Embryology of the domestic cat

[Translated into English by Karin Sandbergen.]

By A.L. Leipoldt
Reprinted with permission.

(Note by PawPeds: The author talks about trimesters in the pregnancy. In the context of this article, a trimester means a period of three weeks.)

Introduction

Since the beginning of the 19th century, biologists knew that the early developmental stages of the different classes of vertebrates were very much alike and that embryos will only start to become really different from each other at a later stage. Karl Ernst von Baer (1791 - 1876), the most prominent embryologist of that time, formulated a number of universal rules on the basis of these observations. The most well-known is Baer's law and goes: "general characteristics a species shares with other vertebrates appear earlier in the embryo than the characteristics specific to the species in question" (vertebrates are birds, reptiles, fish, mammals, amongst others...).

Thus the development of the limbs of all vertebrates is, in essence, similar and only later on during the development, differences become noticeable between wings, paws, arms and legs. This similarity in development was one of the phenomena that inspired to Charles Darwin the idea that different species of animal, which, amongst others, resemble each other in this manner, have a mutual origin (The origin of species, 1859).

Therefore, embryology is an interesting subject. Especially comparative embryology has already fascinated many people. Of course, the embryos of two different vertebrates can always be distinguished from each at any stage, if only because their DNA does not match. The observations by Darwin and von Baer, that these embryos, as regards their appearance, are so highly alike during their early development, is not only fascinating but gives food for thought, especially to those people, who are of the opinion that there is a huge difference between man and (any other) animal.

Fertilisation of the ovum

The beginning of an embryo's development is coitus, the mating, of course. The cat, different from a human being, for example, is an induced ovulator, the ovulation(s) of a cat only take place during (and as a consequence of) the mating. During mating millions of sperm are ejaculated into the cat's vagina, but only a few hundred of them reach the Fallopian tube. Thanks to coitus one or more ova are released in the female cat (this is called ovulation), which "travel" through the Fallopian tubes to the uterus. Fusion of ova and sperm takes place in the Fallopian tubes. Not every random sperm will fuse with a female cat's ovum, recognition will take place, so that only the sperm of a male cat can form a fertilised ovum (zygote) with the a female cat's ovum. If animal species are very much alike and are closely related as a species, a zygote can be formed by parents, who belong to different animal species.

We will only discuss here what happens to the zygote, whose parents belong to the animal species Felis Domesticus. A cat's zygote is not as quickly formed as in a mammal that ovulates independently from coitus, but it does not last longer than about twenty-four hours. With cats it turned out that 20 to 28 hour after mating, zygotes (fertilised ova) of 13mm on average could be flushed out of the Fallopian tubes.

First trimester of the pregnancy

After a successful fertilisation, the first step is cell division. That one fertilised ovum divides itself through mitosis into two cells. Mitosis is a cell division in which two copies develop that each have the same cell core. Genetically, those cells are identical to each other. In female cats this first division takes place after 60 to 68 hours after coitus. These first cells are called blastomeres. After this first division, there will be a division every 10 to 14 hours. Division will soon become asynchronous, so not every cell will divide if another divides too. It has been established in different kinds of mammals that every one of the cells has the complete ability to grow into a full-term individual at the two-cell or four-cell stage. Identical "real" human twins are formed by the separation of two cells after the very first division. Very rarely, but it seems to happen, identical quadruplets occur in humans.

After the four-cell stage, not all cells are "totipotent" anymore. Therefore, identical (monozygotic) octuplets will not occur. Divisions continue, there will be more and more cells. This stage is called the cleavage stage, because little growth takes place, only division (the embryo does not grow yet). Of course, the amount of nuclear material will increase, because every new cell has a nucleus. The lump of cells consisting of blastomeres is called a blastomerula. There are approximately 30 cells that form a sphere, four days after fertilisation. This is called the morula, "mulberry". The diameter is virtually the same as the diameter of the blastomerula.

After about six days, the so-called blastula develops by forming a cavity, surrounded by 60 to 80 cells, which are 0.6mm in diameter. This cavity is the beginning of what will later become the digestive tract. The outer cells of the blastula are called trophoblast; these cells are the beginning of the placenta. In humans, implantation of the embryo takes places in the uterine wall at the end of the first week. Whether this moment also applies to cat embryos, I do not know.

Before that time, trophoblasts produce gonadotropin, a hormone that ensures that the endometrium is ready for implantation of the embryos. The embryos have nestled themselves for the next, very important phase: gastrulatio. Gastrulatio is an exceptional process. Different zones of the blastula fold, and form three more or less distinguishable layers: the ectoderm, endoderm and mesoderm. Ectoderm is the designation for the outer cells of the body: therefore, the skin (epidermis), but the central nervous system also has an ectodermal origin. The endoderm consists of cells that (will) make up the digestive tract, the gastrointestinal system, then. The mesoderm, finally, are the cells, which will make up the muscles, skeleton and internal organs. The first mesodermal structure that will be formed is the chorda, the spinal column. Meanwhile, growth continues and the embryo becomes egg-shaped. The dimensions are approximately 1.5mm by 1mm.

