Gametogenesis: The process of formation of gametes is Called gametogenesis.
It takes place in gonads. Male gonad is testis while female gonad is ovary. The male gamete is Sperm while female gamete is ovum or egg.Gametes are haploid. Thus , meiosis division takes place during their formation.
Types of gametogenesis: It is of two types; Spermatogenesis and oogenesis.
• spermatogenesis: The process of formation of male gametes or sperms is called Spermatogenesis. It takes place in testis and gets completed in 3 phases;
• Multiplication phase,
• Growth phase and
• Maturation phase.
• Multiplication Phase: The epithelial Cells lining the Seminiferous tubules of testis are germinal in nature and Called primordial cells or primary germ Cells. These primary germ Cells undergo repeated mitotic division to form a number of cells called spermatogonia.
• Growth Phase: Each Spermatogonium absorbs nutrients and become larger in size. It's size increases by 4 times during growth phase. The new Cell is called primary Spermatocyte.
• Maturation Phase: The Primary spermatocyte undergoes first maturation division which is meiotic division to form two secondary Spermatocyte. These secondary spermatocyte undergoes second maturation division to form four haploid Cells Called spermatids.
The Process of formation of spermatids from primordial Cells is Called spermatidogenesis.
These spermatids undergo some structural modifications to form sperm. This process is Called spermiogenesis.
Thus, Four sperms are formed from one spermatogonia.
spermatogenesis = spermatidogenesis + spermiogenesis
Importance of spermatogenesis :
• Number of chromosomes is reduced to half. It helps to maintain Parental chromosome number even after fusion of gametes.
• Four sperms are formed from one Spermatogonia.
Structure of sperm:
It is single haploid cell with structural modifications and Consists of acrosome, head, body and tail.
• Acrosome: It is apical Cap like structure and Jelly like. It is modified golgi body. It produces sperm lysin which penetrates egg membrane.
sperm lysin is Hyaluronidase ( In amphibians and dissolves hyaluronic acid) , Acrosin ( In mammals which is also known as Zona lysin which dissolves Zona pellucida) while zona radiata penetrating enzyme (in Reptiles)
It Contains Calcium ions and some amount of magnesium ions.
• Head: It is anterior part and consists of nucleus of it's size.
• Body: It is main part and consists of Centriole, mitochondria and Cytoplasm.
Two Centrioles; proximal Centriole and distal Centsiole are present.
The proximal Centriole lies near to head and it enters inside egg where helps in cleavage by forming Spindle fibres.As centriole is absent in egg.
The distal Centriole is Present at Posterior end of body and forms axial filament of tail.
About (8-10) mitochondria are Present and arranged spirally. They Provide energy to Sperm.
• Tail: It is Cytoplasmic extension of body with filaments arranged in 9+2 manner. It makes sperm migratory.
The Sperm remains Covered by plasma membrane.
• oogenesis The process of formation of egg or ovum or fade is called oogenesis.
It takes place in ovary and gets CompIeted in 3 Phases:
• Multiplication phase,
• Growth Phase and
• Maturation phase.
Multiplication Phase: The germinal epithelial cells of ovary are Called primordial Cells. These primordial Cells undergo the process of repeated mitotic division to form a number of oogonia.
Growth Phase: The oogonia absorbs nutrients and become larger in size about 10 times and are called primary oocyte.
• Multiplication Phase: The primary oocyte undergoes meiotic division , the maturation division. By first maturation division which is reductional and unequal division , two haploid Cells of different size are formed. The larger cell is called secondary oocyte while Smaller cell is called polar body ( First Polar body). Both undergoes second maturation division which is simple mitotic division. The secondary oocyte divides by unequal division to form larger Cell or ovum or egg and Smaller cell Called second Polar body.
The first polar body also divides to form two polar bodies. Thus, at the end of oogenesis one ovum and three polar bodies are formed. The polar bodies eventually degenerate or are non-functional.
• Importance of oogenesis:
⇒ Reduction in number of Chromosomes to half number helps to maintain Parental chromosome number even after fertilization.
⇒ only one functional ovum is formed and most of the nutrients with Cytoplasm is retained in it.
Structure of Egg: It is single haploid Cell and is spherical in Shape. It's diameter is about 2mm and is non- motile.
Egg remains Covered by vitelline membrane inner to it nucleus, cytoplasm, other cell organells and yolk are Present.
The anterior half is pigmented due to melanin and Contains cytoplasm, nucleus & All organells. This half is Called animal hemisphere. The posterior half Contains nutritive substance yolk and this helf is Called vegetative hemisphere.
The Centre of animal hemisphere is Called animal pole while Centre of vegetative hemisphere is Called vegetative pole.
The eggs are released Out in the form of secondary oocyte. Thus, each egg Contains a polar body beneath the vitelline membrane near to animal pole.
In water, animal pole remains upside while vegetative pole remains down side.
• Lecithalily: Amount of Yolk in egg is called lecitlality.
Types of eggs on the basis of lecithality.
• Alecithal Egg: Egg without yolk. Eg: Eutherian mammals like Man, Rabbit etc.
• MicrolecithaI egg: Egg with very less amount of Yolk. Eg. Echinodermates like sea urchin , starfish etc.
• Mesolecithal egg: Egg with moderate amount of Yolk. Eg. Frog's egg.
• Macrolecithal egg: Egg with large amount of Yolk. Eg: Arthropods, Reptiles, Birds etc.
Types of eggs on the basis of distribution of yolk:
• Isolecithal or Homolecithal Egg : Yolk is uniformely distributed. eg: Echinodermates and protochordates.
• Telolecithal egg: yolk is concentrated towards vegetative pole. Nucleus and major parts of cytoplasm are present towards animal Pole as in mesolecithal and macrolecithal egg.
• Centrolecithal egg: Yolk is present at Centre of ovum and is Surrounded by cytoplasm. eg: Arthropods.
• Discoid or meiolecithal egg: Yolk occupies almost Whole Part of egg except a small disc of active Cytoplasm. eg: Reptiles, Birds etc.
• Heterolecithal egg: Yolk is unevenly distributed.
Types of eggs on the basis of presence or absence of Shell:
• Cleidoic egg: Eggs with Shell as in Birds & Reptiles.
About 94% Shell is made up of Calcium Carbonate.
Eggs may remain covered by albumen, membrane or Shell.
• Non-cleidoic egg: Eggs without shell. eg: Frog's egg.
⇒ In Frog; mesolecithal, telolecithal and non-cleidoic egg is present.