Radiolab: "The Primitive Streak"

Date: September 23, 2016
Hosts: Molly Webster & Robert Krulwich
Theme:
This episode delves into the science, ethics, and history surrounding the “14-day rule”—a legal and moral boundary governing how long scientists can grow human embryos in the lab. The show explores why this line was drawn, what happens just before and after 14 days of development, and the scientific advances that now make this rule especially urgent and relevant.

Episode Overview

Radiolab embarks on an investigative journey into how we understand human development at its earliest stages, what “the primitive streak” is and why the 14-day rule has become a flashpoint for science, ethics, law, and philosophy. Featuring interviews with pioneering scientists and ethicists, the episode delves into how new breakthroughs challenge society's boundaries about experimenting with human origins.

Key Discussion Points & Insights

1. Setting the Stage: The 14-Day Rule

• Introduction to the Rule: There is an international guideline that human embryos can only be grown for 14 days in the lab (01:12, 02:51).
• Host Curiosity: Molly admits she didn’t know about this rule or any of the details behind human embryo research, sparking the episode's investigation (02:04, 02:08).

"In this universe are twins and souls and mice and something called the primitive streak. And they're all gathered together around a certain rule."
— Molly Webster (02:17)

2. Historical Backdrop: Law, Ethics, and Early Science

• Roe v Wade and Medical Ethics: 1973’s Roe v. Wade and scandals in medical research (Tuskegee Study, forced sterilizations) brought ethical questions around human experimentation to the forefront (03:52, 04:32).
• Rise of IVF: The birth of “test tube baby” Louise Brown via IVF in 1978 created new fears and possibilities regarding reproduction (05:05, 05:32).
• Committee Formation & Leroy Walters: The government created the Ethics Advisory Board, bringing together various experts (including ethicist Leroy Walters) to set the first boundaries on embryo research (06:13, 06:38).

3. Why 14 Days?

• Leroy Walters' Reasoning: Walters outlined arguments leading to the 14-day rule (07:27):
  • Natural Loss: Before 14 days, ~50% of embryos are lost naturally (07:34, 07:54).
  • Biological Identity: Before 14 days, embryos can still split (twinning) or recombine; only at 14 days do they become truly unique individuals (08:11, 08:20).
  • The Primitive Streak: Around day 14, the “primitive streak” appears—marking the first organizational sign of a body’s axis/spinal cord (08:33, 08:47).
• Global Impact: Although Walters’ report gathered dust, its recommendations spread as a guideline internationally (09:48, 09:56).
• Practicality: The rule felt safe, as labs weren't technologically able to grow embryos that long—until recently (10:11).

"It also seemed like a safe marker, a safe boundary."
— Leroy Walters (09:56)

4. The Scientific Black Box: What Happens Between Days 7 and 14?

• Invisibility of Early Development: No one has witnessed this phase, as detection of pregnancy often happens too late, and imaging is impossible at this time (10:31, 11:33).
• Textbook 'Black Box': The period between implantation and the “primitive streak” is termed a "black box" in textbooks (11:42).

5. Breakthrough in the Lab: Growing the Embryo to Day 13

• Pioneering Work of Magdalena Zernicka-Goetz: At Cambridge, she and her team spent years recreating the womb’s environment in a dish, succeeded with mice, then two human embryos (11:47, 13:01, 13:40).
• Success: One embryo survived and developed to day 13 (13:40).
• Observation: For the first time, scientists could watch previously unseen stages of human development (13:55).

6. Firsthand Glimpse into the Lab

• Visit to Rockefeller University: Molly tours Ollie Rivenloo’s lab in New York, observing actual embryos at day 12 and using advanced microscopes to view them at different stages (16:31, 18:16).
• The Embryo's Appearance: Embryos at this stage are minuscule—just a “grain of sand,” or like “a pinpoint of light” (18:23).

