Formation of the Basic Body Plan

Question 1

Explain formation of the primitive streak and its components

Answer 1

Day 15

1. Thickening, primitive streak, containing a midline groove forms along the midsagittal plane of the embryonic disc
2. Along the length of the primitive streak, the primitive groove becomes deeper and more defined
3. the cranial end of the primitive streak is expanded to form the primitive node
4. the primitive then forms a depression called the primitive pit, which is continuous caudally with the primitive groove

Question 2

What does the primitive streak define?

Answer 2

All major body axes

• primitive streak forms midline of embryonic disc, thus the cranial to caudal axes, and medial- lateral axis

left-right axis

Question 3

Gastrulation definition

Answer 3

The process of forming endoderm, ectoderm and mesoderm from the epiblast layer

Question 4

Stages of gastrulation

Answer 4

1. epiblast cells undergo ingression, moving in through the primitive streak
2. Epiblast cells undergo epithelial to mesenchymal transformation EMT
3. The ingressing epiblast cells invade the hypoblast and displace the cells
4. Hypoblast is eventually completely replaced by a new layer of cells called the definitive endoderm
5. Epiblast cells migrate through the primitive streak and diverge into the space between the epiblast and definitive endoderm- form third layer called intraembryonic mesoderm
6. Remaining epiblast cells constitute the ectoderm
7. By week 4 primitive streak disappears

Question 5

What is the difference between epithelial cells and mesenchymal cells?

Answer 5

Epithelial- sheets of regular shaped cells tightly interconnected to one another at their lateral surfaces.

Mesenchyme- more more irregular, stellate loosely connected cells

Question 6

What happens during EMT?

Answer 6

epiblast cells become more flask shaped, elongate, extend rootlike processes called pseudopodia which allow them to migrate through the primitive streak

Question 7

What happens between the ectoderm and endoderm?

Answer 7

Two depressions are formed, one at cranial and one at caudal end.

• oropharyngeal membrane and cloacal membrane, exclude mesoderm
• eventually bind to gut tube which form the oral cavity and opening of the anus respectively

Question 8

How does the endoderm induce the formation of the mesoderm?

Answer 8

Mesoderm induction requires TGF beta and FDF families of secreted growth factors. Deregulate production of E cadherin, preventing the binding of epiblast cells so they can move into the primitive streak

Question 9

Two important regions of the embryo explained

Answer 9

Nieuwkoop centre- induces the formation of the organiser-

Spemann-Mangold organiser- sends out signals to pattern the newly formed mesoderm into its medial and lateral subdivisions
-primitive node in humans

Question 10

What does the ectoderm differentiate into?

Answer 10

Central neural plate and peripheral surface ectoderm

Question 11

In which direction does the differentiation of cells take place?

Answer 11

Cranial to caudal

Question 12

Teratogenesis definition

Answer 12

Abnormalities of physical development

Question 13

Example of a teratogenic condition + explained

Answer 13

Sirenomelia

- insufficient caudal mesoderm

Question 14

notochord definition

Answer 14

Flexible rod made out of a material similar to cartilage, which eventually forms the vertebral column

Question 15

Stages of notogenesis

Answer 15

1. Prospective notochord cells, formed of mesoderm, migrate cranially at the midline to form the notochord process
2. Day 18 the notochordal process fuses with the endoderm to then become the notochord plate
3. provides a passage from the amniotic sac to the yolk sac- forming the neurenteric canal
4. Eventually, two edges of notochordal plate fuse to become the notochord which ends up in the mesoderm

Question 16

What does the notochord eventually form?

Answer 16

Nucleus pulposus

Question 17

What is another role of the notochord?

Answer 17

Secretes the patterning signal sonic hedgehog, which initiates tissue development and differentiation

Question 18

Organisation of mesoderm

Answer 18

para-axial mesoderm slightly lateral to
notochord (eventually forms somites)- thickened plate

intermediate mesoderm

lateral plate mesoderm- eventually forms splanchnic, somatic and coleum mesoderm- remains thin

Question 19

Function of intermediate mesoderm

Answer 19

• produces urinary and genital system

temporarily holds paraxial and lateral plate mesoderm together

Question 20

Stages of differentiation of lateral plate mesoderm

Answer 20

Day 17
1. lateral plate mesoderm splits into two layers: ventral layer associated with endoderm and dorsal layer associated with ectoderm

2. ventral layer gives rise to splanchnic mesoderm- forming mesothelial coverings of viscera
3. dorsal layer- somatic mesoderm- gives rise to inner lining of body and parts of limbs
4. middle layer forms the coelom, which is the mesodermally lined cavity between the outer body wall and gut

Question 21

Where does mesoderm begin differentiating?

