Asymmetric Cell Division

ASYMMETRIC CELL DIVISION: BINOMIAL IDENTITIES
FOR AGE ANALYSIS OF MORTAL VS. IMMORTAL TREES

Colin Paul Spears and Marjorie Bicknell-Johnson

INTRODUCTION

The generalized Fibonacci numbers arise in models of growth and death [15], with
interesting applications in medical sciences and statistics, such as dose escalation strategies in
clinical drug trials [21]. Bronchial airway segments follow a Fibonacci pattern of bifurcation [7].
Experimental growth of tumor nodules can follow Fibonacci ratios related to dynamics of
intratumoral pressure [20]. The associations of plant phyllotaxis and patterns of invertebrate
growth with the Fibonacci series remain charming but puzzling connections to biology.
Mechanistically, dislodgement, diffusion, and contact pressure models can be successfully applied
to describe macroscopic growth patterns 1L7,231, but specific cellular rationales for such recursive
patternings have been wanting.

In kinetic analysis of cell growth, the assumption is usually made that cell division
yields two daughter cells symmetrically. The essence of the semi-conservative replication of
chromosomal DNA implies complete identity between daughter cells. Nonetheless, in bacteria,
yeast, insects, nematodes, and plants, cell division is regularly asymmetric, with spatial and
functional differences between the two products of division [16]. The binary bud-scar growth of
Saccharomyces occurs with regular asymmetric surface marker evidence, enabling modeling of

events of the lifespan [11]. Mechanisms of asymmetric division include cytoplasmic and
membrane localization of specific proteins or of messenger RNA, differential methylation of the
two strands of DNA in a chromosome, asymmetric segregation of centrioles and mitochondria,
and bipolar differences in the spindle apparatus in mitosis.

777
G. E. Berg*m et al. (eds.), Applications of Fibonacci Numbers, Volume 7, j77-391.
@ 1998 Kluwer Academic Publishcrs. Printed in the Netherlands.

378 C.P' SPEARS AND M. BICKNELL-JOHNSON

Asymmetric binary cell division can be described by the generalized Fibonacci numbers
r.. = G.-r=L' c)2'
{Gr}, G, - Gn-t*Gn-" with starting conditions Go = 0, Gr=Gr=
In the limiting case of c = 2, immortal and mortal identity asymmetric binary trees may be
represented as in Fig. l, in which the filled symbols are replicating or stem cells (Sr) and open
symbols are immature, non-replicating cells.

Gn

I ..v
1
2
^'
3
b tr,},-'

/

/*.'

Figure 1. Immortal and mortal identity trees: above, the Fibonacci case. Below, the
interchange of vertices that creates the G, identity.

The symbols denote different generations. One of the products of division is parentJike
and the other, daughter-like. An identity between immortal and mortal G, sequences results
from the operation of decreasing the lag period for maturation by one cell cycle unit of time,
from c - I in immortal division to the mortal lag of c - 2, and setting the lifespan (with death
of parent in childbirth) as L=2c-2; L is equal to 2(c-2) degree two vertices plus 2 degree
three vertices. In Fig, L, at Gn=c=2t the degree two vertex has been exchanged with the
degree three vertex, by rearrangement in the skeletonized subtrees as shown in Fig. 1. Mature

cells of mortal identity trees show continuous binary production of daughter cells for c

consecutive cell cycles, L--c*("-2), These rules preserve the distribution and sum of the
degrees of the vertices, so that a bijection exists between the immortal asymmetric binary tree
and the mortal asymmetric binary tree, c ) 2, n2l,

ASYMMETRIC CELL DIVISION: BINOMIAL IDENTITIES FOR AGE... 379

Thus, in the Fig. 1 example, both trees have 6 leaves, 2 degree two vertices, and 4
degree three vertices. Note that the Fig. L mortal Fibonacci tree is all stem cells: we define stem

or ,9. cells as replicating cells, i.e., the progenitor or potential parent of another cell. For a
given tree of size G, it is immediately apparent that the number of degree one vertices is
Gn + l, the number of degree three vertices is G. - 1, and since the sum of all vertices is equal
to G. 1"- l, the number of degree two vertices is G.*" -2Gn-I.
G. values over time n are not only the population sums, but also represent the number
of paths from root vertex at n = 0 to leaves of these ordered trees. G6 begins with the (/c - l)st
entry of 1 (represented by the dotted line, Fig. 1) for the mortal tree, us. &th for the immortal
tree, which normalizes n= L at Gn=2. Among positive integers, the c = 2 Fibonacci mortal
series is unique among the mortal identity trees in having no maturation lag, and for which
Sr=Gt Our convention in tree display is that sequential generations are alternated left and
right.

