DIGESTIVE SYSTEM

The Development of the Digestive System

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Developmental Biology• is the study of the sequence of events from fertilization

of a secondary oocyte by a sperm cell to the formation of an adult organism.

Developmental Biology1. Embryonic period

– From fertilization through the 8th week of development

2. Fetal period– begins at week 9 and continues until birth.

3. Neonatal period– first 28 days after birth Aia Bayawa

Prenatal Development

1. First Trimester

2. Second Trimester

3. Third Trimester

Embryonic Period

• Major developmental events that occur during the embryonic period.

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1st Week of Development

• characterized by several significant events including fertilization, cleavage of the zygote, blastocyst formation, and implantation.

Fertilization1. Corona radiata2. Zona pellucidaSyngamy • Fusion of a sperm cell with a secondary

oocyte Polyspermy• fertilization by more than one sperm cell

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Cleavage of the ZygoteCleavage• Rapid mitotic cell divisions of the zygote.

Blastomeres• The progressively smaller cells produced by cleavage.

Morula• Successive cleavage eventually produce a solid

sphere of cells.

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The Embryo• Developmental stage from the start of cleavage until the

ninth week• The embryo first undergoes division without growth• The embryo enters the uterus at the

16-cell state• The embryo floats free in the uterus temporarily• Uterine secretions are used for nourishment

The Blastocyst• Ball-like circle of cells• Begins at about the 100 cell stage• Secretes human chorionic gonadotropin (hCG) to produce the

corpus luteum to continue producing hormones• Functional areas of the blastocyst

– Trophoblast – large fluid-filled sphere– Inner cell mass

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Blastocyst Formation

1. Inner cell mass– is located inside the blastocyst and

eventually develops into the embryo.

2. Trophoblast– is an outer superficial layer of cells that

forms the wall of the blastocyst.

– develop into the fetal portion of the placenta, the site of exchange of nutrients and wastes between the mother and fetus.

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Development from Ovulation to Implantation

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The Blastocyst• Primary germ layers are eventually formed

– Ectoderm – outside layer– Mesoderm – middle layer– Endoderm – inside layer

• The late blastocyst implants in the wall of the uterus (by day 14)Implantation

1. Decidua basalis

– is the portion of the endometrium between the embryo and the stratum basalis of the uterus

2. Decidua capsularis

– is the portion of the endometrium located between the embryo and the uterine cavity.

3. Decidua parietalis

– is the remaining modified endometrium that lines the noninvolved areas of the rest of the uterus Aia Bayawa

Development After Implantation

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2nd Week of Development• 2 layers of the trophoblast:

1. Synsytiotrophoblast

2. Cytotrophoblast

The 2 layers of trophoblast become part of the chorion.

2 layers of the Inner Cell Mass

1. Hypoblast (Primitive endoderm)

2. Epiblast (Primitive ectoderm)

• Cells of the hypoblast and epiblast together form the bilaminar embryonic disc.

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Development of the Amnion

• A thin protective membrane that develops from the epiblast.

Development of the Yolk Sac

• Cells of the hypoblast migrate and become flat, thin membrane forms the wall of the yolk sac, formerly called the blastocyst cavity.

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Development of the Chorion

• The extraembryonic mesoderm, together with the 2 layers of the trophoblast

( cytotrophoblast and syncytiotrophoblast) form the chorion.

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3rd Week of Development

• Begins a 6-week period of very rapid embryonic development and differentiation.

• 3 primary germ layers are established and lay the groundwork for organ development in weeks 4-8.

3 Primary Germ Layers

1. Endoderm

2. Mesoderm

3. EctodermAia Bayawa

Endoderm• Epithelial lining of GIT (except the oral and anal

cavity) and the epithelium of its glands

• Epithelial lining of the urinary bladder, gallbladder, and liver

• Epithelial lining of the pharynx, eustachian tubes, tonsils, larynx, trachea, bronchi, lungs

• Epithelial lining of thyroid gland, parathyroid glands, pancreas, thymus

• Epithelial lining of prostate and bulbourethral (Cowper’s) gland, vagina, vestible, urethra, Bartholin’s glands Aia Bayawa

Mesoderm• All skeletal and cardiac muscle tissue and most

smooth muscle tissue• Cartilage, bone, and other connective tissue• Blood, red bone marrow, lymphatic tissue• Endothelium of blood and lympahatic vessels• Dermis of skin• Fibrous tunic and vascular tunic of eye• Middle ear• Mesothelium of thoracic, abdominal, and pelvic

cavities• Epithelium of kidney and ureters• Epithelium of adrenal cortex• Epithelium of gonads and genital ducts

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Ectoderm• All nervous tissue• Epidermis of skin• Hair follicles, arrector pili muscles, nails, epithelium of

skin glands (sebaceous and sudoriferous), and mammary glands

• Lens, cornea, internal eye muscles• Internal and external ears• Neuroepethelium of sense organs

• Epithelium of oral cavity, nasal cavity, paranasal sinused, salivary glands , and anal canal

• Epithelium of pineal gland, pituitary gland, and adrenal medulla

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Neurulation• The processes in the formation of the

neural plate, neural folds, neural tube.

Brain Vesicles of the Neural Tube

1. Prosencephalon (forebrain)

a) Telencephalon

b) Diencephalon

1. Mesencephalon (midbrain)

2. Rhombencephalon (hindbrain) a) Metencephalon

b) MyelencephalonAia Bayawa