Embryology

د. مها السماك

Lec.1
Embryology
The aim :
Gives idea about human development.
Clarifies the gross anatomical features of organs.
Helps to understand congenital malformations.
Embryology can be divides into two types :
General : general development of all the body (especially in the first months).
Special : development of organs and systems from the beginning to the end.

Gametogenesis (spermatogenesis and oogenesis) : It is the formation of sperms and mature ovum.

The development of a human being begins with fertilization which is a process by which the sperm from the male and the oocyte from the female unite to form new organism (the zygote).

Germ cells undergo chromosomal changes and morphological changes to be ready for fertilization.

Chromosomal changes : reduction in the number of the chromosomes to half that in the somatic cells (by meiotic division).


Somatic cell : 46 chromosome (single).
23 pairs or diploid number of chromosome.
2n DNA.
½ of chromosome from father.
½ of chromosome from mother.
44 chromosomes (22 pairs) are autosome.
2 chromosomes (one pair) is sex chromosome (either XX or XY).
Reduction is accomplished by meiotic division, which done in two stages :

First meiotic division :

Replication of DNA in the germ cells from 2n to 4n and each of the 46 chromosome is a doubled structure and contains 2 chromatids.
Pairing of homologous chromosome (homologous pair mean 4 chromatids)
Interchange of chromatid segments (cross over) between the two paired homologous chromosome when split from each other.

After the splitting completed, each daughter cell contain one member of each chromosome pair and thus has 23 double structured chromosome and 2n DNA
Second meiotic division : the 23 double structured chromosome divide at the centromere and each newly formed daughter cell receives 23 chromatid (single chromosome) and 1n DNA (half that in the somatic cell).
The purpose of meiotic division :
Provide each germ cell (gamete) with haploid number of chromosome (23 chromosome) and half of the amount of DNA of a normal somatic cell (1n DNA), so in fertilization there will be restoration of the normal number of chromosome (i.e. 46) and DNA (2n).
Allow exchange of block of genetic material.

Different between primary spermatocyte and primary oocyte :

primary spermatocyte has two cell with 22+1X chromosome and two cell with 22+1Y chromosome, while the primary oocyte has four cells with 22+1X chromosome, only one is oocyte and the three other are polar bodies.