repro01 - normal placenta



January 2019



Signalment: Adult female cat


HISTORY: This cat was clinically normal and pregnant


HISTOPATHOLOGIC DESCRIPTION (Slide A): Uterus and placenta: The uterus is complete with a perimetrium, myometrium and endometrium. Extending from the endometrium are maternal septa that interdigitate with the labyrinthine villar projections from the fetal chorioallantois. Below these interdigitations, the endometrium is composed of a deep glandular zone consisting of dilated uterine glands and cellular debris and the superficial junctional zone with terminal lamellae, maternal vessels, and glandular secretions. Fetal tissues consist of the allantoic membrane, normal edematous allantochorionic connective tissue containing large vessels, and lamellae/villi lined by trophoblasts. Multifocally, maternal vascular endothelium directly apposes fetal trophoblasts, seperated by a thin acellular layer of stroma. In an area of smooth placentation (chorion laeve), the fetal tissues are characterized by sparse to indistinguishable allantochorionic connective tissue and a chorionic epithelial layer consisting of columnar cells with basal nuclei.


MORPHOLOGIC DIAGNOSIS: Uterus, placenta: Essentially normal tissue, breed unspecified cat, feline.


Signalment (JPC # 2879193): Adult, female, horse


HISTORY: This horse was clinically normal and pregnant.


HISTOPATHOLOGIC DESCRIPTION (Slide B): Uterus and placenta: The uterus is complete with a perimetrium, myometrium and endometrium. The endometrium is composed of a glandular zone consisting of dilated uterine glands containing cellular debris, separated by loosely arranged connective tissue. Extending from the endometrium are crypts, which are formed by the interdigitation of chorioallantoic villi with the endometrium in a microcotyledonary arrangement,imparting a nodular appearance to the materal fetal interface. Fetal tissues include the allantoic epithelium (lining the allantoic cavity), edematous allantochorionic connective tissue containing large vessels,fetal microcotyledons lined by trophoblastic epithelium that interdigitates with the maternal endometrium to form an epitheliochorial attachment.


MORPHOLOGIC DIAGNOSIS: Uterus, placenta: Essentially normal tissue, breed unspecified, equine.


Signalment (JPC #4056477-00): Adult, female, sheep


HISTOPATHOLOGIC DESCRIPTION (Slide C): Uterus and placenta: Two sections are present, with one exhibiting a placentome and the other the intercotyledonary/ intercaruncular space. In the placentome, the uterus is complete with a perimetrium, myometrium and endometrium. The endometrium is composed of a glandular zone consisting of flattened uterine glands separated by loosely arranged connective tissue. The surface of the endometrium is forming crypts (caruncle) which interdigitate with the fetal chorionic villous projections (cotyledon) which together form the placentome. The chorionic villi are lined by trophoblastic epithelial cells that multifocally have lost cellular borders to form multinucleated syncytiotrophoblasts which contact the maternal endometrium to form an epitheliochorial attachment. Subjacent to the trophoblast epithelium is the edematous allantois connective tissue containing large vessels and areas of hemorrhage (marginal hematoma). The allantoic epithelium is indistinct. The intercaruncular uterus has attenuated or indistinct endometrium, flattened endometrial glands and prominent myometrium. The intercotyledonary chorioallantois is thin with an edematous stroma lined by flattened chorionic epithelium.


MORPHOLOGIC DIAGNOSIS: Uterus, placenta and placentome: Essentially normal tissue, breed unspecified, ovine


Signalment (JPC # 4055712-00): Adult, female, dog


HISTOPATHOLOGIC DESCRIPTION (Slide D): Placenta: The chorioallantois is present forming a labyrinthine or lamellar interconnecting network of trophoblast epithelial cells and multifocal multinucleated syncytiotrophoblasts often in direct apposition to maternal blood vessels which are lined by hypertrophic endothelial cells. There are multifocal areas of hemorrhage (stangant materal blood), with formation of hematoidan crystals, and maternal vessels are often congested. When apparent, the endometrial epithelium and connective tissue are replaced by necrotic cell debris and fibrin. Subjacent to the chorioallantoic trophoblastic epithelium, the allantois is edematous and contains numerous congested blood vessels. The allantoic epithelium is flattened and attenuated or multifocally cuboidal.


