8 - Embryogenesis and Development Flashcards
Cleavage
cell division in early embryo
Gastrulation
cell movements which produce gut and 3 primary germ layers
difference between early embryonic and somatic cell cycle
somatic cycle: 4 phases
early embryonic: alternates between S and M phases without intervening G1 and G2
What is morphogenesis?
• The regulation of the pattern of anatomical development.
What is organogenesis?
• Formation of organs.
What is a morula?
• Tight collection of 32 cells at the beginning of differentiation.
What is a blastula?
• An animal embryo at the early stage of development when it is a hollow ball of cells.
What is a blastocoel?
• The fluid-filled cavity of a blastula.
What is a blastomere?
• (After the first division) it’s a cell formed by cleavage of a fertilized ovum.
What is meroblastic cleavage?
• Incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds.
What is holoblastic cleavage?
• Cleavage that completely divided an egg.
What happens in the cleavage of sea urchins?
- Fertilised sea urchin eggs have vitelline membranes.
- Rapid divisions of blastomeres (all the same size) create a morula.
- Cells move around the outside and start to condense at one side of the structure (the bottom); these grow into a hollow structure.
- Equal holoblastic cleavage (divides entire cell into 2).
- Blastomeres become smaller with each division.
why is there unequal holoblastic division in amphibians?
Blastomeres in animal pole are smaller than blastomeres in vegetal pole because of presence of yolk in vegetal hemisphere.
amphibian cleavage steps
● Cleavage 1 and cleavage 2 are perpendicular and (equal holoblastic) but nuclei are displaced ‘animal-ward’
● Cleavage 3 is perpendicular/equatorial but (unequal holoblastic)
● Gives rise to a more rapidly dividing animal pole
What happens in the gastrulation of sea urchins?
• We have a blastula with a single layer of cells on the outside and a centrally located blastocoel.
- Cells at the vegetal pole thicken to form the vegetal plate and these fold inwards to form a tube called the archenteron which will form the endoderm (giving rise to the gut).
- Some cells detach and migrate into the blastocoel.
- These are called mesenchyme cells and will form the mesoderm.
- Cells remaining on the surface form the ectoderm. The open end of the tube at the vegetal pole is called the blastopore.
- Contractions of filopodia of mesenchyme cells pull the tube through the blastocoel.
- Eventually this tube fuses with the wall of the embryo giving rise to the mouth.
chick cleavage
meroblastic = cleavage plane doesn’t bisect yolk
what occurs after the cleavage stage
blastula formation
what occurs after the blastula formation
gastrulation
what does gastrulation form
primary germ layers: ectoderm, mesoderm, endoderm
ectoderm
- Epidermis of Skin
- Nervous system
- Pituitary gland
- Adrenal medulla
- Jaws and teeth
- Germ cells
mesoderm
- Notochord
- Muscular system
- Skeletal system
- Circulatory and lymphatic systems
- Excretory and reproductive systems
- Dermis of skin
endoderm
- Epithelial lining of gut and associated organs
- Epithelial lining of respiratory, excretory and reproductive tracts
gastrulation in sea urchins
begins w/ small invagination in blastula =>
cells continue moving toward invagination =>
elimination of blastocoel => gastrula
gastrulation in amphibians
- blastopore begins to form
- involution
- mesoderm insinuates itself between ectoderm and endoderm (no outpocketing)
- chordamesoderm forms notochord
Gastrulation in chicks
- Cells move through the streak and replace hypoblast cells
- Cells moving through the streak move into the space to create mesoderm
First cleavage division
The first cleavage division 24-30 hours after fertilisation. Holoblastic division. Two blastomeres are formed.
What happens during implantation?
- Once the blastocyst forms, it begins to attach itself to the wall of the uterus (approx. day 6-7).
- Blastocyst secretes enzymes to burrow into the endometrium.
- Inner cell mass next to the endometrium.
What is the structure of human blastocysts?
- Inside of blastocyst is called the Inner Cell Mass (ICM).
- ICMs will become the embryo, which is identifiable at 4-6 days of human gestation.
- Outside of blastocyst have cells called trophoblasts.
- Trophoblasts go on to become the placenta.
- Blastocyst begins to implant into the uterus and continues to develop.
- Next stage is the formation of two cell layers: epiblast + hypoblast
Epiblast
- Outer layer: these cells go on to form the embryonic tissue.
- Surrounded by an amniotic cavity.
Hypoblast
- Primitive endoderm (endo meaning inner).
- Gives rise to extraembryonic tissues.
- Faces onto the yolk sac.
What happens during gastrulation of human blastocysts?
- Around days 12-16, gastrulation results in the formation of a trilaminar embryo.
- Primitive streak forms in dorsal epiblast & this structure defines anterior/posterior and left/right of embryo.
- Epiblast cells (top layer) converge on the primitive streak/groove.
- Individual proliferating cells move through PS (C) and displace the hypoblast to create the endoderm.
- Later cells move through to create the mesoderm.
- Cells spread out laterally and anteriorly (B).
Day 0
fertilization
day 1
2 blastomere cells
day 2
4 cell stage
Day 3
early morula forms
Day 4
embryo has progressed to a morula to develop a blastocyst, 32 cell stage
Day 5
hatching
- the blastocyst digests a hole in the zona pellucida and emerges.
Day 6
Blastocoel formation: the fluid-filled cavity inside a blastula
Days 7-10
implantation in uterine wall
Blastocyst
stage of early development in mammals that consists of a hollow ball of cells
days 12-15
Gastrulation
- Primitive streak forms in dorsal epiblast and defines anterior/posterior and left/right of embryo.
- Cells from primitive node produce the notochord
