VSC262 Embryogenesis and Placentation

• Embryogenesis and Placentation are the topics for the lecture VSC262 202530.

Learning Objectives

• Discuss embryo growth from conception to parturition.
• Briefly describe why and how Maternal Recognition of Pregnancy (MRP) is necessary.
• Discuss placental formation, classification, and function.
• Discuss placentation similarities and differences in domestic species.

Terminology

• Syngamy is the fusion of male and female pronuclei.
• An embryo is an organism in early development, without anatomical form or organogenesis, and unrecognizable as a species.
• Conceptus is the product of conception, including the embryo during early development, extra-embryonic membranes, fetus, and placenta.

Placenta Formation

• The placenta provides attachment between fetal and maternal tissues.
• Placenta functions include exchanging nutrients and wastes between mother and fetus.
• The placenta aids in hormone production.
• The placenta may act as a "shock absorber" to protect the developing conceptus.

Steps Before Attachment

• Development within the zona pellucida (cleavage).
• Hatching of the embryo.
• Maternal recognition of pregnancy.
• Formation of extra-embryonic membranes.

Pre-Attachment Embryo Development

• Ootid is the oocyte after fertilization but before nuclear fusion.
• Male and Female Pronuclei are the genetic components of the ootid.
• The first and second polar bodies are expelled during meiosis.
• Syngamy is the fusion of pronuclei.
• A zygote undergoes cleavage, resulting in daughter cells also known as "Blastomeres".
• The embryo changes from 4 cells, to 8 cells to a morula.
• Morula then becomes a Blastocyst.
• Blastocyst consists of the Inner Cell Mass (ICM), Blastocoele and Trophoblast.

Hatching of the Blastocyst

• Hatching is governed by growth and fluid accumulation in the blastocyst.
• Enzymes produced by trophoblastic cells weaken the zona pellucida.
• Contraction of trophoblastic cells causes intermittent pressure, rupturing the zona pellucida.

Post-Hatching

• The embryo is free-floating and subject to the uterine environment.
• Survival requires adequate luteal function.
• Survival requires adequate progesterone synthesis.
• The uterus needs to be progesterone responsive.
• Uterine "milk" histotroph is a nutrient-rich secretion.

Extra-Embryonic Membranes

• The pre-attachment period is long, and rapid embryonic growth is due to the formation of extra-embryonic membranes.
• Formation of extra-embryonic membranes is essential for embryo attachment to the uterus.
• Membranes originate from the mesoderm, endoderm, and trophoblast.

Amnion

• The yolk sac regresses in size as the conceptus develops; it is transient.
• Primitive germ cells that migrate to the genital ridge are attributed to the yolk sac.
• The amnion encloses the fetus in a fluid-filled cavity for hydraulic protection and also provides an anti-adhesion material.

Allantois and Allantochorion

• The allantois is a fluid-filled sac that collects liquid waste from the embryo.
• Allantois eventually fuses with the chorion to make the allanto-chorion.
• The allantois contains vessels that connect the fetus with placental circulation.
• The Allantochorion is the fetal contribution to the placenta that provides a surface for attachment (chorionic villi).

Amnionic and Allantoic Fluids

• Amnionic fluid is secreted from the respiratory tract and mouth and amnion itself.
• Amnionic Fluid protects the fetus from external shock and prevents adhesions.
• Amnionic Fluid assists in dilating and lubricating birth passages.
• Allantoic fluid is fetal urine and a secretory activity of the allantoic membrane.
• Apposition of allantochorion with the endometrium helps maintain osmotic pressure of fetal plasma.
• The Allantoic Fluid stores foetal wastes that are not transferable back to the mother.

Maternal Recognition of Pregnancy (MRP)

• MRP prevents oestrus and overt signs of cyclicity, whilst survival also requires adequate luteal function.
• MRP Survival requires adequate progesterone synthesis and cervix closure.
• Survival requires a progesterone-responsive uterus with a quiescent myometrium.
• Uterine "milk" histotroph promotes survival
• Anti-inflammatory helps survival.

Attachment vs. Implantation

• Implantation occurs minimally
• Attachment occurs mainly

Placental Development

• Classification is based on the type of attachment to the mother.
• Classification is based on the number of histological layers.
• Classification is based on the distribution of chorionic villi.

