Transcript Vertebrate Embryology

Embryology
• The study of embryos, encompasses the study
of the development of animals
• Deals with ontogenetic development =
individual organism development, rather than
phylogenetic development = evolutionary
history of an organism
Stages in Ontogenetic Development
1. Gametogenesis = formation and maturation
of sperm and egg (1N = haploid)
2. Fertilization = fusion of sperm and egg to
produce diploid (2N) zygote
3. Cleavage = mitotic cell division of early
embryo, eventually forming a blastula or
blastodisc
Stages in Ontogenetic Development
4. Gastrulation = migration and displacement of single
layer of surface cells, still mitotically active, so that
three distinct layers are usually formed.
• These layers are the Primary Germ Layers  all tissues
and organs in the adult organism may be traced back to
these three layers.
– Ectoderm = external layer. Gives rise to skin and nervous
system.
– Mesoderm = middle layer. Gives rise to muscles,
circulatory system, most of the skeleton, excretory and
reproductive systems, etc.
– Endoderm = innermost layer. Gives rise to digestive tract
and derivatives (lungs, liver, etc.)
Stages in Ontogenetic Development
5. Organogenesis = continuous masses of cells in
the 3 primary germ layers become split into
smaller groups of cells  each of which will
develop into a specific organ or body part of the
animal.
•
Early formation = organ rudiments
6. Growth and Differentiation = growth of organ
rudiments and acquisition of structure and
physiochemical properties allowing them to
function as adult structures.
Stages in Ontogenetic Development
• General Rule: In ontogenetic development,
general features common to all members of a
lineage of animals develop earlier in the embryo
than the more specialized or unique features
characteristic of specific members of the group.
• EXAMPLE: Features characteristic of all
vertebrates (brain and spinal cord, notochord and
vertebrae, segmented muscles) appear earlier in
development than the features distinguishing
various smaller groups (limbs in tetrapods, hair in
mammals, feathers in birds), and these appear
earlier than characters distinguishing Families,
Genera, and Species.
Stages in Ontogenetic Development
• In terms of evolutionary theory – features of ancient origin
appear earlier in development than features of more recent
origin.
– This spawned the historic idea that “ontogeny recapitulates phylogeny”
(formulated by von Baer, 1828; popularized by Ernst Haeckel, 1868  became
known as the biogenetic law), or that in its embryonic development, the
organism passed through previous stages in its evolutionary history.
• However, this “review” is not complete, as many stages in
phylogeny are not present in embryonic development, and
there are modifications in ontogenetic development that
serve as adaptations of the embryo to its environment (e.g.,
extraembryonic membranes).
– Thus, this idea has been disproved regarding the organism as a whole.
• Despite the incomplete review, there are several anatomical
characters or organ systems where this “review” is very
important in defining the evolutionary history of organ
systems.
– Examples include: vertebrate kidney, pharyngeal arches, aortic arches.
Vertebrate
Kidney
Pharyngeal
Pouches
Aortic
Arches
Organ system examples of
“ontogeny recapitulates
phylogeny”
Details of Ontogenetic Development
• Cleavage - During the early stages of cleavage, the cells
(known as blastomeres) show very little growth (e.g.,
zygote and blastula are about the same size).
• Egg Types are important in determining the nature of the
cleavage process
1.
2.
3.
Microlecithal = little yolk, blastomeres equal in size (Mammals,
Amphiouxus)
Mesolecithal = somewhat more yolk (moderate amount).
Blastomeres are unequal in size (Amphibians, lamprey,
lungfish).
Macrolecithal = lots of yolk (Reptiles, Birds, Elasmobranchs)
• There are also terms describing the distribution of yolk
within the egg:
1.
2.
Oligolecithal = yolk evenly distributed (microlecithal)
Telolecithal = yolk concentrated in one hemisphere (meso- and
macrolecithal)
Details of Ontogenetic Development
• Fertilization initiates redistribution of
cytoplasmic contents within the zygote, so
that gradients of cytoplasmic substances exist.
• This results in polarity of the egg: Animal Pole
in relatively clear cytoplasm dorsally, Vegetal
Pole in yolky region ventrally.
• Cleavage results in separation of cytoplasmic
substances previously oriented in gradients
within the zygote.
Cleavage Types
• Holoblastic = total cleavage. The entire egg
divides, as do successive blastomeres.
