Answers of the long previous year question of Paper Developmental Biology of 2020, 2022, 2023.
Developmental Biology 2020
a) Mention the components of cortical granules and describe their role to prevent sperm entropy.
Ans→ Serine protease, the granules, glycosaminoglycan, udux-I, hyaline are the components of cortical granules.
Ca2+ ions fuse with cortical granules and the content of cortical granules release between the plasma membrane and vitelline membrane.
(i) Serine protease: Serine protease clip of the EBR-I receptors on vitelline membrane leading to absence of receptors for sperm binding. And also cut off the proteins that are attached between plasma membrane & vitelline membrane.
(ii) Granules: Cortical granules content fuse with vitelline membranes that form the fertilization envelope.
(iii) Glucoasamino-glycan: Glucosamino-glycan absorbs water cause swelling of the pre-vitelline space leads away the fertilization envelope from the plasma membrane.
(iv) udux I: udx-1 present in the former cortical granules membrane produces H2O2. The H₂O₂ acts on ovop and transglutaminase on the fertilization membrane. This helps in hardening of the fertilization membrane.
(v) Hyaline: Hyaline makes a coat surrounding the egg membrane to support the egg.
b) Write a short note on influence of yolk in cleavage.
Ans→ Mainly yolk provides nourishment to the developing embryo. But presence or absence of yolk and high or low amount of yolk can influence the cleavage pattern.
When a high amount of yolk is present, there is less amount of active cytoplasm left which results in the zygotic nucleus position displaced from the geometrical centre.
When cleavage takes place leads unequal cytokinesis and forms unequal sized blastomeres. High amounts of yolk can retard or inhibit cleavage. Blastomeres with high yolk divide it at a slower rate and remain larger.
The amount of yolk and distribution may determine the cleavage in two ways- 1) Holoblastic cleavage 2) Meroblastic cleavage.
1) Holoblastic cleavage: When yolk amount is low and uniformly distributed, cleavage takes places completely and forms separate blastomeres.
- a. Equal holoblastic cleavage: Complete cleavage that divides the whole egg uniformly seen in alecithal eggs and microlecithal egg.
- b. Unequal holoblastic cleavage: In telolecithal, mesolecithal eggs, the holoblastic cleavage forms unequal blastomeres of micromeres and macromeres.
Meroblastic Cleavage: Here the cleavage furrow is only the active cytoplasm and the yolk is undivided.
- a. Discoidal meroblastic cleavage: In a macrolecithal egg, where a high amount of yolk is at one pole, causes cleavage at the discoidal region of cytoplasm.
- b. Superficial cleavage: In a centrolecithal egg, where yolk is at centre, the cleavage takes place only in peripheral cytoplasm.
c) Briefly describe the hormonal regulation of the follicular phase. Describe the changes in mammalian eggs, just after fertilization.
Ans→ Follicular phase of menstrual cycle starts after the menstrual phase runs for ovulation lasts about 8-9 days. Several hormones influenced the maturation of ovarian follicles.
1. Follicle-Stimulating Hormone (FSH): FSH released by the anterior pituitary gland, FSH stimulates the growth and development of ovarian follicles in the ovary.
2. Oestrogen: Produced primarily by the developing ovarian follicles, oestrogen levels rise gradually during the follicular phase.
Oestrogen stimulates the thickening of the endometrium (the inner lining of the uterus), preparing it for potential implantation of a fertilized egg.
After fertilization, mammalian eggs undergo several rapid and crucial changes:
- Cortical Reaction: Prevents polyspermy by modifying the egg membrane and zona pellucida.
- Zona Reaction: Hardens the zona pellucida, further preventing entry of additional sperm.
- Completion of Meiosis II: The secondary oocyte completes its second meiotic division, forming a haploid ovum and a second polar body.
- Pronuclei Formation: Male and female pronuclei form and migrate towards each other.
