Lesson Eight

Animal Development
教学目的：使学生了解动物的生育过程，胚胎发育的结构特点，使学生掌握相关专业词汇以及主要用法。
教学重点：动物发育过程及相关英语词汇的掌握。
教学难点：专业英语词汇的记忆
讲授方法：以学生翻译为主，老师讲解相关专业知识辅助学生理解
授课时间：5月24日
教学内容：

Production of Sperm and Eggs

In sexually reproducing organisms males and females produce sex cells, known as gametes.  These are swimming sperm in males and ova (eggs) in females.
在性繁殖过程中，生物的雄性和雌性产生性细胞，称配子。雄性是能够游动的精子，雌性是卵子。
The process of sperm production, spermatogenesis, takes place in testes.  The sperm originate in gonial cells (spermatogonia) in the walls of seminiferous tubules.  Spermatocytes produced by mitosis in spermatogonia divide meiotically to generate haploid spermatids.  The mature sperm has a tail, a nucleus containing haploid chromosomes, and a front end with an acrosome, the storage site for enzymes that will aid fertilization.
精子产生过程，即精子发生在睾丸中。精子产生于输精管壁的性母细胞（精原细胞）。精原细胞经有丝分裂再减数分裂产生单倍体精子细胞，即精母细胞。成熟精子有尾部，单倍体染色体组，头部有顶体，内部储存酶类，有助于受精。
Ova, which are produced during oogenesis, are generated in gonial cells (oogonia) of the female's ovaries.  Oocytes then enter a stage of arrest in early meiosis.  At a species-specific later point, a final ripening (ovulation) and the first meiotic division occur.  A second meiotic division, followed by development of the embryo, takes place if the egg is fertilized.
在卵子发生过程中，由卵巢中的性母细胞产生。卵母细胞进入减数分裂的抑制阶段。第一次减数分裂产生一个成熟卵。如果卵受精，那么第二次减数分裂伴随胚胎的发育。
Eggs vary greatly in size from species to species and have complex structures.  Virtually all developing animal ova are surrounded by helper cells, either follicle cells or nurse cells.  Depending on the species, eggs also store varying amounts of yolk, a reservoir of nutrients produced by digestive-gland cells in the mother's body.  Finally, follicle cells or cells of the maternal oviduct provide protective coatings for the egg, including albumen (egg white) and various types of outer membranes and shells.
品种间卵的大小变化很大，并且有复杂的结构。实际上，所有发育过程中的卵都辅助细胞环绕，要么是滤泡细胞，要么是抚育细胞。依赖物种的不同而不同，卵黄贮备也不同，即由母体消化腺细胞产生的营养储备。最后，滤泡或母体输卵管细胞产生保护性的卵膜，包括清蛋白和各种外部膜及壳。
Frog oocytes have served as model systems for studies of oocyte development.  During maturation they produce huge numbers of ribosomes through gene amplification.  Large quantities of mRNA may also be made and stored.
蛙卵母细胞作为卵母细胞发育的研究系统模型。在成熟过程中，通过基因扩增产生大量的核糖体。同时也产生和储备了大量的mRNA。
Fertilization: Initiating Development
Fertilization unites male and female gametes and initiates development.  In some species fertilization is external; in others (including most terrestrial animals) it takes place internally.  The first contact of the sperm head with the egg's jelly coat triggers the acrosome reaction, in which enzymes are released to digest a hole through the egg's protective layers, and the plasma membrane of the sperm is brought into position to bind to the ovum's surface.  After fusion of the egg and sperm plasma membranes, the haploid male nucleus with its chromosomes moves into the egg cytoplasm.  Fusion also triggers the egg's final meiotic reduction divisions.  When sperm and egg nuclei unite, the two sets of chromosomes mingle to create a diploid set.  The fertilized egg is now a zygote.
