MFAC 1503 Session 2 2004 Practical 8: Fetal Membranes and Placenta The laboratory class is coordinated by Dr Brian Freeman. It includes both wet specimens, images of the conceptus and histological sections of embryo, fetal membranes and placenta. The notes are on pp. 22-26 of your Practical Manual (on WebCT). A two-page handout with some simple line diagrams of the developing conceptus and embryo will be distributed in class.
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).
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)
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.
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
Laboratory 11 Covers later stages of development in the embryo/fetus.
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.