Classification by Type of Attachment

• Non-deciduate (adeciduate) placenta: the placenta is attached, the most common type in domestic species.
• Deciduate placenta: the placenta is implanted; found in primates, rodents, and lagomorphs.

Histological Characteristics of Placentas

• Epitheliochorial placenta
• Haemochorial placenta
• Endotheliochorial placenta

Terminology of Placental Layers

• The prefix indicates the maternal side.
• The suffix indicates the fetal side.
• Epitheliochorial includes maternal epithelio and fetal chorial layers.
• All maternal and foetal tissue layers are preserved.

Species and Placental Types

• Mare, sow, and ruminants have epitheliochorial placentas.
• Dogs and cats have endotheliochorial placentas.
• Humans, primates, and rodents have hemochorial placentas.

Placental Membrane Apposition Types of Placentas

• Folded placenta.
• Villous placenta.
• Labyrinthine placenta.

Villous Placenta

• Fetal tissues form villi that may be branched.
• Villi interdigitate with similar maternal villi.
• This increases the surface area for attachment and transfer of nutrients and wastes.
• This is the most common type found in domestic species including the horse.

Distribution of Chorionic Villi

• Diffuse: Mare and Sow.
• Zonary: Dogs and Cats.
• Discoid: Rodents and Primates.
• Cotyledonary: Ruminants.

Cotyledonary

• Fetal side: cotyledon.
• Maternal side: caruncle.
• Placentome = Cotyledon + Caruncle.
• Convex is relating to cow/giraffe, Concave is relating to sheep/goat

Placental and Embryonic Systems and Processes

• Attachment of the outer-most fetal membrane (chorion or allantochorion) to the endometrium, known as (PLACENTATION).
• Brings the maternal & fetal blood streams as close together as possible with blood vessels in the chorioallantois.
• The maternal and fetal circulatory systems remain completely separate.
• There is NO DIRECT CONTACT between maternal & fetal blood.
• Exchange of nutrients and wastes occur between mother & fetus.

Placental Functions

• Stimulate ovarian function.
• Maintain pregnancy.
• Influence fetal growth.
• Stimulate mammary function
• Assist in parturition.
• Fetoplacental hormone production.

Endocrine Organs and Function

• Relaxin and placental lactogen are produced.
• Binucleate Giant Cells are present.
• Seen in Ruminants: Bovine - d 18-20 gestation, Ovine - d 14 gestation
• Pregnancy Specific: Protein B is produced.
• Endometrial cups, seen in horses and produce eCG, occur - d 35 gestation.

Examples of Luteo-Placental Shift

• Species where CL is responsible for full gestation progesterone maintenance: Goat, Camelids, Canid, Pig-sow.
• Species that have a luteoplacental shift to maintain gestation progesterone/progestogen: Mare (About 100 days), Cow (About 120-150 days), Ewe (About 50 days).

Hormones and Activity

• Progesterone is important in pregnancy
• Progesterone stimulates secretions from the endometrial glands.
• Progesterone is responsible for the progesterone block, which inhibits myometrial contractions.
• Progesterone is produced by the CL and the placenta.
• CL is not needed for the entire pregnancy in some species.
• Progesterone produces slight anti-inflammatory effects.
• Oestrogens are also important.
• Other placental hormones are Placental lactogen (Cows, sheep, humans, rats, and mice) and Relaxin (Humans, mares, cats, dogs, pigs, rabbits, and Monkeys).
• Placental lactogen aSimilar action to Growth Hormone which aids Mammogenesis and promote Fetal growth.
• Relaxin softens ligaments in the pelvic region and helps with dilation of the cervix during parturition.

Questions and Answers

• True or False? The aminonic fluid is made up of secretions originating from the fetal respiratory system? Ans: True
• True or False: The yolk sac develops from the primitive endoderm? Ans: True
• What type of placentation is present in the sow? Ans: Epitheliochorial and diffuse
• In the cotyledonary placentation what makes up the maternal contribution to the placentome? Ans: Caruncle
• What makes up the fetal contribution in the cotyledonary placentation? Ans: Cotyledon
• Equine chorionic gonadotrophin is produced by what structure in the equine placenta? Ans: Endometrial cups