雌雄配子结合作用称受精作用，发育开始。某些物种中，是外部受孕；另一些物种，包括大部分陆生动物，是内部受孕。当精子和卵子的胶状膜发生接触时，触发了顶体反应。释放多种酶而将卵保护膜消化出一个洞。精子的原生质膜与卵子的表面连接起来。精卵原生质膜融合后，单倍体雄性核进入卵细胞质。融合也触发了最后的减数分裂。当精卵核结合时，两套染色体混合产生一二倍体，即受精卵。
The egg's cortical reaction serves as a barrier to the entry of more than one sperm.  Initially, there is a temporary change in the egg's electrical state, and the egg cell is activated.  The final stage of the reaction, the rapid elevation of the fertilization membrane, prevents further sperm penetration.
卵皮层阻止其他的精子进入，充当壁垒作用。开始时，卵电位发生暂时变化，卵细胞被激活。反应最后阶段，受精卵的膜快速隆起，阻止更多精子进入。
In some species fertilization is not necessary.  Instead, parthenogenesis takes place: the egg is spontaneously activated and proceeds to normal embryonic development.
对某些物种而言，受精不是必须的。孤雌生殖，即卵自发地被激活并进入正常的胚胎发育过程中。
Cleavage: An Increase in Cell Number
Cleavage, the major developmental event immediately following fertilization, is a special form of cell division (mitosis).  Cleavage produces a blastula, a sheet of cells rounded into a sphere that in most species surrounds a cavity.  In the process, the single-celled zygote is divided into many small cells, and yolk, mRNA, ribosomes, and other materials arc distributed to each cell in precise ways.  The cells of the blastula, called blastomeres, also each receive a full diploid set of chromosomes.
受精后，立即发生卵裂，细胞有丝分裂的特殊过程。卵裂产生一个囊胚，形成的细胞壁球形排列形成一个空腔。在这个过程中，单细胞的受精卵分裂成许多小细胞，卵黄，mRNA，核糖体和其他物质被精确分配到每个细胞中。这些囊胚细胞，称卵裂球，每个细胞都有一套二倍体染色体组。
There are different patterns of cleavage in different species.  The amount of yolk present in the egg is a major factor in determining the pattern: in species having little yolk (such as mammals) the zygote cleaves completely through, forming cells that are roughly equivalent in size.  In frogs, in which the egg has somewhat more yolk, cleavage proceeds more rapidly in regions of the embryo having less yolk.  In bird eggs the yolk is so massive that cleavage divisions are restricted to a tiny area of cytoplasm.
不同物种，卵裂方式不同。决定因素主要取决于卵黄的含量。含有少量卵黄的物种（例如哺乳动物），合子分裂得很彻底，子细胞大致平均分裂。蛙卵，卵黄稍微多点，在胚胎的卵黄较少区域发育更快些。鸟卵卵黄很大，卵裂被限制在细胞质很小区域内。
In many species the precise distribution to blastomeres of molecular determinants in the cytoplasm is crucial to proper development of different cell types in the embryo.  In mammal and bird species the fate of cells is determined by the position of a cell late in cleavage.
细胞质中分子遗传因素精确分配到卵裂球中，对许多物种而言，是发育成胚中不同类型细胞关键所在。对哺乳动物和鸟类而言，细胞的命运最终由细胞分裂后所处的位置决定的。
Gastrulation: Rearrangement of Cells（原肠胚：细胞重排列）
The rearrangement of the blastula into a three-dimensional organism with inner, middle, and outer layers occurs during gastrulation.  The resulting gastrula consists of an outer ectoderm, an inner endoderm, and a mesoderm layer positioned between them.
在原肠胚形成过程中，囊胚重新排列，形成包括内层，中层，外层的三围组织。最终原肠胚由一个外肠胚，内肠胚，和一个中肠胚构成。
Each layer gives rise to specific tissues during embryonic development.  A variation in gastrulation, involving the movement of cells into endodermal and mesodermal positions through the thickened primitive streak, arose in reptiles and can still be seen in bird and mammalian embryos, lending support to the theory that birds and mammals evolved from reptiles.
