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Carnegie Stage 22 Introduction

Stage 22 embryos have a greatest length of 23.4 to 27.5 mm and an estimated postfertilization age of 52 to 55 days. At this stage the eyelids cover approximately half of the exposed part of the eye. The fingers approach those on the other side and sometimes touch and overlap. Laminations in the tectum of the mesencephalon represent the presumptive superior colliculus. Many nuclei, fiber tracts and commissures are evident in the brain. The cochlear duct has formed a complete circle with the tip upturned for the second time. The palatine shelves have moved to the horizontal position above the tongue and make contact with the lower edge of the nasal septum. A lumen is present in a portion of the submandibular duct. All of the vertebral segments have formed and exhibit normal spina bifida. All of the joints of the extremities can be identified. Ossification has begun in the clavicle and long bones. A few large glomeruli are present in the kidney and the paramesonephric ducts fuse with each other near the dorsal side of the bladder trigone.

None of the transversely sectioned stage 22 specimens in the Carnegie collection are satisfactory for use in the DREM database so the specimen selected to represent this stage is #H983 from the Boyd collection located at the University of Cambridge in Cambridge, England. This embryo had a greatest length of 28 mm prior to fixation. We wish to thank Professor Graham Burton for making the specimen available for the project. This male specimen was collected in 1962, fixed in formalin, embedded in paraffin and celloidin, and serially sectioned transverse to the long axis at 10 microns. The sections were mounted on 198 large glass slides. Every other slide was stained with hematoxylin and eosin while the intervening slides were alternately stained with either trichrome or silver. Unfortunately, the records of this specimen's preparation are incomplete so not all of the details of the techniques used to prepare the specimen for microscopic study are available.

There are 2151 sections through the embryo. Approximately every fifth section was digitally captured and the Browse part of the DREM database includes 203 of the 408 section images captured. Approximately every tenth section was labeled, and can be viewed at four magnifications. Several 3D reconstructions have been produced from the aligned section images, however, several factors related to the preparation of the specimen mean that the reconstructions are less than ideal. First of all, there are quite severe shrinkage arifacts apparent in the sections, especially in the head and, combined with the fact that the left cerebral vesicle was damaged during preparation, means that the reconstructions should only be regarded as approximations of the original morphology of the embryo. Additionally, the alternating stains give rise to differing degrees of shrinkage making accurate alignment and registration of the sections very difficult. Animations of the 3D reconstructions and flythrough animations of the aligned sections are also included on the disks.

Source: The Virtual Human Embryo.