Chapter 29

Development and Inheritance

 

Lecture Outline

Development and Inheritance

•      From fertilization to birth

–   fertilization

–   implantation

–   placental development

–   fetal development

–   gestation

–   labor

–   parturition (birth)

 

INTRODUCTION

•      The first two months following fertilization is the period of embryonic development and the developing human is an embryo.

•      From week nine until birth is the fetal development period and the individual is a fetus.

•      Prenatal development is the time from fertilization until birth.  It is divided into three trimesters.

Terminology of Development

Summary

•       Gestation period

–    fertilization to birth (38 weeks)

•       Prenatal period (before birth)

–    embryological development

•    first 2 months after fertilization (embryo)

•    all principal adult organs are present

–    fetal development

•    from 9 weeks until birth (fetus)

•    placenta is functioning by end of 3rd month

•       Neonatal period

–    first 42 days after birth

INTRODUCTION

•      Developmental anatomy is the study of the sequence of events from the fertilization of a secondary oocyte to the formation of an adult organism.

•      Embryology is the study of development from fertilization to the fetal period.

•      Obstetrics is the branch of medicine that deals with the management of pregnancy, labor, and the neonatal period (the first 42 days after birth).

EMBRYONIC PERIOD

From Fertilization to Implantation

First Week of Development

•      Fertilization

–   During fertilization, the genetic material from a haploid sperm cell (spermatozoon) and a haploid secondary oocyte merges into a single diploid nucleus.

–   Fertilization normally occurs in the uterine (Fallopian) tube when the oocyte is about one-third of the way down the tube to the uterus, usually within 12 to 24 hours after ovulation. (Oocyte usually dies in 24 hours)

•      The process leading to fertilization begins as peristaltic contractions and the actions of cilia transport the oocyte through the uterine tube.

–   Sperm swim up the uterus and into the uterine tube by the whip like movements of their tails (flagella) and muscular contractions of the uterus.

Fertilization

•      The functional changes that sperm undergo in the female reproductive tract that allow them to fertilize a secondary oocyte are referred to as capacitation.

•      To fertilize an oocyte, a sperm must penetrate the corona radiata and zona pellucida around the oocyte (Figure 29.1a).

•      A glycoprotein in the zona pellucida (ZP3) acts as a sperm receptor, binds to specific membrane proteins in the sperm head and triggers the acrosomal reaction, the release of the contents of the acrosome.

•      The acrosomal enzymes digest a path through the zona pellucida allowing only one sperm to make its way through the barrier and reach the oocyte’s plasma membrane.

Events Before Fertilization

•       transport the oocyte towards the uterus

–    peristalsis of uterine tube

–    movement of cilia

–    oocyte releases chemical attractants

•       sperm swim towards oocyte

–    flagella

–    prostaglandins (within the semen) stimulate uterine contractions that help propel sperm

•       capacitation (final maturation of the sperm) occurs within female

–    acrosomal membrane becomes fragile

Fertilization

•      Fusion of a sperm with a secondary oocyte is called syngamy.

•      Polyspermy is prevented by chemical changes that prevent a second sperm from entering the oocyte.

Sperm Contact during Fertilization

•       Sperm penetrates the granulosa cells around the oocyte (corona radiata)

•       Sperm digests its way through
the zona pellucida

–    ZP3 glycoprotein binds to sperm head, triggering the acrosomal reaction
(enzyme release)

•       Once a sperm enters a secondary oocyte, the oocyte completes meiosis, and the male pronucleus and female pronucleus fuse forming the fertilized ovum or zygote (Figure 29.1c).

Sperm Contact during Fertilization

•       First sperm to fuse with oocyte membrane triggers the slow & the fast block to polyspermy

–    1-3 seconds after contact, oocyte membrane depolarizes & other cells can not fuse with it = fast block to polyspermy

–    depolarization triggers the intracellular release of Ca+2 causing the exocytosis of molecules hardening the entire zona pellucida = slow block to polyspermy

 

Twins

•      Fraternal twins (dizygotic)

–   independent release of 2 oocytes fertilized by 2 separate sperm

–   genetically as different as any 2 siblings

•      Identical twins (monozygotic)

–   2 individuals that develop from a single fertilized ovum

–   genetically identical & always the same sex

–   if ovum does not completely separate,  conjoined twins (share some body structures)

Cleavage of the Zygote

•      Early rapid mitotic cell division of a zygote is called cleavage (Figure 29.2).

•      The cells produced by cleavage are called blastomeres.

•      Successive cleavages produce a solid mass of cells, called the morula (Figure 29.2).

