Formation of Primitive Streak, Notochord, Neural Tube, Subdivisions of Intraembryonic Mesoderm and Embryonic Folding

Question 2

[3-minute video]: Embryonic folding animation

Answer 2

🤓

Question 3

[2-minute video]: Primary Neurulation

Answer 3

💪

Question 4

Click on Answer for relevant labelled diagrams.

Answer 4

[Diagram 1] [Diagram 2] [Diagram 3] [Diagram 4]

Question 4

Click on Answer to view electron microscope images of the neural tube and notochord.

Answer 4

[Image 1] [Image 2] [Image 3] [Image 4]

Question 5

Give a summary of the first week of development.

Answer 5

1. The conceptus is propelled largely by ciliary activity from the site of fertilization (ampulla of the Fallopian tube) towards the site of implantation (endometrial cavity).
2. Continuous cleavage causes numerical cellular increase, hence compaction and eventually hatching occurs.
3. Zygote → Two-cell → Four-cell → Morula → Blastocyst stages
4. Ends by formation of the embryoblastic and trophoblastic tissues
5. The embryonic & non-embryonic tissues are therefore established.
6. The conceptus is ready for implantation by the end of week 1.
7. [Diagram]

Question 6

Implantation is the process by which the blastocyst attaches and embeds itself into the endometrial lining of the uterine wall. It occurs around day 6-13 after conception.
(a) Invasion of the endometrium is done by the ____________________ layer of the blastocyst.
(b) Outline the steps in the implantation process.

Answer 6

(a) trophoblast
(b) Implantation process:
1. Hatching of the blastocyst
2. Attachment of the blastocyst (note; abembryonic pole leads the process, blastocyst attaches via embryonic pole)
3. Syncitiotrophoblast forms (remember, it secretes HCG that maintains corpus luteum)
4. Invasion and embedding
5. Lacunae formation
6. Epithelial repair
[1-minute video]: Implantation

Further notes:
The embryonic pole is the side of the blastocyst where the inner cell mass forms. The abembryonic pole is the side of the blastocyst opposite to the embryonic pole. It does not contain the inner cell mass. These poles define an embryonic-abembryonic axis, which is important for the orientation and organisation of the developing embryo. [Diagram]

Question 7

What structure is last to form during the second week?

Answer 7

Primitive streak

Question 8

List four complications of multiple gestations.

Answer 8

1. Conjoined twins
2. Prematurity
3. Low birth weight
4. Parasitic twins

Question 9

For the following, state its primordial equivalent and 1st day of establishment.
(1) Placenta
(2) Amniotic sac
(3) Body parts

Answer 9

(1) Placenta
Primordial equivalent: Trophoblast
1st day of establishment: Day 5
(2) Amniotic sac
Primordial equivalent: Bilaminar embryo
1st day of establishment: Day 8/9
(3) Body parts
Primordial equivalent: Primitive streak
1st day of establishment: Day 14/15

Question 10

Conjoined twins come about due to separation at the ________________ stage.

Answer 10

primitive streak

Question 11

How is the primitive streak formed, and where is it located?

Answer 11

It is formed by proliferation of the midline epiblast cells. It is located around the caudal end of the bilaminar embryo.

Question 12

Name the parts of a somite from medial to lateral, and the structures that they develop into.

Answer 12

1. Medial part called the sclerotome forms the vertebrae and the ribs
2. Middle part called the myotome, which forms skeletal muscles
3. Lateral part called dermatome, forms dermis of the skin and spinal meninges

Question 13

What body system develops from the intermediate mesoderm?

Answer 13

The genito-urinary system

Question 14

State the functions of the primitive streak.

Answer 14

1. Provide structural support to the bilaminar embryonic disc
2. Establishes the embryonic axis and bilateral symmetry
3. Direct the process of gastrulation

Question 15

The process of gastrulation occurs by migrations of the cells of primitive streak. The cells migrate downwards and outwards. The migration process is in 2 phases. What layers of the gastrula are formed in which phases?

Answer 15

Endodermal layer: 1st phase (formed by the laterally displaced hypoblast layer)
Mesodermal layer: 2nd phase (formed by cells sandwiching themselves between the formed endoderm & the remaining epiblast cells)
Ectodermal layer: 2nd phase (the remaining epiblast constitute this layer)

Question 16

What is the cause of sacrococcygeal teratomas?

Answer 16

They are caused by persistence of remnants of the primitive streak. Because they are derived from pluripotent primitive streak cells, the tumors contain tissues derived from all three germ layers in varying stages of differentiation.

Question 17

What is a chordoma and what is its cause?

Answer 17

A chordoma is a tumor that develops inside the bones of the skull and spine. These tumors develop from remnants of the notochord during intrauterine development.

Question 18

State three functions of the notochord.

Answer 18

1. Provides signals that are necessary for development of axial musculoskeletal structures and the central nervous system (CNS)
2. Defines the primordial longitudinal axis of the embryo and gives it some rigidity
3. Contributes to the intervertebral discs interposed between the bodies of adjacent vertebrae
4. Induces the process of neurulation (especially primary neurulation)

Question 19

State the remnants of the notochord in humans.