Gastrulatio is a vital and vulnerable moment in the existence of an embryo. At this stage, the genetic material of the embryo itself is expressed. Before that time, processes were mainly controlled by maternal influences (by the mother, therefore) through the material in the egg-cell, originating from the mother. Many embryos die at this stage because of lethal (deadly) gene combinations, producing non-functioning or defective proteins. At this stage the cat's embryo is 1.5mm by 1mm.

During the next stage the organs are developed. The ectoderm forms a skin that covers the entire body and a thickened plate develops, which folds into a groove: the neural crest. Ultimately, this will become a tube, which will submerge below the skin's surface and will differentiate to the central nervous system. We have already (or just!) reached day 13.

Something surprising happens: concentrations of mesodermal tissue, "somites", appear on either side of the neural crest, as a result of which the embryo is divided into identical structures. The embryo has an elongated shape and a rod-shaped structure across its middle, the notochord, the first skeletal element (the spinal cord in development). The number of somites increases.

Dorsal and caudal (head and tail end) can be distinguished well, and already a tail is showing. The endoderm is folded as a tubular structure, the beginning of the gastrointestinal system. A primitive sort of blood circulation also develops.

At 15 to 17 days the cat's embryo is 2mm to 10mm big. The neural tube closes. The head is prominently present and slightly bent towards the caudal side of the embryo. Again, something surprising happens: the embryo develops gill arches, four in total in the cat. The primitive gastrointestinal system is ready and there is the beginning of a mouth. In the pharynx behind the mouth (the oesophagus) deep grooves develop that reach to the ectoderm. Later on, these will disappear again, but partly due to these gill arches, the thought arises that each embryo just repeats evolution. At the same time the cerebellum develops. It is already clear where the eyes and ears will be. The kitten has reached day 18 after the conception. At days 18 and 19 forelegs and hind legs develop. The cerebrum and brainstem are already present in a rudimentary form. The embryo is between 7mm and 18mm long, neck and tail are bent towards the trunk.

Everything happens quickly now. Until the 21st day the following things happen: the front gill arch (left and right) deepens itself and ultimately forms the auditory duct. The would-be eyes have a clear pigmentation and a duct for the olfactory organ deepens itself. The cerebellum is divided by a groove. The forelegs are now divided (as they are in the adult cat), with an internal skeleton, yet the hind legs are still primitive protuberances. The tail lengthens and is curled towards the body. Internally, veins differentiate for the blood circulation and nerves. The larynx, bronchi and lungs can also be discerned. Oesophagus, stomach and intestines are formed. Pancreas, thyroid gland, kidney and liver are also developed. On the side of the back, vertebrae are formed. The primitive spinal cord continues into the tail. Even the genital system is already present, albeit primitively.

The embryo is now 10mm to 24mm long. It may be clear that, especially in the early stages, people should be very careful with possible damaging substances. Vaccination with a live (attenuated) virus can be harmful to the embryo and also the use of, for example, remedies for fungi can be detrimental. Antibiotics are only to be administered if strictly necessary. Particularly Baytril should be avoided. Of course, the female cat's life takes precedence over the life of the kittens and naturally the female should only be mated if she is 100 percent healthy. But even pregnant cats can fall ill.

In the meantime the female cat shows signs that she is pregnant: her nipples become slightly larger and pinker than they were before. After 21 days, an experienced person can determine by way of palpation (a way of feeling and touching carefully) if the female cat is pregnant. At this stage, the foetuses are still so small (the size of a large pea), they can be distinguished among themselves and can therefore be counted. Some female cats suffer from morning sickness as a consequence of hormonal changes in their bodies.

Second trimester of the pregnancy

Over the next days (21 to 23 days) the upper lip is formed and eyelids begin to develop. Earlobes are formed. At the end of the legs, toes can be distinguished as a fan of dark matter against a lighter background. The genitalia are developed further. Internally, the skeleton and the muscles differentiate further and the ribs are formed, amongst others. The skeleton that is formed, still consists of cartilage. The face develops, tongue and palate are formed. Thyroid gland, parathyroid gland and heart develop, and an organ that most people at best only know as "sweetbread" from haute cuisine. The common biomedical term for sweetbread is thymus. This is situated in the chest cavity between the breast bone and the heart. The thymus has a central function in the immune system and is responsible for the production of so-called T-cells. If the thymus is absent in newborn animals, they produce hardly any antibodies at all and are therefore very susceptible to infections. But if the thymus is experimentally removed from animals (mice) when they are a few days old, these problems will not occur so much: the lymphocytes that are produced in the thymus and move on to the spleen and lymph glands, probably have already passed on the necessary information about what is foreign to spleen and lymph gland. The embryo now measures 13mm to 30mm.

