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Carnegie Stage 5b Introduction

Stage 5b specimens have an estimated postfertilization age of 9 days with a conceptus diameter of approximately 0.1 mm. The distinguishing characteristic at this substage is the presence of numerous irregular, slit like lacunae within the cytoplasm of the syncytiotrophoblast. Most of the lacunae communicate with each other and with the endometrial sinusoids but they contain relatively little maternal blood.

Stage 5b is represented in the database by Carnegie embryo #8004 that was given a grade of excellent. The conceptus is superficially implanted and imperfectly covered by endometrial epithelium. The bilaminar embryonic disc is slightly oval and dorsally concave. The epiblast is composed of pseudostratified, columnar epithelium in which the cytoplasm is beginning to become vacuolated ventrally. The hypoblast is a single layer of cuboidal or polyhedral cells. The gastrulation (primitive) streak could not be identified.

The endometrial stroma shows an early decidual reaction containing large, clear epitheloid, vesiculated cells that have an oval or polyhedral shape. Future villi begin as cytotrophoblastic clumps that project into the syncytiotrophoblast. The irregular shaped chorionic cavity is filled with maternal blood as a result of a defect in the chorionic membrane. This is considered to be atypical. There are few extraembryonic mesoblast lining the inner surface of the chorion. The umbilical vesicle (yolk sac) appears for the first time and becomes limited by a layer named the exocoelomic membrane. The membrane is considered to be the wall of the primary umbilical vesicle and the surrounding meshwork is thought to be extraembryonic endoderm rather than mesoblast. The amniotic cavity is smaller than the primary umbilical vesicle and is almost closed by amniogenic cells.

The embryonic disc measures 0.046 x 0.100 x 0.132 mm. The chorion measures 0.310 x 0.450 x 0.582 mm and the chorionic cavity measures 0.120 x 0.185 x 0.312 mm. ( click here to see a Table of Dimensions from Hertig and Rock, 1945)

The specimen was prepared for microscopic examination in 1942. It was fixed in Bouin's fluid and 70% alcohol, embedded in celloidin paraffin and serially sectioned at 6 µm. The sections were mounted on glass slides and stained with H & E. One hundred of the sections are included in this database. There are 96 sections through the conceptus including 17 sections through the embryonic disc. Structures are identified in every section image.

The morphology of this specimen is well documented in the literature. It was first described by Drs. Hertig and Rock, 1945. Reconstructions of the conceptus and related maternal blood vessels are illustrated in that publication.

The sections have been digitally restored and labeled, and can be viewed at three magnifications. Several 3D reconstructions have been produced from the aligned sections. Animations of these 3D-reconstructions together with flythrough animations of the aligned sections are also included on the disk.

Source: The Virtual Human Embryo.