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Embryonic Development

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Embryonic Development and its Stages

IVSAT 2024

The embryonic development inside a mother’s womb is a miracle of nature. The union of the gametes and the formation of an embryo clasping inside a mother’s womb leading to the development of a child is a scientific marvel to discuss. In this article, we will elaborate on how an embryo develops and what kinds of changes take place in the stages of embryo development.


What is an Embryo?

The earliest stage of a foetus inside a mother’s womb is called an embryo. It is a cluster of cells that develop into a foetus in due course of time. It forms when the male and female gametes fuse to fertilize. At the first stage, a zygote is formed. It is a unicellular life form that tends to multiply rapidly to form an embryo for ten weeks in a woman’s womb.


It also happens in the sexual reproduction of plants. It happens in the flowers when the pollen grains release their haploid cells to fertilize the haploid cells or ovum in the ovary section. An embryo forms seeds and a flower turns into a fruit.


What is Embryonic Development?

The different phases of development of an embryo that takes place inside a mother’s womb or a flower are together called embryonic development. According to the embryonic development definition, a unicellular zygote converts into an embryo due to cell division inside the protective cover. One cell will divide to become 2 in this cell division. It will then form 4 cells that eventually become 8. This is how rapidly cell division occurs resulting in the formation of an embryo.


This stage of development takes place for 9 weeks. The embryo then converts into a foetus as it takes up a more complex form than before. These stages of embryonic development in human beings will be discussed below.


Primary Stage of Embryonic Period

The early embryonic development stage begins after the fertilization of the female eggs with sperm. Both these gametes are haploid cells. They unite to form a single diploid cell called a zygote. Within a few days, the zygote goes through the mitotic cell division process and achieves the primary stage of embryo development.


Later in the embryonic period, the cells start to divide to form blastomeres. These are clusters of cells. With the ongoing mitosis, the number increases from 2, 4, 8, 16, and so on. These blastomeres constitute an embryo. Nearly 8 to 16 of these units together form a morula.


A morula then keeps on dividing mitotically to form a blastocyst. Its outer layer prepares to attach to the internal uterine wall. It is called the trophoblast and the uterine wall is known as the endometrium. It continues till the 2nd week of fertilization.


On progressing further in this period, the attached blastocyst then starts differentiating into more cells to form an embryo. In this stage, two different kinds of cells appear. The outermost cells comprise the extra-embryonic tissue and the internal cells together form the embryo in two weeks.


It is then the placental development is witnessed from the chronic villi that attach the trophoblast with the uterine wall. This placenta is the ultimate connection between the mother’s amniotic system and the embryo. It helps in the exchange of blood, nourishment, and respiratory gases. The rapid development of the embryo into the foetus is then witnessed.


In the 3rd week, a unique transformation is seen. It is called gastrulation where the germinal layers start to appear. As per the embryonic development definition, three different layers endoderm, mesoderm, and ectoderm develop in the embryo.


Endoderm forms the internal organs such as the liver, pancreas, lungs, bladder, and gut lining. The ectoderm forms the spinal cord, brain, nervous system, skin, nails, hair, etc. The mesoderm forms the connective tissues, circulatory system, muscles, bones, kidneys, and gonads.


This is the elementary pregnancy and embryonic development stage. It further proceeds to the advanced stages of development.


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Advanced Stages of Embryonic Development

  • 4th week – The formation of the heart occurs. It starts functioning by beating. The optic pits and arm buds become visible in the embryo.

  • 5th week – The size of the embryo becomes 4 mm. It takes a C-shape with the development of inner ears, pharyngeal arches, pancreas, liver, gall bladder, and spleen.

  • 6th week – The size becomes 8 mm. It starts to develop eyes, ears, leg buds, flat paddles as hands, along with the precursors of kidneys and stomach.

  • 7th week – the size becomes 13 mm. The lymphatic system starts to develop along with the primary sex organs. The lungs, legs, and arms lengthen in size.

  • 8th week – the size increases to 20 mm. The embryo develops external ears, nipples, along with hair follicles. All the other vital organs start to develop at this stage.


These are the stages of embryonic development elaborately described along with the detailed emergence of the organs in it. Understand the development of these organs step by step right from the beginning of embryo formation.

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FAQs on Embryonic Development

1. What is the prime difference between a foetus, a baby, and an embryo?

An embryo is a cluster of cells that witnesses significant development. If you refer to any early embryonic development PDF, you will understand how the organs start to develop to give it a foetal shape at the 9th week. Once the foetus is formed, it grows for another 7 months to form a fully grown baby with all the organs intact and ready to take birth.

2. What is the function of the placenta?

The placenta is the connection between the mother’s womb and the embryo. It works as an endocrine gland in the late embryonic development stages to secrete growth hormones and sex hormones in order to continue the gestation period safely. It also acts as a channel to get nutrition from the mother along with the exchange of respiratory gases. The embryo depends on the placenta for survival and growth.


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