Principles of Bone Development in Horses - Kentucky Equine Research (2023)

Few animals are as precocious as the horse. Within 20 minutes of birth, a foal can be on its feet, and within hours it can be ready to run at speeds no human athlete will ever achieve. At this stage of life, despite this exceptionally early development, horses have only 17% of their mature bone mineral content, but they also have only 10% of their final bone mineral content.body weight🇧🇷 The relationships between growth, nutrition, strength and bone development, body weight and forces exerted on bones are orchestrated in careful balance when optimal growth is achieved.

The selection and breeding of horses for desirable qualities has been practiced for over 2,000 years. However, most of what we've learned about horse growth has been recorded over the last 20 to 30 years. In 1979, Dr. Harold Hintz on Windfields Farm growth data for 1992 foals from birth to 22 months of age. Records show how quickly foals grow. Thoroughbreds and other light horse breeds reach 84% of their adult height by six months of age. Assuming an adult thoroughbred has 16 hands, the six month old piglet has approximately 13.2 hands. At 12 months, this horse reached 94% of its adult height, or about 15 hands, and at 22 months, it almost stopped growing, reaching 97% of its full height, at about 15.2 hands.

Mature weight is reached more slowly; during thefirst six months of life, the foal gains 46% of its adult weight. Assuming an adult weight of 500 kilograms, the six-month-old baby weighs about 230 kilograms. By 12 months they will have reached 65% of their adult weight (715 pounds or 325 kg) and by 22 months they should have reached 90% of their adult weight (990 pounds or 450 kg). The average daily gains described by Hintz are the same as those recommended by the National Research Council (NRC) for moderate growth. The NRC reports that six-month-old piglets with an estimated adult weight of 1,100 pounds were gaining 1.43 pounds (0.65 kg) per day. The 12-month-old pups gained 0.5 kg (1.1 lb) per day, and the 18-month-olds gained 0.35 kg (0.77 lb) per day.

X-ray studies of bone mineral absorption in horses aged from one day to 27 years have shown that maximum bone mineral content (BMC) is not reached until the horse is six years old. Comparison of the rate of bone mineralization with age reveals a pattern that more closely resembles weight gain than height. By the age of six months, the horses had reached 68.5% of the mineral content of an adult horse, and by the age of one year, they had reached 76% of their maximum BMC. Bone is a much more dynamic tissue than it appears at first glance; However, complete bone mineralization lags behind growth in height and weight.

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bone development

A basic understanding of the bone development process helps explain the intricate nature of growth in horses. There are two anatomical types of bones in the skeleton. Flat bones are generally protective and include the skull, jaw, and parts of the pelvis. Long bones are found in the limbs and include the cannons, arms and thighs. Flat and long bones are functionally different and develop through two distinct processes. Flat bones develop by intramembranous ossification and long bones develop by endochondral ossification. The development of the long bones is of great general interest because of its influence on the health of the horse.

External examination of the bone cannon reveals broad appendages at the ends of the bone (epiphyses), a cylindrical tube tapering to a central girdle in the middle (midshaft or diaphysis), and a developmental zone (metaphysis and physis). 🇧🇷 In growing long bones, the physis or growth plate is a layer of proliferating cells and expanding cartilage matrix. The growth plate calcifies, remodels and is replaced by bone at the end of bone growth. In cross-section, the bone marrow cavity is a central cylindrical tube. The girdle of long bones contains the most densely calcified area of ​​cortical bone. Seen from the waist towards the metaphysis and epiphysis, the cortex thins and the interior is filled with a network of thin, calcified tubes known as spongy or trabecular bone.

There are two surfaces where bone comes into contact with soft tissue: an outer surface (periosteum) and an inner surface (endosteum). These surfaces are lined with layers of osteogenic or bone-forming cells. The endosteum is metabolically more active due to structural differences in compact and trabecular bone. Between 80 and 90% of the volume of compact bone is calcified, while only 15 to 20% of trabecular bone is calcified.

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Trabecular bone is in close contact with bone marrow, blood vessels and connective tissue. The endosseous bone surface represents 70% of the interface with soft tissues, such as muscle. The strength of compact bone helps it perform its primarily mechanical function, and trabecular bone is more changeable and metabolically active. Bones are composed of collagen fibers, usually oriented in a preferred direction, and non-collagenous proteins. Fusiform or platelet-shaped crystals of hydroxyapatite are found in the collagen fibers they contain and in the ground substance. They are normally oriented in the same direction as the collagen fibers. Fiber orientation changes from layer to layer in mature bone, giving bone a lamellar structure. When bone forms rapidly during development, the collagen fibers become loosely packed and randomly oriented.