MORPHOLOGIC DIAGNOSIS: Uterus, placenta: Essentially normal tissue, breed unspecified, canine.


Signalment (JPC # 4070866-00): 7-year-old female paint horse


HISTORY: This horse was diagnosed with peritonitis and pyrexia prior to necropsy


HISTOPATHOLOGIC DESCRIPTION (Slide E): Uterus, endometrial cup: The endometrium is multifocally lost and endometrial glands are widely separated by numerous large, often elongated specialized trophoblasts with abundant eosinophilic vacuolated cytoplasm and one or two vesiculate nuclei with a single prominent nucleolus. Trophoblasts are admixed with numerous lymphocytes and plasma cells with fewer macrophages, multinucleated giant cells and neutrophils. Endometrial glands are often ectatic and contain moderate to abundant homogenous eosinophilic material (secretory product). Large vessels are congested and the lamina propria is edematous and contains dilated lymphatics. There are lymphocytes and plasma cells occasionally surrounding vessels in the lamina propria and myometrium.


MORPHOLOGIC DIAGNOSIS: Uterus, endometrial cups: Essentially normal tissue, paint, equine.



·      The placenta provides fetal nutrition, respiration, and waste removal, synthesis of substances required by the fetus, enzyme production necessary for the uterine attachment, and hormone production (estrogen and progesterone) to maintain pregnancy

·      Only eutherian mammals have a true placenta composed of fetal membranes and endometrium

·      Metatherians (marsupials) form a yolk sac placenta that lacks intimate contact with the uterine mucosa

·      Prototherians or monotremes (platypus and echidna) are egg layers

·      Fetal membranes:

·      Yolk sack: This forms as an outpouching of the endodermal embryonic midgut and nourishes the embryo before the definitive placenta forms; the yolk sac then regresses; yolk sac endoderm is the source of primordial germ cells that later migrate to the fetal gonad

·      Somatopleure: Composed of the chorion and amnion

·      Chorion: This forms from somatic mesoderm in combination with trophoblasts and covers the entire conceptus; it is initially avascular until combination with the allantois provides vascularization

·      Amnion: Formed from ectoderm and covers the embryo

·      Ectodermal epithelium of amnion and chorion is initially squamous

·      Chorionic epithelium becomes cuboidal and forms the trophoblast

·      In some species, some trophoblast cells lose their cytoplasmic borders and form syncytia, termed syntrophoblasts; those that retain their cytoplasmic borders are termed cytotrophoblasts

·      Allantois: Forms as an evagination of the embryo hindgut

·      Expands to fill the extraembryonic coelom; fuses with the amnion to form the allantoamnion and with the chorion to form the allantochorion

·      This fusion triggers vascularization of the amnion and chorion

·      Umbilical vessels form from allantoic splanchnic mesoderm, which serves as the origin of the fetal vessels of the placenta

·      In the ruminant and pig, the allantois incompletely fills the coelom and the amnion fuses with the chorion to form an amniochorion



Classification based upon the fetal extraembryonic membranes:

·      Choriovitelline (yolk sac) placenta: Yolk sac wall (endoderm) fuses with the chorion (trophoblast and mesoderm) and then apposes the endometrium

·      Transitory and of little importance in domestic animals

·      Mice, rats, rabbits, and guinea pigs have inverted yolk sac placentae with the yolk sac directly exposed to the uterine lumen; this is involved in fetal immunoprotection

·      Allantochorionic placenta: Formed by fusion of the allantois and chorion


Classification based upon the degree of uterine destruction:

·      Nondeciduate placenta: Trophoblasts do not significantly invade or destroy the endometrium; at parturition there is virtually no loss of maternal tissue; most domestic animals are nondeciduate

·      Deciduate placenta: Trophoblasts invade and destroy superficial endometrium, which responds by forming large, polygonal cells, called decidual cells, whose precise function is unknown; at parturition there is loss of maternal tissue and maternal hemorrhage; dogs and cats have mildly deciduate placentas; rodents and primates have fully deciduate placentas


Classification based upon the area of fetal-maternal attachment:

·      Frondose chorion: Surface forms projections that increase the surface area and come in contact with the maternal tissues

