Unit 7: Development Biology

7.1 Gametogenesis

Spermatogenesis

Spermatogenesis is the process by which male germ cells (

1. Spermatogonia (2n) – Stem cells located at the basal compartment of the seminiferous tubules; they undergo mitosis to self‑renew and produce primary spermatocytes.
2. Primary spermatocyte (2n) – Enters meiosis I; after DNA replication, each cell contains duplicated chromosomes.
3. Meiosis I – Homologous chromosomes separate, yielding two secondary spermatocytes (n) each with a haploid set of chromosomes (each chromosome still consists of two sister chromatids).
4. Meiosis II – Sister chromatids separate, producing four spermatids (n).
5. Spermiogenesis – Spermatids differentiate into motile spermatozoa: formation of the acrosome, condensation of nuclear DNA, development of the flagellum, and removal of excess cytoplasm.
6. Spermatozoa (sperm) – Fully functional male gamete capable of fertilizing an oocyte.

Hormonal control:

• FSH (Follicle‑Stimulating Hormone) – Acts on Sertoli cells to support spermatogenesis and stimulate androgen‑binding protein production.
• LH (Luteinizing Hormone) – Stimulates Leydig cells to secrete testosterone.
• Testosterone – Essential for the completion of meiosis and spermiogenesis; maintains the spermatogenic environment.

Duration: Approximately 64–72 days in humans from spermatogonium to spermatozoon.

Example: In a healthy adult male, about 100–200 million spermatozoa are produced daily.

Diagram:

Oogenesis

Oogenesis is the formation of the female gamete (ovum) in the ovaries. Unlike spermatogenesis, it yields typically one large ovum and small polar bodies per germ cell, involves prolonged arrest periods, and is completed only after fertilization.

1. Oogonia (2n) – Diploid stem cells that proliferate by mitosis during fetal development.
2. Primary oocyte (2n) – Formed when oogonia enter meiosis I and arrest in prophase I until puberty.
3. Meiosis I (resumes at puberty) – Completes just before ovulation, producing a secondary oocyte (n) and the first polar body (n) (usually degenerates).
4. Meiosis II – Begins after ovulation but arrests at metaphase II; completes only upon sperm entry, yielding the ovum (n) and a second polar body (n).
5. Unequal cytokinesis – Ensures the oocyte retains most cytoplasm, organelles, and nutrients needed for early embryonic development.

Hormonal control:

• FSH – Stimulates granulosa cell proliferation and estrogen synthesis.
• LH – Triggers ovulation and luteinization of the follicle.
• Estrogen – Promotes follicular growth and induces LH surge.
• Progesterone – Prepares the endometrium for implantation and maintains the luteal phase.

Timeline: Oogonia multiply during fetal life; primary oocytes are formed before birth and remain arrested until puberty. Each menstrual cycle typically releases one secondary oocyte; meiosis II completes only if fertilization occurs.

Example: A female is born with ~1–2 million primary oocytes; by puberty this number declines to ~300,000, and only about 400–500 will ovulate during reproductive life.

Diagram:

7.2 Development of Frog

Fertilization

In frogs, fertilization is external and occurs in water. The sperm penetrates the jelly coat of the egg, triggering a series of events that activate the egg and prevent polyspermy.

1. Sperm meets egg – Sperm binds to receptors on the vitelline envelope.
2. Acrosome reaction – Release of enzymes (e.g., hyaluronidase) that digest the jelly coat, allowing sperm to reach the plasma membrane.
3. Fusion of membranes – Sperm nucleus enters the cytoplasm.
4. Cortical reaction – Cortical granules release contents that modify the vitelline envelope, forming a fertilization block (prevents additional sperm entry).
5. Effects:
   • Egg activation – resumption of meiosis II.
   • Restoration of diploidy – fusion of sperm and egg pronuclei.
   • Initiation of cleavage – calcium wave triggers mitotic divisions.

Formula (simplified): Egg (n) + Sperm (n) → Zygote (2n) where n = haploid chromosome number.

Diagram:

Cleavage

Cleavage consists of rapid mitotic divisions without overall growth, partitioning the zygote cytoplasm into smaller blastomeres.

• Holoblastic cleavage – The entire egg divides because the yolk is moderately and evenly distributed (mesolecithal egg).
• Stages:
  1. 2‑cell stage – first meridional division.
  2. 4‑cell stage – second meridional division perpendicular to the first.
  3. 8‑cell stage – third division (equatorial) producing four micromeres and four macromeres.
  4. 16‑cell stage – continued divisions leading to a morula.

Example: In Xenopus laevis, the first cleavage occurs ~90 minutes after fertilization at 22 °C.

Diagram:

Morulation

The morula is a solid ball of cells (typically 16‑32 blastomeres) with little internal cavity. Cells are tightly packed, and polarity begins to emerge (animal vs. vegetal poles).

Diagram:

Blastulation

Blastulation transforms the morula into a blastula by forming a fluid‑filled cavity called the blastocoel.

• Blastocoel – Central cavity that provides space for cell movements during gastrulation.
• Blastoderm – Single layer of cells surrounding the blastocoel.
• Animal pole – Region where the nucleus is concentrated; gives rise to ectoderm.
• Vegetal pole – Yolk‑rich region; gives rise to endoderm.

Formula (cell number estimate): Blastula ≈ 128–256 cells (varies by species).

Diagram:

Gastrulation

Gastrulation reorganizes the blastula into a multilayered gastrula with three primary germ layers: ectoderm, mesoderm, and endoderm. Key movements include epiboly, invagination, and convergent extension.

1. Epiboly – Animal cap cells (future ectoderm) spread and thin over the vegetal hemisphere.
2. Invagination – Cells at the dorsal vegetal region inward fold to form the blastopore, the future anus.
3. Formation of germ layers:
   • Ectoderm – Outer layer; gives rise to epidermis and nervous system.
   • Mesoderm – Middle layer; forms notochord, somites, kidneys, heart, blood, and muscles.
   • Endoderm – Inner layer; produces gut lining, liver, pancreas, lungs.
4. Archenteron – Primitive gut formed by the invaginated endoderm; opens via the blastopore.

Example: In Xenopus, the dorsal lip of the blastopore acts as the organizer (Spemann’s organizer), inducing neural tissue formation.

Diagram:

Organogenesis

Organogenesis follows gastrulation; the germ layers differentiate and organize into specific organs and structures.

• Notochord – Derived from dorsal mesoderm; serves as a flexible axial rod that induces overlying ectoderm to form the neural plate.
• Nerve Cord (Neural tube) – Ectodermal neural plate folds inward, fusing to create the neural tube, which differentiates into the brain and spinal cord.
• Coelom – Formed by splitting of lateral plate mesoderm; creates the body cavity that houses viscera.
• Somites – Segmented blocks of paraxial mesoderm that give rise to vertebrae, skeletal muscle, and dermis.

Key inductive interactions:

1. Notochord → neural plate induction (via secreted factors like Noggin, Chordin).
2. Somites → dermis and muscle differentiation.
3. Endoderm → gut tube formation and organ budding (lung, liver).

Diagram:

Summary of chromosomal changes during gametogenesis: