Model of Human Embryo Development

Context: Israel researchers have successfully created a “human embryo” in the lab without using traditional egg and sperm.

Fetal Development: A Process that Create Life

• Fertilisation: The journey of embryo development begins with fertilization, which occurs when a sperm cell from a male and an egg cell from a female merge.
  • Fertilization typically takes place in the fallopian tube, forming a zygote, which is the initial single-celled structure that carries the full genetic material (46 chromosomes) of the future individual. 

• Cleavage: After fertilization, the zygote undergoes a series of rapid cell divisions called cleavage.
  • These divisions produce a cluster of cells called a morula, which eventually develops into a blastocyst. 

• Blastocyst Formation: The blastocyst is a hollow ball of cells with an outer layer known as the trophoblast and an inner cell mass.
  • The trophoblast will give rise to structures like the placenta, while the inner cell mass forms the embryo itself. 

• Implantation: The blastocyst travels through the fallopian tube into the uterus, where it undergoes implantation. Implantation involves the attachment of the blastocyst to the uterine lining (endometrium).
  • The trophoblast cells play a crucial role in this process, as they invade the endometrial tissue to establish a connection with the mother’s blood supply. 

• Gastrulation: Gastrulation is a fundamental process that takes place during embryonic development.
  • It involves the reorganization of cells in the inner cell mass into three primary germ layers: the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). 
  • These germ layers give rise to different tissues and organs in the body. 

• Neurulation: Neurulation is a specific event during gastrulation that gives rise to the nervous system. The neural tube forms from the ectoderm and eventually develops into the brain and spinal cord. 

• Organogenesis: Organogenesis is the process of organ formation, where the three germ layers differentiate and give rise to specific organs and structures.
  • It includes the development of the heart, lungs, liver, kidneys, and various other organs. 

• Fetal Development: Once the major organ systems have formed, the developing embryo is referred to as a fetus.
  • The fetal period involves further growth and maturation of the organs and tissues. 

• Birth: Human embryo development culminates in childbirth, typically occurring around the 38th to 40th week of pregnancy. 

New Development in Research on Embryo Development 

Lab Grown Embryo 

This lab-grown embryo is considered one of the most complete models of a 14-day-old human embryo. 

• Multiple research teams are working on developing similar human embryo-like models, although none fully replicate the entire early embryo development process.
• Lab-grown embryo models are not intended for use in pregnancy, their purpose is to study the early stages of fetal development.

Method used by the Israel’s Researchers

• Researchers in Israel utilized a combination of stem cells and chemicals. 
• They achieved this by utilizing stem cells, which are early cells capable of differentiating into various cell types.
  • These stem cells spontaneously assembled to form an embryo-like structure, mimicking the molecular characteristics of an early embryo.
• A small portion of this mixture spontaneously assembled into various types of cells, including those forming the fetus, providing nutrients to the fetus, establishing the body’s developmental plan, and creating structures like the placenta and umbilical cord. 
• One challenge they faced was that only 1% of the mixture assembled spontaneously, making the process relatively inefficient.

Challenges to Embryo research  

• While they mimic aspects of development, they are not surrogate human embryos.
• Ethical constraints make it challenging to study the early stages of embryo development after implantation in the uterus.
• In most countries, it is legally accepted to destroy these models after studying them and attempts to implant them are not permitted.
• There is no way for scientists to ethically research the early stages of development of an embryo, as it is difficult to study it after it implants in the uterus.

14 Days Limit on Embryo Research 

• The 14-day limit on embryo research was proposed in the UK in 1979. 
• It coincides with the natural endpoint of embryo implantation. After this point, cells begin to differentiate, and the embryo takes on the characteristics of an individual. 
• This limit is based on ethical considerations, particularly related to the development of the “Primitive Streak,” which signifies the transition from radial to bilateral symmetry in the embryo.

Importance of the research

• Lab-grown embryo-like models serve as alternatives for studying early developmental processes.
• These models allow scientists to explore genetic, epigenetic, and environmental effects on developing embryos, providing insights into disease mechanisms and potential genetic treatments.
• Research during the initial days of embryo development is critical as most miscarriages and birth defects occur during this period.
• Studying these stages can enhance the understanding of genetic and inherited diseases and potentially improve in vitro fertilization success rates.
• Embryo models have provided insights into errors during DNA duplication.
• These models revealed that errors can occur earlier in the duplication process than previously assumed, impacting subsequent cell division. 
• These models help scientists understand the roles of various genes in fetal development, enabling manipulation for research purposes.

Terms

Human Embryo	A human embryo is the earliest stage of human development, typically referring to the first eight weeks after conception. It is characterized by the rapid division and differentiation of cells as the embryo develops into various tissues and organs.
Stem Cells	Stem cells are undifferentiated cells that have the unique ability to develop into different types of specialized cells in the body. In this context, they are used to create the embryo model.
Embryonic Stem Cells	Embryonic stem cells are pluripotent cells derived from the inner cell mass of an early-stage embryo. They have the potential to become any cell type in the body.
Differentiation	Differentiation is the process by which stem cells or precursor cells transform into specific cell types with distinct functions and characteristics.
Foetus	The fetus is the later stage of human development after the embryo, typically beginning around the ninth week of pregnancy and lasting until birth.
Placenta	The placenta is an organ that develops during pregnancy and provides nutrients and oxygen to the developing fetus while also removing waste products.
Umbilical Cord	The umbilical cord is a flexible tube-like structure that connects the fetus to the placenta, allowing for the exchange of nutrients, oxygen, and waste between the fetus and the mother.
In Virto Fertilisation	IVF is a medical procedure in which an egg is fertilized by sperm outside the body, typically in a laboratory dish, and then implanted into the uterus to facilitate pregnancy.
Genetic Code	The genetic code is the sequence of DNA bases that determines the genetic information and traits of an organism.
Epigentics	Epigenetics refers to changes in gene expression and cellular characteristics that are not caused by alterations in the DNA sequence itself but are influenced by external factors and chemical modifications.
Chromosomes	Chromosomes are thread-like structures in the cell nucleus that contain genetic information in the form of DNA. They are responsible for carrying and transmitting genetic traits.
Gene Manipulation	Gene manipulation involves altering the genetic material of an organism, often for research purposes, to understand the role of specific genes in development and function.