lecture 16: invertebrate stem cell systems and evolution of stem cells

Question 1

Do C. elegans have stem cells?

Answer 1

• C. elegans exist as males and hermaphrodites
  
  • no distinct females
• the only dividing cells in adults are found in the gonads
• nematode
• unsegmented round worm
• complete cell lineages have been mapped
• hermaphrodites produce both sperm and eggs
• self-fertilising
• gonad has a region of where cells undergo mitosis, and a meiotic zone
• initial meioses produce sperm which are stored in the spermatheca
• subsequently switch to producing oocytes
• as each oocyte passes through the spermatheca it is fertilised by one of the sperm that had been produced earlier
• continue to produce germ cells throughout their lifetime in the mitotic zone
• stem cell region that continues to produce these mitotic cells
• distal tip cell = stem cell niche

Question 2

What is the function of the distal tip cell?

Answer 2

• acts as a stem cell niche in the gonad and keeps germ cells in a mitotic stem cell-like state
• elongated cell that contacts all the cells in the mitotic zone
• signalling from the distal tip cell up-regulates mitosis-specific genes
• signal from the distal tip cell that activates notch signalling in the mitotic zone
• notch signal activates a protein that shuts down mitotic repressors
• when the cells move away from this signal meiosis signals get produced

Question 3

What organs in adult Drosophila contain stem cells?

Answer 3

• neuroblasts
  
  • asymmetric divisions
  • segregation of cell components
  • limited number of cell divisions
  • so perhaps more stem-cell like/progenitor cells
• ovary
  
  • real stem cells that continue to produce germ cells throughout the lifetime of the organism
• testis
  
  • real stem cells that continue to produce germ cells throughout the lifetime of the organism
• midgut
• hindgut
• renal (M.T.)
• blood (HL)

Question 4

What was the first stem cell niche to be identified?

Answer 4

• in the Drosophila ovary
• drosophila females have germline stem cells
• germarium = part of ovary → probably around 20 or so in each drosophila female, feed oocytes into the ovary
• niche cells present in red, germline stem cell in light green, daughter stem cells in dark green (cystoblasts)
• somatic stem cells allow growth and reproduction of follicle cells (encapsulate a group of germ cells)
• one of these will go on to form the oocyte, the others are nurse cells

Question 5

How are germline stem cells regulated in the ovary of the drosophila?

Answer 5

• terminal filament cells secrete a cytokine-like molecule UPD (unpaired)
• stimulates production of a BMP molecule in Cap cells
• Cap cells form the formal niche in that they are the cells that contact the GSCs but they need the signal from the terminal filament cells in order to function
• BMP molecule is received by the GMCs (BMP receptor)
• intracellular signalling cascade → shut down of production of a molecule called BAM
• as the cells move away from the niche no longer under influence of BMP so start to produce BAM → differentiation
• adherens junctions between GSCs and cap cells

Question 6

What is the adult drosophila testis?

Answer 6

• two testes in adult male
• blind ended tube
• apical part of the tube is where the stem cells are found

Question 7

What can be observed using molecular markers in the testis?

Answer 7

• the physical relationship between germline stem cells (S), somatic hub (*) and somatic stem cell progenitors (P) or stertoli-like cyst cells (C)
• slightly different system to the female
• niche = hub cells
• germ line stem cells are found in a rosette or ring around that niche
• GSCs divide in a particular orientation so that the daughter that gets pushed away from the niche is the one that differentiates
• strict orientation of the mitotic spindle
• somatic stem cells also divide
• surround the daughter of the GSC

Question 8

What is the current model of mollecular signalling in the testicular stem cell niche?

Answer 8

• UDP secreted by hub cells
• cyst stem cells also contribute to the niche for GSCs
• niche doesn’t need to be a static structure
• UDP is received by the cyst stem cells
• upregulation of BMP molecule → downregulation of BAM in GSC
• daughter pushed out of niche → upregulates BAM
• differentiation does not occur immediately
• BAM levels increase until a threshold is reached at the 16 cell stage – triggers differentiation

Question 9

What happens with a loss of BAM in the respective stem cell populations?

Answer 9

• loss of Bam (mutant) in an ovary causes a proliferation of germline stem cells
• loss of Bam (mutant) in a testis causes a proliferation of spermatogonia (16-32-64 etc)

Question 10

How is the adult drosophila midgut epithelium maintained?

