Embryology: Development, Types, and Processes
Embryology
Embryonic Development
Embryology studies the development of the embryo until the formation of an adult individual.
Local Development
Based on the site of embryo development, animals can be divided into:
- Ovulíparos: Animals whose females lay eggs in water, with external fertilization (e.g., some fish and amphibians).
- Oviparous: Animals with internal fertilization, where the female lays an egg that develops externally using nutritional reserves (e.g., reptiles, birds, and various invertebrates).
- Ovoviviparous: Animals whose embryos develop inside an egg within the female, using the egg’s reserves (e.g., some fish and invertebrates).
- Viviparous: Animals, typically mammals, whose embryos develop in the mother’s uterus, receiving nourishment and oxygen through the placenta (e.g., mammals).
Types of Eggs
Vertebrate eggs are classified according to the quantity and distribution of yolk:
- Oligolécito/Alécito: Eggs with little yolk spread throughout the cytoplasm, found in mammals and marine invertebrates (e.g., sponges, corals, starfish). These eggs divide completely and equally.
- Heterolécito: Eggs with more yolk than oligolécitos, concentrated in the vegetal pole (e.g., amphibians, some fish, and most shellfish).
- Megalécito/Telolecito: Eggs full of yolk, except in the animal pole where the nucleus lies. Segmentation occurs only in the animal pole, forming a cell disc (e.g., birds, reptiles).
- Centrolecithal: Eggs with yolk in the central region. The nucleus divides within the yolk before migrating to the periphery (e.g., arthropods).
Segmentation
The initial divisions of the egg, called segmentation or cleavage, produce blastomeres. A mass of blastomeres is called a morula. Segmentation type depends on the amount of yolk:
- Holoblastic Equal: Forms eight equal blastomeres, occurring in alécitos and oligolécitos eggs.
- Holoblastic Unequal: Forms blastomeres of different sizes, occurring in heterolécitos eggs.
- Meroblastic Discoidal: Divisions occur only in the disc region, occurring in telolecithal eggs.
- Meroblastic Superficial: Cells form on the egg’s surface, occurring in centrolecithal eggs.
Direct and Indirect Development
- Direct Development: The egg hatches into a miniature adult (e.g., reptiles, birds, mammals).
- Indirect Development: Animals with little yolk in their eggs hatch as larvae, which undergo metamorphosis to become adults (e.g., some amphibians and invertebrates).
Gastrulation
During gastrulation, the embryo forms three germ layers: ectoderm, endoderm, and mesoderm. The archenteron (primitive gut) forms, communicating with the outside through the blastopore.
In protostomes, the blastopore becomes the mouth. In deuterostomes, it becomes the anus.
Organogenesis
Tissues differentiate from germ layers to form organs:
- Ectoderm: Nervous system, skin, glands, nails, feathers, eye lens, tooth enamel.
- Mesoderm: Dermis, heart muscle, skeletal and smooth muscle, cartilage, bone, blood, bone marrow, pleura, peritoneum, pericardium, urogenital organs.
- Endoderm: Digestive tract, respiratory system, thyroid, parathyroid, thymus, bladder, urethra.
Embryonic Attachments
Embryonic attachments aid development:
- Yolk Sac: Provides nutrients.
- Amnion: Protects the embryo.
- Allantois: Stores waste and facilitates gas exchange.
- Chorion: Outer protective layer; forms the placenta in mammals.
Placenta
The placenta facilitates exchange between maternal and fetal blood, providing nutrients, oxygen, and antibodies, and removing waste. It also produces hormones like estrogen and progesterone.
Human Embryonic Development
The embryo implants in the uterus around six days after fertilization. The trophoblast forms the chorion, while the inner cell mass forms the embryo and other attachments. The placenta develops, and after eight weeks, the embryo becomes a fetus. Birth occurs around 40 weeks.
Twin Formation
- Monozygotic Twins: Genetically identical twins from a single fertilized egg.
- Dizygotic Twins: Genetically different twins from two fertilized eggs.