每个胚层产生特殊的胚胎组织。密实的原肠胚通过细胞运动进入内胚层和中胚层，爬虫，鸟，哺乳动物的胚胎都如此。因此提出这样的理论，鸟类和哺乳动物是由爬虫进化而来的。
Organogenesis: Formation of Functional Tissues and Organs
The organs and tissues of the embryo arise during organogenesis as cells inside the embryo and on its surface become specialized.  Organogenesis actually includes two closely linked processes, morphogenesis and differentiation.  During morphogenesis cells and cell populations change shape: an example is neurulation in vertebrate embryos, in which the edges of the flat neural plate fold upward and fuse, forming the beginnings of the hollow brain and spinal cord.  During differentiation cells mature so that they may perform separate functions.  This maturation may include taking on a function-related shape, such as the long, spindly shape of skeletal-muscle cells.  Cell differentiation also results in responsiveness-the ability of a cell to be regulated within the organism through the action of hormones, neurons, and other signals.
胚经过内部细胞和表面细胞特化后，器官形成，发育成器官和组织。器官形成实质上包括两个联系紧密的过程，即形态发生和分化。在形态发生期间，细胞和细胞群体形状发生改变：例如，脊椎动物的神经胚，扁平神经板的边缘向上折叠并融合，是中空大脑和脊髓神经形成的开端。在分化期间，细胞成熟，执行各自功能。成熟包括功能相关的成型过程。例如，细长的骨骼肌肉细胞。细胞分化也产生应答能力，即细胞受生物体内激素，神经和其他信号的调控过程。
Embryonic Coverings and Membranes
The embryos of land vertebrates are enclosed within four extraembryonic membranes that afford protection while still permitting the exchange of gases, nutrients, and other materials.
脊椎动物的胚胎被4层膜包被，起保护作用，但仍能交换气体，营养和其他物质。
Growth: Increase in Size
Growth in embryos is largely due to an increase in the number of cells rather than to an increase in the size of individual cells.  In many species the extent of embryonic growth is limited by the availability of food (yolk).  In animals that develop entirely free of the maternal body, such as frogs and insects, the embryo give rise to a larval stage that can feed itself and later undergo metamorphosis to attain the adult stage.  In many species the most spectacular growth phase takes place during the juvenile and adolescent phases of the life cycle.  Actual growth generally stops once the organism reaches its typical adolescent phases of the life cycle.  Actual growth generally stops once the organism reaches its typical adult size, although replacement of dead cells may continue.
胚的生长很大程度取决于细胞数量的增加而不是单个细胞大小的改变。在大多数物种中，胚的生长是受卵黄的量限制的。在动物细胞中，完全依赖于母体，例如蛙和昆虫，胚产生幼虫状态，后来经历变态而进入成年状态。在许多物种中，在少年和青春期，发生特别显著的生长。一旦细胞进入成年状态和成年大小，生长就停止了，尽管死细胞的替代不断发生。
A special type of growth, regeneration of lost body parts, can take place in adults of some species.  Prior to such regeneration cells in stump tissue undergo dedifferentiation.  They lose their functional phenotype, divide rapidly, and generate a population of cells that will regenerate the lost part. Compensatory hypertrophy is a different, temporary growth response in which residual tissue increases in mass and cell number: cells undergo mitosis but do not dedifferentiate.
一个特殊类型的生长，再生失去的部分躯体，发生在某些物种的成体中。在再生之前，残肢组织细胞经历了去分化过程。他们失去功能表型，快速分裂，产生细胞群来再生失去的部分。代偿式肥大是一个不同的，暂时的生长应答过程，残余组织在体积和细胞数量上增加：细胞经历了减数分裂，但没有去分化过程。
Aging and Death: Final Developmental Processes
Aging is an ongoing, time-dependent developmental process in which body parts deteriorate.  Proposed causes include the degeneration of collagen (the fibrous proteins of the connective tissues) and limits on the number of times cells can divide.  Other theories focus on a decline of the immune system or on the accumulation of lipofuscins (aging pigments).
