Embryo Development: Mammals, unlike birds, reptiles or amphibians, do not lay eggs but develop their progeny within their bodies. Especially a human baby takes nine months to be born, which is one of the most extended times in terms of other living beings for Embryo Development. An Embryo is present inside the fertilized egg that develops into a new being. This process is known as Embryo Development, where the organism develops from a single-celled zygote to a multicellular organism.

Embryo Development: Definition

During sexual reproduction, fertilisation results in the formation of a zygote which begins to develop into an embryo. The development of an embryo through different stages is called Embryo Development. This process differs in plants and in animals. The zygote divides repeatedly to give rise to a ball of cells. The cells then begin to form groups that develop into different tissues and organs of the body. This developing structure is termed an Embryo.
In Biology, Embryonic Development is also called Embryogenesis. Embryonic Development is the development of a single-cell (zygote) to a multicellular organism (foetus) characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. There are many stages that can be observed during the whole process of embryo development.

Embryo Development in Humans Diagram

Fig: Embryo and Fetal Development in Humans

Stages of Embryo Development in Humans

Embryonic Development in humans can be understood by studying the three stages:
1. The Germinal Stage
2. Embryonic Stage
3. Fetal Stage

1. The Germinal Stage:

Fig: Development and Embedding of the Embryo in the Uterus

The germinal stages include fertilisation, cleavage, blastulation and implantation. It can be understood in the following steps:
(a) Zygote formation and development of an embryo from the zygote
(b) Ball of cells
(c) Embedding of the embryo in the uterus

(a) Zygote formation and development of an Embryo from the zygote
i. Fertilisation is a process where the male gamete and female gamete fuse to form a zygote.
ii. A zygote is a single diploid cell, it undergoes many mitotic divisions without any significant growth in size. This process is called cleavage.

(b) Ball of cells
i. After multiple cell divisions occur, a dense ball of cells is formed called the morula. Typically, morula is a \(16\)-celled stage.
ii. Morula changes to the Blastocyst, which burrows itself into the lining of the uterus and forms a connection with the maternal tissue to get nourishment for its growth and development. This is called implantation. Blastulation is the process of development of the morula prior to implantation.

(c) Embedding of the Embryo in the uterus
i. Blastocyst comes in contact with the uterine wall and adheres to it, embedding itself in the uterine lining via the trophoblast cells. The process is called implantation.
ii. Post successful implantation, the Blastocyst adheres to the endometrium. The superficial cells of the trophoblast fuse with each other to form a multinucleated body called Syncytiotrophoblast that tightly secures the Blastocyst to the uterine wall.
iii. The Syncytiotrophoblast secretes human Chorionic Gonadotropin (hCG), forms villi for implantation, and in later stages forms chorion, amnion, and the foetal part of the placenta. The implantation completes at the \({\rm{2nd}}\) week of fertilisation.
iv. The embryo is joined to the trophoblastic shell with the help of a narrow connecting stalk which later develops into the umbilical cord to attach the placenta and Embryo.
v. As the Syncytiotrophoblast starts to penetrate the uterine wall, the inner cell mass (embryoblast) also develops.
vi. The embryonic stem cells present in the embryoblast can develop into any one of the three germ layer cells that can give rise to all the tissues and organs.

2. Embryonic stage:
This stage includes gastrulation, neurulation and organogenesis.
a. The Gastrulation process starts around the \({\rm{3rd}}\) week of fertilisation. During this, the embryo or inner cell transforms into \(3\) germinal layers, which means that the bilaminar disc is transformed into a trilaminar disc, these three layers are named ectoderm, mesoderm, and endoderm.  These cells transform and get differentiated to all the tissues and organs of the body such as nerve, blood, muscle, bone, digestive tract, etc.
b. Neurulation refers to the folding process where the neural plate is transformed into the neural tube, and this takes place during the fourth week. During this stage, the ectoderm gives rise to neural and epithelial tissue, now the gastrula is known as neurula.
c. Organogenesis is the development of organs that begins in the third week to the eighth week.

3. Fetal Stage:
In this stage, the organs inside the body of the baby begin to develop and can be identified individually. This is called the fetal stage and the embryo is now called a foetus. It continues to grow inside the uterus for a certain period, once it is fully developed it is ready for birth. Different Developments during different weeks of pregnancy in humans.

