Embryology: Egg Types and Development

• This study material covers the subject of embryology

Embryology

• Embryology is the science that studies the formation and development of embryos
• Vertebrates go through similar developmental stages
• Variations in embryonic development occur due to differences in egg cell types
• The rate of cleavage, which is the cell division that forms the embryo, is inversely related to the amount of yolk present

Types of Eggs

• Eggs are classified by yolk amount and distribution

Classification by Yolk Amount

• Alecithal eggs have no yolk and are commonly found in mammals and invertebrates
• Oligolecithal eggs have small amounts of yolk, as seen in amphioxus
• Mesolecithal eggs have a medium amount of yolk, as seen in elasmobranches, fish, reptiles, and birds
• Polylecithal or megalecithal eggs have large amounts of yolk, also found in fish, reptiles, and birds

Classification by Yolk Distribution

• Isolecithal or homolecithal eggs have evenly distributed yolk and are found in microscopic mammalian eggs
• Telolecithal eggs have unevenly distributed yolk, often concentrated on one side, as found in fish, amphibians, reptiles, and birds
• Uneven yolk distribution causes polarity, creating an animal pole and a vegetal pole

Oviparity and Viviparity

• Oviparous animals lay eggs that contain enough yolk to nourish the developing embryo
• Young hatch fully formed in species with massive yolk, like birds
• Young hatch in a larval stage in species with less yolk, like frogs
• Viviparous animals give birth to live young
• Eggs are retained within the mother's body during embryonic development
• In viviparous sharks, the mother provides protection and oxygen while the egg stores nourishment, a condition known as ovoviviparity
• Placental mammals have embryos dependent on maternal tissues for nourishment and waste removal, a condition called euviviparity

Fertilization

• Internal fertilization is the union of egg and sperm inside the female's body
• This is common in viviparous vertebrates and when eggs are covered by an impenetrable shell
• External fertilization occurs outside the vertebrate's body
• The process is common in oviparous fish, frogs, and toads
• Male urodeles deposit a spermatophore, which the female retrieves to fertilize the eggs internally despite lacking a copulatory organ

Cleavage

• Cleavage is the mitotic division of the zygote into blastomeres and is the first stage of embryonic development

Types of Cleavage

• Determinate cleavage, also known as mosaic cleavage, is common in protostomes where the fate of cells is set early
• Each blastomere cannot develop into a complete embryo
• Indeterminate cleavage, also known as regulative cleavage, occurs when a cell has complete cytoarchitectural features
• Characteristic of deuterostomes where the original cell divides and each resulting cell can develop into a whole organism

Additional Types of Cleavage

• Holoblastic cleavage occurs with little yolk
• Four major cleavage types are observed in isolecithal and mesolecithal cells
• Radial cleavage occurs in deuterostomes
• Spindle axes are parallel or perpendicular to the oocyte's polar axis
• Spiral cleavage is conserved
• Group, Spiralia, includes annelids, molluscs, and sipuncula - First two cell divisions' result in four macromeres, or blastomeres
• Bilateral cleavage bisects the zygote into left and right halves, creating mirror images
• Divisions of blastomeres are complete and separate in bilateral holoblastic cleavage
• In contrast, blastomeres stay partially connected in bilateral meroblastic cleavage
• Rotational cleavage involves a normal first division and one daughter cell divides meridionally while the other divides equatorially
• Mammals display rotational cleavage with sparsely distributed yolk
  • Cells require immediate implantation for nutrients
• Equal holoblastic cleavage has the egg and subsequent cells splitting into approximately equal parts, as in in amphioxus
• Unequal holoblastic cleavage results in unequal daughter cells as in a frog embryo
• Micromeres form at the animal pole and macromeres at the vegetal pole
• Meroblastic cleavage occurs with large amounts of yolk
• Cell undergoes partial cleavage including discoidal and superficial types
• Discoidal cleavage involves cleavage furrows that do not penetrate the yolk
• Embryo forms a blastodisc on top of the yolk
  • Commonly found in monotremes, birds, reptiles, and fish
  • Layer of cells in contact with the yolk is known as the syncytial layer
• Superficial cleavage involves mitosis without cytokinesis, resulting in a polynuclear cell
• Nuclei migrate to the egg's periphery, and plasma membrane partitions them into individual cells
  • Occurs in arthropods with yolk (centrolecithal egg cells) in the cell center
  • This cleavage type can promote synchronicity in timing
• In meroblastic cleavage, division occurs only in the living matter on one side of the yolk mass, as in birds and reptiles

Stages of Development

• Blastula is an embryo shaped like a hollow ball
• Walls consist of single layer of cells
  • Cavity is called the blastocoel
• Blastula of polylecithal eggs is a disc of cells lying on the yolk
• Gastrula forms when one side of the blastula pushes inward
• Partially obliterates the blastocoel
• Forms a cuplike stage
• New cavity is called the archenteron - Opening to the outside is the blastopore
• Gastrulation is the dynamic process of cellular movements
• Endoderm, mesoderm, and notochord cells of the blastula migrate, generating primary germ layers for organ formation
• Cell migrations are formative movements
• Rapid cell proliferation provides the supply of additional cells for these events

Germ Layers

• Ectoderm forms the epidermis, epithelial lining of mouth and anus, cornea and lens of eye, nervous system, sensory receptors, adrenal medulla, and tooth enamel
• Mesoderm forms the notochord, skeletal system, muscular system, muscular layer of stomach and intestine, excretory system, circulatory and lymphatic systems, reproductive system, dermis of skin, lining of body cavity, and adrenal cortex
• Endoderm forms the epithelial lining of the digestive and respiratory tracts, urethra, urinary bladder, reproductive system, liver, pancreas, thymus, and thyroid and parathyroid glands

Neurulation

• Neurulation establishes the central nervous system
• Central nervous system derives from the neural ectoderm where the notochord and segmental mesoderm are located
• Begins near the blastopore with the cephalic end beyond the notochord
• Neural plate forms and differentiates
• Marginal band elevates forming pair of neural folds
• These bound a neural groove
• Anterior end of the neural groove becomes the widest, being the future brain while the rest is the future spinal cord
• Neural plate sinks, the neural tube folds grow, and a process of closure near the caudal end of the brain occurs
• The formations result in the neural tube, the central nervous system is ectodermal, dorsally located, and hollow
• The cavity becomes the neurocoel
• Neurocoel becomes ventricles of the brain and the central canal of the spinal cord