Embryology

Question 1

Describe the positional changes of the spinal cord in the embryo, fetus, neonate and adult. May use a set of well labelled diagrams as your answer (4x2 = 8)

Answer 1

Description by Keith Moore (2016) is as follows: Positional Changes of Spinal Cord
The spinal cord in the embryo extends the entire length of the vertebral canal. The spinal nerves pass through the intervertebral foramina opposite their levels of origin. Because the vertebral column and dura mater grow more rapidly than the spinal cord, this positional relationship of the spinal nerves does not persist. The caudal end of the spinal cord in fetuses gradually comes to lie at relatively higher levels. In a 24-week-old fetus, it lies at the level of the first sacral vertebra.The spinal cord in neonates terminates at the level of the second or third lumbar vertebra. In adults, the cord usually terminates at the inferior border of the first lumbar vertebra. The spinal nerve roots, especially those of the lumbar and sacral segments, run obliquely from the spinal cord to the corresponding level of the vertebral column. The nerve roots inferior to the end of the cord (medullary cone) form a bundle of spinal nerve roots called the cauda equina.

Question 2

The neural portion of the eyeball is considered to be an outgrowth of the brain. Justify this statement by giving an overview of the embryological development of the eyeball. You may use a set of well labelled diagrams as your answer (10)

Answer 2

Description by Keith Moore (2016) is as follows:
The eyes begin to develop in 22-day embryos when optic grooves appear (Fig. 18-1 A and B). The eyes are derived from four sources: Neuroectoderm of the forebrain, Surface ectoderm of the head, Mesoderm between the previous two layers, Neural crest cells

Description follows thus: Early stages of eye development. A, Dorsal view of the cranial end of an embryo at approximately 22 days shows the optic grooves, which are the first indication of eye development. B, Transverse section of a neural fold shows the optic groove in it. C, Schematic drawing of the forebrain of an embryo at approximately 28 days shows its covering layers of mesenchyme and surface ectoderm. D, F, and H, Schematic sections of the developing eye show the successive stages in the development of the optic cup and lens vesicle. E, Lateral view of the brain of an embryo at approximately 32 days shows the external appearance of the optic cup. G, Transverse section of the optic stalk shows the retinal fissure and its contents. The edges of the retinal fissure are growing together, thereby completing the optic cup and enclosing the central artery and vein of the retina in the optic stalk and cup.

Question 3

Explain how the term “branchiomotor” differs from “somatic efferent” when describing the origins of muscle groups on the head and neck, and give two examples for each of the two groups (6)

Answer 3

Skeletal muscles of the head and neck fall into 2 groups. Some originate from pre-/ peri-/ postauricular somites and occipital somites - somatic efferent - e.g. extra-ocular eye muscles and muscles of the tongue. Others originate from branchial / pharyngeal arches and are special visceral efferent or branchiomotor - e.g. muscles of mastication, facial expression, pharynx, larynx.

Question 4

State the embryonic origin of the cranial and spinal meninges (2)

Answer 4

“The embryonic origin of the meninges varies across species. In birds (and probably in mammals), the spinal meninges are derived from the somitic mesoderm, the brainstem meninges from the cephalic mesoderm, and the telencephalic meninges from the neural crest.”

Arch Anat Cytol Pathol. 1998;46(3):153-69.Embryonic and fetal development of structures associated with the cerebro-spinal fluid in man and other species. Part I: The ventricular system, meninges and choroid plexuses. Catala M. Service d’Histologie-Embryologie et Cytogénétique, Groupe Hospitalier Pitié-Salpêtrière, Paris, France.

Somitic & cephalic mesoderm = 1 mark, neural crest cells = 1 mark

Question 5

Name and briefly describe the two mechanisms of ossification of the bones of the skull (4)

Answer 5

Intramembranous ossification – flat bones of skulls – connective tissue membrane laid down, various ossification centres

Endochondral ossification – base of skull – cartilage model laid down with various ossifications centres

Question 6

Name the fontanelles present at birth (4 x½ = 2)

Answer 6

Anterior, posterior, mastoid, sphenoid

Question 7

What is the function of these fontanelles? (1)

Answer 7

Sliding of the skull bones over each other during the birth process or room for expansion as brain develops

