Developmental Anatomy – Vet Med Review Class Notes

Phases of Ontogenic Development

  • Ontogenic development – processes transforming a fertilized egg into a new adult individual.
    1. Gametogenesis – formation of gametes via spermatogenesis (males) and oogenesis (females).
    2. Fertilization – union of sperm and egg; occurs in ampullary region of oviduct.
    • Capacitation – sperm maturation in the female tract; completed in the isthmus.
    • Acrosomal Reaction – fusion of sperm plasma membrane with acrosomal membrane; enables spermatozoa to penetrate the zona pellucida.
    • Cortical Reaction – oocyte’s defense against polyspermy via zona and vitelline blocks.
    • Syngamy – fusion of male and female pronuclei → zygote.
    1. Cleavage – mitotic divisions forming blastomeres within the zona pellucida.
    2. Gastrulation – formation of the three germ layers.
    3. Organogenesis – organ formation from germ layers.
    4. Growth and histological differentiation – tissue maturation and expansion.

Cleavage and Blastulation

  • Types of cleavage
    1. Holoblastic – complete; whole of the egg becomes subdivided into blastomeres.
    2. Meroblastic – incomplete; only a part of the egg is subdivided into blastomeres.
  • Morula – solid ball of cells.
  • Blastula/blastocyst – hollow ball of cells; consists of:
    1. Inner cell mass (embryoblast) – a collection of cells localized on one pole (end) of the blastula; gives rise to the embryo proper.
    2. Trophoblast (trophoectoderm) – surface cells of the blastocyst; gives rise to embryonic part of the placenta and attaches the developing embryo to the uterine wall.
    3. Blastocoele – fluid cavity.
  • Embryo transfer is best done on the blastocyst stage.
  • Blastoderm (blastodisc) – in avians; composed of two regions:
    1. Area pellucida – central region; overlies the subgerminal cavity; where embryo develops.
    2. Area opaca – peripheral region; overlies and digests the yolk which nourishes the embryo.
  • Notochord – rod-shaped aggregate of cells located cranial to the primitive streak; adult fate is to become the nucleus pulposus of intervertebral discs.
  • Type of Egg
    • Based on the Amount of Yolk
    • Oligolecithal (Microlecithal) – Eggs with a small amount of yolk; Examples: Mammals, Molluscs; Cleavage: Holoblastic.
    • Mesolecithal (Medialecithal) – Eggs with moderate yolk; Examples: Amphibians (frogs, toads), some fishes; Cleavage: Holoblastic.
    • Polylecithal (Macrolecithal, Megalecithal) – Eggs with massive yolk reserves; Examples: Birds, reptiles, most fishes; Cleavage: Meroblastic.
    • Based on Yolk Distribution
    • Isolecithal – Eggs with evenly distributed yolk; Examples: Mammals, Molluscs; Cleavage: Holoblastic.
    • Centrolecithal – Eggs where yolk is centrally located with cytoplasm forming an external layer; Examples: Insects, arthropods; Cleavage: Meroblastic.
    • Telolecithal – Eggs where yolk displaces the embryo-forming cytoplasm into a small area at the animal pole; Examples: Birds, reptiles, most fishes; Cleavage: Meroblastic.

Gastrulation

  • Epiblast – the upper layer of the inner cell mass; gives rise to all three germ layers (ectoderm, mesoderm, endoderm).
  • Hypoblast – the lower layer of the inner cell mass; forms part of the extraembryonic membranes, such as yolk sac endoderm.
  • Primitive streak – defines the longitudinal axis of the embryo and indicates start of germ layer formation.
  • Germ Layer Derivatives – Parts:
    1. Primitive node (Hensen’s node) – a rounded structure at the cranial end of the streak; organizer region; initiates/regulates formation of notochord and mesoderm.
    2. Primitive pit – small depression at the center of the primitive node.
    3. Primitive groove – depression that runs along the length of the primitive streak.
    4. Primitive ridge – raised lateral edges of the primitive streak formed by migrating epiblast cells.

