Читать книгу Human Developmental Biology - Danton Inc. O'Day - Страница 3

Chapter 1

An Introduction and Overview

Developmental biology is the field of biology that involves the study of how organisms grow and develop. The chapters in this book focus on human developmental biology by taking a linear approach. The book starts with the formation of sperm and eggs and finishes with the formation of specific tissues and organs. All of the major topics of developmental biology are addressed using well characterized and interesting examples.

The first part of the book begins with an overview of development and progresses to the origin and formation of the sex cells. Here we learn about factors (determinants) that determine the fate of the future gametes. After an overview of male and female anatomy, the processes of oogenesis (females) and spermatogenesis (males) are detailed to reveal how eggs and sperm form. The fundamental differences in structure and function of sperm and eggs will be discussed as well as the timing of the events. How each of these cells specialize (cell differentiation) at the cellular and molecular levels also will be detailed. The differentiation of eggs and sperm will provide the first examples of how cells specialize. There are an estimated 230 different types of cells in the body. Many examples of cell differentiation will be covered in subsequent chapters.

The gametes merge at fertilization to begin the processes of embryonic development. But fertilization isn't a random encounter between eggs and sperm with a chance result. Sperm are guided to the egg and, once there, a series of intercellular communications mediated by species-specific molecules guarantee that only the proper sperm will bind to and penetrate the egg. Fertilization is a highly controlled process. Often humans want to intervene in the process of fertilization to prevent it, so techniques of contraception have developed. Sometimes things go wrong and fertilization doesn't occur. In such cases, couples (and individuals) may opt for in vitro fertilization (IVF) which involves many of the new Assisted Reproductive Technologies (ART).

Successful fertilization, in vivo or in vitro, leads to the initiation of early embryonic development that begins with rapid divisions of the zygote called cleavage. Cleavage is a specialized type of mitosis that produces daughter cells called blastomeres. After a period of time the cleaving embryo becomes a multicellular blastocyst. The blastocyst can implant in the uterine wall to set up the placental relationship for further development. The human body is a complex of different organs, tissues and cell types that are arranged in a very precise spatial relationship. During embryonic development the body plan is established through the morphogenetic movements of gastrulation and neurulation. Gastrulation establishes the primary germ layers of the embryo: endoderm, mesoderm and ectoderm. While this is occurring, neurulation starts and ultimately results in the formation of the neural tube, the precursor of the brain and spinal cord. The forth germ layer, the neural crest, also forms during neurulation. The neural crest is involved in the formation of a diversity of cell types including pigment cells, various nerves and bone, among other cell types and tissues.

While the morphogenetic events of gastrulation and neurulation are occurring, the embryo is very susceptible to outside influences. During these critical periods agents called Teratogens, such as carcinogens, viruses and alcohol, can cause birth defects. Other abnormalities are caused by genetic defects.

Various developmental factors continue to come into play to regulate the differentiation of cells as exemplified by nerve cell differentiation. Cellular interactions also mediate the formation of tissues and organs. To understand how tissues and organs develop and the role of induction, we focus on two major examples. First limb development is detailed including the signaling events that occur. Then eye development becomes the organ of focus. The book finishes up by covering the topic of late inductions that occur not only during embryonic development but also throughout our lives. These include how hair, teeth and certain glands develop. The embryonic development of human mammary glands is also covered.

It will become clear that human developmental biology is a very dynamic and exciting topic. The central terms and concepts in this field have been introduced which will become clearer as each chapter in the book unfolds. So, before we continue, let’s take a quick summarized look at human development from birth to death.

Overview of Development

Gametogenesis: the formation of sperm and eggs.

Fertilization: fusion of a sperm and egg to form a zygote.

Cleavage: special, rapid mitotic cell divisions of the zygote to form a multicellular embryo.

Morphogenetic Movements: gastrulation and neurulation initially form primary germ lines establishing cell lineages in appropriate arrangements for further development.

Cell and Tissue Differentiation: cellular interactions regulate gene activity leading to specialization of cell types, tissues and organs.

Pattern and Polarity: humans have left-right, front-back (dorsal-ventral), top-bottom (anterior-posterior) symmetry that is reflected in the development of tissues and organs.

Growth and Maturation: the fetus grows as development continues while post-embryonic development is characterized by growth and sexual maturation (leading us back to gametogenesis and fertilization and the life cycle).

Senescence (aging) and Death: are the final developmental events of life leading to the end of the individual.

These key terms and concepts should be learned and understood before progressing in this book. Some of these topics are not covered (e.g., aspects of growth and maturation; senescence and death) in this volume because its primary focus is on embryonic development.