Twenty one to 22 day feline embryos
Keywords: anatomy, embryology, feline, embryo, exocelom, yolk sac, allantois, amnion
A feline pregnancy, 21 to 22 day into gestation. In this case, the tract was removed from a feral cat so the actual duration of gestation was not known. The length of gestation was verified by comparing the development of these embryos with embryos of known gestation in Knospe's publication of 2002 (see reference).
The tense, spherical shapes of an early feline pregnancies are evident in this image. These lend themselves to easy identification during transabdominal palpation in cats. The spherical shapes become less distinct after 35 days of gestation. In this pregnancy, two of the embryonic-placental units have been opened. It was of course necessary to transect the chorioallantois and release the allantoic fluid to expose the embryos within their amnions.
A feline pregnancy, 21 to 22 day into gestation. In this case, the tract was removed from a feral cat so the actual duration of gestation was not known. The length of gestation was verified by comparing the development of these embryos with embryos of known gestation in Knospe's publication of 2002 (see reference).
The tense, spherical shapes of an early feline pregnancies are evident in this image. These lend themselves to easy identification during transabdominal palpation in cats. The spherical shapes become less distinct after 35 days of gestation. In this pregnancy, two of the embryonic-placental units have been opened. It was of course necessary to transect the chorioallantois and release the allantoic fluid to expose the embryos within their amnions.
The embryo at left has been partially removed from the uterus. The soft tissue indicated by the black arrow is the inner chorioallantoic surface of the zonary band of the placenta. The blue arrow indicates the same tissue but in that case, it is in situ within the uterine lumen.
The diagram below illustrates the cardinal features of an embryo at this stage of development. Note that the chorioallantois has not yet elongated. Elongation becomes evident as pregnancy advances.
A reminder: The outermost membrane in all embryos is the chorion. The allantoic sac lies inside the chorion. The fluid-filled allantoic sac covers the amnion either partially or completely depending on the species. Among the domestic species, coverage over the amnion is only complete in horses. Where the allantois lies adjacent to the chorion, it gradually fuses with that membrane. That fusion results in the formation of the chorioallantois or allantochorion (either term is acceptable). The amnionic membrane also fuses with the allantoic membrane, forming the amnioallantois. Except in texts on embryology, amnioallantois is a term seldom seen in the literature; instead it is commonly referred to as the amnion. The exocelom is a substantial cavity, lying between the yolk sac and the allantois and in an early embryo such as this, between the amnion and allantois as well (see the green color code in the images). The presence of the exocelom is often neglected although it persists and is obvious as a space in the umbilical cord even at the time of parturition in carnivores. It is not seen in advanced gestation in farm animals.
Image size: 2776 x 2212 px
The image below shows an embryo taken from the top image. To help the viewer understand the structures that are visible here, a key has been created directly below this image.
Image size: 1956 x 1398 px
The image below is a key for the image above. Recall that the allantochorion has been removed, so a double line (the membrane is a two-part structure) was added by the author to show its approximate position before the embryo was exposed. Also note that the zonary band of the placenta has been turned back to expose the embryo. If the zonary band was to be replaced, returned as shown by the yellow arrow, it would cover all the structures seen here.
Image size: 978 x 699 px
The paper by Knospe, C. 2002 included under references, contains numerous referenced benchmarks in organ development in feline embryos at 21 to 23 days of age . Paraphrasing some of those benchmarks:
The eyelids have started to form and the external ear has been established. The outlines of the future digits appear as rays. The genital protuberances have formed and the skeleton and muscles have begun to differentiate. The tongue and the primitive larynx are formed and the thymus, thyroid glands and heart appear. Pleural cavities are developing around the lungs and the lungs already show bronchi. The stomach is visible and the liver is developing. Of particular interest here is the fact that the indifferent gonads have developed but both the mullerian and wolffian ducts are still present.
The image below shows another embryo viewed from its anti-mesometrial aspect. In the carnivores, the yolk sac lies adjacent to the mesometrium. The embryo and its membranes lie against the backdrop of the placental attachment zone. Because the allantochorion had to be transected to reveal this view, the allantois and its fluid have been lost. Three fluid filled cavities remain. The halo of the amnion lies closest to the embryo. Below that is the elongated yolk sac. With a circumference larger than that of the yolk sac is the clear outline of the exocelom.
Image size: 1720 x 1165 px
Reference:
Knospe, C. 2002. Periods and Stages of the Prenatal Development of the Domestic Cat Anat. Histol. Embryol. 31, 37-51
