Bio - Developmental Biology
Fundamental Concepts of Cleavage in Developmental Biology
Cleavage: This is defined as rapid cell division occurring without changing the total mass of cells. The result of this process is the production of subsequently smaller cells known as blastomeres.
Axis of Cleavage:
Radial cleavage: In this type, the cells are aligned in a vertical axis. This is characteristic of deuterosomes.
Spiral cleavage: This involves offset cells that spiral around an axis. This is characteristic of protosomes.
Fate of Cells during Cleavage:
Determinate (mosaic) cleavage: This occurs when blastomeres have a decided fate early on.
Indeterminate (regulative) cleavage: In this type, blastomeres do not have a pre-set fate. This process results in totipotent cells.
Evenness of Embryo Division:
Holoblastic cleavage: This is a process where the entire embryo divides evenly. It typically occurs in animals with little yolk, such as humans and sea urchins.
Exception: Frogs have a significant amount of yolk but still undergo holoblastic cleavage; however, it is uneven and exhibits polarity.
Meroblastic cleavage: This refers to partial cleavage where the embryo is not evenly divided. It occurs in animals with a large amount of yolk, including birds, fish, and reptiles.
Polarity in Meroblastic Cleavage: It exhibits an animal pole, which is the site of active cleavage, and a vegetal pole, which consists mainly of yolk and undergoes negligible division.
Progression of the Zygote and Early Embryonic Stages
Morula: A ball of blastomeres that forms when the embryo reaches the cell stage.
Blastula: This stage forms at the cell stage and is characterized by containing a blastocoel, which is a hollow, fluid-filled center.
Blastocyst (Mammals): At this stage, the cells of the blastula begin to divide and differentiate. Cleavage occurs as the fertilized egg travels to the uterus. By the time the fertilized egg reaches the uterus, it has arrived at the blastocyst stage.
Gastrulation and the Formation of Germ Layers
Gastrulation: This is the process involving the formation of a trilaminar embryo. It occurs when epiblast cells invaginate inwards through the primitive streak to form three distinct germ layers: the endoderm, mesoderm, and ectoderm.
Archenteron and Blastopore: As cells invaginate, they create an opening called the blastopore. This leads to the formation of the archenteron (the gut tube), which is the center cavity that eventually becomes the digestive tract.
Ectoderm (Outer Germ Layer) Derivatives:
Central Nervous System (CNS), including the brain and spinal cord.
Peripheral Nervous System (PNS), neural crest cells, and neural ganglia.
Sensory parts of the ear, eye, and nose.
Epidermis layer of the skin, hair, and nails.
Mammary and sweat glands.
Pigmentation cells.
Enamel of teeth.
Adrenal medulla.
Mesoderm (Middle Germ Layer) Derivatives:
Bone and skeleton.
Skeletal, smooth, and cardiac muscle.
Cardiovascular system.
Gonads.
Adrenal cortex.
Spleen.
Notochord: This structure induces spinal cord formation from the ectoderm.
Endoderm (Inner Germ Layer) Derivatives:
Epithelial lining of the digestive, respiratory, and excretory systems.
The PLTT organs: Pancreas, Liver, Thyroid and parathyroid, and Thymus.
Stem Cell Potency and Differentiation
Stem Cells: These are undifferentiated cells that possess the potential (potency) to become many different types of cells.
Totipotent stem cells: These have the capability to become any cell in the organism.
Pluripotent stem cells: These can differentiate into any of the germ layers.
Multipotent stem cells: These can only differentiate into a few cell types belonging to a specific tissue type.
Determination: This is the stage where a cell becomes committed to a specific fate. Although the cell has not yet changed in its physical appearance or function, it has irreversibly established the regulatory instructions that will guide future gene expression. This process leads to selective gene expression, resulting in differences in cell function.
Differentiation: This is the subsequent stage where the cell begins to express those predetermined genes. This leads to the production of specific proteins and functional characteristics that give the cell its specialized phenotype.