IMMORTAL TREES 1. GENERATION AND AGE ANALYSES OF IMMORTAL
ASYMMETRIC BINARY TREES BY G- = G-
-1 + G

o AO
k
0 L2
Go=Gr-l+Gn{,c=4
I 1
1 I
1 1
0 1 1

1 11 2

2 L2 3

3 13 4
4 L4 b

5 15 1 7

b L6 3 10
7 L7 6 L4
8 r8 10 1.9

Figure 2. Immortal Gn = Gn= t * G. _ n.

380 C.P. SPEARS AND M. BICKNELL-JOHNSON

Binomial coefficients are conveniently c-adjusted to give horizontal rows of entries to
describe Gn=Gr-I*Gn-.. Forexample,forc=4,thenumberarrayandtreeareshownin
fig. 2. For each b and (n,&) = (kc-c* 1,&) the entry is L, that is there is a c-step displacement

downward. This gives horizontal inventory of all cells at time n, in population G' by &th
generation.

Traditionally, the G, =Gn_t*Gr-" recursion is associated with slants of Pascal's

triangle, for which there is an extensive number theoretic literature [3,5,6,9,12-14], but here the
rising diagonals are converted to horizontal rows' We set n = | at Gn= 2, which normalizes the
start of population growth to Gn=2 for variable c; more thorough accounting of starting
condition values could set n = 0 at the top of the zeroth column (as in the T^ ease, below)' or
at fr = - 1 (in which case, however, the rather unbiological Gr values 1, 0, 0' ' ' occur).

It is readily shown that
n+
1))
Gn_*Gn_.=5J" - -&1)(e -
(c
Gn= (r)

Spreadsheet labeling of all age groups of these immortal trees by maturation and

replicative status is obtained by c repetitions of /c columns in double left-justified array, as in
d,=L1213 for c = 3 in Table 1, which presents a comparison between single-column, generation
sums and the stem-cell array for Gn= Gn-t* Gn-s'

An expression for the stem cell array is:

""
=i ri=,("-&("-J)-d+
1) (2)

ASYMMETRIC CELL DIVISION: BINOMIAL IDENTITIES FOR AGD... 381

Table I
lmmortal c-Adjusted Arrays

Generational Sums Stem Cell Array
(Single Columns) (c-Column Repeats)

k 0 | 2 34 000
G"d
I I
I I
0 I I
I ll 2 I
2 t2 J ll
t l3 4 ll I
4 t4 I 6 l1 2 I
5 l5 3 9 ll 3 2l
6 l6 6 l3 ll 4 321
7 t7 l0 l19 ll 5 433 I
8 l8 l5 428 ll 6 s46 3

Table 1. Age analysis of immortal asymmetric binary trees. Left, immortal G, single column
array for c = 3 in which entries are the total for each /cth generation for a given cell cycle time,
n, by (1). Right, c-column-repeat horizontal distribution of cells by maturational age' by (2).
The right-most column entry within each (& * l)th generation is the number of stem or
replicating cells (5.), with the number of youngest, newborn cells in the left-most entry within a
given ftth generation. For example, for G' = 19, the 7 first generation cells include 1 newborn,
1 adolescent, and 5 S. cells, and the 10 second generation cells include 4 newborn, 3 adolescent,
and 3 S. cells.

2. MATRIX AGE ANALYSIS OF IMMORTAL BINARY TREES

We recently described combinatoric identities of cxc matrices whose elements obey the

Gn=Gn-t *G.-" generalized Fibonacci recursion equation [1]. Matrices of order c give a

facile approach to age analysis of the immortal asymmetric binary tree. For example, for c = 4,

