Veterinary Embryology – Core Concepts

Twenty question-and-answer flashcards covering fundamental concepts of veterinary embryology, including developmental stages, germ layer fates, fertilization mechanisms, placentation, organogenesis, and comparative reproductive features across species.

What are the six sequential phases of ontogenic development in sexually reproducing animals?

1) Gametogenesis 2) Fertilization 3) Cleavage 4) Gastrulation 5) Organogenesis 6) Growth & Histological Differentiation

During which period (and approximate duration) does organ primordia form in the dog, cat, sheep and pig?

The Embryonic Period – from fertilization until ~30 days when primordial organs are established (≈60 days in horse, cattle, and human).

How many functional gametes are produced per meiotic cycle in spermatogenesis versus oogenesis?

Spermatogenesis yields four spermatozoa; oogenesis yields one ovum (plus polar bodies).

What is the correct developmental sequence of ovarian follicles?

Primordial follicle → Primary follicle → Secondary follicle → Tertiary (Graafian/Mature) follicle.

Which domestic species exhibit induced ovulation rather than spontaneous ovulation?

Cats, rabbits, camels, and ferrets are induced ovulators.

Where does fertilization normally occur in mammals and in birds?

In mammals: the oviduct (fallopian tube); in birds: the infundibulum.

Name the two physiological blocks that prevent polyspermy after fertilization.

1) Zona block (biochemical change in zona pellucida) 2) Vitelline block (reduced fusion ability of oocyte membrane).

Differentiate holoblastic cleavage from meroblastic cleavage.

Holoblastic: complete division of the entire egg into blastomeres (typical of micro-/mesolecithal eggs). Meroblastic: incomplete division; only part of the yolk-rich egg cleaves (typical of megalecithal eggs).

In a mammalian blastocyst, what do the outer trophoblast and inner cell mass (embryoblast) give rise to?

Trophoblast → fetal membranes (amnion, chorion, part of placenta); Inner cell mass → embryo proper, yolk sac, allantois.

List the three primary germ layers and provide one adult derivative of each.

Ectoderm – epidermis or brain; Mesoderm – skeletal muscle or kidney; Endoderm – lining of digestive or respiratory tract.

What adult structure is the remnant of the embryonic notochord?

The nucleus pulposus of intervertebral discs.

Which extra-embryonic membranes arise from somatopleure and which from splanchnopleure?

Somatopleure → Chorion and Amnion; Splanchnopleure → Yolk sac and Allantois.

Contrast monozygotic with dizygotic twins in terms of origin.

Monozygotic (identical) twins arise from one zygote that splits; dizygotic (fraternal) twins arise from two separate zygotes developing simultaneously.

Centric (superficial) implantation is characteristic of which major domestic groups?

Ungulates and carnivores show centric/superficial implantation of the blastocyst.

Which placental type retains all three maternal tissue layers and is found in pigs, horses and cattle?

Epitheliochorial placenta – maternal epithelium, connective tissue, and endothelium are all present.

What structure at the limb bud tip drives proximodistal outgrowth and in what order are regions added?

The Apical Ectodermal Ridge (AER); regions form proximal → distal: girdle (shoulder/hip) → stylopod (humerus/femur) → zeugopod (radius-ulna/tibia-fibula) → autopod (manus/pes).

Which primitive heart region becomes the ascending aorta and pulmonary trunk?

The Truncus arteriosus differentiates into the ascending aorta and pulmonary trunk.

The functional metanephric kidney develops from the interaction of which two embryonic structures?

The ureteric bud (outgrowth of the mesonephric duct) and the metanephrogenic (metanephric) mesenchyme.

Which pharyngeal arch gives rise to the muscles of facial expression and is supplied by which cranial nerve?

Second pharyngeal arch; innervated by cranial nerve VII (facial nerve).

Name the four fetal membranes of amniotes and state one primary function for each.

Chorion – gas exchange/interface with uterus; Amnion – fluid cushion protecting embryo; Yolk sac – early nutrition & blood/germ cell source; Allantois – waste storage and later vascular placenta formation.