Question 8

At what age are all the fontanelles closed? (½)

Answer 8

By two years

Question 9

Give an overview of what is meant by “secondary neurulation” in an embryo. May use a well labelled diagram (5)

Answer 9

Formation of the neural tube inferior to the second sacral level by secondary neurulation. Mesoderm invading this region during gastrulation condenses into a solid rod called the caudal eminence, which later develops a lumen. At the end of the sixth week, this structure fuses with the neural tube. https://clinicalgate.com/embryology-and-brain-development

Question 10

Give a short description of a meningomyelocoele (2)

Answer 10

Bulging fluid-filled “sac” on surface of the body – contents could be meninges, cerebrospinal fluid, neural elements (caudal end of spinal cord).

Question 11

Based on your description of neurulation, briefly explain the process of development of a meningomyelocoele (5)

Answer 11

Explain how the overlying ectoderm and mesoderm need to seal off the neural tube from the surface by growing over the top of the neural tube. Ectoderm forms epidermis; mesoderm forms dermis, hypodermis, connective tissue, bone (vertebrae) and skeletal muscles. Failure of this process will result in elements bulging onto the surface of the body – in this case meninges and neural elements [ Possible tethering of the neural tube, leading to hydrocephalus later.]

Question 12

Briefly define the term “prosencephalisation” (1)

Answer 12

Considerable development of the cerebral hemispheres – in both size and amount of cerebral cortex (neurones; sulci & gyri).

Question 13

Name another portion of the brain that is also well developed alongside prosencephalisation (½)

Answer 13

Cerebellar hemispheres, or neocerebellum.

Question 14

Give a short description of the advantages of the advanced developments mentioned above (4)

Answer 14

Prefrontal cortex – various functions; aspects of sensory and motor homunculi, planning and reflection, language and speech, non-verbal communication, problem solving exercises, powers of reasoning, variety of creative tasks, fine motor movements - examples, quality of memories, and so forth.

Question 15

Explain why, from the perspective of embryological development, the cerebellum is unable to initiate motor activity (2)

Answer 15

Cerebellum develops from the alar plate of the neural tube – sensory / afferent neurones only

Question 16

Give an overview of anencephaly from a developmental perspective (6)

Answer 16

Anencephaly is a form of neural tube defect (NTD) that hampers the normal brain and skull development. Babies affected by this condition are often stillborn or die within a few hours or days of birth. The neural tube ideally folds and closes by the end of the first month (the third and fourth weeks) of pregnancy to form the embryo’s brain and spinal cord. Sometimes, the anterior neuropore of the tube fails to fold and close, leading to the lack of major parts of the brain and skull. Arrested brain development when cerebrum comes into direct contact with amniotic fluid.
Presentation: absence of the bones covering the top and back of the head, bones missing from the sides and front of the head, folded ears, abnormal facial features, cleft palate, congenital heart defects, a ‘frog eye’ appearance due to the absence of the cranial bones. Associated complications often include polyhydramnios due to the baby’s inability to swallow the fluid.

Question 17

Explain why an encephalocoele would most commonly affect either the frontal or the occipital regions of the skull (both pictured below) (4)

Answer 17

Multi-site closure of neural tube in humans; encephalocoele will appear at the junctions of regions 2 & 3 and 2 & 4.

Question 18

Give an overview of neurulation with respect to the spinal cord. You may use diagrams if you wish (10)

Answer 18

Above diagram will suffice. Also need to say induction of neurulation by notochord and process begins at day 22 and completed by day 25 (closure of rostral neuropore) and day 27 (closure of caudal neuropore).

Question 19

Describe the embryological development at the level of the fourth ventricle that result in changes to the positions of the alar and basal plates. Use this information to explain the positions of four different cranial nerve nuclei of your own choice. You may use a well labelled diagram (8 marks for description and diagram + 4 marks for 4 nuclei = 12)

Answer 19

Students are to describe or draw the scheme below, which illustrates how the alar plates move lateral to the basal plates, and the pia mater is stretched over the roof of the 4th ventricle. They then choose 4 cranial nerve nuclei and position them on their diagram according to the quality of the impulse: GSA, SSA, GVA and SVA in the alar plates, and SE, GVE and SVE (branchiomotor) in the basal plates.

