O-233 Cell-cell interactions within germ cell nests reveal the mechanism of primordial follicle formation in humans
Authors: Czukiewska S , Fan X , Mulder A , Jost C , Koning R , Chuva de Sousa Lopes S 
Published: June 2022

Abstract

Study question
How interactions between human fetal female germ cells (hFGCs) and pre-granulosa cells (pre-GCs) affect the hFGCs transition from ovarian cord to primordial follicles (PFs).

Summary answer
Progressive engulfment of hFGCs by pre-GCs and establishment of cell-cell junctions and cell-cell adhesions between hFGCs and pre-GCs leads to PF formation in humans.

What is known already
In mice, development of FGCs is synchronized due to formation of cysts. Cysts comprise of sister FGCs that remain connected via intercellular bridges until germ cell nest breakdown is initiated and PFs are formed. In contrast, hFGCs differentiate asynchronously. In second trimester human ovaries, mitotic, RA-responsive, meiotic and oogonia hFGCs are present in designated parts of the ovary that corresponds to their differentiation stage. Meiotic hFGCs enter prophase I that is divided into 4 substages: leptotene, zygotene, pachytene, diplotene/dictyate. hFGCs in diplotene are ready to form a PF with pre-GCs and become dormant.

Study design, size, duration
We collected in total 17 second trimester human fetal ovaries between 14 and 22 weeks of gestation from elective abortions. We have analyzed cellular structures of hFGC and pre-GCs by transmission electron microscopy and investigated different cell adhesion and cell junction proteins by immunostaining.

Participants/materials, setting, methods
Materials: human fetal ovaries between 14 and 22 weeks of gestation.
Methods: characterization of tissue morphology, immunofluorescence staining, whole mount staining, confocal microscopy, super-resolution microscopy, transmission electron microscopy (TEM), cell counting

Main results and the role of chance
Meiotic hFGCs formed germ cell cysts consisting of multiple sister hFGCs connected by intercellular bridges. Furthermore, we discovered that a subset of hFGCs formed syncytia containing several nuclei (2–4 nuclei). Syncytium formation was caused by cytokinesis failure when an in intercellular bridge failed to be formed. In addition, in some cases hFGC syncytia were connected to other syncytia or mono-nucleated hFGCs by an intercellular bridges. Those multinucleated syncytia did not display signs of apoptosis or morphological abnormalities. Investigation of physical interactions between hFGCs and pre-GCs revealed that as hFGCs enter prophase I, pre-GCs (FOXL2+, VIM+, CDH1+) form membrane protrusions (MPs) and progressively engulf them. First, CDH1+ and VIM+ domains within a pre-GC are mixed, but as engulfment progresses two distinct domains are formed. MP formation is possible due to mesenchymal-epithelial hybrid state and active Hippo pathway in pre-GCs. Inside the cords, hFGCs highly express CDH2, while CDH2 is lowly expressed in pre-GCs. At diplotene, hFGCs (TP63+) downregulate CDH2 and upregulate CDH1. Expression of CDH1 in both diplotene hFGC and pre-GCs causes increase of cell-cell adhesion and enables PF formation. In addition, progressive formation of tight junctions (TJP1+) and cellular interdigitations between pre-GCs and meiotic hFGCs reinforce PF structure.

Limitations, reasons for caution
Due to technical limitations and lack of in vitro culture protocols that support meiotic hFGCs development and survival as well as human PF formation in vitro, we cannot experimentally validate roles of cadherins and other cell junction and cell adhesion proteins in PF formation

Wider implications of the findings
In depth understanding of PF formation process in terms of physical interactions between hFGCs and pre-GCs will be crucial to design in vitro conditions that will support PF formation and our findings will be key to achieve assembling of PFs in artificial human oocytes (gametes) created from pluripotent stem cells.

Trial registration number
not applicable