Preface The book entitled “Fundamentals of Veterinary Developmental Anatomy” has been prepared as per the Veterinary Council of India syllabus and regulations. This book mainly emphasizes on development and differentiation of various organs of domestic animals and has been prepared from several sources including personal experiences and text books of Human and Veterinary Embryology. In this book every chapter is summarized with some important points at the end of the each chapter which shall be useful to the students. The text material of this book is supported by well drawn illustrations. As an aid to understand gross anatomical at aspects in relation to the organogenesis, special efforts have been made to make explicit connections between developmental events and the adult anatomy. Teratology is an important aspect of the Developmental Anatomy. Hence, some important congenital abnormalities also have been mentioned in this text book.

1.1 Historical background of the Embryology Embryology is a branch of Anatomy. It is the study of growth and differentiation undergone by an organism in the course of its development from a single fertilized egg cell into a highly complex and independent living being like its parents. It explains how one develops before he is born. Important changes also take place after birth and continue to occur even to adult stage. eg: Replacement of RbC and WbC is a continuous developmental process and reverting to activities seen in prenatal condition is seen in healing of wounds. So the term ‘Developmental Anatomy’ which includes prenatal and postnatal development is preferred over term embryology as Developmental Anatomy, includes prenatal and postnatal development and as the term embryology refers truly prenatal (Intra uterine) development. Aristotle (384-332/322 BC) proposed two theories regarding origin of life

Development of the face includes the development of orbital, nasal and oral regions. Development of face involves five facial primordia during the embryonic period of development. These primordia include the single frontonasal prominence and the paired maxillary and mandibular prominences. The frontonasal prominence forms two sets of bilateral ectodermal thickenings, the nasal and lens placodes. Frontonasal prominence shows nasal component (of ridges) and a frontal component. A thickned area of ectoderm olfactory/ nasal placode appears at the medial tips of frontonasal prominence.

Alimentary canal : The endoderm is formed by delamination in mammals and birds. This endoderm surrounded by splanchnic mesoderm forms the gut. The endoderm underlying the embryonic disc and serving as a flat roof to the early yolk sac becomes gut endoderm. At 20th day a rapidly expanding embryonic disc begins to fold into a cylindrical embryo. The gut endoderm also participates as a component layer. Folding first into the head end and then into the hind end of elongating embryo this endoderm necessarily takes the form of the two internal blind tubes. The open end of the each tube where it becomes continuous with the yolk sac is called an intestinal portal while the tubes are named the foregut and hindgut. The caudal end of the hindgut is cloaca. An intermediate region opening ventrally into the yolk sac through the narrower yolk stalk is midgut.

The development of the upper respiratory system composed of nasal cavities, naso pharynx and larynx is directly related to the development of the face. The nasal cavities are derived from ectoderm while nasopharynx and larynx are endodermal in origin. The lower respiratory system (trachea and lungs) is a derivative of the embryonic pharynx. The epithelial lining of the lower respiratory system is endodermal in origin, while the surrounding connective tissue and cartilage are mesodermal in origin. The endoderm and underlying mesoderm from the floor of foregut i.e caudal part of pharynx grow caudally in midline and form the laryngo-tracheal groove at about the level of the branchial arches IV -VI from which the larynx and trachea arise. The laryngotracheal tube consists of an inner endodermal lining and an outer layer of splanchnic mesoderm. It grows into underlying mesenchyme. Ingrowths from the sides of the groove at about the level of the branchial arches IV - VI form a pair of ridges that grow towards each other and fuse forming a septum changing the groove into a tube with a blind end caudally. The rostral end opens into the floor of the pharynx. The blind caudal end expands in a bilobed manner into mesenchyme forming bronchial buds the primordia of right and left lungs while the groove is closing. larynx : A pair of elevations, aryteniod swellings appears on the floor of the pharynx along midline at the level of branchial arch IV. The hypobranchial eminence is just infront of the arytenoid swellings. The depression between the hypobranchial eminence and the arytenoid swellings becomes the lumen of the larynx. The caudal part of the hypobranchial eminence forms the epiglottis. The arytenoid swellings and adjacent branchial arch mesenchyme form the other laryngeal cartilages and musculature. The thyroid cartilage arises from branchial arches III and IV. The other cartilages are derived from mesenchyme of branchial arches IV, V & VI. The intrinsic musculature of larynx is derived mostly from the branchial arch V and VI. With the exeption of the epiglottic cartilage the cartilages and muscles of the larynx are formed by the end of embryonic period.

