Unit 2 Histology and Embryology

Key to the Exercises

1. Preparation

Enjoy a video clip. You may watch it twice to achieve a good understanding. After watching, please answer the following questions.

1) The skin.

2) Epidermis.

3) Keratin.

4) The times of the images being magnified by the microscope.

Script

Look, a bare hand. We sometimes lay our forehead on it when we feel tired. The

envelope of the body, a living border between the inside and the outside, between the wet

and the dry, the skin is an assembly of regular cells. It's covered with a network of

furrows and plateaus. The translucent spheres are drops of sweat which have reached the

surface through pores. The skin thus evolves, breathes and perspires, getting rid of

certain wastes of the body. This tormented landscape is our epidermis, magnified 150

times within it the layers of cells are in constant renewal. Cells are already dead when

they reach the surface, disposed of and replaced by others produced in deeper layers.

Here is a hair, magnified 180 times its root, invisible on the screen, loses itself in the

epidermis. A slight cut, this gray area at the center of the image, allows us to distinguish

the various cell layers that constitute the epidermis. As we move deeper, we come across

living cells whose nucleus we can sometimes distinguish, those regular bubbles we can

see at the bottom of the screen. Above, flat cells, emptied of their content ! constitute the

upper layer of the epidermis. Within these, we find a very resistant protein, keratin  ·

which can also be found in hair, and nails, and teeth. Here we are now at the heart of the

epidermis. The membranes form a protective waterproof barrier. For this very purpose,

they are joined together by something like a press stud, this dark crescent at the center

of the image. Yet we can also see how they are arranged in lipid layers placed side by

side1limiting exchanges with the outside. our microscope has unfortunately now reached

its limits. Beyond this, nothing more is visible for the moment.

ll. Reading

Comprehension Check

Pair work Discuss and answer the following questions. Try not to look back at the text.

1) The largest organ of the body is the integument, which is composed of skin and skin

appendages--nails, hair, sweat glands, and sebaceous glands. (Para. 1 )

2) Skin serves as a protective barrier against injury ( e. g., abrasions, cuts, burns),

infectious pathogens, and ultraviolet radiation, and assists in body temperature

regulation, vitamin D synthesis, ion excretion, and sensory reception (touch and pain).

(Para. 1 )

3) Four or five. In a deep to superficial direction, they are the stratum basale ( or

germinativum). The stratum spinosum, the stratum granulosum, the stratum lucidum

(absent in thin skin but present in thick skin) rand the stratum corneum. (Para. 2)

4) Blood vessels in the dermis and eccrine sweat glands can regulate temperature. (Para.

3 &. 5)

5) Apocrine sweat glands are less coiled than eccrine sweat glands, and the lumen of the

secretory tubules in the former has a wider diameter and larger, more numerous

myoepithelial cells than that in the latter. (Para. 6)

6) The cells of both parts contain keratin ; however, the most superficial cells of the

stratum corneum desquamate while the cells of nail plates do not. (Para. 2 & 9)

Language Practice

1. Read the text carefully and then fill out the missing information in the following short

passage. The beginning letter of each missing word has been given.

1) integumentary 2) regulating 3) epidermis

4) stratum 5) keratin 6) corneum

7) shed 8) dermis 9) vessels

10) subcutaneous 1 1) sweat 12) sebaceous

13) sebum 14) follicle

2. Paraphrase the following sentences. Pay special attention to the underlined parts.

1) Four or five layers, which are clearly different from one another, make up the

epidermis.

2) The next layer is the stratum spinosum. It is several cells thick and has polyhedral

cells that steadily and continuously become flatter toward the surface.

3) A fixation artifact decreases the size of the cell. This condition makes the projecting

parts on the cell surface more prominent, and these prominent parts become spines

or prickles. For this reason, we call them prickle cells.

4) Nerve fibers covered by a myelin sheath provide nerve endings to the epidermis and

sensory receptors enclosed in a capsule in the dermis...

5) The surface under the uncovered and hidden parts of the nail plate is the nail bed.

3. Identify the expression from each of the following sentences which corresponds in

meaning to one of the Chinese expressions given below. (These sentences are well written.