382 C.P. SPEARS AND M. BICKNDLL-JOHNSON

r
1 o 1 519
2 1 2 726
3 I 3 1036
4 1 4 1,4 50
The Pascal-triangle-like construction is apparent with (r,s) = (r,s - 1) + (r - l,s),r > l;
(1,")-(1,s-l)+(4,s-1). Many identities exist in these anays [1]. Each entry G, is the
sum of the horizontal row entries, beginning with s = 0, ending in the (r - l)th entry just above
Gr. The circled (s, r) = (0, 1) entry is set to l, for combinatorial convenience; thus the bottom
row contains sums from (r,o) to (r,s), with the sum ofthe elements in the bottom row equal to
the (1,s*1) entry at the top of the next column. There is also a ucolumn sum' rule; each
entry G. is the sum of c consecutive column entries beginning with Gnec_z) and ending with
Gn(c Thus, G. = 131 cells at time n of asymmetric immortal division by
- r).
Gn=Gn-t*Gr_s is comprised of 19*26+36*50 cells, in order, from oldest to youngest
non-replicating age, except for 50 (Gr-.*r) which is the number of replicating cells since this
is the sum of all prior entries up to Gr_2.q1,
The organism C. elegans is a small nematode with more than a thousand somatic cells
in initial development to adulthood, of which exactly 131 cells are regularly programmed for
death [22]. Thus, one could conjecture that these 131 cells are comprised of 50 stem cells, 36
newborn, 26 youngster, and 19 juvenile cells, in the immortal Gn=Gn_r*Gn_t model. In
the mortal identity array for c = 4, below, these would be 58 stem, 42 newborn, and 31 juvenile
cells, which numbers are conceivably relevant for subpopulations [22]. The immortal model may
be most applicable to budding yeast populations [11] and plant cells. The asymmetrical division
of bacterial species is typically associated with non-reproductive sporulation [16].

3. GENERATION AND AGE ANATYSES OF MORTAT IDENTITY ASYMMETRIC
BINARYTREES BY G- =G-_1+G
In most normal cells, a programmed, discrete lifespan exists in oilro. Several
intracellular proteins, such as p53, control both the lag before cell cycling and the lifespan [10].


Mutations in such genes can shorten lag and concurrently block programmed cell death (also
known as apoptosis, after the Greek word for dying leaves falling from trees), which helps
immortalize cells, such as malignant transformation of mammalian cells. Thus, those mortal
asymmetric trees with unperturbed growth curves (G, os. n) that are typically identical to the

immortal case enable statements about the relative contributions of lag uis-a-ois lifespan (c us.
.[) on growth. In plant cells, however, although apotosis can occur after toxic stress [4], an
immortal model of asymmetric binary division, or mortal models with very long lifespan,
combined with parastichy behaviour are more relevant for pursuing clues in Fibonacci/Lucas
phyllota:<is [17].

Gn=Gr-rIGn-. mortal identity asymmetric binary tree for
Figure 3 shows the
c=4,L=2c-2 = 6, with maturation lag c-2 = 2, and the associated stem cells,5, and dying
cells Dr.
qGoS.Do

100
100
0 110
1 2 L 0
2 310
3 421
4 520
D 730
61 051
Figure 3. Gn=Gn-t*Gn-q mortal (.t = 6) asymmetric tree
in which Gn= Sr*Sr-r * Sn-2,

The number of stem cells is obtained from the starting conditions S, = 0 at n =- 1,

,5n=1 at n=0, 1, and 2 and summation of c-1 consecutive ,Sr terms. Thus,

Gn = Gn_t* Gn_q = ,Sn + Sn _ 1 +,9n - 2.

.9; seeuences also describe other age groupings (..8., Dr= Sn-zc+z) adjusted
according to starting points for that given age, similar to the assumptions of [15]' Since the
stem cells of the mortal identity trees include the terminal vertex degree-two dying cells,
Sn- Dn= Gn-c+1.

A natural partitioning or sectioning of mortal trees into age units based on lifespan
(2c-2) units is a fundamental difference, of course, from immortal trees. Table 2 is the
- | * G, - S, =
spreadsheet array for mortal Go = Gn L 4, and four age grouping-columns


repeat to describe all ages by cycle or n values.

Equation (3) describes the Table 2 array of Lpartitioned mortal cells for c : 3, with I
representing the generation identifier. Ages d from L to 2c - 2 all apply to the same generation,
Since lag is c-2 = 1, entries in the d = 1 columns are newborn, non-dividing offspring, and
d ) 2 are all .9. cells of increasing age to the right.

t*l
c,= I ,Lt,-',(,)
ft>1
("- t*- 1)('-e1; i+r-d- 1)
(3)

Table 2
Lifespan-Partitioning of Mortal Identity Gn = Gn _ r * Gn _ e

n
1

L

I 1 2
2 11 3
3 11 1 4
4 1 11 1 6
5 11 2L I
b 1 321 1 13
7 232 31 1 19
8 L23 631 4 I 28
9 L2 763 10 4L L41
10 1 67 6 16 r041 5160

Table 2. Partitioning of L=2c-2=4, of Gn=Gn_r*Gr_s mortal identity asymmetric
binary tree array, The number of column repeats is equal to .t. The spreadsheet rule for
formation is that c vertical entries are entered .t steps to the right and c 1 steps down from
-
the top of the summed entries. From left to right within each & generation, there are c 2
columns(s) of immature cells, plus c columns of S, cells of increasing age, Entries in d L
-
-
columns are D. cells.

