1) General Zoological Data
The squirrel monkey is widely used in research and has been classified as comprising 5 separate species by Wilson & Reeder (1992). Numerous subspecies or different names have been employed for this animal whose phenotype can differ appreciably between animals from different regions of the Amazon basin. There is also controversy, as some authors have comprised all squirrel monkeys in one species. The variability of karyotypes alone (deBoer, 1974) argues against such simplification, however. A more recent splitting of the group has been provided by Groves (2001) who gives detailed description of their phenotypes and distribution. Gotch (1979) stated that the name "Saimiri" derives from the Brazilian/Portuguese for small monkey.
This small South American cebid monkey weighs up to 1,000 g, is arboreal prefers insects, small reptiles and birds as well as fruit (see Lima & Ferrari, 2003), and is a diurnal animal. It associates in relatively large groups (over 100 animals) in the tropical rainforest and rarely comes to the floor of the forest. It is endowed with one of the largest brains for body size in primates (1/9th is CNS) and has been widely used in numerous experimental protocols. It has been useful especially in neural studies and for research into parasitology and virology. Many Primate Research Centers employ this anima for research, and so do many private laboratories. There is a huge literature on the results of many biological studies conducted in this species that has seen marked reduction in the wild and some subspecies of which are endangered now. Hof et al. (2002) who wrote about CNS aging of primates suggested that squirrel monkeys live more than 25 years.
General Gestational Data
Gestation lasts from 146-175 days and produces generally a single young. Twins are very uncommon. The newborn of this gestation weighed 140 g, but Hayssen et al. (1993) mentioned size up to 247 g.
Dukelow et al. (1983) spent much effort to define the growth of preimplantation oocytes growth and effects on cleavage if in vitro fertilization after superovulation (16 h post hCG). They found no significantly increased aneuploidy as result of their manipulations. I have been unable to find publications on early implantation in this widely used species.
4) General Characterization of the Placenta
I have had only one delivered placenta available from a term infant that died neonatally. In addition, slides of a serially-sectioned implanted placenta were reviewed. The two placental disks, characteristic for this species and other cebidae, weighed 22 g and measured 5x4x0.5 cm each. Their surface vasculature was connected around the lateral aspect of the free membranes. This is a trabecular/villous placenta with hemochorial relationship to the maternal circulation. While Mossman (1987) mentioned the existence of villous hematopoiesis, none was found in the specimens I examined. The disks are infiltrated by a few large maternal arteries that have stiff walls, infiltrated by fibrinoid and are surrounded by syncytium (Luckett, 1974; Gruenwald, 1972).
Details of fetal/maternal barrier
The slender villi are covered by syncytiotrophoblast as seen in the next photograph. While there is cytotrophoblast below the syncytium, at term it is so delicate that it cannot be demonstrated.
The cord inserts near the center of one of the two disks. While this disk is often larger than the second one, that was not the case in this placenta. The cord had no twists and possessed two arteries, one vein and no ducts. It is covered by a very thin amnion. The length of umbilical cords in Saimiri was given as 8.5 cm by Spatz (1968).
7) Uteroplacental circulation
The free membranes connecting the disks lack atrophied villi, unlike the condition in the human placenta. This, however, has been the case in most bilobed primate placentas. It carries the connecting vessels laterally between the two disks. The membranes have an inner single layer of epithelium without squamous metaplasia that is planted on a thin layer of connective tissue. It is loosely attached to the chorionic membrane that carries the blood vessels. Outside of the chorion is a small layer of cytotrophoblast and a very minimal amount of decidua capsularis. There are no vitelline or allantoic remnants.
Trophoblast external to barrier
Cytotrophoblast, equivalent to the "extravillous trophoblast" of human gestations superficially invades the decidua beneath the two disks and is also found to alter the maternal arterioles in a manner very similar to human placental sites. Deeper invasion does not occur.
There is complete decidual transformation of the endometrium and relatively few extravillous trophoblastic cells can be found deep in the endometrium. At the site of connection to the endometrium, numerous vacuolated extravillous trophoblastic cells are present intermixed with a small amount of fibrinoid.
Squirrel monkeys have no menstruation; instead, there is a well-defined estrus cycle. Ovaries have a large amount of interstitial gland tissue (Mossman & Duke, 1973).
There is no subplacenta. The absence of atrophied villi in the free membranes remains unexplained.
It is likely that such studies have been performed in Primate Research Centers, but I have not been able to access fundamental information.
15) Pathological features
16) Physiologic data
17) Other resources
18) Other remarks - What additional
Information is needed?
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