IMPLANTATION: Process by which the conceptus attaches to and interacts with uterine endometrium; invasion occurs only in invasive placental types.

Types of Implantation:
• Central / Superficial — blastocyst lies in uterine lumen; attaches superficially. Mare, sow, ruminants.
• Eccentric — blastocyst lies in a fold of endometrium. Some rodents.
• Interstitial — blastocyst burrows into endometrium. Primates, including humans.

TWINNING:
• Monozygotic (Identical): One zygote splits — same genome, same sex.
• Dizygotic (Fraternal): Two separate ovulations + fertilizations — different genomes, may be different sex.
• Freemartin: female calf twin of a male in cattle — sterile due to vascular anastomoses sharing AMH from male twin.

COMMON CONGENITAL ANOMALIES: Cleft palate, atresia ani, schistosomus reflexus, persistent urachus, contracted tendons, perosomus, white heifer disease.

IMPLANTATION, TWINNING & TERATOLOGY

I. IMPLANTATION:

Definition: Attachment and species-specific interaction of the conceptus with endometrium; invasion is limited to invasive placental types.

Stages:
1. Apposition — blastocyst hatches + makes loose contact with endometrium.
2. Adhesion — trophoblast forms tight junctions with endometrial epithelium.
3. Invasion (in deciduate species only) — trophoblast eats into endometrium.

Types of implantation:
• Central (Superficial / non-invasive): Blastocyst stays in uterine lumen, attaches to surface. Examples: mare, sow, cattle, sheep — all the large domestic species.
• Eccentric: Blastocyst settles in a fold of mucosa; mucosa grows over it. Some rodents.
• Interstitial: Blastocyst tunnels into endometrium and is buried. Examples: primates (human, monkey).

Timing:
• Cow: ~day 19-22 (slow, superficial).
• Mare: conceptus mobility continues until fixation at about day 16; epithelial attachment develops thereafter, while endometrial cups form around days 35–40. These events should not be collapsed into a single 'implantation day'.
• Bitch: ~day 17-18 (fast).
• Sow: ~day 13-14.
• Ewe: ~day 15-16.

II. MATERNAL RECOGNITION OF PREGNANCY (must occur before luteolysis):
• Cow: Trophoblast secretes interferon-tau (IFN-τ) on day 16-17 → blocks PGF2α release → CL maintained.
• Mare: The mobile conceptus traverses the uterus repeatedly until fixation around day 16; mobility and suppression of endometrial PGF2α release are essential, but the definitive molecular signal remains incompletely identified.
• Pig: Conceptuses secrete oestradiol → diverts PGF2α.
• Bitch: No maternal recognition needed — CL is independent of pregnancy in dogs.

III. TWINNING:

A. Monozygotic (Identical) Twins — a single zygote splits. The timing-based chorionicity/amnionicity pattern below is a general mammalian model and is not absolute across domestic species:
• Splitting at 2-cell to morula stage → 2 embryos with separate placenta + amnion.
• Splitting at blastocyst stage → 2 embryos sharing placenta but separate amnion.
• Splitting after embryonic disc → 2 embryos sharing placenta + amnion (rare).
• Same genome, same sex, very similar phenotype.
• Used in cattle by deliberate embryo splitting.

B. Dizygotic (Fraternal) Twins — two ovulations + two fertilizations:
• Common in sheep, goats, dogs, cats, sows (litter pregnancies).
• In single-ovulating species (cattle, mare, primates) it is unusual.
• Each twin has its own placenta usually.
• Different genomes; may be opposite sex.

C. Conjoined Twins: Incomplete monozygotic split → twins fused at some level. Rare but may be born alive (e.g. craniopagus, thoracopagus, ischiopagus).

D. Freemartin Syndrome (CATTLE):
• Heterosexual twins (one male + one female) in cattle have a vascular anastomosis between their placentas (~90% of cases).
• Anti-Müllerian Hormone (AMH) + testosterone from the MALE twin reach the FEMALE twin via shared circulation.
• Female twin's reproductive tract is masculinised → underdeveloped uterus, blind vagina, sometimes ovary-like testes → STERILE 'freemartin'.
• Diagnosis: vaginal length probe; karyotype shows XX/XY chimerism (lymphocytes mix between twins).
• Less common in mare, sheep, goat (placentas don't anastomose as readily).

IV. CONGENITAL ANOMALIES (common in domestic species):

Genetic / Chromosomal:
• Freemartinism (Cow) — covered above.
• Cryptorchidism — undescended testis.
• Atresia coli — segmental colonic agenesis (Holstein heritable in some lines).
• White Heifer Disease — a sex-limited inherited Müllerian-duct defect associated with particular white-coat lineages.

Developmental defects:
• Cleft Palate / Lip — failure of palatine processes to fuse.
• Atresia Ani — anal membrane fails to rupture.
• Patent Urachus — allantoic stalk fails to close → urine leaks from navel in foal/calf.
• Persistent Right Aortic Arch — vascular ring around oesophagus → regurgitation in puppies.
• Patent Ductus Arteriosus / VSD — among the recognized congenital heart defects; relative frequency varies by species, breed and population.
• Hydrocephalus — excess CSF; brachycephalic dogs.
• Spina Bifida — failed neural tube closure caudally.
• Schistosomus Reflexus — fetal monster with reversed body wall + everted viscera; common dystocia in cattle.
• Perosomus Elumbis — partial agenesis of caudal spine + hindlimbs.
• Contracted Tendons / Limb Deformities — flexor tendon contracture; common in foals.
• Achondroplasia / Dwarfism — defective endochondral ossification (Dexter cattle, bulldogs).
• Hare Lip (Cleft Lip) — incomplete fusion of upper lip (Charolais cattle, brachycephalic dogs).

Teratogens (cause congenital anomalies):
• Drugs: agents such as griseofulvin or warfarin; anaesthetic risk must be assessed by the specific agent, dose and gestational stage.
• Plants: Veratrum californicum (cyclopia in lambs); locoweed; lupines (crooked calf disease).
• Viruses: BVDV (cattle — cerebellar hypoplasia), BTV, Akabane, Schmallenberg, parvovirus (kittens — cerebellar hypoplasia).
• Nutritional: iodine deficiency (goiter), vitamin A excess, copper deficiency (sway-back lambs).
• Radiation, hyperthermia.