LEARNING OUTCOMES 1. describe the formation of the tubular embryo by creation of body folds 2. note the juxtaposition of ectoderm and endoderm at the oral plate and cloacal membrane 3. describe the formation and fusion of the amnion to create a protective bubble around the embryo 4. be aware of the vestigial nature of the yolk sac in mammals with regard to nutrition but its importance in terms of haematopoiesis and its transient significance in contributing to the choriovitelline placenta 5. show the development of the allantois as a bud of the gut tube and its importance in the chorio-allantoic placenta 6. understand the different histological forms that the foetal/maternal placenta interface can take 7. understand the different anatomical forms that the foetal/maternal placenta interface can take 8. Note the emphasis on glucose and amino acids as energy and growth resources in the foetus and explain how the placenta has an endocrine function in ensuring these resources are directed to the foetus 9. Explain the special foetal adaptations that ensure adequate perfusion of foetal tissues with oxygen MEMBRANOGENESIS AND PLACENTAL FUNCTION

Ectoderm Neural tube Mesoderm Endoderm The flat embryo begins to fold downwards at the sides and at the front and back to enclose a primitive gut Ectoderm Neural tube Mesoderm Endoderm Oral plate Regions of brain Hindgut Cloacal plate Longitudinal view

The folding process not only creates the body form but also the extra- embryonic structures of the yolk sac, allantois, amnion and chorion Longitudinal view

Chorio-allantoic placenta Yolk sac Allantois Amnion Chorion Longitudinal view The chorio-vitelline placenta is temporary (or absent) and is replaced by the chorio-allantoic placenta

Folds also undercut the sides of the body Mesoderm Neural tube Ectoderm Endoderm Gut tube Yolk sac Somatopleure Splanchnopleure Transverse view

Video of chick embryo (50h) showing body folds and amnion In the chick, the formation of the head fold precedes that of the tail and the formation of the body sides progresses caudally

Histological classification of placentas is based on the degree of removal of the maternal layers EPITHELIOCHORIAL Maternal endometrial epithelium intact (horse,pig) SYNEPITHELIOCHORIAL Syncytium of maternal epithelium and Chorion (ruminants) ENDOTHELIOCHORIAL Removal of endometrial epithelium And connective tissue (dogs, cats) HAEMOCHORIAL Removal of maternal endothelium (human, some rodents)

Gross anatomical classification of placentas is based on the pattern of contact between chorion and endometrium DIFFUSE Uniform distribution of chorionic villi over contact surface (horse, pigs) COTYLEDONARY Villi restricted to defined area (cotyledons) (ruminants)

ZONARY Girdle of chorionic villi around middle of chorionic sac (dogs,cats) DISCOIDAL Disc-shaped area on chorionic sac (humans, rodents)

The haemochorial placenta Umbilical vein Umbilical arteries Maternal blood pool Maternal venule Maternal arteriole Chorionic villi A A B from Johnson, Essential Reproduction Foetal capillaries B Notice the expansions at the’turnaround’ to allow slower blood flow and better equilibration with maternal blood The haemochorial placenta shows the intimate juxtaposition of foetal and maternal blood allowing efficient exchange

Glucose is the dominant energy yielding substrate for the foetus with little use of fatty acids

To some extent the foetal-placental unit programmes maternal metabolism to ensure that it meets the needs of the foetus

Although fatty acids are little used by the foetus for energy they are essential for growth and also for laying down fat reserves 1. Fatty acids transported via maternal (or foetal) serum albumin 2. (a) Triacylglcyerols contain mostly palmitate (b) Palmitate will also be formed from excess glucose (c) Epitheliochorial placentas have poor rates of diffusion of fatty acids and neonates (eg calf and piglet) have little body fat compared to the haemochorial model (human) 3. TAG deposits in both white and brown adipose tissue. Brown fat essential for thermogenesis in neonate 4. Crucial here are the essential fatty acids 18:3 (  9,12,15) 18:2 (  9,12) 20:4 (  5,8,11,14)

As with glucose, the foetal-placental unit programmes mammalian metabolism to ensure that it meets the Nitrogen needs of the foetus

Several foetal adaptations contribute to the ability of the foetus to deliver sufficient oxygen to its tissues

The foetal cardiovascular system is adapted to providing well-oxygenated blood to the brain in spite of intermixing of venous return and an incompletely divided heart OXYGEN SUPPLY - FOETAL ADAPTATIONS 2 1. Numbers are partial pressures of oxygen in mm Hg 2. Low vascular resistance in placenta takes 45% of cardiac output 3. Blood returning to right atrium is a mixture of oxygenated umbilical blood and venous return from trunk and limbs 4. Crista dividens directs this better oxygenated blood through foramen ovale for preferential delivery to brain via left ventricle 5. The poorly oxygenated blood from the brain is directed to the right ventricle and then via ductus arteriosis to the dorsal aorta

The haemogobin of foetal red blood cells has a higher affinity for oxygen than that in maternal blood

REFERENCES Cunningham JGC (2002) Textbook of Veterinary Physiology (Saunders) Guyton and Hall (2005) Textbook of Medical Physiology (Elsevier) Johnson MH (2007) Essential Reproduction (Blackwells)