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Placental type
Placental type
Codeledonary Placenta
Codeledonary Placenta
Sheep have a cotyledonary placenta, which is comprised of multiple placentomes rather than a single large area of exchange (10,28). The choronic villi are organized into distinct, localized patches called cotyledons (10). Key components of the placenta include:
Cotyledon: The fetal side of the placentome. (10,28)
Caruncle: The maternal side, composed of specialized thickenings in the uterine mucosa. (10,28)
Placentome: The functional unit formed by the cotyledon and caruncle. There are anywhere from 75-125 placentomes present in sheep. (10,28)
Figure 1. Caruncles in an incised non-pregnant sheep uterus (left) and cross sections through placentomes from a midgestation sheep pregnancy (right).
Bowen, R. (2000) Placentation in Ruminants (Cattle, sheep, ..). Placentation in ruminants. https://vivo.colostate.edu/hbooks/pathphys/reprod/placenta/ruminants.html
Functions and Role in Pregnancy
Functions and Role in Pregnancy
Nutrient and Gas Exchange:
Facilitates the transport of oxygen, nutrients, and antibodies from the mother to the fetus. (10,28)
Removes carbon dioxide and metabolic waste products from the fetus to the maternal circulation. (10,28)
Hormonal Secretion:
Produces hormones such as estrogen, eCG, progesterone, and placental lactogen, which are crucial for maintaining pregnancy and supporting fetal growth.
Progesterone:
Sheep placenta produces sufficient levels by day 50 to sustain pregnancy without the corpus luteum. (28)
Increases blood of the pregnant female and decline a few days prior to birth (29)
Estrogen:
Increases blood of the pregnant female and decline a few days prior to birth. (10,28)
Increases gap junction in myometrium, oxytocin receptors in endometrium, and mucus production in cervix. (29)
eCG:
Has FSH- and LH-like activities. (29)
High levels stimulate ovulation to form accessory CL and induce formation of supplementary CL. (29)
Placental Lactogen:
Secreted in large amounts in sheep, with high levels detected from day 50 of gestation onward. (28)
Somatotropic (fetal growth) and lactogenic (mammary function) activities (29)
Immunological Barrier:
Provides a controlled environment where maternal immune cells are less likely to attack the fetus while still allowing limited antibody transfer. (10,28)
Fetal Development:
Ensures an appropriate supply of nutrients and gases for the developing fetus, particularly critical in the later stages of gestation when growth is rapid. (10,28)
Provides shock protection for fetus. (29)
Placental Changes Throughout Development
Placental Changes Throughout Development
Figure 2. Various perspectives of a placentome which is composed of maternal and fetal tissues for nutrient exchange within the placenta. These placentomes change in size and vascularity throughout gestation.
Senger, P. L. (2015). Pathways to pregnancy and parturition (3rd ed.). Current Conceptions Inc.
Morphological changes
Morphological changes
The following morphologic, or physical, changes occur within the ewe placenta as gestation progresses:
The concentration of connective tissue decreases to minimize the distance between the uterine and placental structures (30)
Mononucleate trophoblast cells differentiate into binucleate trophoblast giant cells which help form syncytial cells (28)
Binucleate cells (BNCs) migrate to the uterine luminal epithelium (LE) to fuse with the cells of the LE to form trinucleate trophoblast-syncytial hybrid cells (30)
The formation of binucleate cells becomes more uniform across the uterine-placental interface as development continues (30)
The trinucleate trophoblast-syncytial cells enlarge into syncytial plaques (30)
As the concave placentomes form, the chorionic villi protrude into crypts of the caruncular tissue around day 30 of gestation (10)
The cotyledons of the placentomes increase many-fold in diameter to provide a large surface area to support placental transfer of nutrients between the dam & the fetus (10)
Figure 3. Histological classification of the ewe placenta at Day 17 (top) vs Day 35 (bottom) of gestation. These two images demonstrate the different structures visible at various stages of pregnancy.
Seo, H. (2024). Placentation in Ruminants Lecture. University of Maryland Department of Animal Sciences.
