Mechanisms for the generation of gas-exchange surface area in rat lung.

During development, rat lung seems to make use of the mechanisms of 1) expansion, 2) replication, and 3) subdivision to increase its gas-exchange surface area. To quantitate the contribution of each one of these mechanisms from birth to adulthood, four models of lung development have been mathematically evaluated: 1) existing saccules at birth septate (subdivide) during the first 2 postnatal weeks, creating smaller alveoli that undergo a continuous expansion up to adulthood; 2) same as the previous model except that new alveoli are also formed by means other than septation after the second postnatal week; 3) same as the previous model except that the alveoli formed by means other than septation start forming immediately after birth; and 4) saccules continue to be formed up to adulthood and subsequently septate, forming alveoli. In an evaluation of these models mathematically, it was found that expansion followed by replication is the most important mechanism for the generation of gas-exchange surface area in all four models in the maturing lung. Although the contribution of septation is important during early postnatal life, its importance decreases with age.

Development of gas-exchange surface area in rat lung. The effect of alveolar shape.

During development, the surface area of the gas-exchange region of the lung can increase by: (1) expansion, (2) subdivision (septation), (3) replication, or (4) change of shape of the basic gas-exchange units (saccules or alveoli). We evaluated the shape of these units in rat lung from birth to adulthood. A shape factor (phi g) was defined in terms of the surface area (Sg) and the volume (Va) of the average unit, using the expression phi g = SgVa-2/3. We studied the lungs of untreated animals and of animals exposed to or treated with hyperoxia (> 95% O2) and/or dexamethasone, each of which is known to inhibit septation in early postnatal life, and with deferoxamine, which protects the lung against the inhibitory effect of hyperoxia. The values found for the shape factor showed no significant difference with regard to age or treatment. This finding suggests that: (1) saccules and alveoli are formed with a certain predetermined shape (close to a hemisphere), (2) any enlargement with time is isotropic, (3) alveolar shape is insensitive to the drastic treatments used, and (4) change of shape is not a mechanism used to increase the gas-exchange surface area of the developing rat lung.

The formation of alveoli in rat lung during the third and fourth postnatal weeks: effect of hyperoxia, dexamethasone, and deferoxamine.

Terminal gas-exchange units in the lung of many species are, at birth, relatively large structures termed saccules. Saccules septate postnatally forming smaller units that constitute the final alveoli. In the rat, septation occurs intensively during the first 2 postnatal wk after which it has been considered to stop. Treatment with dexamethasone or exposure to hyperoxia during the first 2 postnatal wk markedly inhibits septation as evidenced by the formation of fewer and bigger alveoli than in normally developed rats. Deferoxamine, an iron chelator, has been reported to protect the lung from the effects of exposure to hyperoxia in early postnatal life. In this study, we investigated the effects of these treatments during the 3rd and 4th postnatal wk, that is, after the early period of rapid alveolarization. Our results show that treatment with dexamethasone no longer had any inhibitory effect on alveoli formation; that exposure to hyperoxia continued to inhibit the formation of new alveoli, resulting in bigger and less numerous alveoli; that treatment of animals exposed to hyperoxia with deferoxamine still protected their lungs against hyperoxic inhibition; and that elastin fiber length density in the lung was significantly reduced in hyperoxic-exposed animals. These results suggest that septation continues beyond the 2nd postnatal wk and does not stop abruptly at age 14 d in air-breathing rats and that hyperoxic inhibition of alveolarization during the 3rd and 4th postnatal wk is due to the inhibition of septation of existing or newly generated gas-exchange units during that period of lung development.

A model of postnatal formation of alveoli in rat lung.

In rats, and many other species, most lung alveoli are formed after birth. Septation of the large air saccules existing at birth has been considered as the main mechanism for alveoli formation. However, other undefined means of alveolarization have also been postulated to account for the large increase in gas-exchange surface area that takes place in the lung as the rat grows larger. Moreover, recent results show that the majority of alveoli in rat lung are formed by means other than septation of saccules existing at birth, but these mechanisms have not been identified up to the present. In this study, a mathematical model of alveolarization in rat lung is presented. The model is based on three postulates: (a) new saccules continue to be formed up to adulthood according to certain rules; (b) all these saccules subsequently septate generating a certain number of alveoli; (c) once formed, the saccules (and alveoli) do not change in volume, but newly-formed saccules are larger than the preceding ones according to a given law. The model accurately predicts the experimentally-known values at different ages of total alveolar volume, alveolar number, volume of the average alveolus, gas-exchange surface area, and alveolar volume distribution for normal rats and for rats in which septation is inhibited by treatment with dexamethasone or hypoxia during the early postnatal weeks of life.

Alveolar size, number, and surface area: developmentally dependent response to 13% O2.

