Of course, this means that only the suckled gland becomes more responsive to prolactin. This is an example of local control of one gland independent of the others. Phase 3 mammary tissue histologically looks like fully lactating tissue of eutherian mammals, while phase 2 tissue looks more like the prepartum mammary tissue. Binding of prolactin is significantly further enhanced during the 3rd phase compared with the 2nd phase. In addition, concurrent with the transition from the 2nd to 3rd phases of lactation, there is a transient increase in prolactin concentration in the blood.
Oxytocin stimulates milk ejection in marsupials as in eutherian mammals.
However, there is a progressive decline in sensitivity of the lactating gland to oxytocin as lactation progresses. Any gland at a later stage of lactation would not respond the these low oxytocin levels. In contrast, in late lactation when the joey returns to the teat to suckle intermittently, but vigorously, a greater release of oxytocin may occur causing milk ejection in that gland. Although changes in milk composition occur during lactation in eutherian mammals, those changes are minor compared with the changes observed in marsupial milk. By day 182 most milk sugars are oligosaccharides.