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Ch. 42: Embryonic Development
FERTILIZATION = union of a sperm and an egg to form a zygote
1) Sperm has to use enzymes in its acrosome to digest its way through the corona radiata
and into the zona pellicida
2) Head of sperm binds to a specific receptor on egg’s plasma membrane
3) Triggers a quick depolarization of the egg’s plasma membrane  electrical impulse that
kills any other sperm that are attached and prevents polyspermy (“Fast Block”)
4) “Slow Block” to polyspermy then occurs, where vesicles release cortical granules into the
zona pellucida turning it into an impenetrable fertilization membrane
5) Entry of the sperm into the nucleus triggers the release of Ca+ ions, which activates the
cell’s metabolism and protein synthesis
EMBRYONIC DEVELOPMENT
Clevage (mitotis and cytokinesis with little growth in between) results in a Morula (solid ball of
cells)  Blastula (hollow ball of cells)
Eventually, gastrulation occurs and the cell dips in at one point = blastopore (which will
eventually become the anus in mammals)

Forms three germ layers:
o Endoderm  will become the digestive tract
o Ectoderm  will become skin and nervous tissue
o Mesoderm  will become most tissues in between

Surrounds the coelom = internal body cavity
DEVELOPMENTAL PROCESSES
Three processes: Growth, Cellular Differentiation, and Morphogenesis
Cellular Differentiation = when cells become specialized

Zygote cell and its first divisions are totipotent = have the ability to form any kind of
body cell

When they begin to specialize, they don’t lose genes, but turn off certain genes while
leaving others functional = differential gene expression

Involves two processes:

Cytoplasmic Segregation = division of maternal determinants between cells during the
course of mitotic events of cleavage
o Maternal Determinants = substances in the cytoplasm of the egg that contain
substances that influence the course of development

Ex. Gray Crescent of frog embryos contains different chemical signals
than other parts of the blastula

Induction = ability of one embryonic tissue to influence the development of another
tissue
o Ex. Gray Crescent of frog embryos becomes the dorsal lip of the blastopore

Primary Organizer = necessary for complete development

Cells closest to it become endoderm, then mesoderm and ectoderm
as you move away from it

Marks the dorsal side of the frog, so is necessary in determining body
symmetry
Morphogenesis = when cells associate to form tissues and tissues give rise to organs

Depends on pattern formation of cells during division

Most genes that direct pattern formation are highly conserved throughout the animal
kingdom

Egg polarity results from maternal determinants and the protein products of these genes
diffuse away from the areas of high concentration in the embryo, forming a gradient =
Morphogens

Anterior and Posterior must be established first
o Ex. Bicoid proteins are more concentrated at the anterior end, and prevents
formation of posterior end structures. Nanos protein is more concentrated at the
posterior, and is required for abdomen formation

Segmentation Patterns established second
o Gap genes = divide the anterioposterior axis into broad regions (“Gap genes
because if removed, entire segments of the embryo fail to develop”)
o Activated by the anterior/posterior morphogens, and in turn activate the pair-rule
genes


Establish boundaries for each segment
Homeotic (Hox) genes = selector genes because they select what body parts will be made
in each segment
o Homeobox = common structural feature of homeotic genes found in all animals
o Hox genes code for transcription factors to turn on specific genes for that segment

Apoptosis = programmed cell death
o Important in morphogenesis

Ex. Fingers and toes

Occurs in C. elegans 131 times during development