It is suggested that you read them aloud and learn them by heart. )

1)vary regionally因部位(不同)而异

2)serve as作为

barrier against抵御······的屏障

assist in协助参与

3)attach...To将······附在······上

4) (be) parallel to 与  ·  ·  ·  ·  ·  · 平行

5)(be) absent in在······中缺如不存在于······中

(be)present in出现于

6) (be) involved in 参与

7)in response to对······应答对······有反应

8)(be)lined by由······衬里(覆盖内表面)

9) (be) oblique to 与  ·  ·  ·  ·  ·  · 成斜角

4. Translate the following sentences into English with some expressions you have identified

in exercise 3.

1) osteoclasts are involved in the breakdown of bone tissue to release minerals needed

by the body or to allow for reshaping and repair.

2) The membranes of the smooth endoplasmic reticulum serve as surfaces for the

attachment of many enzyme systems.

3) An increase in the number of reticulocytes indicates increased red blood cell

formation, as in response to hemorrhage or destruction of red blood cells.

4) The acrosome covers half of the nuclear surface and contains enzymes to assist in

penetration of the egg during fertilization.

5) Sometimes chromosomes break, and pieces of one chromosome attach to those of

another.

6) The most superficial layer consists of densely packed fibers of type 1 collagen that

are arran9ecl almost parallel to the articular surface.

7) Follicles generally atrophy with increasing age and may be absent in the very

elderly.

8) The inner surface of the bone is lined by a thinner, more cellular endosteum.

9) Hematopoietic stem cells present in the liver are in greater numbers before birth

than afterward,

1o) This type of epithelium is a barrier against different types of injury, microbial

invasion and water loss.

11 ) Where fibers are oblique to the line of pull, muscles may be triangular or pennate.

12) Differentiation of transplanted neural precursors varies regionally in adults'

striatum.

Text B

comprehension Check

Answer the following questions. Try not to look back at the text.

1 ) It means a fertilized oocyte, a highly specialized, totipotent cell. (Para. 1 )

2) Differentiation and growth of tissues and organs occur during the fetal period. (Para. 2)

3) Prenatal development of embryos and fetuses is generally covered by the study of

embryology. (Para. 2)

4) What can help the parents not to put blame on themselves is the explanation of the

developmental basis of abnormalities by health care providers who understand the

common congenital anomalies and their causes. (Para. 11 )

5) People are concerned about the social, ethical, and legal implications of human cloning and

the possibility that cloning may result in birth defects and serious diseases. (Para. 13)

6) They are used to refer to the relationships to the head and caudal eminence ( tail ),

respectively. (Para. 15 )

Language Practice

Translate the following sentences into English. You are required to use the phrase (s) after

each Chinese sentence in constructing the corresponding English sentence.

1 ) Bacterial cells that express one phenotype can be transformed into cells that express a

different phenotype.

2) Developmental anatomy is the field of embryology concerned with the changes that

cells, tissues, organs, and the body as a whole undergo from a germ cell of each parent

to the resulting adult.

3 ) Embryology bridges the gap between prenatal development and obstetrics.

4) Embryology is of practical value in helping understand the causes of variations in

human structure.

5) Regulation of cell numbers is essential for maintaining proper physiological balance

among the various tissues and cell types.

6) These techniques are now widely used in research laboratories to address such diverse

problems as the genetic regulation of morphogenesis : the temporal and regional

expression of specific genes, and how cells are committed to form the various parts of

Human embryonic stem ( hES ) cells hold great potential for cell therapy and

7) regenerative medicine because of their pluripotency and capacity for self-renewal.

III. Video Watching and Speaking

Clip One

please Joltow tite video clip to take (I close look at the human skin land do the exercises

below.

