Basics of embryology


Embryology is the study of how essential embryonic structures develop. The pre-embryonic and embryonic stages, which span the first eight weeks of pregnancy, are followed by the fetal stage, which lasts until birth. Primordial germ cells develop during the embryonic stage and migrate to the developing gonads. Further development into fertile oocytes and spermatozoa (gametes) occur via meiosis at different points in time in males and females. While embryonic stem cells can, like adult stem cells, replace regenerative tissue, they are pluripotent and can differentiate into almost any tissue type. They are therefore of great interest in the development of new therapeutic approaches. For more information on the morphogenesis of organ systems see embryogenesis.


All organ systems develop during the embryonic period, whereas organ maturation occurs during the fetal period.

Germ cell development (gametogenesis)

Primordial germ cells develop very early during embryogenesis. They arise from the wall of the yolk sac and migrate to the developing gonads during the 4th week of development. Primordial germ cells undergo meiosis to form mature sex-specific gametes (oocytes and spermatozoids)

Primordial germ cell development


  • Definition: The sequence of cells that develop into mature germ cells (gametes), which pass on genetic material to progeny.
  • Process: meiosis of primordial germ cells within gonads

Four functional spermatids or an oocyte with three polar bodies are produced from one primordial germ cell!



Meiosis occurs in both sexes as part of germ cell development. Meiosis results in genetic recombination and the number of chromosomes is reduced from diploid to haploid. This ensures that there is no tetraploid set of chromosomes after sexual reproduction.

Numerical chromosomal aberrations are caused by nondisjunction of chromosomes during meiosis, i.e., failed separation of homologous chromosomes (meiosis I) or sister chromatids of a chromosome (meiosis II). Chromosomal aberrations can lead to death of the embryo or syndromes such as Down syndrome or Klinefelter syndrome.

Stages of meiosis I

Stage Number of cells Chromosome set DNA content
Prophase I 1 2n 4C
Metaphase I
Anaphase I
  • Homologous chromosome pairs separate by disintegration of the chiasmata
Telophase I 2 1n 2C

For chromosome pairing during prophase, the homologous chromosome segments must be arranged opposite one another. Both sex chromosomes (X and Y chromosomes) differ greatly in their structure. Only a short homologous sequence (pseudoautosomal region) at the distal short arm enables their pairing and crossing over!

Stages of meiosis II

Stage Number of cells Chromosome set DNA content
Prophase II 2 1n 2C
Metaphase II
Anaphase II
Telophase II 4 1n 1C

For more details about cell biology see the cell cycle and general oncology.

All the oocytes that will ever be produced are formed during the fetal period, whereas sperm production begins at puberty and never stops.

Stem cells

As a result of new breakthroughs in research, stem cells are currently a major focus of interest. Because of their ability to self-renew and differentiate into various cell types, stem cells are one of the most important cell types in an organism. In addition to embryonic stem cells, there are adult stem cells, although these are more differentiated.

  • Definition: Stem cells are cells capable of self-renewal and differentiation into specialized cells.
  • Cell division
  • Characteristics
    • Totipotent (omnipotent): ability of a cell to differentiate into all cell types
    • Pluripotent: ability of a cell to differentiate into nearly all cell types
    • Multipotent: ability of a cell to differentiate into more than one cell type, but not all cell types
  • Classification

Hematopoietic stem cell transplantation is used to treat hemato-oncologic conditions, e.g., leukemias.

Molecular biology of development

Embryogenesis is controlled by various factors, signal molecules, and hormones. The most important terms are covered in the table below:

Subgroup/components Function
Transcription factors Homeobox proteins (Hox protein)

  • Craniocaudal organization of embryo
  • Positioning of limbs
  • Development and patterning of the skin and limbs
Pax proteins

Zinc finger proteins
  • Activation or inhibition of genes for growth and differentiation
Helix-loop-helix proteins
  • Regulation of muscle development
Testis-determining factor (TDF)

  • Development of male gonads
  • Responsible for anti-Müllerian hormone expression through induction of the Hox gene Sox9
Growth factors TGFβ family
EGF -family
  • Differentiation of the nervous system
FGF -family
  • Lengthening of the limbs
IGF -family
  • Stimulates skeletal and muscle growth
NGF -family
  • Neuronal survival and differentiation
Hedgehogs (e.g., Sonic hedgehog gene)
Wnt (e.g., Wnt-7 gene)

Cell adhesion molecules

Immunoglobulin superfamily (e.g., NCAM, or neural cell adhesion molecule)
  • Binding of a neuron to a neighboring cell
Intercellular channels Nexus (gap junctions)
  • Enable communication between neighboring cells.
  • Signaling molecules (e.g., retinoic acid) are able to cross channels and regulate gene activity.
Extracellular matrix Collagen, glycosaminoglycans, proteoglycans, glycoproteins
  • Components formed by cells that function in the binding, transport, and distribution of signaling molecules
Relevant hormones involved in development Androgens (testosterone)

Cellular processes of development

Process Description Examples
  • The process by which primitive (e.g., stem cells) become specialized cells
  • Joining of two or more cells, epithelium, or tissue
  • Movement of cells from one place to another
    • Three theories for migration
      • Migration of cells by amoeboid movement
      • Migration of cells due to folding of the embryo (shift cells from one place to another)
      • Migration of cells via the bloodstream
  • Increase in the number of cells (mediated by cell division)
  • Kaplan. USMLE Step 1 Anatomy Lecture Notes 2016. Kaplan Publishing; 2015.
  • Le T, Bhushan V,‎ Sochat M, Chavda Y, Abrams J, Kalani M, Kallianos K, Vaidyanathan V. First Aid for the USMLE Step 1 2019. New York, NY: McGraw-Hill Medical.
  • Sadler TW. Langman's Medical Embryology. LWW; 2018.
  • Dudek. High-Yield Embryology. Wolter Kluwers.
last updated 04/21/2020
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