the development of synovial membrane from knee joint were studied by electron microscope in human fetuses ranging from 20mm to 260mm crown rump length(40 days to 30 weeks of gestational age). At 40mm fetus, developing synovial tissue was observed
in
homogenous homogenous interzone as a vascular mesenchyme around the periphery. The primitive joint space was appeared after the intermediate layer of the interzone in direct contact with chondrogenic layer at 60 mm fetus. Differentiation of the
synovial
membrane coincided with clarification of the interzone in direct contact with chondrogenic layer at 60mm fetus. Differentiation of the synovial membrane coincided wit clarification of the cavity. When dilatation of the synovial cavity occured,
the
two
types of synovial cells were well identified. Tyep A cells contained numerous vacuoles and vesicles, while type B were well endowed with rough endoplasmic reticulum. At 100mm fetus, type A cells with a markedly attenuated cytoplasm were found as
well as
those cells which contained pinocytotic vesicles and vacuoles. By 150-200mm fetuses a majority of the intimal cells were type B. These cells were characterized by abundant rough endoplsmic reticulum and well developed Golgi complex. In contrast,
A-type
cell had numerous filopodia, pinocytotic vesicles lysosomes, and large vacuoles containing amorphous material. At 260mm fetus, the intimal cells were well developed and plentiful. The most marked difference between the synovial membrane of
full-term
fetus and adult was the large amount of collagen in the latter. During fatal period, the B-cells were most numerous cells type in the intimal cells. The B-cells were clearly distinguishable from the A-cells by their content of extensive rough
endoplasmic reticulum and well developed Golgi complex.
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