- Pronuclei Fusion: The male and female pronuclei fuse to form a single diploid nucleus, the zygote.
- Cytoplasmic Rearrangement: Cytoplasmic contents are rearranged to prepare for the first mitotic division.
- Cleavage Begins: The zygote undergoes its first cleavage, forming a multicellular embryo: Morula→Blastula→Gastrula
a) Classify eggs depending on the amount of yolk present in them. What is vitellogenesis?
Ans→ On the amount of yolk, eggs are classified as 4 types: 1. Alecithal egg; 2. Microlecithal Egg; 3. Microlecithal egg & 4. Macrolecithal egg.
1. Alecithal egg: When yolk is absent in the egg then it is called alecithal egg.
Example: Humans and most mammals.
2. Microlecithal Egg: Egg with a very small amount or negligible amount of yolk present is called microlecithal egg.
Example: Sea urchin, star fish, amphioxus etc.
3. Mesolecithal Egg: Egg containing moderate amount of yolk is called mesolecithal egg.
Example: Amphibians
4. Macrolecithal Egg: Egg containing a large amount of yolk is called a macrolecithal egg.
Example: Reptiles, Birds.
Vitellogenesis: Vitellogenesis is the process of yolk formation and deposition in the developing oocytes (egg cells) of oviparous animals.
b) What is primitive streak? Describe its role in megalecithal egg’s gastrulation with proper diagram.
Ans→ Primitive streak is an early developmental structure in embryos where cells migrate and organize into the three germ layers.
In case of megalecithal eggs, such as those of birds and reptiles, the role of the primitive streak in gastrulation is slightly different due to the large amount of yolk present.
1. Formation of the Blastoderm: The blastoderm, a disc of cells forms on the top of the yolk mass. The primitive streak develops on the blastoderm.
2. Gastrulation Initiation: The primitive streak serves as the starting point for gastrulation. Cells from the blastoderm begin to migrate and move towards the primitive streak.
3. Ingression of Cells: Cells from the blastoderm ingress (move inward) through the primitive streak, reaching the underlying yolk mass.
4. Formation of Gering layers: As cells ingress through the primitive streak, they organize into the three germ layers- ectoderm, mesoderm and endoderm. This process. establishes the basic body plan of the embryo.
5. Differentiation and organ formation: Once the germ layers are established, cells differentiate further to form specific tissues and organs as development progresses.
c) Write a note on the prevention of polyspermy during fertilization.
Ans → Polyspermy prevention mechanisms begin immediately after the entry of first sperm into the zona pellucida layer of ovum. The process involves fast block and slow block to ensure that only one sperm successfully fertilizes the egg.
Fast Block Polyspermy:
1. Membrane Depolarization: Upon fusion of the sperm with the egg’s plasma membrane, there is a rapid influx of sodium cons (Na+) into the egg, causing a temporary depolarization of the egg membrane.
2. Prevention of Additional Sperm Binding: This depolarization makes the egg membrane less receptive to further sperm binding. It essentially acts as an immediate barrier to polyspermy, preventing additional sperm from fusing with the egg’s membrane.
The fast block is rapid but temporary, lasting only a few seconds to minutes. It provides immediate protection against polyspermy.
Slow Block Polyspermy (Cortical Reaction):
1. Cortical granule release: After the penetration of the sperm into the egg, the egg releases cortical granules from the egg’s cytoplasm to the previtetine space.
2. Modification of Zona Pellucida: The contents of the cortical granules, including – Serine protease, Glucosamino-glycan udux I, Hyaline etc. that modify zona pellucida into an impermeable layer.
3. Zona Block: The modification of the zona pellucida forms a physical barrier known as zona block which permanently prevents other sperm from binding to and penetrating the layer.
4. Enzymatic Degradation: The enzymes released during cortical reaction also degrade the sperm binding proteins on the zona pellucida.
Developmental Biology 2023
3.a) Classify placenta based on the degree of foetal and maternal tissues. State the role of syncytiotrophoblast during implantation.
