Cuniculture: Rabbit Biology

			Introduction : the main bones
			General bone structure and bone growth
			Muscles and muscle growth
			Apipose tissue

Introduction : the main bones

For a complete description of the skeleton and muscles, specialized works such as the Atlas d'Anatomie du Lapin, published by Barone et al. (1973) are advised (Latin vocabulary, French and English). Also, articles such as those written by Cantier et al. (1974), establish correspondence between the names of the skeletal muscles adopted by different authors.

Figure 5 : Skeleton of a rabbit (Soucre : Barone et al., 1973)

Skeleton bones

The main bones appear in figure 5 opposite with names =>
Below example of a rabbit skeleton presentation (figure 5a)

Skull and dentition

Figure 6 is the sagittal section of the skull. The size of the nasal sinuses, which occupy one third of the volume of the inside of the skull, is worth mentioning. A side view of the skull is illustrated in Figure 7a and a photo of a rabbit skull is give on figure 7b


Figure 6: Sagittal cut of a rabbit's skull. ( Source: Barone et al., 1973).	Figure 7a: A side view of the skull. (Source: Barone et al., 1973).	Figure 7b : Photo of the skull of a rabbit

Figure 8 shows the implantation of the teeth. Rabbits have two pairs of incisors in the maxilla, and one pair in the mandible which has enabled zoologists to clearly distinguish between rabbits (and lagomorphs in general), and rodents that only have one pair in the maxilla and one pair in the mandible. In the rabbit, the second pair of incisors is situated behind the first and therefore, completely hidden.
All incisors are covered with a layer of enamel which is thinner at the back than the front, thus allowing the rabbit to sharpen its teeth by filing the upper ones against the lower ones. The anterior face has a longitudinal furrow.
There are no canines and a strong diastema separates the incisors from the premolars (3 + 2 pairs) and molars (3 + 3 pairs).

In all, there are 2 pairs of incisors, 3 pairs of premolars, 3 pairs of molars in the maxilla and 1 pair of incisors, 2 pairs of premolars and 3 pairs of molars in the mandible making a total of 28 teeth, of which, 26 are only functional. As in the case of all lagomorphs, the teeth are deeply embedded in the bones but have no roots (figure 8).
Their growth is continuous throughout the animal's life. A measurement of the growth rate of the incisors gave a value of 2 mm per week for the maxilla and 2.4 mm for the mandible. That of the molars and premolars (also called "jugal" teeth) is much slower and has been estimated at only 2 mm per month (~ 4 times slower) (Shadle, 1936).

The kits are born with milk teeth (incisors and premolars), which fall out at around 18 days of age and are almost immediately replaced by the permanent ones.

Figure 8 : View of the implantation of rabbit`s teeth. The bone was separated so that the base of the teeth can be seen. (Source: Barone et al., 1973).

The vertebrae and bones of the limbs

The skeleton has 7 cervical vertebrae, of which the first two are the atlas (carrying the head) and the axis which plays a primordial role in the rotation of the head. The 12 thoracic vertebrae are joined to 12 ribs. Only the first 10 ribs are ventrally attached to the sternum to form the thoracic cage. The other two ribs are called "floating ribs". The 7 lumbar vertebrae are followed by the sacrum which consists of 4 fused sacra vertebrae. The sacrum contains the pelvic bones. Immediately after there are 15 coccygeal vertebrae, the last 10 of which form the tail.. General organisdation of vertebrae is descibed in figure 5. Detailed illustration of vertebral bone is available on figure 9a.

