Biology Ch. 47

• Four basic processes mold embryos: cell proliferation, cell specialization, cell interaction, and cell movement.
• Development occurs at many points in the life cycle of an animal.
• Embryonic development involves shared stages, occurring in a set order in many animal species.
• Biologists use model organisms to study development because they are easy to study in the lab.

Fertilization

• Fertilization results in the formation of a diploid zygote from a haploid egg and sperm.
• Molecules and events at the egg surface are crucial to the fertilization process.
• Sperm penetrates the egg's protective layer.
• Egg surface receptors bind to sperm molecules.
• Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei.

Acrosomal Reaction

• The acrosomal reaction is triggered when the sperm meets the egg.
• The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg.
• Recognition between the sperm and egg triggers fusion of plasma membranes.
• Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy.
• Polyspermy block does not occur in mammals.

Cortical Reaction

• Fusion initiates the cortical reaction.
• Seconds after sperm binding, vesicles beneath the egg plasma membrane release contents, forming a fertilization envelope.
• The fertilization envelope acts as a slow block to polyspermy.
• The cortical reaction requires a high concentration of calcium ions (Ca2+) in the egg.
• A change in Ca2+ concentration triggers the cortical reaction.
• Ca2+ spreading correlates with the fertilization envelope's appearance.

Egg Activation

• A rise in calcium concentration increases cellular respiration and protein synthesis in the egg cell.
• Rapid metabolic changes activate the egg.
• The sperm nucleus fuses with the egg nucleus.
• Cell division begins about 90 minutes after fertilization.
• Proteins and mRNAs needed for activation are already present in the egg.

Fertilization in Mammals

• Fertilization in mammals and terrestrial animals is internal
• Sperm travels through a layer of follicle cells surrounding the egg to reach the zona pellucida.
• Sperm binding triggers a cortical reaction.
• The first cell division occurs 12-36 hours after sperm binding.

Cleavage

• Fertilization is followed by cleavage, a period of rapid cell division without growth.
• Cleavage partitions the cytoplasm of one large cell into smaller cells called blastomeres.
• The blastula is a ball of cells with a fluid-filled cavity called a blastocoel.

Cleavage Patterns in Frogs

• Cleavage is asymmetric due to the distribution of yolk in frogs.
• The vegetal pole has more yolk, and the animal pole has less.
• Yolk distribution greatly affects the cleavage pattern.
• The first two cleavage furrows in the frog form four equally sized blastomeres.
• The third cleavage is asymmetric, forming unequally sized blastomeres because of the yolk.

Cleavage Patterns in Other Animals

• Holoblastic cleavage involves the complete division of the egg.
• It occurs in species with little or moderate yolk (sea urchins, frogs, mammals).
• Meroblastic cleavage involves the incomplete division of the egg.
• It occurs in species with yolk-rich eggs (reptiles and birds).
• In Drosophila and other insects, multiple rounds of mitosis occur without cytokinesis.
• Initial development is carried out by RNA and proteins deposited in the egg.
• After cleavage, the egg cytoplasm divides among many blastomeres, each capable of producing RNA to program cell metabolism and development.

Morphogenesis

• Morphogenesis is the process by which the animal body takes shape.
• Gastrulation involves the movement of cells from the blastula surface into the embryo's interior.
• Organogenesis is the formation of organs.

Gastrulation

• Gastrulation rearranges blastula cells into a three-layered embryo called a gastrula.
• Cells take up new positions and acquire neighbors during gastrulation.
• The three layers produced by gastrulation are called embryonic germ layers.
• The ectoderm forms the outer layer.
• The endoderm lines the digestive tract.
• The mesoderm partly fills the space between the endoderm and ectoderm.
• Diploblasts have ectoderm and endoderm, and triploblasts also have mesoderm.

Gastrulation in Frogs

• Each germ layer contributes to a set of structures in the adult animal.
• Frog gastrulation begins when cells on the dorsal side of the blastula invaginate.
• This position is opposite to the position where the sperm entered the egg.

Gastrulation in Chicks

• The embryo is composed of an upper epiblast and a lower hypoblast layer prior to gastrulation.
• During gastrulation, epiblast cells move towards the midline of the blastoderm and then into the embryo toward the yolk.
• The midline thickens to form the primitive streak.
• The hypoblast cells contribute to the sac that surrounds the yolk and a connection between the yolk and the embryo, but does not contribute to the embryo itself.

Gastrulation in Humans

• Human eggs are small, with very little yolk.
• Fertilization occurs in the oviduct, and development begins as the embryo moves to the uterus.
• A blastocyst, the human equivalent of the blastula, is formed.
• The inner cell mass is a cluster of cells at one end of the blastocyst.
• The trophoblast is the outer epithelial layer of the blastocyst that initiates implantation but does not contribute to the embryo.
• The inner cell mass forms a disk with an inner epibalst layer, and an outer hypoblast layer.
• Following extraembryonic implantation the trophoblast continues to expand, and a set of membranes are formed.
• These enclose specialized structures outside of the embryo.
• Gastrulation involves the inward movement from the epiblast through a primitive streak.
• After gastrulation, the embryonic germ layers have formed.

Developmental Adaptations of Amniotes

• Land vertebrates form four extraembryonic membranes: chorion, allantois, amnion, and yolk sac.
• These membranes provide a life-support system for further embryo development.
• The chorion functions in gas exchange.
• The amnion encloses the amniotic fluid.
• The allantois disposes of waste and contributes to gas exchange.
• The yolk sac encloses the yolk.
• Reproduction outside aqueous environments required the development of the shelled egg of birds/reptiles or the uterus of marsupial/eutherian mammals.
• In both, embryos are surrounded by fluid in the amnion.
• Reproduction is allowed on try land because of this.
• Mammals and reptiles including birds are called amniotes for this reason.

