5.1.1 Skeletal Muscle Development and Growth

The musculoskeletal structure of birds can vary tremendously based on whether they fly or not. The majority of the muscles can be concentrated on the torso for birds that rely primarily on flight to move, as is the case in grouse, whereas birds that rely mostly on running, such as the pheasant and quail, have larger leg muscles [1]. Skeletal muscle cells are elongated, multinucleated cells, called myofibers. At hatch, chicks possess fully differentiated skeletal muscle cells, and muscle growth that occurs thereafter is due to an increase in muscle fiber size [2].

5.1.2 Embryonic Development of the Skeletal Muscle

During early stages of embryonic development, the primordial muscle cells, called myoblasts, concentrate in a block called the myotome. As development progresses, the myoblasts migrate from the myotome into their muscle beds where they continue to proliferate. Some myoblasts fuse with each other to form primary multinucleated muscle fibers, others continue to proliferate and eventually fuse into secondary muscle fibers. The formation of muscle fiber is complete at hatch and subsequent muscle growth is due to the enlargement of existing fibers in a process called hypertrophy [2, 3].

Chicks, for most common gamebirds, are precocial and possess relatively well‐developed leg muscles at hatch. This is due to high levels of circulating maternal thyroid hormone during embryonic development and highly active thyroid during the perinatal period [4, 5].

5.1.3 Muscle Growth Post Hatch

Muscle hypertrophy occurs by the fusion of satellite cells located between the basement membranes of muscle fibers [6, 7]. The proliferation potential of satellite cells is higher in younger animals and diminishes as the animal gets older. Much of what is known about factors regulating the proliferation of satellite cells is derived from in vitro studies in cultured chicken and turkey embryonic myoblast satellite cells [1]. Among these factors, insulin‐like growth factor (IGF) was found to enhance chicken myoblast differentiation and turkey myoblast proliferation [8]; fibroblast growth factor (FGF) was found to enhance turkey myoblasts and satellite cell proliferation [9, 10]; platelet‐derived growth factor (PDGF) was found to enhance chicken myoblast proliferation [11, 12]; hepatocyte growth factor (HGF) was found to activate and enhance proliferation of dormant chicken and turkey satellite cells [13, 14]; transforming growth factor beta (TGF‐beta) was found to inhibit both proliferation and differentiation of turkey satellite cells [15] but when added with FGF, it leads to chicken myoblast differentiation [16]; and myostatin was found to inhibit turkey and chicken myoblast and satellite cell differentiation [17, 18].

5.1.4 Maternal Inheritance of Muscle Growth Property

Muscle morphology is an important determinant of meat quality, and focusing on fast growth rates and muscling alone results in poor meat quality that displays pale, soft, exuding meat – PSE. When cooked, PSE results in soft, poor water binding and poor juiciness. Muscle morphology defined as extracellular matrix spacing and muscle fiber size was determined to be inherited from the mother in turkeys [19]. Although these studies were done in turkeys, maternal inheritance of muscle morphology could be a contributing factor in muscle morphology in gamebirds, so selection of sires and dams during breeding should be carefully considered.

5.2.1 Bone Modeling

Bone acquisition during growth is referred to as bone modeling. It occurs in one of two ways: endochondral ossification or intramembranous ossification. During endochondral ossification, a cartilage anlagen is first formed, in a process referred to as chondrogenesis, followed by its replacement by bone. During embryonic development, progenitor cells that undergo chondrogenesis derive from the ectoderm and mesoderm. These mesenchymal cells condense and differentiate into cartilage‐secreting cells known as chondroblasts. The resulting cartilaginous primary skeleton anlagen grows quickly and is subsequently turned into bone, during prenatal and postnatal growth. This process, referred to as endochondral ossification, is mediated by the activity of chondroclasts, that resorb the cartilage, and osteoblasts, that deposit bone [21].

The second process by which bone modeling occurs is intramembranous ossification, in which bones grow through a mesenchymal template without having to go through the cartilage state first. During this process, condensed mesenchymal cells differentiate directly into osteoblasts at a number of different sites simultaneously and start depositing bone [22].

5.2.2 Long Bone Growth

Long bones grow in both thickness and length. Increase in bone thickness (appositional growth) occurs through the formation of new bone on the outer surfaces of existing bone; increase in bone length (longitudinal growth) is achieved through the activities of the cartilaginous growth plates. Growth plates can be divided into several distinct zones: resting, proliferating, maturing, and terminally differentiated chondrocytes Appositional growth is the result of osteoblast activity on the outer layer of the bone where bone is deposited in successive laminar patterns. This process is coupled with osteoclast activity that causes the resorption of the inner surface of the bone.

5.2.3 Medullary Bone

Medullary bone is a secondary bone tissue that is exclusive to birds [23]. It deposits in spicules in the cavities of long bones and serves as a labile calcium reservoir to support the formation of eggshell in the uterus of the female. The contents of the medullary bone fluctuate with the ovulatory cycle and the position of the egg in the oviduct [24, 25]. When the egg is in the infundibulum, isthmus, or magnum, the bone‐building osteoblasts, under the action of estrogen, actively deposit medullary bone; when the egg transitions to the uterus (eggshell gland), osteoblasts cease depositing bone, while the bone‐resorbing osteoclasts, under the activity of parathyroid hormone, start their bone resorption activity to release calcium. The freed Ca is then deposited on the eggshell, which in turn serves as a reservoir for the developing embryo. The amount of Ca in each eggshell typically represents about 10% of the total body Ca stores in the bird [26]. In avian species that lay a lot of eggs, such as the commercial laying hen, the calcium demand for eggshell formation can get too high and result in the mobilization of calcium from cortical bone. This leads to thinning of the bone and osteoporosis [27].

