Conception and development of the embryo and the fetus Nursing Lecture

development of the embryo and the fetus
Pregnancy
Conception and development of the embryo and the fetus.
The human being is conceived by the union of a female germ cell, the egg or ovum with a male germ cell, the spermatozoon. The germ cells  (gametes), become fused into one cell or zygote which develop into the embryo. The subsequent human development is divided into  three essential phases.
Growth: Increase in size, involves cell division and elaboration of cell products.
Morphogenesis: development of form. Formation of tissues and organs.
Differentiation: Maturation of physiologic processes, resulting in the organs being able to perform specialized functions.Development of gametes.
The gametes are formed by the process of cell division. The human body is made up of millions of cells called somatic cells. The gonads contain special cells called gametes. The male testes produce spermatozoa and the female ovary produces ovum. The somatic cells contain 46 chromosomes – 22 pairs of autosomes and 1 pair of sex chromosomes XY (male) and XX (female). The gametes contain only half the number of chromosomes. 22 autosomes + one X chromosome in the ovum and 22 autosomes + one X or Y chromosome in the sperm.Development of gametes.
The cells multiply by two processes
Mitosis: The process by which body cells replicate to produce two new identical cells. The cells have diploid number (n = 46) of chromosomes.
Meiosis: The process by which the male and female germ cells are produced. The gametes have a haploid number (n = 23)of chromosomes.
– Meiosis is essential for sexual reproduction.

Gametogenesis: formation and development of male and female gametes. Gametogenesis in man is called spermatogenesis and in women is called oogenesis.

Maturation of ovum and sperm
ovum
– At birth each ovary contains a huge number of undeveloped ova (approximately 400.000).
– These are large rounded cells with clear cytoplasm and nucleus in the center.
– Each ovum is surrounded by a layer of few small, flattened or spindle shaped cells. The whole structure is called follicle but at birth or during underdeveloped state it called primordial follicle.
Maturation of ovum and sperm
– As proliferation of cells continues an important fluid called follicular fluid develops between them.
– During puberty the cells within the follicle produce estrogen.
– During female reproductive cycle, several follicles develop but only one of these is selected for complete maturation and ovulation.

Maturation of ovum.
primordial germ cells

primordial germ cells
Gametogenesis
Gametogenesis
Structure of the ovum

Structure of the ovum
The ovum has a nucleus With 23 chromosomes surrounded by cytoplasm and an outer cell membrane. It is surrounded by a translucent coat, zona pellucida. Very close and connected to zona pellucida is a layer of follicular cells called corona radiata. Outer to this is a more loosely arranged layer of follicular cells called cumulus oophorus. The ovum is 0.2 mm. In diameter and barely visible to the naked eye.
Maturation of sperm
Production of spermatozoa is initiated and maintained in seminiferous tubules of the testes.
Spermatogonia in seminiferous tubules of the testes differentiates to(1) spermatocyte which undergo 1st meiotic division to produce (2) spermatocyte then 2nd mitotic division take place to produce to(4) spermatide that matures into spermatozoa.
Each ejaculation about 300 million of spermatozoa are discharged into the vagina.

Maturation of sperm
Maturation of sperm
Production of spermatozoa is initiated and maintained in seminiferous tubules of the testes.
Spermatogonia in seminiferous tubules of the testes differentiates to(1) spermatocyte which undergo 1st meiotic division to produce (2) spermatocyte then 2nd mitotic division take place to produce to(4) spermatide that matures into spermatozoa.
Each ejaculation about 300 million of spermatozoa are discharged into the vagina.

Structure of the sperm
Structure of the sperm
Mature sperm cell is composed of a nucleus, cytoplasm and cell membrane. Sperm is free motile and shaped with a head, middle piece (body) and a tail. The nucleus is within the head. The head has a flattened, oval shape. There is a cap like structure covering the head called acrosome. The acrosome is lysosome that forms around the anterior portion of the nucleus which helps in fertilization. The body contains the mitochondria. The tail helps free movement of the sperm in liquid medium. The entire cell is covered by a plasma membrane.

Fertilization and implantation

Fertilization & Implantation
At the time of sexual intercourse thousands of spermatozoa are deposited in the female reproductive tract. Sperms are motile in the female reproductive tract for few hours to 24-48 hrs. Only one sperm fertilizes the ovum in the fallopian tube. The ovum is viable for 24 hrs after ovulation. The spermatozoa are conditioned to fertilize an ovum. Their plasma membrane and outer acrosomal membrane are removed, releasing a hormone called hyaluronidase. This process is called capacitation.

