The Questions of Developmental Biology

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The first hormone anti-Müllerian duct hormone (AMH; also referred to as Müllerian-inhibiting substance, MIS) destroys the Müllerian duct. The second hormone testosterone masculinizes the fetus, stimulating the formation of the penis, scrotum, and other portions of the male anatomy, as well as inhibiting the development of the breast primordia. Thus, the body has the female phenotype unless it is changed by the two hormones secreted by the fetal testes. We will now take a more detailed look at these events. The developing gonads The gonads embody a unique embryological situation. All other organ rudiments can normally differentiate into only one type of organ. A lung rudiment can become only a lung, and a liver rudiment can develop only into a liver. The gonadal rudiment, however, has two normal options. When it differentiates, it can develop into either an ovary or a testis. The path of differentiation taken by this rudiment determines the future sexual development of the organism. But, before this decision is made, the mammalian gonad first develops through a bipotential (indifferent) stage, during which time it has neither female nor male characteristics. In humans, the gonadal rudiments appear in the intermediate mesoderm during week 4 and remains sexually indifferent until week 7. The gonadal rudiments are paired regions of the intermediate mesoderm; they form adjacent to the developing kidneys. The ventral portions of the gonadal rudiments are composed of the genital ridge epithelium. During the indifferent stage, the genital ridge epithelium proliferates into the loose connective mesenchymal tissue above it (Figure 17.3A,B). These epithelial layers form the sex cords. The germ cells migrate into the gonad during week 6, and are surrounded by the sex cords. In both XY and XX gonads, the sex cords remain connected to the surface epithelium. If the fetus is XY, the sex cords continue to proliferate through the eighth week, extending deeply into the connective tissue. These cords fuse, forming a network of internal (medullary) sex cords and, at its most distal end, the thinner rete testis (Figure 17.3C,D). Eventually, the sex cords now called testis cords lose contact with the surface epithelium and become separated from it by a thick extracellular matrix, the tunica albuginea. Thus, the germ cells are found in the cords within the testes. During fetal life and childhood, the testis cords remain solid. At puberty, however, the cords will hollow out to form the seminiferous tubules, and the germ cells will begin to differentiate into sperm.
The cells of the seminiferous tubule are called Sertoli cells. The Sertoli cells of the testis cords nurture the sperm and secrete anti-Müllerian duct hormone. The sperm are transported from the inside of the testis through the rete testis, which joins the efferent ducts.
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PART 1. Principles of development i
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PART 2: Early embryonic development
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15. Lateral plate mesoderm and endo
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Hox Genes: Descent with Modificatio
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The question of growth. How do our
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Embryonic homologies One of the mos
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absent (Figure 1.16A). Over 7000 af
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Such growth, in which the shape is
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One way to search for the chemicals
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2 3 hours as adults, and they must
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for the following spring's breeding
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VADE MECUM Amphibian development. T
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The formation of a cap is a complex
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In Chlamydomonas, recognition occur
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organism with two distinct and inte
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Aggregation is initiated as each of
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The mathematics of such oscillation
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gonidia to undergo a modified patte
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Sponges develop in a manner so diff
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Embryology provides an endless asso
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Environmental sex determination Sex
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Protection of the egg by sunscreens
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The Developmental Mechanics of Cell
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Autonomous specification was first
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In postulating this model, Weismann
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Insofar as it contains a nucleus, e
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Other tissues may use the same grad
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will give rise to its particular or
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Sydney Brenner (quoted in Wilkins 1
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The results of their experiments we
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This observation led Steinberg to p
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into a single layer of cells (Takei
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6. In conditional specification, th
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3. Geneticists had to explain pheno
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These events are not the normal rou
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differentiated; each of them contai
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federal proscription of human cloni
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ecombinase enzymes are active only
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In situ hybridization But where was
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damaged.) The second primer is adde
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By using the appropriate restrictio
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These homozygous mutant mice lacked
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Phenotype Manipulation This technol
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The second approach is candidate ge
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developmental genetics is different
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This gene consists of the following
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In addition to these basal transcri
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Enhancers are critical in the regul
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The third functional region of MITF
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A fourth enhancer sequence, located
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The discovery of the human globin L
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The results of this procedure are o
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In vertebrates, the presence of met
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chromatin that remains condensed th
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Second, knocking out one Xist locus
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types of cells (Kleene and Humphrey
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(Sxl) gene,* while the autosomes pr
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Table 5.3. Some mRNAs stored in ooc
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determine the anterior-posterior ax
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6. Cell-cell communication in devel
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The inducing tissue does not need i
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Instructive and permissive interact
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mouth, whereas the frog tadpole pro
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are utilized throughout the animal
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includes the TGF-β family, the act
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The RTK spans the cell membrane, an
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members of this family: the C. eleg
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The Wnt pathway Members of the Wnt
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The Hedgehog pathway is extremely i
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A series of mutations has been foun
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The Nature of Human Syndromes As we
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vertebral column deformities, myopi
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The mammalian homologue of CED-4 is
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In the absence of Notch gene transc
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extracellular matrix (Figure 6.32).
