Learning Reproduction in Farm Animals

08-Early Embryonic Development and Placentation

Leaning Objectives:

To understand how the embryo, fetus and placenta develop during gestation.
To understand how the placenta functions to maintain the fetus.

Time comparisons from ovulation during cleavage for different farm species.

	1 cell	8 Cell	Blastocyst	Enter Uterus	Length of
Species	(hours)	(Days)			Gestation
Cattle	24	3	8	3.5	281
Horse	24	3	6	5	337
Sheep	24	2.5	7	3	148
Swine	14-16	2	6	2	114

Embryo Development - Differentiation

Certain organs that have been identified as forming from specific germ layers.

Germ Layer	Organs
Ectoderm	1. Central nervous system 2. Sense organs 3. Mammary glands	4. Sweat glands 5. Skin 6. Hair 7. Hooves
Mesoderm	1. Circulatory system 2. Skeletal system 3. Muscle	4. Reproductive systems (male and female) 5. Kidney 6. Urinary ducts
Endoderm	1. Digestive system 2. Liver 3. Lungs	4. Pancreas 5. Thyroid gland 6. Most other glands 7. Primordial germ cells

Development of Extraembryonic Membranes

Extraembryonic Membranes (Day 18)

Function of Fetal Membranes and Fluids

Yolk sac: Early nutrient supply for early embryo.

Amnion: Protects fetus from injury, provides lubrication for parturition, reservoir for urine and wastes.

Allantois: Fuses with chorion (chorio-allantoic placenta), carries blood vessels of placenta, reservoir for nutrients and wastes. Umbilical cord attaches fetus to allantois.

Chorion: Attaches to uterus, absorbs nutrients from uterus, allows maternal/fetal gas exchange, Produces hormones.

Attachment of the placenta to the uterus. (Implantation)

Cow Sow Ewe Mare

Days 30-35 12-20 18-20 50-60

Types of Implantation.

A. Superficial - Chorion fuses to endometrium

B. Interstitial

Maternal Recognition of Pregnancy

Definition:

Establishment of Pregnancy Sow Mare Cow Ewe

Days of Estrous Cycle

Embryonic Signal

Placental lactogens (Protein Hormones)

High levels in the last 1/3 of gestation.

High levels facilitate higher milk production.

Types of placental attachments

High surface area at placental-uterine attachment.

Fetal Nutrition (Farm Animals)

Placenta highly permeable to water and electrolytes i.e., stored in and exchange between allantoic and amniotic fluids.

Placenta converts glucose to fructose.

Fetal fat formed from fatty acids transferred across the placenta.

Fetus makes proteins from transferred amino acids.

Water soluble vitamins cross the placenta easier than fat soluble.

Minerals are transferred across the placenta.

Antibodies to fight disease do not cross the placenta - the newborn will get these antibodies from the first milk - Colostrum.

Hormonal Control of Gestation

Progesterone

Main Hormone
Causes uterine quiescence and histotroph production.

Sources of Progesterone During Gestation

		Gestation
Sheep	CL to day 50 then placenta	148 days
Cow	CL throughout gestation. However, after ~ day 215 placenta and adrenal are producing enough progesterone to maintain pregnancy.	280 days
Sow	All from CL	114 days
Mare	1st CL produces progesterone up to ~ 180 days, FSH stimulates follicle growth and eCG causes CL's to form from follicles. They secrete progesterone and are active from day 40-180. Progesterone is low until 270 days- rises in last 5 months due probably to placental progesterone production.	337 days

Mare Hormone Levels During Pregnancy