1. Complete the following chart to illustrate the basic structure of the skin.

stratum corneum

epidermis)

basal layer

thermal receptors

Skin<                      Meissner's corpuscles

dermis       free nerve endings

sweat glands, hair follicles and blood vessels

2. Match the following items with their corresponding functions.

1 ) D) 2) C) 3)A) 4 ) B)

3. Omitted.

Script

The body's largest organ-the skin-serves as a protective coat, guarding I

structures from the outside environment. It is composed of two distinct layers of

the epidermis and the dermis. The epidermis consists of an outer sheet of deal

called the stratum corneum, and a deeper section of rapidly dividing cells, called the basal

layer. When dead cells of the stratum corneum were damaged or scraped off during body

activity, cells from the basal layer rise to replace them. The epidermis serves as the

body's initial barrier to invading foreign substances. Directly below the basal layer is the

thicker dermis layer, made of dense connective tissue. Embedded within the dermis are

several kinds of important sensory receptors, including thermal receptors, which detect

temperature change, Meissner's corpuscles which are sensitive to touch, and free nerve

endings which detect pain and tissue damage. The dermis also contains many other

structures, including sweat glands, hair follicles and blood vessels.

Clip Two

Watch the video clip and do the exercises below.

1. Multiple choice.

1 ) B 2)D 3)B 4 ) C 5)D

2. Fill in the blanks in the following paragraph.

1) attaches 2) placenta 3) intermingles

4) nutrients 5) rejected 6) organism

7) genetic 8) abdomen 9) belly button

10) pregnancy 11 ) intense 12) doubles

13) 21 14) kicks 15) beats

Script

As the cells divide, the embryo continues its journey along the fallopian tube. About

one week after conception, it reaches the uterus which is prepared to receive it with the

network of blood vessels and glands. By this time, the embryo has become a fluid-filled

sphere of nearly one hundred cells. Some of the cells become finger-like projections that

anchor the embryo to the uterus to draw nourishment and oxygen and to rid the embryo

of wastes. The anchoring cells secrete a hormone that will prevent the disintegration of

the lining. There will be no menstrual period. Three weeks after fertilization, though

still smaller than a grain of rice, the embryo has a primitive heart. Through the

following weeks, its tissues and organs will develop. In four weeks, it looks like this.

Five weeks, about the size of a pea. Six weeks. Seven weeks. At two months, it is called

a fetus and is obviously human, though only the size of a gumdrop. When the embryo

attaches to the uterus, a complex structure of vessels and cells forms, which is called the

placenta. A network of the embryo's blood vessels closely intermingles next to the

mother's blood supply. While their blood doesn't actually mix. Materials such as

oxygen, nutrients and waste products can pass between mother and child. If their blood

were to mix, the embryo would be rejected as the foreign organism that, in fact. It is.

Don't forget it shares only half of its genetic constitution with its mother. From the

placenta emerges the umbilical cord which leads to the abdomen of the fetus. When the

cord is removed at birth, the belly button is created. During pregnancy, the placenta and

umbilical cord serve as pathways for everything the fetus needs to grow. By the third

month. All the organs of the fetus are essentially in place and it enters the period of

intense growth. During the fourth month, the fetus doubles in size. Its muscles twitch,

By the fifth month, it is nearly 21 cm long, about 9 inches. It kicks occasionally. Its heart

beats about 14o times per minute, twice the speed of an adult.

IV. Writing

Improve the following sentences : putting the action in the verb. The underlining may serve

you as a clue.

1 ) At the basal surface, hemidesmosomes aid in attaching the cells to the underlying

basement membrane.

2) Fibrillinmicrofibrils attach the lamina densa to elastic fibers.

3) The rod shaped granules fuse with the plasmalemma.

4) A deficiency of melanin may also occur.

5) When eccrine sweat glands are involved in emotional sweating, they are controlled by

postganglionic sympathetic neurons that release norepinephrine.

V. Autonomous learning

Vocabulary Development

1. Fill in each of the following blanks with an appropriate word.

1 ) tissues 2) forms 3) nucleus 4) ingestion

5) attracting 6) fat 7) skin 8) melanin

9) keratin 10) fetus

2. Match each of the following terms with its meaning and write the appropriate letter to the

left of each number.

