Chapter 47 Bio 109

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prevailing developmental theory of the 18th century

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preformation

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current developmental theory

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epigenesis

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Developmental theory in which there is progressive development

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epigenesis

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An organisms development is determined by

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its genome
the differentiation of its molecules

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Is the specialization of cells in their structure and function

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cell differentiation

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Process in which the cell takes shape

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morphogenesis

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Major steps in development

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fertilization
Cleavage
Gastrulation
Organogenesis

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Main function of fertilization

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to bring the haploid nuclei of sperm and egg together to form a diploid zygote

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activates the egg

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fertilization

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modifies the zona pellucida as a slow block to polyspermy

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cortical reaction

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Cortical Reaction process

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1. The sperm migrates through the coat of follicle cells and binds to receptor molecules in the zona pellucida of the egg. (Receptor molecules are not shown here.)
2. This binding induces the acrosomal reaction, in which the sperm releases hydrolytic enzymes into the zona pellucida.
3. Breakdown of the zona pellucida by these enzymes allows the sperm to reach the plasma membrane of the egg. Membrane proteins of the sperm bind to receptors on the egg membrane, and the two membranes fuse.
4. The nucleus and other components of the sperm cell enter the egg.
5. Enzymes released during the cortical reaction harden the zona pellucida
6. which now functions as a block to polyspermy.

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in humans what part of sperm comes into the egg

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the entire sperm

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along with 1/2 the genomen human sperm contributes

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centriole

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fertilization is followed by

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cleavage

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A period of rapid cell division without overall cell growth occurs

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cleavage

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Cleavage partitions the cytoplasm of one large cell into many smaller cells called

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blastomeres

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16-64 cell stage

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morula

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the eggs and zygoes of many animals except mammals have

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definate polarity

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2 poles

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vegal
Animal

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yolk pole

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vegal

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embryo pole

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animal

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Occurs in species whose eggs have little or moderate amounts of yolk , such as sea urchins and frogs

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holoblastic

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Occurs in yolk rich eggs

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Meroblastic

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rotate toward the point of sperm entry

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plasma membrane and point of cortex

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is exposed and marks the dorsal surface of the embryo

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gray crescent

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Cleavage occurs more rapidly in the

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animal pole than in the vegetal pole

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the first two cleavages are

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vertical

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third cleavage is

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horizontal

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16-64 cells near animal pole divide more frequently solid ball of cells

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morula

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The bastocoel forms within the morula creating a

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blastula

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in meroblastic most of the cells volume is yolk with but there is a small disk of cytoplasm were cells divide more rapidly called

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zygote

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a
mass of cells that rests on of the yolk mass

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blastoderm

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three germ layers

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ectoderm
mesoderm
endoderm

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outermost germ layer

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ectoderm

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Lines the embryonic digestive tract

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mesoderm

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Partly fills the space between the endoderm and ectoderm

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mesoderm

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Rearranges the cells of a blastula into a three-layered embryo, called a gastrula, that has a primitive gut

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gastrulation

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a three-layered embryo that has a primative gut

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gastrula

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Gastrulation
begins with

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a blastula

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the _______ of blastopore (small indented crease) pushes inward (involution)

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dorsal
lip

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inward push of blastophore

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involution

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process of gastrulation

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1.Gastrulation begins with a blastula-dorsal lip of blastopore (small
indented crease) pushes inward (involution)
2.Lip of blastophore starts becoming circular. Three germ layers start
forming. Blastocoel shrinks
3.Late gastrulation. Circular blastopore surrounds
yolk plug

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upper layer in chick

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epiblast

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Lower chick layer

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hypoblast

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The notochord forms from

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mesoderm

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the neural plate forms from

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ectoderm

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Mesoderm lateral to the notochord Forms

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blocks called somites

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Organogenesis

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Early organogenesis. The
archenteron forms when lateral folds pinch the embryo away from the yolk. The
embryo remains open to the yolk, attached by the yolk stalk, about midway along
its length, as shown in this cross section. The notochord, neural tube, and  somites subsequently form much as they do in
the frog.
Late organogenesis. Rudiments of most major organs have already formed in this chick
embryo, which is about 56 hours old and about 2–3 mm long (LM).

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organisms that Develop within a fluid-filled sac that is contained with a shell or
the uterus

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amniotes

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amniotes extra embryonic membranes

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chorion
amnion
Allantois
yolk sac

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protects
the embryo in a fluid-filled cavity that prevents dehydration and cushions
mechanical shock. (meso and ecto)

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amnion

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meso and ectoderms make up

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amion
chorion

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functions as a disposal sac for certain metabolic wastes produced by the embryo. The membrane of the allantois also functions with the chorion as a respiratory organ.

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allantois

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meso, ecto, endo make up

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allantois

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the membrane of the allantois exchange gases between the embryo and
the surrounding air. Oxygen and carbon dioxide diffuse freely across the egg’s
shell.\

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chorion

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a stockpile of nutrients stored in the egg. Blood vessels in the yolk sac membrane transport nutrients from the yolk into the embryo. Other nutrients are stored in the albumen (the “egg white”).

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yolk sac

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composed of endo and meso

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yolk sac

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mammals exhibit _______ cleavage

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holoblastic cleavage

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mammals eggs show ____ polarity

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no obvious

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Early embryonic development in human has four stages

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(a) After completion of cleavage, Blastocyst reaches the uterus
(b) The trophoblast, the outer epithelium of the blastocyst. Initiates implantation in the uterus and the blastocyst forms a flat disk of cells. the Blastocyst implants
(c) As implantation is completed, gastrulation begins. and the extra-embryonic membranes start to form
(d) By the end of gasturaltion. A three layered embryo with four extra-embryonic membranes has formed

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developmental fate of cells Two general principles

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a) During early cleavage divisions embryonic cells must somehow become different from one another
b) Once initial cells asymmetries are set up subsequent interactions among the embryonic cells influence their fate, usually by causing changes in gene expression (induction)

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are general territorial diagrams of embryonic development-

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fate maps

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Axes of the basic body plan In nonamniotic vertebrates

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Basic instructions for establishing the body axes are set down early, during oogenesis or fertilization.

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Axes of the basic body plan In amniotes,

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local environmental differences Play a major role in establishing initial differences between cells and later the body axes

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Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres. Result is

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(c) Blastomeres that receive half or all of the gray crescent develop into normal embryos

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Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere.

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(c) blastomere that receives none of the gray crescent gives rise to an abnormal embryo without dorsal structures.

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Result of additional dorsal lip

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(2) Grafting the dorsal lip of one embryo onto the ventral surface of another embryo results in the development of a second notochord and neural tube at the site
of the graft.

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Secretes fibroblast growth factor (FGF) proteins.

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i) Apical ectodermal ridge (AER

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Required for limb growth and patterning along the proximal-distal axis.

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AER

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Required for pattern formation along the dorsal-ventral axis.

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AER

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Secretes Sonic hedgehog, a protein growth factor

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Zone of polarizing activity (ZPA)

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Required for pattern formation of the limb along the anterior posterior axis

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ZPA