At 23 to 25 days, the toes of the foreleg, which were already visible, but still formed a lump, start to separate from each other. On the hind legs, which are a little behind in comparison with the forelegs, the would-be toes can be distinguished like a fan of dark matter against a lighter background, as happened before with the forelegs. Kidneys, adrenal glands and genitalia further differentiate. Within the primitive spinal cord, grey and white matter is separated and surrounded by a membrane. Grey matter mainly consists of nerve cells. The grey colour comes from the nucleus of those cells. White matter consists of attenuated nerve fibres, which contain a white, fatty substance, called "myelin". This white matter shrouds the grey matter in the spinal cord. Axons and ganglia differentiate. In the head the jaws, palate, tongue and salivary glands develop. Within the brain the pituitary gland forms itself, an important organ that produces several different hormones: Vasopressin, which is relevant to the body's water balance, Oxytocin, which plays a role at birth and afterwards at lactation and then a number of hormones involved with the regulation of other glands: thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH) and luteinising hormone (LH), which both have an effect on the sex glands. In addition to this the pituitary gland also secretes growth hormone, which plays a very important, guiding role in the growth (of the young animal) as well as in its metabolism.

Stage 25 to 28 days, 21mm to 40mm. At this stage the embryos are in fact virtually complete mini cats and should rather be called foetuses. Everything develops further and differentiates even more. The head develops cheeks, a chin, a nose and a mouth. The area where the navel will be is reduced. The peritoneum, the pulmonary membrane and the diaphragm are formed. The carpal bones, toes and ribs are still cartilaginous, but are starting to calcify. The teeth are formed (hidden away in the jaw).

Things are beginning to take shape. The foetuses are now also growing rapidly and most "parts" are in their designated place. During the phase of 28 to 32 days, they are 25mm to 50mm big and it becomes difficult to determine the number of foetuses. Over the eye (that has been formed), an eyelid develops itself and there are small, triangular ears. The inner ear starts to differentiate into a complicated structure with which the animal can later hear; the eardrum is formed. On an image, the foetus already looks like a kitten. The ossification of several parts, still consisting of cartilage and where bone should usually be, goes on. The bronchi differentiate more and more. In female foetuses the womb develops. The liver, already present, is getting "lobed". The bladder develops. The size of the foetus is now 25mm to 50mm.

The foetuses keep on growing, the head is and will stay relatively big. The female cat now grows visually bigger, certainly if she carries multiple foetuses. We have now arrived at the period of 32 to 38 days. The size of the foetuses is now 35mm to 60mm. The skin feels smooth, there is no fur yet. The external genitalia are formed. The toes develop nails. The eye differentiates further, the iris is formed, amongst others.

Third trimester of the pregnancy

From days 38 to 44 the skin differentiates. It becomes thicker and starts to wrinkle. The ears become bigger, the tail grows longer. The inner ear is formed, but is still of cartilage. The alveoli are developed. The pituitary gland becomes lobed. Within the intestines, intestinal villi are growing. The eyelids are complete and present, the eyes are closed. The embryo is 50mm to 80mm in size.

On day 44 the embryo measures 59mm to 94mm and fur is developing, the foetus is covered with a silky layer. From day 48 (65mm to 125mm), possible pigmentation is visible.

At last, the final stage from day 58 on. This is the stage just before birth. All organs are sufficiently fully-grown to make the foetus viable. A normal delivery takes places between 59 and 69 days after conception. Before day 59, the kittens will not yet be viable enough to survive without special medical help. If gestation lasts longer than 69 days, it will be wise to consult a vet. If the female cat does not mind that her temperature is taken rectally, you can take her temperature every morning: from 12 to 24 hours before delivery her temperature drops from its normal reading of 38.5 degrees Celsius to 37.5 degrees Celsius. A normal delivery starts with cervical dilation. The cat will produce some mucus from her vagina, but it is possible you will not notice this, because she will clean this up herself, if she is able to reach with her big belly. This stage can last up to 36 hours, especially with a primipare female cat (i.e. a female cat that has her first litter). It gets serious when she gets contractions. You can feel these strong contractions very well, when you place a hand on her belly. The most important thing you can do is: being there… Most female cats like it when you keep them company. Delivery of the first kitten often takes the longest. Do not panic too soon, just observe that the female cat stays calm. As long as the mother is not agitated or panicky, most of the time there is nothing going on. It is always pleasant, when both you and the female cat are inexperienced, to get support from an experienced breeder.

However, giving birth is something that is very natural and most of time no complications will happen. Still, it is wise to inform your vet about the approaching happy event. If you should need veterinary assistance, it is good to know who you can call (somehow, kittens like to be born in the middle of night...).

And if everything has gone well... Congratulations...

Sources

• C. Darwin. In The Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life, First Edition. Chapter 13: Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.
• C. Knopse. Periods and Stages of the prenatal development of the domestic cat. Anat. Histol. Embryol. 31,37-51, 2002 Blackwell Wissenschafts-Verlag, Berlin, ISSN 0340-2096. PDF-file: http://www.vetmed.uni-muenchen.de/anat1/english/2002_ck1.pdf.
• C.H. WaddingtonGeorge Allen & Unwin LTD. Principles of Embryology. 1954.