Embryo Development in Humans and Other Animals

There are several stages of development of a new born baby which begin as a fertilized egg then develops into a blastocyst, an embryo and eventually a fetus. Embryo development in humans where the journey of our life begins with a single cell which later divides itself into a mass of many cells together. The scientific name that has been given for the process of embryo development is called embryogenesis. This is nothing but the process which follows fertilization.

What is Fertilization?

Fertilization is the process that takes place in the fallopian tubes of the female reproductive system in humans. There is a series of processes that follows for the formation of the embryo before it reaches the uterus which helps in its growth and development.

From one of the ovaries one egg is released during each normal menstrual cycle about 14 days after the menstrual period.  This is known as ovulation. The mucus in the cervix becomes more elastic and fluid which allows the sperm rapidly into the uterus during ovulation. Fertilization results if sperm penetrates the egg. Cilia are tiny hairs like lining the fallopian tube that propels the fertilized egg towards the uterus through the tube. It further moves down the fallopian tube to the uterus and the cells of the zygote divide repeatedly. Zygote enters  the uterus in around 3 to 5 days. The cells continue to divide in the uterus becoming a blastocyst which is a hollow ball of cells. About 6 days after fertilization The blastocyst implants on the wall of the uterus.

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How Does Fertilization Start?

The process starts in one of the fallopian tubes where the egg cell has fertilized. This fertilized egg cell now divides itself into two cells followed by four and so on. This results in the formation of a ball-like structure that contains many cells inside it. This ball-like structure is referred to as a morula when the cell number is sixteen. This further divides into a structure called the Blastula. 

What is Blastula?

The Blastula is nothing but a stage wherein the cells have arranged themself in a manner that forms a hollow mass structure. Wherein the cells align themselves into the end leaving a cavity within. This later gets filled with a fluid, the structure will look like below.

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What is Blastocyst?

This is the next stage in the Embryo development, this is the stage where-in the Embryo will have cells that have got differentiated and will even differentiate further. The differentiation is a process where one cell gets differentiated from other cells in its vicinity. As a result, we will have a whole new variety of cells from these differentiated cells once after successive divisions. The Blastocyst stage is nothing but a mass containing these differentiated cells. 

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What is Implantation?

As this cell grows further the body of the embryo further develops. For its proper growth and development, the embryo will require proper nutrition. This nutrition is derived from the mother’s body. The blastocyst needs to get connected to that location where it can derive this nutrition. This is the most important stage where the implantation comes into the picture. This is the stage where the blastocyst attaches itself inside the mother’s body at a fixed position inside the uterus, where it implants itself. This is the place where the baby grows.

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What is Placenta?

The main reason for the embryo to attach itself is as below:

1. Deriving nutrition

2. Shedding away waste

3. Exchange of oxygen and carbon dioxide

All of this is served through the barrier, which is an organ called the Placenta. We can say that the placenta connects the baby with the mother. The embryo is all set to grow into a baby.

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What is Fetal Stage?

The stage where the organs inside the body of the baby start developing such that they can be identified is called the fetal stage. The embryo is now called the fetus. The fetus will now continue to grow in the uterus after taking the required amount of time. When the growth is fully completed then the child will be all set to enter the beautiful world. This is how embryogenesis takes place in most of the animals.

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Development of the Foetus

The embryo is considered a foetus at the end of the 8th week after fertilization. At this stage the structures have already been formed and developed.

The following are markers during pregnancy:

• The fetus feels the entire uterus by 12 weeks of pregnancy.

• The sex can be identified by the 14th week.

• The pregnant women can feel the fetus move by 16 to 20 weeks. Women, pregnant for the first time can feel it around this time but those who have been pregnant before can feel it in 2 weeks.

• By about 24 weeks the fetus has a chance of survival outside the uterus.

• Until near the time of delivery the lungs continue to mature. Throughout pregnancy the brain accumulates new cells.

How the Fetus Develops Stage By Stage

Month One

As the fertilized egg grows, a watertight sac forms around it gradually filling it with fluid. This is called the amniotic sac. This very much helps in cushioning the embryo. Simultaneously the placenta also develops which is a round flat organ that helps in transferring nutrients from the mother’s body to the embryo intern transferring waste from the embryo into the mother's body through a tube that is attached to the placenta with the embryo known as Umbilical cord. A primitive phase will take form with large dark circles for eyes. The mouth, lower jaw, and throat are developing. Here blood cells will take shape and the process of circulation will begin. The tiny heart will beat 65 - 75 beats per minute by the end of the fourth week. By the end of the month, the embryo will be quarter-inch long, which will be smaller than a grain of rice when compared.

