Anatomy of Pregnancy 2: Anatomical Features of Pregnancy in the Sow, Mare, Ruminant and Carnivores

Describe what happens to blastocysts in litter-bearing species such as the sow after they enter the uterine horns.

Such as sows

• Spherical blastocysts dispersed along uterine horns by peristaltic movements

• Blastocyst expands to form thin tube (1m long)

• Initiates maternal recognition

What happens in the sow if there is less than 3 embryos present?

• No maternal recognition of pregnancy, results in pregnancy termination

What kind of placenta does the sow have?

Diffuse, epithelio-chorial, non-deciduate placenta

Describe the following characteristics of the sow placenta:

• Diffuse

• Epithelio-chorial

• Non-Deciduate

Diffuse: Villi spread all throughout

• Placental zone - with villi

• Paraplacental zone - small area around edges with no villi

• Also avascular extremity (No vasculature)

Epithelio-chorial: Histology, number of layers between fetus and mother

• No invasion of endometrium

• Survival depends on nutrition - surface area of contact

• Chorionic vesicle expands to contact neighbours.

• Surplus die & reabsorbed.

Non-deciduate:

• No loss of maternal tissue when afterbirth shed

Describe what happens to the blastocyst when it enters the mare uterus.

• Enters uterus approx day 6.

• Remains spherical - protective capsule

• Known as embryonic vesicle

• Initially very MOBILE, covers the full length of the uterus each day

When can the mare blastocyst be visualized in the mare?

• 10 days (Will have to look all throughout the uterus)

Describe when the embryonic vesicle stops it’s motion in the mare reproductive tract and thus continues to develop.

• Embryonic vesicle enlarges

Day 16:

• Becomes lodged in body horn junction

• Can be on side independent of CL

Day 21:

• Amnion visible

• Heartbeat visible on ultrasound

Day 30:

• Palpable as swelling at body horn junction

Describe the features of development occurring throughout the following stages of pregnancy in the mare.

• Day 28 - 33

• Day 40

• Day 60 - 100

• Day 28-33:

  • Allantois enlarges

  • Yolk sac regresses

  • Embryo moves dorsally in vesicle

  • Choriovitelline placenta functional

  Day 40:

  • Chorio-allantois engulfs yolk sac

  • Umbilical cord forms

  • Contains remnant of yolk sac

  • Foetus moves ventrally as elongates

  Day 60 - 100:

  • Foetus too deep in abdomen to identify

Describe features of the mare placenta.

Diffuse, epitheliochorial, non-deciduate placenta

• **Foetal membranes shed < 6hrs after birth.

  • Failure to release membranes requires urgent vet treatment

How long does the mare take to form a placenta.

• Not completely formed until day 140

• Takes over from ovary in maintaining pregnancy

• No septum uterus

• Chorio-allantois expands into second horn

• Foetus mobile within allantois, has very long umbilical cord

• Microcotyledons form, which are microvillous tufts present only during pregnancy, focal areas with increased villi

• Are microscopic

What are the hippomanes?

• Allantoic calculus, accumulations of waste products which have mineralized within allantoic fluid

• Is a normal finding

• Post-birth in mares

Describe what (And when) happens when the morula enters the uterus in the cow.

• Morula enters uterus approx day 4

• Becomes blastocyst approx day 8

• Embryo located in centre of horn, however enlongates in the same side as CL, but in time extends into non-pregnant horn

• Amnion forms approx. day 18

• Chorio-allantois expands into non- pregnant horn

Describe the placenta present in a cow.

Cotyledonary, epithelio-chorial, non-deciduate placenta:

Describe the following features of the cow placenta:

• Caruncle

• Cotyledons

• Placentome

• Caruncle - located on endometrium (mushroom shaped)

• Cotyledons:

• chorionic villi clumped in circular areas

• develop opposite caruncles

• Present on surface of chorion on fetal membranes

Grouping of caruncle and cotelyedons:

• Placentome:

• interdigitation of foetal cotyledon + maternal caruncle

• Chorionic villi invade crypts in caruncle

• Begin to develop from approx d 30

Describe when and how we can identify pregnancy in the cow.

Rectal Ultrasound:

• Pregnancy first reliably identified day 21

• later than mare

• Elongated vesicle

• Ddx from oestrous fluid in uterus

• Routinely performed > day 30

• Very accurate

• corpus luteum

• fluid filled structure in the uterus (same side as CL)

• Embryo + heart beat

Describe how the placenta appears in the ewe.

