Birth Defects

Due to the complicated nature of development many things can go wrong, resulting in birth defects. These can defects can arise from a number of causes including genetic abnormalities, trauma, and substance abuse (including alcohol and tobacco), but usually are the result of an error during development. This section will cover the most common birth defects in basic detail. Defects involving the failure of neuropores to close (neural tube defects) have already been covered in the neurulation section of the website.

Ectodermal Dysplasia

Ectodermal dysplasia is a genetic condition in which ectodermal structures (hair, teeth, nails, and sweat glands) are abnormal. Patients suffering from this condition usually present with a lack of sweat gland, no hair, and abnormal teeth. As these people are unable to sweat they cannot go out into sunlight otherwise they would experience hyperthermia.

Ectodermal Dysplasia


Down's Syndrome

Down’s syndrome is also known as trisomy 21 as the syndrome is caused by a third copy of chromosome 21. People born with this syndrome have stunted growth, mental retardation, and have a specific set of facial characteristics (small chin, oblique eyes, and flat nose). Down’s can be genetically screened for whilst the foetus is still developing, although this is a controversial area.

Down's Syndrome


Hydrocephalus

Hydrocephalus results from either excessive production or impaired drainage of cerebrospinal fluid (CSF). The impaired drainage is usually due to a blockage or narrowing in the brain ventricles, often narrowing of the cerebral aqueduct (congenital aqueductal stenosis). In new-borns the increased intracranial pressure pushes the skull bones apart, and the head swells to an incredible size. When hydrocephalus arises in adults the brain suffers a lot more from the increased pressure as the skull bones are fused and unable to move. Hydrocephalus is treated by surgical implantation of a shunt.

Hydrocephalus


Gastroschisis

Gastroschisis occurs during the fourth week of development when the embryo begins folding. The defect arises from an incomplete folding of the lateral sides of the embryo, resulting in a split anterior abdominal wall, allowing the internal viscera to protrude into the amniotic cavity during development. The defect is treated by surgically relocating the viscera back into the abdominal cavity.

Gastroschisis


Congenital Omphalocele

Congenital omphalocele occurs when the midgut fails to retract back from the umbilical herniation which should happen during Week 11. The persistence of the intestines in the proximal part of the umbilical cord is due to the failure of the abdominal walls to fuse at the umbilicus. The size of the hernia and its contents can vary, and treatment is by surgery.

Omphalocele


Turner Syndrome

Turner syndrome is a chromosomal abnormality caused by the meiosis of gametes, with the foetus lacking one sex chromosome making their genotype 45X (instead of 46XY/46XX). It is rare to survive until birth with this condition, only 1% of foetuses with this abnormality survive. The phenotype of this syndrome is female; however the female usually doesn’t develop secondary sexual characteristics and frequently requires hormone replacement therapy (HRT). Typical features of the syndrome include short stature, a webbed neck, and a broad chest.

Turner Syndrome


Klinefelter Syndrome

Klinefelter syndrome is a trisomy of the sex chromosomes, 47XXY to be exact. The sufferer has a male phenotype due to the presence of a Y chromosome. Typical characteristics of this syndrome include small testes, tallness, lower intelligence, aspermatogenesis (no sperm production), and gynaecomastia (development of breasts) in 40% of sufferers.

Klinefelter Syndrome


Androgen Insensitivity

Androgen insensitivity, also known as testicular feminization, is the inability of cells to respond to androgens which are crucial for guiding the development of the gonads and external genitalia. Sufferers have a male genotype (46XY) but as the body has malfunctioning/absent androgen receptors the default female gonads/genitalia develop and they appear as a normal female phenotype. However, the reproductive system doesn’t develop into a fully-functioning female system despite developing normal female sexual characteristics at puberty. Instead, the sufferer has a blind-ending vagina and doesn’t menstruate, and has testes which descend into the inguinal region. This condition occurs in approximately 1 in 20,000 live births.

Androgen Insensitivity