Endochondral ossification occurs mainly during foal growth before birth. Longitudinal growth occurs in the growth plate in several zones after birth. The first zone contains quiescent chondrocytes. The second zone houses the chondrocytes, which divide and synthesize the matrix; It's called the proliferative zone. The hypertrophic zone is where maturing chondrocytes enlarge and produce vesicles of calcium and alkaline phosphate matrix. The fourth zone is the calcified zone. The mineralization process begins when capillaries and stem osteogenic cells invade the chondrocyte columns. Chondrocytes in the newly calcified area die. Osteogenic cells proliferate and differentiate into osteoblasts. Osteoblasts are arranged along the remnants of cartilage trabeculae and produce collagen and the ground substance of the matrix components.

Membrane-bound cell bodies released by chondrocytes and osteoblasts facilitate mineralization. These are known as ECM vesicles and contain calcium and phosphorus. They also provide enzymes that can break down mineralization inhibitors. Alkaline phosphatase hydrolyses phosphate esters, increasing phosphatase concentration, which in turn increases mineralization. Osteoblasts transform into osteocytes or bone cells in the calcified matrix. Osteocytes are embedded deep in the bone in osteocytic lacunae. A network of thin tubules that runs through the entire bone matrix connects the osteocytes.

The bone cells associated with bone resorption are osteoclasts, giant multinucleated cells found in contact with calcified bone. Osteoclasts resorb bone by means of lysosomal enzymes. Enzymes are excreted by a wavy edge. Absorption begins with the digestion of hydroxyapatite crystals, which contain calcium and phosphorus.

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The growth process of long bones involves the development of bone tissue and the simultaneous resorption of bone tissue. As bone develops in the growth plate, it forms the metaphysis. The metaphysis expands outward with a narrow shell. Growth must occur in connection with the structural and metabolic functions of the bone. This is achieved through continuous appositional growth on the endosteal side of the bone and active removal of bone by osteoclasts on the periosteal surface. This brief description of bone development should help clarify the intricate and metabolically sensitive nature of the process.

birth rates

Optimal growth rates may vary slightly between breeds, but all young horses have several critical considerations for bone growth and development. Extremely rapid growth caused by overfeeding (especially energy) has been linked to this.developmental orthopedic disease(DOD) and lack of robustness. Periods of slow or diminished growth followed by rapid growth are particularly dangerous. Imbalanced levels of calcium, phosphorus, and minerals have been linked to DOD. Certain types of forced training also seem to cause problems with bone development.

Significant bone development begins before birth and continues beyond 18 months of age. The period between three and nine months seems to be the most precarious for the foal in terms of DOD. During this time, serious illnesses can develop that can limit a horse's sporting potential. It is important to monitor growth rates and assess the foal's skeletal development. Steady and moderate growth.along a typical growth curveseems to be the best method to reduce development problems.

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Kentucky Equine Research (KER) has been weighing and measuring foals, weaners and puppies in central Kentucky on a monthly basis for over 15 years. These records, combined with numbers from universities and Windfields Farm in Canada, resulted in a vast body of comparative growth data. These data were formulated in software developed to monitor growth and make comparisons with databases containing records of thousands of foals. Observations from research tests and practical experience have led to the conclusion that slow and steady growth is best for horses. Kentucky Equine Research has reported the occurrence of DOD in Thoroughbred foals. A total of 271 foals were monitored. Ten percent of foals were diagnosed with DOD.

osteocondrite dissecante(OCD) Necklace lesions were diagnosed at a median age of 102 days in mares (2% affected) that were small (7 pounds or 3.2 kg below average) at 15 days of age. Knee and shoulder OCD lesions were diagnosed at an average age of 336 days. About 2% of foals were affected and tended to weigh 12 pounds (5.5 kg) above average at 25 days and 31 pounds (14 kg) above average at 120 days. Foals that developed OCD injuries to the hock tended to be heavier than average at birth, weighing 18 pounds (8 kg) above average at 15 days. They had higher average daily gains for up to 240 days and weighed 34 pounds (15 kg) above average at that age.

Kentucky Equine Research nutritionists recommend managers (1) record birth weight; (2) express weights as a percentage of a reference; (3) Do not allow deviation of 15% or more from thereference weights🇧🇷 (4) maintain DOD records and management changes; (5) weigh monthly; (6) do not overfeed lactating mares; (7) ensure adequate participation; and (8) consider early weaning if foals are growing too fast.

All efforts to support stable and continued growth in foals are important to limit skeletal problems. Foals that have had slow growth followed by prolonged outbreaks are at particular risk of DOD.

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