5.3.1 Metanephric Kidney Development and Osmoregulation in the Embryo

Osmoregulation in the avian embryo involves a balance between water loss, due to evaporation though the eggshell, and water gain through increased metabolic activity as development advances. Maintaining a balance between water loss and water gain is crucial for avoiding osmotic stress, dehydration, and embryonic death [30]. The metanephric kidney and the chorioallantoic membrane (CAM) work together to regulate water and electrolyte balance [31]. Early in embryonic development, the CAM is the major regulator of water and ion balance [32]. As development progresses, the mesonephros, which is then replaced by the metanephric kidneys, takes over osmoregulation as well as nitrogen secretion. During this time, nitrogen secretion switches from urea to uric acid [31].

5.3.2 The Urinary System in the Adult

The urinary system in birds consists of paired kidneys and ureters that empty into the urodeum of the cloaca. The role of the kidney is to filter blood, excrete waste products and ions, and reabsorb needed substances like glucose. The avian kidney is divided into three divisions: the anterior, middle, and posterior lobes [33]. Each lobe is a collection of units referred to as lobules. The outer part of the lobule is called the cortex, which constitutes about >75% of the lobule, and the inner part of the lobule is called the medulla, which constitutes <15% of the lobule; the rest is blood vessels. The functional unit in the kidney is a long convoluted tubule called the nephron [34]. Because birds do not have a urinary bladder, the output of the kidneys is emptied into the cloaca. It then moves in a retrograde fashion toward the large intestine where more water, sodium, and potassium are reabsorbed [35, 36].

In birds, the cloaca and lower intestine serve a similar function to that of the distal nephron in mammals, making fine adjustments in water and sodium reabsorption from the urine.

5.3.3 Urine Concentration and Response to Dehydration

Unlike mammals, the ability of birds to produce hyperosmotic urine is limited. Domestic chickens, for instance, can produce urine that is only twice the osmolality of their plasma [37]. This limited ability is attributed to the fact that the majority of the nephrons in their kidney (85%) are reptilian type nephrons, meaning they lack a loop of Henle, which is the part of the nephron that gives mammalian type nephrons the ability to excrete salt. When birds are subjected to extreme dehydration that results in acute osmotic load, the kidneys respond by slowing down their glomerular filtration rate (GFR) [38] and by reducing the number of functioning reptilian type nephrons [34]. When the bird is severely dehydrated, GFR can be reduced as much as 65%. GFR is controlled by the pituitary‐secreted antidiuretic hormone arginine‐vasotocin (AVT). AVT affects the GFR by decreasing the number of filtering loopless nephrons, by constricting the vasculature around the nephrons [34] and epithelium of renal tubules [39].

5.4.1 Gaseous Exchange

The function and anatomy of the respiratory system of gamebirds are not much different from other birds [42]. Cycles of inspiration and expiration bring air into the respiratory tract where the gaseous exchange occurs in the paleopulmonic parabronchi of the lung in a humidified environment. Gas exchange does not occur in the air sacs; however, the air sacs act as a bellows to move the air through and out of the respiratory tract. Breathing is an active process as the lung is not expansible and birds do not have a diaphragm. Muscle contraction is needed for both inspiration and expiration to create pressure differences for airflow. Contraction with inspiration moves the sternum forward and down, and the vertebral ribs move to expand the sternal ribs and abdominal cavity, creating negative pressure. During expiration, contraction with a different set of muscles decreases abdominal volume [40, 41].

The pattern of airflow through the respiratory tract means that two cycles of inspiration and expiration need to occur to move an initial volume of air into the tract and then its subsequent exit through the upper part of the tract [41, 43]. With both inspiration and expiration, airflow through the paleopulmonic parabronchi is unidirectional from caudal to cranial. Upon inspiration, airflow is to the cranial and caudal air sacs and air leaves the paleopulmonic parabronchi to the cranial air sacs; with expiration, air leaves the cranial air sacs for exhalation through the trachea and air from the caudal air sacs passes cranially to the parabronchi of the lung.

5.4.2 Ventilation Control

Ventilation changes in response to blood pH or arterial gases (PaO₂ and PaCO₂) can be species dependent. In response to changes from normal, ventilation frequency and/or tidal volume will adjust. Sensing is done by chemoreceptors with central, arterial and intrapulmonary receptors located in the brain, carotid artery, and lung, respectively. The arterial chemoreceptors detect changes in PaO₂, PaCO₂, and blood pH. Intrapulmonary receptors are very sensitive to PCO₂, assessing on a breath‐by‐breath basis. Overall, the system is more sensitive to changes in CO₂ than O₂ [44]. Under conditions of increased CO₂ in inspired air, frequency and tidal volume increase. When a decrease in PaCO₂ or intrapulmonary PCO₂ occurs, ventilation frequency is decreased. With hypoxia (low O₂), while typically not experienced, ventilation is initially increased, although once PaCO₂ starts to decrease, ventilation frequency will also decrease.

5.4.3 Respiration and Acid–Base Balance

Shifts in blood pH can have a number of negative effects on the animal’s system. The respiratory system is able to respond quickly to these shifts. By changing ventilation frequency, blood pH can be modified through PaCO₂ adjustments [40]. Circulating levels of CO₂ are a primary determinant of blood pH. In the blood, CO₂ is present in different forms, about 5% as dissolved in arterial blood, combined with hemoglobin, and the majority in the form of bicarbonate ion (HCO3⁻). To produce bicarbonate, CO₂ combines with water to form carbonic acid that dissociates to bicarbonate and hydrogen ions under enzymatic action as demonstrated in the following equation:

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