The role of germ cell loss during primordial follicle assembly: a review of current advances

Abstract

In most female mammals, early germline development begins with the appearance of primordial germ cells (PGCs), and develops to form mature oocytes following several vital processes. It remains well accepted that significant germ cell apoptosis and oocyte loss takes place around the time of birth. The transition of the ovarian environment from fetal to neonatal, coincides with the loss of germ cells and the timing of follicle formation. All told it is common to lose approximately two thirds of germ cells during this transition period. The current consensus is that germ cell loss can be attributed, at least in part, to programmed cell death (PCD). Recently, autophagy has been implicated as playing a part in germ cell loss during the time of parturition. In this review, we discuss the major opinions and mechanisms of mammalian ovarian PCD during the process of germ cell loss. We also pay close attention to the function of autophagy in germ cell loss, and speculate that autophagy may also serve as a critical and necessary process during the establishment of primordial follicle pool.

Keywords: Apoptosis; Autophagy; Germ cell cyst; Germ cell loss; Primordial follicle assembly.

Figure 1

The number of germ cells in mouse and human ovaries at different stages.

Figure 2

Electron microscopy depictions of apoptosis and autophagy in 2dpp mouse germ cell cysts. A depicts a 2-cell germ cyst within which one is undergoing apoptosis. B depicts the magnified section denoted by the dashed yellow box in A. C and D depict an autophagosome observed in a germ cell within a cyst.

Figure 3

Survival of the chosen oocyte, showing that the mouse germ cells receive organelles from neighboring cyst cells and build a Balbiani body to become oocytes. Autophagy is regulated by mTOR signaling; Atg7 or BECN1 deficiency will interdict the formation of the autophagosome, causing germ cell loss.