老化是一个持续过程，随时间发育，身体部分恶化。可能原因包括胶质的退化（纤维蛋白）和能分裂活细胞的限制。另外理论认为，免疫系统的免疫下降或脂褐素的堆积造成的。
课后习题：阅读材料一
  Glossary
sperm精细胞
Cells that develop from the spermatids by losing much of their cytoplasm and developing long tails; the male gamete.
ovum 卵，卵子（复数,ova）
An unfertilized non-motile female gamete.  In many animals it is produced in the ovary.
spermatogenesis 精子发生
The specific name given to the gametogenesis process that leads to the formation of sperm.
oogenesis 卵子发生-
The specific name given to the gametogenesis process that leads to the formation of eggs.
ovaries  卵巢，子房
The female sex organs the produce haploid sex cells, the eggs or ova.
yolk 卵黄，蛋黄
The store of food material, mostly protein and fat, that is present in the eggs of most animals
oviduct 输卵管
The tube (fallopian tube) that carries the primary oocyte to the uterus.
gene amplification 基因扩增
A temporary dramatic increase in the numbers of a particular gene in a genome during one developmental period.
fertilization 受精
The joining of haploid nuclei, usually from an egg and a sperm cell, resulting in a diploid cell called the zygote.
Zygote受精卵
A diploid cell that results from the union of an egg and a sperm.
acrosome reaction 顶体反应
That series of observable, structural changes undergone by a sperm when in the vicinity of an ovum in the oviduct.  Specifically, many openings appear in the sperm head membrane, through which the contents of the acrosome appear to be released.
parthenogenesis ['pa:einE]u'd3enisis]单性生殖，孤雌生殖
Reproduction in which eggs develop normally without being fertilized by a male gamete, producing an individual usually genetically identical to the parent.
cleavage 卵裂
The mitotic division of the zygote that occurs immediately after fertilization and produces a ball of smaller cells without an overall increase in size.
blastula 囊胚，囊胚泡
The stage of the early animal embryo that succeeds cleavage and precedes gastrulation.  A blastula is usually made up of a hollow ball of cells, with a wall one to several cells thick; the central cavity is the blastocoel.
blastomere 卵裂球
One of several small cells formed from an animal zygote during cleavage.
gastrulation 原肠胚形成
The process in the animal embryo by which a blastula is converted into a gastrula, with the laying down of the germ layers.
gastrula 原肠胚
The stage in the development of animal embryos that follows the blastula and results from gastrulation. The cells of a gastrula are differentiated into the germ layers and the central cavity opens to the exterior by the blastopore.
ectoderm 外胚层，内层
The outermost of the germ layers of metazoan embryos.  It develops mainly into epidermal tissue, the nervous system, sense organs, and (in lower forms) the nephridia.
endoderm 内胚层，内层
The innermost of the germ layers of metazoan embryos.
Mesoderm中胚层，中层
The middle layer of the three germ layers of triploblastic animal embryos.  It develops into cartilage, bone, muscle, blood, kidneys, and gonads.
primitive streak 原肠胚
A longitudinal band of embryonic mesodermal cells that develops within a dorsal groove along the length of the gastrula of mammals and birds.
organogenesis 器官形成
The formation of organs.
morphogenesis形态发生，形态建成
The developmental processes leading to the characteristic mature form of an organism or part of an organism.
differentiation 分化，鉴别
The complex of changes involved in the progressive diversification of the structure and functioning of the cells of an organism.  For a given line of cells, differentiation results in a continual restriction of the types of transcription that each cell can undertake.
neurulation 神经胚形成
Developmental stage in an embryo during which the cells differentiate into the brain and spinal cord.
metamorphosis 变态
The transformation from larval to adult form.
regeneration 再生
The replacement by an organism of tissues or organs that have been lost.
dedifferentiation  去分化，脱分化
The loss of differentiation, as in the vertebrate limb stump during formation of a blastema.
compensatory hypertrophy 代偿式肥大
See  regeneration.