Events Within the Egg

•      Sperm entry, triggers oocyte to complete meiosis II and dump second polar body

•      Once inside the oocyte, the sperm loses its tail & becomes a male pronucleus

•      Fusion of male & female haploid pronuclei is the true moment of fertilization

•      Fertilized ovum (2n) is called a zygote

–   zona pellucida still surrounds it

Formation of the Morula

•       Rapid mitotic cell division of embryo is called cleavage

•       1st cleavage in 30 hours produces 2 blastomeres

•       2nd cleavage on 2nd day

•       By 3rd day has 16 cells

•       By day 4 has formed a solid
ball of cells called a morula

Blastocyst Formation

•      As the number of cells in the morula increases, it moves from the site of fertilization down through the ciliated uterine tube toward the uterus and enters the uterine cavity.

•      The morula develops into a blastocyst, a hollow ball of cells that is differentiated into

–   a trophoblast (which will form the future embryonic membranes)

–   an inner cell mass or embryoblast (the future embryo)

–   an internal fluid-filled cavity called the blastocele (Figure 29.2e).

Development of the Blastocyst

•      A blastocyst is a hollow ball of cells

–   enters the uterine cavity

    by day 5

–   outer covering is the

     trophoblast

–   inner cell mass

–   fluid-filled cavity is
the blastocele

•      Trophoblast & part of inner
cell mass will develop into

     the fetal portion of placenta

•      Most of the inner cell mass will become embryo.

Stem cell research and therapeutic cloning

•      Stem cells are unspecialized cells that have the ability to divide for indefinite periods and to give rise to specialized cells.

•      Pluripotent cells such as those of the inner cell mass can give rise to many different types of cells.

–   Scientists hope to remove pluripotent cells and use them to grow tissues to treat particular diseases.

•      Scientists are also studying adult stem cells.

–   Studies have suggested that stem cells in human adult bone marrow are pluripotent and therefore have potential clinical significance.

Implantation

•      The blastocyst remains free with the cavity of the uterus for two to four days before it actually attaches to the uterine wall.

•      The attachment of a blastocyst to the endometrium occurs seven to eight days after fertilization and is called implantation (Figure 29.3).

 

•      Trophoblast develops 2 distinct layers:

–   syncytiotrophoblast secretes enzymes that digest the endometrial cells

–   cytotrophoblast is distinct layer of cells that defines the original shape of the embryo

•      Trophoblast secretes human chorionic gonadotropin (hCG) that helps the corpus luteum maintain the uterine lining

 

 

Implantation

•      Following implantation the endometrium is known as the decidua and consists of three regions: the decidua basalis, decidua capuslaris, and decidua parietalis.

•      The decidua basalis lies between the chorion and the stratum basalis of the uterus. It becomes the maternal part of the placenta.

•      The decidua capsularis covers the embryo and is located between the embryo and the uterine cavity.

•      The decidua parietalis lines the noninvolved areas of the entire pregnant uterus.

•      The major events associated with the first week of development are summarized in Figure 29.5.

Clinical Application

•       Ectopic pregnancy refers to the development of an embryo or fetus outside the uterine cavity.

•       Most occur in the uterine tube

–    usually in the ampullar or infundibular portions

–    some occur in the ovaries, abdomen, uterine cervix, or broad ligaments.

•       Common causes are blockages of uterine tube such as tumors or scars from pelvic inflammatory disease

•       symptoms are missed menstrual cycles, bleeding & acute pain

•       Twice as common in smokers because nicotine paralyzes the cilia

 

•       Depending on the location of the ectopic pregnancy, the condition can become life threatening to the mother.

Development of the Trophoblast

•       trophoblast è syncytiotrophoblast and cytotrophoblast (Figure 29.6a) è part of the chorion as they undergo further growth (Figure 29.11 inset).

•      The cells of the inner cell mass differentiate into two layers that form a flattened disc referred to as the bilaminar embryonic disc (Figure 29.6a).

•   hypoblast (primitive endoderm)

•   epiblast (primitive ectoderm)

Beginnings of Organ Systems(Gastrulation)

•       Day 8

–    cytotrophoblast forms amnion & amnionic cavity

•    cells of inner cell mass on amnionic cavity form ectoderm

•    cells bordering on blastocele form endoderm

–    ectoderm & endoderm together form embryonic (bilaminar) disk

•       Day 12

–    endodermal cells divide
to form a hollow sphere
(yolk sac)

–    cytotrophoblast cells
divide to fill the spaces
surrounding the yolk

    sac with extraembryonic

    mesoderm

•    spaces develop in that layer to form future ventral body cavity

Primary Germ Layers

•      Day 14 --cells of embryonic disc produce 3 distinct layers

•   endoderm è epithelial lining of GI & respiratory

•   mesoderm è muscle, bone & other connective tissues

•   ectoderm è epidermis of skin & nervous system

Development of the Amnion

•      Amniotic fluid protects the developing fetus and can be examined in a procedure known as amniocentesis.