EXTRAEMBRYONIC MEMBRANES

  • Extraembryonic membranes – continuous with the embryonic tissues but not part of the embryonic body proper; shed or absorbed at hatching or birth.
  • EEM – Description and Function
    • Yolk sac – connected to fetal intestines; early source of nutrition; remnant becomes Meckel’s diverticulum after birth.
    • Chorion – outermost membrane; direct contact with maternal endometrium; external surface has numerous villi; forms fetal part of the placenta; facilitates maternal-fetal exchange.
    • Amnion – innermost membrane surrounding the fetus; cushions and protects the embryo with amniotic fluid; prevents tissue adhesion; called the second water bag; expelled after allantois during parturition.
    • Allantois – extension of embryonic bladder via urachus; collects and stores liquid waste; aids in fluid regulation; known as the first water bag.

IMPLANTATION

  • Implantation – attachment of the developing embryo to the endometrium; nidation.
  • Hormonal milieu:
    • High levels of estrogen cause the endometrium to proliferate.
    • Progesterone renders the endometrium receptive to the blastocyst.
  • Types of implantation
    • Interstitial – blastocyst invades and embeds completely within the endometrium.
    • Eccentric – blastocyst is lodged in a fold of the uterine lumen.
    • Centric (superficial) – blastocyst elongates and remains in the uterine cavity.
    • Delayed implantation (Seasonal/Obligate) – implantation paused to align with favorable birth seasons (e.g., mink, seals, bears).
    • Facultative/Lactational – postponed due to lactation; prevents overlap of litters (e.g., rodents, insectivores).

PLACENTATION

  • Placenta – formed by the apposition of fetal and maternal tissues; functions include exchange of gases, nutrients and electrolytes; hormone production and transmission of maternal antibodies.
  • Placentome – in ruminants; fusion of fetal cotyledon and maternal caruncle; typical counts: ~90–100 in sheep, ~70–120 in cattle.
  • Types by fetal-maternal membrane type
    • Choriovitelline – yolk sac placenta; temporary; Marsupials.
    • Chorioallantoic – allantois fuses with chorion; main functional placenta; most mammals.
  • Gross distribution of chorionic villi
    • Diffuse – villi uniformly spread across chorion; Examples: Horses, pigs.
    • Cotyledonary – villi confined to cotyledons that attach to uterine caruncles; Ruminants.
    • Zonary – belt-like zone around the chorion; Carnivores.
    • Discoidal – disc-shaped area; Primates, rodents.
  • Degree of attachment to uterine mucosa
    • Apposed, Non-deciduate – loose; no maternal tissue lost at parturition; Examples: Pigs, horses, ruminants.
    • Conjoined, Deciduate – intimate connection; maternal tissue shed during birth; Examples: Humans, dogs, cats.
  • Tissue layers between fetal and maternal blood
    • Epitheliochorial – intact endometrium contacts intact chorion; Examples: Horses, Pigs.
    • Synepitheliochorial – fetal chorion contacts endometrial connective tissue.
    • Endotheliochorial – fetal chorion contacts maternal endothelium.
    • Haemochorial – maternal blood directly bathes chorionic surface.
    • Examples: Primates, Rodents.

NEURULATION

  • Neurulation – process by which the neural plate forms a neural tube.
    • Primary neurulation – folding of ectoderm to form anterior neural tube.
    • Secondary neurulation – formation of a solid medullary cord caudally that cavitates into a tube.
  • Neural plate – thickened ectoderm along dorsal midline.
  • Neural groove – midline depression of neural plate.
  • Neural tube – fusion of neural folds; precursor of the central nervous system.
  • Neural crest – cells from the neuroectoderm that migrate to the mesoderm; form peripheral structures; sometimes referred to as the fourth germ layer.
  • Neuropores – rostral and caudal temporary openings at neural tube ends; allow communication with amniotic cavity.
  • Teratology – Neural Tube Defects
    • Cranioschisis – closure defect of the brain.
    • Rachischisis – closure defect of the spinal cord.
    • Spina bifida – failure of vertebral arches to close dorsal to the spinal cord; occulta (intact spinal cord) or cystica (meninges and/or spinal cord herniate).
    • Meningocele – meninges protrude through the defect.
    • Meningoencephalocele – meninges and brain tissue herniate.
    • Meningohydroencephalocele – herniation includes the ventricles.
    • Anencephaly – absence of telencephalon; skull vault fails to form.
    • Exencephaly – brain is exposed due to failed closure.

