Embryonic Origins and Development of the Germline

Embryonic Origins: Germline development is identical in every embryo at the start. Divergence occurs only once the germ cells reach the gonads.
Impact on Future Fertility: The specification of the germline lineage, the proliferation of early cells, their maintenance, and the differentiation of primordial germ cells (PGCs) have a profound impact on the number and function of future germ cells. Correct setup in the embryo optimizes adult fertility.
Infertility Statistics in Australia:
- $1$ in $6$ couples in Australia suffer from infertility.
- The cause is shared equally between sexes.
Environmental and Biological Factors:
- Causes include disease (e.g., endometriosis) and age.
- Testicular sperm production continues into advanced age (e.g., Mick Jagger had a child at approximately $74$ years old).
- Ovarian fertility is significantly impacted by age; fertility drops at roughly $35\,years\,old$ and is largely gone by the mid- $40s$ , with rare exceptions.

The First Decision: The germline is one of the first decisions made by the embryo. Following the zygote and blastocyst stages, the embryo hatches and attaches to the uterine wall to begin gastrulation.
Preserving Pluripotency: Germ cells are set aside before gastrulation to maintain a high degree of pluripotency. While other cells are directed toward differentiated derivatives of the ectoderm, mesoderm, or endoderm, germ cells are protected from these differentiation signals.
The Lifecycle of Germ Cells: The process follows a sequence: Primordial germ cell specification $\rightarrow$ Gonad formation $\rightarrow$ Birth $\rightarrow$ Adult development $\rightarrow$ Gametogenesis (starting at puberty) $\rightarrow$ Fertilization and development of a new embryo.

Specification Phase:
- In the mouse embryo, approximately $6$ cells express the gene BLIMP1.
- BLIMP1 Function: It suppresses genes that would normally be switched on by gastrulation cues, preventing these cells from following somatic cell pathways and maintaining pluripotency.
Commitment Phase:
- A subset of these specified cells expresses the gene Stella, indicating official commitment to the germline.
Migration Timeline (Mouse):
- $E4.5$ : Germline begins developing in the primitive epiblast cells (starting with $6$ cells).
- Proliferation: The lineage must proliferate significantly from the initial $6$ cells to establish a full lineage.
- Active and Passive Movement: Migration is both active (cells follow cell-cell signaling pathways) and passive (aided by the physical turning of the embryo).
- $E10$ : PGCs have reached the gonads and begun further differentiation.
- $E11.5$ : Full colonization of the gonad occurs.

Fluorescence Imaging: Researchers can tag PGCs with a fluorescent gene to track their movement in culture under a microscope.
Migration Path: Germ cells move down through the midline of the embryo and then distribute into each gonad on either side of the abdomen.
Experimental Disruptions: Studies that disrupted germline homing demonstrated that lost germ cells result in sub-optimal fertility.

The Colonization Point: This is where germ cell fates diverge based on the gonadal microenvironment.
Ovarian Pathway (Oogonia):
- PGCs enter the ovary and become oogonia.
- They enter meiosis and arrest at Prophase I until puberty.
- Meiosis restarts at puberty but is not completed until fertilization.
Testicular Pathway (Gonocytes):
- PGCs enter the testis and become gonocytes.
- They do not enter meiosis immediately; they undergo mitosis and then arrest.
- Mitosis resumes at birth, but meiosis does not begin until puberty.

Week $4$ : PGCs migrate from the yolk sac into the developing embryo with significant proliferation occurring during the journey.
Week $6$ to $8$ : PGCs colonize the human gonads.
Week $12$ (Female): Ovarian germ cells differentiate into oogonia and arrest in Prophase I of meiosis.
Birth (Male): Testicular germ cells (gonocytes) undergo mitosis and arrest; at birth, they are termed intermediate spermatogonia or prespermatogonia.

Embryonic Origins of Adult Tumors: Although testicular tumors manifest in young men, they develop during the embryonic stage when PGCs are highly vulnerable to environmental impacts.
Rising Incidence: Testicular cancer rates are rising due to changing environmental factors affecting PGC specification and migration.
Pathology:
- Something goes wrong during development where gonocytes remain as gonocytes instead of becoming spermatogonia.
- These cells become resistant to differentiation signals.
- Carcinoma in situ: This is the precursor state found in young boys where cells resemble embryonic gonocytes.
Malignancy at Puberty: Hormonal and structural changes at puberty trigger the progression from carcinoma in situ to clinical malignancy:
- Seminoma Tumors: Cells remain relatively undifferentiated.
- Non-seminoma Tumors: Cells begin to differentiate randomly into mesodermal, ectodermal, and endodermal tissues.

Self-Examination: Individuals with testes should perform periodic checks for changes or pain. While testicular cancer is rare, early detection is vital.
Prognosis: Testicular cancer is highly treatable, boasting a $90-95\%$ recovery rate.
Case Study: Lance Armstrong returned to win the Tour de France five times after being diagnosed with Stage $4$ testicular cancer.

Activity: The session concluded with a review of material via "Questions part two" to reinforce the concepts of germline specification, migration, and differentiation before moving to the final portion of the lecture.