Peroxisomes are single membrane-limited cell organelles that are involved in numerous metabolic functions. Peroxisomes do not contain
DNA; the matrix and
membrane proteins are encoded by the nuclear genome. It is assumed that new peroxisomes are formed by division of existing organelles. The present article gives an overview of microscopic studies and recent unpublished results dealing with peroxisome biogenesis in mammalian fetal liver and presents data on peroxisomes in oocytes. Cytochemical (
catalase and D-aminoacid
oxidase activity) and immunocytochemical data in rat and human liver (
antigens of
catalase, the three peroxisomal beta-oxidation
enzymes,
alanine: glyoxylate aminotransferase, peroxisomal
membrane proteins with molecular weights of 42 and 70 kDa) indicate that during embryonic and fetal development the peroxisomal population undergoes a differentiation with respect to the composition of the matrix and to the size and number of the organelles. In the youngest stages, rare and small peroxisomes are present, into which the matrix components are imported in a sequential way. The import seems asynchronous in peroxisomes of the same hepatocyte. The size and number of the peroxisomes increase during liver development. In rat and human liver, no morphological or immunocytochemical evidence for an elaborate network of interconnected peroxisomes ("reticulum") was found. Instead, peroxisomes presented as individual organelles, which occasionally show membrane extensions. The importance of the metabolic functions of peroxisomes in human liver is emphasized by the
peroxisomal disorders. In the liver of affected fetuses, the microscopic features associated with the defect can already be recognized; i.e., either
catalase containing peroxisomes are absent and
catalase is localized in the cytoplasm (in fetuses affected with
Zellweger syndrome or with
infantile Refsum disease) or peroxisomes are present but they are abnormally enlarged (e.g., a fetus affected with
acyl-CoA oxidase deficiency). In the quail ovary, numerous peroxisomes are observed in the oocyte and in the granulosa cells during follicle maturation, but not in the full-grown egg. Thus, the mechanism of peroxisome inheritance remains unresolved.