Formation of Embryonic Membranes

•       Yolk sac

–    site of early blood formation

–    gives rise to gonadal stem cells (spermatogonia & oogonia)

•       Amnion

–    develops from the epiblast

–    thin, protective membrane called the amnion

–    Initially the amnion overlies only the bilaminar embryonic disc; as the embryo grows it eventually surrounds the entire embryo creating the amniotic cavity (Figure 29.11a inset).

–    surrounds embryo with fluid: shock absorber, regulates body temperature & prevents adhesions

–    fluid is filtrate of mother’s blood + fetal urine

–    May be examined for embryonic cells (amniocentesis)

Amnion, Yolk sac, Chorion, allantois

 

•      Chorion

–   becomes the embryonic contribution to the placenta

–   derived from trophoblast & mesoderm lining it

–   gives rise to human chorionic gonadotropin (hCG)

•      Allantois

–   outpocketing off yolk sac that becomes umbilical cord

 

 

Development of the Yolk sac

•      The hypoblast cells migrate and become the exocoelomic membrane.

•      The hypoblast and the exocoelomic membrane form the yolk sac.  (Figure 29.6b)

 

•      The yolk sac has several important functions.

–   transfers nutrients to the embryo

–   early source of blood cells

–   produces primitive germ cells, which will become spermatogonia and oogonia.

Amnion, Yolk sac, Chorion, Allantois

Amnion, Yolk sac, Chorion, Allantois

Development of Sinusoids

•      ninth day

–   blastocyst is completely embedded in the endometrium

–   syncytiotrophoblast expands and small spaces called lacunae develop within it (Figure 29.6b).

•      twelfth day

–   lacunae fuse to form lacunar networks (Figure 29.6c).

–   Endometrial capillaries around the developing embryo become dilated and are referred to as sinusoids.

•      The synctiotrophoblast erodes the sinusoids and endometrial glands permitting maternal blood to enter the lacunar networks.

•      After the extraembryonic mesoderm develops, several large cavities develop in the extraembryonic mesoderm.  These cavities fuse to form the extraembryonic coelom (Figure 29.6c)

21 Days

Development of the Chorion

•      The chorion develops from extraembryonic mesoderm and the two layers of the trophoblast (Figure 29.6c).

•      The chorion becomes the principal embryonic part of the placenta.

•      The chorion secretes hCG, an important hormone of pregnancy (Figure 29.16).

Parts of Endometrial Lining

•      Decidua = all of endometrium lost as placenta

–   equals all of the endometrium, except stratum basalis

•      Decidua basalis---portion of
endometrium deep to chorion

•      Decidua capsularis---part of
endometrial wall that covers
implanted embryo

•      Decidua parietalis---part of
endometrial wall not modified
by embryo until embryo bumps into it as it enlarges

•      Decidua capsularis fuses with decidua parietalis

Decidua

Umbilical Cord

•      Contents

–   2 arteries that carry blood to the placenta

–   1 umbilical vein that carries oxygenated blood to the fetus

–   primitive connective tissue

•      Stub drops off in 2 weeks leaving scar (umbilicus)

Placenta Previa

•      Placenta is implanted near or covering os of cervix

–   occurs in 1 to 250 live births

•      May lead to spontaneous abortion, premature birth or increased maternal mortality

•      Major symptom is sudden, painless bright red vaginal bleeding in the 3rd trimester

•      Cesarean section is preferred delivery method

Fetal Ultrasonography

•      Transducer emits high-frequency sound waves

–   reflected sound waves converted to on-screen image called sonogram

–   patient needs full bladder

•      Used to determine fetal age, viability, growth, position, twins and maternal abnormalities

Third Week of Development

22-28 days

28 days

Placenta & Umbilical Cord

•      Placenta forms during 3rd month

–   chorion of embryo & stratum functionalis layer of uterus

•      Chorionic villi extend into maternal blood filled intervillous spaces --- maternal & fetal blood vessels do not join & blood does not mix

–   diffusion of O2, nutrients, wastes

–   stores nutrients & produces hormones

–   barrier to microorganisms, except some viruses

•   AIDS, measles, chickenpox, poliomyelitis, encephalitis

–   not a barrier to drugs such as alcohol

•      Placenta detaches from the uterus (afterbirth)             

 

 

Gastrulation

•      During gastrulation the two-dimensional bilaminar embryonic disc transforms into a two-dimensional trilaminar embryonic disc consisting the three primary germ layers

–   ectoderm

–   mesoderm

–   endoderm

•      Gastrulation begins with the development of the primitive streak (Figure 29.7c).

