Developmental changes in mitochondria during the transition into lactation in the mouse mammary gland. II. Membrane marker enzymes and membrane ultrastructure

The activity of cytochrome oxidase (an inner mitochondrial membrane marker) in mouse mammary gland homogenates was found to increase five- to sixfold from late pregnancy to day 8 of lactation, while that of monoamine oxidase (an outer membrane marker) increased only about 25%. The specific activity of cytochrome oxidase in the isolated mitochondria decreased slightly over the same period while the specific activity of monoamine oxidase decreased fivefold. This reflects the fact that both cytochrome oxidase and mitochondrial protein are increasing at a much greater rate than is monoamine oxidase activity. Mixing experiments preclude the possibility that the release or removal of an inhibitor or stimulator produces the changes in enzymatic activity. The cytochrome oxidase to monoamine oxidase ratio was followed throughout the pregnancy-lactation cycle in total mammary homogenates, isolated mammary parenchymal cells, and isolated mammary mitochondria. In each preparation the pattern was the same with little change in the ratio until late pregnancy; and then a three- to fourfold increase occurred and the values reached a maximum by day 8 of lactation. These experiments were interpreted as demonstrating that the observed enzymatic changes are reflective of alterations in the mitochondria of the mammary parenchymal cell population. Electron micrographs of mid-pregnant and mid-lactating mammary parenchymal cells in situ were prepared, and distinct changes in the mitochondrial morphology noted. The most significant and obvious change is the large increase in the number of inner membrane cristae and an increase in matrix density in the lactating gland cell. Therefore, both enzymatic and morphological studies support the concept of an expansion of the mitochondrial inner membrane during presecretory differentiation in the mouse mammary parenchymal cell.

Developmental changes in mitochondria during the transition into lactation in the mouse mammary gland

Mitochondrial biogenesis in the parenchymal cell of the mouse mammary gland appears to occur in two distinct phases: replication during cell proliferation, and maturation during cell differentiation. This study of the mitochondrial maturation phase in the mouse gland demonstrates a significant increase in organelle density on isopycnic sucrose gradient centrifugation during the transition from late pregnancy to day 8 of lactation. Differential fragility to high sucrose concentrations or changes in mitochondrial lipid composition do not satisfactorily explain the density increases. When organelle densities were assessed by centrifugation under iso-osmotic conditions with Ficoll gradients in 0.25 M sucrose, the mitochondria from pregnant glands were observed to be more dense than those from lactating glands. The two mitochondrial populations were also found to differ in their response to changes in sucrose concentration in the Ficoll gradients. When sucrose concentration was increased, the density of both pregnant and lactating gland mitochondria increased nonlinearly, the increase being greater with the lactating gland organelles. By use of mathematical models, the differing response was interpreted as a change in the density and osmotic activity of the mitochondrial internal compartment (inner membrane plus matrix space). We have proposed that the changes reflect a large expansion of the inner mitochondrial membrane and perhaps the matrix material during the transition into lactation in the differentiating parenchymal cell.

Diabetic complications in a patient with coexisting anterior hypopituitarism

We present a patient who developed insulin-requiring diabetes several years after the onset of symptoms of anterior hypopituitarism. It is likely that the hypopituitarism protected him against the development of diabetic retinopathy and glomerular basement membrane thickening but not against neuropathy and atheroma. The significance of this in relation to the growth-hormone-microangiopathy hypothesis is discussed.

Junctional ultrastructure in isolated synaptic membranes

The ultrastructure of synaptic junctions in whole brain tissue and isolated synaptic membranes has been compared. Type 1 junctions are present in the isolated membranes,readily identified by the presence of dense-staining material associated with the postsynaptic membrane, but the dense projections present at the presynaptic membrane in intact tissue are absent. Type 2 junctions are not easily recognized because of the absence of prominent junctional densities, but apposed membranes with the appearance of type 2 junctions are seen in isolated membrane preparations. Junctions without dense-staining material are also seen among SYNAPtosomes and survive the hypotonic conditions used during isolation of the membranes. It thus seems probable that both type 1 and 2 junctions are present in isolated synaptic membrane preparations. In type 1 junctions after isolation,the postsynaptic thickening and cleft substance are together seen to be composed of an array of 200 A dense-staining subunits spanning the postsynaptic unit membrane. The relationship of this structure to the ultrastructure of the cleft substance and postsynapticthickening in intact tissue is discussed.