Data from: SMAD4 promotes somatic-germline contact during murine oocyte growth

Development of the mammalian oocyte requires physical contact with the
surrounding granulosa cells of the follicle, which provide it with
essential nutrients and regulatory signals. This contact is achieved
through specialized filopodia, termed transzonal projections (TZPs), that
extend from the granulosa cells to the oocyte surface. Transforming growth
factor (TGFβ) family ligands produced by the oocyte increase the number of
TZPs, but how they do so is unknown. Using an inducible Cre recombinase
strategy together with expression of green fluorescent protein to verify
Cre activity in individual granulosa cells, we examined the effect of
depleting the canonical TGFβ mediator, SMAD4. We observed a 20-50%
decrease in the total number of TZPs in SMAD4-depleted granulosa
cell-oocyte complexes, and a 50% decrease in the number of newly generated
TZPs when the granulosa cells were reaggregated with granulosa cell-free
wild-type oocytes. Three-dimensional image analysis revealed that TZPs of
SMAD4-depleted cells were also longer than controls and more frequently
oriented towards the oocyte. Strikingly, the transmembrane proteins,
N-cadherin and Notch2, were reduced by 50% in these cells. SMAD4 may thus
modulate a network of cell adhesion proteins that stabilize the attachment
of TZPs to the oocyte, thereby amplifying signalling between the two cell
types.