Month Two

Here the facial features of Embryo continue to develop. The ear starts with a fold of skin at the end at the side of the head. There will be a formation of tiny buds that will eventually grow into arms and legs. Here fingers, toes, and eyes are also beginning to form. At this stage the neural tube that is the brain, the spinal cord, and other neural tissues of the central nervous system are well-formed. Along with this the digestive tract and the sensory organs begin to develop. Finally the bones begin to replace the cartilage. Here the head of the embryo is large when compared to the rest of the body. By the end of the second month, the Embryo is about one inch long and weighs about one thirty of an ounce. After completion of six weeks, you will be able to detect the heartbeat in the Embryo. After the completion of eight weeks, the Embryo is called a Fetus. 

Month Three

At this stage, the development of the baby’s arms, hands, fingers, feet, and toes will be fully completed. At this stage of time, the baby will be able to open and close fists and mouth. Here we will observe, the fingernails and toenails will start to develop and external ears will be formed. At this stage of the process, the baby’s reproductive organs will start to develop,  but even at this stage the baby's gender will be difficult to distinguish in the ultrasound. At the end of the third month, the baby will be fully formed. All the organs and extremities will be present and will continue to mature in order to be fully functional. Here we can observe the circulatory and the urinary systems start working along with this, the liver produces bile. At the end of the third month, the baby will be about four inches long and will be weighing about one ounce. At this stage, we can say that the baby’s critical development has taken place, and the chances of a miscarriage drop considerably. 

Month Four

This month we will be able to hear the baby’s heartbeat with the help of an instrument called the Doppler. Here we will observe fingers and toes are well defined. Eyelids, eyebrows, eyelashes, nails, and hair will also be formed. At this stage, the density of teeth and bones grows, the baby will be able to suck his or her thumb, yawn, stretch, and make faces. At this point of time, the nervous system will begin to function. The reproductive organs and genitalia are fully developed. The sex of the baby can be found out via ultrasound. At the end of this month, the baby will be about six inches long and weighs about four ounces.

Month Five

This is the point where you will be able to feel the baby’s movements as he or she will be developing muscles and exercising them. In medical terminology, the first movement is called quickening. At this stage of development hair begins to grow on the baby’s head. The baby’s back, shoulders, and temples will be covered by short hair that is called Lanugo. This hair usually protects the baby and usually is shed by the end of the first week of life. Vernix Caseosa is the white coating that covers the baby’s skin. This cheesy substance protects the baby from the long exposure to the amniotic fluid. This layer of coding is shed just before the birth. By the end of the fifth month, the baby will be about ten inches long and will be weighing about one and a half to one pound.

Month Six

At this stage of development, the baby's skin will be reddish in color, wrinkles and veins will be visible through the baby’s translucent skin. We can even observe the baby’s fingers and toe prints. Here even the eyelids begin to part and the eyes open. Now the baby begins to respond to sound by moving and the pulse will increase. Here you will be able to feel the jerking movements when the baby hiccups. If the baby is born prematurely after the twenty-third week it will be able to survive with intensive care. At the end of the sixth month, the baby will be twelve inches long and will be weighing about two pounds.

Month Seven:

During this period of time, the baby will continue to mature and develop reserves of body fat. At this point, the baby’s hearing is fully developed. The baby will change positions frequently and respond to stimuli, including sound pain and light. Here the amniotic fluid begins to diminish. On the completion of the seventh month, the baby will be fourteen inches long and weighs about two to four pounds. After the seventh month, if the baby is born prematurely, it will be able to survive.

Month Eight

Even at this stage, the baby will continue to mature and develop reserves of body fat. At this point of time, you will observe that the baby is kicking more. By this time, the baby's brain will develop rapidly and the baby will be able to see and hear. Except for the lungs which are still immature, most of the internal systems will be well developed. By this stage, the baby will be eighteen inches long and will be weighing almost five pounds.

Month Nine 

At this point of time, the baby’s lungs are nearly fully developed. The baby’s reflex is well coordinated so that he or she can close the eyes, blink, turn the head, and grasp firmly. He or she will be able to respond to sounds, light, and touch. Now you can say that the baby is ready to enter the world. On arriving at this stage, the baby moves less due to lack of space. Now the baby begins to position itself for labor and delivery. The baby will drop down into the pelvis. Usually, the baby’s head is towards the birth canal. By the end of this month, the baby is eighteen to twenty inches long and will weigh about seven pounds.

Conclusion:

This article focuses on stages of Embryo Development. Students can refer to this for a thorough understanding on the topic. The PDFs are easily downloadable through the website and for free.