Download Chapter 47. Concept 47.1 Pgs. 992-998 Cleavage –Fig. 47.7.

Chapter 47. Concept 47.1 Pgs. 992-998
Cleavage –Fig. 47.7.
- S ( DNA synthesis) and M phases of cell cycle.
- Skip G1 and G2 so no protein synthesis.
- NO enlargement of embryo.
- Results in division of cytoplasm into blastomeres (each with own nucleus).
Fert. Egg to 4Cell (2nd division) to Morula (5-7 divisions. Latin for mulberry – lobed
appearance) where fluid dilled cavity forms = blastocoel) to Blastula (hollow ball of
cells).
- Now different regions of cytoplasm in different blastomeres so stage set for further
development.
- Eggs have polarity, except mammals. Polarity defined by uneven distribution of
cytoplasmic substances (proteins, yolk).
- Yolk key factor in many animals:
- Concentrated in towards one pole of egg (vegetal) and less towards animal pole.
- Fig. 47.8 Amphibian body axis and Fig. 47.9 Cleavage in Frog.
- Animal Hemisphere is darker due to melanin granules.
- Vegetal Hemisphere has no melanin granules.
- Following fertilization the rearrangement of egg cytoplasm (cortical rotation)
results in the formation of body axis. The grey crescent is the marker for the dorsal side.
- First two divisions are vertical to give 4 equal blastomeres from animal to
vegetal poles. Blastocoel located in animal hemisphere.
- Third division is horizontal results in 8- cell embryo. The uneven distribution of
yolk pushes the mitotic apparatus towards the animal pole so get 4 blastomeres in animal
hemispheres that are smaller than those in vegetal hemisphere.
- This is the general cleavage pattern seen in all deutrosomes. If there is little
yolk then the blastocoel is centrally located.
Birds Fig. 47.10.
- Cleavage is restricted to yolk-free cytoplasm so yolk is uncleaved (meroblastic
cleavage). Even cleavage in sea urchins and frogs (holoblastic).
Gastrulation:
- Sea Urchin ( FIg. 47.11) , Frog ( Fig. 47.12) and Bird ( Fig. 47.13).
- Gastrulation is the movement of cells that results in the formation of the gut and a
three layered embryo.
- This occurs because of changes in cell motility, cell shape and cellular adhesions
to other cells and the extracellular matrix.
- The three-layered embryo (gastrula) is made up of three embryonic germ layers.
- Ectoderm forms outer layer of gastrula, endoderm lines digestive tract and
mesoderm partially fills space between ectoderm and endoderm.
- Invagination is the movement of cells inward and this forms a blunt ended tube
called the archenteron, which is the primitive gut. The open end of the
archenteron will become the anus and is called the blastopore. The second
opening that forms when the archenteron touches the inside of the ectoderm is the
mouth.
Organogenesis:
- Involves more localized morphogenic changes in tissue and cell shape.
- Fig. 47.14. Early organogenesis in frogs. Fig. 47.15 Chick embryo.
- Notochord develops from dorsal mesoderm.
- Dorsal ectoderm thickens due to signals from notocord and forms neural plate.
- Signals from notochord to ectoderm above result in that region becoming the
neural tube. Infolding and pinching off of neural plate generates the neural tube.
Neural crest gives rise to neural crest cells.
- Neural crest cells are found only in vertebrate embryos. They migrate out to
different parts of the body and become teeth, peripheral nerves etc.
- Neural folds are two ridges that form the lateral edges of the plate.
- Mesoderm next to notochord undergoes condensation resulting in blocks called
somites.
- Somites arranged serially along length of notochord. Some become migratory
cells and move to new location. Also form the muscles associated with axial
skeleton.
- Serial origin of axial skeleton and muscles reinforces idea that chordates
essentially segmented animals.
- Lateral to somites the mesoderm splits into 2 layers that form lining of body
cavity or coelom.
Adult derivatives of three embryonic germ layers in vertebrates are: Fig. 47.16.