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EMBRYOLOGY
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CHAPTER 2:Gametogenesis:Conversion of Germ Cells into Male and Female Gametes.
Clinical Correlates
Primordial Germ Cells and Teratomas
Teratomas
Teratomas are tumors of disputed origin that often contain a variety of tissues, such as:
bone
hair
muscle
gut epithelia
and others
Origin of Teratomas
It is thought that these tumors arise from pluripotent stem cells that can differentiate into any of the three germ layers or their derivatives.
Possible Sources of These Tumors
1. Primordial Germ Cells (PGCs)
Some evidence suggests that primordial germ cells (PGCs) that have strayed from their normal migratory paths could be responsible for some of these tumors.
2. Epiblast Cells
Another source may be epiblast cells, which give rise to all three germ layers during gastrulation.
Important Point
Both PGCs and epiblast cells are pluripotent, meaning they can form tissues derived from all three germ layers.
Tissues Found in Teratomas
Tissues within these tumors may include:
gut
bone
skin
teeth
and so forth
These tissues represent derivatives of all three germ layers.
Oropharyngeal teratoma
These tumors may arise from:
PGCs
epiblast cells
Both are pluripotent.
Quick Concept Summary
Teratomas = tumors containing many different tissues.
They arise from pluripotent cells.
Possible origin:
Primordial germ cells that migrate abnormally
Epiblast cells during gastrulation
Tumor may contain tissues from all three germ layers.
Birth Defects and Spontaneous Abortions: Chromosomal and Genetic Factors
Chromosomal Abnormalities
Chromosomal abnormalities, which may be numerical or structural, are important causes of birth defects and spontaneous abortions.
It is estimated that 50% of conceptions end in spontaneous abortion, and 50% of these abortuses have major chromosomal abnormalities.
Thus, approximately 25% of conceptuses have a major chromosomal defect.
Most Common Chromosomal Abnormalities in Abortuses
45,X (Turner syndrome)
Triploidy
Trisomy 16
Together, chromosome abnormalities and gene mutations account for approximately 30% of the total number of birth defects.
Numerical Abnormalities
The normal human somatic cell contains 46 chromosomes.
The normal gamete contains 23 chromosomes.
Somatic cells = diploid (2n)
Gametes = haploid (n)
Euploid = any exact multiple of n (e.g., diploid or triploid)
Aneuploid = any chromosome number that is not euploid, usually:
Extra chromosome (trisomy)
Missing chromosome (monosomy)
Origin of Numerical Abnormalities
Abnormalities in chromosome number may originate during meiotic or mitotic divisions.
In meiosis, two members of a pair of homologous chromosomes normally separate during the first meiotic division, so each daughter cell receives one member of each pair.
Sometimes, separation does not occur (nondisjunction), and both members of a pair move into one cell.
As a result of nondisjunction:
One cell receives 24 chromosomes
Other cell receives 22 chromosomes (instead of 23)
At fertilization:
Gamete with 23 chromosomes + gamete with 24 or 22 chromosomes → individual with 47 chromosomes (trisomy) or 45 chromosomes (monosomy).
Nondisjunction can occur during first or second meiotic division of germ cells and may involve:
Autosomes
Sex chromosomes
In women, the incidence of chromosomal abnormalities, including nondisjunction, increases with age, especially 35 years and older.
Structural Abnormalities: Translocations
Sometimes, chromosomes break, and pieces attach to another chromosome.
Types of Translocations:
Balanced – breakage and reunion occur, no genetic material lost → individuals are normal
Unbalanced – part of chromosome is lost → altered phenotype
Example:
Unbalanced translocations between long arms of chromosomes 14 and 21 during meiosis I or II → gametes with extra chromosome 21 → cause of Down syndrome.
Translocations are common between chromosomes 13, 14, 15, 21, 22 because they cluster during meiosis.
Trisomy 21 (Down Syndrome)
Cause: Extra copy of chromosome 21 (trisomy 21)
Features of Down syndrome:
Growth retardation
Intellectual disability (varying degrees)
Craniofacial abnormalities
Upward slanting eyes
Epicanthal folds
Flat facies
Small ears
Cardiac defects
Hypotonia
Other risks:
Increased chance of leukemia, FOR MORE🚀📚 Want the COMPLETE NOTES of CHAPTER 2 – EMBRYOLOGY
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