Introduction

In sections of the stage 13/14 embryo, 2 components of the placenta and fetal membranes can bee seen.

The amniotic membrane and amniotic space filled with amniotic fluid surrounds the entire embryo. and can be seen in nearly all sections (A1 to G7).

Components of the fetal placental cord and associated blood vessels can also be seen at the umbilical herneation (E2 to F7). Note though that some vitelline components can aslo be seen in some earlier sections.

Both the amniotic sac and embryonic umbilical (placental) region can be seen in the near midline longitudinal (sagittal, parasagittal) sections (G6 and G7).

Stage 13/14 Embryo (Pig)

Use the section list below to access individual slices of the embryo.

A1 | A2 | A3 | A4 | A5 | A6 | A7

B1 | B2 | B3 | B4 | B5 | B6 | B7

C1 | C2 | C3 | C4 | C5 | C6 | C7

D1 | D2 | D3 | D4 | D5 | D6 | D7

E1

Components of the fetal placental cord and associated blood vessels can be seen at the umbilical herneation (sections E2 to F7). Note also lower end of embryo to right. E2 | E3 | E4 | E5 | E6 | E7 | F1 | F2 | F3 | F4 | F5 | F6 | F7

G1 | G2 | G3 | G4 | G5

Both the amniotic sac and embryonic umbilical (placental) region can be seen in the near midline longitudinal (sagittal, parasagittal) sections (G6 and G7). G6 | G7

Development Overview

• Placenta

  • (Gk. plakuos= flat cake)

• embryonic/maternal organ
• villous chorion/decidua basalis
• continuous with amniotic and chorionic sacks
• Dimensions
  • discoid up to 20cm diameter and 3 cm thick (term)
  • weighs 500-600 gm
• Shapes
  • accessory placenta, bidiscoid, diffuse, horseshoe
• maternal and embryonic surface
• delivered at parturition
• retention may cause uterine hemorrhage
• Maternal Surface
  • Cotyledons
  • form cobblestone appearance
  • originally placental septa formed grooves
  • covered with maternal decidua basalis
• Fetal Surface
  • umbilical cord attachment
  • cord 1-2 cm diameter, 30-90cm long
  • covered with amniotic attached to chorionic plate
  • umbilical vessels branch into chorionic vessels
  • which anastomose
• Chorionic Villi-Primary Villi
  • week 2
  • trophoblastic shell cells
  • syncitiotrophoblasts, cytotrophoblasts
  • frorm finger-like extensions
• Chorionic Villi- Secondary Villi
  • week 3
  • extraembryonic mesoderm grows into villi
  • covers entire surface of chorionic sac
• Chorionic Villi- Tertiary Villi
  • mesenchyme differentiates into blood vessels and cells
  • forms arteriocapillary network
  • fuse with placental vessels
  • developing in connecting stalk
• Chorionic Villi
  • Stem
    • or anchoring villi
    • cytotrophoblast cells attached to maternal tissue
  • Branched
    • or terminal villi
    • grow from sides of stem villi
    • region of main exchange
  • surrounded by maternal blood in intervillous spaces
  • maternal sinusoids
• Chorionoic Villi
  • originally cover entire chorionic surface
  • become restricted to decidua basalis region
  • Frondosum
  • Capsularis
• Maternal Decidua basalis reaction
  • deposition of glycogen
  • proliferation of blood vessels
• Placental- Histology
  • Placental- Histology UNSW Histology- slide 96
• Placental Function
  • 4 layers separate maternal and fetal blood
    • syncitiotrophoblast
    • cytotrophoblast
    • villi connective tissue
    • fetal capillary endothelium
  • 3 main functions
    • metabolism
    • transport
    • endocrine
• Placental Metabolism
  • synthesizes
    • glycogen
    • cholesterol
    • fatty acids
  • provides nutrient and energy
• Placental Transport
  • gases and nutrition
    • oxygen, carbon dioxide, carbon monoxide
  • water, glucose, vitamins
  • hormones, mainly steroid not protein
  • electrolytes
  • maternal antibodies
  • waste products
    • urea, uric acid, bilirubin
  • drugs and their metabolites
    • fetal drug addiction
  • infectious agents
    • cytomegalovirus, rubella, measles, microorganisms
• Placental Endocrine
  • Human chorionic gonadotrophin (hCG)
    • like leutenizing hormone, supports corpus luteum
  • Human chorionic somatommotropin (hCS)
    • or placental lactogen
    • stimulate mammary development
  • Human chorionic thyrotropin (hCT)
  • Human chorionic corticotropin (hCACTH)
  • progesterone and estrogens
    • support maternal endometrium
  • relaxin