The radius and the cubitus (also called the ulna, the longest of the two foreleg bones) in the anterior limb are in contact but not fused (Barone et al., 1973). However, in the posterior limb, the tibia and the perone are completely fused in the distal part. The "remaining" part of the perone is called the fibula. Illustation of bones of the 2 legs is available in figures 9b and 9c


Figure 9a : Vertebrae of a rabbit	Figure 9b and 9c : Bones of the forelimb and of the hindlimb of a rabbit

The oganisation of pectotal and of the pelvic grids are illustrated in figure 9d and 9f. The figure 9e contains photos of the scapula (pectoral gridl)


Figure 9d : Pectoral girdle with details of the scapula	Figure 9e ; Photos of the scapula (both sides)	Figure 9f : Pelvic girdle - Ventral & ½ dorsal views

General bone structure and bone growth

B0nes have two different structures. On the periphery there is a hard dense "layer", which is the compact bone. The middle part consisting of small cavities is the spongy bone whether compact or spongy, adult bone tissue is laminar. The structure of the compact bones rests on a beautiful order of concentric laminae. The laminae in the spongy bones form an irregular pattern of fine rows. The bone marrow inside the bones is responsible for producing components in the blood and immune system, that is, red blood cells, white blood cells and platelets.

Bone cells

Three types of cell are involved in the life and metabolism of the bone tissue. These are the osteocytes, osteoblasts and osteoclasts, fed by a network of blood vessels.

The osteocytes are cells trapped in the bone matrix but which communicate with each other by fine extensions connected together. These cells are "mechano-receptors" sensitive to pressure variations exerted on the bone matrix.

The osteoblasts are found on the surface of the trabecular bone and they synthesize the elements of the bone matrix, as well as facilitating their calcification. They communicate with each other and with the matrix osteocytes, via a system of specialised cellular unions.

The osteoclasts are cells with several nuclei. They locally reabsorb the surface of the bone trabeculae in the form of small erosions, which are the origin of an initial reabsorption area. They are not related to the other bone cells by unions but are paired by soluble mediators that move from one cell to another.

Skeleton growth

Initially, the development of the skeleton is ensured by points of ossification (from fibrous cartilage membranes). The combined work of the osteoblasts ensures the growth of the bones in diameter and thickness by continually renewing the bone mass. The osteoblasts are predominantly active during the growth period. The conjugation cartilage at the base of the epiphysis of each bone (or epiphyseal plate) is responsible for growth in length. After 140-150 days, growth in length stops, the length of the rabbit stabilizes and the epiphyseal plate "closes", that is, the conjugation cartilage disappears, mainly due to the action of steroid hormones (Gilsanz et al., 1988). Average skeletal growth speed is slightly slower than that of overall body mass from birth to 5-6 weeks of age : allometric coeficient a = 0.82 (Cantier et al., 1969). After this age, speed of growth of the skeleton is is about one half of that of the whole body mass :a: 0.55, and skeleton growth stops when the rabbits are 5-7 months depending of the breed.

Muscles and Musclar growth

Quantitatively speaking, the most important muscular masses are found in hindquarters. These are the loins and thighs. The skeletal muscles ensure the animal's movements and for this reason are attached to the skeleton in 2 or 3 places, so that movement is generated when they contract.

From the "meat" point of view, the Longissimus dorsi muscle has the greatest mass (Swatland, 1994). From the anatomical point of view, it is a succession of different muscles: the Long dorsal, Long spinal and the Longissimus lumborum (Cantier et al., 1974).

Very diverse in shape, size and function, skeletal muscles are also characterized by strong tissue heterogeneity.. The nerve tissue (afferent motor fibres and efferent sensitive fibres) ensures muscular contraction and relaxation control. The blood vessels guarantee the supply of nutrients and oxygen and remove products from the cellular catabolism. The muscle fibres constitute 75 to 90% of the muscle volume and are the base element of the striate skeletal muscle (Gondret, 1997). They are covered in a delicate sheath of conjunctive tissue called endomysium which is rich in collagen (figure 9g).

Figure 9g : Diagram of the structure of a muscle - Source : Gondret (1997)

Muscle fibers and muscular growth

The muscle fibers are grouped together in primary bundles covered in a fine conjunctive sheath called perimysium. The adipose cells, called intracellular adipocytes, are found amongst the primary bundles. The primary muscular bundles are in turn grouped together into secondary bundles, covered in a thicker sheath of perimysium. The epimysium encloses all the thick secondary bundles to form the cover/envelope of the muscle itself. The conjunctive section at the muscle ends forms the tendons and aponeuroses responsible for binding the muscle to the skeleton. The areolar tissue between the interfibrilar spaces forms an intricate network rather like a more or less developed elastic mesh. Its function is to provide the muscle with a framework capable of organizing the muscular fibres, packing the nervous and vascular elements and providing the necessary lubricated surfaces for the muscular fibres to move amongst each other and amongst the bundles (Doutreloux, 1992).

