Each oocyte in regions 3 - 5 is completely surrounded by a f o l li c u l a r epithelium . This epithelium is represented by a single layer o f f o l l i c l e c e lls along the sides o f oocytes. Near the poles o f oocytes, the
epithelium forms transverse p a rtitio n s across the ovariole called plugs (Davey, 1965), th a t separate adjacent oocytes. Each plug is several c e ll layers deep.
(a) Shapes and dimensions o f f o l l i c l e c e lls
At the s ta r t o f growth phase 3, f o l l i c l e c e lls are cuboidal in shape with lengths o f about 10 pm. At the end o f th is growth phase and during growth phase 4, the f o l l i c l e c e lls have become much more elongate and narrow (length about 20 pm; width about 5 pm). At the s ta r t o f growth phase 5 the c e lls become more or less squamous in shape (apico- basal lengths about 20 pm; widths about 30 pm) and v ite llo g e n ic channels run between adjacent c e lls (Anderson, 1964).
(b) U ltra s tru c tu re o f f o l l i c l e c e lls
During growth phase 3 f o l l i c l e c e lls contain large numbers o f microtubules and m icrofilam ents. These are situated alongside each other ju s t beneath and close to the outer surfaces o f f o l l i c l e c e lls
(those most d is ta n t from the oocyte) at the level o f the apical
desmosomes (Fig, 7). The m icrofilam ents (about 6 nm in diameter) and some o f the microtubules are grouped in bundles o f varying sizes. Some
microtubules occur 's in g ly '; they are associated with microfilaments and are situated up to about 0.5 pm from the nearest adjacent microtubule. The lo n g itu d in a l axes o f the microfilamentatous bundles and nearly a ll the microtubules run c irc u m fe re n tia lly at r ig h t angles to the lon gitudinal
(polar) axes o f elongating f o l li c l e s and th e ir oocytes, and p a ra lle l to the outer surfaces o f the f o l l i c l e c e lls . Some o f the tubules and
filam ents are associated with apical desmosomes (Fig. 8). Others run at r ig h t angles to the plane o f the outer surfaces o f f o l l i c l e c e lls along side the la te ra l f o l l i c l e c e ll membranes (Fig. 9). Microtubules are also found extending along the in te r io r s o f in te rd ig ita tio n s between adjacent f o l l i c l e c e lls . Tubules occur at other levels in f o l l i c l e c e lls a t a v a rie ty of o rie n ta tio n s . Such microtubules are sparsely d is trib u te d and nowhere do they form a w e ll-o rie n te d and concentrated array lik e th a t at the outer surfaces o f these c e lls .
A very s im ila r s itu a tio n exists in f o l l i c l e c e lls during growth phase 4 except th a t the microtubules and especially the microfilaments are more closely packed together (Figs. 10 & 11). The attachment o f the c irc u m fe re n tia lly oriented filam ents to desmosomes (Fig. 12) and to the la te ra l membranes o f adjacent f o l l i c l e c e lls provides an interconnected network o f filam ents throughout the f o l l i c u l a r epithelium (Fig. 13).
During growth phase 5, the apical microfilamentous bundles have become reduced in diameter to about 60 nm. The a p ic a lly situated micro tubules and desmosomes are absent. Thin microfilamentous bundles and microtubules (usually occurring 's in g ly ') fo llo w the shapes o f the devel oping v ite llo g e n ic channels along the la te ra l f o l l i c l e c e ll membranes
(Fig. 14). The stru ctu re o f these channels has not previously been described u ltr a s tr u c tu r a lly . Channels develop during stage 5 at points
24 where three c e lls meet (Fig. 14). F o llic le c e lls also become detached from the tunica propria at such points (Fig. 17). Later on during growth phase 5 the extent o f surface detachment from the tunica increases (Fig. 15). Furthermore, at f o l l i c l e c e ll bases,the micro v i l l i are withdrawn thus creating a space between the oolemma and the f o l li c u l a r epithelium . Channels are f i l l e d with flo c c u le n t m aterial (Fig. 16).
(c) E p ith e lia l plug c e lls
Microtubules and bundles o f microfilaments are found at the apices o f plug f o l l i c l e c e lls d ir e c tly beneath the tunica propria. The
tubules and filam ents appear to be in tim a te ly associated with la te ra l f o l l i c l e c e ll membranes and run p a ra lle l to the outer surfaces o f these f o l l i c l e c e lls and at r ig h t angles to the oocytes' polar axes (Fig. 22).
In addition rather la rg e r bundles of microtubules run alongside the la te ra l membranes o f the c e lls . They run p a ra lle l to the lo n g i tudinal axes o f the f o l l i c l e c e lls , and meet th e ir la te ra l membranes at various points along the lengths o f these membranes (Fig. 23). Large numbers o f microtubules run r a d ia lly w ith in the f o l l i c l e c e lls (Fig. 24) i. e . p a ra lle l to the lon g itu d in a l axes o f individual f o l l i c l e c e lls and r a d ia lly with respect to the lon g itu d in a l axis o f an o va rio le. These tubules also meet the la te ra l membranes at various points (Fig. 25).