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A collecting duct receives tubular fluid from several nephrons and participates in the final adjustment of urine. Collecting ducts merge as they pass through the renal medulla, conducting the remaining fluid toward the renal outlet. Although continuous with the renal tubules, the shared collecting duct system is not technically part of an individual nephron.^[ref]

Its epithelium can change its membrane permeability to water. Vasopressin promotes the presence of aquaporin-2 in the apical membrane facing the tubular lumen. When an appropriate osmotic pressure difference favours outward movement, water crosses toward the surrounding interstitial fluid and can return to the blood. The channel changes access to an osmosis pathway; it does not itself pump water against that driving difference.

A 1993 study by K. Fushimi and collaborators identified a collecting-tubule water-channel Complementary DNA, then tested the encoded channel by gene expression in Xenopus oocytes. Expression increased osmotic water permeability. Localization to the apical region of collecting-tubule biological cells connected that functional result to its site in the kidney.^[ref]

The regulated movement was tested more directly in 1995. Investigators measured osmotic water permeability in isolated perfused rat collecting ducts, then used immunoelectron microscopy to examine channel distribution in the same tubules before, during and after vasopressin exposure. Increased permeability accompanied more channel labelling at the apical membrane relative to intracellular vesicles. Withdrawing vasopressin reversed both changes.^[ref]

This links a physiological change to membrane trafficking: exocytosis can add channel-bearing membrane to the surface, while endocytosis can retrieve it. The amount of channel at the correct surface, as well as its presence somewhere inside the epithelial cell, determines the available water pathway.

Freeze-fracture electron microscopy reveals structures along a fracture through a frozen specimen, commonly within a biological membrane. The exposed surface is shadowed with a thin platinum-carbon deposit; supporting carbon stabilizes the replica, which is cleaned and examined by transmission electron microscopy. The image is of this replica, not a direct view through the original thick tissue.^[ref]

A practical preparation rapidly freezes tissue, fractures it in a cold vacuum apparatus, deposits the replica and transfers the cleaned film to a microscope grid. Membranes often split through their hydrophobic interior, exposing particles and complementary depressions associated with membrane proteins. The fracture plane is not generally the original outer cell surface.

This method is useful because it displays lateral membrane organization that a thin cross-section can hide. However, freezing artifacts, shadowing direction and fracture-path selection affect appearances. Combining replica structure with immunoelectron microscopy can help identify selected proteins, while an unlabeled intramembrane particle should not be assigned a molecular identity from shape alone.

Immunoelectron microscopy uses antibody binding to locate selected molecular targets in an electron microscope image. A common method couples an antibody, directly or through a secondary reagent, to an electron-dense gold particle. The particle supplies visible contrast while the antibody supplies molecular recognition.^[ref]

For example, to ask whether a membrane protein is concentrated in one cellular compartment, label thin sections, count gold markers in that compartment and compare with its area and with background regions. Thirty markers over $3{\mu \mathrm{m}}^2$ give a raw density of ten per square micrometre; three over an equal-area negative control give one. That difference is evidence about labeling specificity, not automatically an absolute protein count.

Controls include omitting the primary antibody and testing material lacking the target. Fixation can destroy or hide binding sites, and the antibody-marker complex displaces the visible particle from the target molecule. An unlabeled region may therefore reflect inaccessible antigen rather than absence of the molecule. Molecular specificity and spatial resolution require separate checks.

The median eminence is a region at the base of the hypothalamus where neurosecretory terminals release regulatory hormones near the first capillary bed of the hypophyseal portal system. It is an interface between hypothalamic neurons and the vascular route to the anterior pituitary. A GnRH terminal here is a site of release; the neuron's soma need not occupy the same location.^[ref]

Access to the capillaries also depends on neighboring glial cells, including tanycytes. In a 2004 study, female-rat tissue containing the median eminence was incubated for 30 minutes with or without added arginine, a substrate for nitric oxide production. Immunoelectron microscopy identified GnRH-containing terminals. In treated preparations, the investigators observed structural changes bringing terminals into direct contact with the space beside the capillaries. Complementary cell-culture experiments implicated signals from endothelium in glial remodeling.^[ref]

The comparison is illuminating because the route is more than a fixed pipe connecting two glands. The arrangement of neurons, glial cells and blood vessels can alter access to the release surface. A microscopic structural observation and a measurement of circulating hormone nevertheless remain different kinds of evidence: proximity alone does not quantify how much GnRH enters the blood.

Occludin is an integral membrane protein concentrated at tight junctions, where neighbouring epithelial or endothelial cells regulate passage between them. It has four membrane-spanning segments and a long cytoplasmic tail. Occludin is a junction component rather than a synonym for the entire sealing structure.

In 1993, Mikio Furuse and colleagues raised monoclonal antibodies against a junction-enriched fraction from chick liver. Immunoelectron microscopy placed the recognized protein directly at membrane-contact points of tight junctions; sequence analysis suggested four membrane crossings. These combined biochemical and spatial observations identified occludin.^[ref]

Later, expression of claudin-1 or claudin-2 in fibroblasts reconstituted extensive junction strands and recruited occludin. This separates the capacity to build a strand from the presence of one particular strand-associated protein.^[ref]

A podocyte is a specialized epithelial cell covering the outer surface of a glomerular capillary within the renal corpuscle. It belongs to the visceral portion of the glomerular capsule. Its branching extensions end in podocyte foot process structures, with a slit diaphragm spanning the gaps between neighboring processes. These structures cooperate with the basement membrane and capillary endothelium in the glomerular filtration barrier.

A 1999 experiment located nephrin more precisely than ordinary tissue staining could. Antibodies recognized the protein in glomerular extracts, and immunoelectron microscopy placed small gold labels in the slits between podocyte processes. The protein was therefore localized to a particular junctional region, rather than uniformly over the cell body.^[ref]

This provides an illuminating scale comparison: a light-microscope image can identify a glomerular region, while a molecular label in an electron-microscope section can assign a protein to one component of its filtration surface. Localization supports a structural role but does not, by itself, measure how rapidly water or each solute crosses the complete barrier.