Weeks	Embryonic Development
\({\rm{1st}}\) Week	Implantation begins in the very first week after fertilization.
\({\rm{2nd}}\) Week	Implantation is completed in the second week. Hormones like HCG, progesterone, estrogen, etc., are produced to maintain pregnancy.
\({\rm{3rd}}\) Week	Gastrulation followed by neurulation takes place resulting in the formation of tri-germinal layers and notochord. The amniotic sac is formed and protects the Embryo-like a cushion.
\({\rm{4th}}\) Week	By the end of the first month, A primitive phase will take form with large dark circles for the eyes whereas other facial organs like the mouth, lower jaw and throat begin to develop. The heart is formed and starts to beat.
\({\rm{5th}}\) Week	The Embryo looks like \({\rm{C}}\)-shape, Organogenesis leads to the development of various organs like liver, spleen, gallbladder, etc. The inner ear begins growing too. Now the size of the Embryo is around \({\rm{4}}\,{\rm{mm}}{\rm{.}}\)
\({\rm{6th}}\) Week	Size of the Embryo now becomes about \({\rm{8}}\,{\rm{mm}}{\rm{.}}\) Development of facial features like eyes and nose continues. Hands and legs develop as well. Stomach and kidney precursors start developing. Organs like kidneys and stomach begin to form.
\({\rm{7th}}\) Week	Size increases to about \({\rm{12 – 13}}\,{\rm{mm}}{\rm{.}}\) Lungs, primary sex organs and lymphatic system start unravelling. Legs and hand fingers are visible too.
\({\rm{8th}}\) Week	Almost all the organs start advancing by this week. The external portions of the ear begin to be seen, nipples and hair follicles also develop.

Embryo Development in Animals

This process of Embryo Development is similar to humans in most animals. Embryonic Development is observed to occur in two different methods:
1. Direct Development:  Most of the animals directly give birth to a small young one that further grows into an adult with time. This occurs when animals undergo internal fertilisation (fertilisation inside the body). The number of offspring is less in this type of development as complete development of an organism requires a lot of energy that is generally obtained from the mother during the whole pregnancy. This phenomenon is seen mostly in mammals and reptiles.
2. Indirect Development: In this type of development, the Embryo develops in an egg in the form of a larva. In these animals, soon after the fertilisation, the zygote divides repeatedly and travels through the oviduct. As it passes through the duct, many protective layers are developed around it forming a hard shell. Once the protective layer or hard shell is formed, birds lay their eggs. The parent bird provides warmth to these eggs for the development of the embryo, this process is called incubation. When a chick is fully developed inside the egg, it breaks the shell and comes out. Animals such as a frog or other water animals in which external fertilisation (outside the body) takes place, development of the embryo also takes place outside the female body.

Summary

Embryo Development is an important life process in both plants and animals. Seeds in plants and fertilised eggs in animals act as a container for the Embryo. It is the containers that are important rather than the Embryo present inside them. In humans, it requires a period of around \(9\) months for the complete development of an Embryo.

Frequently Asked Questions (FAQs) on Embryo Development

Let’s look at some of the commonly asked questions about Embryo Development:

Q.1. What is the definition of an Embryo?
Ans: An Embryo can be defined as an organism in the early stages of development. An Embryo undergoes multiple stages of development to develop into a new organism.

Q.2. What are the stages of Embryonic Development?
Ans:  Embryonic Development is called Embryogenesis. The main stages of Embryonic Development are:
a. The Germinal Stage
b. Embryonic Stage
c. Fetal Stage

Q.3. What is the difference between Embryo and Foetus?
Ans: An Embryo is the earliest stage for the development of an organism. An Embryo has the ability to develop into a new being. It is referred to as a Foetus at the beginning of the ninth week of development after fertilization of the egg. Which is nearly the eleventh week of pregnancy in humans.

Q.4. What is the process of Embryo Development?
Ans: In Biology, Embryonic Development is also called Embryogenesis. Embryonic Development is the development of a single-cell (zygote) to a multicellular organism (foetus) characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. There are many stages that can be observed during the whole process of Embryo Development. The process of Embryonic Development varies in animals and plants.

Q.5. Does the Embryo have a heartbeat?
Ans: The heart is the first organ to start functioning during Embryonic Development. The heartbeat of an Embryo can be heard at about \(5-6\) weeks of pregnancy using ultrasound.

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