Table 3 presents a single column per generation array, and a stem cell (c
- l)-column-
repeat array for spreadsheet display of asymmetric mortal identity Gn= Gr_r * Gn_", c = 4.

ASYMMETRIC CELL DIVISION: BINOMIAL IDENTITIES FOR AGE..' 385

As in the immortal case given in Eq.(2) and Table 1, d represents the column repeats, from 1 to
(" - 2), for newborn cells up to the oldest immature age then c ages for ,9, cells. Sums for d =I
to 2c - 2 in Eq. (3) at a given k give the single-column entries of the mortal array in Table 3.

Table 3.
G n = G r- r * G. -4 Mortal Identity Arrays

Generational Sums Stem Cell Arrav

Single Columns c-l Column Repeat

k0 2

God 12I t2I
I I

I I
0 I I
I ll )
2 t2 3 I

3 t3 4 ll
4 4 I 5 l2 I
5 4 3 7 l3 2l
6 4 6 l0 4 321
7 3 0l t4 3 43 3 I
8 2 34 l9 2 346 3l
9 I 510 26 I 2310 63
Table 3. Age analysis of mortal identity asymmetric binary trees, by maturation age to
replicative status ,5r. Left, single column anay for mortal identity tree (c = 4): entries are
generation sums. The spreadsheet rule for formation is that c vertical entries of a &th
generation are summed and entered c - I steps lower in the (& + l)st generation. The number of
vertical 1s in the /cth generation is equal to .t. Right, c - L column-repeat array for mortal
identity trees, Column entries form a left lower diagonal array. The spreadsheet rule of
formation is that c vertical entries are entered c-l steps down and c-1 steps to the right,
The maturational age distribution of cells reads horizontally from left to right for newborn plus
maturing (from 1 to c-2) and ,5, cells. .9. cells are summed without regard to age, in the
rightmost column (D) within each generation,

The stem cell, lower diagonal array of the mortal identity asymmetric binary tree in
Table 3 is obtained from the lifespan-partitioned, upper diagonal array (such as the example in
Table 2, where c = 3) by summation of the rightward c columns within each i generation,


A likely more familiar, combinatorial interpretation of (3), for the j value sums is that
entries are the number of compositions of n into & parts [8].

4, T^OR TRIBONACCI-TYPE ARRAYS AND TIMDSYMMETRIC MORTAL TREES

Equations related to familiar ?- arrays [2,3,6, 19] may be used to vary.t and c
independently and keep the convention that columns of c-adjusted spreadsheet arrays represent
generations. In contrast to the identity trees and arrays, here the arrays present the initial
conditions for determination of the behaviour of the trees os. the rules of tree formation
determining the number arrays, above. Time-sgmmetric, brt spatially asymmetric, binary
division with no maturation lag (c = 1) of mortal trees may be described using 7- or
Tribonacci-type equations of the form Gn=Gn_"*Gn_c_r+...+Gn_(c*m_1) where rn is
theorderof thearrayof coefficientsintheexpansionof (1 *t*a2+..'+am- 1)tforrn, n20.
In our arrays such as Figure 4, label units of downward column displacement between successive
increasing /c generations as c (with tree maturation lag as c - 1.), and let the order rn of the
array be equal to lifespan .t to write

c. = (-] t) ("-nt" -')-
tt)
F D(-
r), (4)

Figure 4 illustrates a T anray and mortal binary tree for ?u with .t = 6 and c = 3. To
^
preserve * as the generation identifier, c)2, one solution is to have the offspring of a dying cell

show no maturation lag and produce self-generation for c - 1 divisions, here two consecutive
divisions (starred). This may be viewed as a mutational event, with temporary loss of
maturation to the next generation. Such a hyperproliferation parameter could represent "crisis'
periods in aged normal fibroblast populations, in which subpopulations fail to differentiate to
programmed cell death. When c = 1 (no maturation lag), Eq. (5) results, which then represents
tirne-symmetric, but spatially asymmetric binary division, with .t ) 2 as the only variable:

c,=
F I(-,r'(-]') ("1") (5)

When -t = 2, the Fig. 1 mortal Fibonacci tree results, and .t = 3 gives the ersatz 'Treebonacci'
case [5].