Cotyledonary, epithelio-chorial, non-deciduate placenta

Describe the typical pregnancies we may see in ewes.

• High incidence of twin / multiples depending on genetics

• Transuterine migration occurs - makes it easier to support more than one pregnancy, developing embryos can migrate throughout uterine horns

• Twins in different horns

• Synchorial fusion is normal

• Fusion of allantoic blood vessels does not usually occur though

• Freemartinism not a problem (cf calves)

• Caruncles convex / donut shaped

Describe how pregnancy detection occurs in the ewe?

Pregnancy detection:

Abdominal ballotment

• Towards end of pregnancy

• Pressing on abdomen, fetus may bounce back against wall

Ultrasound

• 70-105 days

• Placentomes a problem beyond this, difficult to assess pregnancy after this point

• Foetal numbers estimation

• Good for management in late gestation

Describe the distribution of the embryonic vesicles in litter producing species like the Bitch / Feline.

Litter producing - multiple conceptuses

• Embryonic vesicles mobile till d13

• Even distribution along both horns

• No maternal recognition of pregnancy, due to prolonged CL

Describe what occurs through each stage of pregnancy in the bitch.

Day 16:

• First visible using transabdominal ultrasound

• Series of anechoic (black) spherical vesicles

Day 21:

• Embryo visible + Heartbeat

• Chorio-vitelline (yolk sac) placenta functional

Day 24:

• Chorio-allantois complete

• Yolk sac regresses

• Persists to term as remnant in umbilical cord

• Haematopoeitc function

When can early pregnancy detection be performed in the Bitch?

Early pregnancy detection:

• Routine ultrasound examination 28 days after last mating

• Need to keep in mind there can be up to a week before conception, even after mating

• Minimum d21 embryos present

• Ddx - vesicles from cysts

• No need to repeat examination

• Palpation day 21-31

• Not possible in all bitches

• Series of 'walnuts'

Describe the type of placenta present in the bitch.

Zonary, endothelio-chorial, deciduate placenta

Describe features of the zonary bitch placenta.

• Most of chorion has no villi = paraplacental area

• Villi located in zonary band:

• Encircles equatorial region of chorionic sac.

• Adjacent band of endometrium proliferates to match this chorion

  • Villi invade endometrial glands:

• Destroys glands, maternal epithelium and connective tissue (Deciduate)

What are the areas of hemorrhage surrounding the zonary band in Bitches? Why do they occur?

Marginal haematomas: (Haemophagus organ)

• Areas of maternal blood vessel destruction

• Develop along periphery of the zonary band

• May also be spotted through zonary band

• Foetal cells bathed in pooled maternal blood

• Absorption of breakdown products

• (Cf haemochorial placenta)

• Uteroverdin = blood breakdown products

• Dog - Green

• Cat - Brown

For what applications can ultrasound be used for in the Bitch?

Applications of Ultrasound:

• Pregnancy detection

• Counting foetuses!

• Foetal viability

• Estimate gestational age/ date of whelping:

  • Compare with published charts

  • Day 35 - Foetal anatomy identifiable

  • Day 42 - mineralisation of skeleton

• Measurements:

• vesicle diameter

• crown rump length

• trunk diameter

• bi-parietal diameter

• Identify retained foetuses

For what applications can radiography be used for in the Bitch?

Radiography:

• Before d 42 - cannot distinguish early pregnancy from other causes of an enlarged uterus eg.pyometra

• After d 42 - presence of mineralised skeletons

• Difficult to assess viability / gestational age

• Foetal numbers - count skulls!

• Identify retained foetuses

At what size are pups palpable in the bitch?

>d50

• Unreliable d31-50 as vesicles confluent

How are pups/kittens delivered (In what membrane)?

• Delivered in the amnion

  • Mother tears the membrane from pup / kitten

Describe general features of the deciduate placenta in the Bitch/Feline and colors of the uterine fluid.

Deciduate placenta:

• Placental labyrinth - interface with embryo/mother

• Maternal tissue shed with placenta

• Green uterine fluid in bitch (uteroverdin)

• Brown uterine fluid in queen

What is synchorial fusion?

Fusion of paraplacental areas of adjacent fetuses