I'm working with nrf51 board, I'm using I2C to communicate with an accelerometer (see the scheme), the communications are successful but I dont see any pull up resistors in the lines
Real IIC requires pullup resistors. Many microcontrollers have optional internal pullup resistors on some pins. If there is a micro on the bus, and that's usually the case, then it could be switching its pins between pullup and active low.
The pullups in a micro are usually higher resistance than what you'd use for IIC, but for a short bus all on one board when you know the bit rate is low enough, this can be a legitimate thing to do. I've done it.