The ovum released from the ovary is captured by the fallopian tube and is transported into the uterus. Fertilization takes place in the FT.
Changes after fertilization
The spermatozoon penetrates through the peripheral layer of cells surrounding the ovum. The acrosomal enzymes dissolve the zona pellucida making a channel through it. The sperm looses the membrane over the half of the head. After the penetration is complete a physiologic barrier is developed around the ovum preventing entry of other sperms. The nucleus of the ovum and sperm change into pronuclei, which are fused together to form the fertilized ovum or zygote. The chromosome number of the zygote is 46. ( 23 from sperm and 23 from ovum). The fertilized ovum is transported into the uterus.
Fertilization is artificially possible to ovum after a short incubation of sperm in a defined media, without entering the female reproductive tract . This is used in invitro fertilization.
Growth of zygote in the fallopian tube
Soon after fertilization zygote undergoes mitosis forming 2 cells each with 46 chromosomes. Each of these again undergoes repeated mitosis resulting in 8, 16, 32, 64, … cells. The process of rapid cell divisions is called cleavage. At the 8-16 cell stage, the zygote reaches the uterus. The fertilized ovum spends 4 days in the uterine cavity develops into blastocyst, which is embedded into the uterus. This is called implantation. Implantation takes place 7 days after ovulation. Usually implantation occur at the upper part of the posterior uterine wall. Implantation in the lower part of the uterus leads to placenta previa.
Fertilization & Implantation

Implantation
The trophoblast is responsible for embedding the ovum. This process is carried out by some enzymes. The ovum is deeply sunk into the lining epithelium of the uterus, surrounded by tiny pool of blood. Finger like projections develop out of the trophoblastic layer called chorionic villi, extend into this blood filled spaces. Chorionic villi contain blood vessels that are connected to the fetus for its oxygen and nutrition.
The cells of the chorionic villi secrete the hormone human chorionic gonadotropin (hCG), which maintains the production of progesterone by the corpus luteum.
Stages of development of the fertilized ovum
Ovum: From ovulation till fertilization.
Zygote: The cell that results from the fertilization of ovum by the spermatozoon. (From fertilization to implantation)
Blastomere: The cells that result from rapid mitotic divisions (cleavage)
of the zygote.
Morula: The solid ball of cells formed by 16 or so blastomeres. Morula develops into two different layers of cells – blastocyst and trophoblast.
Blastocyst: Developing morula with an inner and outer cell mass and fluid collected in between.
Trophoblast: The outer cell mass of the blastocyst; feeding layer; develops into chorion and later into placenta and membranes.
Embryoblast: The inner cell mass which develops into the embryo.

Stages of development of the fertilized ovum
Embryo: From implantation to 5-8 weeks, during which all essential structures are developing and definite form is being assumed.
Is the most critical period and are vulnerable to environmental influences (e.g., teratogens such as drugs, viruses, radiation,  infection).

Fetus: From 5-8 weeks of gestation until term.
Associated structures – Yolk sac
The yolk sac is small in humans. It develops as a second cavity in the blastocyst after 8-9 days of conception. It forms primitive blood cells at the first 6 weeks until fetal liver takes this function.
Associated structures – The decidua
After fertilization under the influence of the HCG produced by the trophoblast cells the corpus luteum continue producing progesterone. Thus the endometrium becomes very thick and vascular and it is called decidua.
The decidua has 3 parts-
Decidua basalis: Part of the endometrium lying directly under the embryo where the trophoblast is embedding.
Decidua capsularis: The portion encapsulating the trophoblast.
Decidua vera (parietalis): The remaining endometrium lining the inner uterine wall.
The decidua
Decidua
Associated structures – Chorionic villi
Tiny finger like projections arising from the trophoblast enter into the uterine endometrium. They are called chorionic villi.

The outer trophoblstic layer of chorionic villus is called syncitiotrophoblast and produces the placental hormones. – HCG, HPL, estrogen and progesterone.
The inner trophoblastic layer of chorionic villus is called cytotrophoblast or Langhans’ layer protecting the fetus from maternal infections other than viruses. This layer disappears after 4th or 5th month of pregnancy and infections like syphilis occurring in the late pregnancy can cause fetal damage.
Chorionic villi have a central core of connective tissue containing fetal capillaries.

