History of Embryology

BIO 308 - Embryology

Embryology

• What is in a name?

– Old name - Developmental Anatomy

– New name - Embryology

– Other names - Developmental Biology, Experimental Embryology

Ways of Studying Embryology

• Anatomical approaches

• Experimental approaches

• Genetic approaches

Comparative Embryology
An Anatomical Approach

• Aristotle’s work - 4th century B.C.E.

– Oviparity - born from eggs - birds, frogs and most invertebrates

– Viviparity - live birth - eutherian mammals, or ovoviviparity (eggs hatches inside the body as in certain reptiles and sharks)

Comparative Embryology

• Aristotle’s work

– Cells division patterns

• Holoblastic cleavage - entire egg divides into smaller cells (frogs and mammals)

• Meroblastic cleavage - only part of the egg is destined to become the the embryo - other portion is the yoke for nutrition (chicks)

– Also discovered the function of the placenta and umbilical cord

William Harvey

• 1651 -All animals originate from eggs

• First to see blastoderm of chick

• Also saw islands of bloods cells forming before the heart

Marcello Malpighi

• 1672 - published microscopic account of chick development

– Neural groove (becomes neural tube)

– Muscle-forming somites

– First circulation of the arteries and veins - to and from the yoke

Great debates of embryology - Epigenesis

• Aristotle and Harvey

• Organs formed de novo - “from scratch”

• Kaspar Friedrich Wolff (1767) - observed chick development - heart and blood vessels develop anew in each embryo

Great debates of embryology - Preformation

• Organs in miniature form within the egg and sperm

• Support from Malpighi

• Organisms did not develop but rather “unrolled”

• Before cell theory - no lower limit to cell size

The end of Preformationism
Primary Germ Layers

• Discovered by Pander

• Triploblastic - three layers

• Ectoderm - outer layer of embryo

– Forms skin & nerves

• Endoderm - innermost layer

– Digestive tube & associated organs (also lungs)

• Mesoderm - sandwiched between ectoderm and endoderm

– Blood, heart, kidneys, gonads, bones and connective tissue

• Diploblastic - two layers - porifera & cnidarians

Karl Ernst von Baer

Extended Panders’s work!

Four Principles

1. General features of a large group of animals appear earlier in development than do the specialized features of a smaller group

• All vertebrate embryos have gill arches, notochords, spinal cords and primitive kidneys

Karl Ernst von Baer

Four Principles

2. Less general characteristics are developed from the more general, until finally the most specialized appear

• All vertebrates initially have the same kind of skin - only later does the skin develop fish scales, reptilian scales, bird feathers or hair, claws and nails of mammals

Karl Ernst von Baer

Four Principles

3. The embryo of a given species, instead of passing through the adult stages of lower animals, departs more and more from them

• Visceral clefts of embryonic birds - not like gill slits in adult fish - rather they resemble visceral clefts of embryonic fish & other embryonic vertebrates

• They become gill slits in fish and eustachian tubes in mammals

Karl Ernst von Baer

Four Principles

4. Therefore the embryo of a higher animal is never like a lower animal, but only like its early embryo.

• Human embryos never pass through a stage equivalent to an adult fish or bird

• Early human embryos share characteristics in common with fish and bird embryos

Fate Mapping

• Important program in descriptive embryology - tracing cell lineages

• Sometime single cells can be followed - in others groups of cells are labeled - then you see what that area of the embryo becomes

• Fate maps - these diagrams map larval or adult structures onto regions of the embryo from which it arose

Gastrulation Fate Maps

Vital Dye Marking

• Vogt (1929) - traced fates of amphibian eggs with vital dyes

• Mixed dye and agar on micro slides

• Placed small piece of agar in embryo to stain cells

• Could then trace cell movements

Radioactive Labeling

• One embryo injected with radioactive thymidine

• Second (host) embryo is grown under similar conditions

• Section of interest removed from radioactive embryo and grafted onto the host embryo

• Viewed via autoradiography - photographic emulsion coats micro slides of the embryo tissue - see grains of reduced sliver (dark specs) over the cells that are radioactive

• Fluorescent dyes can also be used

Genetic Marking (Fate Mapping)

• Create mosaic embryos from several species

• A chimeric embryo of quail cells in a chick embryo works well

• The quail cell chromatin looks different and also cell specific antigens that are quail specific - allow you to find individual quail cells in the chick embryo

Cell Migration

• Fate maps have demonstrated extensive cell migration during development

• Melanocytes originate in neural crest cells

• Mart Rawles (1940) transplanted neural crest cells from a pigmented strain into an unpigmented strain of chickens

Neural Crest Cell Migration

Evolutionary Embryology

• Darwin saw embryonic resemblances as very strong evidence for genetic connectedness of different animal groups

• Larval barnacles are very similar to larval crabs and thus barnacles are arthropods not molluscs

Evolutionary Embryology
Embryonic Homologies

• Homologous structures are those organs whose underlying similarity arises from being derived from a common ancestral structure

• The wing of a bird and the forearm of a human are homologous

• Analogous structures are whose similarity comes from their performing a similar function, rather than arising from a common ancestor

• Wing of an insect and the wing of a bird are analogous

Homologies of Structure