Gametogenesis in Mammals Spermatogenesis and Oogenesis  |Notes

Gametogenesis in Mammals Spermatogenesis and Oogenesis |Notes

Gametogenesis in Mammals Spermatogenesis and Oogenesis Gametogenesis is arbitrarily designated as the first stage of embryonic development. Jump inside to more about the Gametogenesis in Mammals Spermatogenesis and Oogenesis | Zoology Notes.

Gametogenesis in Mammals Spermatogenesis and Oogenesis

In embryology, the gametes are usually discussed first, as they provide both the blue print and the raw material from which embryo is formed. The prerequisite for the formation of an embryo is Gametogenesis a process which occurs in the reproductive organs of mature adults, by which sex cells or gametes of male and female individuals are produced.

The female gamete is usually non-mobile, larger and nutrient filled cells, the ovum or egg. The female gamete must be competent to be fertilized, which means that it must develop a number of specialized properties to enable it to interact with the sperm. Whereas the male gamete is usually small and mobile sex cell, the spermatozoon or sperm. Formation of male gametes or spermatozoa in the testes is known as Spermatogenesis and on the other hand female gametes or ova as Oogenesis.

In some animals, the spermatozoa and ova are produced by a single individual which is called hermaphrodite or monoecious or bisexual, whereas in other the two types of gametes are produced by different individuals, and such individuals are called dioecious or unisexual. This type of separation of individuals is called gonochorism.

LETS talk about Spermatogenesis from this topic Gametogenesis in Mammals Spermatogenesis and Oogenesis

Spermatogenesis

Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing harmone [GnRH]. This, if you recall, is a hypothalamic haromne. The increase in level of GnRH then act at the anterior pituitary gland and stimulates secretion of two gonadotropin – lutenising harmone (LH) and Folical stimulating harmone(FSH). LH acts at the leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process pf spermatogenesis. FSH act on the sertoli cells and stimulates secretion of some factors which help in the process of spermatogenesis.

Mammalian spermatogenesis is a highly synchronized, regular, long and extremely complex process of cellular differentiation by which a spermatoginal ”stem cell ” is gradually transformed into highly differentiated haploid cell ‘spermatozoon’. This differentiation involves three distinct classes of germinal cells the spermatogonia, the spermatocytes, and the spermatids, which usually are arranged on concentric layers in the seminiferous tubules.

In the adult mammals spermatogensis is a continuous process, which can be divided into two distinct phases each characterized by specific morphology and biochemical changes of nuclear and cytoplasmic components.The two phases are :

Formation of spermatids

This phase of spermatogenesis is further subdivided into three phases.

Maturation phase –

The undifferentitation of germ cells present in the seminiferous tubules of the testes are called the spermatogonia or sperm mother cells. During this phase the diploid spermatogonia, which are situated at the periphery of the seminiferous tubule, multiply mitotically to forms primary germs cells of the germinal epithelium lining the seminiferous tubules. And also to give rise to new spermatogonial stem cells and then jumps to the growth phase. This phase is also known as Proliferation and renewal of spermatogonia.

Growth phase –

on sexual maturity of animals, some spermatogonia stop dividing and grow in size by accumulating cytoplasm and replicating DNA, and are then termed the Primary spermatocytes which is still diploid in number. During this phase, a limited growth of spermatogonia takes place; their volume becomes double. Other spematogonia continue to divide mitotically and produce more spermatogonia for later spermatogenesis. The growth phase is also known as ‘SPERMATOCYTOGENESIS’ as it give rise to spermatocytes. the primary spermatocytes enter into the next phase namely, maturation phase.

Maturation phaseC

The primary spermatocytes enter into the prophase of first meiotic division or maturation division.

• The first meiotic nuclear division is followed by the division of cytoplasm or cytokinesis.

• It divides the primary spermatocyte into two haploid, secondary spermatocytes

• Both secondary spermatocytes undergo meiosis II or second maturation  which is mitosis and produces 4 haploid spermatids.

• These spermatids, though having a haploid set of chromosomes, are still not capable of functioning as male gametes.