•      Cells of the epiblast move inward below the primitive streak and detach from the epiblast (Figure 29.7b).

Gastrulation

•      The primary germ layers form all tissues and organs of the developing organism (Table 29.1)

•      A solid cylinder of cells the notochord also develops (Figure 29.8).  It plays an important role in the process of induction.

•      The oropharyngeal membrane that will eventually connect the mouth cavity to the pharynx and the remainder of the gastrointestinal tract appears (Figure 29.8 a, b).

•      The cloacal membrane that will form the openings of the anus and urinary and reproductive tracts also appears.

•      The allantois, a vascularized out pouching of the yolk sac extends into the connecting body stalk (Figure 29.8b).  It is not a prominent structure in humans (Figure 29.11a inset).

Neurulation

•      The notochord induces the ectodermal cells over it to form the neural plate (Figure 29.9a)

–   neural plate è the neural folds and neural groove that will fuse to form the neural tube (Figure 29.9d).

–   Ectodermal cells migrate è neural crest (Figure 14.26) which give rise spinal and cranial nerves and their ganglia, autonomic nervous system ganglia, the meninges of the brain and spinal cord, the adrenal medullae, and several skeletal and muscular components of the head.

Neurulation

•      The head of the neural tube è three primary vesicles

–   prosencephalon

–   mesencephalon

–   rhombencephalon (Figure 14.26)

•      Later the secondary vesicles will develop. 

–   telencephalon

–   diencephalon

–   metencephalon

–   myelencephalon.

•      Neural tube defects (NTDs) are caused by arrest of the normal development and closure of the neural tube.  These include anencephaly and spina bifida (Clinical Application).

Development of somites

•      The somites, a series of paired, cube-shaped structures, develop from the mesoderm.

•      Eventually 42-44 pairs of somites will develop.

•      Each somite has three regions (Figure 10.20b).

–   Myotome

–   Dermatome

–   Sclerotome

Development of the intraembryonic coelom

•      Small spaces in the lateral plate mesoderm fuse to form a larger cavity, the intraembryonic coelom. 

•      This cavity splits the lateral plate mesoderm into two parts called the splanchnic mesoderm and the somatic mesoderm (Figure 29.9d).

–   The intraembryonic mesoderm divides into the pericardial, pleural, and peritoneal cavities.

–   Splanchnic mesoderm forms portions of the heart, respiratory and digestive systems.

–   Somatic mesoderm gives rise to bones, ligaments, and dermis of the limbs and the parietal layer of the serous membranes.

Development of the cardiovascular system

•      Angiogenesis, the formation of blood vessels, begins in the extraembryonic mesoderm in the yolk sac, connecting stalk, and chorion.

–   initiated when angioblasts aggregate to form isolated masses of cells referred to a blood islands (Figure 21.32). 

–   Angioblasts form the walls of the blood vessels

–   Spaces in the blood islands from the lumen of blood vessels.

•      The heart forms in the cardiogenic area of the splanchnic mesoderm.

•      The mesodermal cells form a pair of endocardial tubes (Figure 20.18).

–   The tubes fuse to form a single primitive heart.

Development of the chorionic villi and placenta

•      Chorionic villi develop as projections of the cytotrophoblast that eventually contain blood filled capillaries (Figure 29.10b).

•      Blood vessels in the chorionic villi connect to the embryonic heart by way of umbilical arteries and veins (Figure 29.10c).

•      The placenta has a fetal portion formed by the chorionic villi of the chorion and a maternal portion formed by the decidua basalis of the endometrium (Figure 29.11a)

Development of the chorionic villi and placenta

•      Functionally the placenta allows oxygen and nutrients to diffuse from maternal blood to fetal blood that carbon dioxide and wastes diffuse from fetal blood into maternal blood.

–   also serves as a protective barrier

–   stores nutrients

–   secretes several important hormones

•      The connection between the placenta and the embryo is the umbilical cord (Figure 29.11a).

•      After the birth of the baby, the placenta detaches from the uterus and is therefore termed the afterbirth.

Clinical Application

•      Placenta previa is a condition in which part or the entire placenta becomes implanted in the lower portion of the uterus, near or over the internal os of the cervix.  If detected during pregnancy (either by ultrasound or as a result of sudden painless bright red vaginal bleeding during the third trimester), cesarean section is the preferred method of delivery.

Fourth week of Development

•      Embryonic folding converts the embryo from a flat, two-dimensional trilaminar embryonic disc to a three-dimensional cylinder.

•      Development of the somites and the neural tube occurs during the fourth week.

•      Several pharyngeal (branchial) arches develop on each side of the future head and neck regions (Figure 29.13).