Embryology is the study of the development of an organism from conception to birth. The prenatal period spans 38 weeks and includes the embryonic period from fertilization to 8 weeks, and the fetal period from 9 weeks until birth. During the embryonic period, the three germ layers are established which will give rise to all organs. Organogenesis occurs from weeks 4 through 8. The fetal period involves growth and maturation of organs. A basic understanding of embryology assists with learning anatomy and diagnosing congenital conditions.

1. Human Embryology
Chapter 1. Introduction
SHAFIM AHMED

2. Embryology
 What is Embryology?
1. Definition: the study of the origin and development
of an organism.
2. focuses on the development that occurs in the
prenatal period (gestational period)

3. Embryology
 Prenatal period: before birth
• 38 weeks(266days,9months) from conception to birth
(average) “fetal” age
• Gynecologic timing has been from LMP (last menstrual
period) therefore refers to 40 weeks “gestational”
age
 Date of conception has been difficult to time
 LMP is on average two weeks before ovulation

4. prenatal period
 “Embryonic” period: ( embryo)
1.extends from fertilization through 8 weeks.
week 1 starts with fertilization and extends through the first cell
divisions that produce an embryo with one germ layer.
week 2 produce two germ layers.
week 3 produce three germ layers and folds those a recognizable
three dimensional body form.
week 4 through 8 (month 2) are the period of organogenesis.

5. 2.Week 4 is particularly important, as the initial set-up of all
organ systems is completed in this week.
All the major events of organ formation and
formation of body regions occurs during the
period of organogenesis, which is
completed in the embryonic period.

7. “Fetal” period: 9 weeks – birth (fetus)
• The major events that occur during the fetal period involve
growth in the size of organs by rapid increase in cell number, as
well as biochemical functional maturation.
• Most organs begin with a basic form of functioning during the
fetal period, but it is only for development of the organ itself
usually. The most important is exception is the cardiovascular
system. Mature functional status is not achieved in most organ
untill after birth.

9. Prenatal Development
Embryonic
development
fertilization - 8 weeks
All major organs formed
Fetal development
9 weeks – birth
Organs grow larger and
become more complex
Postnatal DevelopmentPostnatal Development
time period from fertilization to birth = gestationgestation

10. Relative Size of Human
Conceptus
Figure 28.1

11. Four mammalian embryos at variousFour mammalian embryos at various
stages of development (HAECKEL):stages of development (HAECKEL):
• A PigA Pig
• B CowB Cow
• C RabbitC Rabbit
• D HumanD Human
““Please, note the similarities inPlease, note the similarities in
the different developingthe different developing
mammalian species!”mammalian species!”

12. Embryonic body axis terminology
1.The first axis formed is the cranial-caudal axis
The cranial-caudal axis is established while the
embryo is still a flat disk or sheet of cells, long
before it fold into the three-dimensional structure.
This axis runs from the future head to the future
”tail” end.

13. 2. The dorsal-ventral axis is established very shortly
thereafter.
The second major axis the dorsal-ventral, is
formed as the body folds. This axis defines,
respectively, the future” back” and “front” sides of
both embryonic and adult bodies.

14. Significance of Embryology in medicine
 The knowledge of the basic principles ofThe knowledge of the basic principles of
development will assist the student in understandingdevelopment will assist the student in understanding
relationships taught in gross anatomy, histology, andrelationships taught in gross anatomy, histology, and
pathology courses, and will later serve the clinicianpathology courses, and will later serve the clinician
in diagnosing certain conditions and in counselingin diagnosing certain conditions and in counseling
congenital malformations (significance incongenital malformations (significance in
gynecology/andrology and in molecular biology!).gynecology/andrology and in molecular biology!).

15. Content
 General Embryology
1.Fertilization
2.Cleavage
3.Implantation
4.Formation Of The Germ Layers
5.Folding Of The Embryo
6.Derivatives Of The Germ Layers
7.External Appearance Of The Embryo And Fetus
8.Placenta
 Special Embryology
Systems development