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mammary gland tissue. The binding o
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In some cells, gene transcription r
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PARTE 2. Early embryonic developmen
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The means by which sperm are propel
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The sperm released during ejaculati
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Lying immediately beneath the plasm
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The mechanisms of chemotaxis differ
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Once the sea urchin sperm has penet
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Removal of these threonine- or seri
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undergo the acrosomal reaction with
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Gamete Fusion and the Prevention of
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The fast block to polyspermy. The f
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envelope to expand and become the f
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The calcium ions responsible for th
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Thus, the conversion of NAD + to NA
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eticulum (McPherson et al. 1992). T
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cAMP-dependent protein kinase activ
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These cells divide to form embryos
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(Figure 7.34; Elinson and Rowning 1
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6. In many species, eggs secrete di
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One consequence of this rapid cell
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Table 8.1. Karyokinesis and cytokin
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provides a classification of cleava
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This occurred at precisely the time
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These rapid and invariant cell clea
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The identities of the signaling mol
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Ingression of primary mesenchyme Fu
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But these guidance cues cannot be s
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lastocoel wall (Dan and Okazaki 195
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The orientation of the cleavage pla
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stage, the remaining cells divide n
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embryos can produce these cells, bu
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Early Development in Tunicates Tuni
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(or inactivating) specific genes. T
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occur under laboratory conditions.
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the germ cells. Using fluorescent a
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Conditional specification As we saw
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eggs. Coda This chapter has describ
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9. The genetics of axis specificati
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Following cycle 13, the oocyte plas
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Thus, at the end of germ band forma
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Classic embryological experiments d
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posterior region of the unfertilize
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Further studies have strengthened t
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The Hunchback protein also works wi
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transcription factor to activate an
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The gap genes The gap genes were or
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Table 9.2. Major genes affecting se
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The development of the normal patte
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However, the mechanism by which the
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As we saw above, the gap gene and p
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can substitute for Ultrabithorax an
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The Translocation of Dorsal Protein
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The Gurken signal is received by th
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This fate map is generated by the g
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first glimpses of the multiple leve
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10. Early development and axis form
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While these cells are dividing, num
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The next phase of gastrulation invo
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Black and Gerhart (1985, 1986) let
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The convergent extension of the dor
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During early gastrulation, three ro
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Spemann concluded from this experim
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Hans Spemann and Hilde Mangold: Pri
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We will take up each of these quest
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Moreover, the injection of exogenou
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These Nodal-related proteins will a
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The diffusible proteins of the orga
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Follistatin. The fourth organizer-s
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Frzb and Dickkopf. Shortly after th
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Moreover, when dorsal blastopore li
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The relationship between these thre
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Nodal protein activates expression
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11. The early development of verteb
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Gastrulation in Fish Embryos The fi
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If one conceptually opens a Xenopus
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Left-right axis formation Little is
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thereby forming the secondary hypob
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This region, the germinal crescent,
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Axis Formation in the Chick Embryo
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The mechanisms for neural induction
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*Arendt and Nübler-Jung (1999) hav
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Compaction The fifth, and perhaps t
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Modifications for development withi
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The ectodermal precursors end up an
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These include the genes for transcr
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Experimental analysis of the hox co
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Kessel and Gruss (1991) found this
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Mice homozygous for an insertion mu
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7. In birds, gravity is critical in
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This tissue forms the amnionic sac
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12. The central nervous system and
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surrounding them. As much as 50% of
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Cell wedging is intimately linked t
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Fig 12.6 Human neural tube closure
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The anterior-posterior axis The ear
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Ventral patterning of the neural tu
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The time of this vertical division
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layer (Wallace 1999). Each Purkinje
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However, if these cells are transpl
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Neuronal Types The human brain cons
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Neurons transmit electrical impulse
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Development of the Vertebrate Eye A
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In addition, there are numerous Mü
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are shed and are replaced by new ce
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Just as there is a pluripotent neur
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13. Neural crest cells and axonal s
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The Trunk Neural Crest Migration pa
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Recognition of surrounding extracel
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However, D. J. Anderson's laborator
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Neural crest cells from rhombomeres
