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AHMB1 > 5: Birth Defects > Flashcards

5: Birth Defects Flashcards

1
Q

What are birth defects?

A

Malformations that become obvious at birth and have developed during embryonic and foetal development

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2
Q

What is embryogenesis?

A

The process by which a fertilised egg develops into a mature embryo
It involves a series of highly co-ordinated steps that result in the formation of basic structures and organs of an organism

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3
Q

What are some common defects?

A

Defects in limbs, heart, and spinal cord
Also face, intestine, stomach, and sexual organs

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4
Q

What causes birth defects?

A

Around 25% of all abnormalities have a genetic origin
Can be mutations (mis-sense, nonsense, deletions, absence of chromosomes, etc)
Other environmental factors can include drugs, viruses, and radiation
Usually birth defects are a result of both genetic and environmental factors

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5
Q

What is foetal alcohol syndrome (FAS)?

A

Growth, mental, and physical problems that may occur in a baby when a mother drinks alcohol during pregnancy
Specific symptoms include a typical appearance of the face including eyes, jaw and lips
Poor prenatal growth
Decreased muscle tone and poor co-ordination
Delayed development and problems in thinking, speech, movements and social skills
Sometimes causes heart defects

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6
Q

What is cyclopia?

A

A rare congenital disorder characterised by the formation of a single, centrally located eye
Typically results in stillbirth
Nose is often located on top of the eye (proboscis)
Cerebral cortex does not form two separate hemispheres (holoprosencephaly)
Can be caused by environmental as well as genetic factors

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7
Q

What is disprosopia?

A

The duplication of face structures
Results from abnormal development during embryogenesis

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8
Q

What is Shh?

A

Sonic hedgehog
Morphogen (a signalling molecule that helps organise tissue development)
Secreted by signalling cells to bind to receptors on target cells
Crucial for patterning tissues during development

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9
Q

What is Ptc?

A

Patched
Transmembrane receptor that normally inhibits the pathway when Shh is not present
When Shh binds to Ptc, it relieves inhibition and promotes activation of Smo

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10
Q

What is Smo?

A

Smoothened
A receptor that, when activated by Shh signalling, triggers downstream signalling
Initiates signalling pathways that lead to activation of Gli proteins

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11
Q

What is the Gli complex?

A

Gli proteins are transcription factors that mediate the effects of the Shh pathway
When active, they enter the nucleus and regulate gene expression, influencing cell growth and differentiation

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12
Q

What does the Shh signalling pathway do?

A

Plays a critical role in embryonic development, cell differentiation, proliferation, and patterning of tissues, including the nervous system and limbs

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13
Q

How do Shh mutations cause cyclopia?

A

Shh signalling is blocked when mutated
This expresses Pax6 and prevents the splitting of the pre-chordal plate into two separate regions
Therefore only one eye develops

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14
Q

What conditions can mutations in Shh cause?

A

Holoprosencephaly (including cyclopia)
Limb abnormalities
Defects in inner organs
Growth defects in bones
Medulloblastoma
Carcinomas
Rhabdomyosarcoma
Prostate, pancreatic, and breast cancers

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15
Q

What is medulloblastoma?

A

The most common form of brain cancer in children
Originates from the cerebellum
Four subtypes, each of which is caused by abnormal Shh signalling

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16
Q

What drug is an antagonist of sonic hedgehog signalling?

A

Cyclopamine
Inhibits Smoothened, therefore prevents activation of Gli proteins

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17
Q

Describe how Shh mutations cause medulloblastoma.

A

Shh signalling controls the proliferation of GCPs (granular cell precursors) in the external granule layer (EGL), which is a key part of cerebellar development
During normal development, Shh stimulates GCPs to proliferate and differentiate into neurones that migrate to the internal granule cell layer (IGL)
In medulloblastoma, Smo remains persistently active even in the absence of Shh
This leads to activation of Gli transcription factors and drives uncontrolled proliferation of GCPs in the EGL
This leads to excessive growth, causing a tumour growth in the cerebellum

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18
Q

Describe the skeletal patterns of the tetrapod limbs.

A

Stylopod: The upper segment of the limb, e.g., humerus in the arm or femur in the leg
Zeugopod: The middle segment, e.g., ulna and radius in the arm or tibia and fibula in the leg
Autopod: The distal segment, e.g., carpals and digits in the hand or foot

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19
Q

What is the AER?

A

The apical ectodermal ridge
A thickened band of ectoderm at the top of a developing limb bud in vertebrate embryos
Plays a crucial role in limb development ensuring proper proximal-distal limb outgrowth (from shoulder to finger)

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20
Q

What is ectoderm?

A

One of three primary germ layers formed during early embryonic development. It gives rise to various tissues and structures including skin, the nervous system, and sensory organs

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21
Q

What are the different types of AER?

A

Early AER (formed at initiation of limb bud development)
Late AER (maintains signalling for distal limb structures)
Forelimb AER (controls development of the forelimb)
Normally AER is continuous and forms a smooth, uninterrupted ridge, however in mutations, defects can be caused including missing digits

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22
Q

What is acheiropodia?

A

A lack of formation of the distal extremities
Results from loss of late AER

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23
Q

What are FGFs?

A

Fibroblast growth factors
A family of proteins that play essential roles in regulating various aspects of cell growth, development, and repair
They are involved in many physiological processes, including embryogenesis
These can substitute for AER function

24
Q

What is FGF8?