Histological Changes
Histological Changes
Placentation is classified based on histological analyses that determine:
The number & type of uterine & placental cell layers
The extent of trophoblast invasion into the uterine tissue
Degree of trophoblast differentiation
Number of trophoblast layers that separate the maternal & fetal blood (28)
Different structures are visible under histological analysis depending on the day of gestation. The following structures can be identified under a microscope at a specific day: (28)
Day 17: caruncle, intercaruncular endometrium, conceptus, luminal epithelium (LE), glandular epithelium (GE), and blood vessels (BV)
Day 20: caruncle, intercaruncular endometrium, and the conceptus
Day 30: caruncle, intercaruncular endometrium, and the chorioallantois
Day 35: caruncle, intercaruncular endometrium, and the cotyledon
Day 50: cotyledonary villi, caruncle, and main layers forming the placental barrier
Day 70: cotyledonary villi and the caruncle (28)
Pregnancy
Pregnancy
Duration
Duration
Gestation lasts about 147 days (5 months), with slight variation depending on the breed. (25)
Gestation is divided into three trimesters, each lasting approximately 49 days, with key developmental milestones occurring in each phase. (25)
Physiological Changes
Physiological Changes
Early Stage
Early Stage
(Days 0–30): Fertilization leads to embryo implantation, supported by increased progesterone levels. (26)
Middle Stage
Middle Stage
(Days 31–100): Fetal growth accelerates, and ewes may gain weight with minor udder development. (26)
Late stage
Late stage
(Days 101–147): The fetus grows significantly, the ewe’s abdomen expands noticeably, and colostrum production begins. (26)
Fetal Numbers
Fetal Numbers
First-time ewes often carry a single lamb.
Most breeds commonly produce twins.
Triplets or larger litters are less frequent but more likely in prolific breeds like Finnsheep. (25)
Fetal Development
Fetal Development
First Trimester
First Trimester
Organs begin forming, and the fetus measures 3–5 cm by Day 30.
By Day 49, the fetus is a more recognizable shape, with visible facial features and early development of internal organs
The limb buds form and initial skeletal structures are laid down, although they remain largely cartilage at this stage. (26)
Second Trimester
Second Trimester
Skeleton and body structures develop.
The skeleton begins to ossify (cartilage turns to bone).
Muscle growth allows for increased movement, and the fetus starts to look more like a lamb.
The fetus grows rapidly during this time, reaching about 10 cm by Day 50 and about 30 cm by Day 98. (26)
Third Trimester
Third Trimester
The fetus continues to grow rapidly, developing wool, muscle, and fat, reaching a birth weight of 3–5 kg, influenced by breed and litter size.
The lungs mature, and the fetus becomes increasingly capable of breathing and surviving outside the womb.
The fetus reaches about 35 cm in length by Day 120. (26)
Parturition
Parturition
Parturition Occurs in 3 Main Steps:
Initiation of Myometrial Contractions
As the fetus approaches the space limitations of the uterus, it triggers the release of adrenal corticotropin (ACTH) from the fetal pituitary which stimulates the fetal adrenal cortex to secrete adrenal corticoids. (10)
Increased levels of fetal corticoids cause an endocrine change in the ewe, stimulating the conversion of progesterone to estradiol by enzymes. (10)
The conversion of progesterone to estradiol triggers the placenta to synthesize PGF2a. (10)
Increased estradiol and prostaglandins cause to myometrium to become more active causing uterine contractions. (10)
As pressure in the uterus increases, the fetus will rotate so that its head and front limbs are facing the rear of the dam for easier delivery. (10)
The myometrial contractions push the fetus towards the cervix, applying pressure to it. This pressure activates pressure-sensitive neurons which synapse with oxytocin-producing neurons in the hypothalamus to trigger the release of oxytocin. (10)
The increase in oxytocin facilitates further myometrial contractions to push the fetus against the cervix, creating a positive feedback loop of oxytocin release in response to pressure on the cervix. (10)
As the myometrial contractions peak, the fetus enters the cervical canal in preparation for birth. (10)
Expulsion of Fetus
PGF2a, released from the placenta, stimulates the synthesis of relaxin which causes the connective tissue in the cervix to soften and pelvic ligaments to relax. (10)
As the uterine contractions intensify, the feet and head of the fetus begin to press against the fetal membranes, causing them to rupture. (10)
When the fetal membranes rupture, they release the amniotic and allantoic fluids that were in them. These fluids lubricate the birth canal, facilitating the passage of the fetus. (10)
As the fetus begins to enter the birth canal, head first, it experiences reduced oxygen levels which triggers increased fetal movement. (10)
The increased movement from the fetus causes further myometrial contractions as well as abdominal contractions. The combination of these contractions will push the fetus through the birth canal and out of the ewe. (10)
Expulsion of Fetal Membranes
After the lamb is delivered, the uterus will continue to contract but to a lesser degree as during the previous stage. (10)
Blood flow to placentomes is restricted by vasoconstriction of blood vessels supplying the area. (10)
Proteolytic enzymes and immune cells break down the connective tissue and collagen in the placentome. (10)
Eventually all of these factors will cause the placental cotyledon to detach from the maternal caruncular regions and the membranes will collect in the uterine lumen. (10)
Uterine contractions and gravitational forces will cause the membranes to be expelled through the birth canal. This generally happens within a couple of hours after birth. (10)
Timeline of Reproduction
Timeline of Reproduction
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