Genetic Retention and Reprogramming: Differentiated cells retain the entire genome. Nuclear reprogramming can be used to restore a differentiated cell to an earlier, less specialized state.
Organogenesis and Neurulation
Organogenesis: The process of the formation of new organs.
Neurulation: This refers to the development of the nervous system. An embryo undergoing this stage is referred to as a neurula.
The notochord stimulates the ectoderm to thicken, forming a neural plate.
The neural plate folds in on itself to create the neural fold or neural groove.
Continued folding results in a hollow tube known as the neural tube, which eventually differentiates into the CNS.
Neural Crest Cells: Some cells roll off during this process and migrate throughout the body to form teeth, craniofacial bones, skin pigmentation, and other structures.
Somites: Masses of mesoderm cells located alongside the notochord which eventually become the vertebrae and skeletal muscles (forming the axial skeleton).
Embryonic Membranes and Support Structures
Amnion: The innermost layer of the membrane surrounding the embryo. It secretes amniotic fluid, which serves as a water cushion to protect the embryo.
Amniotes: Animals that possess an amnion, including reptiles, mammals, birds, and marsupials.
Anamniotes: Animals that lack an amnion, as the surrounding water environments act as a natural cushion (e.g., frogs and fish).
Chorion: The outermost layer of the embryonic membrane.
Placental Mammals: It forms the fetal half of the placenta, which is the platform for the exchange of gases, nutrients, and waste.
Egg-laying Animals: It acts as a membrane for gas exchange located just underneath the egg shell.
Allantois: A sac that buds off of the archenteron and is responsible for waste storage and disposal.
Placental Mammals: It transports waste to the placenta, eventually becomes the umbilical cord, and forms the urinary bladder in adults.
Egg-laying Animals: It initially stores uric acid and later fuses with the chorion to assist with gas exchange.
Yolk Sac:
Placental Mammals: Does not actually contain yolk. It functions temporarily until the placenta is fully formed, providing early nutrients and serving as the primary site of first blood cell formation.
Egg-laying Animals: Contains yolk and supplies all necessary nutrients required by the developing embryo.
Reproductive Strategies and Developmental Influences
Reproductive Methods:
Oviparity: Offspring develop in eggs that hatch outside of the mother's body. There is no placental connection (e.g., birds, fish, and reptiles).
Viviparity: Offspring develop inside the mother's body and receive nourishment via a placental connection (e.g., mammals).
Ovoviviparity: Offspring develop in eggs that hatch within the mother's body. There is no placental connection, and embryos rely entirely on the yolk sac for nutrition (e.g., some snakes and amphibians).
Internal Fertilization: Animals that utilize internal fertilization typically have more complex anatomies, higher rates of reproductive success, and produce fewer gametes.
Embryonic Induction: This occurs when organizers secrete chemicals that influence what neighboring cells will become in the future.
Homeotic Genes: These genes decide which part of the embryo develops into what structures by regulating the formation of body axes and body structures in the proper location during early development.
Homeobox: A common DNA sequence that is homologous across different organisms and contains homeotic genes.
HOX Genes: A specific subset of homeotic genes responsible for the anterior-posterior (head-to-tail) positioning of body parts. They act as a Master Controller, turning different gene expressions on and off.
Egg Cytoplasm Determinant: If the egg cytoplasm is unevenly distributed (establishing animal and vegetal poles), an axis is created that influences how the embryo divides during cleavage.
Apoptosis: Programmed cell death essential for normal development. In the fetus, it removes webbing between fingers; in adults, it helps prevent cancer.
Temperature-dependent Sex Determination: In some reptiles, sex is determined during development by ambient temperature rather than chromosomes.
Pattern I: Development results in males at cold temperatures and females at warm temperatures (e.g., turtles).
Pattern II: Development results in females at both low and high temperatures, while males are produced at intermediate temperatures (e.g., crocodiles).