Embryonic Development

The human embryonic development process is also known as human embryogenesis. This process shows embryo formation or development generally takes place. This process generally gets initiated by cell division and cellular differentiation of the embryo at the primary stage of development of the embryo. 

Embryonic development begins with Germinal stage

Fertilisation

During copulation (coitus) semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix,enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube.The ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilisation takes place. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary isthmic junction. This is the reason why not all copulations lead to fertilisation and pregnancy. The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.

Embryogenesis

Formation of embryo from zygote includes following stages :

• Cleavage
• Blastulation
• Gastrulation
• Foetal development

Cleavage (or Segmentation)

Takes place during passage of zygote through the fallopian tube to uterus.

It is the mitotic cell division in zygote with process of cleavage :

Process of Cleavage

The first cleavage furrow appears at animal-vegetal axis (meridional)

2nd division is also meridional but at right angles to the first one forming four blastomeres.

3rd division is equatorial, resulting in eight blastomeres.

4th furrow is again meridional forming a 16 cell stage called morula.

5th furrow is equatorial though this may differ in different animals.

The synchrony of division is maintained upto 64 cell stage.

Blastulation 

Rearrangement of blastomeres forms blastula (blastodermic vesicle) with blastocoel

At this stage the zona pellucida disintegrates and rapid growth of embryo begins

The blastocoel signifies :

The site where ancestral embryos used to lodge nutrients as yolk.

Space used to acquire more and more fluid with increasing dimensions of the embryo. This results in the expansion of the outer layer of the blastodermic vesicle (also called blastocyst) as a thicker layer. 

After one week of fertilisation, blastocyst gets implanted in the uterine wall.

Peripheral cells of the morula form trophoblast or trophectoderm which draws food for the embryo from the uterine wall. This has only trophic and protective function and forms foetal membranes and part of the placenta and has no role in the formation of embryos.

Inner cell mass forms the body of the embryo, initially distinct as outer epiblast and inner hypoblast. The trophoblastic cells above the epiblast are called cells of Rauber.

Now, the blastula develops trophoblastic villi and gets attached to the endometrium of uterus.

The uterine wall at this time undergoes profound changes to form decidua, distinct as :

Decidua basalis – between embryo and uterine serosa

Decidua capsularis – between embryo and lumen of uterus

Decidua parietalis – remaining part of decidua.

Implantation of the Blastocyst

Implantation is the attachment of the blastocyst to the uterine wall. It occurs after 7 days of fertilisation. The portion of the blastocyst where the inner cell mass is located lies against the endometrium of the uterus. The blastocyst sinks into a pit formed in the endometrium and gets completely buried in the endometrium.

The embedded blastocyst forms a villi to get nourishment.

The cells of the inner cell mass differentiate into two layers :

A layer of small, cuboidal cells known as the hypoblast layer and a layer of high columnar cells, the epiblast layer .

Both the hypoblast and epiblast form a flat disc called the embryonic disc. The trophoblast cells divide and differentiate into two layers -an inner layer of mononucleated cells, the cytotrophoblast and An outer, multinucleated zone without distinct cell boundaries, the syncytiotrophoblast or syncytium. At the same time a small cavity appears between the epiblast and trophoblast. This cavity enlarges to become the amniotic cavity.

Gastrulation / Formation of Germ Layers

The sequence of forming respective germinal layers is different from frog, the order is as follows:

Formation of Endoderm

With increasing size of the inner cell mass and blastodermic vesicle a group of cells detach (delamination) from here and push into the blastocoel, get arranged along the inner wall of blastocoel, divide rapidly and form a complete inner layer of endoderm.

Now the erstwhile blastocoel becomes the lumen of archenteron. This differentiates into two parts – one within the body of the embryo, forming a gut and the other projects out as distal yolk sac.

After shifting of endodermal cells the rest of the inner cell mass forms embryonic disc.

Formation of Mesoderm.

At the caudal margin of the embryonic disc the cells divide rapidly to become thick mass. Subsequently, these cells start migrating in the area behind endoderm to form mesoderm.

Formation of Ectoderm

After the formation of mesoderm the remaining cells of the embryonic disc proliferate and get arranged in a layer outside the mesoderm to form ectoderm.

Foetal development

Foetal membranes or extra embryonic membranes are formed of embryonic tissue (trophoblast) that lies outside the embryo.

These membranes are found in amniotes (reptiles, birds and mammals).

The developing foetus soon becomes enclosed by four foetal membranes, amnion, chorion, allantois and yolk sac.

Amnion and chorion are formed by the fusion of the amniotic folds arising from the two ends of the foetus whereas allantois is produced from embryonic hindgut.

1.   Amnion is formed of mesoderm on the outside and ectoderm inside. It has no blood vessels. Space between amnion and foetus is called amniotic cavity and it contains amniotic fluid. Amnion protects the foetus from mechanical shock.

2.  Chorion is formed of ectoderm externally and mesoderm inside. Alongwith the allantois it participates in the formation of placenta. Space between amnion and chorion is externally embryonic coelom. It completely surrounds the embryo and protects it by providing nourishment and removing waste.

3.   Allantois consists of mesoderm on the outside, and endoderm internally. It extends to fuse with chorion and forms a structure rich in blood vessels called the allanto-chorion which gives rise to foetal part of the placenta. It helps in respiration, nutrition and excretion.

4.  Yolk sac is a membranous sac attached to an embryo near allantois; having yolk in egg laying animals. In mammals the yolk sac functions as the site of blood cell formation until about the 6th week, when the liver takes over this role. Thereafter the yolk sac starts to shrink.

Placenta is an organic connection between the foetus and uterine wall.It is contributed by both foetal and maternal tissue.It act as a barrier between the foetus and the mother

Although there is no blending of the two blood supplies, the placenta acts as an ultrafilter; soluble inorganic and organic materials, nutrients, hormones, antibodies against diphtheria, smallpox, scarlet fever, measles, etc. can pass from the mother to the foetus.

It helps in exchange of gases between mother and foetus and elimination of nitrogenous discharge and other waste of foetus.

Conclusion

Embryonic development is also called embryogenesis. It is a series of steps where multiple processes occur, such as ovulation, fertilisation, cleavage, blastulation, implantation, gastrulation, etc. It is a complex process where fertilised eggs start developing into an embryo. It takes about 4 days to develop a human embryo after an egg is fertilised. It takes about 12-14 hours to divide actively. Which later eventually undergoes development and gives rise to a foetus in a human form.