Plenary lectures 11 1 THE BASAL LAMINA I N EPITHELIAL-MESENCHYMAL MORPHOGENETIC INTERACTIONS, M. B e r n f i e l d ( S t a n f o r d , C a l i f . , U S A ) . The g e n e r a t i o n of s t r u c t u r a l form of s e v e r a l embryonic e p i t h e l i a l o r g a n s , s u c h as t h e l u n g , k i d n e y , mammary and s a l i v a r y g l a n d s , o c c u r s by a common s e q u e n c e . t h e l i a l c e l l s , and d u r i n g d e v e l o p m e n t , t h e bud u n d e r g o e s a d i s t i n c t i v e s e q u e n c e of f o l d i n g and b r a n c h i n g which r e s u l t s i n a morphology t h a t i s c h a r a c t e r i s t i c of t h e o r g a n . l u l a r l o o s e c o n n e c t i v e t i s s u e , o r mesenchyme, which i s a b s o l u t e l y r e q u i r e d f o r t h e morphogenesis. m a t e r i a l s , f i b r i l l a r c o l l a g e n and a w e l l - d e f i n e d b a s a l l a m i n a . Each o r g a n arises a s a rounded bud c o n s i s t i n g of a l a y e r of e p i - The e p i t h e l i a a r e s u r r o u n d e d by a h i g h l y c e l - Between t h e e p i t h e l i u m and mesenchyme l i e amorphous With mouse embryonic s a l i v a r y g l a n d s , t h e mesenchyme, amorphous m a t e r i a l s a n d f i b r i l l a r c o l l a g e n c a n b e removed by m i c r o d i s s e c t i o n i n c o l l a g e n a s e , y i e l d i n g e p i t h e l i a which r e t a i n t h e b a s a l l a m i n a . Such e p i t h e l i a m a i n t a i n normal m u l t i - l o b u l a r morphology, b u t r e q u i r e r e c o m b i n a t i o n w i t h mesenchyme f o r c o n t i n u e d b r a n c h i n g m o r p h o g e n e s i s . by b r i e f t r e a t m e n t w i t h nanogram amounts of t e s t i c u l a r h y a l u r o n i d a s e . t r e a t m e n t c a u s e s t h e c e l l s t o round up and r e s u l t s i n d i s r u p t i o n of c e l l u l a r a d h e s i o n s , d i s o r g a n i z a t i o n of c y t o s k e l e t a l s t r u c t u r e s a n d , when t h e e p i - t h e l i u m is recombined w i t h mesenchyme, t h e l o s s of m u l t i - l o b u l a r morphology. These o b s e r v a t i o n s s u g g e s t t h a t t h e b a s a l l a m i n a i s r e q u i r e d f o r m a i n t e n a n c e of normal morphology. However, d e l a y i n g r e c o m b i n a t i o n w i t h mesenchyme f o r 2 h r s , d u r i n g which t h e e p i t h e l i u m r e p l a c e s t h e l a m i n a , r e v e r s e s t h e a l t e r a - t i o n s and p r e v e n t s t h e l o s s of morphology. Thus t h e mesenchyme may h a v e a p r o p e r t y which i s d e l e t e r i o u s t o e p i t h e l i a l r e c o v e r y from h y a l u r o n i d a s e , b u t i s n o r m a l l y i n v o l v e d i n morphogenesis. The l a m i n a c a n b e c o m p l e t e l y removed Such The l a m i n a c o n t a i n s g l y c o s a m i n o g l y c a n (GAG) and l a b e l i n g s t u d i e s show t h a t c a . h a l f of t h e l a b e l e d GAG i s h y a l u r o n i c a c i d , c h o n d r o i t i n - 4 - i s t w i c e c h o n d r o i t i n - & s u l f a t e and c h o n d r o i t i n i s i n t r a c e amounts. U l t r a s t r u c t u r a l s t u d i e s u s i n g t a n n i c a c i d f i x a t i o n and r u t h e n i u m r e d s t a i n i n g reveal t h e l a m i n a t o b e composed of components i n o r d e r e d p e r i o d i c a r r a y s i n t i m a t e l y a s s o c i a t e d w i t h t h e plasmalemma. The b a s a l l a m i n a may c o n s i s t , i n p a r t , of s u p r a m o l e c u l a r complexes of h y a l u r o n i c a c i d and p r o t e o g l y c a n which a r e Upsala J Med Sci 82 112 Plenary lectures organized into an extracellular scaffolding that imposes structural form on the epithelium. In intact glands, the GAG in the basal lamina is rapidly turning over. Laminar GAG is lost more rapidly at the distal ends of the lobules, the sites of greatest cell proliferation and change in cell shape, and less rapidly within the interlobular clefts. This differential rate of Loss results in more GAG accumulating within the clefts and less on the lobules. The pattern of GAG metabolism is not secondary to lobular growth, nor is it altered by inhibition of cell proliferation or by inhibitors of collagen secretion or cross-linking. A similar pattern is seen during the branching morphogenesis of several embryonic epithelia, consistently reflecting the morphologic changes characteristic of each organ, but is not observed on unbranched epithelia. Thus, differential laminar GAG turnover is associated with branching morphogenesis. The pattern of laminar GAG metabolism occurs only in the presence of mesen- chyme. The mesenchyme is not involved in the synthesis, deposition or organization of the lamina, but culture in combination with mesenchyme causes a loss of GAG from the epithelium. The mesenchyme possesses activity which removes laminar GAG, producing a heterogeneous mixture of small molecular weight components. The activity requires living mesenchymal cells and is not duplicated by medium conditioned by mesenchyme. mesenchyme, therefore, is responsible for the degradation involved in the differential turnover of laminar GAG. The Since embryonic salivary epithelia require a GAG-rich basal lamina to maintain morphology, and require mesenchyme for continued branching morpho- genesis, it is likely that the effect of the mesenchyme on GAG within the lamina is involved in the changes in epithelial morphology which occur during development. Upsulu J Med Sci 82