Group 1 1 ) d 2) e 3) a 4) b 5) c

Group2 6) j 7) I 8)f 9) g 10) h

3. Give the meaning for each of the following terms.

1 ) before operation

2) above the stomach

3) across the abdomen

4) a poor condition of health caused by a lack of food

5) visible with the naked eye

Supplementary Listening

script

A woman's reproductive cell is commonly called an "egg", but the correct term

"oocyte". Likewise, a man's reproductive cell is widely known as a "sperm", but the

preferred term is "spermatozoon". Following the release of an oocyte from a woman

ovary in a process called ovulation, the oocyte and spermatozoon join within one of the

uterine tubes which are often referred to as fallopian tubes. The uterine tubes link a

woman's ovaries to her uterus or womb. The resulting single-celled embryo is called a

zygote, meaning "yoked or joined together". The zygote's 46 chromosomes represent the

unique first edition of a new individual’s complete genetic blueprint. This master plan

resides in tightly coiled molecules called DNA. They contain the instructions for the

development of the entire body.

Translation of the Texts

Text A

体被系统

体被是人体最大的器官，由皮肤，以及指甲、毛发、汗腺、皮脂腺这样的皮肤附属物组成。一个成年人皮肤重量大约是3-5千克，总表面积约是1.5-2平方米。其厚度，因部位的不同差异较大，大约在0.5-3毫米之间：背部最厚，眼睑处最薄。除了作为一个保护性屏障来抵御损伤(例如：擦伤、割伤、烧伤)、传染性病原体以及紫外线辐射之外，皮肤还可以协助机体进行体温调节、维生素D合成、离子排泄，以及感觉(触觉和痛觉)接受。并且，皮肤具有相当强的再生能力。皮肤由表皮和真皮构成。还有一层疏松的皮下结缔组织，即皮下组织，将皮肤附着在各种皮下结构上，也使身体绝大部分的皮肤可以运动（图1）。

 

1. 表皮

表皮主要由被称作角质形成细胞的细胞构成，角质形成细胞占据了表皮细胞的90%以上。其他的表皮细胞有黑素细胞、梅克尔细胞和朗格汉斯细胞。

表皮分为4或5个独特的层。基底层在最深处，由一层附着于基底膜上的紧密排列的嗜碱性的立方形到柱形的上皮细胞构成。这些细胞有着卵圆形的细胞核，经常呈现出有丝分裂相；它们不断进行细胞分裂，以替代移出表皮的细胞。（由深及浅）第二层是棘层，为多层细胞，其中的多面体细胞由深及浅逐渐变得扁平。(组织)固定导致细胞皱缩，人为地造成细胞突起更加明显，呈棘状或刺状，“棘细胞”因而得名。第三层是颗粒层，由3-5层扁平的细胞组成，其细胞轴与表皮表面平行排列。它们含有大量嗜碱性的透明角质颗粒。颗粒层的表面，即第四层，是一层薄薄的、半透明、弱嗜酸性的细胞层，被称为透明层。透明层只出现在厚皮肤里，而在薄皮肤中缺如。它由几层紧密连接且无核无细胞器的扁平细胞构成。表皮最外层，即角质层，由一些死亡的、无核角质细胞构成；其厚度在不同区域有所变化。一种叫做角蛋白的蛋白质代替了胞浆填充细胞。角质层最表层的细胞通过被称为脱落[原文为desquamation，是否改为“脱皮”？]的过程不断脱离皮肤[不改]（图2A–C）。

 

“基底层或生发层”改为“基底层(或生发层)” 图2B 光学显微镜下位于真皮和表皮交界处的厚厚的皮肤层。最外层的角质层以厚厚的角蛋白层为表征。伸入到表皮层的真皮层乳头包含乳头层的疏松结缔组织。伸入到表皮层的真皮层乳头包含乳头层的疏松结缔组织。H染色。 图2C 光学显微镜下进一步放大后显示的厚皮肤的表皮。表皮层中不断更新的上皮细胞显示从基底层向外发展的进行性分化和角蛋白形成过程。图像显示基底层、棘层（又棘细胞层）、颗粒层以及一小部分角质层的主要特征。图像底部显示出部分真皮层。H染色。

