3: Basic Embryology

What is the prenatal period?

The time between conception and birth

How long does the embryonic period last?

8 weeks

What major event marks the end of the embryonic period?

All major organs are in place

What is the primary focus of the fetal period?

Growth and complexity of organs

How long does the fetal period last?

30 weeks

What major changes occur by the end of the embryonic period?

All major organs are in place, and the embryo looks distinctly human.

Embryonic period

The first 8 weeks of prenatal development during which all major organs are formed.

What is the basic body plan of the adult human described as?

A tube within a tube

By which month of development is the basic body plan evident?

Month 2

What separates the outer body wall and the inner tube in the abdomen?

The peritoneal cavity

What are the two layers of the skin?

Epidermis and dermis

What structures form from the inner tube in the thoracic region?

Respiratory structures

Where do the kidneys lie in relation to the dorsal body wall?

Directly deep to the dorsal body wall

What do the limbs consist mostly of?

Bone, muscle, and skin

What does the outer body wall consist mostly of?

Trunk muscles

What cavities are found in the thorax?

Pleural cavity around the lungs and pericardial cavity surrounding the heart

Epidermis

The outer layer of the skin

Dermis

The inner, leathery layer of the skin

Peritoneal cavity

The serous cavity in the abdomen separating the outer body wall from the digestive tube

Parietal serosa

The outer lining of the body cavities

Visceral serosa

The inner lining covering the organs within the body cavities

What is the term for the early division sequence of cells in the zygote?

Cleavage

During which week does the neural tube begin to close?

Week 3

What structure does the neural crest form?

Sensory nerve cells

Which part of the mesoderm forms the kidneys and gonads?

Intermediate mesoderm

What does the splanchnic mesoderm give rise to?

Heart and most blood vessels

What is the function of the trophoblast in the blastocyst?

Helps form the placenta

What does the endoderm become?

Inner epithelial lining of the gut tube

What is the primitive streak?

A raised groove on the dorsal surface of the epiblast

Which germ layer forms the brain and spinal cord?

Ectoderm

What is the role of the notochord?

Defines the body axis

Where does fertilization of an oocyte by a sperm generally occur?

In the lateral third of the uterine tube

What is the term for the early division sequence that produces a large number of cells for the embryo?

Cleavage

Zygote

A fertilized oocyte, which is the initial cell formed when a sperm cell unites with an egg cell

Neurulation

The process by which the neural tube is formed from the ectoderm, which will develop into the brain and spinal cord

What does the neural tube become?

The brain and spinal cord

What are somites and what do they form?

Somites are blocks of mesoderm that form the segmented structures of the outer tube, such as vertebrae and ribs

Coelom

The body cavity formed between the two layers of the lateral plate mesoderm, which will become the serous cavities of the ventral body cavity

What does the splanchnic mesoderm give rise to?

The entire wall of the digestive and respiratory tubes, except the inner epithelial lining, as well as the heart and most blood vessels

What is the primitive streak and what does it form?

A raised groove on the dorsal surface of the epiblast that forms the three primary germ layers: ectoderm, mesoderm, and endoderm

What does the ectoderm develop into?

The brain, spinal cord, and epidermis of the skin

What is the significance of the notochord?

It defines the body axis and is the site of the future vertebral column

During which weeks does the fetal period occur?

Weeks 9 through 38

At how many weeks after conception do normal births typically occur?

38 weeks

At what week can infants usually survive without lifesaving measures if born prematurely?

Week 30

What is a common complication for premature infants born as early as 22 weeks?

Respiratory distress

What major processes occur during the first half of the fetal period?

Cells are still differentiating into specific cell types to form the body’s distinctive tissues and complete the fine details of body structure.

What is considered a premature birth?

A birth that occurs before 38 weeks after conception.

What are some elevated risks for premature infants that survive?

Rates of visual problems and mental retardation.

What is the study of the 38-week prenatal period called?

Embryology

During which period is the basic body plan established?

The first 8 weeks (embryonic period)

What are the primary functions of the fetal period?

Growth and maturation

What structure does the inner cell mass of a blastocyst become?

The embryo

What is the term for the early cell division without growth that produces the morula?

Cleavage

What are the three layers formed during gastrulation?

Ectoderm, mesoderm, and endoderm

What does the notochord signal the ectoderm to form?

Neural tube

By the end of which week do the mesodermal layers condense into somites, intermediate mesoderm, and lateral plate?

Week 3

What shape does the embryo take on during week 4?

Tadpole shape

When do limb buds appear during embryonic development?

Around day 28

Morula

A solid ball of cells resulting from the division of a fertilized ovum, and from which a blastocyst is formed

Blastocyst

A structure formed in the early development of mammals, consisting of an inner cell mass that becomes the embryo and a trophoblast that forms part of the placenta

Gastrulation

The process during early embryonic development where the embryo forms three layers: ectoderm, mesoderm, and endoderm

Primitive streak

A structure that forms on the surface of the early embryo, marking the beginning of gastrulation

Notochord

A rodlike structure that defines the longitudinal body axis and later forms each nucleus pulposus of the vertebral column

Somites

Segmented blocks of mesoderm that later develop into the vertebral column and associated structures

Amnion

Membrane that forms a fluid

Blastocyst

Stage of early embryonic development; a hollow ball of cells; the product of cleavage.

Cleavage

An early embryonic stage consisting of rapid cell divisions without intervening growth periods; begins with a fertilized ovum and produces a blastocyst.

Ectoderm

Embryonic germ layer that forms both the outer layer of the skin (epidermis) and nervous tissues.

Embryo

The developing human from week 2 through week 8 after fertilization.

Endoderm

An embryonic germ layer that forms the lining and glands of the digestive and respiratory tubes.

Fertilization

Fusion of the sperm and egg nuclei.

Fetus

Developmental stage lasting from week 9 of development to birth.

Induction

The influence exerted by a group of cells on the differentiation of adjacent cells or on the development of an embryonic structure.

Inner cell mass

Accumulation of cells in the blastocyst from which the body of the embryo derives.

Mesoderm

The embryonic germ layer that gives rise to most structures in the body, including the skeleton, muscles, dermis, connective tissues, kidneys, and gonads.

Neural crest

Embryonic tissue derived from ectoderm that migrates widely within the embryo and gives rise to sensory neurons, all nerve ganglia, melanocytes, and other structures.

Oocyte

Immature egg undergoing the process of meiosis.

Retroperitoneal

External or posterior to the peritoneum.

Somite

A mesodermal segment of the body of the embryo.

Trophoblast

External layer of cells in the blastocyst (early embryo); forms the embryo’s contribution to the placenta.

Uterine tube

Tube through which the ovum travels to the uterus; also called fallopian tube and oviduct.

Yolk sac

Embryonic sac that stores a tiny quantity of yolk and gives rise to the lining of the digestive tube; also gives rise to the primordial germ cells and the blood cells.

Zygote

Fertilized egg.