128 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 4. Ultrastructure of fetal guinea pig epidcrmis at 56th (lay of gestation turally (Fig. 4) by an increased size and number of keratohyalin gTanules, by the aggregation of tonofilaments into well-defined bundles, and by the appearance of a more differentiated corneum layer. The stratum granu- losurn is more evident but still does not provide a continuous mature layer underlying the corneum. The epidermis of the 61-day guinea pig fetus (Fig. 5) has achieved both structural and functional maturity. At near-term, the ultrastruc- ture of the fetal guinea pig epidermis is virtually indistinguishable from that of the adult animal. Protein-Sulfhydryl in the Barrier In the final elaboration of keratin from soluble precursors, the oxi- dation of protein-sulfhydryl (--SH) groups to form cystine bridges is gen- erally assumed to play a prominent role. In the present study we have tried to gain insight into the relation between the participation of pro- tein --SH in the keratinization process and the development of the func- tional barrier. The results of the --SH analyses on fetal guinea pig epidermis are given in Table I the relationship of --SH content to the barrier forma- tion in the developing epidermis is shown in Fig. 6.

FETAL EPIDERMIS 129 •::•": • Z'..•.•' ...... :•.... •-.• .... '•%• .•.... ..... 7 ,•:.• • "•::: :•:.'.'•-'•¾. • •.' •::•. •. • •,•:• .. .. ..%•....• •%.•.• '•7 :? ..& "y .... • .•. • .• ::i : .. ':• .... ...... . . -.. -..:-•.• _• •:. • . .. z... •.2':.." :• ...:•.. ' ': .. ' ' - L•v' ".v--'•:. ß ': ....• :-. :•:.... .... "•.':• • .• •'•:. .. ' .-- •:,.. " -: •: .•: --:': •.-. • .:•3,•: ':::•3• • "' r h. :z.' ..?-.• :•:.•.. ß ß • • :.•- .'• * .. • ,• .:"8 •.•.'.. :"•q'•Y m'-. ' • ' '• : •'... • •5..e• :: ...... • .•:.. z ..... . • •e.. •.v .... ,•.•. -•:K..... •7.• '•-L ' •K3 •-'-* • •:• ' •..-• '.:.' • • :" '::•": ':' :.7 "•1'5:• .-. -:• •:• ::•i'•:•....•b4 •4:. "•:- e?• '•' "'•?:7..7 -•:' '" "• :: ..... : '::3•V •- .. • .•77 .:•.• •:...•::....•.•:,:.•. :t-:•..: ' ß :-7..•..• ...... •:• • ...... "•g .... •:' "--•:•' • •'• •: • •-' ' -"•:--•' i'•:• '•.•.•7 , '•' Figure 5. Ultrastructure of fetal guinea pig epidermis at 61st day of gestation In the guinea pig fetus, epidermal --SH was not detected analytically until the 50th day of gestation. Coincident with the beginning of bar- rier development and the appearance of the •anular and horny layers, epidermal --SH began to increase rapidly. A peak in the --SH level was seen at about the time when ba•ier development was almost complete (S8• a•y). The epidermal --SH values observed at any particular time during fetal •owth will represent a net difference between --SH formation and --SH involvement in reactions (possibly oxidative) essential for the rapid growth and differentiation of the developing epidermis. In the early stages of hazier development the rate of synthesis is predominant, re- sulting in a sharp increase in epidermal --SH. Just before term, •e second process, by which --SH is converted to another form, becomes