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The development of the electromyogram (EMG) of tibialis anterior (TA) and medial gastrocnemius (GM) during locomotion was studied in normal rats from the onset of quadruped walking (postnatal day 10, P10) until P42. The objectives were to relate signal properties of the EMG and coordination of muscle activity to functional development of the hindlimb, which proceeds rapidly around P15. Both the EMG characteristics and the activation pattern showed marked changes with age. Initially, the EMG bursts were irregular and protracted. The activity level in the two muscles, in particular in GM, seemed to be low. Until P14, the motor units of GM showed a tendency towards synchronization. The EMG of TA consisted of an adult-like interference pattern from the youngest age studied. Although co-contraction of TA and GM was sometimes observed until P14, reciprocal activation of the muscles was evident at all ages. The timing of the alternating pattern became more accurate with age. The activity level in both muscles increased markedly from P15. These changes were reflected in the form factor (coefficient of variability) of the EMG and, to a lesser extent, in the power spectra. The time course of these changes bears a close temporal relationship to the development of locomotion. These results suggest that the degree of muscle activation is a decisive factor with respect to locomotor development. It is proposed that this is related to the maturation of supraspinal control.

A panel of monoclonal antibodies directed against various lymphoid and non-lymphoid cell subsets was used to study the lymph nodes of human fetuses of 16-40 weeks. B cells were of intermediate size and were present at all ages in primitive follicles and in the outer cortex. The fetal B-cell immunophenotype is indicative of an intermediate stage of development, just preceding the differentiation to mature B cell. Forty to sixty per cent Leu1+ B cells were observed in the follicles until the end of the second trimester. At all stages, T cells showed an immunophenotype similar to type III thymocytes, different from adult peripheral T cells, with a marked predominance of CD4+ T cells. Leu7+ NK cells were generally absent. OKIa+ interdigitating reticulum cells were present in T-cell areas. Some axillary lymph nodes showed strongly CD1+ dendritic cells, probably Langerhans' cells. Macrophages and granulocytes were present in varying numbers. Altogether, our results indicate that fetal lymph nodes are quite well differentiated at an early fetal age, although T and B cells do not (yet) show adult immunophenotypes. The expression of the CD38 antigen may be a main marker related to the immaturity of fetal T and B cells.