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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 ...

Baboon placentae were studied which ranged in age from 13 to 40 days. Implantation in the baboon is superficial and a single, discoid, villous hemochorial placenta is formed. No chorionic villi develop peripheral to the placenta and there is no decidua capsularis. Trophoblastic tissue within the placenta is of two types, cyto-and syncytiotrophoblast. The chorionic villi are short and straight at 13 days but have begun to branch by 16 days. The villi consist of a mesenchymal core surrounded by a single layer of cytotrophoblast which is in turn surrounded by a layer of syncytiotrophoblast. Cytotrophoblastic cells from the distal tips of the villi spread over the maternal surface of the placenta and form a complete cytotrophoblastic shell against the uterine stroma. Maternal blood enters and leaves the intervillous space through clefts in the cytotrophoblastic shell which connect with arteries and expanded venules within the endometrium. A limited decidualization of the uterine stroma takes place.Early stages of baboon placentogenesis have been described by Noback ('46) who reported on a single specimen from late in the first month of gestation, by Gilbert and The purpose of this paper is to describe the structural changes which occur in the baboon placenta from the appearance of the chorionic villi until the definitive embryonic placenta is established. MATERIALS AND METHODSA total of 66 placentae ranging in insemination age from 13 to 40 days were employed in the present study. The distribution of the specimens was as follows: 13-15 days, 5 specimens; 16-18 days, 7 specimens; 19-21 days, 8 specimens; 22-25 days, 16 specimens; 26-30 days, 17 specimens; 31-40 days, 13 specimens. The placentae were obtained by cesarean section according to the method of Claborn et al. ('67). The insemination age was based on single matings of 2 to 12 hours duration with the day of mating considered day zero, or in cases where mating extended over more than a one-day period the third day preceding perineal deturgescence was designated day zero (Hendrickx and Kraemer, '68).The placentae were fixed in 10% buffered formalin, Bouin's fluid or FAA (5 parts formalin, 5 parts glacial acetic acid and 90 parts 80% ethyl alcohol), dehydrated, embedded in Paraplast and sectioned at 6 to 10 p. Younger specimens were sectioned serially with the embryo intact and stained with hematoxylin and eosin. Older embryos were removed and sectioned separately and serial sections were made from blocks of placental tissue. In these cases, alternate sections were stained with hematoxylin and eosin, and Mallory's triple, Masson's trichrome or Enders-Mossman-Groat placental stain. OBSERVATIONSImplantation in the baboon is superficial and central with chorionic villi formed only over the embryonic pole of the blastocyst, the original site of attachment to the uterine mucosa. This results in the formation of a single discoid or slightly ovoid placenta with a membranous, avillous paraplacental chorion and no decidua capsularis. Placentation in the baboon wil...