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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.
organ systems see embryogenesis.

Definition: Embryology is the study of prenatal development.
Classification :
According to chronological development (week of development) :
1. Pre-embryonic period (1st to 2nd week of development): fertilization, implantation, implantation of the blastocyst in the uterus, amnion formation.

2. Embryonic period (3rd to 8th week of development): development into a human form, organogenesis.

3. Fetal period (9th week of development to birth):
growth and differentiation of tissue and organs formed during the embryonic period.
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 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  
Time: 4th embryonic week
Site: yolk sac wall
Process: Diploid primordial germ cells develop and migrate to the developing gonads of the urogenital folds.
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.

Oogenesis (development and maturation of the ova)

An oocyte with 2–3 polar bodies are produced.
Spermatogenesis (sperm cell development):
 Four functional spermatids are produced.

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.
germ cell maturation in which four daughter cells with a recombinant genome are produced from one germ cell.
random division of the diploid set of  chromosomes to haploid daughter cells to ensure genetic diversity of offspring.
 For meiosis to occur, DNA is replicated during the S phase of interphase.
Meiosis occurs in two major phases:
a. Meiosis I (equatorial division):

Synapsis (syndesis): pairing of homologous chromosomes
Genetic recombination: Maternal and paternal copies of the chromosomes initially divide into daughter cells. → genetic diversity.
Crossing over: exchange of DNA segments between homologous chromosomes.
Number of chromosomes is reduced: The diploid set of chromosomes is divided in half when the homologous chromosomes separate from one another.
Disjunction: without centromere splitting → 2 haploid daughter cells, each with 23 chromosome pairs .

Meiosis II (nuclear division):

 Both sister chromatids of each chromosome of the haploid set separate. → Generally corresponds to the phases of mitosis.
Disjunction: with centromere splitting .
Cell number: 1 mother cell → 4 daughter cells
Set of chromosomes: diploid mother cell (2n) → haploid daughter cell (1n).
DNA content: 4 chromatids of the mother cell (4C) → 1 chromatid per daughter cell (1C).
Sex-specific features
Oogenesis: female oocytes are produced before
birth; however, meiosis I and meiosis II are
interrupted by a resting phase.
Meiosis I arrest: arrest in prophase I; resting
phase until ovulation (1o oocyte).
Metaphase II arrest: arrest of meiosis II during
metaphase II; resting phase continues until
fertilization (2o oocyte).

with the onset of puberty, the testes begin to continuously produce sperm.

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
1. Prophase I

- Leptotene
Chromosomes condense and become visible.
Chromosomes attach to the nuclear envelope.
- Zygotene
Homologous chromosome pairing by convergence of chromosomes.
Synaptonemal complex formation.
- Pachytene Crossing over: The chromatids crossover (form chiasmata) and exchange genetic material. → genetic recombination of homologous chromosomes.
- Diplotene
The synaptonemal complex disappears.
Homologous chromosomes are present in the single form.
Chromosomes are held together by the chiasmata (crossover site).
- Diakinesis
The nuclear membrane disintegrates.
Start of spindle apparatus formation
   Number of cells : 1.
Chromosome set: volume 2n.

2. Metaphase I
 Chromosome pairs align at the equatorial
3. Anaphase I
Homologous chromosome pairs separate by disintegration of the chiasmata.
4. Telophase I
Stage :
Cell membrane pinches in at the equator → division into two haploid daughter cells.
Number of cells : 2
Chromosome set: n

     Stages of meiosis II
1. Prophase II
Chromosomes condense.
Spindle apparatus formation.
Number of cells : 2
2. Metaphase II
Chromosomes align at the equatorial plane.
3. Anaphase II
The two chromatids of each chromosome are separated at the centromere.
Chromatids migrate to the cell poles.
4. Telophase
Stage :
Reformation of the nuclear envelope of daughter cells.
Cell membrane pinches in at the equator.
Division into two haploid cells each with respective single chromatids.
Number of cells : 4

 !! 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
Symmetric division: self-replication → both daughter cells possess the stem cell properties.
Asymmetric division: differentiation → one stem cell is a copy of the mother stem cell and the other daughter cell is a precursor cell with differentiated properties.
1. 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 nearly all cell types.
Multipotent: ability of a cell to differentiate into more than one cell type, but not all cell types.

Embryonic stem cells (ESCs)
Origin: from the inner cell mass in the blastocyte stage
Degree of differentiation: pluripotent cells that can only develop into embryonic cells, but not trophoblastic cells.
Adult stem cells
Origin: differentiated tissue with high potency (e.g., bone marrow)
Degree of differentiation: pluripotent cells that provide a supply of cells for regenerative tissue.

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

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