The reproductive sequence of fertilization, implantation, development, and birth is the activity in which an organism develops from a single-celled zygote to a multi-cellular organism.

The early stages of embryonic development begin with fertilization. Fertilization is the process in which haploid gametes (sperm and ovum) fuse to form a diploid cell called a zygote. To ensure that each zygote has the correct number of chromosomes, only one sperm can fuse with one egg. For fertilization to occur, sperm must be deposited in the vagina so that it can swim through the cervix and into the uterus, and then up into the fallopian tube where fertilization of the ovum happens. After fertilizing the developing cell that is now called an embryo, it repeatedly divides several times until a solid ball of cells called morula forms. As the cells in the morula continue to divide, they begin to move towards the outer edges of the ball, until it becomes a hollow ball of cells called the blastula. 

Upon reaching the uterus, blastula embeds itself in the thickened endometrium (lining of the uterus) by the process called implantation. Once it implants itself in the uterine lining, pregnancy begins. 

The next stage in embryonic development is the formation of the body plan. During gastrulation, the blastula undergoes a process called gastrulation, in which the three germ layers form, mainly, the ectoderm, mesoderm, and endoderm. The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to columnar cells and internal organs. 

Gastrulation leads to the formation of the three germ layers that give rise to the different organs in the animal body, a process called organogenesis. The embryo becomes the fetus that grows inside the uterus and further develops and continues until birth. Different organs take part in the development of the many organ systems of the body. Sometimes full development, as in the lungs, continues after birth. 

During pregnancy, the unborn baby (fetus) depends on its mother for nourishment and oxygen. Since the fetus doesn’t breathe air, his or her blood circulates differently than it does after birth. At birth, significant changes take place. The umbilical cord is clamped, and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of air, the lungs start to expand, and the lungs’ ducts both close. The baby’s circulation and blood flow through the heart now function like an adult’s. 

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Key Points

• Fertilization involves the union of the sperm and egg cells in the fallopian tubes.

• After fertilization, cell division starts to form the developing embryo in stages, including the morula and then the blastula.

• Implantation of the embryo takes place in the endometrium of the uterus.

• Organogenesis is the formation of organs from the germ layer, which gives rise to specific tissue types. 

• The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to columnar cells and internal organs.

• The embryo becomes the fetus that grows inside the uterus and further develops and continues until birth.

• The fetus depends on its mother for nourishment and oxygen.

• At birth, the first breaths of air, the lungs start to expand, and the lungs’ ducts both close; the circulation and blood flow through the heart now function like an adult’s. 

Key Terms

zygote: a fertilized egg cell

fertilization: The act of fecundating or impregnating animal or vegetable gametes

implantation: The embedding of the fertilized ovum into the uterine wall

fetus: an unborn offspring of a mammal

umbilical cord: is a narrow tube-like structure that connects the developing baby to the placenta

ovum: the female reproductive cell

morula: is an early-stage embryo consisting of 16 cells

blastula: an animal embryo at the early stage of development when it is a hollow ball of cells

gastrulation: single-layered blastula is reorganized into a multilayered structure known as the gastrula

organogenesis: the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism.