Biological Cycles: Seed Plant Reproduction and Oocyte Development Stages

Spermatophyte Cycle: Seed Plant Reproduction

Seed plants (Spermatophytes) are highly evolved organisms. Their life cycle involves distinct stages of reproduction.

The Sporophyte Stage and Male Structures

The Sporophyte is the entire plant, consisting of roots, stems, leaves, and flowers. In Angiosperms (flowering plants), the male reproductive part is the androecium, which is composed of stamens.

Anatomy of the Stamen

• A stamen consists of the anther and the filament.
• The anther contains two cavities called thecae.
• These thecae house the sacs that represent the microsporangia.

Male Gamete Formation (Pollen)

Inside the microsporangia, microspore mother cells form, which develop into pollen grains. When the microspore germinates, it originates the pollen tube. The pollen grain represents the male gametophyte, containing the male sperm nuclei necessary for fertilization.

Female Gamete Formation and Fertilization

The female reproductive structure is the gynoecium, which consists of modified leaves called carpels. The gynoecium is differentiated into three main parts:

• The ovary
• The style
• The stigma

Inside the ovary are the ovules (macrosporangia), where the megaspores are formed. When the megaspore germinates, it originates the embryo sac. This sac represents the female gametophyte and contains the egg cell (oosphere).

Fertilization occurs when a sperm nucleus fuses with the egg cell, resulting in an embryo. This embryo develops within the seed, which, upon germination, yields a new plant.

Structure and Development of the Egg Cell (Oocyte)

The egg cell (oocyte) is the female sex cell, characterized by specific structural components and a complex developmental process within the ovary.

Anatomy of the Mature Egg

The oocyte consists of a nucleus called the germinal vesicle. The cell exhibits polarity:

• The side containing the nucleus is often termed the animal pole.
• The opposite side is the vegetative pole.

Inside the nucleus, there are often two dense spots called nucleoli. The cytoplasm is largely occupied by nutrient reserves, commonly referred to as yolk.

Protective Layers of the Oocyte

The cytoplasm is surrounded by a series of protective layers, listed from inside out:

1. The vitelline membrane (or yolk membrane)
2. The zona pellucida
3. The corona radiata

Ovarian Follicle Development Stages

Female sex cells develop in the cortex of the ovary within cavities called follicles. The name of the follicle varies depending on the developmental stage of the cell it contains:

• When the cell is immature, the cavity is called the primary follicle.
• When a secondary oocyte is formed inside, the structure is known as the Graafian follicle.
• When the Graafian follicle releases the oocyte during ovulation, the remaining structure is called the corpus luteum (or yellow body).
• When the corpus luteum degenerates, the residual structure is called the corpus albicans.