Introduction to developmental anatomy-embryology Dr Peter Gichangi BSc, MBCHB, MMED(O/G), Ph.D.

Objectives • Describe: –Developmental period –Embryological terminologies –Significance of embryology –Descriptive embryological terminologies

Beginning • Human development is a continuous process that begins when an oocyte (ovum)is fertilized by a sperm (spermatozoon). • Cell division, cell migration, programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, a zygote, into a multicellular human being.

Changes • Although most developmental changes occur during the embryonic and fetal periods, important changes occur during later periods of development: • Infancy, childhood, adolescence, and early adulthood • Development does not stop at birth. – Important changes occur later.

DEVELOPMENTAL PERIODS • It is customary to divide human development into prenatal (before birth) and postnatal (after birth) periods. • Most visible advances occur during the third to eighth weeks of embryonic development. • During the fetal period, differentiation and growth of tissues and organs occur. • The rate of body growth increases during this period.

Figure 1-1 Early stages of development. Development of an ovarian follicle containing an oocyte, ovulation, and the phases of the menstrual cycle are illustrated. Human development begins at fertilization, approximately 14 days after the onset of the last normal menstrual period. Cleavage of the zygote in the uterine tube, implantation of the blastocyst in the endometrium (lining) of the uterus, and early development of the embryo are also shown. Beginning students should not attempt to memorize these tables or the stages (e.g., that stage 3 begins on day 4 and stage 5 on day 7).

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Figure 1-2 The embryonic period terminates at the end of the eighth week; by this time, the beginnings (primordia) of all essential structures are present. The fetal period, extending from 9 weeks to birth, is characterized by growth and elaboration of structures. Sex is clearly distinguishable by 12 weeks. Fetuses are viable 22 weeks after fertilization, but their chances of survival are not good until they are several weeks older. The 11- to 38-week fetuses shown are approximately half of their actual sizes. For more information, see Chapter 6.

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Carnegie Stages of Early Hunan Embryonic Development (weeks 1-8)

Age (days)•

External Features

1 23 4·5 6 7·12 17 19 23

Fertilized oocy1e 1 0.1 Morula (4 16 cells) 2 0.1 Free blastocyst 3 0.1 Attachment of blastocyst to endometrium 4 0.1 Impal ntation,bllaminar embryo wtih primary yolk sac 5 0.10·.2 Trilaminar embryo with primitive streak,chorionic villi 6 0.2 0.3 Gastrulation, formation of notochordal process 7 0.4 Hensen's node and prlmitlve pit. notochord and 8 1·1.5 neurenteric canal,appearance of neural plate,neural folds,and blood islands Appearance of first somites,deep neural groove, 9 1.5 2.5 elevation of cranial neural folds,early heart tubes Beginning of fusion of neural folds,formation of optic 10 2 3.5 sulci,presence of first two pharyngeal arches,beginning heart beat. curving of embryo Closure of cranial neuropore, formation of optic vesicles, 11 2.5·4.5 rupture of oropharyngeal membrane Closure of caudal neuropore, formation of pharyngeal 12 3·5 arches 3 and 4, appearance of upper limb buds and tail bud, formation of otic vesicle Appearance of lower limb buds,lens placode,separation 13 46 of otic vesicle from surface ectoderm Formation of lens vesicle,opt c cup, and nasal pits 14 5·7 Development of hand plates,primary urogenital sinus, 15 7·9 prominent nasal pits,evidence of cerebral hemispheres Development of foot plates,visible retinal pigment, 16 8·11 development of auricular hillocks,formation of upper lip Appearance of finger rays,rapid head enlargement,six 17 11·14 auricular hillocks, formation of nasolacrlmal groove Appearance of toe rays and elbow regions,beginning of 18 13·17 formation of eyelids,tip of nose distinct. presence of nipples Elongation and straighten!ng of trunk, beginning of 19 16 18 herniation of midgut Into umbilical cord Bending of arms at elbows, distinct but webbed fingers, 20 18·22 appearance of scalp vascular plexus,degeneration of anaI and urogenitalmembranes Longer and free fingers,distinct but webbed toes, 21 22·24 inditterent external genitalia Longer and free t s.better development of eyelids and 22 2 3 28 external ear More rounded head, fusion of eyelids 23 27-31

