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.

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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.

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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.

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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.

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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.

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Reference:

Knospe, C. 2002. Periods and Stages of the Prenatal Development of the Domestic Cat Anat. Histol. Embryol. 31, 37-51

Feline placentation

Keywords: feline, placenta, anatomy

Feline gestation is approximately 64 days, starting about a day after breeding (cats are induced ovulators). Therefore, feline gestation is marginally longer than canine gestation.

Using body, crown-rump and head diameters from various published sources, the pregnancy below was estimated to be approximately 53 days of age. The first image serves as a key for the main image. Unfortunately, the manner in which the fetuses were arranged for photography suggests that they were somehow attached to one another. They were in fact, three separate fetuses.

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In these specimens, the allantochorionic membranes of these fetuses were transected and everted (turned inside out), releasing the allantoic fluid from each fetus and exposing their internal chorionic surfaces (in reality, their allantochorionic surfaces because the allantois and chorion fuse as gestation progresses). The amnionic fluid remains within the amnions of the fetuses.

The gross anatomy of feline placentation is described as "zonary" and is described in greater detail elsewhere in LORI.

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Note the pellets of meconium; probably an indication of fetal stress. In advanced gestation, it is not uncommon to find yellowish amniotic fluid which in turn, has stained the fetus a yellowish color. Meconium is often found in the lungs of those fetuses as well.

Meconium staining is generally associated with fetal stress but a literature search will reveal that the pathophysiology is not at all clear. Fetuses with meconium staining are not always in state of acidosis and the overriding sympathetic tone associated with stress does not explain anal relaxation and increased peristalsis.

As shown above, the extra amnionic umbilical cord is not well defined in cats. The same is true of dogs as illustrated elsewhere in LORI. One could argue that these species do not even have extraamnionic cords. This is because the exocelom spreads out widely where the base of the exocelom and yolk sac remnant attaches to the placental zonary bands.

As mentioned elsewhere, the feline placenta does not have a green hematophagus border as is seen in canids. However, the mechanism for iron absorption appears to be similar in both species

Surgical anatomy of the feline tract

Keywords: surgery, ovariectomy, comparative, feline, ligaments, anatomy

A feline reproductive tract dissected in situ, suspended under water. Water has swelled some of the connective tissue obscuring fine blood vessels. The color contrast and saturation of the tract has been increased to delineate structures.

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First, note that the uterus is bicornual with a very short uterine body, similar to that in bitches. The bladder has been reflected ventro-caudally to show the uterine body. The ureters can be seen at the base of the bladder.  

In both queens and bitches, there is no major (middle) uterine artery such as the arteries seen in the ruminants and horses. Mesometrial vascular supply is only surgically significant, in pregnant queens and bitches, in older animals, and those with uterine pathology, Therefore, transection of the mesometrium in a cranial-to-caudal direction in nulliparous queens and bitches will usually not result in serious hemorrhage.

Close inspection of the image shows that the left kidney is situated slightly more caudal than the right kidney. The left ovary in turn, is also more caudal than the right. Therefore it is sometimes suggested that the left ovary is easier to remove and should be be removed first, while the abdominal incision is still stretching. However this difference in location is only slight and may not be important during surgery. 

Fortunately there is far less adipose tissue around the uterus and ovaries in queens and the depth of the abdomen is generally less than in dogs, so ovariectomy is generally more straightforward in queens than bitches.

Ligamentous strictures surrounding the ovaries are of surgical importance. In queens, the utero-ovarian ligament (also known as the proper ligament of the ovary) is fragile in comparison to that of bitch Therefore caution should be exercised when grasping it to elevate the ovary in preparation for ovariectomy. As noted by Hill and Smeak (2010) rupture of this structure may not only cause hemorrhage but can also fracture the ovary, result in an ovarian remnant syndrome (see below). It is also important to realize that one usually cannot apply enough tension to the utero-ovarian ligament of queens to allow safe rupture of the suspensory ligament as has been described for bitches. That technique for bitches is well described by Hill and Smeak as well. Therefore, only light tension should be applied to the utero-ovarian ligament in queens, while the suspensory ligament is transected with scissors. This allows  the ovary and its vascular pedicle to be elevated for safe removal. This approach also prevents painful sub-peritoneal hemorrhage due to tearing of the peritoneal blood supply at the base of the ovarian suspensory ligament.

Unlike bitches, queens do not have ovarian bursae that surrounds the ovaries completely.In the image above, one can see the surface of the ovaries  This is not possible in bitches As alluded to already, rough handling the the utero-ovarian ligament in queens may fragment their ovaries. However, it is also possible to fragment a feline ovary merely by handling it. In such cases, the absence of a complete ovarian bursa allows ovarian fragments may be carried on instruments and the surgeon's gloves to other areas of the abdomen. The omentum and peritoneal surface are well vascularized, allowing ovarian fragments to seed and grow into accessory ovarian tissue. This is the essence of the feline ovarian remnant syndrome.

Reference.

Hill, L.N. and Smeak D.D 2010. Suspensory ligament rupture technique during ovariohysterectomy in small animals. Vetlearn. Compendium for continuing education for Veterinarians. June 2010. E1-8

The feline vestibule and vagina.

Keywords: anatomy, cat, feline, vagina

It is sometimes stated that a swab can be used to stimulate the feline cervix  to cause LH release and ovulation. However, the cat has a vagina that is too narrow to accommodate the tip of a common rayon swab. Only the portion of the tract that is caudal to the external urethral orifice (Ex U.O. below) i.e. the vestibule, can accommodate a swab tip. Interestingly, the distance from the vulva lips to the cervix appears to be too long for a feline penis. Cranial to the Ex U.O. it also becomes too narrow to accommodate the penis. This anatomy suggests that the short, barbed feline penis only locks into the vestibule and that  a tom cat ejaculates some distance caudal to the cervix.

Legend: VL = Vulva lips, Cx = cervix.

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