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28 Cards in this Set

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Ontogeny
history of life of an organism: its cycle of development, growth & maturation
Heterochrony
# Define

* change in relative rate of development of characters in phylogeny
* trait may have developed faster or slower in ancestral taxon
* displacement of a feature in an organism relative to the time that feature appeared in an ancestor

# Significance

* easy way to achieve major evolutionary change (fewer genes altered)
Paedomorphosis
#
# loss of 1 or more ontogenetic stage(s)
# larval or immature features of ancestors become adult characters
# i.e. the retention of a more flexible juvenile morphology
# Methods

* speed development of reproductive organs so sexually mature "juveniles" form
* delay or slow down development of somatic organs; its sexually mature but some other "somatic" traits look unfinished
Peramorphosis
* Define
* "old looking"
* addition of new developmental stage(s) in descendants
* increase in specialized complexity of adult
* Methods
o somatic structure appears early & develops fast
+ Example - overdeveloped/huge Irish elk antlers
o period of ontogeny extended; new structures can be added
Egg Yolk: Evolutionary Trends
* vertebrates, in general, have larger yolks in vertebrates than ancestral invertebrates or other Chordates, this correlates to the larger body sizes of many vertebrates
* microlecithal eggs are secondarily derived in therian mammals; i.e. ancestral trait was lost in most vertebrates, but "reacquired" in therian mammals
Types of Eggs (3)
# Variation - within each category listed below, the proportion of yolk varies with body size, numbers of offspring, development patterns
# See Taxonomic variation in table below
# Microlecithal (Oligolecithal) - little yolk in egg, very small

* Non Craniate Taxa - Echinodermata; Urochordata, Cephalochordata
o

# Mesolecithal - moderate amount of yolk
# Macrolecithal - large amount of yolk
Zygote - single cell after fertilization
A. Yolk Distribution

* Isolecithal?
Telolecithal?
Isolecithal

* Define - yolk divided "equally" into all future cells; seen in microlecithal eggs
* Taxa - Theria, Urochordata & Cephalochordata

Telolecithal

* Define - yolk concentrated at vegetal pole; little yolk gets in cells at animal pole
* Taxa - all vertebrates with mesolecithal & macrolecithal eggs
Cleavage

* Holoblastic?
complete cleavage through animal & vegetal poles

* possible only in eggs with little yolk (i.e. microlecithal or mesolecithal eggs)
Cleavage: Meroblastic?
incomplete, only animal pole divides into cells, yolk doesn't
Cleavage: Discoidal?
an extreme form of meroblastic cleavage

* very tiny cells are produced at animal pole; huge yolk mass
Evenness of Cell Size
# Equal - all cells are equal in size & yolk content
# Unequal- animal pole cells are small & vegetal pole (yolk) cells larger

* Pattern
o mesolecithal (yolk cells are larger than animal pole cells)
o macrolecithal (yolk mass does not divide at all)
Blastula?
Define - hollow ball of cells forms as cleavage continues
Blastula Formation: Mesolecithal (Amphibian) Pattern
# Blastomeres

* newly formed cells of this hollow ball
* upper "animal" cells, smaller than lower "yolk" filled cells, large

# Blastocoel - large hollow cavity inside blastomeres
Blastula Formation: Macrolecithal (Aves & most Amniotes) Pattern
Blastodisc

* no true blastula forms
* small cluster of tiny, (animal pole cells) on top of solid mass of yolk form blastodisc
* separates into top & bottom layers or sheets of cells with a narrow blastocoel space
o Epiblast = upper layer source of all 3 germ layers
o Hypoblast = lower layer; will be displaced by endoderm
Gastrula

A. Major Events
# Gut formation - archenteron (lining made of endoderm)
# Embryo becomes bilaterally symmetrical, elongated & cephalized

* gastrulation may overlap with neurulation

# Formation of 3 germ layers - endoderm, ectoderm & mesoderm

* ectoderm cells are the exterior layer of cells
* endoderm cells migrate inside "above" yolk cells
* mesoderm cells form a layer between endoderm & ectoderm layers
Extra Germ Layer Terminology
Mesenchyme?
Ectomesenchyme?
o mesenchyme - separate mesoderm cells surrounded by extracellular matrix (may be found in embryos & adults)
o ectomesenchyme - neural crest cells that invade mesoderm layer during development
Gastrulation: Mesolecithal (Lissamphibian) Pattern
B. Mesolecithal (Lissamphibian) Pattern

