Developmental Anatomy (Embryology) – BCSE Study Guide

Overview and Clinical Importance

Developmental anatomy (embryology) is the study of how organisms develop from fertilization through birth. For veterinarians, understanding embryology is essential for diagnosing congenital anomalies, managing reproductive issues, and providing accurate prognostic information to clients. This topic represents approximately 2-3 questions on the BCSE within the Anatomy domain.

Key clinical applications include: recognizing teratogenic periods when fetuses are vulnerable to developmental defects; understanding species-specific placentation types for reproductive management; identifying fetal circulation shunts (patent ductus arteriosus, patent foramen ovale) as congenital heart defects; and counseling clients about inherited disorders.

1. Gametogenesis

Gametogenesis is the process by which diploid precursor cells undergo meiosis to produce haploid gametes (sperm and ova). This process differs significantly between males (spermatogenesis) and females (oogenesis).

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules of the testes and begins at puberty. The process is continuous and produces millions of sperm daily. Key features include:

• Spermatogonia (stem cells) undergo mitosis to maintain the germ cell population
• Primary spermatocytes (2n) undergo Meiosis I to form secondary spermatocytes (n)
• Secondary spermatocytes undergo Meiosis II to form spermatids (n)
• Spermatids undergo spermiogenesis (morphological transformation) to become mature spermatozoa
• Sertoli cells provide structural and nutritional support; blood-testis barrier protects developing sperm

[Include Image: Figure 1. Spermatogenesis process showing progression from spermatogonium to mature spermatozoon]

Oogenesis

Oogenesis begins during fetal development and is arrested at various stages until ovulation. Key differences from spermatogenesis:

• Oogonia proliferate by mitosis ONLY during fetal life
• Primary oocytes begin Meiosis I but arrest in prophase I (dictyate stage) until puberty
• At ovulation, Meiosis I completes, producing a secondary oocyte and first polar body
• Meiosis II begins but arrests at metaphase II until fertilization
• Fertilization triggers completion of Meiosis II, producing the ovum and second polar body

[Include Image: Figure 2. Oogenesis process showing primary oocyte through ovulation and fertilization]

Comparison: Spermatogenesis vs. Oogenesis

2. Fertilization and Early Development

Fertilization

Fertilization is the union of sperm and ovum to form a diploid zygote. In most domestic species, fertilization occurs in the ampulla of the oviduct. The process involves several critical steps:

• Capacitation: Sperm undergo changes in the female reproductive tract to acquire fertilizing ability
• Acrosome reaction: Release of enzymes to penetrate the zona pellucida
• Sperm-egg fusion: Contact with oocyte membrane triggers cortical reaction
• Block to polyspermy: Zona reaction and vitelline block prevent additional sperm entry
• Pronuclei formation: Male and female pronuclei form and merge

Early Embryonic Development

Following fertilization, the zygote undergoes a series of cell divisions (cleavage) without overall growth:

[Include Image: Figure 3. Early embryonic development stages from zygote to blastocyst]

Gastrulation

Gastrulation is the process that establishes the three primary germ layers that give rise to all tissues and organs:

3. Placentation Types by Species

The placenta is a temporary organ that facilitates exchange of nutrients, gases, and wastes between mother and fetus. Placental classification is based on two criteria: (1) gross shape and (2) histological layers between maternal and fetal blood.

Classification by Gross Shape

[Include Image: Figure 4. Comparison of placental shapes: diffuse, cotyledonary, zonary, and discoid]

Classification by Histological Layers (Fetomaternal Interface)

The number of tissue layers separating maternal and fetal blood determines placental invasiveness:

Species-Specific Placentation Details

Ruminants (Cattle, Sheep, Goats)

• Cotyledonary placenta with 70-120 placentomes in cattle; 90-100 in sheep
• Placentomes = fetal cotyledon + maternal caruncle
• Binucleate cells (BNCs) produce placental lactogen and pregnancy-associated glycoproteins
• Syndesmochorial (modified epitheliochorial) in some classifications

Equine

Horses have a diffuse, microcotyledonary epitheliochorial placenta with unique features:

• Endometrial cups (days 36-120): produce eCG (equine chorionic gonadotropin)
• Allantochorion forms microvilli that interdigitate with endometrium
• Hippomanes: allantoic debris sometimes found in amniotic fluid

Carnivores (Dogs and Cats)

Dogs and cats have zonary endotheliochorial placentas:

• Marginal hematomas at placental edges: provide iron for fetal development
• Green pigment (uteroverdin) in dogs from biliverdin breakdown
• Lamellar zonary structure provides efficient exchange

[Include Image: Figure 5. Cross-section of cotyledonary placenta showing placentome structure]

4. Organogenesis

Organogenesis is the process of organ formation from the three germ layers. This is the most critical period for teratogenic sensitivity, as disruption during this time causes structural birth defects.

