Chapter 47 Animal Development

Nicole Gallup

Embryonic Development• Genomes of zygote and differences btwn early embryonic

cells determine development

• Cytoplasmic Determinants – Uneven distribution of maternal substances in the unfertilized egg

• Differences between cells because of their location in the embryo

• Cell Differentiation – specialization of cells form and function, caused by gene expression

• Morphogenesis – process by which an embryo takes shape and cells are in the appropriate locations

Embryonic Stages• Fertilization – When Gametes (sperm and egg) unite

• Cleavage – Rapid Cell divisions after Fertilization. S phase (DNA synthesis) and M phase (mitosis). Skips protein synthesis

• Gastrulation – Morphogenetic phase Drastic rearrangement of the cells of the blastula. Forms a three-layered embryo with a primitive gut.

• Organogenesis – When regions of the three-layered embryo develop into fundamental organs

Fertilization Vocab• Acrosomal Reaction - discharge of a sperm’s acrosome when

it is near the egg

• Acrosome – Vesicle at the tip of sperm, helps sperm penetrate the egg

• Fast Block to Polyspermy – Depolarization of egg membrane after sperm binds to vitelline layer. Prevents more sperm from entering

• Fertilization Envelope - the changed vitelline layer – prevents other sperm from entering the egg

• Slow Block to Polyspermy – Formation of fertilization envelope and other changes, opposite of Fast block, lasts longer

Fertilization• Fertilize externally – eggs and sperm are released at the same time.

• Sperm touches egg’s jelly coat – triggering release of acrosome – hole is formed in jelly

• Acrosomal process forms – protrudes from sperm, penetrates jelly coat, binds to receptors on egg cell – aka acrosomal reaction

• Hole made in vitelline layer – allows contact and fusion of gamete plasma membranes – membranes depolarize forming Fast block

• Sperm nucleus enters cytoplasm of egg – then slow block forms

Cleavage Vocab• Blastomer – smaller cells that the embryo divides into

• Morula – cluster of cells after the first 5-7 divisions

• Blastocoel – a fluid filled cavity

• Blastula – hollow ball of cells

• Yolk – stored nutrients – distributed differently in all embryos

• Vegetal Pole – The pole that the yolk is most concentrated

• Animal Pole – Opposite pole, very little yolk

Cleavage• After fusion of gametes cytoplasm rearranges forming 1 body

axis. Other axes form later

• First 2 divisions are meridional (Vertical) = 4 blastomers of equal size

• Third division is equatorial (Horizontal) = 8 blastomers of unequal size – Animal hemisphere = small cells, Vegetal hemisphere = lager cells

• Blastula is located in the Animal Hemisphere

Gastrulation Vocab• Gastrula – 3 layered Embryo

• Germ Layers – The 3 layers produced.

• Ectoderm – Outer layer

• Endoderm – Inner Layer

• Mesoderm – Partly fills space between Ecto and Endo

• Invagination – When cells fold inward

• Archenteron – Primitive Gut

• Blastopore – Opening in the archenteron, develops into the anus.

Gastrulation• Complicated mechanics – Large amount of yolk & blastula is

more than 1 cell thick

• Begins on back side of Blastula – cells begin to invaginate in the line along the region

• Dorsal Lip – The Dorsal side of the blastopore

• Lip extends and invagination continues until the two ends on the blastopore meet on the ventral side

• Involution – When future endoderm and mesoderm cells on the surface roll over edge of the lip into the interior of the embryo

Gastrulation• Inside – cells move away from blastopore and become germ

layers and blastocoel collapses

• Yolk Plug – Large food-laden endodermal cells surrounded by blastopore

• End of Gastrulation, circular lip of blastopore encircles plug, cells on surface becomes the ectoderm

• Anus forms from the blastopore and mouth develops at the opposite end.

Organogenesis Vocab

• Notochord – Formed from dorsal mesoderm

• Neural Tube – when neural plate curves inward – rolling into itself

• Neural Crest – band of cells along border of Neural tube

• Somites – Paired blocks of mesoderm lateral to notochord

Organogenesis• First organs to take shape – neural tube and notochord

• Signals from notochord to ectoderm cause ectoderm to become neural plate

• Cells from neural crest migrate to all parts of the body – form peripheral nerves, teeth, skull bones

• Some somites become wandering cells – go to new locations.

• Organogenesis continues – cell differentiation continues to refine organs

Neural Plate formation

Neural Tube Formation

Somites

Morphogenesis• Major aspect of development in animals – involves

movement of cells.

• Changes in shape involve reorganization of the cytoskeleton. Cytoskeleton drives cell migration.

• Cells that move 1st drag others behind them – directs movement of a sheet if cells

• Convergent Extension – morphogenetic movement – cells of tissue layer rearrange, sheets become narrow (converge) and become longer (extend)

Extracellular Matrix• Extracellular Matrix (ECM) – Mixture of secreted glycoproteins

outside plasma membrane of cells – trigger/guide movement

• Some ECMs promote migration, providing specific molecular anchorage for moving cells

• Others keep cells on correct paths – inhibiting migration – use nonmigratory cells

• Cell Adhesion Molecules (CAMs) – glycoproteins – help cell migration and stable tissue structure

• Cadherins – important cell-to-cell adhesion molecule.

Developmental Fate of Cells

• Development requires a combo of morphogenetic changes and the timely differentiation of cells in specific location

• 2 general principles

– Early cleavage divisions – Embryonic cells must become different from each other

– Once initial cells asymmetries are set up, subsequent interactions among the embryonic cells influence their fate – usually causing changes in gene expression

A Cell’s Fate

• Fate Maps – diagram of embryonic development – reveals future development of individual cells/tissues

• A cell’s fate can be changed by moving the cell to a new location

• 2 Important conclusions

– Specific tissues of the older embryo can be attributed to certain early “founder cells”

– As development proceeds a cell’s developmental Potential becomes restricted

Establishing Cellular Asymmetries• Establishing basic body plan is 1st step in morphogenesis – a

prerequisite for the development of tissues/organs

• Totipotent – describes a cell that can become any part of an organism

• Zygote’s pattern of cleavage affects the fate of cells

• Progressive restriction of potency is a feature of development in all animals

• The tissue-specific fates of cells in late gastrula are fixed

Inductive Signals

• Cell division creates cells that differ from each other the cells then influence each other’s fate (induction)

• Pattern Formation – development of an animal’s spatial organization, arrangement of organs/tissues – influenced by inductive signals

• Positional Information – Molecular cues – control pattern formation

Limbs• Limbs begin as bumps of tissue called Limb buds

• Buds – consist of a core of mesoderm tissue covered by a layer of ectoderm – 2 organizer locations affect limb’s development

• Apical Ectodermal Ridge (AER) – 1 organizer – thickened area of ectoderm at the tip of the bud

• Zone of Polarizing Activity (ZPA) – other organizer – block of mesodermal tissue located underneath ectoderm – posterior side of the bud is attached to body

Citations• http://www.vcharkarn.com/uploads/0/80.jpg• http://3.bp.blogspot.com/_NDw_XebDkYI/S7ApTP1gibI/AAAAAAAAAO4/

aYitNrMkyWo/s1600/cleavage.jpg • http://bio1152.nicerweb.com/doc/class/bio1152/Locked/media/

ch47/47_12FrogGastrulation.jpg • http://bio1151.nicerweb.com/Locked/media/

ch47/47_14FrogOrganogenesis_CL.jpg