Species-specific differences in follicular antral sizes result from diffusion-based limitations on the thickness of the granulosa cell layer.

The size of mature oocytes is similar across mammalian species, yet the size of ovarian follicles increases with species size, with some ovarian follicles reaching diameters>1000-fold the size of the enclosed oocyte. Here we show that the different follicular sizes can be explained with diffusion-based limitations on the thickness of the hormone-secreting granulosa layer. By analysing published data on human follicular growth and granulosa cell expansion during follicular maturation we find that the 4-fold increase of the antral follicle diameter is entirely driven by an increase in the follicular fluid volume, while the thickness of the surrounding granulosa layer remains constant at ∼45±10 µm. Based on the measured kinetic constants, the model reveals that the observed fall in the gonadotrophin concentration from peripheral blood circulation to the follicular antrum is a result of sequestration in the granulosa. The model further shows that as a result of sequestration, an increased granulosa thickness cannot substantially increase estradiol production but rather deprives the oocyte from gonadotrophins. Larger animals (with a larger blood volume) require more estradiol as produced by the ovaries to down-regulate follicle-stimulating hormone-secretion in the pituitary. Larger follicle diameters result in larger follicle surface areas for constant granulosa layer thickness. The reported increase in the follicular surface area in larger species indeed correlates linearly both with species mass and with the predicted increase in estradiol output. In summary, we propose a structural role for the antrum in that it determines the volume of the granulosa layer and thus the level of estrogen production.

Organisation of B-cell receptors on the cell membrane.

B-cell receptors (BCRs) have been reported to organise into oligomeric clusters on the B-cell surface, and mutations, that are likely to interfere with such clustering, result in B-cell unresponsiveness. This has led to the suggestion that pre-formed BCR clusters may be crucial for B-cell signalling. However, neither the size nor the fraction of BCRs organised in such clusters have yet been determined in experiments. Hence, the authors use a statistical approach to predict the membrane organisation of BCRs, based on available experimental data. For physiological parameters, most BCRs will organise into supramolecular polymers that comprise about five receptors where the non-covalent interactions are mediated by the IgH transmembrane helix. A reduction in the density of IgM to 2-5% of the normal density, a characteristic of anergic MD4 B cells, strongly reduces IgM polymerisation, and it is suggested that impaired BCR clustering may be responsible for the unresponsiveness of anergic B cells.

Formation of the B cell synapse: retention or recruitment?

Interaction of B cells with membrane antigen results in the formation of the B cell synapse: the B cell receptor (BCR) and antigen concentrate in the contact zone while CD45/B220 and the phosphatase SHP-1 are excluded. This study shows that, unlike in T cells, synapse formation does not require active transport processes (while subsequent antigen extraction and IgM downregulation do). The synapse architecture depends on the available protein ligands in the contact zone. Thus Syk, IgM and Fc receptor accumulation require the presence of ITAM-bearing BCRs, membrane antigen and membrane (IgG-containing) immune complexes, respectively. Remarkably, non-bound proteins are frequently not only homogeneously distributed but excluded from the contact zone. These results suggest that proteins mainly reach the contact zone by undirected diffusion, and in order not to be expelled by molecular crowding they require capture by and fixation to a binding protein.

B cells acquire antigen from target cells after synapse formation.

Soluble antigen binds to the B-cell antigen receptor and is internalized for subsequent processing and the presentation of antigen-derived peptides to T cells. Many antigens are not soluble, however, but are integral components of membrane; furthermore, soluble antigens will usually be encountered in vivo in a membrane-anchored form, tethered by Fc or complement receptors. Here we show that B-cell interaction with antigens that are immobilized on the surface of a target cell leads to the formation of a synapse and the acquisition, even, of membrane-integral antigens from the target. B-cell antigen receptor accumulates at the synapse, segregated from the CD45 co-receptor which is excluded from the synapse, and there is a corresponding polarization of cytoplasmic effectors in the B cell. B-cell antigen receptor mediates the gathering of antigen into the synapse and its subsequent acquisition, thereby potentiating antigen processing and presentation to T cells with high efficacy. Synapse formation and antigen acquisition will probably enhance the activation of B cells at low antigen concentration, allow context-dependent antigen recognition and enhance the linking of B- and T-cell epitopes.