The kidneys develop from the nephrotome (intermediate mesoderm). The series of nephrotomes forms urino-genital ridge which splits up into an lateral nephric and a medial genital ridge, which project into the coelom from the dorsal wall. This is the short segment of mesoderm that connects somite with lateral mesoderm. Three sets of kidneys are formed during embryologic development in vertebrates viz., pronephros, mesonephros, and metanephros. First two sets are temporary structures and third set becomes the kidneys found in the adult.

Indifferent stage: Primordia are present for both sexes (gonads and mesonephric and paramesonephric ducts), although genetic sex is determined at fertilization. Gonads : are structuarally indistinguishable prospective testis and ovaries. The first sign of gonadal development is a swelling called the gonadal ridge on the medial side of the middle part of the mesonephros. It enlarges due to arrival of primary germ cells. Cells of mesonephric tubules invade the ridge to form clusters and besides surround germ cells resulting in gonadal cords. The primordial germ cells of these gonads arise from the endodermal cells of the yolk sac. These migrate to the gonadal ridge through the dorsal mesentry. Migration of primordial germ cells from the yolk sac endoderm into the gonadal ridges initiates testicular (if SRY gene is present) or ovarian development. The primordial germ cells form the sex cells, called gonadal blastema. The somatic blastema (non-germ cell) forms the other parenchymal elements of the gonads. The germ cells are capable of amoeboid-like movement.

Cardiovascular system consists of heart and blood vessels which include both arteries and veins. Blood vessels formation occurs in two stages i.e vasculogenesis and angiogenesis. Splanchnic mesodermal cells lining the yolk sac form clusters, referred to as blood islands. Vasculogenesis, the formation of blood vessels from blood islands of of Pander, commences during the third week of gestation in domestic animals first in yolk sac and later in the allantois.Development of blood vessels involves a complex series of events during which endothelial cells differentiate, proliferate, migrate and become organized into vascular network. Angiogenesis describes budding and sprouting of new vessels from existing vessels. This process a fundamental requirement for embryological development, continues postnatally.

Lymph vessels arise from mesoderm by vasculogenesis and angiogenesis in common with the arteries and veins. Initially six primary lymph sacs develop in the late embryonic period. They are paired jugular sacs lateral to the internal jugular veins, a single retroperitoneal sac developing close to the root of the mesentry, the cisterna chyli dorsal to the retroperitoneal sac and a pair of iliac sacs at the junction of the iliac veins. Lymphatic vessels draining the head, neck and fore limbs arise from jugular sacs. Drainage of lymph from the pelvic region and hind limbs is through the iliac sacs, while the retroperitoneal sac and cisterna chyli drain the viscera. At a later stage of lymphatic development, the lymph sacs become interconnected by a series of lymphatic vessels.The plexus between the jugular sacs and the cisterna chyli gives rise to the thoracic duct which opens into the jugular veins. lymphatic organs : The lymphatic organs are fully developed and functional before birth. Immunocompetent cells can be detected in most lymphatic organs by the end of the second trimester of gestation. lymphnodes : The lymphnodes are formed by the partitioning of small lymph vessels or sacs. Therefore, lymphnodes are usually found in related groups or in chains along the course of a lymph vessel. The partitioning is initiated by the ingrowth of mesenchymal processes into the lymph vessel. The mesenchyme forms a network of lymph sinusoids that communicate with the lumen of the lymph vessel. The mesenchyme also forms a reticular fiber frame work that outlines and supports the sinusoidal network. At this time a blood vessel enters the developing lymphnode. The point of entrance establishes the location of the hilus. At the periphery of the node the mesenchyme condenses to form a capsule. The mammalian lymphnodes consists of cortical and medullary regions. The cortical parenchyma contains lymphnodules, while the central medullary region contains anastomosing cords of lymphoid tissue. The structure of the porcine lymphnode differs from other domestic animals in that the lymphnodules are centrally located with the cords at the periphery. The flow of lymph in the porcine lymphnode is in the opposite direction to that in other domestic animals, with lymph entering at the hilus and leaving from the cortex.