2. 真皮

真皮，是一个富含血管的结缔组织，能为皮肤提供机械支持、柔韧性和抗张强度。血管提供营养并参与体温调节。厚厚的真皮层分为两层：表面的乳头层由疏松结缔组织构成，含有I 、III型胶原纤维，同时还散在分布着弹性纤维、结缔组织细胞以及丰富的毛细血管网；深处的网织层由不规则致密结缔组织构成，由粗大的、交织成束的以I型为主的胶原纤维交织成网。除成纤维细胞之外，真皮里其他的结缔组织细胞包括巨噬细胞、肥大细胞、脂肪细胞、浆细胞和淋巴细胞。

真皮里还有丰富的神经分布，包括复杂的感觉神经网络，以及支配着汗腺、血管平滑肌和立毛肌的传出性交感神经。有髓神经纤维向表皮和真皮内的有被囊的感受器发出神经末梢，这些感受器包括梅氏小体和帕西尼小体。梅氏小体位于真皮乳头层，是介导触觉的机械性刺激感受器；帕西尼小体是较大的有被囊感受器，位于真皮层的更深部和皮下组织里。

3. 皮肤的附属器

外泌汗腺（简称汗腺），是单式卷曲的管状腺体，由分泌部分和狭窄些的分泌管道部分构成。外泌汗腺受胆碱能神经系统支配，主要通过蒸发散热，起到调节和保持体温的作用。它们也可以协助分泌一些离子[不改][原文为iron excretion，是否应为“离子排泄”？]。在正常情况下，作为对环境热刺激和情绪刺激的应答，外泌汗腺每天可以产生500-750ml或者更多的汗液（图3）。

 

图3 光学显微镜下真皮层的外分泌汗腺。在卷曲的腺体分泌部分的横截面和斜剖面中，分泌细胞有相对较淡的细胞质包绕着中央显著的内腔。可以看到一些较小、染色较深的导管剖面和它们特有的双立方形上皮细胞。周围真皮中含有丰富的毛细血管。H染色。

顶泌汗腺，也被称作气味汗腺，是一些大的、有分支的腺体，分布在腋窝、阴囊、包皮、小阴唇、乳头和肛周。它们不如外泌汗腺那样卷曲，而且很多小卷曲会相互融合，形成相互缠绕的管道网络。这些分泌管道的囊状内腔排布着单层立方上皮。此外，相对于外泌汗腺，顶泌汗腺有着更大的直径和体积更大、数量更多的肌上皮细胞。这些细胞与分泌上皮共用一个基底膜。

毛覆盖了身体表面除嘴唇、手掌、足底等处的绝大部分。毛发生于表皮层，穿过真皮层，并且常常延伸到皮下的结缔组织里。每根毛发都由游离的毛干和毛根组成。毛根的下端被管状的毛囊包裹。毛囊包括表皮部分（上皮组织）和真皮部分（结缔组织）。每个毛囊的长轴通常与表皮平面有一定倾斜角。毛是角质化的线状物。分布的身体部位不同，毛呈现出不同的粗细和长度（图4A–B）。

 

Henle layer 亨勒层。图4A 毛囊皮脂腺单位

 

图4B 光学显微镜下接近表皮的薄皮肤。显示立毛肌和毛皮脂单位部分微相关。因为放大比例，此图看不到毛干。皮脂腺及其导管(箭头所指的方向) 开口于毛囊上方末端。外根鞘与表皮在表面上是相延续的。起于下层真皮的立毛肌从毛囊底部斜向延伸至真皮乳头层。H染色。

皮脂腺与毛密切相关，在真皮内，位于毛囊和立毛肌之间。它们是一些全泌腺，其分泌产物中的一种叫做“皮脂”，由细胞分解后的富脂成分组成。大多数皮脂腺通过一条汇入毛干附近的毛囊上部的管道排空自己的分泌物。这些单式的或分支状的泡状腺染色浅，呈卵圆形。每一个腺泡都有一层薄的结缔组织囊包裹，通常数个腺泡开口于一个公共管道。管道内层覆盖的复层扁平上皮延续于外层的毛囊上皮根鞘（图5）。

 

