Anatomy and physiology

Development of the embryo from fertilization to birth

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Posted By SUMIT SHARMA

If you are going to have a baby! It is always special, exciting, wonderful, and joyful moments. Sometimes, you curious to know the development of the embryo throughout pregnancy. 

In my last article, we knew the process of fertilizationIn this article, we will see how the embryo develops in the entire pregnancy.  

Generally, the development of the embryo begins with fertilization. It starts when your periods are not coming. It is called the Last Menstrual Period (LMP). This end of female periods begins the story of baby development. 

Your pregnancy calculates from the first day of LMP. From this LMP, your gynecologist may easily calculate the expected date of delivery (EDD).

On average, there are 38-40 weeks (or 9 months or 280 days) of the whole pregnancy. These weeks are generally divided into three trimesters, which is also called the gestation period

During this period, the child grows into the uterus. Let’s know each trimester or human embryo stage in detail –

 

First trimester (1st to 3rd month) or (0 to 13 weeks)

It is the most crucial part of your pregnancy. The maximum chances of miscarriage and birth defects occur during this trimester. You need to take more care of yourself. Let’s see how your baby grows week by week – 

1st and 2nd week of pregnancy

Your body prepares for ovulation as usual. The first two weeks take ovum maturation from LMP. On the 14th day, ovulation takes place and the ovum fuse with sperm.

 

3rd week of pregnancy (1st week after fertilization) 

Ovulation to implantation (Germinal stage)

Once the ovum gets fertilized, it becomes a single-celled-zygote. It undergoes mitotic division (cell division) after the conception of 24-36 hours. 

This process of cell division is called cleavage, where cell starts to multiply –

Single-cell (zygote)

2 cells stage

4 cells stage

8 cells stage

 

These eight cells are loosely arranged. On the 4th day after conception, it converted into 16 cells stage called morula (solid mass of blastomeres). 

These cells are covered with the zona pellucida membrane. As soon as the morula enters the uterine cavity, fluid penetrates inside the morula through zona pellucida. 

Gradually, it forms a fluid-filled cavity into the morula. This fluid-filled cavity is called a blastocele

On the 5th day from conception, the cells develop into a blastocyst (32 cells stage). These cells are closed together at one side is called embryoblast (inner cell mass). 

Blastocyst also formed a single layer of large flattened cells outside called trophoblast (outer cell mass). 

Blastocyst (hollow ball of cells) = embryoblast + blastocele + trophoblast + zona pellucida membrane

After that, blastocyst hatching occurs. Eventually, the zona pellucida membrane starts to ruptures and disappear. 

As soon as the zona pellucida membrane is shed, the blastocyst gets an L-selectin molecule around the trophoblast surface for further protection. These selectin molecules look like a finger-like projection which helps for implantation. 

On the 7th day after conception, the L-selectin molecule bind to the decidua functionalis layer (superficial layer of endometrium uterus). Then, it invades into the endometrium layer. Here, the implantation is finished. 

At the end of this week, your baby is just a blastocyst. 

 

4th week of pregnancy (2nd week after fertilization) – bilaminar germ disc

Development of inner cell mass (embryoblast)

As the week starts, the blastocyst is partially embedded in the uterine wall. The inner cell mass (embryoblast) of the blastocyst divides into two layers – hypoblast and epiblast. These two layers make together bilaminar germ (embryonic) disc.

development of the embryo

This embryoblast helps in the formation of the embryo. These layers also form two cavities for further protection of the embryo.  

  • Epiblast layer forms an amniotic cavity 
  • Hypoblast forms yolk sac

 

The amniotic fluid help in the development of the embryo. In this fluid, the embryo can easily move and float. 

Amniotic fluid act as a shock absorber. It protects the embryo from external trauma and injury. Amniotic fluid also contains antibodies that prevent your baby from infection. 

Whereas the yolk sac provides nourishment to the growing embryo until the placenta forms. 

 

Development of outer cell mass (trophoblast)

Meanwhile, on another side, the outer cell mass (or trophoblast) starts to divides into two layers – cytotrophoblast (inner) and syncytiotrophoblast (outside). 

The trophoblast helps in the development of the placenta

Syncytiotrophoblast releases human chorionic gonadotropins (hCG) hormone in blood immediately after implantation. This hCG goes to the ovary and stimulates the corpus luteum to produce more estrogen and progesterone continuously.

