EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
5. Anomalies (abnormalities) of Gametogenesis
Review of MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D.
- Morphologic anomalies
- ABNORMAL SPERMATOZOA can be seen even in normal semen. Up to 10% of sperm in an ejaculate may be grossly abnormal. They generally do not fertilize oocytes due to their poor motility and fertilizing capabilities. They generally do not affect fertility unless their numbers exceed 20%
- Double sperm forms may be the result of failure of disjunction during spermatogenesis
- The number of abnormal sperm may be increased by x-rays, severe allergic reactions, antispermatogenic agents, and other factors
- UNUSUAL CELL TYPES IN A FETAL OVARY
- Oocyte with 2 nuclei-usually fail to mature
- Two oocytes in the same follicle-infrequent in the human femal Most of these never mature
- Chromosomal anomalies: abnormalities occur during meiosis when distribution of the chromosomal material between the gametes takes place
- ANOMALIES INVOLVING THE AUTOSOMES (SOMATIC CHROMOSOMES): meiotic division includes a stage of chromosome pairing. This provides the possibility of nondisjunction where homologous chromosomes may fail to separate and pass to opposite poles, resulting in some germ cells with 24 chromosomes, while others have only 22
- If a germ cell with 24 autosomal chromosomes unites with one that has a normal complement (23), a zygote with 47 chromosomes is formed - trisomy (presence of 3 representatives of a certain chromosome rather than 2)
- If a germ cell with 22 autosomal chromosomes fuses with a normal one, the zygote ends up with 45 chromosomes - monosomy (presence of only 1 representative of a particular chromosome instead of the usual pair)
- ANOMALIES INVOLVING THE SEX CHROMOSOMES
- The same type of abnormalities as described under IIA are seen her Certain cells have no sex chromosomes, while others have 2 (or sometimes even more)
- Generally speaking, chromosome abnormalities more often affect the female gametes. The greater vulnerability of the female as compared with the male gametes is due to sex differences in the chronology of maturation, despite almost identical fundamental mechanisms
- Chronology of gametogenesis
- IN THE MALE: the fundamental difference apparent between the male and female is that there is an unequal duration of meiosis in the 2 sexes. In the male, meiosis comes about within several days
- IN THE FEMALE: the process begun during fetal life is suspended for a "considerable" time, almost a dozen years. It is this very delay which may result in the number of chromosomal abnormalities seen in the female
