WEEK 20 - development of animal body plans + formation of the coelum

what happens straight after fertilisation

first cleavages occur, forming a blastula

what is cleavage

increase in no. of cells without increase in cell mass

what does gastrulation do + involve

lay out the body plan

involves cell shape, changes, and cell movements

establishes the 3 Germ layers - endoderm, ectoderm, mesoderm

what are the 3 germ layers

endoderm, ectoderm, mesoderm

what is a blastopore

region where cells forming endoderm and mesoderm communicate with eachother

what happens to blastopore after gastrulation

archenteron breaks through front

protosomes - blastopore is mouth opening

deuterosomes - blastopore becomes anus1

chick gastrulation

blastodisc is a single-celled hollow ball

gastrulation occurs via a primitive streak (posterior to anterior)

Hensen’s node is functional equivalent of blastopore lip

what is neuralation

formation of a neural tube - developing brain + spine

oricess of neuralation

mesoderm condenses to form notochord along midline

induction of neural ridge by notochord

morphogen gradient - Sonic hedgehog

progression to neural fold, neural tube closure

neural tube forms brain + spinal cord

morphogen signal from notochord patterns neural tube dorso-ventrally

what direction is the neural tube patterned

dorso-ventrally

how is the neural tube patterned

opposing gradients of sonic hedgehog and BMP/Wnt

what are somites

precursors to bones of the skeleton

what do somites do

pairs form in a regular pattern through oscillatory signalling and lateral inhibition mechanisms

• vertebrae and limbs form in specified places along the AP axis

what happens for limb outgwoth

morphogen gradient from the apical epidermal ridge

head formation

at phylotypic stage all vertebrate embryos have gill slits

embryonic pharyngeal arches get modified

parts form human inner ear

other term for coelum

ventral body cavity

what is the coelum

body cavity located between the alimentary canal + the body wall, lined with mesoderm which surrounds internal organs

what does the dorsal cavity contain

spinal cord, facial cavities, cranial space

features of an acoelomate

no enclosed body cavity

• space between gut + body wall filled with mesenchyme cells

• layer of mesoderm-derived muscle runs beneath ectoderm layer

example of acoelomates

flatworm, jellyfish, sea anemones, parasitics (tapeworn)

features of a pseudocoelomate

fluid-filled body cavity

• internal organs suspended in fluid, no attachment to body wall

• layer of mesoderm derived muscle runs around body walls

• internal organs in contact with fluid due to no mesodermal peritoneal membrane

examples of pseudocoelomates

roundworm, nematodes, rotifers, parasitics (ascans, hookworm)

features of coelomates

enclosed body cavity which develops as 2 layers of mesoderm

• coelom is lined with a mesodermal peritoneal membrane which surrounds internal organs

• layer of mesoderm muscle lines body wall

• peritoneum separates internal organs from coelomic fluid

examples of coelomates

annelid worms, molluscs, insects, vertebrates

function of the coelom

provides space for 3D growth of complex organ structures

protective environment for organs of ventral body cavity

allows muscle groups to grow independently of body wall (heart, lung smooth muscle, diaphragm)

produces coelomic fluid

function of coelomic fluid

supports growth + movement of internal organs

provides medium for movement of immune cells (coelomocytes)

acts as hydroskeleton to support movement

facilitates transport of gases, nutrients, waste

evolutionary origin of protosome origin

spiral cleavage

embryo has 1 (monoblastic) ir 2 (diploblastic) cell layers

evolutionary origin of deuterosome origin

radial cleavage

embryo has 3 cell layers (tripoblastic)

bilateral symmetry

what is spiral cleavage

animal pole blastomeres are rotated with respect to those of the vegetal pole

what is radial cleavage

animal pole blastomeres are symmetrically orientated with those of the vegetal pole

what is gastrulation

the process of reorganisation of single-layered blastula into a multi-coloured gastrula consisting of 3 germ layers

what is the ectoderm

skin + appendages, tooth, neural crest + tube

what is the mesoderm

peritoneal membrane, connective tissue/bone, skeletal/smooth muscle

what is the endoderm

epithelial gut lining, epithelial lining of lung, secretory tissues of coelum

coelom formation in protosomes

Schizocoely

mesoderm splits to create coelomic cavity

coelom formation in deuterosomes

Enterocoely

differentiation of endodermal placodes into mesoderm

mesoderm forms pockets which bud off the endoderm and extend to form the coelomic cavity

4 areas of the coelomic cavity

transverse fold (septum)

pericardial cavity

pleuroperitoneal cavity

peritoneal cavity

what is the transverse fold (septum)

separates pericardial + pleuroperitoneal cavities

what is the pericardial cavity

mesoderm-lined cavity around heart

what is the pleuroperitoneal cavity

mesoderm-lined cavity around all visceral organs other than the heart

what is the peritoneal cavoty

mesoderm-lined cavity of abdominal/pelvic region

what separates the pleural and peritoneal cavities

diaphragm - muscular coelomic fold