·      Smooth chorion: Areas without projections

·      Diffuse placentation: The chorionic villi are present over the entire surface of the endometrium (sow, mare, camel, llama)

·      Cotyledonary placenta: Isolated tufts of branched chorionic villi (cotyledons) attach to preformed endometrial prominences (caruncles), together they form placentomes (ruminant)

·      Zonary placenta: Chorionic projections occupy a ring‑like band extending transversely around the circumference of the fetus (carnivores)

·      Discoid placenta: One or two disk-shaped areas of the chorion are attached to the endometrial stroma (primate, rabbit, rodent)


Classification based upon the fetal-maternal interhemal barrier:

·      There are a maximum of six layers of tissue separating fetal and maternal circulation: (1) maternal capillary endothelium, (2) maternal connective tissue, (3) uterine epithelium, (4) chorionic epithelium (i.e. trophoblast), (5) allantochorionic connective tissue (mesenchyme), and (6) allantochorionic capillary endothelium

·      Epitheliochorial placenta: All six layers present (sow, ruminant, horse)

·      Syndesmochorial placenta: Five layers are present; the uterine epithelium is eroded; this type occurs focally in ruminants after the establishment of an epitheliochorial placenta and in pathological circumstances

·      Endotheliochorial placenta: Uterine epithelium and connective tissue are eroded, leaving only four layers (carnivores)

·      Hemochorial placenta: All maternal layers are eroded leaving trophoblasts in direct contact with maternal blood (primate, rabbit, rodent)


Configuration of maternal and fetal tissue interface:

  • Folded: Macroscopic undulations of fetal tissues (plicae) and microscopic ridges (rugae in pigs; lamellae in carnivores) interlock with corresponding infoldings (fossae) in the endometrium
  • Villous: Branched chorionic villous protrusions interdigitate with corresponding maternal crypts (horse, ruminant, human)
  • Labyrinthine: Trophoblasts form an intercommunicating network with maternal capillaries


Summary of placentation by species:


Fetal/Maternal Attachment

Maternal/Fetal Interface

Uterine Distruction

Interhemal Barrier














































Placental variations in individual species:


·      Early in development the yolk sac is large, it regresses after day 20

·      Trophoblast epithelium is columnar at the base of the folded ridges and becomes flattened at the tips; areola-gland complexes form over uterine gland openings

·      Maternal areolae surround uterine glands; fetal areolae are associated structures composed of villi

·      Uterine “milk” (histiotrophe) forms from degenerate cells and accumulates between uterine and fetal tissue



       Initially the conceptus is unattached to the uterus and is held in place only by increased uterine tone

·      The allantochorion and adjacent uterine epithelium undergo specialized folding, forming microcotyledons over most of the placenta after day 60; spaces similar to areoli surround the microcotyledons opposite ducts of the endometrial glands

·      Chorionic girdle: This refers to rapidly proliferating trophoblasts between the regressing yolk sac and the developing chorioallantois; trophoblasts become binucleate and invade the endometrium around day 37, destroying uterine endothelium and implanting in the stroma to form endometrial cups

·      Endometrial cups: Scattered over the placental surface, protrude into the allantoic cavity

  • Form from detachment and invasion of chorionic girdle trophoblasts into the uterine stroma of the pregnant mare
  • Up to 5 cm in diameter, discrete structures in the superficial endometrium composed of large, binucleate, eosinophilic, polyhedral cells, intermingled with uterine glands
  • Responsible for the production of equine chorionic gonadotropin (ECG), which stimulates development and stabilizes hormonal function of the corpora lutea and is found in the serum of mares between 40 and 120 days gestation
  • Peak hormone production occurs around 60 days gestation and degeneration and eventual sloughing begins around 75 days gestation
  • Lymphocytes and plasma cells are common at the periphery of the cups in early formation which progresses to diffuse mixed inflammation by the time regression occurs, and the fetal cells undergo necrosis with the necrotic cells eventually forming “chorioallantoic pouches” on the surface of the chorioallantois near where the umbilicus attaches

·      Hippomanes are soft, dark brown masses that usually float free in the allantoic fluid or may be lodged in fetal membranes and consist of concentric rings of amorphous material composed of mostly mucoprotein and calcium phosphate from cellular debris

