Prenatal and Postnatal Growth and Development of Cranial and Facial Structures
This document provides an overview of head and neck development from early embryogenesis through postnatal growth. It begins with definitions of growth and development and introduces concepts like fertilization, cleavage, formation of the germ layers and notochord.
Fundamental Concepts of Embryology
Embryology is the study of development of an individual before birth. Every individual spends the first nine months (266 days or 38 weeks to be exact) of its life within the womb (uterus) of its mother. During this period it develops from a small one-celled structure to an organism having billions of cells, where numerous tissues and organs are formed and come to function in perfect harmony.
The prenatal period can be divided into three distinct stages:
- Period of ovum
- Period of embryo: During these two months we call the developing individual an embryo. The most spectacular of these changes occur in the first two months; the unborn baby acquires its main organs and just begins to be recognizable as human.
- Period of fetus: From the third month until birth we call it a fetus.
Defining Growth and Development
To understand these processes, one must look at the scientific definitions provided by various experts.
The Concept of Growth
- "An increase in size or number." - Profitt (1986).
- "Increase in size, change in proportion & progressive complexity" (KROGMAN).
- "Change in any morphologic parameter which is measurable" (MOSS).
- "Growth signifies an increase, expansion or extension of any given tissue." - Pinkham (1994).
The Concept of Development
- "Development is a progress towards maturity" – Todd (1931).
- "Development connotes a maturational process involving progressive differentiation at the cellular and tissue levels" – Enlow.
Early Prenatal Development
Development begins with fertilization, which is the fusion of sperm and ovum resulting in the formation of a zygote. This process typically takes place in the ampulla of the uterine tube. Following fertilization, the egg undergoes rapid division through a stage called the Morula. Eventually, fluid seeps into the morula to form a Blastocyst.
Implantation begins at day 7, involving the embedding of the blastocyst into the uterine lining. During this stage, the trophoblast forms syncytial trophoblast which erodes into the endometrium, and lacunae and primary villi are formed to create placental tissues.
The following table outlines the viability of gametes essential for this process:
| Gamete Type | Viability Duration |
|---|---|
| Oocyte | 12-24 h |
| Sperm | 12-48 h |
Development of Craniofacial Structures
The development of the head and neck involves the pharyngeal apparatus, which consists of pharyngeal arches, pharyngeal pouches, and pharyngeal grooves/clefts. These structures lead to the formation of the face, tongue, and jaws.
Specific developmental milestones include:
- Development of the upper lip and upper jaw.
- Development of the lower lip, lower jaw, philtrum and external ears.
- Development of the nose, eyes, and palate.
- Development of the tongue and associated oral structures.
Postnatal Growth and Factors
Postnatal growth continues the progression towards maturity. This phase includes the growth of the cranial vault, cranial base, and the nasomaxillary complex or maxilla and mandible. Various theories explain these changes, such as the Servosystem Theory / Cybernetic Theory by Petrovic and the Etiological basis of malocclusion theories.
Key aspects of postnatal study include:
- Methods of studying growth and principles of growth.
- Identification of the growth spurt.
- Process of ossification.
- Environmental and genetic factors affecting growth.