ZOO: DSC-353 Reproductive and Developmental Biology
Unit 4: Late Embryonic Development
1. Fate of Germ Layers - Fate Map and Organiser Concept
Following gastrulation, the three primary germ layers differentiate into specific tissues and organs.
Fate Map
A Fate Map is a diagrammatic representation showing the future destination of various cells or areas of a blastula or early gastrula.
- Ectoderm: Gives rise to the nervous system, epidermis of the skin, and sense organs.
- Mesoderm: Develops into the circulatory system, muscles, bones, and excretory system.
- Endoderm: Forms the lining of the digestive tract, respiratory system, and associated glands (liver, pancreas).
Organiser Concept
The Organiser (specifically the Spemann-Mangold Organiser in amphibians) refers to a region of the embryo that can induce the development of the primary embryonic axis and coordinate the differentiation of surrounding tissues.
2. Extra-embryonic Membranes in Birds and Mammals
These are specialized membranes that develop outside the embryo proper to provide protection, nutrition, and waste management.
3. Implantation of Embryo in Humans
Implantation is the process by which the blastocyst attaches to and embeds within the endometrium of the mother's uterus.
- Timing: Occurs approximately 6–9 days after fertilization in humans.
- Process: The Trophoblast cells of the blastocyst secrete enzymes that digest the uterine lining, allowing the embryo to sink into the vascularized tissue.
- Hormonal Support: Progesterone is essential for maintaining the uterine lining for successful implantation.
4. Placenta: Structure, Types, and Functions
The placenta is a temporary organ formed by the association of maternal and fetal tissues for physiological exchange.
Structure and Functions
- Nutrition: Transfer of nutrients from mother to fetus.
- Respiration: Exchange of Oxygen and Carbon dioxide.
- Excretion: Removal of fetal metabolic wastes.
- Endocrine: Secretes hormones like hCG, Estrogen, and Progesterone to maintain pregnancy.
Classification of Placenta
- Based on Extra-embryonic membranes: Yolk sac placenta or Chorio-allantoic placenta.
- Based on Distribution of Villi: Diffuse, Cotyledonary, Intermediate, or Discoid.
- Based on Histology (Maternal-Fetal Barriers):
- Epitheliochorial: Least invasive (e.g., Pig).
- Endotheliochorial: Villi contact maternal blood vessel endothelium (e.g., Carnivores).
- Hemochorial: Villi are bathed directly in maternal blood (e.g., Humans).
Exam-Oriented Focus
Exam Tip: For classifications of the placenta, always remember that Humans have a Discoid, Hemochorial, and Deciduate placenta. These terms are frequently asked in objective and short-answer questions.
Frequently Asked Questions (FAQs)
- Q: What are the three germ layers?
A: Ectoderm, Mesoderm, and Endoderm.
- Q: Which extra-embryonic membrane protects against shock?
A: The Amnion.
- Q: What is a "Hemochorial" placenta?
A: A type of placenta where fetal villi are in direct contact with maternal blood.
Common Mistake: Do not confuse the Yolk Sac in birds and humans. In humans, the yolk sac is vestigial (does not contain yolk) but remains important for early blood cell formation.
Mnemonics
- E-M-E: Ectoderm (External - Skin), Mesoderm (Middle - Muscles/Blood), Endoderm (Entirely inside - Gut).