Answer 21

Cranial to caudal direction

Question 22

Somite definition

Answer 22

block-like condensations of mesoderm

Question 23

What occurs in the head region?

Answer 23

Mesoderm forms a band of cells that remain unsegmented

Question 24

Explain somite pair formation sequence and days

Answer 24

Day 20 - first pair of somites form at the head-trunk border

remainder form in the cranial-caudal progression- finishing on day 30

Approximately 42-44 pairs form, flanking the notochord from the occipital region to the tip of the embryonic tail

Several caudal most somites disappear- forming 37 pairs in total

Question 25

What do somites form?

Answer 25

most of the axial skeleton- form limb buds vertebral column, occipital bones, voluntary musculature of neck body wall limbs and dermis of neck

Question 26

Segmentation definition

Answer 26

Formation of serially repeated and functionally equivalent units

Question 27

Stages of somitogenesis

Answer 27

1. The presomitic paraxial mesoderm is patterned using two molecular activities known as the 'clock' and ;wave front' 2. The segmentation clock controls the expression of cyclic genes. 3. these include the notch family and Wnt pathway 4. Outer shell of the somite formed by mesenchymal to epithelial transformation 5. core of mesenchymal cells then contains a cavity called the somitocoel that fills with the proliferating mesoderm cells

Question 28

How is the spacing of the segments achieved?

Answer 28

Wavefront is generated using a gradient of Fgf8, which is transcribed in the tail bud. Cells migrate out of the tail bud into the presomitic mesoderm High concentration in tail, low in head, thus controlling the position of the somite boundaries along the cranial/caudal axis

Question 29

What two regional components does the somite form originally?

Answer 29

ventromedial region- sclerotome forms vertebral body and intervertebral disc dorsolateral region- dermomyotome forms dermis and skeletal muscle

Question 30

Explain structure of sclerotome

Answer 30

- left and right sclerotome from the same segmental level fuse and engulf notochord - becomes segmented in the rostral and caudal directions by a von Ebner's fissure - one half forms single vertebral body - other half forms intervertebral disc

Question 31

Explain structure of dermomyotome

Answer 31

Divided into a dorsal and ventral half dorsal- dermatome ventral- myotome that will further vide into dorsal and ventral half

Question 32

Structure and function of myotome

Answer 32

dorsal- epaxial ventral- hypoaxial- muscles of the ventrolateral body wall, girdle, limb and Tonge

Question 33

How is sclerotome formed?

Answer 33

1. Notochord secretes Sonic Hedgehog, along with noggin (a Bmp inhibitor) also secreted by the notochord 2. Express the factor Pax1- a paired box transcription factor which characterises the vertebral bodies and vertebral discs. 3. Pax 1 induces epithelial to mesenchymal transformation

Question 34

How is dermamyotome formed?

Answer 34

1. dorsal neural tube and surface ectoderm produce various Wnts, which induce dermamyotome. 2. Wnts express Pax3 which allows the dermamyotome further develop and form the myotome 3. Wnt6 from ectoderm also maintains epithelial characteristics of the dermatome 4. long range Shh signalling is needed for the initial specification of the epaxial part of the myotome 5. Further patterned by a gradient of Bmp4, which is secreted by the lateral plate mesoderm and noggin secreted by the neural tube and notochord (which attenuates the gradient)

Question 35

Proof of the importance of the notochord in sclerotome formation?

Answer 35

Remove notochord and add it to mesoderm of another organism. Will form a second vertebral column

Question 36

What will the dermatome eventually form?

Answer 36

Dermis- connective tissue and fat

Question 37

Teratology definition

Answer 37

The study of abnormal development in embryos and the causes of congenital malformations or birth defects

Question 38

What are the most sensitive periods of time in relation to teratogenesis?

Answer 38

First two weeks- teratogenic agents will kill the embryo, rather than cause congenital malformations During organogenesis- between days 15 and 60, agents most likely to cause major congenital malformations- during most rapid cell deviation

Question 39

Example of teratogenic agent + defect

Answer 39

Thalidomide- caused malformation of organs and limbs. Thalidomide is antiangiogenic thus embryos cannot receive enough oxygen. Also inhibits Tumour Necrosis Factor alcohol- fetal alcohol syndrome and retardation

Question 40

Name of mesodermal-ectoderm structures

Answer 40

Somatopleure

Question 41

Name of mesodermal-endoderm structures

Answer 41

Splanchopleure

Question 42

What two forms of mesoderm are formed first?

Answer 42

Cardiac mesoderm and extra embryonic mesoderm which helps form around the embryo