An electron microscope study of cytoplasmic organelle transformations in developing mouse oocytes

The development of the mouse oocyte during the primordial, primary and secondary follicular growth stages was studied by means of the electron microscope.During the early stages of oocyte maturation, mitochondrial multiplication takes place along with an apparent temporary transition from round to oval shape. The internal structure of many of the mitochondria is altered by separation of membranes of a crista to form a vacuole. This enlarges to pear-shaped configurations and gradually it forms so large a structure as to result in compression of adjacent cristae, thereby altering the entire appearance of the organelle.Dense round bodies encapsulated by a single membrane are found in the cytoplasm of oocytes of primary follicles near the periphery. The Golgi complex appears in primary follicle oocytes as an aggregation of vesicles. Gradually the number of lamellae in the complexes increase and these organelles become more peripherally located. The "Balbiani yolk nuclei" apparently is represented by a conglomeration of Golgi complexes and are present only in primordial and young primary follicle oocytes.The endoplasmic reticulum appears in the early stages only as rough-surfaced vesicles. At later stages individual cisternae become prominent. Apparently, a modified form of E. R. appears during maturation of the secondary follicle oocyte.Multivesicular complexes, each consisting of two components, small vesicles and larger vesicles enclosing microvesicles (multivesicular bodies), were commonly found during all stages of oocyte growth. The secondary follicle oocytes frequently contain multilamellar bodies. These are commonly found in juxtaposition to the multivesicular complexes and also near the egg periphery and occasionally near the nuclear envelope.