Developmental Biology (Zoology): gametogenesis, development of frog

Spermatogenesis is the process of formation of haploid spermatozoa (sperms) from the diploid spermatogonia (germ cells) in the seminiferous tubules of the testes. It is completed in three phases: multiplication, growth and maturation.

1. Multiplication phase (spermatocytogenesis) The diploid (2n) primordial germ cells, called spermatogonia, multiply by repeated mitotic divisions to produce a large number of spermatogonia. Some of these enlarge and are called primary spermatocytes (still diploid, 2n).

2. Growth phase Each primary spermatocyte grows in size, accumulates nutrients and prepares for meiosis. It is now ready for the maturation (meiotic) division.

3. Maturation phase (meiosis)

• First meiotic division (Meiosis I): Each primary spermatocyte (2n) undergoes meiosis I to form two secondary spermatocytes (haploid, n). This is a reductional division.
• Second meiotic division (Meiosis II): Each secondary spermatocyte undergoes meiosis II (equational division) to form two spermatids (haploid, n). Thus one primary spermatocyte gives rise to four spermatids.

4. Spermiogenesis (differentiation) The spermatids do not divide further; they undergo morphological transformation into motile spermatozoa (sperms) by developing a head, neck, middle piece and tail, and losing most of the cytoplasm. This change is called spermiogenesis.

5. Spermiation The mature sperms are finally released from the Sertoli cells into the lumen of the seminiferous tubules — this release is called spermiation.

Hormonal control: Spermatogenesis is stimulated by FSH (acts on Sertoli cells) and testosterone (from Leydig cells), with LH stimulating testosterone secretion. It occurs at about 15–16 years of age and continues throughout life.

Schematic diagram:

        Spermatogonium (2n)
                │  mitosis (multiplication)
                ▼
        Primary spermatocyte (2n)
                │  Meiosis I (reductional)
                ▼
        Secondary spermatocytes (n)  (2)
                │  Meiosis II (equational)
                ▼
        Spermatids (n)  (4)
                │  spermiogenesis (differentiation)
                ▼
        Spermatozoa / Sperms (n)  (4)

Key points to remember: one primary spermatocyte → 4 sperms; meiosis I is reductional, meiosis II is equational; spermiogenesis = spermatid → sperm; spermiation = release of sperms.

Blastula: A hollow ball of cells with a fluid-filled cavity, the blastocoel, formed from the morula in embryonic development.

Formation of blastula in frog

• After fertilisation, the zygote divides by repeated mitosis (cleavage). The first two cleavages are vertical and the third is horizontal, above the equator, giving 8 cells: four small micromeres at the animal pole and four large macromeres at the vegetal pole.
• Further divisions give a solid ball of cells, the morula.
• Fluid then collects between the cells in the animal half, and the cells rearrange to form a cavity, the blastocoel, pushed towards the animal pole. The embryo is now a blastula with a roof of small micromeres and a floor of large yolk-laden macromeres.

Why micromeres divide faster than macromeres

Micromeres contain less yolk, so there is less resistance to the cleavage furrow and the cytoplasm is more active. Macromeres are heavily loaded with yolk, which slows cleavage.

(c) Coelom

The space between the somatic (parietal) and visceral layers of mesoderm in the developing frog embryo is the coelom — the true body cavity lined by mesoderm.

Oogenesis is the formation of haploid female gametes (ova) from diploid germ cells (oogonia) in the ovary. It has three phases.

1. Multiplication phase: Primordial germ cells in the germinal epithelium of the foetal ovary divide repeatedly by mitosis to form many diploid (2n) oogonia. In humans this is completed before birth.

2. Growth phase: Each oogonium enlarges, stores yolk and nutrients and becomes a diploid primary oocyte. It starts meiosis I but stops at prophase I and stays arrested (surrounded by follicle cells, as a primary follicle) until puberty.

3. Maturation phase:

• From puberty, under the influence of FSH and the LH surge, one primary oocyte completes meiosis I each month. The division is unequal: it gives a large secondary oocyte (n) with almost all the cytoplasm and a tiny first polar body (n).
• The secondary oocyte starts meiosis II but stops at metaphase II and is released at ovulation.
• Meiosis II is completed only if a sperm enters. It again divides unequally into a large ovum (ootid) (n) and a second polar body. The first polar body may also divide.
• Thus one oogonium gives one ovum and three polar bodies, which degenerate.

Schematic diagram

        Oogonium (2n)
             │  mitosis (multiplication)
             ▼
   Primary oocyte (2n)   growth, arrested in prophase I
             │  meiosis I
             ▼
  Secondary oocyte (n) + First polar body (n)
             │  meiosis II (after sperm entry)
             ▼
      Ovum (n) + Second polar body (n)

The unequal divisions conserve the yolk-rich cytoplasm in a single large ovum for nourishing the embryo.

