Carnegie Collection of human embryos

Carnegie embryo 8171. Early lacunar stage (Stage 5b)
Courtesy of Dr. Allen C. Enders

An important source for human embryology, including implantation and formation of the placenta, is the Carnegie Collection now housed at the Human Developmental Anatomy Center in Washington D.C. The core of this collection is the carefully dated series of embryos first described by Hertig, Rock and Adams (here).

The Virtual Human Embryo is an online ressource based on the serially sectioned embryos in this collection and includes 3D reconstructions. It covers all 23 Carnegie stages in the first 8 weeks of embryonic development and cannot be too highly recommended.

Carnegie Embryo 7801. Showing extraembryonic coelom (eec)
and secondary yolk sac (sys) (Stage 6)
Courtesy of Dr.Allen C. Enders

Now a group in Amsterdam has used the Carnegie Collection to develop an additional annotated digital atlas of human development (described here). They also utilized material from the Boyd Collection at the Centre for Trophoblast Research in Cambridge.

They make two claims. First that representations in textbooks have become increasingly schematic. This is demonstrably true. Second that the descriptions in standard texts are often based on extrapolation to humans from animal models. It is hard to assess if the latter truly is the case. For example Human Embryology by Hamilton, Boyd and Mossman (previous post) was based on the human embryos in the possession of the three authors. In Germany there was a strong tradition to cover the embryology of all vertebrates, concluding with the human, exemplified by Dietrich Starck's Embryologie.

In physiology, on the other hand, animal data often are presented as if they were human. One example concerns oxygen tensions in various parts of the fetal circulation. Pretty much every textbook of physiology has a large illustration of the fetal circulation with data obtained in sheep by Dawes, Mott and Widdicombe. The figure legends often fail to acknowledge the source or the species or both.

Placentation in a marmot: the woodchuck

Woodchuck or groundhog (Marmota monax)
CC Wikimedia Commons

Not much has been written about placentation in squirrels and other sciuromorph rodents - at least in comparison to myomorphs (e.g. mouse, hamster) and hystricomorphs (e.g. guinea pig, capybara). Marmots belong with chipmunks and ground squirrels in the Tribe Marmotini. The most complete description is for the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) (Mossman & Weisfeldt Am J Anat 1939;64:59-109). It was based largely on specimens collected a century ago by Thomas George Lee (background here).

Interhaemal region in the labyrinth of a woodchuck placenta
Marmota monax Courtesy of Dr. Allen C. Enders

The woodchuck has a labyrinthine, haemochorial placenta. In the above section the large channels with maternal red cells are lined by syncytiotrophoblast. A fetal capillary is seen to the right.

Early Development of the placenta in the Colorado chipmunk
Tamias quadrivittatus Courtesy of Dr. Allen C. Enders

There are several ways to make a haemochorial placenta (discussed here). Squirrels go through a transient endotheliochorial phase. The figure is from a chipmunk and shows a maternal capillary in which part of the endothelium has been replaced by syncytiotrophoblast whilst two endothelial cells remain intact (further figures here). In the first instance this creates the equivalent of the spongy zone found in other rodent placentas.

Later the fetal mesoderm grows into the trophoblast bringing with it the fetal capillaries. To start with the outgrowths are fingerlike (villi) as can be seen in a recent publication on the woodchuck (here). Nearer term, however, the labyrinth occupies most of the depth of the placenta. The spongy zone is then very thin and occupied by syncytiotrophoblast with clumps of nuclei as well as mononucleate giant cells (Dr. Allen C. Enders, personal communication).

Syncytins are endogenous retrovirus envelope genes (previous post). Two occur in murid rodents and another in South American hystricomorphs. Now a search of the genome of the thirteen-lined ground squirrel has turned up several candidates and further work in the woodchuck has shown one of them to be a bona fide syncytin. By in situ hybridization the gene was not expressed in the labyrinth but rather in the part of the spongy zone that had yet to be reached by the fetal vessels.

The Mammalian Ovary

Mossman and Duke 1973

It is now 40 years since publication of Harland Mossman's "other" book (previous post). It differs in many respects from the better known volume on fetal membranes. In particular it is richly illustrated with photomicrographs. Most of the specimens were in the collections of its authors. The Harland W. Mossman Collection is conserved at the University of Wisconsin Zoological Museum and there is a searchable data base (here).

Kenneth L. Duke was an alumnus of Brigham Young University at Provo, Utah. On retiring from Duke University he sent his collection there. Mercifully it has remained intact although it has not been catalogued. Much of the material will be from Duke's yearly field trips to The Great Basin but there is also material from South East Asia. There are placentas from some species including the Philippine colugo.

Comparative Morphology of the Mammalian Ovary takes as a starting point the red squirrel (Tamiasciurus hudsonicus) before dealing with a number of topics across the full range of mammals. For individual taxa the synoptic tables at the back of the book are unsurpassed as a source. The footnotes to these tables indicate where they rely on Duke's Collection now at BYU.

Disclosure: I am adjunct curator of The Harland W. Mossman Collection.