A Borg nursery in 2376
A maturation chamber, also informally known as a Borg nursery, was a device used by the Borg Collective to accelerate the development of adolescent humanoids until they were ready to service the Collective as drones. Among those who were placed in such chambers were Annika Hansen (Seven of Nine), Icheb, Maryl and a young boy Seven of Nine met in Unimatrix Zero. (TNG: "Q Who"; VOY: "Collective"; VOY: "Unimatrix Zero")
Children assimilated by the Borg were placed in maturation chambers where they spent the following seventeen cycles. Once a drone disengaged from a maturation chamber, it would await instruction from the Collective, in order to gain a designation and a purpose. (VOY: "Mortal Coil", "Drone")
A Borg nursery in 2365
Maturation chambers were first encountered in 2365, when Commander William Riker, Lieutenant Commander Data, and Lieutenant Worf beamed aboard a damaged Borg cube to investigate after its initial attack upon the USS Enterprise-D. Riker referred to the chambers as a "nursery". This "nursery" consisted of several drawers in the wall, each containing an infant in an early stage of assimilation. The Borg infants did not exhibit either the extensive implants or the body armor of mature drones, nor was it known at the time if Borg infants required food sources other than the electricity consumed by adults. (TNG: "Q Who")
At the time "Q Who" was produced, the back story regarding the Borg increasing their numbers through assimilation was not yet developed. Therefore, Riker's description of the nursery indicates that the Borg reproduced with each other, their babies born as organic lifeforms, and then (after birth) enhanced with mechanical devices. This was later disproved by several episodes of Star Trek: Voyager, which stated that the Borg expand their numbers through assimilation only (see Seven of Nine's quote about the fetal Borg below, for example), capturing babies and children and then placing them in maturation chambers to accelerate their growth.
A fetal Borg drone inside a maturation chamber
After assimilating The Doctor's mobile emitter and sampling DNA from Ensign Mulchaey in 2375, Seven of Nine's nanoprobes created a modified maturation chamber in one of USS Voyager's science labs. When Seven of Nine saw a fetal Borg drone inside this chamber, she indicated that this was unusual, saying, "I don't understand. The Borg assimilate. They do not reproduce in this fashion." The growth rate of this drone, known as One, was twenty-five times faster than normal. The chamber was protected by a force field. (VOY: "Drone")
It remains unknown how the implantation of Borg devices affects the growth of infants to maturity – namely, whether immature drones need to have their implants upgraded, or if said implants adapt to physiological changes in the drone.