REVERSIBLE MOVEMENT OF FLUID ACROSS THE FOETAL LUNG

Differentiation of the sarcoplasmic reticulum and T system in developing chick skeletal muscle in vitro

The electron microscope was used to investigate the first 10 days of differentiation of the SR and the T system in skeletal muscle cultured from the breast muscle of 11-day chick embryos. The T-system tubules could be clearly distinguished from the SR in developing muscle cells fixed with glutaraldehyde and osmium tetroxide. Ferritin diffusion confirmed this finding: the ferritin particles were found only in the tubules identified as T system. The proliferation of both membranous systems seemed to start almost simultaneously at the earliest myotube stage. Observations suggested that the new SR membranes developed from the rough-surfaced ER as tubular projections. The SR tubules connected with one another to form a network around the myofibril. The T-system tubules were formed by invagination of the sarcolemma. The early extension of the T system by branching and budding was seen only in subsarcolemmal regions. Subsequently the T-system tubules could be seen deep within the muscle cells. Immediately after invaginating, the T-system tubule formed, along its course, specialized connections with the SR or ER: triadic structures showing various degrees of differentiation. The simultaneous occurrence of myofibril formation and membrane proliferation is considered to be important in understanding the coordinated events resulting in the differentiated myotube.

Increased acinar complexity with polyhydramnios

We report 4 cases that presented a contradiction in terms of intrauterine lung growth. In all cases there was increased acinar complexity despite an array of influences known to be associated with pulmonary hypoplasia. In 2 of the 4 cases the lungs were severely hypoplastic by weight. Following an analysis of the factors operative in each case regarding lung growth, we conclude that diffuse increased acinar complexity occurs in selected cases with massive polyhydramnios. The observation is of importance because it is consonant with an intimate and dynamic relationship between amniotic fluid volume, lung fluid production, and fetal breathing movements in normal intrauterine lung growth and maturation. Although lung hypoplasia is known to occur in association with polyhydramnios, the reported alterations in lung structure were not previously observed.

Fetal respiration. A review

During recent years respiratory movements by the human fetus have been rediscovered. The types of movements have been defined, and their dependence on some physiologic conditions has been described. Pharmacologic agents influence not only the occurrence of the movements but also their rate and depth. A tidal flow of fluid between lung and amnionic sac has been established, which may play an important role in lung development. Demonstration of this route for exchange of water and soluble substances identifies the lung as an organ very likely involved in exchange between fetal compartments. Additionally, the deposition in the lung of the particulate matter contained in amnionic fluid may not be proof of pathologic aspiration as previously thought.

Formation of elaborate networks of T-system tubules in cultured skeletal muscle with special reference to the T-system formation

Muscle cells, cultured for 1-28 days from 11-day chick embryo breast muscles, often show elaborate, three-dimensional networks of a membranous system. The network consists of tubular units which are quite regularly arranged. The tubular units composing the network are accessible to ferritin particles suspended in the culture medium; this suggests continuity with the extracellular fluid. These networks can be regarded as a special morphological elaboration of the T-system tubules. Such network formations can be seen much more often in well-developed myotubes. The networks usually exhibit a hexagonal pattern, which is formed of tubular units of a constant diameter. However, some early myotubes contain tetragonal networks, which are composed of spherical pockets with channels of lesser diameter connecting the pockets. Networks are also observed which probably represent a transitional form between these two patterns. Myotubes show many inpocketings of the sarcolemma similar to what are commonly referred to as caveolae or micropinocytotic vesicles. The similarity in configuration and dimension of the tubular units of the network to the caveolae leads to the plausible suggestion that repeated caveola-formation from the sarcolemma or T-system tubule may result in formation of these networks. In this connection, a possible mechanism of the T-system tubule formation is discussed.