GAMETOGENESIS

Spermatogenesis

· Spermatogenesis is the process of formation of haploid, microscopic and functional male gametes, spermatozoa from spermatogonia present in testes.

· It occurs in the seminiferous tubules of the testis and consists of the following phases.
(i) Multiplication phase:

· It includes rapid mitosis of primordial germ cells present in the germinal epithelium of seminiferous tubules.

· As a result, a large number of diploid and round sperm mother cells, called spermatogonia, are formed.

(ii) Growth phase:

· Each spermatogonium increases in size (about twice) due to the accumulation of nutritive materials in the cytoplasm and the replication of DNA and forming a diploid primary spermatocyte.

· This phase is of much shorter duration than in oogenesis.

(iii) Maturation phase:

· Each primary spermatocyte by meiosis, I and II, produces four spermatids.

· Spermatids separate out in spermiogenesis.

iv) Spermiogenesis:

· It is the transformation of a non-motile and non-functional spermatid into a motile and functional spermatozoan.

· In its development, the flagellated tail of the sperm projects into lumen while the head is embedded in the Sertoli cell.

· Mature spermatozoan or sperm gets finally detached and is released in the lumen of seminiferous tubules.

Oogenesis

· Oogenesis is the formation of haploid female gametes (ova) from oogonia present in the ovary.

· Oogenesis consists of following phases:
(i) Multiplication phase
· In this phase, primary germ cells of the germinal epithelium of the ovary undergo rapid mitosis to form groups of diploid egg mother cells, called oogonia.

(ii) Growth phase
· One oogonium from the group of oogonia is transformed into a diploid primary oocyte.

· Others from the nutritive follicular epithelium around it.

· This structure is called a primary follicle.

(iii) Maturation phase
· By first meiosis, the primary oocyte divides into two haploid cells.

· One large cell receives most of the cytoplasm and the other smaller one is the first polar body.

· By second meiotic division, secondary oocyte divides to form a large ovum and a small polar body.

· The first polar body also divides into two small polar bodies.

· Due to this, a primary oocyte produces one ovum and two or three polar bodies.

· Polar body disintegrates and only one functional gamete (ovum) remains.

· In the human females, meiosis second is completed only at the time of sperm entry.

VVI Points to Remember for Entrance Exams

· The two centrioles of the spermatids become arranged one after the other behind the nucleus. The anterior one is known as the proximal centriole.
· The proximal centriole is usually located on the neck of spermatozoa. During fertilization, it is introduced to the egg.
· The proximal centriole is required for the first cleavage.
· The posterior centriole is known as the distal centriole. It changes into basal bodies.
· Distal centriole gives rise to the axial filament of the sperm
· Mitochondria from different parts of spermatid arranged in the middle piece around the axial filament.
· Mitochondria in the middle piece provide energy and strength to the sperm for locomotion.
· Golgi complex of spermatid give rise to acrosome.
· Acrosome forms a cap in front of a nucleus containing a lytic agent which dissolves the egg membrane during fertilization.
· Acrosome of mammalian sperm produces sperm lysins called ‘hyaluronidase’.
· If acrosome is removed from sperm, it will fail to penetrate into the ovum.
· The spermatozoa are released into the lumen of the seminiferous tubule by a process called Spermiation.
· During Spermiation, most of the cytoplasm of the spermatozoa is ejected as the residual body and remains embedded in the cytoplasm of the Sertoli cell.
· Primary oocytes undergo growth; the growth phase during oogenesis is comparatively longer. The size of oocytes increases very much.
· During the maturation phase, the primary oocyte (2n) under­goes meiosis I producing two haploid cells (n), the larger one is the secondary oocyte and the smaller one is the first polar body.
· Meiosis II of secondary oocyte results in the formation of functional egg or ovum and a second polar body.
· In oogenesis, only one of the four products of meiosis becomes a functional egg. The rest three are nonfunctional polar bodies that finally degenerate.
· Vitellogenesis is the formation and deposition of yolk in the egg.
· Major chemical components of the yolk are phospholipids and proteins.
· During the phase of maturation, the primary spermatocyte undergoes meiosis I giving rise to two haploids (n) secondary spermatocytes. The secondary spermatocytes undergo meiosis II resulting in the formation of four spermatids.
· Metamorphosis of spermatid to sperms is termed spermiogenesis.
· A spermatid is nonmotile and heavy. It has organelles like mitochondria, Golgi bodies, centrioles, nuclei, etc.
· During spermiogenesis, the weight of gamete is reduced along with the development of locomotory structures.
· Nucleus becomes compact forming the major part of the head of spermatozoa.
· A typical mammalian sperm are flagellated, consisting of four parts namely head, neck, middle piece and tail.
· The human sperm was first seen by Hamm and Leeuwenhoek.
· Tailless, non-flagellate ‘amoeboid’ sperm is found in the roundworm Ascaris.
· Oogenesis is the process of maturation of reproductive cells in the ovary.
· Oogenesis is basically similar to spermatogenesis. It includes a phase of multiplication, phase of growth and phase of maturation
· During the phase of multiplication, the primordial cells in the ovary divide mitotically to form oogonia (egg mother cells).
· Each oogonium divides mitotically to form two primary oocytes

Also, Read our Other Notes Important for Entrance Exams:

• Gametogenesis
• Developmental Biology
• Embryonic Development of Frog