Answer 19

☑ Nucleus pulposus of the intervertebral disks
☑ Apical ligament of dens

Question 20

State five derivatives of the neural crest cells.

Answer 20

1. spinal ganglia and ganglia of autonomic nervous system
2. neurolemma sheaths of peripheral nerves
3. meninges particularly arachnoid and pia mater
4. melanocytes
5. suprarenal (adrenal) medulla

Question 21

State two characteristics of neural crest cells.

Answer 21

(a) Multipotent
(b) Migratory

Question 22

State five derivatives of the ectodermal layer.

Answer 22

1. central nervous system
2. peripheral nervous system
3. sensory epithelia of eyes, ears, nose
4. epidermis and its appendages [.g. mammary glands]
5. pituitary gland [specifically the anterior lobe/adenohypophysis]
6. enamel of the teeth
7. iridal and ciliary muscles of the eye [exception to the skeletal muscles which develop in the paraxial mesoderm]

Question 23

State five derivatives of the endoderm.

Answer 23

1. epithelial lining of digestive and respiratory tracts
2. thyroid and parathyroid glands
3. thymus
4. liver
5. pancreas
6. epithelial lining of the urinary bladder and most of the urethra
7. epithelial lining of the tympanic cavity
8. tympanic antrum

Question 24

State two derivatives of the dermatome of a somite.

Answer 24

1. spinal meninges
2. truncal dermis and subcutaneous tissue

Question 25

State two derivatives of the myotome of a somite.

Answer 25

1. epiaxial muscles
2. hypaxial muscles

Question 26

State three derivatives of the sclerotome of a somite.

Answer 26

1. vertebrae
2. intervertebral disk
3. ribs

Question 27

State four derivatives of the intermediate mesoderm.

Answer 27

1. kidneys
2. ureters
3. gonads
4. vas deferens and epididymis

Question 28

State two derivatives of the somatic layer of the lateral plate mesoderm.

Answer 28

1. parietal peritoneum
2. sternum
3. appendicular skeleton
4. dermis

Question 29

State four derivatives of the splanchnic layer of the lateral plate mesoderm.

Answer 29

1. visceral peritoneum
2. adrenal cortex
3. lymph nodes
4. spleen
5. smooth muscle of the GIT
6. myeloid stem cells
7. cardiac musculature

Question 30

What is the embryonic origin of spina bifida occulta?

Answer 30

failure of the caudal neuropore to close

Question 31

Neural tube defects are commonly associated with what deficiency?

Answer 31

folic acid deficiency (Vitamin B9)

Question 32

“The ____(1)____ forms the floor of the amniotic cavity and is continuous peripherally with the amnion. The ____(2)____ forms the roof of the exocoelomic cavity and is continuous with the thin ____(3)____ membrane. This membrane together with the ____(2)____ lines the ____(4)____.”

Answer 32

1. epiblast
2. hypoblast
3. exocoelomic/Heuser’s
4. primary umbilical vesicle

Question 33

What are the derivatives of the neural tube?

Answer 33

brain and spinal cord

Question 34

Briefly discuss spina bifida occulta.

Answer 34

• mildest form of spina bifida
• NTD resulting from failure of the halves of one or more neural arches to fuse in the median plane
• occurs in L5 or S1 vertebra
• In the minor form, the only evidence of its presence may be a small dimple with a tuft of hair arising from it. An overlying
  lipoma dermal sinus or other birthmark may also occur.

Question 35

Briefly discuss spina bifida meningocele and meningomyelocele.

Answer 35

spina bifida meningocele: protrusion of a meningeal cyst (meninges and CSF) through a defect in the vertebral column (neural tissue is absent in the cyst)
spina bifida meningomyelocele: protrusion of a meningeal cyst containing the spinal cord or nerve roots through a defect in the vertebral column

Question 36

What is rachischisis?

Answer 36

This is a condition whereby part or the whole of the spinal cord is exposed due to incomplete fusion of the vertebrae. It is a neural tube defect resulting from failure of the neural tube to close.

Question 37

State the embryological basis of caudal dysgenesis. [A disorder of gastrulation]

Answer 37

defective caudal mesodermal migration

Question 38

List the anomalies associated with caudal dysgenesis.

Answer 38

Sirenomelia
Vertebral anomalies
Limb fusion

Question 39

State the anomalies associated with neural crest cells.

Answer 39

1. Congenital aganglionic megacolon
2. Disorders of skin pigmentation
3. 1st pharyngeal arch syndromes
4. Some cardiac malformations

Question 40

Outline the process of secondary neurulation and state two anomalies associated with it.

Answer 40

• condensation of mesenchyme to form a neural rod
• cavitation to form the sacral neural tube
• anomalies: junctional neural tube defect and segmental spinal dysgenesis, caudal agenesis and caudal duplication syndrome, myelomeningocele, limited dorsal myeloschisis and congenital dermal sinus, lumbosacral lipomatous malformation, thickened filum and filar cyst, low-lying conus, retained medullary cord, terminal myelocele and terminal myelocystocele.

Question 41

Name four cutaneous stigmata of spina bifida

Answer 41

(a) Skin dimple
(b) Hair tuft
(c) Lipoma
(d) Hyperpigmentation