25 28 29 30 32 33 36 38 41 44 46 49

51 53 56

Carnegie stage Crown-Rump Length(mm)

Pairs of somites

1-3 4-12 13-20 21-29 30-31

EMBRYOLOGIC TERMINOLOGY • Oocyte (L. ovum, egg). The female germ or sex cells are produced in the ovaries. When mature, the oocytes are called secondary oocytes or mature oocytes. • Sperm (Gr. sperma, seed). The sperm, or spermatozoon - male germ cell produced in the testes (testicles). • Zygote. From union of an oocyte and a sperm during fertilization. A zygote or embryo is the beginning of a new human being.

Gestational Age • Calculated from the presumed first day of the last normal menstrual period. • This is the gestational age, which is approximately 2 weeks longer than the fertilization age because the oocyte is not fertilized until approximately 2 weeks after the preceding menstruation.

Cleavage • This is the series of mitotic cell divisions of the zygote that result in the formation of early embryonic cells, blastomeres. • The size of the cleaving zygote remains unchanged because at each succeeding cleavage division, the blastomeres become smaller.

Morula (L. morus, mulberry) • This solid mass of 12 to approximately 32 blastomeres-formed by cleavage. • The blastomeres change their shape and tightly align themselves against each other to form a compact ball of cells-compaction. • Occurs 3 to 4 days after fertilization, just as the early embryo enters the uterus.

Blastocyst (Gr. blastos, germ + kystis, bladder)

• Formed when a fluid-filled cavity, the blastocystic cavity, develops inside the morula. • Blastocyst centrally located cells, the inner cell mass or embryoblast, is the embryonic part of the embryo.

Implantation • The process during which the blastocyst attaches to the endometrium, the mucous membrane or lining of uterus, and subsequently embeds in it. • The preimplantation period of embryonic development is the time between fertilization and the beginning of implantation, a period of approximately 6 days.

Gastrula (Gr. gaster, stomach) • The blastocyst is converted into a gastrula: a three-layered or trilaminar embryonic disc forms structure during the third week. • The three germ layers of the gastrula are: ectoderm, mesoderm, and endoderm • The layers differentiate into the tissues and organs of the embryo.

Neurula (Gr. neuron, nerve) • It is the first appearance of the nervous system and the next stage after the gastrula. • This occurs during the third and fourth weeks.

Embryo (Gr. Embryon) • This is the early developing human. • Embryonic period extends to the end of the eighth week (56 days), by which time the beginnings of all major structures are present. • The size of embryos is given as crown-rump length, which is measured from the vertex of the cranium (crown of head) to the rump (buttocks).

Stages of Prenatal Development • Early embryonic development is described in stages to reflect morphologic characteristics. • Stage 1 begins at fertilization and embryonic development ends at stage 23, which occurs on day 56. • The fetal period begins on day 57 and ends when the fetus is completely outside the mother.

Conceptus (L. conceptio) • This the product of conception. • The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic e.g. the placenta and its associated membranes.

Primordium (L. primus) • The beginning or first discernible indication of an organ or structure e.g. the primordium of the upper limb appears as a bud on day 26. • The terms anlage and rudiment have similar meanings.

Fetus (L. unborn offspring) • After the embryonic period (8 weeks-56 days) and until birth, the developing human is called a fetus. • During the fetal period (ninth week to birth), differentiation and growth of the tissues and organs formed during the embryonic period occur. • These developmental changes are not dramatic.

Abortion (L. aboriri, to miscarry) • A premature stoppage of development and expulsion of a conceptus from the uterus or expulsion of an embryo or fetus before it is viable-capable of living outside the uterus. • An abortus is the products of an abortion (i.e. the embryo/fetus and its membranes).

Trimester • A period of three calendar months during a pregnancy. • Obstetricians commonly divide the 9-month period of gestation into three trimesters. • The most critical stages of development occur during the first trimester (13 weeks) when embryonic and early fetal development is occurring.

Postnatal Period • The period occurring after birth. • Infancy refers to the earliest period of extrauterine life, roughly the first year after birth. – An infant aged 1 month or younger is called a newborn or neonate. – The body grows particularly rapidly • total length increases by approximately one half • weight is usually tripled.