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the aortic arch arteries and the se
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Soon afterward, the expression patt
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Ericson and colleagues (1996) have
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That it must be a sort of a surface
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the G growth cone acts abnormally,
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Guidance by diffusible molecules Ne
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There is some reciprocity in scienc
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The activity of the synapse release
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Growth of the retinal ganglion axon
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The complicated nerve fiber circuit
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Snapshot Summary: Neural Crest Cell
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Fetal Neurons in Adult Hosts In 197
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Somites give rise to the cells that
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Eph signaling is thought to mediate
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(The dermis of other areas of the b
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Muscle cell fusion The myotome cell
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The mechanism of intramembranous os
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mitochondrial energy potential (Sha
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Mutations in FGFR1 can cause Pfeiff
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These buds eventually separate from
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Step 2: The metanephrogenic mesench
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Page 423 and 424: Formation of Blood Vessels Although
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Page 427 and 428: In some instances, angiogenesis may
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Page 431 and 432: Blood and lymphocyte lineages. Figu
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Page 435 and 436: In mammalian embryos, food and oxyg
Page 437 and 438: *In some infants, the septa fail to
Page 439 and 440: As Figure 15.27 shows, the stomach
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Page 443 and 444: In mammals, the size of the allanto
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Page 447 and 448: Moreover, as will be detailed in Ch
Page 449 and 450: These cells secrete the paracrine f
Page 451 and 452: Induction of the apical ectodermal
Page 453 and 454: The progress zone: The mesodermal c
Page 455 and 456: The mechanism by which Hox genes co
Page 457 and 458: Thus, while the Hox gene expression
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Page 461 and 462: Between the time when the cell's de
Page 463 and 464: Snapshot Summary: The Tetrapod Limb
Page 465: This environmental view of sex dete
Page 469 and 470: into the mesenchyme, but stay near
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Page 473 and 474: Wnt4: a potential ovary-determining
Page 475 and 476: Anti-müllerian duct hormone Anti-M
Page 477 and 478: in another (see Jacklin 1981; Bleie
Page 479 and 480: Chromosomal Sex Determination in Dr
Page 481 and 482: The sex-lethal gene as the pivot fo
Page 483 and 484: Doublesex: The switch gene of sex d
Page 485 and 486: It is not known whether the tempera
Page 487 and 488: 18. Metamorphosis, regeneration, an
Page 489 and 490: There are no ipsilateral (same-side
Page 491 and 492: Organ-specific response to thyroid
Page 493 and 494: The more T 3 receptors a tissue has
Page 495 and 496: Other species, such as A. tigrinum,
Page 497 and 498: Metamorphosis in Insects Types of i
Page 499 and 500: On one side of the sac, the epithel
Page 501 and 502: During the first larval instar, the
Page 503 and 504: central region producing the Wingle
Page 505 and 506: The molecular biology of hydroxyecd
Page 507 and 508: Since its discovery in 1954, when B
Page 509 and 510: How do they regain the ability to d
Page 511 and 512: It is probable that during normal r
Page 513 and 514: The head activator gradient. The ab
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However, recent studies have indica
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Moreover, if a mutation occured in
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Snapshot Summary: Metamorphosis, Re
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19. The saga of the germ line We be
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In the nematode C. elegans, the ger
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When ultraviolet light is applied t
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Fibronectin is likely to be an impo
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Germ cell migration in birds and re
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EG Cells, ES Cells, and Teratocarci
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come together and are then separate
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The distal tip cell extends long fi
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The initiation of spermatogenesis d
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ecause the flagellum is beginning t
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A partial catalogue of the material
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When a specific 340-base sequence f
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Oocyte chromosomes can be incubated
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The structure of the meroistic ovar
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Periodically, a group of primordial
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Following ovulation, the luteal pha
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20. An overview of plant developmen
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oth the embryo and the mature sporo
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*A small weed in the mustard family
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should provide information on the e
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of hormones. In scarlet runner bean
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embryos demonstrate that isolated p
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When the shoot emerges from the soi
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etween nodes is called an internode
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flowering patterns among the over 3
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genes, class D genes are now being
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Table 21.1. Specific settlement sub
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Predictable Environmental Differenc
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Diapause Many species of insects ha
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The hindwing pigments of the short-
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Ferguson and Joanen (1982) speculat
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Predator-Induced Defenses One survi
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3. Antigens are presented (usually
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the ipsilateral eye. The situation
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*The importance of substrates for l
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Thus, most abnormal embryos are spo
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Retinoic acid as a teratogen In som
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Another pathway to apoptosis involv
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Whether heavy maternal alcohol cons
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Some scientists, however, say that
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Chains of causation Whether in law
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Snapshot Summary: The Environmental
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One could emphasize the common desc
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The search for the Urbilaterian anc
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Hox Genes: Descent with Modificatio
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Changes in Hox gene transcription p
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But within the crustacean lineages,
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Changes in Hox gene number The numb
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Homologous Pathways of Development
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The gradient of Dpp concentration f
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Modularity: The Prerequisite for Ev
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Such a trait would be selected for
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*The lack of such transitional form
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Coevolution of ligand and receptor
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Evidence for this mathematical mode
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It is difficult for a mutation to a
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The population genetics model conta
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However, once one adds development
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A partial list of genes active in h
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