Growth of scleral cartilage

The eye wall of most submammalian vertebrates contains an ocular skeleton including a cup of hyaline cartilage in the sclera. During embryonic development the changes in the size and shape of this plate of cartilage match the growth of the choriod coat, pigmented epithelium and neural retina. In microphthalmic eyes which were experimentally produced by draining the vitreous body from the eye during early development, the sclera is smaller in area but thicker than normal. This is also true in spontaneously occurring microphthalmia . Thus, the area of the sclera adjusts to the size of the eye, probably under the control of mechanical factors generated by an expanding vitreous body. The problem remains whether the total mass of sclera continues to increase at a normal or at a reduced rate in eyes growing at subnormal rates. The growth of the untreated sclera was reflected by increases in its water, dry mass, DNA, hydroxyproline and protein nitrogen. There was a relative dehydration of the sclera during its development. The growth of the untreated sclera was reflected by increase in its water, dry mass, DNA, hydroxyproline and protein nitrogen. There was a relative dehydration of the sclera during it development. The sclerae of experimentally produced microphthalmic eyes of 16 days old chick embryos had a smaller wet weight, dry weight, DNA content, hydroxyproline content and protein nitrogen content than did the sclerae of the normal, unoperated contralateral eyes. The low DNA content indicates a failure of the cell population to increase in these slow growing eyes. On the other hand, the ratios of hydroxyproline and protein nitrogen to DNA, are the same in both the microphthalmic and normal eyes indicating that the mean output of extracellular components per cell is the same in both groups. It seems probable, therefore, that the growth of the sclera is regulated by the expanding vitreous body, and this regulation is mediated by the control of the number of cells which are mobilized into the cell population of the sclera (AU)

The growth of cartilage canals

Investigations into the growth of cartilage canals, in the cartilaginous upper end of the developing tibia in foetal sheep, were conducted along three lines: (1) The blood vessels were injected with visually opaque fluid and the selected cartilage was cleared. As the cartilage increased in size, the side-arms of a T-shaped canal common to all specimens increased their transverse extent. This point to active penetration of the cartilage by the vessels. (2) The vascular pattern within individual canals was examined histologically in serial sections. The tissue at the end of every canal received arterial blood first, perhaps because an active metabolic process (erosive) was taking place there. (3) The tissue between the vessels and the surrounding cartilage was examined histologically and histochemically in serial sections. a) At the end of a canal this tissue was minimal, chondrocyte capsules were breached on the side facing the vessels, chondrocyte glycogen was diminished but not entirely lost close to the vessels and some glycogen was present within the canal, multinucleate chondroclasis with high ribonucleic acid content were also present within the canal. b) Elsewhere along the canal the intervening tissue was greater in amount, glycogen was absent from the chondrocytes close to the vessels and also absent from the interior of the canal, cytoplasmic ribonucleic acid was not increased or near the canal, multinucleate cells were absent. It was concluded that a slow erosize process occurred over the greater part of the canals and produced a gradual increase in its diameters. At the ends of the canals a rapid erosive process contributed to elongation of the canals (AU)

The patterns of cartilage canals

The pattern of vascular canals in the cartilagenous upper end of the developing tibia was investigated in the sheep, goat, rabbit, cat, rat and man. The blood vessels were injected with a visually opaque fluid, and the cartilage was fixed, dehydrated and cleared. Specimens immersed in their clearing fluid were examined visually in transmitted light with a stereoscopic microscope. In each species the upper tibial cartilage showed a constant vascular canal pattern at all stages prior to development of the secondary centre of ossification. The pattern was constant in broad outline only, and allowed great variation in individual details. The canal pattern in any one species differed from the patterns of all other species studied. The different patterns were not obviously related to differences in the size and/or shape of the homologous cartilage in the different species examined. The course and distribution of the vascular canals indicated that the presence of these canals in cartilage cannot be explained soley as the result of passive inclusion of perichondrial cartilage around them (Summary)