Childhood • This is the period from approximately 13 months until puberty. • Just before puberty, growth accelerates-the prepubertal growth spurt.

Puberty • Occurs usually between the ages of 12 and 15 years in girls and 13 and 16 years in boys, • Features – secondary sexual characteristics develop – capability of sexual reproduction is attained • Puberty ends – in females with the first menstrual period or menarche, – in males when mature sperms are produced

Adolescence • The period from approximately 11 to 19 years of age, • Characteristics – rapid physical and sexual maturation. – extends from the earliest signs of sexual maturitypuberty-until the attainment of adult physical, mental, and emotional maturity.

Adulthood (L. adultus, grown up) • Attainment of full growth and maturity, is generally reached between the ages of 18 and 21 years. • Ossification and growth are virtually completed during early adulthood (21 to 25 years). • Thereafter, developmental changes occur very slowly.

SIGNIFICANCE OF EMBRYOLOGY • Bridges the gap between prenatal development and obstetrics, perinatal medicine, pediatrics, and clinical anatomy. • Develops knowledge concerning the beginnings of human life and the changes occurring during prenatal development. • Is of practical value in helping to understand the causes of variations in human structure. • Illuminates gross anatomy and explains how normal and abnormal relations develop.

Teratology (Gr. teratos, monster) • This is part of embryology and pathology that deals with abnormal development (birth defects). • This branch of embryology is concerned with various genetic and/or environmental factors that disturb normal development and produce birth defects.

Historical perspectives

Figure 1-3 Illustrations from Jacob Rueff's De Conceptu et Generatione Hominis (1554) showing the fetus developing from a coagulum of blood and semen in the uterus. This theory was based on the teachings of Aristotle, and it survived until the late 18th century. (From Needham J: A History of Embryology. Cambridge, University Press, 1934; with permission of Cambridge University Press, England.)

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Figure 1-5 A, Photograph of a 1673 Leeuwenhoek microscope. B, Drawing of a lateral view illustrating the use of this primitive microscope. The object was held in front of the lens on the point of the short rod, and the screw arrangement was used to adjust the object under the lens.

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Figure 1-6 Copy of a 17th-century drawing of a sperm by Hartsoeker. The miniature human being within it was thought to enlarge after the sperm entered an ovum. Other embryologists at this time thought the oocyte contained a miniature human being that enlarged when it was stimulated by a sperm.

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DESCRIPTIVE TERMS IN EMBRYOLOGY • Descriptions based on anatomical position • The terms anterior or ventral and posterior or dorsal are used to describe the front or back of the body or limbs and the relations of structures within the body to one another.

Embryos • When describing embryos, the terms dorsal and ventral are used. • Superior and inferior are used to indicate the relative levels of different structures. • For embryos, the terms cranial or rostral and caudal are used to denote relationships to the head and caudal eminence (tail), respectively.

Distance • Distances from the source or attachment of a structure are designated as proximal or distal. • In the lower limb, for example, the knee is proximal to the ankle and the ankle is distal to the knee.

Other planes 1 • The median plane is an imaginary vertical plane of section that passes longitudinally through the body. • Median sections divide the body into right and left halves. • The terms lateral and medial refer to structures that are, respectively, farther from or nearer to the median plane of the body.

Other planes 2 • A sagittal plane is any vertical plane passing through the body that is parallel to the median plane. • A transverse (axial) plane refers to any plane that is at right angles to both the median and coronal planes. • A frontal (coronal) plane is any vertical plane that intersects the median plane at a right angle and divides the body into anterior or ventral and posterior or dorsal parts.

Figure 1-7 Drawings illustrating descriptive terms of position, direction, and planes of the body. A, Lateral view of an adult in the anatomical position. B, Lateral view of a 5-week embryo. C and D, Ventral views of 6-week embryo. E, Lateral view of a 7-week embryo. In describing development, it is necessary to use words denoting the position of one part to another or to the body as a whole. For example, the vertebral column (spine) develops in the dorsal part of the embryo, and the sternum (breast bone) in the ventral part of the embryo.

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