Placental Blood vessels

• form initially in the connecting stalk (then umbilical cord)
  • anastomose in chorioni
  • extend maternally toward chorionic villi
  • extend embryonically to the sinus venosus and dorsal aorta
• Arteries
  • paired
  • carry deoxygenated blood (from dorsal aorta) and waste products to the placental villi
• Veins
  • paired initially then only left at end of embryonic period
  • carry oxygenated blood to the embryo (sinus venosus)

Blood flow through the Embryo

Maternal Blood | -> umbilical vein -> liver -> anastomosis -> sinus venosus -> atria ventricles-> truncus arteriosus -> aortic sac -> aortic arches-> dorsal aorta-> pair of umbilical arteries | Maternal Blood

This is shown on the stage 13/14 pig G6 section.

Parturition (Childbirth) Overview

• Fetal Growth
  • First Trimester (1 - 12 weeks)
    • embryonic and early fetal
  • Second Trimester (13 - 24 weeks)
    • organ development and function, growth
  • Third Trimester (25 - 40 weeks)
    • organ function and rapid growth
• Birth (parturition)
• Textbook
  • Larson (2nd Edition)
    • Chapter 15 p471-488
  • Moore & Persaud (6th Edition)
    • Chapter 7 p129-167
• Fetus to neonate involves three phases
  • Transition Phases
  • late gestation
  • parturition
    • processes needed to establish independent homoeostatic regulation after separation from the placenta.
    • Phases are regulated by a series of fetal and placental endocrine events.
      • Gluckman PD, Sizonenko SV, Bassett NS Acta Paediatr Suppl 1999 Feb;88(428):7-11
• Newborn Homoeostasis
  • Newborn has to establish
  • lung function
  • circulatory changes
  • thermoregulation
  • endocrine function
  • nutrition
  • gastrointestinal tract function
  • waste
  • kidney function
• Childbirth

  • Parturition (L. parturitio = childbirth)

