Summary of Developmental Biology given in 3rd Bachelor of Biomedical Sciences at KU Leuven. Made in 2024. Taught by 3 proffen: An Zwijsen, Callaerts Patrick and tylzanowski przemko.
Developmental Biology
Chapter 1: Fate and Potency
Fate = Describes what a cell will normally develop into. However, having a particular fate does
not imply that the cell could not develop differently if placed in a different environment.
Fate map = Map or diagram based on having followed cell lineage from specific regions of the
embryo to “map” larval or adult structures into the region of the embryo from which they arose.
(=lineage tracing) To study the embryonic origin of various adult tissues and structures. The
"fate" of each cell or group of cells is mapped onto the embryo, showing which parts of the
embryo will develop into which tissue.
• Symmetric divisions = the daughter cells have the same fate
• Asymmetric divisions = the daughter cells have different fates
• Blastomeres = cells of the zygote that will undergo cleavage (controlled by maternal
factors). So this is the first thing that happens after fertilization.
• Embryonic cleavage = The embryo divides into smaller and smaller cells without the
cytoplasmic volume being increased. (no gap phases: no cell growth or protein expression)
Patterns of embryonic cleavage
1. The amount and distribution of the yolk determines at which side (so where) cleavage will
occur and also the relative size of the blastomeres.
2. Factors in the cytoplasm influence the angle of the mitotic spindle and timing of its formation.
Holoblastic cleavage involves the division of the entire egg into blastomeres. The patterns
followed by these cells during cleavage are radial, spiral, rotational, and bilateral.
Meroblastic cleavage involves an early separation between cells at the animal pole and yolk at
the vegetal pole of the egg.
1
,• Vegetal pole = yolk-rich
• Animal pole = cleavage side (less yolk)
• Synctium = a single cell or cytoplasmic mass containing several nuclei, formed by fusion of
cells or by division of nuclei.
Differentiation
= generation of cellular diversity or specialized cell types
Restriction vs. potency
= 2 opposing theories, the potency theory is the right one.
→ Restriction = cells lose all genes except those relevant for their task
→ Potency = the whole genome stays intact, but genes are selectively turned on or off.
Spemann experiment with salamander embryo’s:
=> at 16 cell stage, 1 nucleus was separated from the other nuclei and it was demonstrated that
this 1 nucleus still could develop into a complete embryo. = toti-potent cells
2
,Waddington’s landscape
= concept to demonstrate that cells in an early stage still have the potency to become every type
of cell (= embryonic stem cells) (~ top of the hill), but that during differentiation cells become
more and more restricted to one specific fate (~ going downhill following different paths)
Induced pluripotent stem cells = a type of pluripotent stem cells derived from adult somatic
cells that have been genetically reprogrammed to an embryonic stem cell-like state through the
forced expression of genes and transcription factors important for maintaining the defining
properties of ES cells.
Germ layers
1. Ectoderm (epidermis, CNS)
2. Mesoderm (cardiovascular system, urogenital system, muscles, bone and cartilage)
3. Endoderm (lungs, GI system and associated organs like pancreas, liver, etc.)
=> pluripotent cells will give rise to the 3 different germ layers, and multipotent cells belong to
the same germ layer.
Specification and determination
Specification = The first stage of commitment of a cell or tissue fate during which the cell or
tissue is capable of differentiating autonomously. Specification of (a) cell(s) can be tested when
the cell or tissue is isolated and cultured in a neutral milieu. In this neutral medium, the
autonomous differentiation can be studied. Cell commitment can still be reversed, because
when these cells are placed into a different environment with different neighbors and other
external signals its fate can still be altered.
Determination = Stage of commitment following specification. The determined stage is when a
cell or a tissue is capable of differentiating autonomously even when placed into a non-neutral
environment (so into another environment). The determination stage is often considered
irreversible. The fate is determined and cannot be changed anymore.
3
, Transplantation experiments:
The animal cap assay:
Activin = morphogen
blastula = pluripotent
neutral medium/environment with saline
=> depending on the different concentrations
of Activin cells will differentiate into different
cell types.
=> Specification
During commitment a cell may look
indifferent from its neighbors, but its
developmental fate may have already
become restricted.
Commitment = first specification, then
determination
Transcription factors are often the
determining factors that can turn on the
differentiated cell type features by
regulating expression in a positive or
negative fashion.
= differential gene expression
Genomic equivalence = every somatic cell contains the complete genome
Growth = disproportional during development
Migration
4
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