Formation of Basic Body Plan

Question 1

Gastrulation overview

Answer 1

Week 3: Form 3 germ layers in correct topological position
Form basic body plan/ axes
Blastula containing bilaminar disc becomes multi-layer organism containing trilaminar disc

Question 2

Germ layers

Answer 2

Ectoderm + mesoderm + endoderm
Each layer forms specific tissues/organs
Groups of cells directed to become specific structures at early stage of development

Question 3

Mesenchymal cells

Answer 3

Derive from any germ layer

Form loosely organised tissue, later differentiate to spec. tissue

Question 4

Formation of primitive streak

Answer 4

Forms on dorsal surface of epiblast
Day 15/16 = clearly visibility narrow groove w/ slightly bulging regions on each side due to proliferation/migration epiblast cells to median plane of embryonic disc
Elongate by add cells to caudal end
Proliferation cause primitive node at cephalic end = elevated area surrounding primitive pit

Question 5

Formation of germ layers

Answer 5

Epiblast cells migrate towards primitive streak (epithelial to mesenchymal transition)
Invagination: Cells detach, move through primitive groove
Cells displace hypoblast = endoderm
Cells between epiblast and endoderm = mesoderm
Cells remain in epiblast = ectoderm
Cells migrate beyond disc, meet extra embryonic mesoderm, cover yolk sac and amnion
In cephalic direction pass each side of prechordal plate (between tip notochord and oropharyngeal membrane, derived from first cells migrate through node in midline moving in cephalic direction)
Primitive streak diminish in size

Question 6

Oropharyngeal membrane

Answer 6

Cranial end
Small region tightly adherent ectoderm and endoderm cells
Future opening of oral cavity

Question 7

Cloacal membrane

Answer 7

Same as oropharyngeal
At caudal end
Future opening of anus

Question 8

Primitive node as an organiser (experimental)

Answer 8

Epiblast cell migration through primitive groove controlled by embryonic growth factors
Transforming growth factor-β = primitive streak initiation
Nodal (TBF- β) = mesoderm formation and regulation of transcription factor expression
Fibroblast growth factor (FGF) 8 = Cell migration and specification (Synthesised by primitive streak, control cell movement by down regulate protein E-cadherin [binds epiblast cells together]
Control cell specification into mesoderm)

Question 9

Teratogenesis associated with gastrulation

Answer 9

Initiation of gastrulation = highly sensitive stage for teratogenic insult (cause birth defects)
Caudal dysgenesis/sirenomelia – insufficient mesoderm formed at caudal region
Abnormalities in lower limb, urogenital system, lumbosacral vertebrae
Sacrococcygeal teratomas - primitive streak persists in this region
Clusters of pluripotent cells proliferate and form tumours
Contain tissues from 3 germ layers

Question 10

Notochord formation

Answer 10

Prenotochordal cells invaginate in primitive node: move forward cranially to midline until reach prechordal plate (important organiser of head region)
Prenotochordal cells intercalate w/ hypoblast, midline of 2 cell layers = notochordal plate
Hypoblast replace by endoderm cells
Cells as notochordal plate proliferate and detach from endoderm
Form solid chord of cells = definitive notochord
Elongation of notochord = dynamic process: cranial-to-caudal sequence, notochord and prenotochordal cells (mesenchymal) extended cranially to prechordal plate (caudal to oropharyngeal membrane) and caudally to primitive pit
Where pit forms indentation in epiblast: neurenteric canal connects amniotic and yolk sac cavities temporarily
Cloacal membrane form at caudal end of embryonic disc
Notochord later form nucleus pulposus = inner gel centre of intervertebral discs

Question 11

Notochord

Answer 11

Cellular rod
Defines axes of embryo, basis for development of axial skeleton
Provide rigidity
Indicates future site of vertebral bodies

Question 12

Derivatives of mesodermal germ layer

Answer 12

Initial mesoderm layer = thin sheet loosely woven tissue on each side of midline
Day 17: Differentiate to paraxial (cells close to midline proliferate to form thickened plate) and lateral plate (remain thin)

Question 13

Paraxial mesoderm

Answer 13

Week 3: Paraxial mesoderm organised into segments (somitomeres)
First in cephalic region, proceed cephalocaudally
Somitomere = mesodermal cells arrange in concentric spirals around centre of unit
Head region: somitomeres form in association w. segmentation of neural plate (neuromeres)
Occipital region caudally - somitomeres organise into somites
New somites appear in craniocaudal sequence

Question 14

Somite pairs

Answer 14

Body segmentation 
42-44 pairs at end week 5:
4 occipital
8 cervical 
12 thoracic 
5 lumbar 
5 sacral 
8-10 coccygeal pairs 
1st occipital + last 5-7 coccygeal somites late disappear 
Rest form axial skeleton

Question 15

Intermediate mesoderm

Answer 15

Temporarily connects paraxial and lateral plate mesoderm

Differentiate to urogenital structures

Question 16

Lateral plate mesoderm

Answer 16

Split:
Parietal (somatic) - line intraembryonic cavity
Visceral (splanchnic) - surround organs

Question 17

Parietal (lateral plate mesoderm)

Answer 17

Mesoderm from parietal layer + overlying ectoderm = lateral body wall folds
Lateral folds + head fold + tail fold = close ventral body wall
Dermis (skin in body walls and limbs), bones + connective tissue (limbs), sternum
Sclerotome + muscle precursor cells migrate into parietal layer = costal cartilages, limb muscles, most body wall muscles
Surround intraembryonic cavity, form thin membranes:
Mesothelial, serous, line peritoneal, pleural, pericardial cavities, secrete serous fluid

Question 18

Visceral (lateral plate mesoderm)

Answer 18

Visceral layer + endoderm = wall gut tube

Form thin serous membrane around organs

Question 19

Somite differentiation

Answer 19

Somites originally form from presomitic mesoderm (ball of cells)
Epithelization: cells arrange in donut shape around small lumen
Week 4: cells in ventral, medial walls of somite lose epithelial characteristics, now mesenchymal, shift position, surround neural tube and notochord = sclerotome
Cells at dorsomedial + ventrolateral edge (upper region) = myotome
Cells between myotome = dermatome
Myotome cells become mesenchymal, migrate beneath dermatome = dermomyotome

Question 20

Sclerotome

Answer 20

Differentiate into vertebrae + ribs

Tendon cartilage and bone component of somite

Question 21

Myotome

Answer 21

Precursor muscle cells

Segmental muscle component of somite

Question 22

Dermomyotome

Answer 22

Dermis for skin, back muscles, body wall (intercostal muscles), some limb muscles
Each dermatome and myotome has own segmental nerve component

Question 23

Molecular regulation of somite differentiation (experimental)

Answer 23

Signals for somite differentiation come from surrounding structures:
notochord, neural tube, epidermis, lateral plate mesoderm
Secreted products of NOGGIN genes and sonic hedgehog (SHH), produced by notochord and floor plate of neural tube
Induce ventromedial somite to become sclerotome

Question 24

Situs inversus (clinical)

Answer 24

Major visceral organs mirror normal anatomical positions
Increase congenital heart defects
Many have no associated health issues
Asymmetric body plan