Understanding Human Anatomy and Pathology

An Evolutionary and Developmental Guide for Medical Students

Rui Diogo, Drew M. Noden, Christopher M. Smith, Julia Molnar, Julia C. Boughner, Claudia Alexandra Amorim Barrocas, Joana Araujo Bruno

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781498753913
• Edition: 1
• Topic: Medicina
• Subtopic: Teoría, práctica y referencia médicas

Understanding Human Anatomy and Pathology: An Evolutionary and Developmental Guide for Medical Students provides medical students with a much easier and more comprehensive way to learn and understand human gross anatomy by combining state-of-the-art knowledge about human anatomy, evolution, development, and pathology in one book.

The book adds evolutionary, pathological, and developmental information in a way that reduces the difficulty and total time spent learning gross anatomy by making learning more logical and systematic. It also synthesizes data that would normally be available for students only by consulting several books at a time.

Anatomical illustrations are carefully selected to follow the style of those seen in human anatomical atlases but are simpler in their overall configuration, making them easier to understand without overwhelming students with visual information. The book's organization is also more versatile than most human anatomy texts so that students can refer to different sections according to their own learning styles.

Because it is relatively short in length and easily transportable, students can take this invaluable book anywhere and use it to understand most of the structures they need to learn for any gross anatomy course.

Chapter 1
About the Book

 

 

Anatomy is the oldest formal discipline in medicine, and this book builds upon the exceptional contributions made by outstanding scholars. Among them, we want to pay special tribute to some of the most accurate and most frequently used atlases and dissector manuals for human gross anatomy: Netter’s Atlas of Human Anatomy (6th edition, 2014), Gray’s Atlas of Anatomy (2nd edition, 2014), Grant’s Atlas of Anatomy (13th edition, 2012), and Grant’s Dissector (15th edition, 2015). We are particularly grateful to the authors of these books, and their predecessors, for their remarkable and crucial contribution to the learning and teaching of human gross anatomy. The present book is a way for us to acknowledge our deep gratitude for the amazing work done by these authors and by the authors of similar atlases and textbooks.

To maintain clarity and narrative flow for the students, we have not included citations and parenthetical asides in this book. We are, of course, not trying to deny credit to other authors, as we just acknowledged some of these books, but we provide at the end of this book a brief, summarized list of suggested readings related to the content of each chapter. In fact, the main goal of this book is to provide medical students a unique, and much easier and comprehensive way of learning and understanding human gross anatomy by combining state-of-the-art knowledge about human anatomy, evolution, development, and pathology. However, the purpose is not to provide a huge amount of information that might be interesting for students, but that lies outside of the aspects they will need to learn and understand for their gross anatomy courses and their board exams in particular. The purpose is instead to add the evolutionary, pathological, and developmental information in a way that will reduce the difficulty of, and amount of total time spent in, learning gross anatomy, by making learning more logical and systematic. Also, as the plan is to synthesize data that would normally be available for students only by consulting several books focused on human anatomy, evolution, development, and pathology at the same time, this book was put together in such a way that it is relatively short and easily transportable. So, students can take the book anywhere and use it to learn and understand all the structures that they have to normally learn for any gross anatomy course, with the exception of the internal and sexual organs, as explained above.

Therefore, to describe anatomical structures, we use the brief, direct writing style of “dissector” textbooks frequently used by students, highlighting in bold all the anatomical structures that are normally mandatory for medical students to know, and provide an index where all these structures, as well as other key concepts/terms also highlighted in bold in the text, can be easily found. Concerning the illustrations, we carefully selected a set of anatomical illustrations that follow the style of those used in human anatomical atlases but that, in our opinion, are simpler in their overall configuration and sufficient in number to show and make it easier to better understand the anatomical structures highlighted in bold in the text, without overwhelming the students with visual information. This is particularly important because the uniqueness of this book is that it includes these high-quality anatomical illustrations plus a selective set of illustrations and schemes to go along with the evolutionary, developmental, and pathological parts of the text.