Question 20

Briefly describe the macroscopic appearance of the cortex of the cerebrum in a child with a severe form of FASD. State why the cortex has this appearance (4)

Answer 20

Students should mention microcephaly, a smoother surface to the cerebral hemispheres, smaller gyri; caused by reduced amounts of cortical and subcortical grey matter in the cortex.

Question 21

Give an overview of the origin, migration and roles of neural crest cells. May use well labelled diagrams in your answer (12)

Answer 21

Origin from neural tube during the process of neurulation (days 22 to 27).

Migration and contributions to cranial & sensory ganglia & nerves, adrenal medulla, melanocytes, brancheal arch cartilages, heart - bulbar & conal ridges, meninges, and

cranial mesenchyme & connective tissue.

Question 22

List six cells, tissues or structures that develop from neural crest cells after they have migrated throughout the body (½ each = 3)

Answer 22

Melanocytes, dorsal root ganglia (sensory) of spinal nerves (not “spinal ganglia”), sensory ganglia of cranial nerves (not “cranial ganglia”), adrenal medulla, connective tissue of head (mesenchyme), autonomic ganglia (marks given if several ganglia listed, but we wanted more than just a list of autonomic ganglia to answer the question), meninges (pia mater & arachnoid), branchial arch cartilages, heart – bulbar & conal ridges, Schwann cells, retina

Question 23

Give two (2) examples of congenital anomalies that develop as a result of inappropriate neural crest cell migration sites (2)

Answer 23

Anomalies may affect any of the regions listed: cranial and spinal sensory ganglia and nerves, adrenal medulla, melanocytes, brancheal arch cartilages, heart - bulbar & conal ridges, meninges, cranial mesenchyme and connective tissue; the well-known examples will be anomalies in the heart and Treacher-Collin’s Syndrome (below).

Question 24

Draw and label a diagram of a cross section through the neural tube in an embryo to show the position of the developing neural crest cells. It is important to include the main developing structures surrounding the neural tube (5)

Answer 24

Draw closed neural tube (see question!) with surrounding structures – ectoderm (not neuro-ectoderm or neural plate), neural crest cells (dorsolaterally), mesoderm or somites (laterally), notochord (ventrally)

Question 25

Give an overview of the closure of the neural tube in the human embryo. You may use a well labelled diagram in your answer (6)

Answer 25

The diagram on the left side = multi-site closure of neural tube in humans; primary (1-4) and secondary (5) neurulation.

Question 26

Describe the different types of neural tube defects that occur. Include in your answer the anatomical structures involved in each defect (10)

Answer 26

• May occur on the head (anencephaly, encephalocoele) or on the vertebral column (spina bifida), or both (e.g. severe forms such as myeloschisis, rachischisis) (4)
• Spina bifida may be hidden (spina bifida occulta) or visible as a cyst on the back (spina bifida cystica) (2)
• Spina bifida cystica may contain only CSF (meningocoele, rare) or CSF and neural elements (meningomyelocoele) (4)

Question 27

Draw and fully label a cross section through the developing neural tube in the thoracic region of the embryo, giving a short explanation of the functional implications for each of the structures that you have labelled (10)

Answer 27

Neural tube is divided into:
- ventricular zone – giving rise to ependymal layer, lining the central canal
- mantle layer – giving rise to grey matter (neurones)
- marginal layer – giving rise to white matter (tracts)
Mantle layer is divided by the level of the sulcus limitans into alar plate dorsally and basal plate ventrally –» alar plate = sensory / afferent = dorsal horn, & basal plate = motor / efferent = ventral horn.
Asking for the thoracic region will, hopefully, trigger recall of the lateral horn – autonomic (sympathetic) outflow, and the resulting lateral horn (visceral), versus the dorsal and ventral horns (somatic).
See diagram of the functional anatomy of the spinal cord, below.

Question 28

Outline how the canal of the cranial end of the developing neural tube in the embryo is transformed into the various compartments of the ventricular system of the brain (4)

Answer 28

• Telencephalon – lateral ventricles (I & II)
• Diencephalon – ventricle III
• Mesencephalon – aqueduct of midbrain
• Rhombencephalon – ventricle IV [divided into upper and lower halfs, contributed by the metencephalon and myelencephalon