Embryology

Question 1

Key stages of embryo development

Answer 1

1. Fertilsation
   Sperm cell fuses with egg cell to create zygote
2. Cleavage
   Zygote undergoes rapid cell division without growth
   Creates morula
3. Blastulation
   Morula continues to divide & forms hollow ball of cells (blastocyst)
   Blastocyst composed of 2 cell layers (Inner cell mass – gives rise to embryo ) (Trophoblast – forms placenta & other supporting tissues)
4. Implantation
   Blastocyst attaches to endometrial lining of uterus & begins to implant
5. Gastrulation
   Cells of ICM differentiate into 3 primary germ layers

Question 2

What is the process of gastrulation

Answer 2

1. Formation of primitive streak
   defines axes of embryo
2. Cells from epiblast move towards & through primitive streak (remaining cells form ectoderm)
3. 3 germ layers formed
   ectoderm, mesoderm, endoderm

Question 3

What do the 3 germ layers give rise to

Answer 3

Ectoderm - skin, nervous system & sensory organs
Mesoderm - muscles, bones, blood vessels & reproductive organs
Endoderm - gastrointestinal tract, respiratory system & some glands

Question 4

What is the process of neurulation

Answer 4

1. Notochord formation
2. Neural plate formation
3. Neural tube formation
4. Neural crest formation

Question 5

What are somites

Answer 5

Somites are segments of mesodermal tissue present in developing embryo

Play important role in development of musculoskeletal & nervous system

Question 6

Describe somite development & differentiation

Answer 6

1. Formation
   Formed through process of somitogenesis
   Paraxial mesoderm (located on either side of neural tube) segments into somites in cranial to caudal sequence
2. Differentiation
   Somites undergo differentiation to give rise to different cell types & tissues
   Divided into 3 different regions:
   a. Sclerotome
   Gives rise to axial skeleton (including vertebrae & ribs)
   Sclerotome cells migrate around neural tube & notochord to form vertebral column
   b. Myotome
   Gives rise to skeletal muscles of trunk & limbs
   Myotome cells differentiate into muscle fibres & migrate to respective locations in body
   c. Dermatome
   Gives rise to dermis of skin
   Dematome cells migrate to body wall & diffentiate into connective tissue & skin cells of dermis
3. Neural crest cells
   Somites also give rise to neural crest cells which migrate to various locations in body
   Differentiate into variety of cell types, including neurons, glial cells & pigment cells

Question 7

How are vertebrae formed

Answer 7

1. Notochord formation
   Embryonic precursor of vertebral column
2. Somite formation
3. Somite differentiation
   Sclerotome gives rise to vertebral body, vertebral arch & intervertebral disc
4. Sclerotome reorganisation
   Cells migrate & condense around notochord & developing spinal cord
   Sclerotome cells begin to differentiate into cartilage
5. Ossification
   Process of bone formation
   Replacement of cartilage by bone
   Endochondral ossification forms vertebral body
   Intramembranous ossification forms vertebral arch
6. Intervertabral disc formation

Question 8

Describe differentiation of neurons

Answer 8

1. Expansion of lateral parts of neural tube leads to formation of thick lateral walls & thin roof & floor plates
2. Indentation of neural canal delineates dorsal & ventral columbs within intermediate zone – termed alar & basal plates
3. Alar & basal plates give rise to dorsal & ventral horns of spinal cord
   Notochord & floor plate are sources of signals that induce ventral or dorsal cell types
   Effect dependent on concentration gradient

Question 9

What are the 3 axes of limb bud formation

Answer 9

Proximo-distal (shoulder-finger tip)

Antero-posterior (thumb-little finger)

Dorso-ventral (knuckle-palm)

Question 10

Describe limb bud formation

Answer 10

Limb bud formation begins with proliferation of skeletal precursors from lateral plate & muscle precursors from somites

Cells accumulate under ectoderm creating bulge (limb bud)

Question 11

Describe limb bud outgrowth

Answer 11

Thickening of ectoderm at tip of limb bud = apical ectodermal ridge (AER)
Enduced by mesenchyme cells in core of limb bud
Major signalling centre
Essential for sustained outgrowth & limb development

As bud grows, cells differentiate & cartilaginous structures appear

Proximal structures develop first

Question 12

Describe specification of anterior-posterior axis in limb formation

Answer 12

Thumb anterior & little finger posterior

Organiser = small block of mesoderm = zone of polarising activity

Signalling molecule = sonic hedgehog

Polarising region specifies digit identity via concentration gradient

Question 13

Describe digit formation in limb formation

Answer 13

Apoptosis – programmed cell death

Essential for joints to form & fingers to become separate

Question 14

Describe synovial joint formation

Answer 14

1. Condensation of mesenchymal cells
2. Chondrogenesis
3. Formation of joint cavity
4. Synovial membrane formation
5. Formation of articular cartilage
6. Formation of joint capsule
7. Ligament & tendon formation
8. Innervation

Question 15

What is developmental orthopaedic disease

Answer 15

Broad term used to encompass number of disease states which occur during fetal development

Can be congenital (during or before birth) or acquired (after birth)

Question 16

What are some congenital factors of developmental orthopaedic disease

Answer 16

1. Intrauterine positioning
   Causes angular & flexural limb deformities
   “windswept” foals
2. Teratogenic factors
3. Genetic/breed predisposition
   e.g. Osteochondrosis (OCD) - endochondral ossification defect
   e.g. joint dysplasias
   e.g. Wobbler syndrome – malformed & unstable vertebrae
   e.g. hemivertebrae – vertebrae short & wedge shaped
4. Vascular supply
   Reduction in blood supply can cause angular limb deformities

Question 17

What are some acquired factors of developmental orthopaedic disease

Answer 17

1. Trauma
   Trauma, uneven loading or excessive exercise
2. Nutrition
   Vitamin D or copper deficiency
   Excess zinc
   Abnormal Ca : P ratio