WEEK 3: Embryology of limbs

Question 1

Describe the process of limb development.

Answer 1

1. Limb Bud Formation (Fourth Week):

Limb buds appear as small elevations of the ventrolateral body wall during the fourth week.
Upper limb buds develop opposite the caudal cervical segments, and lower limb buds form opposite the lumbar and upper sacral segments.

2. Limb Bud Elongation and Timing (Fourth to Seventh Week):
   Limb buds elongate through the proliferation of mesenchyme within them.
   Development of upper limb buds precedes that of lower limb buds by about 2 days.
   Upper limbs become visible by day 26 or 27, while lower limb buds appear a day or two later.
3. Three Centers Determining Limb Axes:

Apical Ectodermal Ridge (AER): Determines proximal and distal segments; induces undifferentiated proliferation of underlying mesenchyme. Absence or removal results in growth and differentiation failure (phocomelia) or supernumerary limbs.

Zone of Polarizing Activity (ZPA): Determines cranial to caudal axis (preaxial and postaxial margins).

. Dorsal and Ventral Ectoderm: Determines dorsal and ventral axes.

4. Formation of Hand and Foot Plates (Fifth to Sixth Week):

Distal ends of limb buds flatten into paddle-like hand and foot plates.

By the end of the sixth week, mesenchymal tissue in the hand plates condenses to form digital rays, outlining the pattern of digits.

5. Digital Ray Development and Tissue Breakdown (Sixth to Eighth Week):

Mesenchymal condensations at the tips of digital rays are induced by the AER to form mesenchymal primordia of the bones in the digits.
Intervals between digital rays are occupied by loose mesenchyme.
Mesenchyme in intervening regions undergoes breakdown, forming notches between digital rays.
Separate digits are produced by the end of the eighth week.

6. Role of Apoptosis and BMPs in Tissue Breakdown:

Apoptosis is responsible for tissue breakdown in interdigital regions.
BMPs (Bone Morphogenetic Proteins) mediate apoptosis.
Blocking BMPs can result in syndactyly (fusion of fingers or toes).

7. Limb Extension and Rotation (Early 7th Week):

Limb buds extend ventrally during early seventh week.
Developing upper and lower limbs rotate in opposite directions and to varying degrees.

8. Mesenchymal Model Formation (7th Week):
   Mesenchymal models of the bones form as cellular aggregations when the limbs elongate.
9. Chondrification Centers (5th Week):
   Chondrification centers appear in the fifth week, and by the end of the sixth week, the entire limb skeleton is cartilaginous.
10. Osteogenesis (7th Week Onward):

Osteogenesis of long bones begins in the seventh week from primary ossification centers in the middle of cartilaginous models.
Ossification centers are present in all long bones by the 12th week.
Ossification of carpal bones starts during the first year after birth.

11. Muscle Development (From Dermomyotome Regions):

Myogenic precursor cells from dermomyotome regions of somites migrate into limb buds.
These precursor cells differentiate into myoblasts, the precursors of muscle cells.
Muscle Organization and Differentiation:

As long bones form, myoblasts aggregate to form a large muscle mass in each limb bud.
Initially, muscle mass separates into dorsal (extensor) and ventral (flexor) components.
Muscle tissue splits into flexor and extensor components, undergoing additional splittings and fusions.
The complex pattern of muscles is determined by connective tissue derived from lateral plate mesoderm.

12. Blood Vessels and Ligaments:

Mesenchyme in the limb bud gives rise to ligaments and blood vessels.
Segmentation and Fusion:

Limb musculature is initially segmented according to the somites from which they are derived.
With elongation of limb buds, muscles split into flexor and extensor components, undergoing further splittings and fusions.
The final pattern of muscles is determined by connective tissue derived from lateral plate mesoderm.

Question 2

State the 2 genes involved in limb formation and their function.

Answer 2

Sonic Hedgehog (SHH):

Function: SHH is a signaling protein that plays a crucial role in determining the patterning and growth of limbs during embryonic development.

t is produced in the zone of polarizing activity (ZPA) at the posterior margin of the limb bud.

SHH establishes the anterior-posterior axis (preaxial and postaxial) of the limb and is involved in specifying digit identity. The concentration gradient of SHH across the developing limb bud is essential for the proper development of individual digits.

Homeobox (HOX) Genes:

Function: HOX genes are a family of transcription factors that regulate the anterior-posterior patterning of the developing embryo, including limb development. They control the identity of different segments along the body axis. In limb development, specific HOX genes are expressed in a spatial and temporal manner, helping to determine the identity of various structures along the proximal-distal axis of the limb buds. The precise expression and regulation of HOX genes contribute to the proper formation of different parts of the limbs.

Question 3

There are two main types of limb anomalies:
Amelia
Meromelia

Answer 3

There are two main types of limb anomalies:
Amelia, absence of a limb
Meromelia (Greek, meros, part, and melos, limb), absence of part of a limb.

Question 4

Anomalies or defects of the limbs originate at different stages of development.

Suppression of limb bud development during the early part of the fourth week results in absence of the limbs, amelia.

Arrest or disturbance of differentiation or growth of the limbs during the fifth week results in various types of meromelia.

Outline some causes of limb anomalies.

Answer 4

Causes of limb anomalies

*Genetic factors, e.g., chromosomal abnormalities associated with trisomy 18.

*Mutant genes as in brachydactyly or osteogenesis imperfecta, a severe limb defect with fractures occurring before birth

*Environmental factors, e.g., teratogens such as thalidomide
A combination of genetic and environmental factors (multifactorial inheritance), e.g., congenital dislocation of the hip.

*Vascular disruption and ischemia, e.g., limb reduction defects

Question 5

Describe the following limb anomalies.

Cleft hand and cleft foot:

1. In ectrodactyly (lobster-claw deformities’)s, t
2. Brachydactyly
3. Polydactyly
4. Syndactyly
5. Congenital Clubfoot

Answer 5

1. here is absence of one or more central digits, resulting from failure of development of one or more digital rays. The hand or foot is divided into two parts that oppose each other like lobster claws.
2. Shortness of the digits (fingers or toes) is the result of reduction in the length of the phalanges. This anomaly is usually inherited as a dominant trait and is often associated with shortness of stature.
3. The term supernumerary digits refers to the presence of more than the usual number of fingers or toes. Often the extra digit is incompletely formed and lacks normal muscular development.

If the hand is affected, the extra digit is most commonly medial or lateral rather than central. In the foot, the extra toe is usually on the lateral side. Polydactyly is inherited as a dominant trait.

4. Syndactyly is the most common anomaly of the hand or foot. Cutaneous syndactyly (simple webbing between digits) is the most common limb anomaly. It is more frequent in the foot than in the hand.

-Cutaneous syndactyly results from failure of the webs to degenerate between two or more digits.
-Osseous syndactyly (fusion of the bones-synostosis) occurs when the notches between the digital rays fail to develop; as a result, separation of the digits does not occur.

Syndactyly is most frequently observed between the third and fourth fingers and between the second and third toes. It is inherited as a simple dominant or simple recessive trait.

5. Any deformity of the foot involving the talus is called talipes or clubfoot. Talipes equinovarus is the most common type. Clubfoot is a relatively common anomaly, occurring approximately once in 1000 births.

It is characterized by an abnormal position of the foot that prevents normal weight bearing. The sole of the foot is turned medially (inverted). Clubfoot is bilateral in approximately 50% of cases, and it occurs approximately twice as frequently in males. The cause of clubfoot is uncertain.

Although it is commonly stated that clubfoot results from abnormal positioning or restricted movement of the fetus’s lower limbs in utero, the evidence of this deformation is inconclusive.