Page 161 - 86 human physiology part-2

Chapter 19

Infancy

Infancy is the period that follows the neonatal period and includes the first two years of life.
During this time tremendous growth, coordination and mental development occur. Most infants learn to
walk, manipulate objects and can form basic words by the end of infancy. Another characteristic of
infancy is the development of deciduous teeth.

Deciduous Teeth

Deciduous teeth, otherwise known as milk teeth, baby teeth, or primary teeth, are the first set of
teeth in the growth development of humans and many other animals. They develop during the
embryonic stage of development and erupt - become visible in the mouth - during infancy. They are
usually lost and replaced by permanent teeth, but in the absence of permanent replacements, they can
remain functional for many years. (Concise)

Deciduous teeth start to form during the embryo phase of pregnancy. The development of
deciduous teeth starts at the sixth week of development as the dental lamina. This process starts at the
midline and then spreads back into the posterior region. By the time the embryo is eight weeks old,
there are ten areas on the upper and lower arches that will eventually become the deciduous dentition.
These teeth will continue to form until they erupt in the mouth. In the deciduous dentition there are a
total of twenty teeth: five per quadrant and ten per arch. The eruption of these teeth begins at the age of
six months and continues until twenty-five to thirty-three months of age. The first teeth seen in the
mouth are the mandibular centrals and the last are the maxillary second molars.

The deciduous dentition is made up of centrals, laterals, canines, first molars, and second molars;
there is one in each quadrant, making a total of four of each tooth. All of these are replaced with a
permanent counterpart except for the first and second molars; they are replaced by premolars. These
teeth will remain until the age of six. At that time, the permanent teeth start to appear in the mouth
resulting in mixed dentition. The erupting permanent teeth causes root resorption, where the permanent
teeth push down on the roots of the deciduous teeth causing the roots to be dissolved and become
absorbed by the forming permanent teeth. The process of shedding deciduous teeth and the replacement
by permanent teeth is called exfoliation. This will last from age six to age twelve. By age twelve there
are only permanent teeth remaining.

Deciduous teeth are considered essential in the development of the oral cavity by dental
researchers and dentists. The permanent teeth replacements develop from the same tooth bud as the
deciduous teeth; this provides a guide for permanent teeth eruption. Also the muscles of the jaw and the
formation of the jaw bones depend on the primary teeth in order to maintain the proper space for
permanent teeth. The roots of deciduous teeth provide an opening for the permanent teeth to erupt
through. These teeth are also needed in the development of a child’s ability to speak and chew their
food correctly.

Female

For females puberty is caused by alterations in brain functions that result in an increase in the
secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Increased levels of
GnRH stimulates the secretion of pituitary gonadatrophins FSH and LH causing follicle development
and estrogen secretion. Estrogen is responsible for accessory sex organs and secondary sex

376 | Human Physiology