Molecular Patterning During Development Flashcards
Define cell differentiation.
Process by which embryonic cells become
different from one another, resulting in the emergence of cell types such as muscle, nerve, skin and fat cells.
It is the achievement of a stable terminal state (not just transitory differences) and is characterized by the profile of proteins in that cell.
Identify principles governing cell differentiation.
-Generative program: The embryo does NOT contain a description of the adult, rather it contains a generative program for making it (progressive series of instructions).
-Regulatory proteins work together as a “committee” to control the expression of a eukaryotic gene
-Combinations of a few gene regulatory proteins can
generate many different cell types during development
Identify the main cell stages following fertilisation.
- Morulla (TOTIPOTENT cells, can become any tissues, including placenta)
- Blastocyst (inner cell mass made of PLURIPOTENT cells, can become any tissue except placenta + trophoblast which form placenta)
- (this only applies to cells derived from inner cell mass) Multipotent blood stem cells (can develop into more than one cell type like RBCs and WBCs, but are more limited than pluripotent cells) and other stem cells (e.g. for muscle, or for bone, etc.)
Define Potency, totipotent, pluripotent, multipotent.
Entire repertoire of cell types a particular cell can give rise to in all possible environments.
- Totipotent (toti= whole).
Identical and unrestricted; can give rise to any cell of the body (EMBRYONIC) (e.g. Cells of the very early mammalian embryo) - Pluripotent (pluri= more).
Less potent; can give rise to many cell types but not all (EMBRYONIC) (e.g. Inner cells of the blastocyst) - Multipotent (multi= many)
Give rise to cells that have a particular function (ADULT) (e.g. Blood stem cells;)
Define cell fate.
The fate of a cell describes what it will become in the course of normal development.
When a cell “chooses” a particular fate, it is said to be determined, although it still “looks” just like its undetermined neighbours. Determination implies a stable change - the fate of determined cells does not change.
Describe two stages of commitment (and briefly what happens before that).
Define competence of a cell. How can a cell lose competence.
Ability of a cell to respond to the chemical stimuli.
A cell can lose competence by changes in surface receptor or intracellular molecules
Explain how gene expression changes underlie
cell differentiation/the mechanistic basis of fate decision.
“1. Bivalent (poised or paused) chromatin comprises activating and repressing histone modifications at the same location. This combination of epigenetic marks at promoter or enhancer regions keeps genes expressed at low levels but poised for rapid activation (genes both active and silenced)
- When an ES (embryonic stem) cell receives a signal to differentiate into a specified cell lineage, activation of the specific developmental genes are needed for differentiation. The developmental genes needed will be activated and the other genes that are not required for that cell lineage will be repressed (silenced) through their bivalent domains.
- After the differentiation has taken place, genes needed for differentiation may be silenced (because no longer needed).”
- Developmental regulatory genes such as HOX, SOX, T-box are associated with transcription factors (which can be stimulatory or inhibitory, balance of the two determines how strongly the gene is expressed) which bind upstream to promoter regions and act as developmental regulators. They bind to these regulatory sequences and help differentiate some cells down particular lineage.
- Combinations of a few regulatory proteins can generate many cell types
At which cell stage can see some features change within cells to do with cell date decision ?
4 cell stage
Identify a situation where it is possible to turn determined state back.
Experimentally, can reverse that locked in pattern, take terminally differentiated cells and turn them back
into embryonic stem cells. Through Somatic Cell Reprogramming (by defined factors or therapeutic cloning)
Explain the embryonic development of the somites, myotomes and dermatomes.
Somite formation occur as paraxial mesoderm becomes segmented.
- Cells of paraxial mesoderm have an internal clock
- They go through cycles every 90 minutes defined by notch signalling clock
- Wave of signal passes through the embryo
- When the wave passes cells, they are programmed to change into part of a somite
- If the wave passes cells early in the cycle, they become the front end of the segment (head end)
- If the wave passes cells late in the cycle, they become the tail end of the segment
- Process repeated over and over
- myotome is the part of a somite that develops into the muscles
- dermatome is the part that develops into skin
Identify and define the processes of growth and ossification of fetal bones taking place in fetal development.
- Endochondrial Ossification
- Uses hyaline cartilage as the model for long bone formation.
- Radiologists can determine the skeletal age of a patient by examining the development of epiphyseal plates - Intramembraneous Ossification
- Formation of bone in fibrous connective tissue (which is formed from condensed mesenchyme cells)
- The process occurs during the formation of flat bones such as the mandible and flat bones of the skull
Define mesenchyme.
Generalised embryonic connective tissue derived from mesoderm
Describe the location and function of HOX genes.
