complexity and development

how did we think the embryo forms- history

Aristotle (4th century BC):

• everything in the embryo present from beginning

• new structures arise progressively (epigenesis)

1600: preformation

• all embryos existed since the beginning of time

• Malpighi- “observed” a tiny fully formed chick in the egg from the very beginning

• homunculi exist in sperm

DNA does not provide a blueprint for embryonic development. more a series of instructions.

what a newly fertilised egg needs to do to become a fully formed organism:

-cell division

-growth

-cell death

-cell specialisation (differentiation)

-patterning (knowing where all bits go)

-cell movement

-changes in shape

-communication between cells

part 1- divide, differentiate, die

• cleavage divisions. no growth

• 1-16 cell stage: cells totipotent (can become any cell in the embryo including placenta)

• blastocyst: ICM (inner cell mass) pluripotent

evidence: in identical twins the embryo splits during early development

mouse chimeras: fuse 2 8 cell embryos together. mouse contains cells from both throughout body

-pluripotent- e.g. embryonic steam cells

-multipotent- e.g. haematopoietic stem cells (blood)

-unipotent- e.g. epidermal cells (skin)

-terminally differentiated cells- e.g. blood cells, skin

changes in gene expression are key- haemoglobin in red blood cells, keratin in skin

gene expression is activated or repressed via transcription factors binding to gene regulatory regions.

MORPHOGENESIS- creating form

requires: - cell movement, changes in cell shape, changes in cell adhesion

gastrulation: the process by which an animal embryo transforms from a one-dimensional layer of cells into a three-dimensional structure (the gastrula) with three cell layers:

• ectoderm(outer)

• mesoderm(middle)

• endoderm(inner)

PATTERNING:

-positional information

-cell-cell communication

induction: the [process where a group of cells signals to another group of cells in the embryos, affecting how they develop

organiser: a signalling centre that directs development of the whole embryo or a part of it

anterior → posterior

zone of polarising activity (ZPA): region of posterior limb bud

acts as an organiser for the AP axis of the limb

morphogen: form giving substance whose concentration varies across a gradient

• can directly activate cells at a distance

• produce concentration-dependent responses in receptive cells

HOX CODE:

homeotic selector genes:

• genes located on chromosome in order of expression

• two clusters, expressed in nested pattern

mutations in homeotic selector genes cause homeotic transformations (e.g. legs on head in flies)

hox genes are conserved from flies to vertebrates. give identity to individual segments.

• form anterior to posterior along the anteroposterior (head to tail) axis of vertebrate embryos

• precursor of trunk skeletal muscle, cartilage, tendons, dermis

• form from mesoderm

• form in pairs

• characteristic number for each species

if one hox gene is removed, transformation from that gene to the previous gene occurs