Unlike all other cells in the body, which age and eventually die, a female’s oocytes can give rise to a new individual. Oocytes have, in a sense, achieved immortality. In order to reach this exalted state, each oocyte undergoes a complex program of differentiation within the ovary before it is ovulated and fertilized. This program, explained in Hugh Clarke’s review published in WIREs Developmental Biology, begins with a long period of growth, which in humans requires several months. During growth, the oocyte accumulates vast reserves of messenger RNAs and proteins as well as organelles such as mitochondria. After fertilization, these maternal reserves will provide the newly created embryo with the resources it needs until it can begin to manufacture them itself.
Once the oocyte has completed growth, its differentiation program concludes via a rapid process, lasting about 36 hours in humans, termed meiotic maturation. During maturation, the oocyte undergoes the first meiotic division and prepares for the second division, ensuring that the embryo will possess the correct number of chromosomes.
Growth and maturation of the oocyte occur within a structure termed a follicle, and they depend on continuous communication between the oocyte and the surrounding somatic cells of the follicle. Impaired communication between the oocyte and its somatic environment can lead to infertility, including in humans. Signals from the somatic cells trigger resting oocytes to enter the growth phase. Metabolites transmitted to the oocyte by the somatic cells via intercellular channels termed gap junctions support its continued growth. Signals that regulate the initiation of meiotic maturation are also transmitted via the gap junctions.
The physical contact between the follicle cells and oocyte that is necessary for this communication is mediated through long narrow fingers of the follicle cells that penetrate through an extracellular coat surrounding the oocyte to reach its surface. A recent review describes how communication becomes established and is maintained, explains how the regulatory signals are generated and transmitted, discusses new pathways of communication that have recently been proposed, and highlights the essential role played by the oocyte itself in maintaining communication with the follicular somatic cells, thereby ensuring that its differentiation program is properly executed.
Text kindly contributed by Hugh Clarke.