·      Cervical star: No villi; cervical part of the placenta; most common site of mycotic placentitis



·      Mononuclear trophoblasts have sparse rough ER with many apical mitochondria

·      Giant binucleate trophoblasts lack microvilli and have many cytoplasmic organelles and inclusions, many mitochondria, and well-developed Golgi complexes

·      Have no desmosomes and are mobile within the chorionic epithelium

·      Fuse with uterine epithelium and form trinucleate hybrid cells in which hormone-containing granules are transferred from fetus to dam

·      Secrete progesterone, prostaglandin and placental lactogen

·      Although there are areas where the trophoblast is in contact with maternal connective tissue (i.e. syndesmochorial), in general, two layers of epithelium separate fetal and maternal blood

·      Hematomas develop at the base of the chorionic villi because of erosion of maternal capillaries and extravasation of maternal blood later in gestation

·      Trophoblasts phagocytize the red blood cells and release iron

·      These areas are predisposed to bacterial infection during pregnancy

·      Because of interdigitation of fetal villi in maternal crypts, fetal tissue may be retained within the caruncles after birth

·      Trophoblastic epithelium at the base of villi is generally columnar and becomes cuboidal at the tips

·      The epithelium of crypts is irregular, usually cuboidal, but in the sheep and goat, there are areas, which are syncytial



·      Initially extensive choriovitelline placenta, which invades and erodes uterine tissue, then regresses

·      Yolk sac persists through the pregnancy

·      The definitive placenta is composed of three layers:

·      The labyrinthine layer

·      Junctional zone: Contains terminal lamellae, maternal vessels and glandular secretions (enlarged in the canine)

·      Glandular zone: The deepest layer; contains dilated, secreting glands and decidual cells

·      Marginal hematomas occur randomly; processing of extravasated blood by trophoblasts produces green or brown meconium

·      Syncytiotrophoblasts constitute the majority of the trophoblastic layer


Anomalies and abnormalities of the placenta:

·      Hydrops amnion (excessive amniotic fluid): Volumes 8-10 times normal often associated with fetal monsters and prolonged pregnancies

·      Hydrops allantois (excessive allantoic fluid): Volumes 10-50 times normal

·      Retained placenta: Usually caused by infections or uterine inertia

·      Adventitial placentation: Duplication or fusion of placentomes in ruminants because of inadequate fetal-maternal contact

·      Amniotic plaques: Squamous epithelium, with or without keratinization, on the amnion internal surface, present mostly on the bovine umbilical stump

·      Adenomatous dysplasia/hyperplasia of the chorion and/or allantois: Horses

·      Mineralization: Many species, little significance

·      Allantoic calculus: Cow, Homogenous mucoproteinaceous material attached to allantois or floating freely within the allantoic cavity

·      Yolk sac remnants are often attached to the umbilicus in many species.

·      Amorphous globosis: Frequently haired, solid mass attached to the umbilicus; may be a defective twin

·      Freemartinism: Hypoplasia of the female genital tract when twinned with a male; ruminants; results from shared blood supply and shared hormones (anti-Müllerian hormone)



1.    Aughey E, Frye FL. Comparative Veterinary Histology. London, England: Mason Publishing; 2001: 196-203.

2.    Bacha WJ, Bacha LM. Color Atlas of Veterinary Histology. 3rd 3. Ames, IA: John Wiley & Sons, Ltd.; 2012:244, 254-258.

3.    Banks WJ. Applied Veterinary Histology. 3rd ed. St. Louis, MO: Mosby; 1993:457-468.

4.    Benirschke K. Comparative Placentation. 2007. Available at, Accessed December 20, 2018.

5.    Lewis SH, Benirschke K. Placenta. In: Mills SE, ed. Histology for Pathologists. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkens; 2007:1095-1129.

6.    Ross MH, Pawlina W. Histology A Text and Atlas. 5th ed. Baltimore, MD: Lippincott Williams & Wilkens; 2006:796-801.

7.    Young B, O’Dowd G, Woodford P. Wheater’s Functional Histology. 6th ed. Philadelphia, PA: Elsevier; 2014:370-376.

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