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Carnegie Stage 5a-1 Introduction

Stage 5a specimens have a postfertilization age of 7 to 8 days and are characterized by a trophoblast that is still mainly solid. The greatest diameter of the conceptus is less than 0.5 mm. The blastocystic cavity is flattened probably because of its collapse during implantation. Mesoblast formation begins along the inner side of the trophoblast. The embryonic disc is approximately 0.1 mm in diameter and is composed of two layers, a thick layer called the epiblast and a thin layer called the hypoblast. The amniotic cavity is apparent for the first time and is formed by the curved epiblast of the embryonic disc. The endometrial stroma is edematous. Stage 5a is represented by databases for two Carnegie embryos, a younger one designated Stage 5a-1 and an older one designated Stage 5a-2.

The younger stage 5a specimen is Carnegie embryo #8020 that has an estimated postfertilization age of 7 days and had been given a grade of excellent. The conceptus shows early superficial implantation having eroded the endometrial epithelium but barely penetrated the endometrial stroma. Isolated spaces are present within the syncytiotrophoblast (e.g., sections 50 - 52) that has begun to engulf endometrial gland cells (see sections 50 - 63). Maternal sinusoids have begun to enter the syncytiotrophoblast ( e.g., sections 35 - 41, 59 - 67). A portion of the conceptus is still exposed to the uterine cavity. A small amniotic cavity is visible and amniogenesis is underway. The epiblast of the embryonic disc is composed of polyhedral cells that have no precise pattern of arrangement. Three mitotic figures are evident. The hypoblast is composed of relatively small, darkly stained, vesiculated cells with less distinct boundaries than those of the epiblast. No mitoses are present.

The embryonic disc measures 0.044 x 0.092 x 0.126 mm. The chorion measures 0.125 x 0.300 x 0.450 mm and the chorionic cavity measures 0.044 x 0.186 x 0.288 mm. (Table of Dimensions; Hertig and Rock, 1945)

The specimen was prepared for microscopic examination in 1942. It was fixed in 70% alcohol and Bouin’s fluid, embedded in celloidin paraffin, and serially sectioned at 6 µm. The sections were mounted on glass slides and stained with H & E. Two of the glass slides hold sections through the conceptus. There are 78 sections through the conceptus and 20 sections through the embryonic disc. Structures are identified in every section image.

The morphology of this embryo is well documented in the literature. It was first described by Drs. A. T. Hertig and J. Rock in 1945. Reconstructions of the maternal blood vessels at the implantation site are illustrated in the publication and are reproduced here.

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 fly-through animations of the aligned sections are also included on the disks.

Source: The Virtual Human Embryo.