Nonpregnant female rats were kept in 13% O2 for greater than 3 wk before being bred, throughout pregnancy, and, with their pups, after birth; control rats were only in air. The average volume (v), number (N) and surface area (Sa) of gas-exchange structures (saccules or alveoli) were estimated by stereological means. Saccule conversion to alveoli by septation between age 2 and 14 days was impaired in 13% O2 rats; there was less decrease in v (signifying less septation) and less increase in N (indicating the formation of fewer alveoli) in 13% O2 pups than in air pups. Between age 14 and 40 days, v rose 2-fold in air pups and 1.3-fold in 13% O2 pups; N increased 1.7-fold in air rats and 2.8-fold in 13% O2 rats. In other experiments, 23-day-old rats, previously only in air, were continued in air or were placed in 13% O2 until 44-days-old. At age 44 days, Sa was 25% and v 27% greater in 13% O2 rats than in air rats, but N was the same in both groups. We conclude there are multiple mechanisms for forming alveoli and increasing Sa and these mechanisms exhibit a developmentally dependent response to 13% O2.

Alveolar dimensions and number: developmental and hormonal regulation.

We used three-dimensional reconstruction 1) to determine the effect of treating rats with dexamethasone, from age 4 to 13 days, on alveolar volume (v) and number (Na) and 2) to determine if v, Na, or both change between age 14 and 60 days. At age 14 days, v, Na, and gas exchange surface area (Sa) were (2.7 +/- 0.3) x 10(4) microns3, (20.2 +/- 2.1) x 10(6), and 832 +/- 29 cm2, respectively, in diluent-treated rats; in dexamethasone-treated 14-day-old rats the same parameters were (7.5 +/- 1.1) x 10(4) microns3, (9.9 +/- 1.6) x 10(6), and 733 +/- 16 cm2, respectively. At age 60 days, v, Na, and Sa were (7.1 +/- 0.4) x 10(4) microns3, (60.8 +/- 4.1) x 10(6), and 4,495 +/- 187 cm2 in diluent-treated rats and in rats treated with dexamethasone from age 4 to 13 days, v, Na, and Sa at age 60 days were (15.9 +/- 2.4) x 10(4) microns3, (25.8 +/- 3.8) x 10(6), and 3,424 +/- 203 cm2. We conclude that treatment with dexamethasone from age 4 to 13 days (the period of normal septation) resulted in larger alveoli at age 14 and 60 days, and in diluent- and dexamethasone-treated rats the increase in Sa between age 14 and 60 days is due, at least in part, to the formation of new alveoli.

Aspiration and characterization of predentin fluid in developing rat teeth by means of a micropuncture and micro-analytical technique.

A fluid phase was aspirated in vivo and in vitro from predentin or pulp of developing rat teeth by means of a micropuncture technique. Pooled aspirates (approx. 2 nL) were analyzed for P, Na, K, Ca, Mg, and S by electron probe microtechniques (Lechene and Warner, 1979). Compared with pulp fluid, currently and previously studied cartilage fluids, as well as serum, predentin fluid showed elevated K, depressed Na, Cl, and Ca, as well as increased P. Statistical analysis was possible for only a few groups of comparisons among the elemental profiles. Ultrastructural examination of the aspiration site and of the aspirates showed no evidence of contamination with cell organelles or other formed elements. The micropuncture technique used was a critically precise and laborious procedure; possible contamination with intracellular fluid could not be avoided. The consistently low Mg concentration found in the aspirates, however, supports our view that the samples were primarily extracellular.

Light-scattering study on the influence of link-glycoproteins and lysozyme on the hyaluronate molecular conformation in solution.

The influence of link-glycoproteins and mammalian lysozyme on the configuration and size of the hyaluronate molecule in highly diluted solutions under physiological electrolytic and pH conditions was investigated by light-scattering techniques and confirmed by column chromatography, isopycnic flotation, and boundary centrifugation. It was consistently found that link-glycoproteins induce an increase in the basic structural dimensions of the hyaluronate molecule in solution. It was also found that this increase was reversed or prevented under the action of mammalian lysozyme. Changes in configuration of the hyaluronate molecule could be related to its aggregating capacity when the hyaluronate interacts with proteoglycan subunits. It is postulated that link-glycoproteins induce structural changes in the hyaluronate molecule that might improve its aggregating capacity while mammalian lysozyme prevents or regulates such improvement.

Biomechanical and biochemical properties of dog cartilage in experimentally induced osteoarthritis.

The finding of other investigators that increased water content is often associated with signs of a torn collagen network in human osteoarthritic (OA) cartilage led to this study. In the Pond-Nuki model of post-traumatic OA experimental but not control femoral condylar cartilage showed evidence of breakdown and stiffening of collagen network as assessed by measurement of swelling properties and indentation behaviour respectively. These changes in the unstable knees occurred despite lack of erosion of that surface cartilage ascertained from carbon black mapping and history. The stiffening rather than softening change was therefore attributed to cartilage oedema of the middle and deep certilagenous zones, wherein breakdown of collagen network has been postulated to occur. Because of insignificant reduction of total hexuronate in these cartilages, a proteoglycan (PG) profile of sedimentation coefficients for aggregate (PGA) and subunit species (PGS) was analysed to see if collagen network changes in the dog preceded PG alteration. Despite minimal histological changes our results confirmed previous findings in the tibial plateau cartilage in this model, that PGA was reduced in size and PGS increased in amount. Slight enzymatic breakdown of PGs, or altered synthesis due to cellular responses to either the injury directly or to synovial inflammation, seems necessary to explain such changes in the absence of cartilage erosion.