The heavy release of estrogen and progesterone makes your endometrium layer thicker and more vascularized. 

This thick layer of endometrium produces glycogen and lipids. These glycogen and lipids provide nourishment to the blastocyst.

The hCG eliminates in the urine, which is used to detect pregnancy. It acts as an indicator of pregnancy. The pregnancy test kit (like prega news) is used primarily to detect the hCG content of your urine. If it is present, then it will show a positive result of your pregnancy.

Later, syncytiotrophoblast makes the lacunar system by fusing with small vacuoles. By this lacunar system, mother blood enters through maternal arterioles, and the blood drains out through maternal venules. 

As your baby further develops, the yolk sac starts producing mesoderm. It accumulates outside the yolk sac and amniotic cavity. This mesoderm outside the embryo. So, it is called extraembryonic mesoderm

As time passes, the extraembryonic mesoderm develops inner or outer layer. Eventually, it forms a cavity between the inner or outer layer, called the chorionic cavity

Meanwhile, cytotrophoblast starts to form finger-like projections (primary chorionic villi). These villi expand into the surrounding syncytiotrophoblast.

Gradually, this extraembryonic mesoderm also grows into primary villi, which form secondary chorionic villi. At the end of this week, it converted into tertiary chorionic villi

  • Primary chorionic villi (trophoblast only)
  • Secondary chorionic villi (trophoblast and mesoderm)
  • Tertiary chorionic villi (trophoblast, mesoderm, and blood vessels)

 

This tertiary villus connects with maternal blood. Eventually, it forms a temporary placenta. 

 

5th week of pregnancy (or 3rd week after fertilization) – trilaminar germ disc 

Embryonic stage

This is the stage where a mass of cells known as an embryo. At this stage, the bilaminar embryonic disc (two-layer) converts into a trilaminar embryonic disc (three-layer). This process is called Gastrulation

The embryo, which is having three layers, is called Gastrula. During this gastrulation process, three germ layers form – ectoderm (outer), mesoderm (middle), and endoderm (inner). 

All these three layers are derived from the epiblast. Let’s look at how do these germs layers form. 

Generally, epiblast and hypoblast are tightly fused with each other. The hypoblast layer is just underneath the epiblast. 

Epiblast plays a crucial role in the development of the embryo. This is the part that helps in forming all organs and other body parts of your baby. 

The process of development of the embryo begins when the nucleus of the epiblast starts to proliferates longitudinally at the epiblast surface. It migrates forward from one side to another side. Gradually, this longitudinal line makes primitive streak and primitive node. 

This primitive streak and node release a special chemical – Fibroblast Growth Factor-8. This chemical help in the migration of cells towards primitive streak and node. 

Later on, these cells proliferate from upward to downward, and they become primitive grooves and primitive pits

These cells mix with hypoblast. Gradually, it takes the place of hypoblast and moves laterally (sideways). As time passes, hypoblast cells disappear. These new cells spread below, which is called endoderm

Once the endoderm layer is formed, other cells also migrate downward and make the mesoderm layer. This middle layer formed below the epiblast and above the endoderm. 

The remaining part of the epiblast is called ectodermEach germ layer forms different parts and organs of your baby’s body. This is called organogenesis

An outer layer of the embryo (ectoderm) will form – 

  • Brain, spinal cord (neural tube), 
  • Skin including hair and nails
  • Eyes, nose, and inner ears. 
  • Pituitary, mammary, and sweat glands
  • Enamel of the teeth

 

The middle layer of the embryo (mesoderm) will form – 

  • Muscle tissue 
  • Cartilage and bone 
  • Subcutaneous tissue of the skin
  • Vascular system – heart, arteries, veins, and lymph vessels
  • Urogenital system – kidneys, gonads, and their ducts.
  • Spleen and adrenal glands

 

An inner layer of the embryo (endoderm) will develop – 

  • Gastrointestinal tract
  • Respiratory tract
  • Urinary bladder 
  • Thyroid, parathyroids, liver, and pancreas
  • Tympanic cavity and auditory tube

 

In this week, the heart-beat of your baby can be seen during ultrasonography. 

Almost all organs are entirely formed by the first trimester except the brain and spinal cord. They (brain and spinal cord) continue to grow throughout pregnancy. 

development of the embryo

 

6th week to 10th week of pregnancy (4th week to 8th week after fertilization)

In these weeks, the embryo forms a curved shape. It starts to develop limbs bud. As pregnancy advances, the limb buds of the embryo start to form cartilage. Later, it develops into bones of the legs and hands. 

