Stages of Embryonic Development from Fertilization to Organogenesis
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.
(Image will be Uploaded Soon)
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.
FAQs on Embryonic Development in Humans and Animals
1. What is embryonic development?
Embryonic development is the process by which a single fertilized egg cell develops into a multicellular organism through coordinated cell division, differentiation, and morphogenesis. It begins with fertilization and continues through several key stages:
- Cleavage – rapid cell divisions producing a multicellular embryo
- Blastulation – formation of the blastula with a fluid-filled cavity
- Gastrulation – formation of three primary germ layers
- Organogenesis – development of tissues and organs
2. What are the main stages of embryonic development?
The main stages of embryonic development are fertilization, cleavage, blastulation, gastrulation, and organogenesis. These stages occur in a specific sequence:
- Fertilization – fusion of sperm and egg to form a zygote
- Cleavage – rapid mitotic divisions without growth in size
- Blastulation – formation of the blastocyst (in mammals)
- Gastrulation – establishment of ectoderm, mesoderm, and endoderm
- Organogenesis – development of organs from germ layers
3. What is cleavage in embryonic development?
Cleavage is a series of rapid mitotic cell divisions that increase cell number without increasing the overall size of the embryo. During cleavage:
- The zygote divides into smaller cells called blastomeres
- No significant growth occurs between divisions
- A solid ball of cells called the morula is formed
4. What happens during gastrulation?
Gastrulation is the stage where the three primary germ layers are formed and the basic body plan is established. During this process:
- Cells move inward through a structure such as the blastopore
- The ectoderm forms the outer layer
- The mesoderm forms the middle layer
- The endoderm forms the inner layer
5. What are the three germ layers and what do they form?
The three germ layers are ectoderm, mesoderm, and endoderm, and each gives rise to specific tissues and organs. Their major derivatives include:
- Ectoderm – forms the nervous system, brain, spinal cord, and epidermis
- Mesoderm – forms muscles, bones, blood vessels, heart, and kidneys
- Endoderm – forms the digestive tract lining, liver, pancreas, and lungs
6. What is organogenesis in embryonic development?
Organogenesis is the process by which organs and tissues develop from the three germ layers. It follows gastrulation and involves:
- Cell differentiation into specialized cell types
- Formation of organ primordia (early organ structures)
- Complex tissue interactions and signaling pathways
7. What is the difference between embryonic and fetal development?
Embryonic development involves the formation of basic body structures, while fetal development focuses on growth and maturation of established organs. The key differences are:
- Embryonic period – from fertilization to the end of the 8th week in humans; major organs are formed
- Fetal period – from week 9 until birth; organs grow and become functional
8. How does cell differentiation occur during embryonic development?
Cell differentiation occurs when unspecialized cells become specialized due to selective gene expression. This happens through:
- Activation and repression of specific genes
- Cell-to-cell signaling via morphogens
- Environmental cues within the embryo
9. What is the blastocyst and what is its function?
The blastocyst is a hollow stage of the mammalian embryo that enables implantation into the uterine wall. It consists of:
- An outer layer called the trophoblast, which forms part of the placenta
- An inner cell mass, which develops into the embryo
- A fluid-filled cavity called the blastocoel
10. Why is embryonic development important?
Embryonic development is important because it establishes the body plan and forms all major tissues and organs of an organism. Its significance includes:
- Creation of organized body structures from a single zygote
- Formation of functional organ systems
- Determination of anatomical symmetry and patterning