Answer 10

• maintained by a population of multipotent stem cells
• ISC
• basally located
• largest cells are the enterocytes - absorptive cells of the intestinal epithelium
• also find enteroendocrine cells and enteroblast (transient cell type)
• only cells undergoing division are the stem cells (cf human gut), not TAs
• enteroblast will differentiate to either and enterocyte or enteroendocrine cell depending on what signals it receives

Question 11

What happens with a loss of APC in the midgut epithelium?

Answer 11

• hyperplasia
• stem cells pretty much form a mono layer around the edge of the intestine
• with loss of APC you see multilayering, hyperplasia
• looks like an intestinal polyp

Question 12

What is colorectal or bowel cancer?

Answer 12

• colorectal cancer has the highest incidence of all malignant cancers in australia
• 1/17 lifetime risk for men
• 1/26 lifetime risk for women
• colorectal cancer is the second most common cause of cancer death (after lung cancer)
• increased Wnt signalling in the intestinal epithelium results in adenomatous polyps that develop into colorectal carcinoma
• drosophila could be a good model for this

Question 13

Where did stem cells arise in metazoan evolution?

Answer 13

• ancestral colonial choanoflagellate
• sponges are the most ancestral of the animals (but suggested comb jellies (Ctenophores) may sit basal to porifera)
• if Ctenophores are more basal suggests that muscles must have evolved twice in the evolution of the animal kingdom
• sponges are however very simple animals with stem cells

Question 14

What is the tissue structure of sponges?

Answer 14

• layers of cells but no “real” tissues or organs
• different cell types in layers
• no neurons or muscle
• no epithelial tissue
• loose arrangement of cells
• “mesenchyme”-like
• water pumped in through porocytes into the central cavity
• pumped out hole in top called osculum
• surrounding central cavity are cells called choanoctes
  
  • has a flagellum
  • phagocytosis of food particules
  • passed on to amoebocyte
  • look for all the world like another type of organism: choanoflagellate → suggested sponges are derived from these

Question 15

What is a choanoflagellate colony?

Answer 15

• perhaps the origin of sponges

Question 16

What are properties of archeocytes (amoebocytes) and choanocytes?

Answer 16

• both have stem cell properties
• archeocytes and choanocytes both express a member of the Piwi family of proteins – these proteins are restricted to the germline in most higher metazoans but found in stem cell populations in planaria and cnidaria
• Human Piwi has also been found in haemtopoietic stem cells
• archeocytes can renew themselves and differentiate into various other cell types
• choanocytes can dedifferentiate into archeocytes and also directly produce gametes (archeocytes can also produces gametes)
• Piwi play a role in stabilising the genome

Question 17

What is a model of stem cell evolution?

Answer 17

• the ancestral metazoan was a conglomerate of unicellular flagellates (choanocyte-like)
• proliferation became restricted to a subset of cells, allowing other cells to specialise in function

Question 18

What are the stem cell populations in plants?

Answer 18

• plant meristems
• since stem cells are found in plants it means they either evolved twice or evolved before the animal/plant split
• shoot apical meristem
• root apical meristem

Question 19

What is seen in the growing root meristem?

Answer 19

• the growing root meristem shows zones of cell division, elongation and differentiation
• plants are restricted in that they have cell walls which means they don’t have the same flexibility that animal cells do
• elongation is laying down of microtubules within the cells in a particular direction
• cells grow essentially by expanding along one axis

Question 20

What maintains the meristem?

Answer 20

• cell signalling maintains the meristem, although plants use many different signals to animals
• a short-range signal produced by the Wuschel-expressing cells maintains the meristem and a signal secreted by the meristem (Clavata3) feeds back to limit the size of the Wuschel domain and hence the size of the meristem
• similar feedback systems are used in animals

Question 21

What was metazoan evolution?

Answer 21

• comb jellies in a more basal layer has caused some confusion

Question 22

review points:

Answer 22

• what is the distal tip cell in C. elegans?
• how do the drosophila male and female germline stem cell niches resemble each other and how are they different?
• what is similar about drosophila and vertebrate intestinal stem cell niches (wnt signalling)?
• what are the sponge stem cells (archeocytes, choanocytes)?
• what is a model of stem cell evolution?
• where are plant stem cells located?
• how does a feedback system maintain the meristem?