Human Embryogenesis: Definition & Stages

Human Embryogenesis: Definition & Stages
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  • 0:00 Embryogensis
  • 0:51 Blastocyst Formation
  • 1:15 Implantation
  • 1:44 Gastrulation
  • 2:46 Neurulation
  • 3:48 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
In this lesson, you'll learn about human embryogenesis, the process of reproduction in humans. You'll explore what happens during each stage of embryogenesis, as well as the key terms used to discuss the process.

Embryogenesis?

Embryogenesis is the development of a fertilized egg that occurs early on in pregnancy. After a sperm fuses with an egg, many changes occur in a specific order. The cells divide, reorganize and form layers of tissue that will eventually develop into specific organs. Embryogenesis happens very early during pregnancy, even before a baby is a true fetus.

The first step to embryogenesis is fertilization, where a sperm cell fuses with an egg cell. Together, they form a zygote. This occurs in the fallopian tubes, or the pathway between the ovaries and the uterus in the female reproductive system. Hours after the two cells join together, the zygote begins dividing and moves onto the next stage.

Blastocyst Formation

As the zygote divides, it forms a blastocyst, or a ball of cells. The blastocyst cells keep dividing and eventually form a hollow cavity in the center called a blastocoel. The blastocoel looks like a basketball, with a hollow inside and a thicker coating of cells on the outside. This hollow shape will be important for creating the layers of the body later on.

Implantation

After blastocyst develops, the ball of cells moves through the fallopian tubes and attaches to the uterine wall during implantation. This is where the embryo will continue to grow into a baby. When the blastocyst attaches to the uterine wall, cells in the uterus begin to grow around the blastocyst, forming the amniotic cavity, or a closed sac. Similarly, the uterus will grow around the blastocyst to help it mature.

Gastrulation

Gastrulation is a critical stage where the three different layers of the embryo form. Each layer is destined to become a different set of tissues and organs. During gastrulation, cells at the top of the blastocyst migrate towards a central line called the primitive streak. This creates a cavity called the archenteron that will form the digestive tract.

The area at which the archenteron opens is called the blastopore. This invagination, or indent, causes the formation of three tissue layers: the ectoderm, the endoderm and the mesoderm. The ectoderm becomes the outer layers of the body, such as skin and nails, as well as the nervous system. The endoderm forms the lining of the digestive, urinary and respiratory systems.

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