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.

Organ system examples of “ontogeny recapitulates phylogeny”

VertebrateKidney

PharyngealPouches

AorticArches

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 process1. Microlecithal = little yolk, blastomeres equal in size

(Mammals, Amphiouxus)2. Mesolecithal = somewhat more yolk (moderate amount).

Blastomeres are unequal in size (Amphibians, lamprey, lungfish).

3. Macrolecithal = lots of yolk (Reptiles, Birds, Elasmobranchs) • There are also terms describing the distribution of yolk

within the egg:1. Oligolecithal = yolk evenly distributed (microlecithal)2. 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-).

Fig 5.2 – Cleavage stages in chordates

= holoblastic equal

= holoblastic unequal

= discoidal

= holoblastic equal

Fig 5.3 – Holoblastic unequal cleavage in the bowfin, Amia

Fig 5.4 – Discoidal cleavage in the Zebrafish, Danio rara

http://www.luc.edu/faculty/wwasser/dev/cleavage.mov

Amphibian Cleavage Video

Blastulae1. Microlecithal (Amphioxus) hollow sphere2. 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

Blastulae4. 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.