NERVOUS SYSTEM

  • The nervous system is the first system to initiate its development; functionally, however, it is overtaken by the development of the vascular system, which is the first organ system to gain function.
  • Primitive Cells:
    • Neuroblasts – immature neurons.
    • Spongioblasts – develop into neuroglial cells.
  • Brain Development
    • Pituitary Gland – dual origin:
    1. Rathke’s pouch – ectodermal outpocketing of the stomodeum; gives rise to the adenohypophysis, or anterior pituitary.
    2. Infundibulum – ventral downgrowth of the diencephalon; gives rise to the neurohypophysis, or posterior pituitary.
    • Secondary Brain Vesicle Divisions
    • Prosencephalon → Telencephalon, Diencephalon
    • Mesencephalon → Mesencephalon
    • Rhombencephalon → Metencephalon, Myelencephalon
    • Adult Derivatives (Ventricular Derivatives)
    • Telencephalon – Cerebral hemispheres, olfactory bulb, hippocampus; Lateral ventricles.
    • Diencephalon – Thalamus, hypothalamus, epithalamus (including pineal gland), optic vesicles; Third ventricle.
    • Mesencephalon – Midbrain (tectum and tegmentum), cerebral peduncles, colliculi; Cerebral aqueduct (Aqueduct of Sylvius).
    • Metencephalon – Pons and cerebellum; Upper part of fourth ventricle.
    • Myelencephalon – Medulla oblongata; Lower part of fourth ventricle.

MUSCULOSKELETAL SYSTEM

  • Paraxial mesoderm forms a series of segmented blocks: somitomeres (occipital region) and somites (body region).
  • Somite components and derivatives
    • Sclerotome – medial portion; gives rise to vertebrae and ribs.
    • Dermatome – lateral portion; gives rise to the dermis of the skin.
    • Myotome – middle portion; gives rise to skeletal muscles.
  • Somitomeres migrate into pharyngeal arches to form head muscles (jaw, face, pharynx, larynx).
  • Chondrogenesis – cartilage formation from mesenchymal cells; differentiation into chondroblasts; entrapped cells become chondrocytes; perichondrium formed by surrounding mesenchyme.
  • Osteogenesis – bone formation process.
  • Teratology (congenital defects)
    • Myelodysplasia – general spinal cord malformation.
    • Hydromyelia – dilation of the central canal due to CSF accumulation.
    • Syringomyelia – cystic cavities in spinal cord (e.g., Weimaraners, Manx cats).
    • Diplomyelia – two spinal cords in one vertebral canal.
    • Hydrocephalus – CSF accumulation in ventricles.
    • Microcephaly – small brain due to genetic or prenatal insult.
    • Holoprosencephaly – incomplete division of prosencephalon.
    • Cerebellar hypoplasia – cerebellar underdevelopment; common in certain infections.
    • Arnold-Chiari malformation – downward displacement of cerebellum through foramen magnum.
    • Hydranencephaly – cerebral hemispheres replaced by CSF sacs.
  • Ossification Types
    • Intramembranous – bone forms from mesenchymal cells differentiating into osteoblasts; flat bones (clavicle and skull bones).
    • Endochondral – bone forms from a cartilaginous model.
  • Teratology – congenital defects of the skeleton
    • Achondroplasia – dwarfism from defective endochondral ossification.
    • Osteogenesis imperfecta – fragile bones.
    • Osteopetrosis – abnormally dense bones.
    • Block vertebra, Hemivertebra, Butterfly Vertebra.
    • Scoliosis, Kyphosis, Lordosis.
    • Polydactyly, Syndactyly.
    • Bimelia, Amelia, Ectromelia, Notomelia, Perineomelia, Hemimelia, Phocomelia.
    • Arthrogryposis – fixed joints; may be associated with prenatal infections (Akabane, bluetongue).
    • Hip Dysplasia – abnormal hip joint; common in large breeds.
    • Double muscling – due to myostatin mutation (Belgian Blue, Piedmontese).

