Edorium J Anat Embryo 2016;3:17–27. Danowitz et al. 17 www.edoriumjournals.com/ej/ae REVIEW ARTICLE PEER REVIEWED | OPEN ACCESS A combined approach of teaching head development using embryology and comparative anatomy Melinda Danowitz, Hong Zheng, Adriana Guigova, Nikos Solounias ABSTRACT the evolutionary changes of many structures allows for a greater understanding of the Many aspects of human head embryology reflect human embryology, and removes the need for its evolutionary development. The pharyngeal memorization of seemingly complex processes. arches, a major component of head development, A link to comparative evolutionary anatomy originally functioned in filter feeding and provides context to the purpose and morphology vascular exchange, which is why each arch of primitive structures, and clarifies several has associated vasculature and muscles. The issues in human head development. primitive tongue had few-associated muscles and was responsible for simple movements; the Keywords: Anatomy education, Embryology, Head human tongue evolved post-otic somites that and neck, Pharyngeal arches migrate to the tongue and develop the majority of the tongue musculature. These somites originate How to cite this article outside the tongue, and the motor innervation therefore differs from the general and special Danowitz M, Zheng H, Guigova A, Solounias N. A sensory innervation. In the primitive condition, combined approach of teaching head development the trapezius and sternocleidomastoid belonged using embryology and comparative anatomy. to a single muscle group that were involved in Edorium J Anat Embryo 2016;3:17–27. gill movements; they separate into two muscles with the reduction of certain skeletal elements, but retain the same innervation. Examining Article ID: 100011A04MD2016 ********* Melinda Danowitz1, Hong Zheng2, Adriana Guigova3, Nikos doi:10.5348/A04-2016-11-RA-3 Solounias4 Affiliations: 1BA, Medical Student, Anatomy, New York In- stitute of Technology College of Osteopathic Medicine, Old Westbury, NY, USA; 2DO, Resident, Vascular Surgery, Phila- INTRODUCTION delphia College of Osteopathic Medicine, Philadelphia, PA, USA; 3DO, Fellow, Hematology/oncology, North Shore- Long The development of the human head is of critical Island Jewish, New Hyde Park, NY, USA; 4PhD, Professor, Anatomy, New York Institute of Technology College of Os- importance to the medical students, as a complete teopathic Medicine, Old Westbury, NY, USA. understanding is necessary to comprehend pathologies Corresponding Author: Nikos Solounias, 8000 Northern such as cleft lip and palate, Treacher Collins syndrome, Boulevard, Old Westbury, NY, USA, 11568; Email: nsol- and DiGeorge syndrome, among many others. These [email protected] pathologies have been studied in mice in addition to humans, allowing for the establishment of genetic homologies [1–3]. The embryology of the human head is Received: 06 February 2016 strongly related to the evolutionary history of vertebrate Accepted: 04 March 2016 head and neck. We believe a link to comparative Published: 16 April 2016 evolutionary anatomy is needed, as it provides context to Edorium Journal of Anatomy and Embryology, Vol. 3; 2016. Edorium J Anat Embryo 2016;3:17–27. Danowitz et al. 18 www.edoriumjournals.com/ej/ae the original purpose and development of early structures. combined tissues from all three germ layers. The sections An AAMC 2000 Graduation Survey revealed that 73% of this study are the components of the head. Each is of respondents never took a premedical comparative treated here separately but they all interact and form an anatomy course [4]. The American Association of integrated whole. Many of the interactions take place Medical Colleges and the Howard Hughes Medical simultaneously. Institute recommended that evolutionary biology should be included in medical education [5]. Without BRAIN, HEART, PHARYNX, AND an understanding of the evolutionary history, head NOTOCHORD embryology can be an overly complex topic that involves The head begins to form with interactions between a great deal of memorization. Furthermore, anomalous four basic early structures: brain, heart, pharynx and anatomical presentations often reflect the typical notochord (Figure 1). The brain forms as an involuting morphology in more primitive animals; studies of human set of neural folds from the rostral end of the neural biliary tract and limb musculature variations revealed tube. The original embryological external surface of the that the atypical human structures resembled the normal neural tube will ultimately form the internal folds and condition seen in other animals [6, 7]. ventricles of the brain. The early heart develops anterior The study of evolutionary biology relates not only to to the brain. As