From a quantitative point of view, muscular speed of growth is a bit more rapid than that of the whole body from birth to 10-12 week of age, according to the breed or the line : allometric coefficient a = 1.2. After this period corresponding to the establishment of puberty, muscular growth slows sharply down and is about one half of that of the whole body : allometric coefficient a : 0.50 (Cantier et al. 1969). .Muscular growth ends when the rabbit is 5 to 7 month of age , depending of the breed. However, individual muscles follow different patterns of longitudinal and cross-sectional growth, so that their functional capacities (force, range of contraction) and mutual functional relationships are age-dependent. Fibre type distribution, fibre cross-sectional area and compactness, colour and metabolic characteristics varied according to age. The effects of eventual feeding treatment are low in comparison with age effects.

Muscle fibers and myofibrils

Muscular fibre is an elongated, tubular, plurinucleate cell. It is the basis of motor activity Most of the sarcoplasm in the muscular cell is occupied by contractile elements called myofibrils. There are three different types of filaments in the myofibrils: thick ones (150 Å in diameter), which are basically formed by myosin, and fine ones (70 Å in diameter), formed by actin. They also contain tropomyosin and the troponin complex. The intermediate filaments constitute a network of longitudinal and transversal filaments associated to the sarcomere. When the bundles of fibres go from one end of the muscle to the other, parallel to the main axis, the muscle is described as "fusiform" (i.e., the Biceps femoris). When they form a more or less definable angle, in relation to the main axis, the muscle is "penniform" (from penna, feather); an example of this is the Longissimus lumborum.

Myosin and actin have a key function in the architecture and a key enzymatic function in muscular contraction, during which chemical energy is transformed into mechanical energy. Muscular contraction is produced when the fine actin filaments slip between the thick myosin ones. The energy required for contraction comes from ATP hydrolysis via the myofibrillar ATPase. The ATP is then regenerated by the aerobic or anaerobic metabolism. Depending on the speed at which the fibres contract in the centre of a muscle and on the major type of metabolism working to regenerate the ATP used in the contraction, different types of fibres are differentiated; their functional characteristics are listed in table 3. Bearing in mind the very variable myoglobin content of one type of fibre or another and the fact that some muscles are formed exclusively by one type of fibre, the rabbit's muscles are either red or completely white

Table 3. Characteristics of the different types of muscular fibres. Source: Gondret (1997).

Characters	Types of Fibres
Characters	I	IIA	IIX	IIB
Contraction sped	slow	fast	fast	fast
Fatigue resistance	+++	++	?	?

Motor plate surface	+	+++	?	+++
Sarcoplasmic Reticulum	+++	+++	?	+++
Transversal tubules	+	+++	?	+++
Color	red	red	white	white
Myoglobin	+++	+++	++	+
Capillairy density	+++	++	?	+
Number of mitochondrion	+++	+++	++	+
Collagen content	+++	++	?	++
Section area	+	++	+++	++++

Use of glycogen	+	++	++	++++
Use of lipids	+++	+++	?	+

Myofibrillar ATPase	+++	++	?	+
Hexokinase	+++	++	?	+
Phosphorylase	+	++	?	++
Anaerobic enzymes of glycolyse	+	++	++	++
Oxidative Enzymes (aérobies)	+++	++	++	+

Repartition of main muscles on the rabbit carcass

The main skeletal muscles observable during a dissection are described in figure 9h. For more details it is possible to refer to the original article published in French in 1968 by Cantier and Vezinhet with explanations for each muscle. In this article, for each cited muscle, the French name and the international Latin name are given

Figure 9h : Main skeletal muscles

Pectoral and superficial neck muscles	Superficial neck muscles	Neck and shoulder muscles	Deep nek muscles	Abdominal muscles	Sub-lumbar muscles