The placenta
Placenta has formed through the union of chorionic villi (fetal surface) and the decidua basalis (maternal surface).
Placenta is a disc like organ ,weighs about 500 gm and covers one quarter of uterine wall.
The placenta arises out of the trophoblastic tissue and does the functions of fetal lung, kidneys, gastrointestinal tract and a separate endocrine organ through out pregnancy.

As the syncytiotrophoblasts proliferate and grow into the decidua, they breach the uterine blood vessels. As a result, the maternal blood escapes from the vessels and surrounds the syncytiotrophoblasts, bathing the chorionic villi in a pool of maternal blood. When the fetal blood vessels grow into the chorionic villi (tertiary villi stage), the exchange of nutrients and water supply begin.
The chorionic villi initially develop all over the surface of the blastocyst; however, by the third and fourth month, degenerate except at the side attached to the decidua basalis. The surface of the chorion in contact with the decidua capsularis becomes smooth. The villi towards the decidua basalis continue to grow and develops into the placenta.
Functions of placenta
Transfer of gases or gas exchange (lung).
Transport nutrients (gastrointestinal).
Excretion of wastes (kidneys).
Transfer of heat (skin).
Conjunction of drugs and hormones (liver).
Production of various protein and steroid hormones (endocrine gland).

Placenta structure
A mature placenta consists of two parts: the uterine, or maternal, part and the fetal part. The fetal side is smooth and slick, with the umbilical cord projecting from its surface. The maternal side of the placenta is red and more “bumpy,” showing the shape of the chorionic villi masses (cotyledons). The bumpy shape of the placenta is caused by some chorionic villi attaching directly to the decidua basalis and acting as anchoring villi. The placenta is composed of seven to ten cotyledons.
There is no direct exchange of blood between the embryo and the mother during pregnancy. The exchange is by selective osmosis through the chorionic villi. The pregnant woman should not take any drugs, caffeine, alcohol or nicotine unless prescribed as they can cross into fetal circulation.
Placenta
Placenta

Umbilical cord (Funis)
The blood vessels from chorionic villi join together to form two large arteries and one large vein which form the umbilical cord. The function of the umbilical cord is to transport oxygen and nutrients from the placenta to the fetus and return waste products from the fetus to the placenta. The arteries and vein are bound together by Wharton’s jelly which forms the bulk of the cord. The outer surface is covered with amniotic membrane. There is no nerve supply to the umbilical cord.
The number of arteries and vein in  the umbilical cord should be checked at the time of delivery. 1% of fetuses have only one artery and one vein which may be associated with congenital abnormalities of the heart and kidneys.
Nuchal cord: A loop of cord around the fetal neck. It should be removed before the delivery of the shoulder to avoid traction on the cord and  to ensure unimpaired Oxygen and nutrition supply to the fetus.
Membranes
The surface of the chorion in contact with the decidua capsularis becomes smooth and eventually becomes the chorionic membrane – the outer most fetal membrane. A second membrane is formed under the chorionic membrane is called amniotic membrane which is white and shiny in nature.
The outermost embryonic membrane that encloses amnion, embryo, and yolk sac . This is a thick membrane develop from trophoblast and has many finger like projection, called chorionic villi.
These chorionic villi contain blood vessels that are connected to the fetus and are the sole means by which oxygen and nourishment are received from woman.

Chorion
Chorionic villi invade decidua basalis that enlarge and multiply rapidly to form a chorion frondosum (leafy chorion), this structure become the fetal component of placenta.
Chorionic villi covering the decidua capsularies degenerate and almost disappear, leaving only roughened membrane, the chorion laeve (bald chorion).
chorion laeve and amnion form amniochorion ,an important site for metabolic activity.
Is the second embryonic membrane, originate from ectoderm, a primary germ layer. It is a thin protective membrane that contains amniotic fluid.
The space between amniotic membrane and embryo is the amniotic cavity.
The amniotic cavity is filled with fluid and is called amniotic fluid and it is otherwise called the bag of waters; the fetus floats and move within the space.
Amnion

The amniotic fluid is produced by the amnion and membranes support the amniotic fluid.
The fluid is continually swallowed by the fetus and it is reabsorbed. At term average amount of fluid present is 1L.
Excessive amniotic fluid (more than 2L) is called hydramnios. This is associated with fetal abnormalities – tracheo esophageal fistula or anencephaly.
Fetal urine is excreted into the amniotic fluid adding to volume. A reduction in amniotic fluid volume oligohydramnios (less than 300 is associated with abnormalities of the kidneys.
Functions of amniotic fluid:


Providing a cushion against mechanical injury.
Control embryo temperature.
Permitting symmetrical external growth.
Preventing adherence of amnion.
Providing space for free movement.
By full term ,amniotic fluid volume is 500 to 1000 ml.
Amniotic fluid is slightly alkaline & contains albumin, urea, uric acid, bilirubin, leukocytes,…….