图5 光学显微镜下的一个毛皮脂单位。毛囊的底部末端膨大形成毛球。相关的皮脂腺含有淡色的细胞，这些细胞不断扩大、分解，并开口于毛囊上端的一个管道（箭头所指）。光学仪器使得来自于毛囊基底的毛干呈现黄色。周围真皮是不规则的致密结缔组织。H染色。

 

图 6A 矢状切面 A: Proximal nail fold 近端甲襞 Cuticle 甲上皮 Lunula 甲半月 Nail plate 甲板 Proximal nail matrix 近端甲母质 Dorsal nail plate 甲板背侧 Ventral nail plate 甲板腹侧 Distal phalanx 远节指骨 Hyponychium 甲下皮 图 6B 横切面 B: Dorsal nail plate背侧甲板 Ventral nail plate腹侧甲板 Nail bed 甲床 Eponychium 甲上皮 Lateral nail groove 侧甲沟 图6C 指甲的生长 The proximal nail matrix generates the dorsal layer of the nail plate, and the distal matrix generates the ventral layer. 近端甲母质产生甲板的背层，远端甲母质产生甲板的腹层。 The average growth of toenails is about 1 mm a month. 趾甲平均每月生长一毫米。 The rounded shape of the free edge of the nails is dictated by the shape of the lunula. After avulsion of a nail, the free edge of the new one grows parallel to the lunula. 甲半月的形状决定了指(趾)甲的自由边缘的圆形形状。当原来的指（趾）甲撕脱后，新的指（趾）甲的自由边缘会平行于甲半月生长。

指甲是手指（脚趾）末节指（趾）骨背面的表皮的角质层的修饰物。这些微凸、半透明的指甲板是由多层角质化的鳞状上皮细胞紧密排列形成的。这些细胞含有坚硬的角质蛋白，而且不会脱落。指甲板暴露部和隐藏部的底面是甲床。它由表皮的生发层和其下的致密真皮构成，缺乏皮下组织，但却紧紧附着在末端指(趾)骨的骨膜上（图6A–C）。

Text B

人体发育过程

人体的发育是一个连续的过程，始于女性的卵子与男性的精子结合而完成受精。随后，细胞分裂、迁移、程序性死亡、分化、生长和细胞重排将这个高度特化的全能细胞——受精卵转化为一个多细胞的人体。尽管发育过程中的绝大多数变化出现在胚胎和胎儿时期，重要的变化仍会发生在此后的各个发育期：婴儿期、儿童时期、青少年期甚至成年早期。人体发育并不会止于人出生后。除了生长之外的一些重要变化发生在出生之后（如牙齿和女性乳房的发育）。

1 发育阶段

习惯上将人体发育分为出生前阶段和出生后阶段。在出生前阶段，最明显可见的发育变化出现在胚胎发育的第三至八周。组织和器官的分化和生长发生在胎儿期。此时，身体的生长（速）率增加。

2 胚胎学的重要性

从字面上看，胚胎学是研究胚胎的科学。但实际上胚胎学泛指胚胎以及胎儿的出生前发育。

发育解剖学是关于细胞、组织、器官以及整个机体在双亲的各一个生殖细胞发育为成年个体的过程中所经历的所有变化的胚胎学领域。出生前发育远远快于出生后，并会导致惊人的变化。

畸胎学是胚胎学以及病理学的一个分支，研究的是异常发育（出生缺陷）。这一胚胎学的分支学科涉及扰乱正常发育、导致出生缺陷的各种遗传和（或） 环境因素。

胚胎学

·架起出生前发育与产科学、围产科学、儿科学、临床解剖学之间的桥梁。

·扩充与人类生命起源、出生前发育阶段的变化相关的知识。

·对帮助我们理解人体结构变异产生的原因具有实用价值。

·阐明大体解剖学，并解释正常和异常的关系是如何发展的。

临床医师掌握人体正常发育以及发育异常的原因的相关知识是十分必要的，它可以为胚胎和胎儿正常发育提供最大可能性。产科学的许多现代实践都离不开应用胚胎学。对于产科医生而言，特别关注的胚胎学话题是：排卵、卵细胞和精子的转运、受精、植入、胎儿与母体的关联、胎儿的血液循环、发育的关键时期以及导致出生缺陷的原因。除了对母亲的照料，产科医生还守护着胚胎和胎儿的健康。胚胎学对于儿科医生的意义是显而易见的，因为他们的部分病人出现因发育异常导致的出生缺陷如膈疝、脊柱裂、先天性心脏病等。

