Periodization of intrauterine development

The periodization of intrauterine development can be based on several different characteristics:

• according to time − but we usually do not know the exact moment of fertilisation;
• according to the size of the concept − the disadvantage here is individual differences, so it mainly applies statistically;
• according to the number of somites - a fairly accurate method, but it can only be used between the 20th and 35th weeks (before the 20th week there are none, after the 35th week they already merge);
• by weight - there are also individual differences, in addition, the embryo cannot be weighed based on an ultrasound examination;
• according to morphological characteristics - the most accurate method, can be compared with other animal species, e.g. Jirásk's periodization.

Prenatal development can be divided into three main periods. The first period is the embryonic during this period cells, tissues and organs are created, it lasts about 8 weeks. The next period is the fetal. The last period is around the birth and is the perinatal period.

Stages of development according to Jirásek

1. Unicellular − begins with fertilisation, ootid, zygote, day 1.
2. Blastomeric − furrowing of the oocyte begins (it divides, but its volume does not increase), a structure called the morula is formed, 2.−3. day.
3. Blastoderm - a blastocyst is formed (fluid begins to enter the space between the cells until a uniform cavity is formed), 4th day.
4. Double-layer target − epiblast, hypoblast, are formed in the 2nd week.
5. Three-layer target with axial structures - 3 germ sheets are formed, 3rd week.
6. Tubular embryo - formation of somites, closure of the neural tube, 4th week.
7. C-shaped embryo − the embryo bends into lordosis, the formation of limbs, 5.−6. week.
8. Late embryonic period − limbs (including fingers) are formed, at the end the eye slits close → end of the embryonic period.
9. Fetal period - the end of the first trimester (after the end of the embryonic period), the second and third trimesters.
10. Perinatal period.

Links

Related articles

• Prenatal period: The first week of embryo development • Second week of embryo development • Third week of embryo development • Fourth to eighth week of embryo development

References

• SADLER, Thomas, W.. Langmanova lékařská embryologie. 1. Czech edition. Praha : Grada, 2011. pp. 414. ISBN 978-80-247-2640-3.

• VAJNER, Ludek. General Embryology I [lecture for subject Embryology and Developmental Biology, specialization General Medicine, 2nd Faculty of Medicine Charles University]. Prague. 6/11/2011. Avaliable from <<https://dl1.cuni.cz/course/view.php?id=1055>>.