for intervals of one hour to twenty-eight days after injection. 2. The region of dividing cells is defined and evidence given for a zone of reserve cells at the top of the cartilage columns. 3. The difficulties of quantitative grain count studies are discussed, and some approximate values are given for the generation time and mitotic cycle periods of the cartilage plate cells. 4. Some further evidence is given about the life cycles of the osteoblast and the osteoclast.

A number of cell kinetic techniques using labelled thymidine and autoradiography have been applied to study growth cartilage in the rat tibia during ageing. No change in the duration of the synthesis phase was found from 4 to 13 weeks of age but there was a reduction in cell proliferation rate during this period. Measurements of labelling index, proliferation zone size and height of hypertrophic cells were used to calculate the growth rate of the bone from 7 days to 1 year. The results agreed well with radiographic measurements of bone growth.

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Twisting and bending deformities of the long limb bones of growing birds are common. In domestic poultry they are associated with rapid growth. In order to examine the variation in bone growth rates amongst the Class Aves as a step towards learning about the aetiology of growth deformities, we collected data on this and several other variables from a large sample of species. Adult tarsometatarsal length (ATL) was proportional to adult weight (W)0.36. The exponent did not differ significantly from 1/3. Mean legginess (L), defined as ATL/W1/3 (i.e. ATL.W‐1/3), was 80 mm.kg‐1/3 but varied according to the habitat occupied; terrestrial and arboreal birds had relatively longer legs than aquatic and aerial ones. ATL growth rate (GR) averaged 20 mm.d‐1 and was not correlated with W. It was highly influenced by pattern of development, being on average about three times greater in altricial compared with precocial birds of the same adult weight. Amongst birds of the same developmental pattern, GR was linearly related to L. Tarsometatarsal growth rate varied from 0.35 to 60 mm.d‐1 in the Class Aves and there was no evidence of intrinsic limits to GR from our results. How rapidly such bone growth rates are achieved remains to be determined.

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