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:
(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.
(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
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