Fine Structural Markers of Human Oocyte Quality in Assisted Reproduction

Review Article

Austin J Reprod Med Infertil. 2014;1(1): 5.

Fine Structural Markers of Human Oocyte Quality in Assisted Reproduction

Nottola SA1*, Macchiarelli G2 and Familiari G1

1Department of Anatomy, Histology, Forensic Medicine and Orthopaedics, Sapienza University, Italy

2Department of Life, Health and Environmental Sciences, University of L´Aquila, Italy

*Corresponding author: Nottola SA, Department of Anatomy, Histology, Forensic Medicine and Orthopaedics, Section of Human Anatomy, Electron Microscopy Unit, Laboratory “Pietro M. Motta”, Sapienza University of Rome, Via Alfonso Borelli 50, Rome 00161, Italy

Received: September 30, 2014; Accepted: November 27, 2014; Published: December 02, 2014


After completing its maturation, the human oocyte acquires very peculiar fine structural features. In this review we describe by LM, TEM and SEM the nuclear and cytoplasmic changes, as well as the changes in the texture of the ZP, occurring in the human oocyte during the final stages of maturation. Oocytes were obtained during ART cycles and destined to morphological evaluation after the informed consent of the patients. The critical analysis of the data reviewed leads us to emphasize that: 1. Both the completion of the maturative changes in the preovulatory period and the absence of degenerative alterations in the cytoplasmic microdomains of the human mature oocyte ultimately render the female gamete competent for fertilization; 2. Several minute cellular aberrations, detectable only by electron microscopy analysis, may occur in the human oocyte as the consequence of the application of ART protocols (IVM and cryopreservation, in particular) and could be responsible for ART failures, even affecting early embryo development.

Keywords: Oocyte; Assisted reproduction; In vitro maturation; Cryopreservation; Human; Electron microscopy


ART: Assisted Reproductive Technology; GV: Germinal Vesicle; GVBD: Germinal Vesicle Breakdown; IVM: In vitro Maturation; LM: Light Microscopy; M-SER: Mitochondria-smooth Endoplasmic Reticulum; MII: Second Metaphase; MV: Mitochondria-vesicle; PVS: Perivitelline Space; SEM: Scanning Electron Microscopy; SER: Smooth Endoplasmic Reticulum; TEM: Transmission Electron Microscopy; ZP: Zona Pellucida


The positive outcome of ART procedures strictly depends on the oocyte quality, evaluable in terms of maintenance of full morphofunctional integrity during its complex and long maturative iter. In fact, in vivo and in vitro as well, the completion of oocyte maturative changes in the preovulatory period and the absence of degenerative alterations in the oocyte cytoplasmic microdomains both ultimately render the female gamete competent for fertilization.

Aim of this paper is to briefly review the process of the periovulatory maturation of the oocyte in humans, giving particular emphasis to its fine structural morphology, as revealed by LM, TEM and SEM [1- 7]. Electron microscopy observations in particular, associated with clinical, epidemiological, biological, molecular and biochemical data, are useful in the assessment of the structural integrity of the oocyte, in order to define a proper “oocyte health” state. Electron microscopy, in fact, can highlight not only obvious defects but even minute subcellular alterations in the oocyte microdomains. These minimal alterations cannot be diagnosed by other morphological approaches routinely used in ART laboratories and could be responsible, in some cases, of unexplained failures of ART procedures [1]. Recently, electron microscopy, applied to the study of human oocytes subjected to different protocols aimed at the preservation of female fertility, has also provided some useful and objective information regarding oocyte sensitivity to cryopreservation [8-17] and/or to IVM [15].


The data reported in this review mostly derived from structural and ultrastructural observations carried out by our research groups on human oocytes obtained during ART cycles and evaluated at the light of the specific literature at regard. All the samples, destined to electron microscopy observations after the informed consent of the patients, were fixed in 0.5% - 2% glutaraldehyde in phosphate buffer and processed for LM, TEM and SEM as detailed elsewhere [6, 8-10, 13,18,19].

The Human Mature Oocyte: Fine Structural Parameters

General features

Observed by LM in equatorial sections, the human mature oocyte shows a rounded shape and a diameter of about 100 μm. The ooplasm has a homogeneous texture and contains numerous, uniformly dispersed organelles. Vacuoles, variable in size and location, are only occasionally observed in healthy oocytes. The oocyte is surrounded by the ZP, a glycoprotein matrix that forms a continuous and uniform layer and has a thickness of about 17 μm. The oocyte and ZP are separated by a thin PVS, in which the first polar body is located, visible only in particularly favorable sections (Figure 1). In the same sections the MII chromosomes are also frequently observed, aligned on the meiotic spindle [8-10,13]. Further details on each of these structures will be provided in the next paragraphs.