• To become functional spermatozoa, they have to undergo a process of differentation or specialization called, Spermioteliosis or Spermiogenesis

2. Spermiogenesis

• A sperm or spermatozoon is a very active and mobile cell.

• To provide great deal of mobility to the sperm, all extra materials are discarded and a high degree of specialization take place in sperm cell.

• Sperms consists of two parts: Head and a vibratile tail and spermiogenesis can be divided into two processes; (i)Formation of Head of spermatozoon:

• The two major parts of sperm head are the nucleus and the acrosome a)Changes in Nucleus:

• Nucleus loses its entire fluid content, all its RNA, nucleolus and most of proteins.

• Only DNA is retained.

• Shape of nucleus also changes – instead of usual spherical form the nucleus becomes elongated and narrow.

b)Acrosome Formation:

•Acrosome of a sperm is derived from Golgi apparatus of a spermatid.

• The Golgi apparatus of an early spermatid consists of a series of cisternae arranges concentrically and aggregations of small vacuoles.

• During acrosome formation, one or more vacuoles start enlarging and inside the vacuole appears a small dense body called proacrosomal granule.

• Vacuole increases in size by fusing with small vacuoles containing proacrosomal granule as such proacrosomal granule also increases  in size.

• Vacuole orients itself on the tip of elongating nucleus and is called Acroblast and the proacrosomal granule is now called Acrosomal granule

• Vacuole loses its liquid content and its wall extends over the acrosomal granule and front half of nucleus, covering both in a double membrane sheath called the Cap of spematozoon

• Rest part of Golgi is eliminated from sperm as “Golgi rest” or “residual body” with some cytoplasm

HORMONAL CONTROL OF SPERMATOGENESIS

Spermatogenesis is under the control of endocrine hormones. It begins at the time of puberty due to large scale secretion of gonadotrophin releasing hormone or GnRH by hypothalamus. It acts on anterior pituitary to produce two gonadotrophins: luteinising hormone(LH) and follicle stimulating hormone(FSH). LH, also called ICSH( Interstitial cell stimulating hormone) act on interstitial or leydig cells which produce androgens like testosterone which is essential for the formation of sperms.

Under the influence of FSH, Sertoli cells develop androgen-binding protein(ABP). The later helps in concentrating testosterone in the seminiferous tubules for spermiogenesis.

            STRUCTURE OF HUMAN SPERMATOZOA

It is a microscopic, motile, uniflagellate male gamete which in humans is about 60 µm long. A covering of plasma membrane covers all around it. A sperm has four parts- head , neck ,middle piece and tail.

HEAD- It is the anterior, broad and flattened part of the sperm. Head consists of two parts nucleus and acrosome

The sperm nucleus occupies most of the available space of the head.It is the shape of the nucleus that ultimately decides the shape of the sperm head.It is enveloped by nuclear membrane.

ACROSOME occupies the anterior tip of the sperm. It forms a cap like structure called head cap. It occupies the space between the anterior half of the nucleus and the plasma membrane of sperm tip. It is formed from the Golgi complex and is covered by a membrane of its own

Acrosome consists of a number of hydrolytic enzymes which help in tissue lysis and this facilitates the penetration of the sperm into the egg. Acrosome and anterior half of nucleus are covered by a fibrillar sheath galea.

NECK – It is the smallest part of spermatozoan and may be indistinct. It is formed of two centrioles perpendicular to each other and is formed from the centrosome of spermatid. Each centriole is a micro tubular triplet structure having 9 + 0 arrangement.

Proximal centriole lies in a depression in the posterior surface of the nucleus and is perpendicular to main axis of the sperm. Distal centriole is along longitudinal axis of the sperm. Centrioles form spindle for the first cleavage of zygote. Distal centriole acts as basal body and gives rise to axoneme or axial filament of the sperm-tail.

MIDDLE PIECE

It lies behind the neck and is cylindrical in the human sperm. It is formed of a mitochondrial spiral, nebenkern, around the proximal part of axoneme. The mitochondria are the carriers of the oxidative enzymes and the enzymes which are responsible for oxidative phosphorylation. So the middle piece is the powerhouse of a sperm. Posterior half of nucleus, neck and middle piece of sperm are covered by a sheath, manchette.

