Embryonic development is the process by which a fertilized egg develops into a fully formed human being. It begins with fertilization and continues through a series of well-organized stages inside the uterus. The fetal stage begins after the embryonic stage (after 8 weeks of pregnancy) and continues until birth.
This period is called the fetal period, during which the body systems formed earlier grow, mature, and become functional.
Prenatal human development is divided into two major stages:
- Embryonic stage (fertilisation to 8 weeks)
- Fetal stage (9th week to birth)
Embryonic Stages
Embryonic development is the process by which a single-celled zygote develops into a multicellular embryo with distinct organs and body structures. The period from fertilization to 8 weeks is called the embryonic period, during which all major organs are formed.
Early Embryonic Development
- Cleavage: The zygote divides repeatedly by mitosis to form 2, 4, 8, and 16 cells. The cells formed during cleavage are called blastomeres.
- Morula: An embryo with 8–16 blastomeres is called a morula.
- Blastocyst: The morula continues dividing and forms a blastocyst. The blastocyst consists of Trophoblast – the outer layer that helps in implantation and placenta formation, and Inner cell mass – forms the embryo.
Gastrulation
Gastrulation begins during the third week of development. The formation of germinal layers is known as gastrulation. Later, these germ layers differentiate into different tissues and organs.
- Ectoderm—nervous system, brain, spinal cord, skin epidermis, hair, nails, etc.
- Mesoderm- connective tissues, muscles, bones, circulatory system, kidneys, gonads, and notochord.
- Endoderm- lining of the digestive and respiratory tracts, liver, pancreas, urinary bladder, and lungs.
Organogenesis (Weeks 4–8)
This is the period of organ formation.
- Week 4 – Heart begins beating
- Weeks 5–6 – Eyes, ears, limb buds develop
- Week 7–8 – Most major organs are formed; embryo shows human appearance
By the end of the 8th week, the embryonic stage is complete.
Fetal Stage
From the 9th week onwards, the embryo is called a fetus.
- During this stage, the organs grow, mature, and become functional
- By the 9th month, the fetus is fully developed and ready for birth.
Week-wise Human Embryonic Development
Weeks | Description |
|---|---|
Second Week | The trophoblast develops further, and early placenta formation begins. |
Third Week | Gastrulation occurs, and the formation of ectoderm, mesoderm, and endoderm. |
Fourth Week | The heart begins to beat (the first functional organ) |
Fifth Week | The gall bladder, ears, spleen, pancreas, and pharyngeal arches begin to develop. |
Sixth Week | The embryo develops eyes, nose, stomach, and early kidney structures. |
Seventh Week | The embryo develops primitive sex organs, lungs, and the lymphatic system. |
Eighth Week | Hair follicles and external ears begin to develop. |