发育异常是导致婴儿期死亡的主要原因。人体结构和功能发育的知识对于理解新生儿期的生理变化和帮助处于病痛中的胎儿、婴儿是必不可少的。外科手术的发展，特别是针对胎儿、围产期及儿科年龄段患者的手术的发展使得人体发育知识更具有临床重要性。如今对于胎儿实施外科手术已成为可能。理解和纠正绝大多数的先天异常必须依赖正常发育和可能产生的异常发育的相关知识。对于常见先天异常及其原因的认识也使得内科医生、牙科医生和其他医疗工作者能够解释异常情况的发育基础，这常常能消除父母的内疚感。

3 人体发育的分子生物学

分子生物学领域的快速发展，使得一些先进技术得到广泛应用（如重组DNA技术、嵌合模型、转基因小鼠和干细胞操作）。这些技术如今广泛应用于实验室研究以解决各种问题：如形态发生的遗传调节、特定基因的暂时的和区域性的表达、以及细胞是如何定向（分化）形成胚胎的各个部分的。我们首次开始理解：在正常和异常发育过程中，特定基因是何时、何地、以何种方式激活和表达的。

1997年，Ian Wilmut和他的团队运用体细胞核移植技术第一次成功克隆了哺乳动物——绵羊“多利”。此后，其他动物也陆续地被从培养的成体分化细胞成功克隆。由于涉及社会、伦理和法律等方面的问题，对于“克隆人”的兴趣引发了很大争议。另外，也存在这样的担忧——克隆人可能导致有先天缺陷和严重疾病的婴儿的出生。

人类胚胎干细胞具有多能性，既能自我更新，又能分化为特定的细胞类型。人类胚胎干细胞的分离和程序化培养为治疗退行性、恶性肿瘤性和遗传性疾病提供了很大的可能性。

4 胚胎学的描述性术语

部分标准的拉丁文形式的术语给出了对应英文，如sperm (spermatozoon)。临床常用的以人名命名的术语在括号内出现，如输卵管(fallopian tube)。解剖学和胚胎学使用了一些与位置、方向相关的术语，均以人体不同的平面为参照。所有关于成人的描述都是基于“身体直立，双上肢下垂于躯干的两侧且掌心向前”的假设姿势(图1A)。这便是解剖学姿势。术语“前”或“腹侧”与“后”或“背侧”用于描述身体或四肢的前、后以及身体内部结构的相对位置。当描述胚胎时，常用到术语“腹侧”和“背侧” (图1B)。“上方”和“下方”用于指示不同结构的相对水平(图1A)。对于胚胎而言，用术语“颅侧（嘴侧）”和“尾侧”来表示相对于头端和尾端的位置关系(Fig. 1B)。相对于一个结构的起源或附着的位置用“近端”和“远端”来表示。如在下肢，膝关节位于踝关节的近端而踝关节位于膝关节的远端。

正中矢状平面是一个假想的将人体分为左、右两部的纵切面(图1C)。术语“外侧”和“内侧”分别指身体结构相对于正中矢状面较远或较近。矢状面是指任何一个经过人体的平行于正中矢状面的垂直平面(图1C)。横断面（轴面）是指任何一个与正中矢状面以及冠状面均成直角的平面(图1D)。额状面（冠状面）是指任何一个与正中矢状面垂直相交的平面(图1E)，它将人体分为前（腹侧）以及后（背侧）两部分。

 

图1. 图片说明人体的体位，方向和截面。A．成人的侧面解剖体位。B．五周胎儿的侧面图。C 和D. 六周胎儿的腹侧图。E．七周胎儿的侧面图。在描述生长的过程时，有必要使用一些词语来表示出一个部位相对于另一个部位或者相对于整个身体的位置。例如，脊柱是在胎儿的背部生长，而胸骨则是位于胎儿的腹部。