"Holy crap. That’s really small. That's like a grain of sand." — Molly Webster (18:30)

• Day-by-Day Changes: The lab team guides Molly through images showing the embryo’s rapid complexity from day 8 to day 13, noting cell specialization and the emergence of the epiblast—the precursor to all tissues (19:12, 20:59), and the origin of the placenta (20:50).

7. Scientific Revelations and Mysteries

• Embryo's Self-Organization: It turns out that, up to day 14, embryos can organize themselves without maternal input—a surprising result (21:42, 22:08).
• Preparing for Maternal Connection: Between days 12 and 14, embryos begin developing pathways for maternal attachment, starting the “hookup” to the network of the mother’s body (22:39).
• Pregnancy Hormone (hCG): At day 10, the embryo starts releasing the hormone that pregnancy tests detect, essentially broadcasting its presence (23:12, 23:30).

"The thing it's reading is that hormone... boom, boom, boom, boom, boom... I'm here, I'm here. Look at this hormone." — Molly Webster (23:35)

8. The Human Experience of Scientific Discovery

• Awe of the Unknown: Scientists liken seeing these early developmental stages for the first time to receiving “the digital image of the Hubble telescope” (23:59).
• The Abrupt Stop: To comply with the rule, experiments are halted and embryos preserved at day 14 (24:43). There’s a strange emotional dynamic: a desire to keep going is tempered by an ethical line (25:28).

"You want to open it, but you have to be ready to see what's in it." — Ollie Rivenloo (25:53)

9. The Rule Under Pressure: What Now?

• Technological Advance Outpacing Law: Now that 14 days is attainable, a debate emerges: should we extend the limit? (26:13, 26:40).
• Ethical and Religious Diversity: Scientists express humility and reluctance to decide where the line should be, recognizing various religious beliefs and societal attitudes (28:12, 28:20):
  • Catholicism: Soul at conception
  • Judaism/Islam: Significance of heartbeat
  • Buddhism: Individuality at cord cutting
  • Hinduism: Life as a circle, not a linear origin

"What origin? A circle does not have an origin." — Ollie Rivenloo (28:20)

• The Desire for Knowledge vs. Social Responsibility: Scientists voice their intrinsic curiosity but stress the importance of societal consensus and sensitivity.

Notable Quotes & Memorable Moments

• "There is a place in the development of the human being that no scientist has seen? ... No."
  — Robert Krulwich & Molly Webster (11:33)
• "This is this kind of period, which in the textbook you will find described as a black box."
  — Magdalena Zernicka-Goetz (11:42)
• "The human embryo will behave in a self organizing manner in a complete absence of maternal inputs, at least for 14 days."
  — Ollie Rivenloo (21:55)
• "When people receive the digital image of the Hubble telescope… when I look inside of our own anatomy at the time where nobody knows that we even exist, it’s the same as looking at dimensions that we have never imagined we will ever see because we didn’t even know they exist."
  — Ollie Rivenloo (23:59)
• "If technology has now moved it a whole other week... does that mean the rule’s going to have to change?"
  — Robert Krulwich (26:13)

Timestamps for Key Segments

• Introduction to 14 Day Rule: [01:12]–[03:02]
• Leroy Walters and Committee History: [06:13]–[09:48]
• Defining the Primitive Streak and Embryo Individuality: [08:33]–[09:05]
• What Happens Between Days 7 and 14: [10:31]–[11:47]
• Magdalena’s Breakthrough Growing the Embryo: [13:01]–[13:55]
• Lab Tour and First Glimpse of Embryos: [16:31]–[19:11]
• Interpreting Embryo Images and Key Discoveries: [19:12]–[23:12]
• Debate on Extending the Rule and Societal Implications: [26:13]–[30:02]

Closing Thoughts

The episode closes with a sense of wonder, humility, and caution: modern science, while making the invisible visible, must continually balance its curiosity with deep ethical wisdom and respect for varying beliefs about what it means to be human. The intersection of scientific possibility and human values is only getting more intense as technology advances.