Human Embryology


Begins with the fertilization of an egg by a sperm. A fertilized egg is called a zygote.



           the sperm approaching the ovum


The fertilized egg divides into two cells and then into four cells. These are embryos at the 2-cell and 4-cell stage.


The embryo will continue to divide until it forms a solid

 ball of 64 cells called a morula. Human embryology is

 just like the embryology of all other animals until the

 morula stage. Cells in the morula stage are totipotent

 stem cells.


What is a stem cell?

·      A cell in the body whose job has not yet been



What is meant by totipotent?

·      a cell with the “total” potential to develop into

           a new individual. They can become ANY cell

   in the body.


The morula will then hollow out forming a blastula.



Four days after fertilization, hatching occurs. The

blastula emerges from the hardened zona pellicida (ZP).


Four to six days after fertilization, cells within the

Blastula will become differentiated into two types of

cells, embryonic stem cells and trophoblasts.   The

inner cell mass (ICM) contains the 40 embryonic stem

cells that will become the fetus and amniotic sac. The trophoblasts will become the placenta.

·      These cells of the blastula are pluripotent.


What is meant by pluripotent?

·      a cell with the “most” potential to develop into

different cells. They can become ALMOST ANY

cell in the body.





T= Trophoblasts                        


Pluripotent stem cells in the ICM will differentiate into

 4 major branches.


1.  HSC – Hematopoetic Stem Cells

(all blood cells)

Erythorocytes            Lymphocytes

Neutrphils                 Eosinophils

Basophil                           Moncytes


2.  MSC – Mesenchymal Stem Cells

 (mesenchyme cells)

Bone cells                  Cartilage cells

Tendon cells              Muscle cells

Adipose cells                    Marrow cells

3.  ESC – Endodermal Stem Cells

 (digestive tract & lungs)

Esophagus                 Pancreas

Liver                          Intestines

Stomach                           Lungs

4.  NSC – Neural Stem Cells

(brain and nerve cells)

Neurons                     Oligodendrocytes

Glial cells                  Astrocytes


·      These four major branches will differentiate into

210 types of specialized somatic (body) cells.



On day 49, about eight weeks after fertilization, the embryo becomes a fetus which contains fetal stem cells. Fetal stem cells are also pluripotent.


                     the unborn child at 6 weeks                  the unborn child at 8 weeks            





Stem cells from the placenta and adult stem cells are multipotent.



What is meant by multipotent?

·      a cell with “much” potential to develop into

different cells. They can become A LIMITED

RANGE of cells in the body.