UNSW Embryology

Beginnings, Growth and Development

© Dr Mark Hill (2008)

Acknowledgements

Practical - Placenta and Fetal Membranes

Practical 3 - Fertilization to Implantation	Practical 6 - Implantation to 8 Weeks	Practical 11 - Fetal Period
	Practical 8 - Placenta and Fetal Membranes

Introduction

MFAC 1503 Session 1 2008 Practical 8: Fetal Membranes and Placenta The laboratory class is coordinated by Dr Murray Freeman. It includes both wet specimens, images of the conceptus and histological sections of embryo, fetal membranes and placenta. The notes of your Practical are in your Manual.   There are additional general placenta resources (not part of today's Practical): Placenta Notes | Placental Histology | Placental Abnormalities | Stage 13/14 | Stage22 | Placental Vascular Beds | Blood | Blood Vessels | Birth |.

Human Embryo showing developing placenta and membranes

Stage 11 (Exalto et al.)

In this first set of images of the stage 11 conceptus we will begin at the outer margin of the chorionic sac and then work inward to the other fetal membranes, finally looking at a cross-section of the embryo and associated fetal membranes.

Carnegie Stage 11 human conceptus, consisting of a chorionic sac of 12-13 mm diameter, which contains a 3 mm long embryo. The blood is in the future umbilical vessels in the connecting stalk (not visible).

The villi now contain embryonic vessels and are defined as tertiary chorionic villi (blood has drained out of these vessels).

The chorionic sac is now 2 developing regions: villous chorion with many villi and the smooth chorion with few villi, site of apposition of the chorionic sac to the decidua capsularis. (from Exalto et al., 1980)

Carnegie Stage 11 human embryo, 3 mm long, (14 somites) showing amniotic and yolk sacs. Note: amniotic sac (relatively tense), yolk sac (less tense), head fold region, region of heart tube ventral to the head fold, and the somites. The opaque walls of the left umbilical vein are seen clearly in the body wall just ventral to the somites. The connecting stalk passes to the right of the caudal part of the embryo across the chorionic sac and out of the picture! The whole structure is bathed by chorionic fluid (from Exalto et al., 1980).

Stage 11 embryo cut longitudinally (rostral to left, caudal right)

Since all embryos are twisted, the "longitudinal" section actually cuts obliquely through the brain and neural tube, showing where these structures open into the amniotic sac, i.e., the cranial and caudal neuropores respectively. Note: (i) The amnion has a double-layer of cells (seen clearly in parts of the section) and encloses the amniotic sac, which contains amniotic fluid. (ii) The amnion attaches to the embryo ventral to the heart. (iii) The wall of the yolk sac is less tense (i.e., more folded) than the amnion. (iv) The yolk sac wall also consisted of two layers of cells originally; the wall thickens in places where blood islands arise. Gastrointestinal tract - approximate regions of the midgut ("roof" region of the yolk sac) and hindgut (Note: The foregut appears to be separated from the midgut by a dense column of tissue, but this is a section through one of the umbilical veins crossing to the heart obliquely from the side of the embryo).

Stage 12

Carnegie Stage 12 human conceptus with embryo (3.4 mm, 27 days)

(a) Note relative sizes of amniotic and chorionic sacs (yolk sac not seen). Tertiary chorionic villi, highly-branched (from Blechschmidt, 1974). Note yolk sac stalk and body stalk are separate structures, i.e., the umbilical cord has not yet formed.

b) Embryo dissected free of amnionic sac and chorionic sac (Kyoto Collection)

(c) Embryo reconstruction showing umbilicus is wide and the walls of the umbilical coelom are stippled. The body stalk component of the future umbilical cord is the thick, cross-hatched region with three vessels (a vein & 2 arteries). Identify: 3rd pharyngeal arch (flexion fold), the intra-embryonic coelom (dashed line) with right-hand entrance (coelomic portal, black) from the chorionic sac, umbilical vein in body stalk, funnel-shaped transition zone between the yolk sac and the yolk sac stalk connecting to the gut. Allantois not indicated. Thick black line: amniotic ectoderm covering the future umbilical cord (reversed drawing from Blechschmidt, 2004)

Stage 6/7 (Nishimura)

Overview of blastocyst implantation in uterine wall during the third week of development.

Carnegie Stage 6-7, human conceptus approximately 16-18 days. (about the time of the first missed menstrual period).

Identify chorionic sac, yolk sac and amniotic sac, all containing different fluids. Identify secondary chorionic villi, maternal blood "spaces" (empty) and uterine tissue (maternal decidua). The embryo lies between the yolk sac fluid and the amniotic sac fluid. Note that the ends ("bases") of some long secondary chorionic villi are attached to the maternal (decidual) tissue by dense clusters of trophoblast cells (the cytotrophoblastic column) - these are anchoring secondary villi. Other shorter, secondary villi are "free-floating" in the maternal intervillous spaces, from which the blood has drained into the large endometrial veins. (Image: Nishimura etal., 1977)

Higher power view of conceptus. Identify the three fluid-filled sacs, the embryo, and the body (connecting) stalk indicating the caudal end of the embryo. Note the thin amnion and the thicker wall of the yolk sac and conversely, the thick ectoderm and thin entoderm. Vascular channels in connecting stalk. The broad elevation in the ectoderm is the expansion dome (primitive single brain bulge) and the cluster of cells at the caudal end of the embryonic disc is the primitive node region. Note the precipitated protein in the extra-embryonic coelom and in the yolk sac, and the start of the formation of the intra-embryonic coelom between ectoderm and entoderm, at the cranial (rostral) end of the expansion dome.

Laboratory: Placenta-fetal membranes

Examine the images of the umbilical cord and developing placenta in Phase 1 Virtual Slides (Female Reproductive System - Fetal Membranes & Placenta)

Scroll down the linked page to SCIENCE PRACTICAL: FETAL MEMBRANES & PLACENTA

Online Resources

UNSW Embryology Notes

Placenta (general notes on placental development)

Human Menstrual Cycle

Week 1 Development

Week 2 Development

UNSW Embryology

Next Practical

BGD Practical - Fetal Development covers later stages in development from the embryo to fetus.

Glossary

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Terms

• anastomosis connection between two tubes.
• 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

These notes and linked materials have been prepared for Educational purposes only.

Please email Dr Mark Hill if you wish to make a comment about this current project.

UNSW Embryology ISBN: 978 0 7334 2609 4

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