DIGESTIVE SYSTEM

  • The first indication in digestive tract formation is the establishment of the endodermal layer of the blastocyst.
  • The primitive gut is a tube lined by endoderm and enveloped by splanchnic mesoderm. It is divided into foregut, midgut, and hindgut.
  • Foregut derivatives and blood supply
    • Pharynx, esophagus, stomach, cranial duodenum.
    • Celiac artery.
  • Midgut derivatives and blood supply
    • Remaining small intestine, cecum, ascending colon, part of transverse colon.
    • Cranial mesenteric artery.
  • Hindgut derivatives and blood supply
    • Transverse colon, descending colon, cloaca forms the rectum and most of anal canal.
    • Cranial mesenteric artery (note: transcript lists this after Hindgut; standard anatomy uses the caudal mesenteric artery for hindgut, but here it is given as cranial mesenteric).
  • Pharyngeal Arch, Pouch and Clefts (pharyngeal apparatus)
    • Pharyngeal arches – 6 in primitive form; in mammals there are 4 well-defined arches (I–IV) with V–VI involvement at varying degrees; fifth arch atrophy; sixth arch fuses with fourth forming a 4th–6th arch complex.
    • Pharyngeal arch I derivatives – mandible, maxilla, zygomatic bone, temporal bone, malleus, incus; muscles of mastication; mylohyoid, rostral digastricus, mylohyoid; tensor tympani; nerve supply: Trigeminal nerve (CN V).
    • Pharyngeal arch II derivatives – stapes, hyoid bones, muscles of facial expression; nerve supply: Facial nerve (CN VII).
    • Pharyngeal arch III derivatives – hyoid bones, stylopharyngeus muscle; nerve supply: Glossopharyngeal (CN IX).
    • Pharyngeal arches IV–VI derivatives – laryngeal cartilages (epiglottic, arytenoid, thyroid, cricoid); nerve supply: Vagus (CN X).
  • Pharyngeal Pouches
    • I – auditory tube, guttural pouch.
    • II – palatine tonsil.
    • III – external parathyroid gland, thymus.
    • IV–VI – internal parathyroid gland, avian ultimobranchial body.
  • Organ development (esophagus to pancreas)
    • Esophagus – elongates from pharynx to stomach.
    • Stomach – fusiform dilation in the foregut; two rotations bring stomach to its definitive abdominal position; attaches to abdominal wall by dorsal (greater omentum) and ventral (falciform ligament) mesogastrium.
    • Liver – develops from ventral endodermal bud; hematopoietic liver occupies most of abdominal cavity during development; physiological umbilical herniation occurs when the growing intestinal loop is displaced out of the abdominal cavity into the extra-embryonic coelom.
    • Pancreas – develops from dorsal (major portion) and ventral (minor portion) endodermal buds.
    • Intestines – first 180° of intestinal rotation occurs during herniation; during hernia reduction, an additional 90° rotates to a total of 270°; final positioning involves a further rotation of roughly 360° total.
  • Esophagus, stomach, liver, pancreas, and intestines are described with their rotations and attachments to mesenteries.

