History, origin, and function of transzonal projections: the bridges of communication between the oocyte and its environment

Abstract

Development and differentiation of a functional oocyte that following fertilization is able to give rise to a new individual requires continuous physical contact with the supporting somatic cells of the ovarian follicle. As the oocyte is surrounded by a thick extracellular coat, termed the zona pellucida, this essential contact is mediated through thin cytoplasmic filaments known as transzonal projections (TZPs) that project from the somatic granulosa cells adjacent to the oocyte and penetrate through the zona pellucida to reach the oocyte. Gap junctions assembled where the tips of the TZPs contact the oocyte plasma membrane, and other contact-dependent signaling may also occur at these sites. Here, I describe early studies of TZPs, which were first identified in the late 19th century, discuss their similarities with classical filopodia, review their structure and function, and compare two models that could account for their origin. Possible priorities and directions for future studies close this contribution.

Reestablishment of transzonal projections and growth of bovine oocytes <i>in vitro</i>

“…1) In vivo growing oocytes collected from early antral follicles (0.5-0.7 mm) and fully grown oocytes collected from antral follicles (4-6 mm). 2) DOs collected from early antral follicles cultured for 12 and 24 hours. 3) DOs cocultured with MGCs 24 hours after denudation, for 24-120 hours.…”

“…1) DOs collected from early antral follicles cultured for 24 hours. 2) DOs cultured with MGCs hours after denudation, for 13 days and OCGCs cultured for 14 days. 3) In vivo fully grown oocytes collected from antral follicles (4-6 mm).…”

<i>In vitro</i> growth of bovine oocytes in oocyte-cumulus cell complexes and the effect of follicle stimulating hormone on the growth of oocytes

“…Oocytes and cumulus cells communicate bilaterally, thereby promoting oocyte growth. Previous studies have suggested that this communication is mediated by paracrine signals and gap junctions existing at the end of transzonal projections (TZPs) as well as extracellular vesicles [ 18 , 19 , 20 ]. Thus, it is assumed that the connections between oocytes and cumulus cells are maintained during in vitro growth culture.…”

In Vitro Production of Embryos from Prepubertal Holstein Cattle and Mediterranean Water Buffalo: Problems, Progress and Potential

“…More recently, the intimate relationship between the oocyte and cumulus cells has been investigated to better define the role of TZPs [ 76 , 77 ]. Although more research needs to be done to determine how the physiology, distribution and retraction of TZPs impacts IVEP outcomes in both prepubertal and adult oocytes, TZPs are known to facilitate communication and the transport of essential molecules between granulosa cells and the oocyte [ 76 , 78 ]. Despite differences observed in the organization of TZPs in COCs from lambs compared to adult ewes, the impact on embryo development remains unclear [ 79 ].…”