EL PROCESO DE TOMA DE DECISIONES a Etapa 1 Identificar el problema

A fte r the release of a terminal oocyte, the remaining f o l l i c l e ( i. e . the f o l l i c u l a r epithelium and tunica propria) is situated at the base o f the ovariole where i t forms a corpus luteum. In spite o f a considerable reduction in volume o f the space previously occupied by the oocyte, no crumpling o f the f o l l i c u l a r epithelium takes place.

Neither is the epithelium separated from the tunica propria. However, the tunica is highly folded. This is an in d ica tio n o f the contraction o f the f o l l i c l e and is a s itu a tio n peculiar to post-ovulation tunica p ropria. In P. americana both an outer granular and an inner fibrous layer are present as in stage 5 (Fig. 50). In R. prolixus too, the inner fibrous layer is present as in f o l li c l e s p rio r to ovulation (Fig. 51), However, there is evidence th a t some degeneration o f the tunica propria layers occurs in 'o ld ' corpora lutea where the folds o f the tunica propria become more conspicuous because o f secondary fo ld in g

(Fig. 52 & 53).

Many o f the c e lls o f the corpus luteum undergo pyknotic degeneration and e x h ib it large nuclei in various stages o f a u to lys is. The degenera­ tio n appears to begin at the centre o f the corpus luteum and then pro­ ceeds outwards towards the tunica. The signs o f degeneration are c le a rly apparent too, in the cytoplasm (Fig. 55).

More im portantly, microtubules are very conspicuous in c e lls not yet showing signs o f degeneration. Large numbers of microtubules run

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p a ra lle l to the long axis o f each c e ll in both P. americana and R.

prolixus (Figs. 56 & 57). Longitudinal and transverse sections indicate th a t the microtubule o rie n ta tio n is radial with respect to the lon g itu d in a l axis o f the corpus luteum. In P. americana, in addition to the r a d ia lly oriented microtubules there are c irc u m fe re n tia lly oriented microtubules. These occur d ir e c tly beneath and p a ra lle l to the outer surfaces o f f o l l i c l e c e lls , and at r ig h t angles to the lon gitudinal axis o f the ovariole (Fig. 58). Although c irc u m fe re n tia lly oriented microtubules occur p r io r to ovulation in P. americana, ra d ia lly arranged microtubules are peculiar to post-ovulation f o l l i c l e c e lls . R adially arranged micro­ tubules in R. prolixus are also unlike the s itu a tio n p r io r to ovulation.

As in the f o l l i c l e c e lls o f P. americana and R. prolixus p r io r to o vu la tio n , c e lls o f the corpus luteum contain bundles o f microfilaments d ir e c tly beneath th e ir outer surfaces (Figs. 58 & 59).

A diagrammatic representation o f the s itu a tio n p rio r to , and a fte r , ovulation in P. americana and in R. prolixus is shown in Figure 60.

DISCUSSION

A continuous and rhythmic reproductive cycle has been observed in P. americana (B e ll, 1969; Maa & B e ll, 1977). This cycle is thought to be co n tro lle d in part by an endogenous 'c lo c k - lik e ' mechanism w ith in ovarian f o l li c l e s (Maa & B e ll, 1977). In R. p ro lix u s , ovulation is not synchronous fo r d iffe r e n t ovarioles w ith in the same in d ivid u a l (Heubner & Anderson, 1972 b). Ovulation must therefore be co n tro lle d by a

mechanism w ith in each o va rio le . The mechanism is very probably an active process since ovulation only occurs once the oocyte is f u l l y mature, i. e . when the chorion is completed. The f o l l i c l e does not increase in size

during chorionation since the chorion develops w ith in the 'space' between oocyte and f o l l i c u l a r epithelium . The volume occupied by the oocyte during th is stage is constant. Something must be d riv in g the oocyte down in to the o viduct, and i t seems th a t the f o l l i c l e is involved.

The tunica propria o f the corpus luteum becomes highly folded.

I t has been suggested (Singh, 1958) th a t ovulation in Locusta m igratoria is followed by 'progressive contraction of the tunica propria through the formation of secondary f o ld s '. However, there is no known case o f a c tiv e ly c o n tra c tile e x tra c e llu la r fib re s . The presence o f cytoskeletal elements w ith in c e lls previously p r a c tic a lly devoid o f these elements and during a time when oocyte growth has ceased is s ig n ific a n t. There is a strong case fo r proposing th a t the f o l li c u l a r epithelium becomes a c tiv e ly c o n tra c tile as i t enters a new phase o f cytoskeletal co-ordination. The radial o rie n ta tio n o f microtubules (in f o l l i c l e c e lls o f corpora lutea) may be responsible fo r the elongated shape o f f o l l i c l e c e lls in corpora lutea compared w ith f o l l i c l e c e lls p r io r to ovulation. The la t t e r are squamous (P. americana) or low columnar (R. p ro lix u s ) in shape and contain no microtubules p a ra lle l to th e ir long axes. Microtubules are spatio-tem porally associated w ith c e ll elongation in a wide range o f

