Embryology

The process of human development begins when a sperm fertilises an ovum. The union of the male and female gametes results in a fertilised cell called a zygote. A zygote is transformed into a multicellular human being through a series of processes such as cell division, cell migration, growth, and cell rearrangement. Most of these changes occur in the embryonic and foetal stages. The field of study related to the formation, growth, and development of an embryo is called embryology. It includes the study of the prenatal stages of human development, which begins from fertilisation, formation of the zygote, development of the embryo and foetus to the birth. 

Fertilisation

The process in which the sperm (male gamete) and the egg (female gamete) fuse together is fertilisation. The union of the two gametes forms a cell known as the zygote. 

The reproductive system releases an egg in the fallopian tube during the ovulation phase of the menstrual cycle. If two eggs are released in the fallopian tube, it may result in fraternal twins. 

During the ovulation phase, the cervical mucus (a fluid produced by the cervix) will thin to form an easier passage for the sperm. The sperm swims through the cervix to reach the fallopian tube, where fertilisation occurs within 24-72 hours after the sperm enters the vagina.

Blastocyst Development

By the fifth or sixth day after fertilisation, the fertilised egg becomes a rapidly dividing ball of cells known as a blastocyst. The inner group of these cells forms the embryo. The outer group of these cells forms an organ known as the placenta, which is attached to the uterus wall.

During maturation, the blastocyst is protected by an outer covering known as zona pellucida. The outer group of cells of the blastocyst lies just beneath this cover and forms the future placenta, which protects and helps develop the foetus in the uterus. The inner group of the cells develops into various organs of the human body, such as skin, liver, muscles, heart, etc. 

The cells of the blastocyst divide and thus grow at a rapid pace and turn into more specialised cells. The structure becomes tight, and the outer covering of the blastocyst (zona pellucida) breaks, thus releasing the blastocyst. The blastocyst then reaches the uterus through the fallopian tube.

Blastocyst Implantation

The blastocyst, on reaching the uterus, implants in the endometrium (a mucous membrane lining the uterus). The outer cells of the blastocyst and the uterine lining together form the future placenta. The function of the placenta is to provide nutrients to the baby and remove its waste.

Embryo Development

After the blastocyst implants into the uterus, it is referred to as an embryo. In this stage, the external structures, as well as the internal organs, start developing.

The lower jaw, mouth and throat develop. Simultaneously, the blood circulation system starts developing and creates a heart tube. The brain and the spinal cord form completely. Other body structures like the ears, eyes, legs, arms, fingers and toes are also shaped. In contrast, the sensory organs and the digestive tract start their development. The formation of the first bones replaces the cartilage. 

The final embryo development stage starts around the tenth or twelfth week of pregnancy. In the final stage of development, the embryo is termed a foetus.

Foetal Development

The final stages of embryonic development start by the twelfth week of fertilisation. This stage is termed the foetal stage. By this stage, all external structures and organs have been formed, although they need to grow. 

The upper and lower extremities of the foetus develop completely at three months of pregnancy. The reproductive organs and the ears and teeth are formed. By the end of the third month, the expansion of the urinary and circulatory systems is complete. By this time, the foetus is about 5 inches in length. 

The foetus starts responding to sound at six months of pregnancy. The foetus is about 12 inches in length and starts changing position by this time. The length of the foetus can go up to 14 inches. 

At around eight months of pregnancy, the foetus forms a baby. The foetus’s brain starts evolving so quickly that the baby starts to hear and see. The respiratory system of the baby still requires maturation at this stage. At around nine months of pregnancy, the baby can move its whole body. During the final stage of pregnancy, the uterus contracts to push the foetus towards the cervix until the baby comes down the birth canal.

Conclusion 

The study of an embryo’s formation, growth, and development is termed embryology. The five stages of embryonic development are fertilisation, blastocyst development, blastocyst implantation, embryo development and foetal development.

The male and female gametes fuse in the fallopian tube to form the zygote in fertilisation. The zygote then turns into a rapidly dividing ball of cells known as a blastocyst, which reaches the uterus through the fallopian tube. The blastocyst implants in the endometrium, that is, the mucus lining of the uterus. The blastocyst’s endometrium and outer cell form the placenta while the inner cells form the embryo. 

The internal organs and the external structure of the baby start forming in the embryo development stage. The embryo enters the foetal stage after the twelfth week of pregnancy. In this stage, the baby’s organs start growing, the baby starts moving, responds to sounds and stimuli. The uterus contracts to push the baby down the birth canal at nine months of pregnancy.