Gr=Gn_.*Gn_c_1... *Gn_(.+tr_r) = G.-s* Gn-4,,.' *G.-a

n Gn
0 1 1
1 1 1
2 I 1
3 I I 2
4 1 2 3
5 1 3 4
6 4 1 5
7 5 3 8
I 6 6 L2
9 5 10 I 16
10 4 15 4 23
11 3 2L 10 34
t2 2 25 20 1 48
13 1 27 35 5 68

Figure 4. Tribonacci-, or ?--type, array and binary mortal tree for T6 or L =6, and c=3.
To preserve i=generation, c)2, one solution is to have the offspring ofa dying parent cell
show no maturation lag and produce self-generation for c - L divisions, here 2 consecutive
divisions (starred).

In Figure 5, taking c =2 and L =2, the resulting
Gk=Gk-.*Gt-("+ L-r)=Gk-z+Gk-3 are column sums of the classic Mann-Shanks
array [18], described by Eq. (6):

c" = ry (r]') (n- kk-2i)
F D(-
(6)

which of course is (a) with c = 2, L = 2. A, ?*-type tree for this case, in which matulation lag
is c - I for the first daughter cell (and thus the second daughter, produced at 'death in
childbirth,' 'back-mutates' just once (c - 1) and has no maturation lag), is shown in Fig. 5, n.

and rb according to [18].

388 C.P. SPDARS AND M. BICKNELL-JOBNSON

k Gr = Q* + Gtr-(c+L-r) = Gt-e + Gt-g
I 0
2 1 1
3 1 1
4 1 1
D 2 2
6 l1 2
I 3 3
8 31 4-
9 14 5
10 61 7
11 43 I
L2 1 10 I L2
I3 10 6 16

Figure 5. I,eft, T2 array for the Mann-Shank series. The spreadsheet rule for formation of
entries n 2 2 is that c = 2 vertical entries are summed to be entered one step to the right and
c = 2 steps downl n and /c are labeled in conformity with [2], so that when Ic is prime, all entries
for that row are evenly divided by their n values. Right, a Mann-Shanks tree, .t = 2, c =2,lag
=c-L, terminal (2nd) offspring maturation lag-0 with c-1 'back mutations'production ol
self-generation. There are four different types of cells per generation, as exemplified by
generation A. Each type ofcell is described by the same sequence, {...1,0,1,1,1,2,2,3,...},
i.e., S1 = Sk-z*S*-s.

The occurrence of unequal dichotomies in plants, such as in Fucus spiralis [17] could
result from asymmetric binary cell division with consequences on a macroscopic scale. Although
a physical representation of our tree structures is only implicit, the spatial relationships among a
given population may echo the temporal patterns, such as by secretion of products that provide
structural scaffolding. From Jean ll7, p. 1421, (The botanist Church (1904) frequently insists
on...the periodic sequence of ones and twos.'. for the explanation of rising phyllotaxis,
analogous to the phenomenon of cell division." Is regular asymmetric cell division, temporal or
spatial, the fundamental mechanism of plant phyllotaxis? Is it the unifying theme behind
mathematical patterning in animals, and if so, could it relate to combinatoric homeostatic
mechanisms that compensate apopotic losses with gain of form and function?

ASYMMETRIC CELL DIVISION: BINOMIAL IDENTITIES FOR AGE..' 389

SUMMARY

Binary cell division is regularly asymmetric in most species' Growth by asymmetric
binary division may be represented by the generalized Fibonacci equation, Gn=Gn-t*Gn-..
Mortal-immortal identities are of interest for study of influences of checkpoint genes with dual
functions for control of programmed cell death os. lag period before cycling. The mortal and
immortal growth models presented give predictions of percentages of cells by age after birth and
by generation. Our models, for the first time at the single cell level, provide rational bases for
the occurrence of Fibonacci and other recursive mathematical phyllotaxis and patterning in
biology, founded on the occurrence of regular asymmetry of binary division'

ACKNOWLEDGMENTS

We are grateful to Rhonda Schruby for extensive and capable literature searching.

Supported in part by NCI CA39629 and the Gary Ford Duthie Fund for Cancer Research.

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AMS Classification Numbers: 11865, 92C15' 11839

Asymmetric Cell Division

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