– Equal = microlecithal eggs; dividing cells are equal in
size
– Unequal = mesolecithal eggs; dividing cells ventrally are
larger than those dorsally
• Meroblastic (Discoidal) = division occurs only in a
small area at the animal pole (becomes the
blastodisc).
• Oligo (Micro)  Holoblastic equal cleavage
(Mammals, Amphioxus)
• Telo (Meso)  Holoblastic unequal (Amphibians)
• Telo (Macro)  Discoidal (Reptiles, Birds,
Elasmobranchs)
Cleavage Patterns
• Regular progression of cleavage divisions:
– Vertical plane → produces 2 cells
– Vertical plane, but rotated 90° → 4 cells
– Horizontal plane → 8 cells
• Position of upper cells relative to lower cells
during cleavage is important to classification:
– Radial Cleavage = cleavages are symmetrical to the first
(Echinoderms and Chordates – cleavage pattern shows
link between these groups, both deuterostomes).
– Spiral Cleavage = cleavages are rotated from thje first
(Annelids, Molluscs, some other invertebrates →
separate evolutionary lineage: Protostomes)
• End product of cleavage is the blastula (micro-,
meso-) or blastodisc (macro-).
= holoblastic equal
= holoblastic unequal
= discoidal
= holoblastic equal
Fig 5.2 – Cleavage stages in chordates
Fig 5.3 – Holoblastic unequal cleavage in the bowfin, Amia
Fig 5.4 – Discoidal cleavage in the Zebrafish, Danio rara
Amphibian Cleavage Video
http://www.luc.edu/faculty/wwasser/dev/cleavage.mov
Blastulae
1. Microlecithal (Amphioxus)  hollow sphere
2. Mesolecithal (Amphibian)  hollow sphere,
wall is several layers thick.
3. Macrolecithal  blastula forms as a plate,
several cell layers thick, on top of the yolk mass
(blastula termed a blastodisc).
• 2 areas of the blastodisc:
– Area opaca = peripheral portion of blastodisc
• attached to the yolk mass
• involved in digestion of yolk and formation of the
extraembryonic membranes.
• doesn't contribute to the embryo
– Area pellucida = central part of blastodisc
• becomes lifted off the yolk mass
• forms the actual embryo
Blastulae
4. Microlecithal (mammals)  blastula becomes
specialized for placental attachment.
• Early division (cleavage) similar to that in
Amphioxus, but later 2 distinct groups of cells
develop:
– Trophoblast = expanded sphere of cells (similar to
Amphioxus blastula)
– Inner Cell Mass = mass of cells lying directly on top of
blastocoel. Similar to macrolecithal blastodisc.
• Trophoblast becomes extraembryonic membranes,
which form the embryonic side of the placenta.
• Inner Cell Mass develops into the embryo.
Fig 5.5 – Cleavage and blastulae in living mammals
Chemical Changes During Cleavage
• Ratio of Nuclear (DNA) to Cytoplasmic Material
– very low in zygote
– reaches adult cell levels, essentially without growth, by blastula
stage.
• Therefore – the amount of DNA in the embryo increases as
division (cleavage) proceeds. Where does this DNA
originate?
– Czihak et al. (1967) - Experiment: radioactive Uridine was given
to early sea urchin embryos  some of it becomes incorporated
into DNA (indicates conversion of RNA to DNA). Later research
showed that conversion was due to the enzyme ribonuclease
reductase, present in the zygote.
– So ... one source of increased DNA is from the RNA, present in the
cytoplasm of the zygote, which is converted to DNA.
– Grant (1958) - Experiment: 14C-glycine injected into zygotes 
some becomes incorporated into DNA by serving as a precursor
in purine synthesis.
– A second source of DNA are precursors (amino acids) present in
the zygote (purines = A,G. pyrimidines = C,T,U).
Chemical Changes During Cleavage
• Protein Synthesis - mostly proteins directly
involved in cell multiplication (e.g., histones,
tubulin  microtubules, ribonucleotide reductase
{RNA→DNA}).
• Experimental:
– Treat cleaving eggs with puromycin (which inhibits
RNA-dependent protein synthesis)  cleavage stops.
– Treat cleaving eggs with Actinomycin D (which inhibits
RNA production) cleavage proceeds normally.
• Conclusion: Protein synthesis uses RNA (all three
varieties) already present in the zygote.