Overview
This lecture covers key concepts in embryology, focusing on types of twins, placental structure and function, fertilization, early embryo development, implantation, and fetal membranes.
Types of Twins
- Dizygotic twins result from two fertilized oocytes, forming two embryos with separate or fused membranes.
- Monozygotic twins arise from a single fertilized oocyte, splitting at morula or blastocyst stages.
Placenta: Structure and Function
- The placenta is a feto-maternal organ with a fetal (chorion frondosum) and maternal (decidua basalis) part.
- Functions include protection, nutrition, respiration, excretion, and hormone production.
- Placenta types classified by extraembryonic tissue, interaction with endometrium, tissue layers, and invasion degree.
Types and Classifications of Placenta
- Choriovitelline placenta found in marsupials; chorioallantoic is definitive in higher mammals.
- Placenta surface types: diffuse (pigs, horses), cotyledonary (ruminants), zonary (carnivores), discoid (rodents, primates).
- Degree of invasion: adeciduate (noninvasive, e.g., horses, pigs) vs. deciduate (invasive, e.g., humans, dogs).
Fertilization Process
- Steps: capacitation, acrosome reaction, adhesion, penetration of zona pellucida, membrane fusion, pronuclei formation, karyogamy.
- Capacitation involves sperm membrane changes enabling oocyte penetration.
- Acrosome reaction releases enzymes, allowing sperm entry.
Early Embryo Development
- Cleavage is mitotic division from zygote to blastocyst, with no overall size increase.
- Morula is a compacted ball of cells; blastulation forms a fluid cavity (blastocyst).
- The inner cell mass becomes epiblast and hypoblast (embryonic disc).
Gastrulation and Germ Layers
- Gastrulation forms three germ layers: ectoderm, mesoderm, endoderm.
- Somites develop from mesoderm and determine embryo age.
Fetal Membranes and Fluids
- Four extraembryonic membranes: yolk sac, amnion, chorion, allantois.
- Amnion forms by folding, cavitation, or inversion depending on species.
- Umbilical cord contains arteries, veins, urachus, and yolk sac remnants.
Implantation and Abnormalities
- Implantation types: centric (superficial), interstitial, eccentric.
- Delayed implantation (diapause) occurs in some species.
- Ectopic pregnancy is abnormal implantation outside the uterus.
Key Terms & Definitions
- Dizygotic twins — twins from two separately fertilized oocytes.
- Monozygotic twins — twins from a single oocyte that splits.
- Placenta — organ connecting fetus and mother for nutrient and waste exchange.
- Chorion frondosum — fetal part of the placenta.
- Decidua basalis — maternal part of the placenta.
- Capacitation — sperm changes enabling fertilization.
- Acrosome reaction — release of sperm enzymes for oocyte penetration.
- Morula — early embryo, solid ball of cells.
- Blastocyst — embryo stage with fluid cavity and differentiated cell mass.
- Gastrulation — process forming three germ layers.
- Somites — segmented mesoderm structures forming body tissues.
- Umbilical cord — structure connecting fetus to placenta.
Action Items / Next Steps
- Review types and classifications of placenta.
- Study the stages of fertilization and early embryogenesis.
- Learn the processes of gastrulation and extraembryonic membrane formation.
- Memorize key terms and their definitions for exams.