TAIL-

It is the longest part of sperm. It is slender and tapering part.

It is formed of two parts:

Central, the contractile and microtubular part called axoneme or axial filament, and outer protoplasmic sheath. The axoneme is formed of 11 proteinous microtubules arranged in 9 + 2 manner. Sometimes, a ring centriole may be present at the junction of the middle piece and flagellum. The tail shows lashing movements which provide forward push to the sperm. Sometimes, the distal part of axoneme is uncovered and is called an end piece.

lets talk about Oogenesis from Gametogenesis in Mammals Spermatogenesis and Oogenesis

OOGENESIS IN MAMMALS

It is the process of formation of haploid ova from the diploid germinal cells in the ovary.

Site of Oogenesis • Oogenesis takes place in the ovaries.

• The ovary is composed of connective tissue called stroma, wrapped by a layer of cuboidal cells, the germinal epithelium, covered by a layer of flattened cells, the visceral peritoneum.

• The stroma consists of an outer cortex and an inner medulla.

• The cortexs contains rounded bodies, called ovarian, or Graafian, follicles at various stages of development.

Stages of Oogenesis

• Oogenesis is basically similar in nearly all animals.

• It consists of three phases i.proliferative, ii.growth and differentiation, and iii.maturation. i. Proliferative Phase

• Certain cells in the germinal epithelium of the ovary are larger than others and also have larger nuclei.

• These cells undergo mitotic divisions, producing undifferentiated germ cells called oogonia, or egg mother cells, in the ovary.

• The oogonia have 2n, or diploid, number of chromosomes.

• The oogonia divide mitotically to multiply their number.

•Some two million oogonia are formed in each foetal ovary. The oogonia form egg tube of Pfluger which grows into the cortex and give rise to a multicellular mass called egg nest.

• Addition of oogonia results in the growth of the ovaries.

• In the human female, the oogonia start developing into primary oocytes by the third month of her fetal life.

ii.The phase of Growth and Differentiation

• One oogonium of the egg nest grows in size and function as primary oocyte, the other cells lose the potential to become the primary oocyte, and form a regular layer, the follicular epithelium, round the primary oocyte to protect and nourish it.

• The enclosed primary oocyte is called primary follicle.

•The oocyte secretes sulphated glycoproteins that accumulate in the space formed by the separation of the follicle cells from the oocyte.

• This material forms a continuous coat, the zona pellucida, around the oocyte.

• The growth phase of the primary oocyte is very long and it may make it several hundred to several thousand times larger than the oogonium.

• When she is born, her two ovaries contain some 400,000 primary oocytes in the prophase of meiosis—I.

• These primary oocytes remain in the prophase—I till the female attains sexual maturity.

• After sexual maturity of the animal, oogonium in a follicle prepares for the formation of an ovum.

•On sexual maturity meiosis—I is resumed and completed at about the time of ovulation, some 13 — 45 years after meiosis began.

• Oogenesis is, thus, not necessarily a continuous process, unlike spermatogenesis. 

• The fluid-filled spaces coalesce, forming a large cavity, the antrum, or follicular cavity.

• The follicles with antra are known as Graafian follicles.

• Growth and differentiation of primary oocyte involve many changes in the nucleus as well as the cytoplasm.

(a)Nucleus •The nucleus of the primary oocyte becomes very large due to increase in nucleoplasm.

• It is often called the germinal vesicle.

Its nucleoli enlarge or increase in number due to excessive transcription of rRNA by the rDNA of the nucleolar organizing regions of the nucleolar chromosomes.

•The germinal vesicle often shifts to the yolk-free animal pole so that the polar bodies are easily separated from the oocyte at this pole.

(b) Cytoplasm •The cytoplasm of the primary oocyte greatly increases in amount. •Large numbers of ribosomes are assembled.

• Mitochondria also increase in number. • Endoplasmic reticulum becomes more elaborate.

• Golgi complexes accumulate.

• The rough endoplasmic reticulum and the Golgi complex give rise to spherical, membrane -bound cortical granules that gather in the peripheral region of the cytoplasm, now called cortex.

