Development

Question 1

Describe cell differentiation

Answer 1

• Process by which embryonic cells become different from one another
• Involves the emergence of cell types such as muscle, nerve, skin and fat cells
• It is the achievement of a stable terminal state
• It is characterised by the profile of proteins in that cell

Question 2

What is the hierarchy of stem cells?

Answer 2

• Totipotent
• Pluripotent
• Multipotent

Question 3

Define potency

Answer 3

The entire repertoire of cell types a particular cell can give rise to in all possible environments

Question 4

Describe totipotent

Answer 4

Cells of the very early mammalian embryo; identical and unrestricted; can give rise to any cell of the body

Question 5

Describe pluripotent

Answer 5

Inner cells of the blastocyst; less potent; can give rise to many cell types but not all

Question 6

Describe multipotent

Answer 6

They give rise to cells that have a particular function e.g blood stem cells

Question 7

What are the two stages of stem cell commitment?

Answer 7

• First stage: specification (reversible)

- Second stage: determination (irreversible)

Question 8

How does a naive cell become specified?

Answer 8

• Intrinsic signal - cell autonomous signal tells the cell who it is
• Extrinsic signal - a chemical or molecule in the environment gives the cell spatial information, tells the cell where it is

Question 9

Describe the progress of a cell during development

Answer 9

• Naive
• Cytoplasmic determinants or induction
• Specified
• Loss of competence for alternative fates
• Determined
• Cell specific gene expression
• Differentiated

Question 10

Define competence

Answer 10

Ability of a cell to respond to the chemical stimuli

Question 11

How can a cell lose competence?

Answer 11

By changes in surface receptor or intracellular molecules

Question 12

What is intramembranous ossification?

Answer 12

The formation of bone in fibrous connective tissue (which is formed from condensed mesenchyme cells.
The process occurs during the formation of flat bones such as the mandible and flat bones of the skull

Question 13

Describe HOX genes

Answer 13

• HOX genes are a related group of genes that are expressed along the long axis of the embryo from head to tail
• They are ancient genes and are therefore present in many animal groups. They have been extensively studied in Drosophila melanogaster

Question 14

Describe limb development initiation

Answer 14

• During embryonic development HOX genes determine the body axis and the position of the limbs along the body axis
• The products of HOX genes belong to a class of proteins known as transcription factors, which bind to DNA, and thereby regulate the transcription of other genes
• Once the cranio-caudal position is set, limb growth is regulated along three axes

Question 15

What are the three axes along which, limb growth is regulated?

Answer 15

• Proximo-distal axis
• Antero-posterior axis
• Dorso-ventral axis

Question 16

On approximately what day of development do upper limb and lower limb buds appear?

Answer 16

• Upper limb - 24

- Lower limb - 28

Question 17

By what week are all major components of the limbs present?

Answer 17

Week 8

Question 18

Describe the rotation of the limbs during development

Answer 18

• Development of the fore and hind limbs is similar
• In week 7 the forelimbs rotate 90 degrees laterally and the hind limbs rotate 90 degrees medially
• Results in the flexor compartments being anterior in the upper limb and posterior in the lower limb
• The sole of the foot is equivalent to the palm of the hand
• Big toe (hallux) is equivalent to the thumb (pollux)

Question 19

What controls proximo-distal development?

Answer 19

Apical Ectodermal Ridge (AER)

Question 20

How is AER involved in limb development?

Answer 20

• It induces the underlying tissue to remain as a population of undifferentiated, rapidly proliferating cells - known as the progress zone
• As cells move further away from the AER they will begin to differentiate into cartilage and muscle
• This differentiation results in proximo-distal development

Question 21

Describe proximo-distal limb development

Answer 21

• HOX-8 controls the position of the limb on the long axis of the body
• Initiation of outgrowth of the fore limb is controlled by the TBX5 gene and FGF-10
• AER secrets FGF4 and FGF8 to maintain the progress zone and the further development of the proximo-distal axis
• As growth progresses, mesenchymal cells are left behind the advancing ridge and so the begin to differentiate

Question 22

What regulates the antero-posterior axis?

Answer 22

Zone of Polarising Activity (ZPA)

Question 23

Describe ZPA

Answer 23

• Cluster of cells near the posterior border of the limb form the ZPA which regulates the AP axis
• It ensures that the thumb grows on the cranial (anterior) side of the limb bud
• ZPA expresses the protein sonic hedgehog (SHH)
• Moves distally with the AER

Question 24

Describe the formation of the dorso-ventral axis

Answer 24

• BMPs in the ventral ectoderm induce EN1
• EN1 represses WNT7 restricting its expression to the dorsal limb ectoderm
• WNT7 induces LMX1 which then specifies the cells to be dorsal

Question 25

What determines the shape of bones?

Answer 25

HOX genes

Question 26

Define amelia

Answer 26

Complete absence of the limbs

Question 27

Define meromelia

Answer 27

Partial absence of the limbs

Question 28

Define phocomelia

Answer 28

Absence of long bones

Question 29

Define micromelia

Answer 29

Segments are abnormally short

Question 30

Define brachydactyly

Answer 30

Short digits

Question 31

Define syndactyly

Answer 31

Fused digits - failure of apoptosis

Question 32

Define polydactyly

Answer 32

Extra digits