Unit 3: ZOO: DSC-353 Reproductive and Developmental Biology

1. Types of Eggs and Egg Membranes

Animal eggs vary significantly based on the amount and distribution of yolk, which determines the subsequent pattern of development.

Classification based on Yolk Quantity

• Alecithal: Eggs with negligible yolk (e.g., placental mammals).
• Microlecithal: Small amount of yolk (e.g., Amphioxus, sea urchin).
• Mesolecithal: Moderate amount of yolk (e.g., amphibians).
• Macrolecithal/Megalecithal: Large amount of yolk (e.g., birds, reptiles).

Classification based on Yolk Distribution

• Isolecithal: Yolk is uniformly distributed throughout the cytoplasm.
• Telolecithal: Yolk is concentrated at one pole (Vegetal pole).
• Centrolecithal: Yolk is concentrated in the center (common in insects).

Egg Membranes

Membranes protect the embryo and facilitate physiological exchange.

• Primary Membrane: Produced by the oocyte itself (e.g., Vitelline membrane, Zona pellucida in mammals).
• Secondary Membrane: Produced by the ovary follicle cells (e.g., Corona radiata).
• Tertiary Membrane: Produced by the oviduct or other maternal organs (e.g., Albumin, shell of a bird's egg).

2. Fertilization: External and Internal

Fertilization is the fusion of male and female gametes to form a zygote.

Process and Changes in Gametes

• External Fertilization: Occurs outside the body, usually in water (e.g., frogs, most fish).
• Internal Fertilization: Occurs inside the female reproductive tract (e.g., mammals, birds).
• Capacitation: Physiological maturation of sperm within the female tract required for fertilization.
• Acrosome Reaction: Release of enzymes to penetrate the egg membranes.

Blocks to Polyspermy

Mechanisms to ensure only one sperm fertilizes the egg.

1. Fast Block: Rapid change in the membrane potential of the egg (depolarization) immediately after sperm contact.
2. Slow Block (Cortical Reaction): Release of cortical granules that harden the vitelline membrane into a fertilization envelope.

3. Planes and Patterns of Cleavage and Blastulation

Cleavage is a series of rapid mitotic divisions where the zygote is converted into a multicellular blastula.

Planes of Cleavage

• Meridional: Plane passes through the animal-vegetal axis.
• Equatorial: Plane passes at right angles to the animal-vegetal axis, dividing it into two halves.

Patterns of Cleavage

Blastulation

The stage following cleavage where a fluid-filled cavity called the blastocoel is formed within the embryo (now called a blastula).

4. Early Development of Chick up to Gastrulation

Bird eggs are highly telolecithal, leading to a specific pattern of development.

• Cleavage: Meroblastic discoidal cleavage occurs only in the small disc of cytoplasm (blastodisc).
• Blastula Stage: Formation of the blastoderm, which separates into two layers: the epiblast (upper) and hypoblast (lower).
• Primitive Streak: A thickening of the epiblast that serves as the site for the beginning of gastrulation.
• Gastrulation: The process of cell migration where the three germ layers (ectoderm, mesoderm, and endoderm) are formed.