Gametogenesis & Spermatogenesis | Zoology Optional Notes for UPSC PDF Download

Introduction

• Gametogenesis is a biological process in which diploid germ cells undergo the process of proliferation and differentiation to produce haploid mature gametes.
  Two types of gametes produced during gametogenesis, male gametes (spermatozoa) and female gametes (ova).
  The process by which male gametes are formed in the testis is termed as spermatogenesis and formation of female gametes in the ovary termed as oogenesis.

Spermatogenesis

• The male gonad known as testis is the site of spermatogenesis.
• Testis is made up of numerous minute elongated and coiled tubules called seminiferous tubules.
• The inner lining of seminiferous tubules is called as germinal epithelium.
• Germinal epithelium of seminiferous tubules consists of proliferative primordial germ cells (primary germ cells) and somatic cells called sertoli cells (nurse cells).

Formation of Spermatids

Formation of spermatids is subdivided in three phases:

• Multiplication phase
• Growth phase
• Maturation phase

Multiplication phase

• This phase is known as proliferation and renewal of sprmatogonia.
• In this phase diploid primordial germ cells (gonocytes) undergo repeated mitotic division and form large number of sperm mother cells called spermatogonia.
• These spermatogonia are the diploid cells found lying next to the basal membrane of the seminiferous tubules.
• These spermatogonia are referred as two types: Type A spermatogonia and type В spermatogonia.
• Type A spermatogonia serve as the stem cells which divide to form additional spermatogonia, whereas type В spermatogonia are the precursors of sperms.

Growth Phase

• During this phase, spermatocytes prepare itself to enter meiosis or reduction division.
• Spermatogonium increase in size by accumulating nutritive materials.
• Sertoli cells serve as a nurse cell and provide nourishment to spermatagonium.
• The enlarged spermatogonia are called primary spermatocytes.

Maturation phase

• Each primary spermatocyte undergoes two successive divisions, called maturation divisions.
• The first maturation division (meiosis-I) is reduction division or meiotic division reducing the chromosome number from diploid to haploid.
• The primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes.
• Secondary spermatocytes undergo the second maturation division (meiosis-II) and form four haploid spermatids, by each primary spermatocyte. These are non functional male gametes.
• To become functional, these spermatids undergo a complex process of transformation to spermatozoa referred as spermiogenesis.

Spermiogenesis

• The non functional spermatids leading to formation of active motile functional spermatozoa through a complex process of cytological and chemical transformation is referred as spermiogenesis.
• All the superfluous materials of the developing spermatozoa are discarded and a high degree of specialization occurs in the sperm cell so as to provide high mobility to male gamete.
• During the process of spermiogenesis, two major parts of sperm are formed i.e. the head and the tail. The process of spermiogenesis is divisible in number of steps:

Formation of head of spermatozoon:

The nucleus and acrosome undergoes following changes to form a sperm head:

Changes in nucleus:

• Nucleus of sperm shrink by losing much of fluid and chromosomes becomes closely packed into a small volume.
• The total ribonucleic acid, nucleolus and most of its proteins are eliminated, leaving only genetic material the deoxyribonucleoprotein.
• The nucleus moves from center to periphery of cell and becomes elongated. In different animals it assumes different shapes which determine perspective shape.

Acrosome formation

• The acrosome of a spermatozoon is derived from the golgi apparatus of a spermatid.
• The golgi apparatus secrets glycoprotein rich granules these granules referred as proacrosomal granules, each granule is enclosed within a membrane limited vesicle of golgi complex.
• These proacrosomal granules fuse to form a single large acrosomal granule. Other vesicle from the golgi apparatus contribute to the growth of structure.
• The enlarged acrosomal granule spreads over the surface of nucleus forming a cap which covers the anterior half or two thirds of the nucleus covering with double membranous sheath.

Formation of the tail of spermatozoon

• The centrosome of a spermatid after the second meiotic division consists of two cylindrical bodies known as centrioles, lying at right angle to each other.
• These two centrioles migrate from the periphery of the cytoplasm inward toward the nucleus.
• One of them, the proximal centriole become localized at the posterior pole of the nucleus opposite the acrosome the other the distal centriole give rise to the axial filament for the flagellum of spermatozoon.
• The mitochondria distributed at the periphery of the cytoplasm, migrate toward the portion of the flagellum inside the cytoplasm and surround it. Further develop into the mitochondrial sheath of the sperm middle piece.

Oogenesis

Oogenesis is the process of formation of haploid female gamete oocytes from the diploid egg mother cells of ovary. Process of oogenesis is subdivided in three phases:

Multiplication Phase

• The germinal cells of ovary in female called primordial germ cells.
• These diploid primordial cells are multiplied by mitosis to form the egg mother cells (oogonia). This phase occurs within the ovary, usually during embryonic, larval or foetal life.
• Before birth most of these germ cells degenerate through a process known as atresia. A few germ cells grow and develop larger cells called primary oocytes.
• The growth of oocytes arrested during the diplotene of meiotic I prophase.
• The duration of meiotic arrest differs among species to species.
• During meiotic prophase, the oocytes grow and acquire developmental information.

Growth phase

• The growth and differentiation of primary oocyte involves accumulation of nutrients and other reserves required during embryonic development.
• The oocyte grows to enormous proportion normally to attain the largest cell of the animal body.
• The growth period of oocyte increases nuclear substances as well as cytoplasmic reserve including yolk.

Growth of nuclear substances

• Due to production of large number of nuclear sap, nucleus of growing oocyte increases in size.
• The nucleus becomes large due to increase amount of the nucleoplasm and is called germinal vesicle.
• The nucleolus becomes large or its number is multiplied due to excessive synthesis of ribosomal RNA by ribosomal DNA of nucleolar organizer region of chromosome.
• Formation of lampbrush chromosomes in fishes, amphibians, reptiles, birds, insects, etc. for rapid yolk synthesis.
• That is directly related with increased transcription of mRNA molecules and active protein synthesis in the cytoplasm.

Growth of cytoplasmic substances

• The amount of cytoplasm of oocyte increases quantitatively and qualitatively.
• The mitochondria increases in number, in amphibian and birds the mitochondria become aggregated in the form of large mitochondrial clouds.
• Endoplasmic reticulum with ribosomes become more complicated, the golgi bodies manufacture cortical granule besides performing their normal function.
• These organelles are necessary for oocyte metabolism.

Vitellogenesis

• The process of formation and deposition of yolk is called vitellogenesis. Yolk is the major nutritional reservoir of the oocyte.
• Yolk is made up of protein, phospholipids and neutral fats. It may be Protein yolk or Fatty yolk and are of 2 types:
• Granular yolk: In invertebrates and lower chordates fine granular yolk distributed entirely in cytoplasm of oocyte.
• Yolk platelets: Yolk occurs in large granules called yolk platelets found in most of vertebrates fishes, amphibians, reptiles and birds.
• The yolk platelets of amphibians oocyte consists of two main proteinaceous substances phosvitin (two molecules) and lipovitellin (one molecule)
• In vertebrates, liver is the site for synthesizing the yolk protein and are then transported to the oocyte via blood circulation to the ovary.
• In insects yolk formation occurs in fat bodies.

Maturation Phase

• It is characterized by meiosis. In this, the diploid and fully grown primary oocyte completes meiosis-I (reductional division) and forms two unequal haploid cells.
• The smaller cell is called first polar body and the larger cell is called secondary oocyte and has nutrient-rich cytoplasm.
• Secondary oocyte undergoes meiosis-II to form two unequal haploid cells. The smaller cell is called second polar body and the larger cell is called ovum.