It is also important to emphasize that this book is also a way for us—all the authors of this book except Drew Noden—to pay tribute to the truly remarkable scientific contribution of Drew to the topics presented and discussed in the book. In fact, several parts of this book were adapted from the excellent notes and diagrams that he has meticulously prepared for the courses that he taught at Cornell University. Thank you, Drew, for this and for your amazing career, as well as your kindness.

Chapter 2
Introduction

2.1 Conserved Features of Vertebrate Embryology

Most anatomical features of human embryos (Plate 2.1) are common to all vertebrate embryos, based on a pattern that emerged over 400 million years ago. Examples include the notochord, a collagenous rod that extends the length of the body axis, and somites, segments of mesoderm arranged on either side of the notochord. The tissues that will form these structures are produced by rearrangements of embryonic cells during a stage called gastrulation, when the embryo is less than two weeks old. These tissues are: (1) the ectoderm, which forms the skin and nervous system; (2) the mesoderm, which forms the musculoskeletal system and circulatory systems; and (3) the endoderm, which forms most of the digestive and respiratory systems. Very soon after gastrulation, organs begin to form. The first structures to appear are the primordia of the brain and spinal cord, backbone and muscles, digestive tube, and cardiovascular systems (Plate 2.2). These early organ systems will expand, a few additional ones added, and by the 10–12-mm stage, all organ systems are in place and most are functional. Which of these structures, based on their location and organization, resemble those found in the adult? For example, do you have somites and a notochord? If not, why are they present in all vertebrate embryos, including humans? The reason is phylogenetic constraint: We, humans, have an evolutionary history from our ancestors, and the organizational configuration that was laid down during early vertebrate evolution has remained with few modifications. The fact that embryos continue to adhere to this very ancient configuration is due to developmental constraint.

Somites are clusters of mesodermal cells that aggregate tightly together in a cranial to caudal sequence on both sides of the hindbrain and spinal cord. Later, these blocks rearrange to form vertebrae and ribs, all the voluntary muscles of the body and limbs, and most axial connective tissues (see Section 6.1). Equally important, somites establish a segmental pattern that is imposed on the spinal cord and peripheral nerves that emerge from it. In the embryo, segmentation allows for the formation of repeated sets of nearly identical tissues, thereby simplifying the “blueprints” necessary to generate each segment. Secondarily, each segment receives signals that give it its unique spatial identity, allowing, for example, cervical and thoracic vertebrae to have different shapes. Segments also define spatial compartments that allow the nerves, muscles, and skeletal structures to contact each other and form stable relationships that persist in the adult. The clear segmentation of adult structures such as vertebrae and the peripheral nerves that emerge from the spinal cord reflects the phylogenetic and developmental constraints established by our ancestors and maintained during earlier developmental stages. Of course, many new structures have been added over the course of vertebrate evolution—limbs are a good example. As limbs evolved, the embryo co-opted nearby progenitor populations derived from somites to form appendicular muscles, which do not maintain their cranio-caudal segmented pattern. However, since the basic pattern of limb musculoskel-etal organization was established, it has remained largely unchanged during early stages of development.

Plate 2.3 summarizes the key anatomical features that define the common embryonic body pattern of vertebrate embryos. These features include: a dorsal, hollow neural tube with regional specializations; a notochord located in the midline, immediately ventral to the neural tube; a series of segmentally arranged somites that later form axial musculoskeletal tissues; a ventral gut tube that forms the lining of organs associated with respiration and digestion; a coelomic cavity that later becomes subdivided into pleural, pericardial, and peritoneal cavities; a ventral heart tube from which blood flows through a series of segmentally arranged arteries; and a body wall that surrounds the embryo except at the umbilicus. The developing embryo is well protected. In mammalian embryos, the first organ system to become functional is the placenta. The placenta is composed of both maternal uterine tissues and membranes derived from embryonic tissues, which establish intimate associations with the lining of the maternal uterus. The placenta is an amazingly complex structure capable of serving functions later carried out by the neonatal liver, kidneys, lungs, endocrine, and digestive systems. While it is usually selective about what molecules are allowed to pass into the embryonic circulation, it is not impermeable. Both acute and chronic exposure to complex environmental chemicals, some viruses, and physical agents presents potential risks to the embryo, and many of these-alone or in combination—have not been rigorously assessed. Additionally, agents that disrupt maternal endocrine and metabolic functions can have severe secondary consequences for embryonic and placental development.