Early Embryonic Development
Early Embryonic Development
Three germ layers
About 10 – 14 days after conception, the blastocyst differentiate into three layers, ectoderm, mesoderm, and endoderm. All tissues, organs, and organ system will develop from these germ cell layers.
germ-layers

Duration of pregnancy
The length of pregnancy may be divided into days, weeks or months.
Estimated date of pregnancy calculated from (LMP) first day of last menstrual period.

Development of the fetus
4 Weeks:
Period missed, embryo 4-5 mm length, foundation of nervous system, GI tract, skin, bones and lung are formed. Limb buds begin to form. Rudimentary eyes, ears & nose appear.

9-12 weeks:
Fetus 50-87 mm length, weighs 45 gm. Fingers and toes distinct. placenta complete. External genitalia show definite characteristics, fetal circulation complete.

17-20 weeks:
Fetus 150-190 mm length, weighs 260-460 gm. Lanugo hair cover all body, fetal movement felt. Eye brow and scalp hair present. Vernix caseosa cover the skin.

26-29 weeks:
Fetus 250- 275 mm length, weighs 910-1500 gm. Red skin, rhythmic breathing movement. Pupillary membrane disappears from eyes. Can survive preterm birth.

30-34 weeks:
Fetus 280- 320 mm length, weighs 1700- 2500 gm. Visible toe nails. Eyelids open. Maximum weight gain. vigorous movement.

38-42 weeks:
fetus 360-500 mm length, weighs 3400- 3600 gm. Smooth skin. Prominent chest. skull bones ossified and joined by sutures. Testes in the scrotum

development of the embryo and the fetus

Fetal Circulation
The fetus must maintain the blood flow to placenta to obtain oxygen and nutrients ad to remove carbon dioxide ad other waste products. i.e. placenta assumes function of fetal lungs.
The  oxygenated blood flow up the cord through the umbilical vein and passes into inferior vena cava ,part of oxygenated blood goes through the liver , but most of it passes through a special fetal structure, the ductus venosus, which connect umbilical vein and the inferior vena cava.
From inferior vena cava , the current flow into right auricle and goes directly to the left auricle through a special fetal structure, the foramen ovale. It then flows into the left ventricle and out through the aorta.
The blood that circulates up the arms and the head return through the superior vena cava to the right auricle again but instead of passing through foramen ovale as before, the current is deflected downward through into right ventricle and through the pulmonary arteries. part of it goes to the lungs for purpose of nutrition but most of it goes into descending aorta through the ductus arteriosus.

The blood in the aorta, with exception of that which goes to head and upper extremities ,passes downward to supply the trunk and lower extremities. Most of this blood find its way through internal iliac or hypogastric arteries and back through the umbilical arteries to the placenta ,where it is again oxygenated.
Fetal circulation
Fetal circulation
Changes in the Fetal Circulation after Birth

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Multiple pregnancy
Most common is twins.
Types
Fraternal (dizygotic): Two ova are fertilized with two different sperms.
Identical (monozygotic): Develop from single fertilized ovum. They have same sex and same genotype (appearance).
Predisposing factors – Dizygotic twins
Maternal age: Increases with age up to 35 years and then decreases suddenly.
Increased parity
Conception that takes place in the first 3 months of a relationship.
Increased coital frequency
Twinning decreases during malnutrition and seasons winter and spring.
Heredity
Monozygotic twins: No predisposing factors.
Complete separation of the cellular mass into two parts is necessary for twin formation. Incomplete division leads to formation of Siamese twins. The number of amnions and chorions present depends on the timing of cell division.

Monozygotic twins – types
Dichorionic-diamniotic: Division within 3 days of fertilization, 2 embryos, 2 amnionionic sacs and 2 chorions.
Monochorionic-diamniotic: Division about 5 days after fertilization, 2 embryos, 2 amnionic sacs and 1 common chorion.
Monochorionic-monoamniotic: Division about 7-13 days after fertilization, 2 embryos, 1 common amnionic sac and 1 common chorion.
Monozygotic twinning is a random phenomenon and incidence is 4 per 1000 live births.
Siamese twins
Siamese twins

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