organeogenesis
- Created by: stg.pepper
- Created on: 30-10-20 15:07
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- Organogenesis
- Zygote to Embryo
- After fertilisation, an zygote undergoes cleavage for 3 days, leaving it with eight cells
- Cells continue dividing and the zygote becomes a morula, then a blastocyst, containing an inner cell mass (of pluripotent cells that form the embryo) and trophoblast cells that form the placenta.
- The blastocyst implants into the uterus after 8 days, secreting enzymes to digest the endometrium it buries itself into.
- Once implanted, an embryo develops extraembryonic membranes
- Yolk sac: first site of blood cell formation
- Amnion: surrounds the embryo, filled with amniotic fluid. Protects the embryo from physical damage and dehydration. The inner most membrane.
- Chorion: becomes the main part of the placenta.
- Allantois: becomes the vascular connection between the embryo and placenta.
- In the first weeks of development, endometrial (decidual) cells nourish the embryo. Between 4 and 12 weeks, the placenta gradually takes over this role,
- Germ Layers
- In the second week of development, cells of the blastocyst's inner mass form a two-layered disc of embryonic cells.
- This becomes a three-layered disc a week later, and the cells go from totipotent to multipotent.
- The three-layered embryo forms an indentation called the primitive streak along its dorsal surface. A node at the tail (caudal) end of the PS emits growth factors that direct cells to migrate through the primitive streak.
- This creates two new layers of cells, making three groups of cells in total.
- The first of the new layers is the endoderm, which displaces the hypoblast and lies adjacent to the yolk sac.
- The second new layer of cells is the mesoderm, the middle layer.
- The cells that remain in the epiblast, that didn't migrate through the PS form the ectoderm.
- This process is gastrulation and happens 14-16 days after fertilisation.
- This creates two new layers of cells, making three groups of cells in total.
- The three-layered embryo forms an indentation called the primitive streak along its dorsal surface. A node at the tail (caudal) end of the PS emits growth factors that direct cells to migrate through the primitive streak.
- This becomes a three-layered disc a week later, and the cells go from totipotent to multipotent.
- Specific organs derive from specific germ layers
- The ectoderm forms the central and peripheral nervous systems, sensory organs, the epidermis, hair, and nails.
- Through the process of neurulation, specialised neuroectodermal tissues along the length of the embryo thicken to form the neural plate, around week 4. The sides of the plate rise to form a fold which converges into the neural tube.
- The tube lies on top of a rod-shaped notochord from the mesoderm, which develops into the nucleus polposus, the inner core of the intervertebral discs.
- Block-like structures called somites form on either side of the tube; they eventually become the axial skeleton, skeletal muscle, and dermis. 44 pairs develop in succession, anterior to posterior, forming muscle, vertebrae, and rib bones. The anterior neural tube dilates and subdivides into vesicles that develop into brain structures around week 4-5.
- The B vitamin folate is extremely important to healthy neural tube development. Deficiency of it in the early weeks of pregnancy can cause neural tube defects such as spina bifida (where the vertebral column fails to close and spinal tissue protrudes from it) and anencephaly (partial or complete absence of brain tissue).
- Through the process of neurulation, specialised neuroectodermal tissues along the length of the embryo thicken to form the neural plate, around week 4. The sides of the plate rise to form a fold which converges into the neural tube.
- The mesoderm forms the skeleton, muscles, connective tissue, heart, blood vessels, and kidneys.
- The endoderm forms the epithelial lining of the GI tract, the liver, pancreas and lungs.
- The heart develops early on, starting off as a tube-like structure that's connected to the chorionic villi via capillaries.
- It begins beating around week 4 and starts to pump embryonic blood a week later, when the liver begins producing red blood cells.
- Between weeks 4 and 5, the eye pits form, the limb buds become visible and the pulmonary system begins to develop.
- Around week 6 of development, the foetus begins making uncontrolled limb movements. The GI system develops too rapidly for the abdomen to contain, so the intestines temporarily loop into the umbilical cord.
- Also around week 6, the hands and feet develop digits as apoptosis (programmed cell death) causes the tissues between fingers and toes to disintegrate,
- Around week 7, the face is more complex in structure, including nostrils, outer ears, and lenses.
- Around week 8, the head is nearly as large as the rest of the body. All the major brain structures are in place, as are external genitalia. Bone replaces cartilage through the process of ossification and the embryonic period ends.
- The ectoderm forms the central and peripheral nervous systems, sensory organs, the epidermis, hair, and nails.
- In the second week of development, cells of the blastocyst's inner mass form a two-layered disc of embryonic cells.
- The process by which an embryo establishes rudimentary structures of all its organs and tissues from the three germ layers.
- Zygote to Embryo
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