ZOO: DSC-353 Reproductive and Developmental Biology
Unit 4: Late Embryonic Development
Table of Contents
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.
| Membrane | Primary Function |
|---|---|
| Yolk Sac | Provides nutrients to the developing embryo (highly developed in birds). |
| Amnion | A fluid-filled sac that protects the embryo from mechanical shock and desiccation. |
| Allantois | Functions in respiration and the storage of nitrogenous waste. |
| Chorion | The outermost layer involved in gas exchange; in mammals, it contributes to placenta formation. |
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
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.
Mnemonics
- E-M-E: Ectoderm (External - Skin), Mesoderm (Middle - Muscles/Blood), Endoderm (Entirely inside - Gut).