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

130 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2 I o Days 40 Figure 6. Epider•nal sulfhydryl levels ("to,tal" and "free") of guinea pig fetus during second half of gestation dominant. While the free --SH usually represents about 70-80% of the total --SH, and shows the same general trends throughout the study, there are wide variations in the relation of the free --SH to the masked --SH (Table III) which appear to be cyclic. Table III Availability of Reactive ("Free") Sulfhydryl Groups During Prenatal and Postnatal Development of the Guinea Pig Body Weight Ratio: Ratio: (g) Free --SH/Total --SH Free --SH/Masked --SH Prenatal period 14 0.69 2.2 25 0.76 3.2 28 0.71 2.5 32 0.54 1.8 41 0.92 11.7 41 0.76 3.1 45 0.79 3.7 55 0.51 1.0 69 0.79 3.6 87 0.62 1.6 98 0.70 2.3 102 0.82 4.5 Neonatal period 95 0.75 3.0