Maturation, the growth and transformation of a single-celled zygote to a multicellular organism, has fascinated humans for centuries. In only a matter of months, a single cell can develop and mature to a complex organism. Although the maturation period for each species is different, the result is the same: a complex living organism.

No matter the species, the early stages of matu­ration are common to almost all animals. The first step is fertilization, in which the male and female sex cells or gametes fuse, creating the single-celled zygote. In species living in an aquatic environment, external fertilization is the usual method, in which the female deposits eggs into the environment to be immediately fertilized by the male. This method of fertilization usually requires courtship and environ­mental cues to be sure a male is present for fertili­zation, as well as to prevent the destruction and drying out of the eggs. In dry environments, the only way for sperm to reach the egg is by internal fertilization. By this method, sperm are deposited in or around the reproductive tract of the female. After fertilization, the newly formed zygote creates a fertilization envelope to prevent polyspermy, or union with more than one sperm cell.

Two distinct development modes begin after fertil­ization, protostome development and deuterostome development. Examples of common organisms that feature protostome development are molluscs and arthropods, while chordates and echinoderms are common examples of organisms that feature deu- terostome development, among many others. From this point, maturation begins with the first cell divi­sions, known as cleavage. Unlike normal cell division, cleavage divisions are rapid, with no time for cell growth between each division. This process virtually partitions off the single-celled zygote into smaller cells called blastomeres, each complete with its own nucleus. In deuterostome cleavage, the cells divide in a radial pattern, in which the planes are parallel or perpendicular to the vertical axis of the embryo. Cleavage in deuterostomes is also indeterminate, meaning these new cells are not yet fated and can form an entire organism if isolated. In protostome development, the cleavage pattern is spiral, in which the planes of division are diagonal to the vertical axis of the embryo. Cleavage in protostomes is also deter­minate, in that these new cells are already fated and cannot form a whole organism when isolated.

This cleavage continues, forming a multicellular ball of blastomeres, known as a morula. Eventually, the number of blastomeres grows to form a single­layered ball of cells known as a blastula, featuring a hollow cavity in the center, known as a blasto- coel. In humans and mammals specifically, this stage forms a blastocyst, with the key difference being the presence of an inner-cell mass within the blastocoel, as well as an outer epithelial lining to the blastocyst, known as the trophoblast. Also, in many other species, different concentrations of yolk, stored nutrients for maturation, tend to off­set cleavage patterns. This is due to high concen­trations of yolk found at the vegetal pole of the blastula and low concentrations of yolk at the ani­mal pole. Thus the blastomeres at the animal pole tend to be smaller then the blastomeres at the vegetal pole. Yolk concentration can be so high that cleavage is hindered and even incomplete, a phenomenon known as meroblastic cleavage. Alternately, cleavage that is unimpeded by yolk and continues to completion is known as holoblas- tic cleavage. This uneven distribution of yolk is very distinct in different species and characterizes how they develop.

Although at these stages of maturation we start to see characteristic differences in each species, gastrulation generally follows cleavage blastulation. Gastrulation is the rearrangement of the blastula to form a gastrula consisting of two or three germ layers: the ectoderm and endoderm, with the meso­derm as the third and middle layer. Gastrulation also forms a primitive gut, known as the arch- enteron. This rearrangement allows the cells to interact in new ways and causes changes in cell shape, motility, and adhesion. One process of rear­rangement is invagination, in which the single­celled blastula buckles into the blastocoel to form a second layer. This invagination causes the direct formation of the archenteron as well as the blasto­pore. This blastopore becomes the primitive mouth in protostomes and the primitive anus in deuteros- tomes. A second mechanism is involution, in which cells “roll” over the lip of the blastopore into the interior of the embryo. The combination of these two processes forms the gastrula. The archenteron and coelom, the primitive body cavity, differ in deuterostomes and protostomes. In deuterostomes, the body cavity formation is described as entero- coelous, in which the mesoderm forms outpockets from the archenteron and forms the body cavity. In protostomes, the formation of the coelom is described as schizocoelous, in which mesoderm near the blastopore split and outpockets to form the coelom.

Organogenesis is characterized by the devel­opment of primitive organs, vessels, and body systems in the embryo. Each of the three germ layers of the gastrula gives rise to the beginnings of very specific organs throughout the body. The ectoderm, the outermost layer of the gastrula, develops into the epidermis (skin) and sense receptors, as well as a majority of the nervous system. The mesoderm, the middle layer of the gastrula, gives rise to the skeletal and muscular system, as well as the reproductive system, circu­latory system, and excretory system. The endo­derm, the innermost layer of the gastrula, forms the epithelial lining of the digestive and respira­tory tracts and the liver pancreas, as well as sev­eral glandular organs. One of the first organs to develop is the neural tube. The ectoderm thick­ens and pinches inward, forming the primitive spinal cord.

In mammals, all of these maturation processes occur during pregnancy, or gestation. The length of the gestational period directly correlates to the size of the growing organism. The typical human gestational period is about 40 weeks. However, the typical gestation period of a rodent may be only 21 days, while the gestation period of a cow averages about 270 days. Gestation in elephants can last as long as 600 days. In humans, pregnan­cies are usually split into three trimesters of about 3 months each. Organogenesis is usually com­pleted by the end of the first trimester. At this point, the embryo is no longer considered a gas- trula, but a fetus; fetuses average only 5 cm in length. In the second trimester, the fetus becomes more active within the womb and grows to approximately 30 cm. The final trimester results in birth of the child, or parturition. It is believed that hormone levels within the blood control labor, the process of birth. However, the mecha­nism behind this is not yet fully understood. Labor consists of three phases: the thinning and dilation of the cervix, the delivery of the baby, and the expulsion of the placenta.

Modern science has provided this insight into these complex processes, but this has not always been the understanding of maturation. As far back as 2,000 years ago, Aristotle proposed the idea of epigenesis, in which the animal develops from a relatively formless zygote. This theory, which was more accurate than most then believed, contrasted with the theory that prevailed to the 18th century, that of preformation. In preformation, it was believed that within the egg or sperm was a pre­formed miniature infant or homunculus and that this homunculus simply grew and matured within the womb until it was born. It wasn’t until the 19th century, with the invention of light micros­copy, that scientists were able to get a more accu­rate understanding of the complex process of maturation.

Christopher D. Czaplicki

See also DNA; Fertility Cycle; Gestation Period; Life

Cycle; Metamorphosis, Insect

Further Readings

Campbell, N. A., & Reece, J. B. (2005). Biology

(7th ed.). San Francisco: Pearson Education.

Ulijaszek, S. J., Johnston, F. E., & Preece, M. A. (1998). Human growth and development. Cambridge, UK: Cambridge University Press.