RESPIRATORY SYSTEM

  • Lung histogenesis – development of bronchi and lungs occurs in distinct stages:
    1. Embryonic period – formation of initial primordia of bronchi and lungs.
    2. Fetal period – development of bronchial tree and early gas-exchange structures, subdivided into:
      a. Pseudoglandular stage – extensive branching of bronchial tree into a compound tubuloalveolar pattern.
      b. Canalicular stage – close contact of capillaries and parenchyma (bronchioles and alveoli).
      c. Saccular stage – formation of terminal saccules due to terminal branching of the airway conducting system.
      d. Alveolar stage – establishment of the blood–air barrier for efficient gas exchange.
    3. Postnatal period – final development of mature alveoli and full lung functionality.

GENITOURINARY SYSTEM

  • The intermediate mesoderm (urogenital plate) gives rise to the urogenital system: kidneys, gonads, and their respective duct systems; epithelial lining of the bladder and urethra is endodermal.
  • Urinary system components
    • Urogenital plate comprises:
    1. Nephrogenic cord – laterally positioned; forms kidneys and ureter.
    2. Gonadal ridge – medially positioned; forms ovaries/testes and female/male genital tract.
  • Kidney development occurs in three overlapping stages (pronephros, mesonephros, metanephros) in a cranial-to-caudal sequence.
  • Congenital defects (reproductive and urinary system overlap in notes):
    • Cleft lip – failure of fusion of maxillary and medial nasal prominences.
    • Cleft palate – failure of fusion of palatine processes.
    • Micro/ Macroglossia – abnormal tongue size.
    • Ankyloglossia – abnormally short tongue.
    • Supernumerary teeth – extra teeth within jawbone.
    • Heterotopic polydontia – extra teeth elsewhere on head.
    • Stenosis – abnormal narrowing of digestive tract.
    • Atresia – failure of canalization; obstruction.
    • Achalasia – failure of cricopharyngeus muscle to relax during swallowing.
    • Meckel’s diverticulum – yolk stalk remnant; may cause colic/peritonitis.
    • Megacolon (Hirschsprung’s disease) – dilatation of colon cranial to aganglionic segment.
    • Imperforate anus – cloacal membrane fails to involute.
    • Omphalocele/Umbilical hernia – loops of intestine outside abdominal cavity.
    • Situs inversus – abdominal or thoracic organs reversed.
  • Embryonic structures and adult derivatives (gonads, ducts, and external genitalia)
    • Primordial germ cells – produce gametes; migrate to gonads during development.
    • Indifferent stage – both sexes share gonads, genital ducts, external features.
    • SRY gene – drives differentiation of bipotential gonad into testis.
    • Gonadal ridge – becomes testes in males; ovaries in females; gonadal cords become seminiferous tubules in males; primordial follicles in females.
    • Mesonephric (Wolffian) duct – becomes epididymis, vas deferens, ejaculatory duct, seminal vesicles; regresses in female.
    • Paramesonephric (Müllerian) duct – becomes oviducts, uterus, cervix, proximal vagina; regresses in male.
    • Urogenital sinus – forms bladder and pelvic/penile urethra in both; contributes to female urethra and vestibule.
  • Mammary Gland Development
    • Mammary ridge (milk line) – ectodermal thickening extending from axillary to inguinal region; mammary buds form at sites and develop into mammary glands; species variation in counts (e.g., 2 in sheep/goat/mare; 4 in cow; 8 in queen; 10 in bitch; 14 in sow).
  • Teratology (general embryology context)
    • Embryonic structures and congenital defects related to organ systems described above.
  • Congenital Defects (reproductive/urinary emphasis)
    • Renal agenesis/hypoplasia – failure of one or both ureteric buds to form.
    • Renal duplications – excessive splitting of the ureteric bud.
    • Ectopic ureter – ureter opens into vagina or urethra.
    • Horseshoe kidney – fusion of kidneys at caudal poles.
    • Congenital cystic kidney – urine accumulation due to nephron uropathy.
    • Patent urachus (urachal fistula) – failure of allantoic stalk to close at birth.
    • Cryptorchidism – failure of testicular descent; common in certain breeds.
    • Hypospadias – urethra opens on ventral penis due to failure of urethral folds to fuse.
    • Epispadias – urethral opening on dorsal penis.