• expelling the fetus, placenta and fetal membranes
• probably initiated by fetus not mother
• Labor
  • uterine contractions and dilation of cervix
  • process under endocrine regulation
• Placenta and Fetal Membranes
  • secundina (L. following)
  • expelled after birth
  • Placenta covered in earlier Lecture
• Labor Abnormalities
  • Premature Labor
    • occurs 7 -10% in humans
    • contributes 75% perinatal mortalities
  • Underdeveloped systems
  • particularly respiratory
    • surfactant
    • hyaline membrane disease
    • see respiratory development lecture
• Uterine Myometrial Changes
  • Smooth muscle fibers
  • hypertrophy not proliferation
  • Stretching of myometrium
  • stimulates spontaneous muscular contraction
  • Inhibiting Contraction
    • progesterone inhibits contraction
  • Stimulating Contraction
    • Increased estrogen levels
      • placental secretion sensitizes smooth muscle
    • Increased oxytocin levels
      • fetal oxytocin release- force and frequency of contraction
      • fetal pituitary
    • prostaglandin production
      • estrogen and oxytocin stimulate endometrial production of prostaglandin
• Labor- Stages
  • stage 1- dilatation
    • uterine contractions 10 minutes apart
    • function to dilate cervix
    • fetal membranes rupture releasing amnion
    • 7 -12 hours (longer for first child)
  • stage 2- expulsion
    • uterine contractions push fetus through cervix and vagina
    • uterine contractions 2-3 minutes apart
    • 20 - 50 minutes
  • stage 3- placental
    • following child delivery contractions continue to expel placenta
    • haematoma separates placenta from uterine wall
    • separation occurs at spongy layer of decidua basalis
    • 15 minutes
  • stage 4- recovery
    • continued myometrial contraction closes spiral arteries
    • 2+ hours
• Labor stage 2- Expulsion
  • Options
  • Vaginal Delivery
    • dependent on pelvic canal size
    • vaginal canal can be enlarged by episotomy
      • cutting peritoneal muscle
      • avoids tearing
      • clinically more often than necessary
  • Cesarean Section
    • surgical incision through abdominal wall
    • 15 - 25 % of all deliveries
    • clinically more often than necessary
• Placental Abnormalities
  • placenta accreta
    • abnormal adherence, with absence of decidua basalis
  • placenta percreta
    • villi penetrate myometrium
  • placenta previa
    • placenta overlies internal os of uterus
    • abnormal bleeding
    • cesarian delivery
• Transition from fetus to neonate
  • Phases
  • late gestation
  • parturition
  • processes needed to establish independent homoeostatic regulation after separation from the placenta.
  • Regulated by fetal and placental endocrine events.
  • Glucocorticoids
  • have an important role in the preparation for birth, including involvement in lung and cardiac development, and the maturation of enzymes in a variety of pathways.
  • Fetal cortisol
  • production is under hormonal control.
    • The transition from fetus to neonate--an endocrine perspective. Gluckman PD, Sizonenko SV, Bassett NS Acta Paediatr Suppl 1999 Feb;88(428):7-11

