Causes and manifestations of developmental anomalies

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Introduction

Development of anomalies (also known as congenital anomalies or birth defects) are structural, functional, metabolic, or behavioral disorders present at birth that arise from abnormal embryonic or fetal development. The study of these abnormalities and their origins is referred to as teratology.

Classification of Anomalies

Malformation

Malformations are primary structural defects causes by abnormal morphogenesis during organogenesis, typically between weeks 3 and 8 of development. Examples include cleft lip and palate, and neural tube defects such as spina bifida and anencephaly

Spina bifida

Disruption

Disruptions result from the secondary breakdown of a normally developing structure. An example is limb amputations caused amniotic bands.

Deformation

Deformations arise due to mechanical forces that alter the shape or position of a body part, such as clubfoot caused by restricted intrauterine space.

Dysplasia

Dysplasia refers to abnormal organization of cells within a tissue, leading to disordered overall structure.

Causes of Developmental Anomalies

Genetic Causes

Genetics contributions include

• Chromosomal abnormalities such as Down syndrome (Trisomy 21) and Turner syndrome (Monosomy X)
• Single-gene mutations
• Multifactorial inheritance involving both genetic and environmental factors

Environmental Causes (Teratogens)

Teratogens are external agents that disrupts development:

• Infectious agents: Rubella, Cytomegalovirus, Toxoplasmosis
• Drugs and chemicals: Thalidomide, Isotretinoin, Alcohol
• Physical agents: radiation
• Maternal conditions: poorly controlled maternal diabetes increases the risk of cardiac and neural tube defects

Mechanisms of Abnormal Development

Abnormal development can result from

• Excessive cell death
• Reduced cell proliferation
• Disrupted cell migration (for example, neural crest migration errors)
• Failed induction between interacting tissues during organ formation
• Mechanical interference, such as constriction by amniotic bands

Manifestations of Developmental Anomalies

Structural abnormalities

These include neural tube defects (spina bifida, anencephaly), craniofacial anomalies such as cleft lip and palate, limb defects (amelia, polydactyly) and congenital heart defects including ventricular septal defect and tetralogy of Fallot.

Cleft defects

Growth abnormalities

Examples include intrauterine growth restriction and excessive fetal growth (macrosomia), often linked maternal metabolic disease.

Functional or metabolic disorders

These may involve neurological impairment (such as intellectual disability or seizures), endocrine defects or renal malformations.

Minor anomalies

Small structural deviations such as microtia or short palpebral fissures may serve as markers of more significant underlying developmental problems.

Sensitive Periods for Teratogenesis

Susceptibility to developmental anomalies varies by stage:

• 0-2 weeks: all-or-none period - either no defect or embryonic loss
• 3-8 weeks: organogenesis - highest risk for major structural anomalies
• After 8 weeks: functional and minor structural defects become more likely

Resources

1. Sadler, T. W. Langman’s Medical Embryology, 14th edition
2. Junqueira, L. C., Carneiro, J. Basic Histology: Text and Atlas, 15th edition
3. Moore, K. L., Persaud, T. V. N., & Torchia, M. G. The Developing Human: Clinically Oriented Embryology.
4. Moore, K. L. The Developing Human.