development progresses, the heart slowly anatomy and embryology, but also to medicine in general. swings posteriorly and many of its arteries will become At the molecular level, evolution helps biologists study incorporated into the pharynx as the pharyngeal arch genetic mutations that allow viruses to infect multiple arteries (Figure 1). Thus, the proximal heart circulation is species, or how pathogens gain virulence [8]. Darwinian medicine also facilitates comprehension about natural selection, and which aspects of the human body are vulnerable to, or protected against disease [5]. Natural selection explains malarial resistance in sickle-cell trait carriers, and can potentially clarify associations between breast cancer and birth control [9]. Studies of medical education curricula in the UK as well as in North America demonstrated an overall need for the incorporation of evolutionary biology into medical courses [10, 11]. The evolutionary origin of the skull, face, and neck has been extensively studied from a scientific standpoint [12– 15]. For example, studies involving transplanting avian embryonic cells and tracking the differentiation of cranial tissues gave insight into the fate of mesodermal and neural crest cells [16]. We aim to link human head embryology with evolutionary history at a level understandable for medical students. We, therefore, provide a brief description of the evolutionary history of each process involved in head development, to provide context to the embryonic structures and ultimate derivatives, and to help students understand why the human anatomy develops from the early embryo the way it does. We currently provide a comprehensive summary of Figure 1: Internal and external view of the developing head the development of the human head. The aim for this and neck at week 4. (A) Ventral view depicting the early heart, article is to be used as a supplement to the various medical maxillary and mandibular processes of arch 1, arch 2, and the oropharyngeal membrane with several perforations. (B) Cross embryology textbooks. We believe the abbreviated yet sectional view showing the relationship between the folding comprehensive summary of head and neck embryology, neural groove, pharyngeal arch contents, and notochord. The supplemented with a section describing the comparative otic placode is depicted before penetrating the head to form the evolutionary anatomy, will enable medical students to otic vesicle. The pharyngeal arch contains an associated artery, better understand the development of the head. vein, nerve, muscle, and cartilage, all of which are capped by an intramembranous ossification. The dorsal aorta is initially a THE GENERAL DEVELOPMENT OF THE bilateral structure with the notochord positioned at the midline; the aortae fuse to form a single vessel during week 4. (C) Lateral HUMAN HEAD view representing the developing heart, head, and neck. Arch 1 The head is formed by the interaction of the three has a dorsal and ventral component, separated by the temporal- basic tissues: ectoderm, mesoderm and endoderm. When mandibular hinge joint (TMJ). (D) Internal view emphasizing the head is fully differentiated, its parts are the result of the interrelationship between the pharyngeal arches and arterial branches, as well as the notochord and developing brain. Edorium Journal of Anatomy and Embryology, Vol. 3; 2016. Edorium J Anat Embryo 2016;3:17–27. Danowitz et al. 19 www.edoriumjournals.com/ej/ae closely related to the head. Pharyngeal, or branchial arches processes (maxillary and mandibular processes) whose are 6 units of complex tissues aligned in a sequence. The derivatives are ultimately connected by a hinge joint pharynx has pouches and clefts along its length. A cleft- (temporomandibular joint) (Figure 1). The upper jaw pouch unit alternates with an arch. The clefts (externally) (maxilla) is not derived from the first arch pharyngeal and the pouches (internally) correspond to each other cartilages. The pharynx is an internal space around which and are separated by a membrane (Figure 2). Cleft 1 is the arches appear to wrap around. The arches and the the only cleft that persists in humans; the first cleft and cleft-pouch units constitute the wall of the pharynx. The pouch fuse to form the tympanic membrane of the ear. median area is the pharyngeal floor, which constitutes All other clefts are obliterated by the down growth of the mucosal surface, but not the internal muscles, of the the platysma muscle from arch 2. Each pharyngeal arch tongue [17]. includes an associated artery, vein, a branchiomeric The anterior part of the pharynx connects to an cranial nerve, muscles, cartilage, and neural crest cells. external tunnel, the stomodeum, which forms under the Pharyngeal arch 1 is the