Baboon placentae ranging in age from 45 to 175 days were studied for progressive changes in their morphology. The baboon embryo implants superficially and develops a single, discoid, villous hemochorial placenta. The remainder of the chorion is membranous and a true decidua capsularis is not formed. A lobular structure is developed during the fetal period. At 45 days the amnion only partially fills the chorionic cavity but by 60 days has expanded and the two membranes are directly contiguous. Trophoblastic tissue is of two types: cytotrophoblast and syncytiotrophoblast. Cytotrophohlast is found primarily in the chorionic plate and cytotrophoblastic shell. The chorionic villi are originally composed of a double layer of trophoblast; cytotrophoblast internally and syncytiotrophoblast externally. The cellular layer gradually disappears so that by full-term the villous walls are formed by a single layer of syncytiotrophoblast. Hofbauer cells are common within the villous cores, diminishing in number toward term. Large amounts of collagenous connective tissue develop in the chorionic plate and in the villi, and fibrin and fibrinoid materials accumulate in the basal plate and anchoring villi. The endometrium is decidualized to its greatest extent by the beginning of the fetal period and undergoes only minor cytological alteration throughout the remainder of gestation.The literature on the histological structure of the baboon placenta during the fetal period of gestation is limited to four publications: Breschet (1845) and Turner (1879) described mainly the gross structure of single specimens with only limited histological observations. Coventry ('23) described the placenta of a baboon killed at 4 months of gestation and Hillemann ('55) described a specimen obtained near term. Observations on the fetal vasculature of full-term baboon placentae were reported by Houston and Hendrickx ('68). Detailed consideration of other primate fetal placentae is limited to man (Wislocki and Bennett, '43; Wislocki and Padykula, '61; B@e, '54; Harris and Ramsey, '66; Boyd and Hamilton, '67) and the macaque (Ramsey and Harris, '66). The purpose of this paper is to describe the histological structure and changes which occur during the fetal period of gestation in the baboon (45-1 75 days) and to compare this information with that of other primates which have been described.
MATERIALS AND METHODSPlacentae utilized in this study were obtained surgically by the method described by Claborn et al. ('67). The distribution of the specimens was as follows:45-60 days, 15 specimens; 60-90 days, 12 specimens; 90-120 days, 18 specimens; 120 days-term, 40 specimens. The tissue was fixed in buffered formalin, Bouin's fluid, or FAA (5 parts formalin, 5 parts acetic acid, 90 parts 80% ethyl alcohol). It was then embedded in paraffin, sectioned at 6-8 p, and alternate slides were stained with hematoxylin and eosin, Mallow's triple, and periodic acid-Schiffs-alcian blue.
OBSERVATIONSImplantation in the baboon is superficial and a decidua capsularis ...
