Ossification of the rat and mouse skeleton in the perinatal period

In the perinatal period the skeletal development of rats and mice showed transient stages of missing to incomplete or largely complete ossification. Alizarin red S staining revealed characteristic morphological patterns of maturation, which differed from one part of the skeleton to another. In the two species examined all the skeletal elements were ossified 2 days after birth. In about 12% of the rats and 90% of the mice an additional sternebra formed in the postnatal period. In a small number of cases abnormal ossification was observed. A few days after term the spontaneous rate of abnormally ossified bones was found to be 0.51% in the rats and 1.40% in the mice. A reduced rate of ossification at a given time of development which is never less within the normal range, may be indicative of a nonspecific retardation of fetal maturation.