Placenta Development Terms

• anastomose-
• angiogenesis- the formation of blood vessels also called vasculogenesis in the embryo
• angioblasts- form clusters or blood islands on surface of yolk sac.
• capsularis-
• chorionic sac- fetal membrane that surrounds the developing embryo.
• cord knotting- umbilical cord knotting occurs in 1%, prevents the passage of placental blood. pseudoknots also occur usually with no effect.
• cotyledons- maternal side cobblestone appearance, originally placental septa formed grooves is covered with maternal decidua basalis.
• cytotrophoblast- extraembryonic cells of trophoblastic shell surrounding embryo, contribute to villi and placental membranes.
• decidua basalis reaction- occurs in maternal endometrium at site of, and following, blastocyst implantation. Seen as a deposition of glycogen and proliferation of blood vessels.
• endocrine- function of placenta:
  • Human chorionic gonadotrophin (hCG)- like leutenizing hormone, supports corpus luteum
  • Human chorionic somatommotropin (hCS)- or placental lactogen, stimulate mammary development
  • Human chorionic thyrotropin (hCT)
  • Human chorionic corticotropin (hCACTH)
  • progesterone and estrogens- support maternal endometrium
  • relaxin- role in parturition, softens ligaments
• fetal drug addiction- occurs when drugs used maternally cross the placental barrier and can establish addiction in the unborn fetus.
• fetal erythroblastosis- (Haemolytic Disease of the Newborn), an immune problem from fetus Rh+ /maternal Rh-, leakage from fetus causes anti-Rh antibodies, which is then dangerous for a 2nd child.
• frondosum-
• haemocytoblasts- stem cells for embryonic blood cell formation
• Haemolytic Disease of the Newborn- see fetal erythroblastosis.
• Human chorionic gonadotrophin- (hCG) like leutenizing hormone, supports corpus luteum
• Human chorionic somatommotropin- (hCS) or placental lactogen stimulate mammary development
• Human chorionic thyrotropin- (hCT) placental derived hormone equivilant to thyroid
• Human chorionic corticotropin- (hCACTH) placental derived hormone equivilant to corticotropin (ACTH) from the pituitary.
• maternal antibodies- antibodies from the mother's immune system that are capable of crossing placental barrier. They can provide immune protection to the embryo, but may also participate in immune disease (fetal erythroblastosis).
• maternal sinusoids- placental spaces around chorionic villi that are filled with maternal blood. Closest maternal/fetal exchange site.
• placenta- (Gk. plakuos= flat cake) describes its typical mature discoid shape (20cm diameter and 3 cm thick at term,weighs 500-600 gm).
• placenta accreta- abnormal, adherence with absence of decidua basalis.
• placental arteries- paired, carry deoxygenated blood and waste from the embryo (dorsal aorta->internal iliacs->PA)
• placental blood- blood found within the placental vessels. Obviously part of the fetal blood, but can be collected at birth for theraputic use containing blood stem cells (see cord blood banks).
• placental blood vessels- form initially in the connecting stalk (then umbilical cord), anastomose in chorioni and extend maternally toward chorionic villi, extend embryonically to the sinus venosus and dorsal aorta.
• placental layers- 4 layers separate maternal and fetal blood: syncitiotrophoblast, cytotrophoblast, villi connective tissue, and fetal capillary endothelium.
• placenta percreta- abnormal, villi penetrate myometrium.
• placenta previa- placenta overlies internal os of uterus, abnormal bleeding, may require cesarian delivery.
• placental veins- paired initially then usually only one left at end of embryonic period, carry oxygenated blood to the embryo (sinus venosus)
• primary villi- develop week 2, consist of trophoblastic shell cells both syncitiotrophoblasts and cytotrophoblasts. Form finger-like extensions into the maternal endometrium.
• protein hormone- usually a protein distributed in the blood that binds to membrane receptors on target cells in different tissues. Do not easliy cross placental barrier.
• relaxin- hormone.
• secondary villi- develop week 3, extraembryonic mesoderm grows into villi, initially covers entire surface of chorionic sac.
• sinus venosus- cavity into which all major embryonic paired veins supply (vitelline, placental, cardinal)
• syncitiotrophoblast- extraembryonic cells of trophoblastic shell surrounding embryo, outside the cytotrophoblast layer, involved with implantation of the blastocyst by eroding extracellular matrix surrounding maternal endometrial cells at site of implantation, also contribute to villi. (dark staining, multinucleated)
• tertiary villi- develop week 4, mesenchyme within secondary villi differentiates into blood vessels and cells, forms arteriocapillary network, fuse with placental vessels developing in connecting stalk.
• trophoblast-
• umbilical cord- fetal attachment cord 1-2 cm diameter, 30-90cm long, covered with amniotic attached to chorionic plate, umbilical vessels (artery, vein) branch into chorionic vessels. Vessels anastomose within the placenta.
• villi- initially outgrowth of the trophoblastic shell which involve other tissues with development. Develop in sequence (primary, secondary, tertiary) with mature villi being stem por branched type.
• virus- small infectious agent able to cross placental barrier. Can infect embryo and cause developmental abnormalities. (e.g. cytomegalovirus, rubella, measles)
• vitelline- Blood vessels cover entire surface of yolk sac and connect to embryo through yolk stalk
  • Arteries- arises from dorsal aorta and contribute to adult GIT arteries.
  • Veins- empties into sinus venosus and contribute to the adult portal system.
• waste products- products of cellular metabolism and cellular debris, e.g.- urea, uric acid, bilirubin

Quick Links

Placenta Pages:

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Genes/DNA Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Urogenital Musculoskeletal Integumentary

Movies ----------------- Embryo Growth Ultrasound Week 1 Week 2 Week 3 Week 4 Stage 13/14 Stage 22 Neural Head and Neck Heart Vasculature Gastrointestinal Urogenital Other Embryos Larsen Internet

Other Embryos ----------------- Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G