Developmental Anatomy PDF

Developmental Anatomy ACCORDING TO ARISTOTLE, the first embryologist known to history, science It is a most beautiful thing to s tudy and wonder remains the beginning of knowledge" (Aristotle, Metaphysics, ca. begins with wonder...

Developmental Anatomy ACCORDING TO ARISTOTLE, the first embryologist known to history, science It is a most beautiful thing to s tudy and wonder remains the beginning of knowledge" (Aristotle, Metaphysics, ca. begins with wonder: "It is owing to wonder that people began to philosophize, the different changes of life, from the 350 BeE). The development of an animal from an egg has been a source of won­ microscopic changes of conception to der throughout history. The simp le proced ure of cracking open a chick egg on the more apparent ones of maturity each successive day of its 3-week incubation period provides a remarkable expe­ and old age. rience as a thin band of cells is seen to give rise to an entire bird. Aristotle per­ FRANKLIN MALL (CA. 1 890) The greatest progressive minds of formed this procedure and noted the formation of the major organs. Anyone can wonder at this remarkable-ye t commonplace-phenomenon, but it is the sci­ entist seeks to discover how development actually occurs. And rather than dis­ embryology have not looked for hypo th eses; they have looked at em bryos. sipating wonder, new Wlderstand.ing increases it. Multicellubr organisms do not spring forth fully formed. Rather, they arise by a relatively slow process of progressive change that we call development. In JANE OPPENHEIMER ( 1 955) nearly all cases, the development of a multicellular organism begins with a sin­ gle cell-the fertilized egg, or zygote, which divides mitotically to produce all the cells of the body. The study of animal development has traditionally been called embryology, after that phase of an organism that exists between fertiliza­ tion and birth. But development does not stop at birth, or even at adulthood. of skin cells (the older cells being slo ughed off as we move), and our bone mar­ Most organisms never stop developing. Each day we replace more than a gram row sustains the development of millions of new red blood cens every minute of our lives. In addition, some ar:-imais can regenerate severed parts, and many frog, or a caterpillar into a butterfly). Therefore, in recent years it has become species lffidergo metamorphosis (such as the transformation of a tadpole into a customary to speak of developmental biology as the discipline tha t studies embryonic and other developmental processes. tions it seeks to answer. Most of the questions in developmental biology have As the introduction to Part T notes, a scientific field is defined by the ques­ been provided to it by its embryological heritage. We can identify three major approaches to studying embryology: Anatomical approaches Experimental approaches Genetic approaches Each of these traditions has predominated during a different era. However, although it is true that anatomical approaches gave rise to experimental approach­ es, and that genetic approaches built on the foundations of the earlier two approaches, all three traditions persist to this day and continue to play a major role in developmental biology. The basis of aU research in developmental biolo­ gy is the changing anatomy of the organism. Today the anatomical approach to 6 CHAPTER 1 development is continually expanded and enhanced by rev­ S. In many species, the organism that hatches from the egg olutions in microscopy, computer-aided graphical recon­ or is born into the world is not sexually mature. Rather, structions of three-dimensional objects, and methods of the organism needs to undergo metamorphosis to applying mathematics to biology. Many of the beautiful become a sexually mature adult. In most animals, the photographs in this book reflect this increaSingly impor­ young organism is a called a larva, and it may look sig­ tant component of embryology. nificantly different from the adult. In many species, the larval stage is the one that lasts the longest, and is used for feeding or dispersal. [n such species, the adult is a The Cycle of life brief stage whose sole purpose is to reproduce. In silk­ One of the major triumphs of descriptive embryology was worm moths, for instance, the ad ults do not have the idea of a generalizable animal life cycle. Each animal, mouthparts and cannot feed; the larvae must eat whether earthworm or eagle, termite or beagle, passes enough so that the adult has the stored energy to Sur­ through similar stages of development. The stages of devel­ vive and mate. Indeed, most female moths mate as soon opment between fertilization and hatching are collectively as they eclose from their pupa, and they fly only once­ called embryogenesis. to lay their eggs. Then they die. Throughout the animal kingdom, an incredible variety 6. In many species, a group of cells is set aside to produce of embryOnic types exist, but most patterns of embryogen­ the next generation (rather than forming the current esis are variations on six fundamental processes: fertiliza­ embryo). These cells are the precursors of the gametes. tion, cleavage, gastrulation, organogenesis, metamorpho­ The gametes and their precursor cells are collecti vely sis, and gametogenesis. called germ cells, and they are set aside for reproduc­ tive function. All the other cells of the body are called 1.. Fertilization involves the fusion of the mature sex cells, somatic cells. This separation of somatic cells (w.hlch the sperm and egg, which are collectively called the give rise to the individual body) and germ cells (which gametes. The fusion of the gamete cells stimulates the contribute to the fonnation of a new generation) is often egg to begin development and initiates a new individ­ one of the first differentiations to occur during animal ual. The subsequent fusion of the gamete nuclei (both development. The germ cells eventually migrate to the of which have only half the normal number of chromo­ gonads, where they differentiate into gametes. The somes characteristic for the species) gives the embryo development of gametes, called gametogenesis, is usu­ its genome, the collection of genes that helps instruct ally not completed until the organism has become phys­ the embryo to develop in a manner very similar to that ically mature. At maturity, the gametes may be released of it parents. and participate in fertilization to begin a new embryo. 2. Cleavage is a series of extremely rapid mitotic divisions The adult organism eventually undergoes senescence that immediately follow fertilization. During cleavage, and dies, its nutrients often supporting the early the enormous volume of zygote cytoplasm is divided embryogenesis of its offspring and its absence allowing into numerous smaller cells called blastomeres. By the less competition. Thus, the cycle of life is renewed. end of cleavage, the blastomeres have usually formed a sphere, known as a blastula. 3. After the rate of mitotic division slows down, the blas­ A Frog's life tomeres undergo dramatic movements and change their All animal life cycles are modifications of the generalized positions relative to one another. This series of exten­ one described above. Figure 1. 1 shows the development of sive cell rearrangements is called gastrulation, and the the leopard frog, Rana pipiens, and provides a good start­ embryo is said to be in the gastrula stage. As a result of ing point for a more detailed discussjon of a representa­ gastrulation, the embryo contains three germ layers that tive life cycle. will interact to generate the organs of the body. 4. Once the germ layers are established, the cells interact with one another and rearrange themselves to produce Gametogenesis and fertilization tissues and organs. This process is called organogene­ The end of one life cycle and the beginning of the next are sis. Many organs contain cells from more than one germ often intricately intertwined. Life cycles are often controlled layer, and it is not unusual for the outside of an organ by environmental factors (tadpoles wouldn't survive if to be derived from one layer and the inside from anoth­ they hatched in the fall, when their food is dying), so in er. For example, the outer layer of skin (epidermis) events. A combination 01 photoperiod (hours of daylight) most frogs, gametogenesis and fertilization are seasonal comes from the ectoderm, whereas the inner layer (the dermis) comes from the mesoderm. Also during organo­ and temperature informs the pituitary gland of the mature genesis, certain cells undergo long migrations from their female frog that it is spring. The pituitary then secretes hor­ place of origin to their finaI location. These migrating mones that stimulate her ovary to make the hormone estro­ cells include the precursors of blood cells, lymph cells, gen. Estrogen then instructs the liver to make and secrete pigment cells, and sex cells. yolk proteins, which are then transported through the ---- -- -  DEVELOPMENTAL ANATOMY 7 - a./ Morul (maleg Sperm Oocyte Location of Ge!m plasm germ ce s ll ( 0 : gamete) ZATION FERTILI-