* formation of a cleft or dorsal lip
* at this location, cells move/migrate over blastula & then migrate inside blastocoel
* opening enlarges into circular blastopore - this opening will become adult anus
* animal pole cells overgrow the yolk filled cells & yolk cells fill the blastopore as a yolk plug
* blastocoel ultimately disappears
* archenteron or small, primitive gut (enteron) forms & is filled with yolk
Gastrulation: Macrolecithal (bird) Pattern
* primitive streak forms an anterior-posterior zone in midline of blastodisc
* anterior-most point of streak is called Hensen's node
* primitive streak & Hensen's node are homologous to blastopore
* epiblast cells turn inward & migrate laterally & anteriorly to form mesoderm & endoderm
* archenteron closes off slowly as embryo "lifts" off of yolk; stays attached to yolk via stalk
Coelom Formation:
Enterocelous vs Modified Enterocoelous
Enterocoelous Coelom Formation

* Taxa - primitive deuterostomes (Echinoderms, primitive chordates) & anterior region of lamprey
* Method - mesoderm folds inward as pockets off of endoderm

Modified Enterocoelous

* Taxa - most vertebrates (including posterior region of lamprey)
* Method
o superficially - solid block of mesoderm splits open to form cavity
o mimics shizocoelous coelom formation of protostomes - annelids, arthropods...
o differs from "true" schizocoely because mesoderm/endoderm in vertebrates are NOT distinct germ layers at the start of gastrulation
Functions of Coelom
# hollow body cavities lined with mesoderm
# tube (gut) within a tube (coelom space)
* membranes that line cavity produce serous fluids that provide lubrication for organ movement
* frees organs from linkage to movement of trunk
Typical Neurulation Events
# Timing - formation of the nervous system often concurrent with gastrulation
# Neural Plate - thicker region of ectoderm forms above notochord
# Neural Folds & Groove - neural ectoderm forms lateral ridges or folds & a deep pit between folds
# Neural Tube - folds sink deeper & folds grow together

* neurocoel or neural cavity remains inside tube later enlarges to form brain's ventricles

# Brain Formation

* tube grows fastest at its anterior end
* brain first expands into 3 enlarged regions & later 5
C. Atypical Neurulation
* Events
o solid keel of ectoderm sinks deep
o then neurocoel cavity forms
* Taxa
o lamprey - Cephalaspidomorphi
o teleosts - highly derived members of the Actinopterygii
Induction of Vertebrae: Normal and abnormal
Normal Event

* presence of neural tube stimulates vertebral formation
* ensures that both of these structures grow as a functional unit

Abnormalities - Anencephaly & Spina Bifida

* abnormal conditions where neural tube fails to close
* 10% of U.S. population suffers some form of spina bifida
* risk decreases when women take 400 mg folic acid daily (before pregnancy begins)
* Variation
o location along spinal cord or brain differs & severity differs
o errors in skin, meninges around spinal cord or vertebral formation
o most severe are cases with incompletely closed tube; neurons can't develop
Neural Crest (ectomesenchyme) Cells: What clades have them, location, function?
* derived trait in vertebrates
* cells that pinch off of neural tube mass early & migrate to # locations in body
* Some Derivatives - branchial skeleton; chromatophores; dentine; # nerve ganglia - sympathetic, parasympathetic, dorsal root (sensory); Schwann cells
Epidermal Placodes
* derived trait in vertebrates
* special regions of ectoderm sink beneath surface of skin
* Some Derivatives - lens of eye; otic capsule; lateral line organs; contributes to some cranial ganglia
Prototheria Development
* Taxa - monotremes - duck-billed platypus & echidna (spiny anteater)
* Typical Amniote Pattern
o Macrolecithal Egg
o Telolecithal Egg & Discoidal Cleavage
o Blastodisc forms on top of yolk
o Primitive Streak & Hensen's node - Gastrulation
Theria Development
* Taxa - marsupials & placental mammals
* Derived Features
o Microlecithal Egg - secondarily derived
o Isolecithal Yolk Distribution & Equal Cleavage - secondarily derived
o Secondarily derived means trait was present in an ancestor [primary derivation], then lost & finally reappears in modified form for second time in a descendant
Eutheria Development
* Blastocyst - new design for "blastula" stage
o Trophoblast [troph = eat]
+ assume it's not homologous to a "trophoblast" layer found in marsupials - development & location differ
+ cells form on outer sphere of cells around future embryo
+ form the earliest attachment of embryo to uterine wall
+ digest uterine tissues for nutrients to feed developing embryo before placenta forms
+ contribute to formation of placenta
o Inner Cell Mass - forms the embryo