Primordial germ cells

Origin and early segregation

The development of primordial germ cells has a number of unique features. In mammals, their segregation occurs at the gastrula stage, which is considered a relatively late timeframe.

A key characteristic of gonocytes is their extragonadal origin. In the human embryo, they initially segregate within the inner cell mass. These cells are direct descendants of totipotent embryonic stem cells. It is important to note their special relationship to the classical germ layers: although primordial germ cells are visualized within the ectoderm, endoderm, or mesoderm, functionally and genetically they are not derivatives of any of them.

Temporary localization before migration

Before reaching the primordia of the future gonads, gonocytes undergo a complex journey. First, they migrate from the embryo proper structures into extraembryonic tissues.

The wall of the yolk sac (specifically, the regions of extraembryonic mesoderm and endoderm) serves as their primary temporary station. Looking at the topography of this process, clusters of primordial germ cells are found in close proximity to the origin of the allantois. It is here that the so-called gonoblast is formed — the primary germinal primordium containing a pool of gonocytes ready for the next leg of their journey.

Mechanisms of migration to the gonads

The large-scale migration of gonocytes back into the embryo's body starts after the 25th day of embryonic development. Nature has provided several redundant mechanisms for the successful movement of these vital cells:

1. Interstitial type (main). Cells actively move through the mesenchyme of various embryonic structures. Navigation is carried out via contact guidance.
2. Passive transport. Part of the cell population enters the systemic circulation and is carried by the bloodstream directly to the gonad primordia.
3. Chemotaxis. It is hypothesized that chemical signals play a guiding role, pointing the cells in the correct direction of movement.

Those gonocytes that traveled passively through the circulatory system penetrate the gonad tissues via diapedesis — they smoothly pass through the vascular wall without compromising its structural integrity.

Homing and ultimate fate

The process by which primordial germ cells finally settle in the gonad primordia is called homing. In humans, it is completed by the 28th–30th day of intrauterine development.

Upon arrival, the gonocytes locate between the cells of the coelomic epithelium. This epithelium forms the microenvironment that provides all the necessary conditions for their survival and further differentiation. However, migration is a complex process, and some cells inevitably deviate from the route. Such "lost" gonocytes cannot survive outside the gonads and are eliminated through programmed cell death — apoptosis.

Subsequently, the surviving cells enter gametogenesis — the formation of gametes, which is accompanied by cycles of mitosis and meiosis. In males, this process is called spermatogenesis and takes place in the testes (forming spermatozoa). In females, it is oogenesis, which is localized in the ovaries and leads to the formation of oocytes. During meiosis (at the pachytene stage), multiple crossing over occurs between homologous chromosomes — single or double exchanges that can involve two, three, or all four chromatids, ensuring genetic diversity.

Class and exam questions

What else to study on this topic

• Morphological differences between spermatogenesis and oogenesis
• Stages of meiosis and molecular mechanisms of crossing over
• Structure of the coelomic epithelium and its role in gonad development
• Mechanisms of diapedesis and cell contact guidance

Other topics in the "Gametogenesis" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.