Ans→ Placenta can be classified based on the degree of involvement of foetal and maternal tissues into 5 main types:
1. Epitheliochorial Placenta:
- There are multiple layers of tissue, between the maternal and foetal blood, including maternal endometrial epithelium and foetal chorionic epithelium.
- Example: Cows, pigs and horses
2. Endotheliochorial Placenta:
- Here the maternal blood comes in direct contact with the endothelial lining of the foetal chorionic capillaries, with no maternal epithelial layers.
- Example: Seen in carnivores like dogs, cats etc.
3. Hemochorial Placenta:
- In hemochorial placentation, the maternal blood directly bathes the foetal chorionic villi, with no intervening tissue layers.
- Example: Humans, primates, rodents. and some other mammals.
4. Syndesmochorial Placenta:
- Maternal blood comes into contact with the foetal trophoblast layer, which is partially eroded to allow for exchange.
- Examples: Some members of rodentia, guinea pig & nutria.
5. Haemodichorial Placenta:
- Maternal blood comes into direct contact with two layers of foetal tissue: the chorionic epithelium and the trophoblast layer.
- Example: Seen in some Primates like lemurs and bush babies.
3.b) Briefly describe the hormonal regulation of the follicular phase. Describe the changes in mammalian eggs, just after fertilization.
Ana→Follicular phase of menstrual cycle starts after the menstrual phase runs for ovulation lasts about 8-9 days. Several hormones influenced the maturation of ovarian follicles.
1. Follicle-Stimulating Hormone (FSH): FSH released by the anterior pituitary gland, FSH stimulates the growth and development of ovarian follicles in the ovary.
2. Oestrogen: Produced primarily by the developing ovarian follicles, oestrogen levels rise gradually during the follicular phase.
Oestrogen stimulates the thickening of the endometrium (the inner lining of the uterus), preparing it for potential implantation of a fertilized egg.
After fertilization, mammalian eggs undergo several rapid and crucial changes:
- Cortical Reaction: Prevents polyspermy by modifying the egg membrane and zona pellucida.
- Zona Reaction: Hardens the zona pellucida, further preventing entry of additional sperm.
- Completion of Meiosis II: The secondary oocyte completes its second meiotic division, forming a haploid ovum and a second polar body.
- Pronuclei Formation: Male and female pronuclei form and migrate towards each other.
- Pronuclei Fusion: The male and female pronuclei fuse to form a single diploid nucleus, the zygote.
- Cytoplasmic Rearrangement: Cytoplasmic contents are rearranged to prepare for the first mitotic division.
- Cleavage Begins: The zygote undergoes its first cleavage, forming a multicellular embryo: Morula→Blastula→Gastrula
3.c) Differentiate between meroblastic and holoblastic cleavage. Mention the importance of primitive streak.
Ans→
| Points | Meroblastic Cleavage | Holoblastic Cleavage |
| 1. Occurrence | Occurs in eggs with large amount of yolk. (Telolecithal or centrolecithal eggs) | Occurs in egg with little or no yolk (Isolecithal or microlecithal egg). |
| 2. Cleavage | Incomplete cleavage | Complete cleavage. |
| 3. Blastula Forms | Forms a disc-like structure called the blastodisc. | Forms a multicellular hollow ball shaped blastula. |
| 4. Distribution | Not uniform distribution of cytoplasmic material. | Uniform distribution of cytoplasmic material. |
| 5. Examples | Eggs of reptiles, birds and insects. | Egg of amphibians, mammals & sea urchins |
Two important roles of primitive streak –
1. Establishment of Body Axes: Through inductive with surrounding cells, the primitive streak helps establish the spatial orientation of the embryo in formations of body axes dorsal-ventral, anterior-posterior, left-right.
2. Initiation of Gastrulation: Primitive steak initiated the gastrulation process by which 3 germ layers formed.
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