c e ll types (Renaud & S w ift, 1964; Tilney & Porter, 1967). Any necessary increase in la te ra l membranes is already present; f o l l i c l e c e ll boundaries are highly convulated p r io r to ovu la tio n . This change in c e ll shape

throughout the f o l l i c u l a r epithelium in a d ire c tio n at r ig h t angles to the f a i r l y r ig id chorionated oocytes' polar axes, together w ith aid from a tunica pro pria , which is perhaps e la s tic and under tension p rio r to oocyte discharge, might create the necessary force required to discharge an oocyte Assistance may be provided by the c irc u m fe re n tia lly oriented microfilaments at the apices o f f o l l i c l e c e lls .

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In R. p ro lix u s , the ovariole sheaths contain myoepithelial c e lls . Since the ovariole sheaths l i e very close to mature f o l l i c l e s , they may also a s s is t in the process o f ovulation. The ovariole sheath o f JP. americana contains no myoepithelial c e lls or any other c o n tra c tile elements and therefore probably exerts no d ire c t force on f o l li c l e s but rather o ffe rs a c o rs e t-lik e support to the ovariole.

The p o s s ib ility remains th a t oocytes are released purely as a re s u lt of the breakdown o f the e p ith e lia l plug which separates f o l li c l e s from the pedicel perhaps by muscular contraction or p e ris ta ls is o f the pedicel and/or oviduct. However, how would these elements 'know' when oocytes are mature? I t seems more l ik e ly th a t the e p ith e lia l plug is a c tiv e ly 'broken through' by the combined forces exerted on the r ig id chorionated egg by the tunica p ro pria , cytoskeletal elements w ith in the f o l l i c u l a r epithelium , and in some cases a muscular ovariole sheath. Thereafter crowding (Raven, 1961) and the e la s tic nature o f the tunica propria

(Singh, 1958; Bonhag & Arnold, 1961; Guthrie & T in d a ll, 1968; Chapman, 1972) may e ffe c t the posteriad movement o f f o l li c l e s down the o va rio le.

The v e rs a tile nature o f the f o l l i c u l a r epithelium is once more apparent. I t seems th a t the f o l l i c u l a r epithelium p rio r to ovulation contains cytoskeletal arrays which appear to be s p a tia lly and temporally co-ordinated fo r shape control during the production o f mature oocytes, but th a t once oocytes are f u l l y mature, the f o l li c u l a r epithelium enters a new phase o f cytoskeletal co-ordination concerned w ith the discharge o f the mature oocytes.

CHAPTER 4

SPATIO-TEMPORALLY CO-ORDINATED EPIDERMAL CELL SHAPING DURING WING DEVELOPMENT IN CALLIPHORA ERYTHROCEPHALA

INTRODUCTION

Waddington' s (1941) examination o f wing development in Drosophila melanogaster indicates th a t epidermal c e lls undertake a remarkable

sequence o f shape changes as the compact highly folded imaginai wing disc expands during pupation to form a fla tte n e d b la d e -like adult wing. The sequence is apparently more complex than any other reported fo r an

epidermis during embryogenesis. The i n i t i a l l y very elongate c e lls shorten, then re-elongate by producing fin e basal extensions. These sub­ sequently shorten,and f i n a l l y c e ll bodies fla tte n ,b u t ye t again produce basal extensions. Such m aterial therefore provides an e xcellent oppor­ tu n ity to examine the ways in which the shaping o f ind ividu a l c e lls is related to overall changes in epidermal shape and surface area, and the ways in which changes in cytoskeletal organisation and c e ll contact are s p a tia lly and fu n c tio n a lly co-ordinated w ith c e ll and tissue shaping. Waddington's account was based on lig h t microscopical examination of sections o f p a ra ffin wax embedded m a te ria l. I t did not provide the

inform ation required to tackle a ll the issues raised above. This chapter provides such information fo r wing development in Calliphora erythrocephala The information has been obtained by a detailed examination o f the morpho­ m etries, h isto lo g y, and u ltra s tru c tu re o f the wing epidermis as an imaginai disc is converted in to an adult wing.

Studies on the development o f Cal1iphora wing discs are not exten­ sive (Lewerenz, 1961; Agrell ,1966, 1968; Sprey, 1970; 1971; Sprey & Oldenhave, 1974; V ijverberg, 1974 a,b). These authors have con-

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centrated mainly on lig h t microscopical investigations o f la rv a l discs or very early stages in évagination. No u ltra s tru c tu ra l studies o f de­

veloping Calliphora wings have been published.

RESULTS

1 THE MAIN STAGES OF WING EPIDERMAL SHAPING, EXPANSION AND GROWTH