Mitochondrial morphology in human fetal and adult female germ cells

Abstract

The aim of this study has been to observe, by electron microscopy, the morphological changes affecting mitochondria and associated organelles in the human female germ cell during oogenesis, maturation and fertilization. In the primordial germ cell (PGC), rounded mitochondria with a pale matrix and small vesicular cristae are disposed near the nucleus and significantly increase in number during PGC migration and settlement in the gonadal ridge, where they differentiate into oogonia. In these early stages of mammalian oogenesis, aggregates of mitochondria are typically clustered around or in close relationship with the nuage. In oocytes at early prophase stage, mitochondria proliferate while aligned along the outer surface of the nuclear membrane, contain a more dense matrix than before, and have lamellar cristae. Oocytes of primordial and primary follicles mostly contain round or irregular mitochondria whose matrix has become very light. These mitochondria show typical parallel, arched cristae, and are clustered near the nucleus with other organelles forming the Balbiani's vitelline body. When follicles grow, the mitochondria of the oocytes become even more numerous and are dispersed in the ooplasm. Both paranuclear accumulation and subsequent dispersion of mitochondria in the cytoplasm are likely to be regulated by microtubules. By ovulation, mitochondria are the most prominent organelles in the ooplasm. They form voluminous aggregates with smooth endoplasmic reticulum (SER) tubules and vesicles. These mitochondrial-SER aggregates (M-SER) and the mitochondrial-vesicle complexes (MV) could be involved in the production of a reservoir of substances or membranes anticipating subsequent fertilization and early embryogenesis. Just after fertilization, the mitochondria of the oocyte undergo a further substantial change in size, shape, and microtopography. In the pronuclear zygote, mitochondria concentrate around the pronuclei. During the first embryonic cleavage divisions, round or oval mitochondria with a dense matrix and few arched cristae are gradually replaced by elongated ones with a less dense matrix and numerous transverse cristae. A progressive reduction in size and number of M-SER aggregates and MV complexes also occurs. In summary, oocyte mitochondria show dynamic morphological changes as they increase in number and populate different cell domains within the oocyte. They form complex relationships with other cell organelles, according to the different energetic -metabolic needs of the cell during differentiation, maturation, and fertilization, and are ultimately inherited by the developing embryo, where they eventually assume a more typical somatic cell form.

Reproductive aging is associated with decreased mitochondrial abundance and altered structure in murine oocytes

“…Increased oxidative stress in aged murine oocytes has been linked to low ooplasmic ATP levels [22]. Undifferentiated appearance of mitochondria in MII oocytes has previously been described; therefore, our finding is consistent with prior reports [12,17]. In MII oocytes mitochondria have a round shape with few well defined cristae.…”

Apparent mitochondrial asymmetry in Xenopus eggs

“…Mowry, unpublished results). Asymmetric distributions of mitochondria have been observed during oogenesis and early embryonic development in several organisms (Mignotte et al, 1987;Bement and Capco, 1990;Elinson et al, 1993;Calarco, 1995;Yost et al, 1995;Bavister and Squirrell, 2000;Motta et al, 2000). Despite the general uniformity of mitochondria, their size, structure, and/or number may be modified to meet changing requirements (reviewed in Bereiter-Hahn and Voth, 1994;Lloreta-Trull and Serrano, 1998).…”

Mitochondrial Fission and Fusion Mediators, hFis1 and OPA1, Modulate Cellular Senescence

“…Mitochondria undergo morphological change during mating (12), meiosis in Saccharomyces cerevisiae (13), and during human spermatogenesis (14) and oogenesis (15). Furthermore, dramatic shape changes of mitochondria occur during cell death.…”