Embryology Cellular and Molecular Mechanisms Involved in Fertilization and Development

Sea Urchin Fertilization unfertilized sea urchin egg Contains digestive enzymes Animation 43-01 fertilized sea urchin egg Vitelline envelope Egg plasma membrane

Sea Urchin Fertilization Acrosomal enzymes digest jelly coat Bindin on acrosomal process contacts receptors on vitelline envelope Acrosomal process forms by actin polymerization Egg plasma membrane forms fertilization cone to engulf sperm head

Preventing Polyspermy Polyspermy = fertilization of egg by more than one sperm Blocked in two ways Fast block to polyspermy: Change in charge across the egg plasma membrane, mediated by influx of sodium ions Slow block to polyspermy: cortical granules release enzymes and water-absorbing materials to form fertilization envelope around egg, mediated by release of stored calcium from endoplasmic reticulum

Sea Urchin Fertilization Cortical granules fuse with plasma membrane and release Enzymes to catalyze reactions that break bonds between vitelline envelope and plasma membrane remove bindin receptors harden vitelline envelope fertilization envelope Substances that absorb water and raise vitelline envelope Sea Urchin Fertilization

Applying Your Knowledge Bindin Cortical Granule Acrosome Vitelline envelope Fertilization cone Which structure of the egg has a receptor for sperm? Which structure is directly responsible for bringing the sperm head into the egg ? Which part of the sperm contains enzymes for digesting through the jelly coat surrounding the egg?

Development A series of progressive changes that generates the structures of the organism throughout its life cycle Includes growth: change in size due to increase in cell number and/or cell size differentiation: cellular specialization morphogenesis: establishing body and organ shapes

Determination Precedes Differentiation Determination: commitment of a cell to a particular fate, occurs by the activation of specific genes Affected by Cytoplasmic segregation Induction Differentiation is the result of determination

Induction: process in which a substance or tissue influences the fate of a group of adjacent cells Optic vesicle induces lens placode Optic cup induces lens Lens induces cornea Transplant of tissue from dorsal lip of blastopore induces second invagination and formation of secondary embryo.

Embryonic Stages Cleavage: series of rapid cell divisions following fertilization, leading to morula: ball of cells blastula: hollow sphere of cells Blastocoel = fluid-filled space Blastula Morula Blastomere = individual cell of blastula

Yolk Content Affects Cleavage Patterns Blastodisc= layer of cells on top of yolk

Cytoplasmic Determinants Influence Cleavage Patterns Orientation of mitotic spindles in early cleavage is dependent on the organization of cytoplasmic determinants in the egg Mosaic Development Regulative Development Cytoplasmic Determinants Influence Cleavage Patterns

Cell Adhesion Molecules Influence Blastula Organization Cell Adhesion Molecules (CAMs) are trans-membrane cell surface proteins that bind to CAMs on other cells. CAMs are involved in cell migration and producing stable tissue structure. Experiments have shown that cadherin (one type of CAM) stabilizes the frog blastula.

Embryonic Stages Gastrulation: cellular movements forming three tissue layers and distinct body axes Invagination Sea Urchin mesoderm blood vessels, muscle, bone, liver and heart Migration nervous system, skin linings of digestive and respiratory tracts primitive gut

Sea Urchin Gastrulation

Cytoskeletal Components Directing Invagination

Embryonic Stages Gastrulation in the Frog Involution

Embryonic Stages Gastrulation in the Chick Migration

Neurulation in the Frog Notochord, derived from mesoderm, induces the overlying ectoderm to form the neural tube Notocord in Chordates First tissue to fully differentiate Forms in mid-dorsal mesoderm (chordamesoderm) Induces formation of neural tube and cranial space in nearby ectoderm (dorsal side) Replaced by spinal column (cartilage and bone from mesoderm) in vertebrates

Cytoskeletal Components Involved with Neurulation

Tissues of the Frog Neurula Differentiation of Mesoderm Mid-dorsal – chordamesoderm Dorsal – somites – segmented parts of body Intermediate – thin layer, contributes to kidney and/or testicular ducts (labeled “nephrotome” in previous slide) Lateral Plate – two sheets join at mid-ventral line Somatic – body side Splanchnic – surroundings of gut Coelom – body space between two layers of mesoderm b

Neurulation in the Chick Embryo

Applying Your Knowledge Gastrula Neurula Blastula Ectoderm Mesoderm Which one represents a hollow ball of cells? Which tissue induces formation of neural structures? Which one involves invagination or involution of cells?

Morphogenesis Pattern Formation: organization of differentiated tissues into specific structures Body segmentation in fruit flies is controlled by gradients of materials established in the egg actions of a series of embryonic genes Animation 19-03

Genes Affecting Body Segmentation Products of Maternal Effect Genes establish gradients in the egg Gap genes define broad areas and regulate Pair-rule genes Pair-rule genes refine segment locations and regulate Segment Polarity genes Segment Polarity genes determine segment boundaries and orientations Homeotic genes define the role of each segment

Mutation in a Homeotic Gene Causes Legs to Grow Where Antenna Should Be

Homology between Genes Controlling Segmentation in Fruit Flies and Mice

Morphogenesis May Require Apoptosis Apoptosis = Programmed Cell Death

Cellular and Molecular Mechanisms Essential to Development Specificity of receptors and enzymes Cytoplasmic organization Cell movement through the action of cytoskeletal elements and cell adhesion molecules Orientation of tissue layers leading to Induction Differential Gene Expression

Applying Your Knowledge Differentiation Growth Morphogenesis Which process leads to a morula? Which process leads to the formation of somites, the precursors to vertebrae? Which process is directly preceded by determination?