The size of the brain continues to grow than the rest of the body. During these weeks, the placenta also develops; it spreads hair-like projection outside the surface to make contact with the wall of your uterus. 

 

11th to 13th week of pregnancy (9th to 11th week after fertilization) 

At this stage, the embryo will be considered a fetus. It will remain a fetus until birth. Your baby starts to developing teeth, fingernails, toenails, external ears, and reproductive organs. 

The fetus’s liver starts to produce bile, and the kidney also starts to excrete urine. The size of limbs gets more longer. 

The completion of the first trimester reduces the chances of miscarriage.  

 

Second trimester (4th to 6th month) or (14 to 26 weeks)

This is the best trimester where the pregnant female does not get much difficulty. You may get a better experience. During this trimester, you will get rid of nausea, vomiting, and morning sickness problem. 

The fetus’s sex can be determined at the 14th week by ultrasound. The testes may appear in a male fetus, whereas the ovary and uterus can be seen in a female fetus. Your doctor can see these features on ultrasound if you are having either a boy or a girl. 

During 16th to 20th week, you can feel the movement of the fetus. In the 20th week, the placenta fully formed.

Your fetus is fully covered with chorionic frondosum. Generally, the placenta acts as a barrier with two components – the fetal portion and the maternal portion. 

development of the embryo

As pregnancy advances, this chorionic frondosum is merged with decidua basalis of the uterus by connecting stalk (number of large arteries and veins). Later, this connecting stalk converted into an umbilical cord. 

Chorionic frondosum (foetal portion) + decidua basalis of uterus (maternal portion) = Complete Placenta

Placenta connects to the baby via the umbilical cord and maternal uterus. This placenta continues to grow throughout pregnancy. Meanwhile, the connection of the placenta is maintained through the umbilical cord.

The placenta has two crucial functions –

1.  Metabolic functions 

  • Exchange of gases – Mother provides oxygen to the fetus, and fetus drains out carbon dioxide to mother.
  • Deliver nutrients and electrolytes – Mother provides nutrients like glucose, amino acids, fatty acids, B-vitamins, etc. 
  • Transmission of maternal antibodies (IgG) – This IgG antibody provides passive immunity. It helps in the protection from infections. 
  • Waste removal – uric acid, urea, etc. 

 

2. Hormonal functions 

  • Human chorionic gonadotropins – pregnancy indicator
  • Estrogen and progesterone – Makes endometrium uterus layer thick and vascularised.
  • Human Placental lactogen (somatomammotropin) – It helps in metabolism, which stimulates lipolysis and gluconeogenesis. It also encourages breast development for milk production. 
  • Thyroid hormones – for fetus brain development. 
  • Release relaxin – it helps in the relaxation of the ligament in the pelvis and widening the cervix. 

 

During the 24th week, a fetus starts to gaining weight. By the end of the second trimester, your fetus will get around 1.5 kg, and the length will be 13 to 16 inches long. 

 

 

Third trimester (7th to 9th month) or (27 to 40 weeks)

This is the final step of your pregnancy. During this trimester, you will feel lots of fetal activity like – 

  • fetus will be more active and start to change position 
  • fetus starts kicking more
  • fetus will get more body fat
  • fetus can see and hear

 

development of the embryoGradually, the fetus head starts to moves into the position of delivery. Meanwhile, your fetus’s lungs continue to mature.

At the time of birth, the average weight of your fetus is around 3000 to 3400 g, and the length is 19 to 22 inches. During the 38th to 40th week, you will feel labor pain; if it does not begin, your doctor will induce labor pain. 

Now, your baby is ready to come into the world.

 

Takeaway 

In this article, we have seen – 

  • Development of the blastocyst
  • Development of the embryo 
  • Development of the fetus 
  • Development of the placenta

 

In this entire journey of pregnancy, we learned how does your baby grows from fertilization to birth. It starts to grow step by step like –  

Zygote (single-cell stage) → 2 cells stage → 4 cells stage → 8 cells stage → Morula (16 cells) → Blastocyst (32 cells) → Embryo → Foetus

If you found this post (development of the embryo) informative, please share it on social media. 

 

Source – T. W. Sadler. Langman’s medical embryology.  9th edition. General embryology, Chapter – 2 to 6. Page-31 to 117.

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