2.2 Causes and Mechanisms of Developmental Pathologies

While we have emphasized the common—constrained—aspects of early development, each species also has unique embryonic and placental structures and functions. Genetic and metabolic activities in the embryo are also highly specialized and stage-specific and, especially at early stages when organ systems are being established, are particularly vulnerable to disruptions. These can manifest as structural alterations, often apparent at birth, but also as more subtle metabolic changes that carry forward into and through adulthood and even subsequent generations. Genetic factors (including polymorphisms and mutations) establish limits and functional boundaries for metabolic pathways and signaling cascades. Within this range, however, environmental influences that skew maternal, placental, or embryonic/fetal function can have substantial and irreparable effects. For the purposes of understanding both normal embryology and developmental abnormalities, the extent to which extrinsic influences-anything outside of the embryo/fetus—affect the embryo or placenta must be considered.

Developmental abnormalities can result from disruptions of cell proliferation, cell movement, cell differentiation, cell survival, or morphogenesis. Abnormalities arising during prenatal stages of development are commonly called congenital abnormalities or birth defects. Some abnormalities may not show clinical signs until the affected systems become functional (e.g., sensory systems, locomotion, reproduction) or stressed (metabolic, endocrine), or they may compromise cell functions in ways that are not recognized until adulthood; for example, increasing risks of cancer or heart disease. Clinically relevant abnormalities are present in 3% of live births, or ~320 babies in the United States every day, and an equal number are subsequently detected prior to the age of one year (data available from the Center for Disease Control website). About half of all neonatal deaths are attributable to identified developmental problems, ranging from gross structural abnormalities to those that are more subtle, such as low birth weight and placental insufficiency. Defects frequently are part of a syndrome, a set of abnormalities that often appear together, and are assumed to result from the same genetic variation or environmental insult. Birth defects contrast with anatomical variations, which are seen in the karyotypically/genetically “normal” population: You will surely see such variations in the human cadavers you dissect in your gross anatomy course.

All structures and functions in embryonic and placental cells and tissues are potentially subject to disruption by outside factors, broadly categorized as teratogens. In the past, emphasis was placed on chemicals or physical factors (e.g., radiation, heat) that reduced viability and caused morphological abnormalities. Now the range of disrupters is much broader, and includes nutritional factors as well as the collective effects of multiple agents, each of which alone is not significantly problematic. It is estimated that 40%–70% of human embryos die and are aborted during the first four weeks of development, mostly during the gastrulation stage when intraembryonic reorganizations, activation of several thousand genes, and initial apposition with uterine epithelium and placental formation all occur simultaneously over a period of less than two days. In one study, it was found that at least half of early aborted embryos have major genetic lesions, such as lost, broken, or extra chromosomes.

Traditional explanations of the causes of defects focus on single-gene mutations or environmental toxins. However, while these factors may be important contributors, most developmental defects cannot be adequately explained by them. We now know that an optimal outcome—a healthy organism, organs, tissues, etc.—requires cooperative and integrated spatial and temporal regulation of every ongoing process within all cells of the embryo. Each of these processes is the result of interactions involving many genes and their products with many elements of their environment and nutrition. Teratogens can target the embryo directly or primarily affect the mother or the placenta. Every cell in each of these sites continually monitors and responds to all aspects of the chemical and physical world around it, using a combination of receptors, channels, pores, and endocytotic vesicles, and then integrates the complex signals it receives through multiple interconnected metabolic and gene-regulatory pathways. These pathways determine whether the cells divide or not, survive or die, maintain the same phenotype or change, and move or remain stationary. Some extrinsic factors (e.g., radiation, some steroid analogs) act directly upon DNA (deoxyribo-nucleic acid, the molecule that stores hereditary information) or other cell structures, but most interfere with the metabolic and signal transduction pathways that integrate and regulate cell activities.

2.3 A Holistic Approach to Anatomy

The points mentioned above emphasize the importance of a holistic approach to developmental and gross anatomy and pathology. Understanding gross a...