Timeline of Major Organ System Development

Key Developmental Structures

Neural Tube Development

The neural tube forms by neurulation and gives rise to the CNS. Defects in closure result in:

• Anencephaly: failure of cranial neural tube closure
• Spina bifida: failure of caudal neural tube closure
• Hydrocephalus: abnormal CSF accumulation

Heart Development

The heart develops from cardiogenic mesoderm through several stages:

• Heart tube formation and looping (D-loop = normal; L-loop = dextrocardia)
• Septation of atria (septum primum and secundum) and ventricles
• Truncoconal septation (aorticopulmonary septum separates aorta from pulmonary trunk)

[Include Image: Figure 6. Neural tube formation (neurulation) and neural crest cell migration]

5. Fetal Circulation and Changes at Birth

Fetal circulation differs dramatically from postnatal circulation because the lungs are non-functional and gas exchange occurs at the placenta. Three key shunts bypass the pulmonary circulation and liver.

Fetal Circulatory Shunts

Fetal Blood Flow Pathway

Understanding the pathway of oxygenated blood in the fetus is critical:

• Oxygenated blood from placenta travels via UMBILICAL VEIN
• Most blood bypasses liver via DUCTUS VENOSUS to IVC
• Blood enters RIGHT ATRIUM where crista dividens directs it through FORAMEN OVALE to LEFT ATRIUM
• Left ventricle pumps to ascending aorta, supplying brain and heart with highest oxygen blood
• Deoxygenated blood from cranial vena cava goes to right ventricle, then pulmonary artery
• High pulmonary resistance shunts most blood through DUCTUS ARTERIOSUS to descending aorta
• Blood returns to placenta via UMBILICAL ARTERIES (branches of internal iliacs)

[Include Image: Figure 7. Fetal circulation showing the three shunts: ductus venosus, foramen ovale, and ductus arteriosus]

Changes at Birth

Birth triggers dramatic cardiovascular changes driven by the first breath:

6. Congenital Anomalies

Congenital anomalies are defects present at birth. They may be genetic (inherited), teratogenic (caused by environmental agents), or multifactorial. Veterinarians must recognize common defects and understand their etiologies.

Classification of Congenital Defects

Common Teratogenic Causes

Species-Specific Inherited Defects

Cattle

• Complex Vertebral Malformation (CVM): Holstein; vertebral defects, arthrogryposis; autosomal recessive
• Bovine Leukocyte Adhesion Deficiency (BLAD): Holstein; recurrent infections; autosomal recessive
• Arachnomelia: Brown Swiss, Simmental; spider-leg appearance; autosomal recessive
• Bulldog calf (Dexter): chondrodysplasia; lethal in homozygotes

Dogs

• Patent Ductus Arteriosus (PDA): common in Maltese, Poodles, German Shepherds; polygenic
• Portosystemic Shunt (PSS): Yorkshire Terriers, Maltese; may be congenital or acquired
• Hip Dysplasia: Large breeds; polygenic multifactorial
• Cleft palate: Brachycephalic breeds

Horses

• Lethal White Overo Syndrome: Paint horses; ileocolonic aganglionosis; homozygous lethal
• Hyperkalemic Periodic Paralysis (HYPP): Quarter Horses (Impressive line); autosomal dominant
• Severe Combined Immunodeficiency (SCID): Arabian horses; autosomal recessive

[Include Image: Figure 8. Examples of congenital anomalies: cleft palate, arthrogryposis, cerebellar hypoplasia]

Memory Aids and Mnemonics

Germ Layer Derivatives

'EEE' for Ectoderm: Epidermis, Eyes (lens), and Encephalon (brain/nervous system)

'MBM' for Mesoderm: Muscle, Bone, Marrow (blood)

'ENDO for internal': Endoderm lines internal tubes (GI, respiratory) and makes internal glands (liver, pancreas, thyroid)

Placenta Types by Species

'Horses and Pigs are Diffuse and Superficial (epitheliochorial)'

'Ruminants have COTS of babies' (COTyledonary)

'Dogs and Cats wear ZONEs (zonary) and Dig in ENDOthelium (endotheliochorial)'

Fetal Shunts

'DVF-FAO-DA' pathway: Ductus Venosus → Foramen Avale → Ductus Arteriosus

Remnants: 'Liga-MENTS': Ligamentum venosum, Fossa ovalis, Ligamentum arteriosum

Cerebellar Hypoplasia Viruses

'Cats get FP, Cows get BVD': Feline Panleukopenia → cats; BVD → cattle. Both cause cerebellar hypoplasia in utero.