-where: expressed along the long axis of the embryo from head to tail
-function: determine the body axis + the position of the limbs along the body axis + the shape of bone
(the products of HOX genes are transcription factors which bind to DNA, and thereby regulate the transcription of other genes (e.g. TBX5, TBX4))
Once the cranio-caudal postion is set, limb growth is regulated along three axes:
* Proximo-Distal axis
* Antero-Posterior axis
* Dorso-Ventral axis
Describe the process of development of limbs (not the role of genes in it, the process itself).
- Upper Limb buds appear on approximately day 24 between somites C5-T1
- Lower limb buds appear on approximately day 28 between somites L1-S2
-In week 7 the forelimbs rotate 90° laterally and the hind limbs rotate 90 ° medially.
* Results in the flexor compartments being anterior in the upper limb and posterior in the lower limb
* The sole of the foot is equivalent to the palm of the hand and big toe (hallux) is equivalent to the thumb (pollux)
-By week 8 all major components of the limbs are present and medial rotation of the LL is complete
Describe the components of limb bud.
- Limb Bud consists of:
- Core of mesenchyme derived from parietal layer of lateral plate mesoderm
- Ectoderm which forms the outer covering of the limb (epidermis)
- Ectoderm is thickened at the ‘apex’ of the developing limb to form the Apical Ectodermal Ridge (AER, organizer of tissue structures)
Describe the development of the Proximo-distal.
- AER controls proximo-distal development
- It induces the underlying tissue to remain as a population of undifferentiated, rapidly proliferating cells- known as the PROGRESS ZONE (of proliferating mesenchyme)
- As cells move further away from the AER they will begin to differentiate into cartilage and muscle
- This differentiation results in proximo-distal development
1) HOX-8 controls the position of the limb on the long axis of the body
2) Initiation of outgrowth of the fore limb is controlled by the TBX5 gene and FGF-10
3) AER secretes FGF4 and FGF8 to maintain the progress zone and the further development of the proximo- distal axis
4) As growth progresses, mesenchymal cells are left behind the advancing ridge (and its influence) and so they begin to differentiate
Describe the development of the antero-posterior axis.
- The antero-posterior axis is regulated by the Zone of Polarizing Activity (ZPA, formed by a cluster of cells near the posterior border of the limb)
- It ensures that the thumb grows on the cranial (anterior) side of the limb bud
- ZPA expresses the protein sonic hedgehog (SHH). ZPA moves distally with the AER
- Adding a ZPA to the limb bud results in mirror image duplication of digits (in chicks)
Describe the development of the Dorso-ventral axis.
- BMPs (Bone Morphogenic Proteins) in the ventral ectoderm induce EN1 protein
- EN1 represses WNT7 restricting its expression to the dorsal limb ectoderm
- WNT7 induces LMX1 which then specifies the cells to be dorsal
What is the consequence in the variations in the combinations of HOX gene ?
Variations in the combinations of HOX genes ensure that the:
* Upper and lower limbs are different TBX5 (upper limb), TBX4 (lower limb)
* Patterns for the proximal (arm) middle (radius and ulna) and distal (hand) are defined
What is the consequence of failure of apoptosis in between digits ?
Syndactyly (webbing)
Identify and briefly define the main limb defects.
- Amelia = complete absence of the limbs
- Meromelia = partial absence of the limbs
- Phocomelia = absence of long bones
- Micromelia = segments are abnormally short
The following ay involve either the hands or feet or both:
* Brachydactyly = short digits
* Syndactyly = fused digits - failure of apoptosis
* Polydactyly = extra digits
* Cleft foot = lobster claw deformity.
Explain the cause and effects of thalidomide.
-Thelidomide is a chemical teratogen
- Mechanism:
a) SHORT EXPOSURE leads to loss of blood vessels, uniform or localised cell death, loss or partial loss of ZPA and AER signaling which later recovers to distalise remaining tissue.
b) PROLONGED OR EARLY EXPOSURE leads to total loss of vessels, widespread cell death, and all signaling lost.
-Effect is phocomelia (flipper-like arms or legs, if short exposure), or amelia (if prolonged or early exposure), associated also with intestinal atresia and cardiac abnormalities
Explain the cause and effects of Holt Oram Syndrome.
Cause: TBX5 mutations
Effect: Defects in limb development: Upper limb deformities, heart defects
Once the the cranio-caudal position is set, limb growth is regulated along what three axes ?
- Proximo-distal axis
- Antero-posterior axis
- Dorso-ventral axis
Limb development defects
just images of amelia and phocomelia