Somites are derived from paraxial mesoderm and they are located lateral to the developing notochord and neural tube. During the third week of gestation in domestic animals, the outlines of somites first become visible and in the fifth week of gestation the somite formation is completed. Somites differentiate into sclerotome, myotome and dermatome. They form skeletal, muscular and dermal elements i.e, vertebral column, muscles and dermis respectively. The connective tissue, cartilage and bone all differentiate from mesenchyme which arises chiefly from mesodermal somites and lateral layer of somatic mesoderm while connective tissue of alimentary canal and lower respiratory tract is derived from splanchnic mesoderm. Mesenchyme is a spongy meshwork composed of branching cells spaces between which are filled by ground substance. The supporting tissues, connective tissue, cartilage and bone are derivatives of mesenchyme in which ground substance is gelly like, stiff and firm respectively.

The integumentary system comprises the skin its hair and glands and modified cutaneous structures hooves, claws, horns, beak and feathers. Although the mammary gland is a skin derived organ, because of its close association with the female genital system it is included in respective chapter. The skin consists of two layers: a superficial layer, the epidermis, which is derived from ectoderm and a deeper layer, the dermis, which develops from mesoderm. Epidermis : The epidermal layer covering the entire embryo initially consists of a single layer of cuboidal cells resting on a basal lamina. Shortly after neurulation, these ectodermally derived cells divide and give rise to a superficial layer of flattened cells, the periderm and underlying layer of cuboidal cells, the basal layer. Further, proliferation of the cells gives rise to an intermediate layer resulting in a multi-layered covering of epidermis. Cells of neural crest and mesodermal origin also contribute to the population of cells found in the skin.

Towards the end of the third week of embryological development, the notochord induces the overlying columnar ectodermal cells of the embryonic disc to become pseudostratified neuroepithelial cells and form a spoon shaped thickening of dorsal ectoderm called the neural plate. The cranially expanded region of the neural plate forms the primordium of the brain, while the narrower region caudal to the brain gives rise to spinal cord. The raised lateral edges of the neural plate form the neural folds while the depressed midline region of the plate forms the neural groove. Cellular proliferation at the medial aspects of the neural folds causes these structures to gradually approach each other in the midline, meet and fuse, forming the neural tube which encloses a central neural canal.

Each optic vesicle grows outwardly until it comes into contact with the surface ectoderm. At this point, the optic vesicle induces the overlying ectoderm to thicken and to invaginate against the optic vesicle. The optic vesicle becomes concave and convert into double walled optic cup. The attachment of optic cup to the brain is called optic stalk. The space between inner and outer walls disappears as they come opposed. Since invagination of optic cup occurs at the ventral margin of the vesicle a groove is formed in ventral rim of the cup which extends along the ventral surface of the optic stalk (choroid fissure) which closes at the end of embryonic period. The groove and the notch in the optic cup are referred to collectively as the optic fissure. The optic fissure begins to close by fusion of its edges soon after the optic cup is completely formed. While it is open, the fissure allows a branch of the choroid artery (Hyaloid artery) into the cavity of the optic cup. With the development of the optic cup the lens placode invaginates into rim of the optic vesicle and forms lens vesicle. The lens vesicle loses contact with the surface ectoderm and positioned at the opening of the optic cup and forms lens.

The defined endocrine glands include the pituitary gland, the pineal gland, the adrenal glands, the thyroid gland and the parathyroid glands. The organs which contain groups of endocrine cells include the pancreas, the testis, ovary and placenta. The Hassall's corpuscles of thymus secrete hormone which contributes to the maturation of T-lymphocytes. Hence the thymus can be considered as having an endocrine function. Pituitary : Refer to Part II, Chapter 2 Thyroid gland : Refer to Part II, Chapter 2 Pineal gland (Epiphysis cerebri) : Develops as a dorsal neuro epithelial diverticulum of the caudal part of the roof of the diencephalon. After its formation the gland remains attached to the diencephalon by a stalk. The neuroepithelial cells differentiate into pinealocytes and glial cells. The developing gland is surrounded by a thin layer of piamater.

Teratology : is the study of abnormal development. If the malformation involves an organ or only a portion of the body, it is called an anomaly. If it is so extensive great deformity of the individual occurs when the animal is designated a monster. etiological basis of teratology : All teratogenic agents directly or indirectly ar;ect the process of cell differentiation. These agents are divided into two broad categories i.e genetic or Intrinsic factors and environmental or extrinsic factors. Genetic or Intrinsic factors : There are several types of genetic factors that result in abnormal development.

Glossary