1. Basic embryology Dr. Nagy Nándor
Semmelweis University, Department of Human Morphology and Developmental Biology

2. Embryology – study of the origin and development of single individual
Prenatal period (before birth):
Embryonic period – first 8 weeks (all major organs formed)
Fetal period – remaining 30 weeks
Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum, egg, which, after fertilization, is then called a zygote. The zygote undergoes rapid mitotic divisions, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

3. Prenatal period:
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

4. Question of time (anatomy vs. gynecology)
Fertilization (anatomy) – clinically usually impossible to determine, uncertain
Last menstruation (gynecology) – certain, but just an estimate, not actual age
(ovulation usually 14 days after mestr., fertilization within 2 days)
Length of pregnancy:
280 days (40 weeks) after the last menstruation,
266 days (38 weeks) after conception
Trimesters:
1st: 0-12 weeks
2nd: weeks
3rd: 25-40

5. Oogenesis
Starts in the embryonic period, arrested it the firs prophase till puberty. The first meiotic division completed upon ovulation, the second division at fertilization. In humans, usually only one oocyte is ovulated in each menstruation period, between puberty and menopause, when further maturation cases.
-4 cells are produced, but only one is functional, the rest (polar bodies) are defective. The oocyte gives the cell organelles the cytoplasm, mitochondria and yolk to the embryo.
Some oocytes may rest in the vulnerable period of arrested meiosis for years, resulting in increased ratio of genetic defects with maternal age.

7. Spermiogenesis
Strong selection: defective cells will not usually reach the site of fertilization
Starts at puberty, continuously active, high number of sperms are produced, ideally no age limit exists.
Four functional, mobile cells are produced.
22X or 22Y: determines the genetic sex of the embryo.

8. Fertilization 1
Spermia must penetrate the corona radiata and the zona pellucida by active movement and by the help of acrosomal enzymes. When the first sperm reaches the cell membrane, they fuse, and the genetic material (male pronucleus) enters the cell. This is the activating signal for the oocyte to finish the second meiotic division.

9. Developmental events in the first 3 weeks after fertilization
Cleavage
Gastrulation
Neurulation
Placenta formation
During the first month of development,
the germ layers of the embryo
and the extraembryonic structures
develops.

10. Two pronuclei unite Cleavage starts
1st week: Ovulation, fertilization, cleavage, transport to uterus, implantation
Two pronuclei unite
Cleavage starts

11. Timing of Cleavage

12. Implantation of the Blastocyst
Trophoblast layer invades the uterus
App. at the 8th day after fertilization
Uterine mucosa must be in receptive stage
(secretory phase)

13. Extrauterine (ectopic) pregnancy

14. Types of twin pregnancy

15. Neurulation in the Human Embryo
ectoderm starts forming brain and spinal cord
Neural tube – a hollow tube pinches off into the body. Cranial part of the neural tube becomes the brain.
Maternal folic acid deficiency causes neural tube defects !!!

16. Week 5-8 – The Second Month of Embryonic Development
Limb buds form
Embryo first looks recognizably human (week 8)
Head is disproportionately large
All major organs are in place

17. By 8 weeks, about 2 months, all major organs are in place in at least a rudimentary form; this is why drugs early in pregnancy are so important to avoid – many cause birth defects; baby is a little over 1” long (below right)