Organogenesis

During organogenesis, various regions of the germ layers develop into rudimentary organs.

• Cells may change shape or migrate to a new location.
• Neurulation is the formation of the brain and spinal cord in vertebrates.
• Neurulation begins as cells from the dorsal mesoderm form the notochord.
• The notochord is a rod extending along the embryo's dorsal side.
• Signaling molecules secreted by the allantois, notochord, and tissues cause to neural plate

Neurulation

• The neural plate rolls itself into the neural tube which becomes the central nervous system (brain & spinal cord) after rolling into a tube.
• Notochord disappears before birth but contributes to parts of the disks between the vertebrae.

Cell Migration in Organogenesis

• Neural crest cells develop along the neural tube of vertebrates and eventually detach and migrate throughout the body
• They form various parts of the embryo like nerves, parts of the teeth, and skull bones.
• Mesoderm lateral to the notochord forms blocks called somites.
• The somites dissociate to form mesenchyme cells, forming the vertebrae, ribs, and muscles associated with the vertebral column

The Cytoskeleton in Morphogenesis

• Cytoskeleton movements can change cell shape or enable migration.
• Microtubules and microfilaments of the cytoskeleton are essential to these events.

Cell Shape Changes in Morphogenesis

• Reorganizing the cytoskeleton changes cell shape during development.
• The contraction of actin filaments at the end of cells cause them to become wedge shaped
• The cytoskeleton also directs convergent extension.
• This is a sheet of cells rearranges to form a longer, narrower shape.
• Cells elongate and wedge between to form columns of cells.

Cell Migration in Morphogenesis

• The cytoskeleton is responsible for cell migration.
• Transmembrane glycoproteins called cell adhesion molecules mediate this.
• Migration involves the extracellular matrix (ECM), a meshwork of glycoproteins and molecules lying outside the plasma membrane of cells.

Programmed Cell Death

• Programmed cell death is also called apoptosis.
• Apoptosis occurs at individual cells, sets of cells, or whole tissues stop developing, die, and are engulfed by neighboring cells.
  • Example: Extra neurons
• Some structures function in early stages but eliminated during later development e.g. tadpole's tail during metamorphosis

Cell Fate

• Determination is the process by which a cell commits to a particular fate.
• Differentiation refers to the resulting specialization in structure and function.
• Cells in a multicellular organism share the same genome.
• Differences in cell types are only from the expression of different sets of genes.

Fate Mapping

• Fate maps are diagrams showing organs/structures that arise from embryo regions.
• Researchers mark individual blastomeres during cleavage and follow the descendants of that cell.
• Ablation (destruction) is used on C. elegans to determine which structures arise from each cell.
• Researchers determined the lineage of each of the 959 somatic cells in the worm.
• Germ cells are specialized cells that give rise to eggs or sperm.
• In C. elegans, complexes (P granules) persist in germ cells throughout development.
• P granules are partitioned into the posterior-most cells with each cleavage.
• P granules act as cytoplasmic determinants, fixing germ cell fate.

Axis Formation

• Bilateral symmetry is found across animals.
• Animal's body plans exhibits asymmetry across the dorsal-ventral and anterior-posterior axes.
• The right-left axis is largely symmetrical.

Axis Fomration in Frogs

• The anterior-posterior axis of the frog embryo is determined during oogenesis.
• The animal-vegetal asymmetry dictates where the anterior-posterior axis forms.
• The dorsal-ventral axis is determined at random.
• Fusion with sperm makes the egg surface rotate with respect to the inner cytoplasm.
• This cortical rotaion brings molecules from one vegetable context with molecules in the inner cytoplasm.
• Cortical rotation leads to expression of dorsal- and ventral- genes.

Axis Formation in Birds, Mammals, and Insects

• In chicks, gravity is involved in establishing the anterior-posterior axis.
• Later, pH differences between the two sides of the blastoderm establish the dorsal-ventral axis.
• In zebrafish, signals in the embryo gradually establish the anterior-posterior axis over a day.
• In insects, morphogen gradients establish the anterior-posterior and dorsal-ventral axes.

Developmental Potential

• Hans Spemann experimented to determine potential
• The first two amphibian blastomeres are toipotent (all cell types can develop)
• Mammal cells reamin totipoten untul eight stages
• Ability to regulate envrinment cause mammal cells to regulate fate
• Specific cells fixed by by the late gastrula stege
• As the fate of cells becomes more clear, cells gain fate by induction'
• Inductive response - signal to to differentiate in type of cell

The "Organizer" of Spemann and Mangold

• Early stages of transplantation caused two types of gastrula and dorsal
• Dorsal flip creates an orgnizer of the embryo

Vertebreat Limb

• Pattern formation caused by signals of arrangement of tissues

• Cues are molecular

• Axes:

  • Proxminal Distal Axis

• Ectodermal ridge is very thick (AER)

• The zone is very active

• (ZPA)- tissue under the ectoderm near body development of ZPA

  • Hedgehog helps form ZPAs

• Hox proteins affect expression

This lesson explores key processes in fertilization and embryonic development. It covers topics such as the cortical reaction, gastrulation, induction, the role of the cytoskeleton, and the acrosomal reaction. Understanding these processes is crucial for comprehending developmental biology.