Embryology is the study of the origin and prenatal development of an organism, comprising the germinal (first two weeks from conception), embryonic (third to eighth week from conception), and fetal period (ninth week from conception until birth). Gestation in embryology refers to conceptional age, i.e., the time period from conception to birth, and should be distinguished from gestational age, an obstetrics term, which is measured in weeks following the first day of the mother's last menstrual period. Primordial cells develop during the embryonic period and migrate to the developing gonads. Further maturation into fertile oocytes and spermatozoa occurs via meiosis during different stages of male and female organism development. Embryonic stem cells are pluripotent and can differentiate into almost any type of tissue, thus being of great interest in the research and development of new therapeutic approaches. For more information on the morphogenesis of organ systems, see the article on .
- Definition: the study of the origin and prenatal development of an organism.
- Timing: The periods of embryonic development are measured from the actual point of conception (i.e. conceptional age).
|Periods of embryonic development |
|Germinal period||1–2 weeks|
|Embryonic period|| |
|Fetal period||9 weeks to birth|| |
Primordial germ cell development
- Occurs during the 4th week of embryonic development
- Primordial germ cells arise from the yolk sac and migrate to the developing gonads of the urogenital folds.
- After meiosis, primordial germ cells mature to sex-specific gametes (oocytes and spermatozoa).
- Definition: the cells that develop into mature germ cells (gametes), which pass genetic material on to progeny
- Process: involves meiosis of primordial germ cells within the gonads
- Definition: germ cell maturation in which four daughter cells with a recombinant genome are produced from one germ cell
Process: Meiosis involves two major phases (referred to as meiosis I and meiosis II) occurring after DNA replication during the S phase of interphase.
Meiosis I (equatorial division)
- Synapsis (syndesis): the pairing of homologous chromosomes
- Genetic recombination: Individual chromosome segments are exchanged between the maternal and paternal chromatids of homologous chromosomes (recombination) in a process known as .
- Chromosome number reduction:
- Meiosis II (nuclear division):
- Meiosis I (equatorial division)
Stages of meiosis I
|Stage||Number of cells||Chromosome set||DNA content|
|Prophase I|| || || || |
|Telophase I|| || || |
Stages of meiosis II
|Stage||Description||Number of cells||Chromosome set||DNA content|
|Prophase II|| || || |
|Telophase II|| || || |
- Definition: cells capable of differentiating into specialized cells (potency) as well as replicating while maintaining their undifferentiated state (self-renewal)
- Cell division
- Totipotency (omnipotency): the ability of a cell to differentiate into all cell types, including extraembryonic (placental) cells
- Pluripotency: the ability of a cell to differentiate into all cell types, with the exception of extraembryonic cells (for example, the cells of the ectoderm, which can differentiate into nerve or skin cells, but not into other germ layer cells)
- Multipotency: the ability of a cell to differentiate into more than one related cell type (for example, hematopoietic stem cells, which can differentiate into myeloid or lymphoid cells)
Embryonic stem cells (ESCs)
- Pluripotent cells that can only develop into embryonic cells, but not trophoblastic cells
- Originate from the inner cell mass during the blastocyte stage
- Adult stem cells
- Embryonic stem cells (ESCs)
|Molecules involved in embryogenesis |
|Types of molecules||Subgroup/components||Function|
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|Growth factors|| |
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|Extracellular matrix|| |
|Relevant hormones involved in embryogenesis|| |
The FGF gene triggers the Fetal Growth of Fingers.
Cellular processes of development
|Cellular developmental processes |
|Process||Description||Examples of the process||Examples of associated pathology|
Programmed cell death
Fragmentation and, thus, the separation of the webbed embryonic fingers and toes
The process by which primitive (e.g., stem cells) become specialized cells
Joining of two or more cells, epithelium, or tissue
Urethral folds of the penis
Movement of the cells to a specific destination
Neuronal cells during fetal brain development
Lissencephaly: underdevelopment of cerebral grooves and folds
Increase in the number of cells (mediated by cell division)