A

A signalling protein that plays a crucial role in development of limbs, secreted by the apical ectodermal ridge to maintain proliferation in the progress zone of the limb bud

25
What is thalidomide?
A drug given to treat morning sickness in pregnant women, led to problems in limb development
26
What is phocomelia?
A rare congenital disorder characterised by severely shortened or absent limbs - results from impaired limb development during early embryogenesis May be a result of disturbed interaction between AER and underlying mesenchyme (embryonic connective tissue)
27
What is polydactyly?
A condition characterised by having more than five fingers on each hand
28
What is the ZPA?
The zone of polarising activity This is responsible for determining the anterior-posterior axis of a developing limb, which corresponds to the thumb to little finger direction in the hand, or big toe to little toe in the foot
29
What protein mediates ZPA activity?
Shh (Sonic hedgehog) Cells exposed to high Shh levels develop into posterior structures (e.g. little finger) Cells exposed to low Shh levels form anterior structures (e.g. thumb)
30
What is syndactyly/synpolydactyly?
A condition where two or more digits are fused together Digits are formed by programmed cell death in the interdigital regions, therefore removing these cells In duck leg primordium, there is limited cell death, leading to webbed feet
31
What are homeotic mutations?
Mutations that transform one body part into another Caused by mutations in the homeotic (Hox) genes E.g. in flies
32
What is primary sex determination?
The first stage in sex determination Determination of the gonads (ovaries or testes) Determined by the karyotype (XX or XY) Occurs at fertilisation
33
What is secondary sex determination?
The development of secondary male/female sex characteristics This is determined by hormones secreted from the gonads Has two major temporal phases: during organogenesis and at puberty
34
What are bipotential gonads?
The status of the gonads at an early stage of development There is no difference between male and female gonads
35
What are the two duct systems associated with the gonads?
Mullerian duct Wolffian duct
36
What happens to the two duct systems of the gonads during male development?
The Mullerian duct degenerates The Wolffian duct remains, forming the vas deferens and epididymis
37
What happens to the two duct systems of the gonads during female development?
The Wolffian duct degenerates The Mullerian duct remains and forms the Fallopian tube and part of the uterus
38
What is the vas deferens?
A tube that carries sperm from the epididymis to the urethra during ejaculation
39
What is the epididymis?
A coiled tube located on the back of each testis that stores and matures sperm cells
40
What is Klinefelter syndrome?
XXY males Symptoms include poor beard growth, breast development, under-developed testes, and infertility
41
What is Turner syndrome?
X0 females Symptoms include characteristic facial features, constriction of aorta, poor breast developement, under-developed ovaries, infertility
42
What is the Sry gene?
The sex-determining region on the Y chromosome
43
What is the Wnt4 gene?
A potential ovary-determining gene XX ovaries which lack Wnt4 express testis-specific markers such as 3B-hydroxysteroid dehydrogenase
44
What enables the development of the Mullerian duct in females?
Oestrogen
45
What are the two hormones that enable formation of the male phenotype?
anti-Mullerian hormone (AMH): leads to the degradation of the Mullerian duct Testosterone: leads to differentiation of the Wolffian duct into the epididymis, vas deferens, and seminal vesicles, as well as developing the scrotum and penis
46
What are the seminal vesicles?
Glands that produce a thick fluid rich in fructose to nourish sperm, their secretion makes up a large part of semen
47
How do steroid hormones act as transcription factors?
They are lipophilic so can enter cells through the plasma membrane and then bind to receptor proteins on the nuclear membrane and enter to form a hormone-receptor complex which then interacts with DNA acting as a transcription factor
48
What is androgen insensitivity syndrome?
When individuals are XY and have the SRY gene They also have testes that make testosterone and AMH Patients lack the testosterone receptor and therefore cannot respond to the testosterone they produce They are able to respond to oestrogen and therefore develop female characteristics instead of male
49
What is pseudohermaphroditism?
When an individual has one type of gonad but the secondary sex characteristic differs from the gonadal sex
50
What is male pseudohermaphroditism?
When an individual has male gonads but female secondary sex characteristics Caused by mutations in the testosterone receptor or mutations affecting testosterone synthesis
51
What is female pseudohermaphroditism?
When an individual has female gonads but male secondary sex characteristics Can be caused by congenital adrenal hyperplasia
52
What are 'true hermaphrodites'?
When individuals contain both male and female gonadal tissue Occurs in a rare exchange where the recombination of X and Y chromosomes leads to the SRY gene being transferred to an X chromosome X chromosomes are randomly inactivated and the inactivated chromosome is passed on to the cell's descendant Due to this, the recombined SRY gene will reside on the active X chromosome on other gonadal cells, and therefore the gonads will contain male and female tissue
53
What is dihydrotestosterone?
A derivative of testosterone Plays a key role in development of male characteristics More active than testosterone Important for development of penis and prostate
54
What is 5-alpha reductase?
An enzyme that catalyses the conversion of testosterone to dihydrotestosterone
55
What is aromatase?
An enzyme that converts testosterone to oestradiole, which later forms oestrogen Aromatase is highly expressed in the placenta Recessive mutations in aromatase gene slead to accumulations of androgens during pregnancy and excess oestrogen production in females, as well as males with female secondary sex characteristics

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