Hind limb, external face	Hinf limb, internal face	Hind limb, external face (deep muscles)	Front limb, externam face	Forearm, external face	front limb, internal face

Adipose tissue

At slaughter age (10-12 weeks), the dissectable adipose tissue, 1.4-1.7% rabbit's live weight, is composed by abdominal (peri-renal) fat (53%), scapular fat (41%) and inguinal fat (6%) . In addition to these organised tissues the presence of small quantities intermuscular adipose tissue (fat nodules) and some intramuscular adipose cells must be mentioned. In the rabbit like in some other species adipose tissue is one location of lipids synthesis (in addition to liver) and of lipiolyse . It acts as a reserve tissue which develops especially once that metabolic requirements of the growth of noble tissues (nervous, skeleton, muscles) have been reached. If in a slaughter rabbit (10-11 weeks) , dissectable adipose tissue represents about 1.3 to 2% of live weight), in some particulaly fat adults, abdominal fat alone can represent up to 25% of live weight or more.

From birth till 5-6 weeks of age, weight development of adipose tissue is slower than that of the whole body mass : allometric coefficient a= 0.82. After this age adipose tissue begins to grow faster than the whole body (a= 1.87) till about 10-12 weeks of age At this period of rabbit’s life, skeleton and muscular tissue developments begin both to slow down and adipose tissue begins to grow very rapidly : a=3.21 till adulthood (Cantier et al. 1969)

The presence of intermuscular adipose tissue inside of the different carcass cuts (fat nodules), and the simultaneous presence of adipose cells inside of muscles are largely responsible of juiciness and of the taste of rabbit meat. The importance and lipid composition of these internal adipose components are sharply correlated with the more visible abdominal fat. For this reason, quality of a rabbit carcass is partly estimated by the importance of visible abdominal fat . To help the estimation of the general adiposity of a carcass a simple system of visual notation was established in France: note 1 for the too lean carcasses up to note 5 for the too fat carcasses. This system is described below on figure 9i

Figure 9i : French system of notation of rabbit's carcass adipodity
Note 1	Note 2	Note 3	Note 4	Note 5

No or little fat The 2 kidneys are uncovered and protruding	Two individualized lateral fat masses reaching the groin in two narrow bands. Absence of fatty cord along the spine The right kidney is lightly covered on the outer side The left kidney adheres to fatty tissue but is not at all covered	Two fairly large lateral fat masses over the entire length abdominal cavity and the presence of a thin fatty cord along the spine. The right kidney is half covered by adipose tissue The left kidney is lightly covered, its outline remains visible	Two thick and wide fat masses running the length of the abdo-minal cavity, and the presence of a very distinct fatty cord along the spine. The right kidney is two-thirds coated, its upper surface remains visible. The left kidney is covered over more than half of its surface	Two thick and wide fat masses encompassing the fatty cord central in the renal area and covering more than half of the saddle surface. The right kidney remains slightly visible. The left kidney is completely covered.

Special case of the newborn

At birth the newborn rabbit contains about 5.8% of lipids, wile later this proportion varies from 10% to 15% according the age or reproduction stage. But if the lipids content is clearly lower, the main diffreence is in the repartion in the young rabbits of fat tissues (energy reserves). The main lipids reserve is in the brown adipose tissue and partily in the liver .Lipids of the white adipose tissue represents only 8% ot the total (figure 09j) . The role of these particular tissues (brown and white adipose tissue) is explained the section 7.4 devoted to the newborn biology . But in few words the brown adipose tissue is a lipids reserve exclusively used for thermoregulation under nervous control. This tissue will disappear some weeks after weaning (modification of type of metabolism). Lipids of all other tissues participe normally in the general metabolism.

Figure 09j : Repartition of lipids of a newborn rabbit in the different tissues and organs (according to Dawkins et al., 1964)

Biology of the Rabbit - Index of the Website


	Biology of the Rabbit by François LEBAS Directeur de Recherches honoraire de l'INRA English revised version of "Biologie du lapin" , translated from French by Cathy R. Martin and Joan M. Rosell Edition 2020

3 - Skeleton and Muscle growth