(c) Cleavage

After fertilization, the frog's zygote immediately undergoes a series of rapid mitotic divisions called cleavage, producing a hollow ball of cells (blastula). Morulation and blastulation come later, and gastrulation follows blastulation.

Neurulation is the formation of the neural tube, the primordium of the central nervous system, from the dorsal ectoderm after gastrulation. In the frog it occurs as follows.

1. Neural plate: Under the inductive influence of the underlying notochord and chordamesoderm, the dorsal ectoderm thickens into a flat neural plate.
2. Neural groove and folds: The plate sinks along its midline to form a neural groove, while its edges are raised as two neural folds.
3. Neural tube: The neural folds move towards each other and fuse in the mid-dorsal line, beginning in the middle and proceeding forwards and backwards. The groove becomes a hollow neural tube. The anterior part enlarges to form the brain and the rest forms the spinal cord; the cavity is the neurocoel.
4. Separation: The tube separates from the overlying ectoderm, which closes over it to form the epidermis.
5. Neural crest: Cells at the junction of the folds form the neural crest, which migrates to form spinal and cranial ganglia, Schwann cells, pigment cells and adrenal medulla.

(b) Epiboly

Epiboly is the gastrulation movement in which the rapidly dividing micromeres spread and overgrow the slower, yolk-laden megameres, eventually covering the whole embryo (as in amphibian blastula).

Coelom: The coelom is the true body cavity lined by mesoderm (mesothelium), lying between the body wall and the alimentary canal. It is filled with coelomic fluid and allows independent movement and growth of internal organs. In frog, it is a schizocoelom.

Formation in frog's development:

1. Gastrulation: The blastula undergoes gastrulation, forming the archenteron and three germ layers — ectoderm, endoderm and mesoderm. The mesoderm arises from the lips of the blastopore.

2. Splitting of mesoderm: The mesoderm splits into an outer somatic (parietal) layer and an inner splanchnic (visceral) layer. The space that appears between these two layers is the coelom. Because it arises by splitting of mesoderm, it is called a schizocoelom.

3. Formation and partitioning of coelomic sacs: The coelom first appears as paired cavities in the lateral plate mesoderm. These enlarge and fuse, and then the coelom is partitioned by the septum transversum into an anterior pericardial cavity (around the heart) and a posterior pleuroperitoneal cavity (around the lungs and viscera). In frog, there are no separate pleural cavities; the pleuroperitoneal cavity is single.

4. Lining: The somatic mesoderm together with ectoderm forms the body wall, while the splanchnic mesoderm covers the gut. The coelom is lined by mesothelium and filled with coelomic fluid, which protects the viscera and allows free movement of the heart and gut.

Organogenesis is the process by which the three primary germ layers (ectoderm, mesoderm and endoderm) formed during gastrulation differentiate and organise into the various organs and organ systems of the embryo.

Neurulation in the development of frog:

1. Formation of neural plate: After gastrulation, the dorsal ectoderm lying above the notochord and dorsal mesoderm becomes thickened and flattened to form the neural plate.

2. Formation of neural groove: The neural plate sinks along its median longitudinal axis, and its lateral edges are raised as neural folds. The depressed middle region is called the neural groove.

3. Formation of neural tube: The neural folds grow towards each other and fuse in the mid-dorsal line, converting the neural groove into a hollow neural tube. The fusion begins in the middle and proceeds anteriorly and posteriorly. The anterior end becomes enlarged to form the brain and the posterior part forms the spinal cord.

4. Formation of notochord: The underlying chordamesoderm forms a solid rod, the notochord, which lies below the neural tube.

5. Separation from ectoderm: The neural tube finally separates from the overlying surface ectoderm, which closes over it to form the epidermis. Some cells at the junction of neural tube and ectoderm form the neural crest cells, which later give rise to ganglia and other structures.

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OR

Allograft (homograft) is the transplantation of an organ or tissue from one individual to another individual of the same species but with a different genetic makeup (non-identical donor and recipient).

Methods used to prevent rejection of transplanted organs:

1. Tissue typing (histocompatibility testing): The donor and recipient are matched for MHC (HLA) antigens so that the graft is as compatible as possible.

2. Immunosuppressive drugs: Drugs such as cyclosporin A, azathioprine, prednisolone and tacrolimus are given to suppress the immune system and prevent it from attacking the graft.

3. Use of antibodies: Antilymphocyte globulin and monoclonal antibodies against T-cells are used to reduce the immune response.

4. Radiation: Local X-ray irradiation of the graft area or lymphoid tissue is sometimes used to suppress lymphocyte activity.

5. Close blood relation donor: Grafts from close relatives (especially identical twins) are preferred, as they are less likely to be rejected.

Why cornea transplantation is easier:

The cornea is an avascular (bloodless) tissue — it has no blood vessels and no lymphatic vessels. Since the immune cells (lymphocytes) reach a graft through the blood and lymph, they cannot easily reach the corneal graft. Moreover, the cornea is separated from the blood supply by the blood–corneal barrier, so the graft antigens are not exposed to the recipient's immune system. Hence, the immune response against the graft is very weak and the graft survives for a long time without much immunosuppression.