• The ovum has a store of cell organelles to start development with. This stockpile relieves the embryo of the need to produce the organelles during the early stages of development.

• Lipids and polysaccharides are also prepared in large quantities.

• Besides the internal synthesis of food materials, the primary oocyte may also receive, proteins, lipids and polysaccharides from outside.

• Some of these materials are absorbed through the plasma membrane from the surrounding intercellular fluid.

• As the oocyte attains its final dimensions, synthesis stops and the rate of metabolism falls to zero

Vitellogenesis. •The yolk is the most prominent cytoplasmic component of the egg in many oviparous animals.

• The process of laying down of yolk in the primary oocyte is called vitellogenesis.

• It takes place in the prophase-I of meiosis.

• Among mammals, egg-laying mammals like Duck-Billed Platypus shows vitellogenesis.

• In all other mammals it is absent.

iii) Maturation Phase. •Like the primary spermatocyte, a full-grown primary oocyte also undergoes first meiotic or maturation division and produces two daughter cells.

• The division of its cytoplasm is, however, very unequal due to the eccentric position of the spindle close to the plasma membrane.

• One daughter cell is extremely small, whereas the other is almost as large as the primary oocyte itself.

• The large cell is called the secondary oocyte.

•It receives almost whole of the cytoplasm and yolk of the primary oocyte.

• The small cell is known as the first polar body.

• It is also called polocyte.

• It consists almost of a bare nucleus.

• Both have the haploid or n number of chromosomes, as the first meiotic division is reductional.

• The secondary oocyte undergoes the second meiotic or maturation division, again with unequal cytoplasmic cleavage.

• This forms a large cell, the ootid, with essentially whole of the yolk and cytoplasm, and a very small cell, the second polar body.

• The ootid and the second polar body are haploid as the second meiotic division is equational.

• The ootid grows into a functional haploid ovum.

• Its nucleus, now called pronucleus, sinks down into the cytoplasm.

• The polar bodies have no function and disintegrate due to lack of cytoplasm and food.

• They are absorbed into the female’s body.

• The ootid grows into a functional haploid ovum.

• Its nucleus, now called pronucleus, sinks down into the cytoplasm.

• The polar bodies have no function and disintegrate due to lack of cytoplasm and food.

• They are absorbed into the female’s body.

• A thin vitelline membrane is secreted outside the plasma membrane of the oocyte.

• It consists mainly of polysaccharides and glycoproteins.

• It protects the egg from mechanical and chemical injuries.

• Growth and differentiation of the primary oocyte occur in the prolonged prophase-I of meiosis.

• The time of completion of meiosis varies in animals.

• In most vertebrates, including humans, meiosis-I is resumed at ovulation and proceeds up to the metaphase of meiosis-II.

• The egg remains at this stage until fertilization, which triggers the completion of meiosis-II.

• All the oogonia in the ovary do not give rise to ova simultaneously.

• They do so singly (humans, cattle) or in groups (rabbit, dogs, cats, etc).

• While one or more oogonia mature, others remain inactive.

Hormonal Control of Oogenesis •In mammals, there is cyclic production and ovulation of ripe eggs.

• Here, the anterior pituitary secretes two gonadotrophins which act synergistically to regulate oogenesis.

• These are follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

• They cause the growth of the follicle, prepare the egg for ovulation, and stimulate secretion of estrogen by the follicle cells

STRUCTURE OF OVUM- • It is the non-motile female gamete. Human ovum is rounded non cleiodic (without shell) and alecithal(without yolk). It is about 100µm in diameter. Ovum is covered by its own membrane called plasmalemma or oolemma. Its cytoplasm is called ooplasm. The outer region of ooplasm contains mucopolysaccharide cortical granules.

Cytoplasm contains microtubules, mitochondria, ribosomes, RNA, proteins, glycogen and lipid particles.Nucleus is large and is called germinal vesicle.Nucleus is generally in metaphase II prior to fertilization

Ovum has two coverings, a non cellular zona pellusida and a multicellular corona radiata. Zona pellusida carries receptor proteins for sperms

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