EMBRYOLOGY

Embryology, what exactly is embryology? It is the science concerning the development of an embryo form the fertilization of the ovum to the fetus stage. Fertilization occurs in a series of steps: contact between the sperm and egg, entry of sperm into the egg, fusion of egg and sperm nuclei, and activation of development .During fertilization, a sperm must fuse with and penetrate the female egg for a successful fertilization. Fusing is the easy part, but penetrating through the egg’s hard protective shell is a problem for sperm. Thus, sperm go through a process called the acrosome reaction. An acrosome reaction is the reaction that occurs in the acrosome of the sperm as it approaches the egg. The acrosome is a cap-like structure over the frontal (anterior) half of the sperm’s head. As the sperm approaches the zona pellucid (glycoprotein membrane) of the egg, which initiates the acrosome reaction, the memebrane surrounding the acrosome fuses with the plasma membrane of the sperm. The contents (surface antigens and enzymes to break through egg’s hard shell) are exposed allowing fertilization to occur. The cortical reaction occurs directly after the acrosomal reaction. It happens when a sperm cell fuses with the egg’s plasma membrane which alters the zona pellucid preventing other sperm from binding and entering the egg. It is the exocytosis of the egg’s cortical granules (secretory vesicles below the plasma membrane). When the sperm is in contact with the egg’s plasma membrane, calcium is released from storage sites in the egg triggering the fusion of cortical granule membranes with the egg plasma membrane. The wave of calcium surrounds the egg and a wave of cortical granule fusion results.

Cleavage is the first step in development for all multicellular organisms. It converts a single-celled zygote into a multicelled embryo via mitosis. It is the division of cells in the early embryo. The blastula is produced by mitosis of the zygote. A blastula is a ball of cells surrounding the blastocoel (fluid-filled cavity). As a result of rapidly dividing cells, their size decreases. However, it increases their surface area to volume ratio to increase allowing more efficient oxygen exchange between cells and their environment. The blastula receives RNA and information carrying molecules to start the differentiation of cells and early development.

Grastrulation occurs next. It is a series of cell migration to positions where they will form three primary cell layers: ectoderm (outer layer), endoderm (inner layer), and mesoderm (middle layer). This single-layered blastula is reorganized into a gastrula. The ectoderm forms tissues such as skin, hair, sweat glands, and epithelium. It also develops the brain and nervous system. The mesoderm forms structures associated with body movement and support. Mesoderm structures include muscles, cartilage, bone, and blood. It also forms kidneys and reproductive organs. Reproductive organs are also developed by the archenteron. The archenteron is the primitive gut that forms during gastrulation in the developing blastula. Lastly, the endoderm forms tissues and organs that aid in digestion and respiration. Endocrine structures such as the thyroid and parathyroid glands are formed by the endoderm. The liver, pancreas, and gall bladder are also developed by the endoderm.

Organogenesis is summed up by its name, the creation of organs. It is the process by which the ectoderm, endoderm, and mesoderm develop into internal organs. For humans, this process usually occurs between the third and eighth week in utero. The germ layers in organogenesis differ by three processes. These three processes are folds, splits, and condensation.  

Let’s take a closer look at fertilization in sea urchins.

Fertilization is external. Most sea urchins have their eggs free floating in the sea, but others keep them on their spines for protection. To prevent the sperm and egg from being washed away they have evolved mechanisms to bring the gametes together. When a sperm cell encounters an egg of the same species, components of the jelly coat bind to specific egg receptors in the plasma membrane. This triggers the release of calcium that facilitates fertilization.

In the sea urchin, early cell divisions are rapid. The proteins that are synthesized during cleavage utilize mRNA found in the cytoplasm provided by the mother. The first three cell divisions bisect the embryo equally; the fourth cleavage divides the cells in the top half equally, but those in the bottom half unequally. The cells continue to divide until the form the blastula.

A sea urchin embryo has ten cycles of cell division to make a single epithelial layer enveloping a blasteocoel. The embryo then begins grastrulation, a multipart process which dramatically rearranges and invaginizes cells to produce three germ layers.

The fertilized egg develops into a free-swimming blastula embryo in as little as twelve hours! The simple blastula transforms into a cone-shaped echinopluteus larva which has elongated arms, nutrients, a cilia to capture food particles. It may take several months for the larva to be fully developed. The larva sinks to the bottom of the sea after development is completed and metamorphoses into an adult in as little as one hour.