Early embryonic development begins with fertilization, where a sperm cell merges with an egg to form a zygote. The zygote undergoes rapid cell divisions called cleavage, forming a multicellular structure called a blastocyst. The blastocyst implants itself into the uterine wall, where it differentiates into an inner cell mass (which will become the embryo) and an outer layer called the trophoblast (which will form part of the placenta).
Gametogenesis :
Definition: Gametogenesis is a process of formation of haploid male and female gamete for sexual reproduction.
Gametes are haploid reproductive cells which carries haploid set of chromosomes formed from diploid gonad.
Phases of Gametogenesis:
1.) Multiplication phase 2.) Growth phase 3.) Maturation phase.
Multiplication phase: In this phase, the primordial germ cells are multiply in number of cells through mitotic cell division.
Growth phase: In this phase, the germ cells Increase their size and volume.
Maturation phase: In this phase, the grown germ cells formed haploid gamete cells by meiotic cell division.
Significance–
- Production of gamete: Male and female gamete are produced by this process.
- Sexual Reproduction: Formation of gametes which is crucial for sexual reproduction.
Types: Gametogenesis are of two types-
- Spermatogenesis: Formation of male gamete- Sperm
- Oogenesis: Formation of female gamete- Ovum.
Spermatogenesis:
Definition: Spermatogenesis is a process of formation of mature sperm cells from spermatogonium germ cells by multiple round of division in male.
Location: Spermatogonium cells of seminiferous tubules of testis.
Phases: Spermatogenesis process divided into two phases.
- 1) Spermalocytogenesis
- 2) Spermiogenesis
1) Spermatocytogenesis: Spermatocytogenesis refers to the process of spermatogonia developing into primary spermatocyte in the seminiferous tubules of the testis.
Phases: Have 3 major phases:
(i) Multiplicative phase (1) Growth phase (ⅲ) Maturation phase.
(i) Multiplicative phase: In this phase, spermatogonium cells are multiply in and form A Type A spermatogonia (A1, A2, A3) and then Type B spermatogonia.
(ⅱ) Growth phase: Type B spermatogenia undergo mitosis division and formed primary spermatocytes.
(iii) Maturation phase: In maturation phase, the primary spermatocytes are undergo first meritic division formed secondary spermatocyte. Then, the secondary spermatocyte undergo second meiotic division and produce haploid cells called as spermatids.
2) Spermiogenesis or Spermateleosis: Spermiogenesis is a process where the spermatids transform or modify into mature elongated spermatozoa or sperm cells.
Oogenesis :
Definition: The process of formation of a ovum from ovary is called oogenesis.
- This process start from embryonic stage of a girl embryo.
- The steps in oogenesis up to the production of primary oocytes occur before birth.
- Primary oocyte get arrest at meiosis I of Prophase I of Diplotene substages.
Phases: 3 phases.
1. Multiplication phase:
- The germinal cells of ovary having diploid number of chromosomes.
- The cell undergo mitotic cell division.
- Resultant daughter cells called oogonia.
2. Growth cell.
- The growth of oogonia occur In this phase cell does not undergo cell division.
- Growth associated with both nuclear and cytoplasm
3. Maturation phase:
- Starts at adulthood (puberty period)
- Primary oocyte —Meiosis I→ Secondary oocyte –meiosis II→ Ootid
Types of Egg :
- Egg has one animal pole, and one vegetal pole.
- Generally on the vegetal pol to side more yolks present.
Types of Egg on amount of Yolk :
1. Alecithal egg: Yolk is absent in the egg.
Example: Human and most of mammals.
2. Microlecithal Egg: Egg contains less amount or negligible amount of yolk is called microlecithal egg.
Example: Sea urchin, star fish, amphioxus etc.
3. Mesolecithal Egg: Egg contains moderate amount of yolk is called mesolecithal egg.
Example: Amphibians
4. Macrolecithal Egg: Egg contains large/ high amount of york is called macrolecithal egg.
Example: Reptiles, Birds.
Types of Eggs an distribution of Yolk :
1.Isolecithal Egg:
- When yolk is distributed equally in the cytoplasm of the egg is called isolecithal egg.
- Example: Amphioxus, Echinodermata.
2. Telolecithal egg:
- When yolk is present only in one end of ooplasm is called telolecithal egg.
- Example: Amphibians, Reptiles, Birds.
3. Centrolecithal egg:
- When yolk is present at the centre is called centrolecithal egg.
- Example: Insects.
Types of egg on the basis of shell :
1. Cleidoic egg:
- When the egg contains a thick and hard outermost shell is called cleidoic egg.
- Cleiodic egg are terrestrial adaptions it weep protect from desiccation.
- Example: Birds, Reptiles, Prototheria
2. Non-Cleidoic egg:
- Egg is not covered by a hard shells only a membrane is present, called e non-cleidoic egg.
- It is aquatic adaption
- Example – All mammals, fish, amphibians.
Egg Membrane :
- The covering which is found outside the plasma membrane of egg is called egg membrane.
- Main functions of membrane is to protect polyspermy.
- Give protection, help to maintain desiccation.
- Egg membrane made up of – protein, carbohydrate, lipid.
Based on the origin, Egg membrane are 3 types :
1) Primary Egg Membrane :
- The membrane which is found around the plasma membrane.
- Formed by ovum itself.
→ Zona pellucida
2) Secondary Egg Membrane:
- The membrane around the primary membrane.
- Secreted by follicular & cells ovary.
–> Chorion (found in insect egg)
–> Corona radiata (mammals)
–> Granulosa & theca membrane (human)
3) Tertiary Membrane:
- Secreted by oviduct or uterus.
→ Jelly coat found in amphibian egg.
→ Albumin layer, shell membrane, shell found in birds ey egg
Fertilization :
- Syngamy – Fusion of male and formale gamete
- Plasmogamy – Fusion of cytoplasm of two gametes
- Karyogamy – fusion of male pro-nuclei & female pronuclei
- Amphimixis – fusion of chromosome of two nuclei
Types of fertilization:
On the basis of occurence. fertilization of 2 types-
1.) External Fertilization:
- When fertilization occur out of the body it is called external fertilization.
- Normally takes place in water.
- Example: Amphia, fish, sea urchin.
2.) Internal fentilization:
- When fertilization occurs inside the body of organisms is called internal fertilization.
- Seen in case of terrestrial organisms.
- Example: Reptiles, Birds, Mammals
In case of human, the site of fertilization is ampulla region of fallopian tube.