(a) Multiplication, growth, maturation

Gametogenesis proceeds through three sequential phases: the multiplication phase (primordial germ cells divide by mitosis to form many spermatogonia/oogonia), the growth phase (cells grow in size and accumulate nutrients), and the maturation phase (meiosis occurs to form haploid gametes).

Oogenesis is the formation of haploid ova from diploid oogonia in the ovary. It has three phases.

1. Multiplication phase: Germ cells in the germinal epithelium of the ovary divide by mitosis to form many diploid oogonia. In humans this is completed before birth.

2. Growth phase: Each oogonium grows by storing yolk and nutrients and becomes a primary oocyte (2n). It begins meiosis I but is arrested in prophase I, surrounded by follicle cells, until puberty.

3. Maturation phase:

• Meiosis I is completed (under FSH and LH, shortly before ovulation) with unequal division into a large secondary oocyte (n) and a small first polar body (n).
• The secondary oocyte starts meiosis II and is arrested at metaphase II; it is released at ovulation.
• If a sperm enters, meiosis II is completed, again unequally, giving a large ovum (n) and a second polar body.
• Hence one oogonium yields one ovum and three polar bodies; the polar bodies degenerate.

Diagram

        Oogonium (2n)
             │  mitosis
             ▼
   Primary oocyte (2n)  (growth; arrested in prophase I)
             │  meiosis I
             ▼
  Secondary oocyte (n)  +  First polar body
             │  meiosis II
             ▼
        Ovum (n)        +  Second polar body

The unequal divisions keep nearly all the yolk and cytoplasm in the single ovum to nourish the early embryo.

(b) Two spermatids

A secondary spermatocyte is haploid (n) and divides by meiosis II to give two haploid spermatids. (Four spermatids come from one primary spermatocyte, since it first gives two secondary spermatocytes.)

The coelom is the true body cavity lined by mesoderm. In the frog it forms by schizocoely, that is, by splitting of the mesoderm.

1. Mesoderm formation: During gastrulation, cells moving in through the blastopore lips form the mesoderm as a layer between ectoderm and endoderm.
2. Splitting of mesoderm: The lateral mesoderm splits into two layers: an outer somatic (parietal) layer lying against the ectoderm and an inner splanchnic (visceral) layer lying against the endoderm (gut).
3. Coelom: The space that appears between the two layers is the coelom. The paired cavities enlarge and join, and the cavity is filled with coelomic fluid.
4. Partitioning: The septum transversum separates an anterior pericardial cavity (around the heart) from the posterior pleuroperitoneal cavity (around the lungs and viscera).

The somatic layer helps form the body wall and the splanchnic layer covers the gut, so the organs can move freely in the coelom.

(a) Morula, blastula, gastrula

After fertilisation the zygote undergoes cleavage to form a solid ball of cells, the morula. A fluid-filled cavity (blastocoel) then develops to give the blastula, and the cells then move by gastrulation to form the three germ layers of the gastrula.

(c) Blastocoel

The blastula is a hollow ball of cells whose central fluid-filled cavity is the blastocoel. Blastopore and archenteron form later in the gastrula, and the neurula is a still later stage.

(c) Involution, epiboly, invagination

In frog gastrulation, cells at the dorsal lip move inwards by involution, the animal-pole cells spread over the vegetal cells by epiboly, and a few cells sink in by invagination to begin the archenteron. Cleavage is a pre-gastrula process.

Fertilization in frog is external and takes place in water, during the breeding season.

1. Amplexus: the male clasps the female and, as she releases eggs into the water, he releases sperm over them, which increases the chance of fertilization.
2. Approach and contact: the egg is surrounded by a jelly coat and vitelline membrane. A sperm reaches the egg by swimming, attracted by chemicals from the egg; the sperm head contacts the egg surface, mostly in the animal hemisphere.
3. Acrosomal reaction: enzymes from the acrosome digest the jelly coat and let the sperm head contact the vitelline membrane.
4. Entry: the sperm nucleus and centriole enter the egg; the tail is left outside. Entry triggers the cortical reaction, in which the vitelline membrane lifts to form the fertilization membrane, preventing polyspermy.
5. Completion of meiosis II in the egg and release of the second polar body.
6. Syngamy: the male and female pronuclei (n) fuse to form the diploid zygote (2n). A grey crescent appears opposite the sperm entry point, fixing the future dorsal side of the embryo.

(b) Mesoderm

In frog, a triploblastic animal, the coelom is a true body cavity that forms within the mesoderm and is lined by mesodermal peritoneum. Ectoderm forms the epidermis and nervous system, and endoderm forms the gut lining. Mesoglea is the jelly layer of cnidarians.