The Submicroscopic Structure of Articular Cartilage in the Adult Pig

Horky D.: The Submicroscopic StrUCture of Articular Cartilage in the Adult Pig. Acta vet. Bmo, 62,1993: 9-18. Articular cartilage collected from the femoral heads of 5 adult pigs of both sexes,_ aged 14-24 months, was studied by light microscopy and transmission and scanning electron microscopy. . . Chondrocytes of the surface layer were oval in shape and sent out short projections into the surrounding pericellular matrix. Their cytoplasm contained numerous lysosomes, transport vacuoles, centrioles and a well·formed Golgi apparatus and small deposits of glycogen. They were arranged in rows or tiers. Chondrocytes of the middle layer were oval cells enclosed by pairs in lacunae. Their nuclei had 1-2 nucleoli of reticular type. The zonula nucleum limitans was well developed. The cytoplasm contained a large number of mitochondria, cisternae of the granular endoplasmic reticulum, a large Golgi field, numerous transport vacuoles, lysosomes and conspicuous glycogen deposits. In the transitional zone, chondrocytes were arranged in tiers perpendicular to the surface. They were smaller in size and the cytoplasm contained, apart from the typical organelles, large bundles of intermediate filaments. Chondrocytes of the deep layer could be distinguished into those characterized by conspicuously large lipid vacuoles and those with homogeneous cytoplasm and small glycogen deposits. The pericellular matrix was well developed in the majority of chondrocytes; in the regions where it was missing the cell membrane was in contact with the intercellular matrix. In the middle layer, cell detritus was seen at the border between pericellular and intercellular matrix Submicroscopic structure, articular cartilage, adult swine Articular cartilage is an avascular, alymphatic and aneural tissue lining the articular bonesurface. Like other connective tissues it consists of cells chondrocytes deposited in an abun-· dance of the intercellular matrix. The chondrocytes account for only 0.01-0.1 % of the total cartilage volume. The intercellular matrix is made up of collagenous fibres, proteoglycans, and. organic and inorganic components. A proper function of the articular cartilage depends on its mechanical properties permitting it to a) transfer and distribute high pressure forces upon the subchondral bone; b) maintain the constant load at a relatively low level; c) facilitate movementat minimal friction (Wright 1969; Freeman and Kempson 1973; Maroudas 1973; Chappuis et a!. 1983; Swann et a!. 1984). Resistance to pressure in cartilage is secured by the structure and arrangement of the intercellular matrix in both its parts (Weiss et a!. 1968; Clarke 1974; Bloebaum and Wilson 1980; Horky 1980; Ghadially 1983; O'Connor et al. 1948; Clark 1990). The chondrocytes havea minimal involvement in the mechanics of articular movement but playa key role in the synthesis. of intercellular matrix which is responsible for the mechanical properties of cartilage and thesliding and lubrication of contact surfaces (Maroudas 1973; Ghadially 1983; Palmoski and Brandt 1984; Poole et al. 1988; Buckwalter et al. 1989; Copf and Czarnetzki 1989; Fife1989). Articular cartilage arises from mesenchyma during the skeletal development as a part of carp· Lilaginous blastema of the bone rudiment. The pre-formed bone rudiment is gradually eroded • but articular cartilage is affected by neither this nor the following ossification process and remains ;as a thin layer on the articular surface (Bonucci 1967; Hanaoka 1976). The condensation of mesenchyma in the blastema takes place in the early embryonic development and in man, according to Gardner and O'Rahilly (1968), chondrification of the femur .is commenced at 6 weeks and the articular cavity appears as a differentiated groove produced by mesenchymal blastema of the bone rudiment at 8 weeks (Ghadially 1983). Our results suggest -that by this time articular cartilage has been completed (Horky 1991a, b). However, data on articular cartilage differentiation in lower mammals, apart from cattle (Horky 1986), have not been . reported in the literature. The differentiation events leading to the formation of articular cartilage before birth, and .eventually producing the highly specialized tissue after birth, are called the maturation process. They are determined by genetic, endocrinologic and nutritional factors (Grondalen 1974c; Silbergeret aI. 1961; Grondalen 1979a, d, e, f) to which the effects of endogenous environments are added in the postnatal period (Ghadially 1981; Perrin et aI. 1987; Wilsman et -al. 1981). Changes in the morphology of articular cartilage are most frequently related to age. They have been amply documented in mice (Silberger et aI. 1976), rats (Mark et aI. 1998), rabbits (Davies et aI. 1962; Barnett et aI. 1963), dogs (Lust et al. 1972; Lust and Sherman 1973; Wiltberger and Lust 1975; Fife 1989), cattle (Horky 1983, 1987; Neame et aI. 1989; Kiefer et aI. 1989) pigs (Grondalen 1974b, c, f; Nakano et aI. 1979a, b; Horky 1989) and man (Horky 1980, 1991a, b; Ghadially 1983). From the studies concerning porcine articular carti• :lage, information on its ultrastructure under physiologic conditions has been provided only by the paper of Bhatnagara et al. (1981), who investigated pigs 20 to 30 weeks old, and by our earlier work (Horky 1991d) on porcine articular cartilage in the early postnatal period. Some of the above mentioned studies have also been concerned with pathological findings at the lumbosacral junction (Doige 1980) or with the growth plate in relation to age (Nakano et aI. 1982; Farnum et aI. (1984). The ultrastructure of articular cartilage of the adult pig, which so far has not been studied, cis dealt with in this communication. Materials and Methods Articular cartilage was collected from the femoral heads of 5 pigs aged 14-24 months to be "studied by light microscopy and transmission and scanning electron microscopy. For transmission microscopy, the tissue samples were further dissected to obtain strips, 1 by 1 by 3 mm in size, which were immediately fixed in a glutaraldehyde solution (400 mmol/l in 0.1 M phosphate buffer, pH 7.4). The tissue was then decalcified with 0.1 M EDTA in 400 -mmol/l solution of glutaraldehyde, pH 7.2, applied twice for 60 min., and then left in the solution ·overnight. In the last bath it was kept for 75 min. and then rinsed in 4 consecutive baths of 0.1 M phosphate buffer, pH 7.4 (30 min. each) and fixed in two baths of 40 mmol/l solution of OsO, :in phosphate buffer, pH 7.4. Semithin sections for light microscopy observations were prepared -by standard techniques (dehydration, immersion and embedding in Durcupan ACM) and stained with methylene blue and Azure II. Ultrathin sections were made using an ultramicrotome (Ultra,cut Reichert), stained with either lead citrate alone or uranyl acetate and lead citrate, and exami• . ned and photographed with a Tesla BS 500 electron microscope.