Structure of a Typical Embryo Sac: Unravelling the Heart of Female Reproduction
Alright, today we're going to
zoom in on the microscopic world of the embryo sac, where the magic of female
reproduction unfolds within the ovule of a flower.
1. Location:
- The embryo sac is located within the
ovule, a structure housed in the ovary of the flower.
2. Initiation:
- The embryo sac develops from the
functional megaspore, which is the result of the meiotic division of the
megaspore mother cell.
3. Types of Embryo
Sac:
- There are two main types of embryo sacs
based on their development process:
- Monosporic:
Develops from a single megaspore.
- Pentasporic:
Develops from five megaspores.
- In most angiosperms, the monosporic type
is prevalent.
4. Cell Types within
the Embryo Sac:
- Egg Cell (Oosphere):
- Located at one end of the embryo sac.
- The egg cell is the female gamete, the
cell that will fuse with a sperm cell during fertilization.
- Synergids:
- Surround the egg cell.
- Synergids play a role in guiding the
pollen tube to the egg cell during fertilization.
- Antipodal Cells:
- Located at the opposite end of the embryo
sac from the egg cell.
- The antipodal cells' function is not
fully understood, but they may have a role in nourishing the developing
embryo sac.
- Central Cell:
- Positioned centrally within the embryo
sac.
- The central cell contains two nuclei,
known as polar nuclei.
- Polar Nuclei:
- Two nuclei within the central cell.
- The fusion of one polar nucleus with a
sperm cell contributes to the formation of endosperm, a tissue that
nourishes the developing embryo.
5. Structure of the
Central Cell:
- The central cell is large and often has
two nuclei (binucleate).
6. Nuclei within the
Embryo Sac:
- The number of nuclei in the embryo sac can
vary, and the arrangement may include one, two, or more nuclei depending
on the stage of development.
7. Fertilization and
Seed Development:
- After pollination and the growth of a
pollen tube, fertilization occurs when a sperm cell fuses with the egg
cell.
- The fertilized egg develops into the
embryo, and the ovule matures into a seed.
8. Importance of the
Embryo Sac:
- The embryo sac is the site where the
female gamete (egg cell) and male gamete (sperm cell) unite, initiating
the process of seed development.