MAMMARY GLAND DEVELOPMENT (REITERATED)

  • Mammary ridge (milk line) – ectodermal band from axillary to inguinal regions; mammary buds form and develop into glands; species-specific counts noted above.

TERATology – ADDITIONAL REMINDERS

  • Teratology encompasses structural defects in multiple organ systems described; etiologies include genetic, nutritional, environmental factors; timing of insult often determines organ system affected.

CARDIOVASCULAR SYSTEM

  • The circulatory system is the first functional organ system to form.
  • Haematopoiesis occurs in three overlapping periods:
    1. Mesoblastic period – yolk sac.
    2. Hepato-lienal period – liver and spleen become major blood-forming organs.
    3. Medullary period – bone marrow becomes the major hematopoietic organ.
  • Heart development
    • Heart forms from a horseshoe-shaped cardiac tube that positions ventrally to the embryonic disc after folding.
  • Arterial System
    • Paired ventral and dorsal aortae develop; bilaterally connected by up to six aortic arches located within pharyngeal arches.
  • Venous System
    • Sinus venosus receives: vitelline veins (yolk sac), umbilical veins (allantois), and cardinal veins (embryo).
  • Congenital defects (reproductive/urinary and cardiovascular notes amalgamated here in the transcript)
    • Double cervix – lack of fusion of paramesonephric ducts.
    • Pseudohermaphroditism – gonads of one sex with some genital characteristics of the other.
    • Gartner’s duct – nonfunctional remnants of caudal mesonephric duct.
    • Paramesonephric duct atresia (White heifer disease) – absence of oviducts, uterus, cervix, and vagina.
  • Primitive Heart components and derivatives
    • Truncus arteriosus – output region of the heart; develops into ascending aorta and pulmonary trunk.
    • Bulbus cordis – becomes right ventricle.
    • Primitive ventricle – becomes left ventricle.
    • Primitive atrium – forms right and left atria.
    • Sinus venosus – receives veins; left sinus venosus becomes coronary sinus; right is incorporated into right atrium.
    • Septation and foramen ovale – septum primum and secundum; foramen ovale becomes fossa ovalis.
    • Aortic arches (I–VI) – various degenerations/persistence:
    • I–II degenerate; III becomes internal carotid and proximal common carotid; external carotid buds from III.
    • IV – left: aortic arch; right: proximal right subclavian.
    • V – rudimentary or absent.
    • VI – proximal part becomes pulmonary artery; right distal part degenerates; left distal part persists as ductus arteriosus (later ligamentum arteriosum).
  • Embryonic Veins and Derivatives
    • Cranial cardinal vein – drains into internal jugular; external jugular and subclavian arise by budding from cranial cardinal veins.
    • Cranial vena cava – arises from caudal segment of right cranial cardinal vein and right common cardinal vein.
    • Vitelline veins – drain liver sinusoids and give rise to portal vein.
    • Umbilical veins – right atrophies; left forms round ligament of liver along falciform ligament.
  • Ductus venosus
    • Develops between the left umbilical vein and the right hepatic vein; shunts blood away from sinusoids toward systemic veins.
  • Congenital Defects (cardiovascular context)
    • Ectopia cordis – heart located in cervical region.
    • Atrial and Ventricular Septal Defects (ASD/VSD) – incomplete septation.
    • Tetralogy of Fallot – VSD; rightward shift of pulmonary outflow; pulmonary stenosis; resultant right ventricular hypertrophy.
    • Persistent truncus arteriosus – failure of truncus spiral ridges to form and fusion.
    • Persistent ductus arteriosus – failure of ductus arteriosus to close at birth; blue baby.
    • Persistent right aortic arch – vascular ring around esophagus/trachea causing swallowing difficulties in young animals.
    • Portosystemic shunts – persistent ductus venosus or anomalous portal Caval/azygos shunts bypass liver.
    • Congenital hereditary lymphedema – failure of lymphatics to connect with venous system.