Animal Reproduction and Embryonic Development
Animal Reproduction: Gamete Formation
Reproductive systems are essential for gamete formation. They consist of gonads (testes and ovaries) and accessory sex organs. In hermaphrodites, ovotestes are present. In insects, the seminal receptacle stores sperm, which is released when the female expels ovules.
Gamete Formation (Gametogenesis)
Gamete formation, or gametogenesis, involves several phases:
- Proliferation: Stem cells divide by successive mitosis to form diploid (2n) future spermatogonia and oogonia.
- Growth Phase: Spermatogonia and oogonia grow to become primary spermatocytes and primary oocytes, respectively, still diploid.
- Meiotic Phase: Primary spermatocytes and oocytes undergo meiosis.
- Differentiation Phase: Spermatids differentiate into spermatozoa, and oocytes mature into ova.
Spermatogenesis
In males, the proliferative phase of spermatogenesis takes place continuously. New spermatogonia continuously grow and transform into primary spermatocytes. After the first meiotic division, each primary spermatocyte forms two secondary spermatocytes (n). These then yield four spermatids (n) at the end of the process.
Oogenesis
In females, the first phases occur during embryonic development. Developing oocytes form primordial follicles, and their activity is arrested until puberty. Each primary oocyte forms one secondary oocyte (n) and a polar body. The second meiotic division of the secondary oocyte leads to a single ovum (n) and a second polar body.
Spermatozoon Structure
The cytoplasm of the spermatid degenerates while the nucleus enlarges to form the head. The Golgi apparatus forms the acrosome, which contains enzymes. These enzymes dissolve the ovum’s membrane. The tail is flagellar, energized by mitochondria.
Ovum Structure
The ovum consists of a zona pellucida, corona radiata, and germinal vesicle.
Fertilization
External Fertilization
In external fertilization, common in fish, ovules are expelled, and the male releases sperm into the aquatic environment to fertilize them.
Internal Fertilization
In internal fertilization, sperm is introduced into the female’s reproductive tract, often through copulation.
Mechanism of Fertilization
When sperm contacts the ovum, it penetrates through the ovum’s layers using enzymes from the acrosome. The sperm nucleus penetrates the ovum’s membrane, and the two nuclei merge to form a zygote.
Reproductive Strategies: Oviparous, Ovoviviparous, Viviparous
Embryonic Development
Segmentation
Embryonic development begins with segmentation, a series of cell divisions. Starting from the zygote, cells divide to form a morula, and later a blastula (which contains a blastocoel cavity).
Gastrulation
During gastrulation, blastula cells rearrange into distinct germ layers. Organisms are classified based on the number of these layers:
- Diploblasty: Organisms with two germ layers: the ectoderm and the endoderm. From these, organs develop. The endoderm defines a cavity, the archenteron, which communicates with the outside via the blastopore.
- Triploblasty: Organisms with three germ layers, where the mesoderm forms between the ectoderm and endoderm. The mesoderm forms parietal and visceral layers, and the space between them is the coelom.
Organogenesis
Organogenesis completes development with the formation of specific organs from the embryonic germ layers:
- From the ectoderm forms the epidermis, nervous tissue, and sensory receptors.
- From the endoderm originates the lining of the digestive and respiratory tubes, and the bladder.
- From the mesoderm forms the heart, dermis, kidneys, gonads, and the skeletal-muscular system.