3. INTRODUCTION 3 In simple terms, pattern formation refers to
the generation of complex organizations of cell fates in space and
time. During embryogenesis, information encoded in the genome is
translated into cell proliferation, morphogenesis, and early stages
of differentiation. Embryonic pattern arises from the spatial and
temporal regulation and coordination of these events.
4. INTRODUCTION 4 In developmental biology, pattern formation
describes the mechanism by which initially equivalent cells in a
developing tissue in an embryo assume complex forms and functions
(Ball, 2009) The process of embryogenesis involves coordinated cell
fate control (Lai, 2004; Tyler and Cameron, 2007). Pattern
formation is genetically controlled, and often involves each cell
in a field sensing and responding to its position along a morphogen
gradient, followed by short distance cell-to-cell communication
through cell signaling pathways to refine the initial pattern. In
this context, a field of cells is the group of cells whose fates
are affected by responding to the same set positional information
cues. This conceptual model
5. Why Pattern Formation? 5 The reliable development of highly
complex organisms is an intriguing and fascinating problem. The
genetic material is, as a rule, the same in each cell of an
organism. How do then cells, under the influence of their common
genes, produce spatial patterns ? Development of an organism is, of
course, under genetic control but the genetic information is
usually the same in all cells. A crucial problem is therefore the
generation of spatial patterns that allow a different fate of some
cells in relation to others (Koch and Meinhardt, 1994).
6. DEFINITION OF TERMS 6 Induction is the stimulation of a cell
to differentiate in response to a stimulus produced by another
cell. It is mediated by inducer substances that diffuse from one
cell to another. It results in cell determination. Determination is
the commitment of a cell to undergo differentiation. It is an
irreversible process but is not accompanied by morphological
changes. Determinants are the cytoplasmic effector molecules that
mediate determination. Differentiation is the variation in the
pattern of expression of a common set of genes to form cells of
diverse morphology and function.
7. 7
8. 8
9. 9
10. 10
11. Time-table of landmarks in early human development 11 Day 1
- cleavage Days 2-4 - morula; free- floating conceptus in uterine
tube Days 5-6 - formation of the blastocyst and embryoblast; -
implantation Week 2 (days 7-14) - formation of the bilaminar embryo
0.1 mm Week 3 (days 15-20) - formation of the trilaminar embryo 1.0
mm Week 4 (days 21-28) Day 21 - formation of neural tube 2.0 mm Day
22 - formation of the heart Day 23 - formation of eye and ear
rudiments Day 25 - formation of branchial arches Day 26 - formation
of upper limb bud Day 28 - formation of the lower limb bud 5.0 mm
Weeks 5 to 9 (2nd month) - Period of organogenesis Week 6 1.0 cm
Week 9 4.0 cm
12. GENES OF PATTERN FORMATION 12 Every organism has a unique
body pattern. This patterning is controlled and influenced by the
HOMEOBOX genes. These specify how different areas of the body
develop their individual structures, e.g. Arms, legs etc.
13. HOMEOBOX GENE 13 Homeotic genes are regulatory genes that
determine where certain anatomical structures, such as appendages,
will develop in an organism during morphogenesis. The expression of
homeotic genes results in the production of a protein (homeodomain)
that can turn on or switch off other genes. This genes act as
Transcription factors.
14. HOX GENE 14 Human hox genes are collected into homeotic
clusters. o There are 4 homeotic clusters, labelled A,B,C and D,
oEach cluster is situated on a different chromosome. o Each
homeotic cluster consists of 13 homeotic
15. The RNA expression pattern of three mouse Hox genes in the
vertebral column of a sectioned 12.5-day-old mouse embryo: the
anterior limit of each of the expression pattern is different Each
Hox gene is expressed in a continuous block beginning at a Specific
anterior limit and running posteriorly to the end of the developing
vertebral column
16. HOX GENE 16 The four numerically corresponding genes for
the four different clusters form a paralogous group. o The hox
genes are responsible for patterning along the antero-posterior
axis. o The genes are expressed sequentially beginning with the
paralogous group 1, which is expressed first o The sequential genes
specify different segments in cranio-caudal sequence extending from
paralogous group 1, which specifies the most cranial structures, to
paralogous group 13, which specifies the most caudal structures. o
Thus the first genes to be expressed specify the most cranial
structures while the last to be expressed specify the most caudal
structures. This is responsible for the cranio-caudal sequence of
development, where the more cranial segments develop slightly
before the more caudal structures. Consequently the upper limb
develops ahead of the lower limb.
17. Clinical Correlates 17 Mutations in genes of pattern
formation leads to a lot of clinical important congenital
malformations and anomalies Aniridia Synpolydactyly Axenfeld-Rieger
syndrome Branchiootorenal syndrome Coloboma Langer mesomelic
dysplasia Lri-Weill dyschondrosteosis Microphthalmia Mowat-Wilson
syndrome Amelia Limb deformities
18. Synpolydactyly 18 Mutation in the HOX D13 gene.
19. Aniridia 19 Aniridia with PAX6 gene mutation.
20. Axenfeld-rieger syndrome 20 mutations in one of the genes
known as PAX6, PITX2 and FOXC1.
21. REFERENCES 21 A. J. Koch and H. Meinhardt (1994).
Biological Pattern Formation : from Basic Mechanisms to Complex
Structures. Rev. Modern Physics 66, 1481-1507 Ball, (2009). Shapes,
pp. 261290. Eric C. Lai (2004). "Notch signaling: control of cell
communication and cell fate" 131 (5). pp. 96573.
doi:10.1242/dev.01074 Melinda J. Tyler, David A. Cameron (2007).
"Cellular pattern formation during retinal regeneration: A role for
homotypic control of cell fate acquisition". Vision Research 47
(4): 501
