Early Embryology/Development - Checked and Complete

Question 1

Describe development of primordial germ cells

Answer 1

During the very early parts of embryogenesis, the primordial germs cells leave the vicinity of the developing embryo to the wall of the yolk sac .

At about 5 weeks they return to the gonadal ridge

Question 2

Describe Spermatogenesis

Answer 2

Two types of spermatogonia are developed from the primordial germ cells at puberty - Type A and B spermatogonia.

Type A spermatogonia undergo mitosis to make more Type A spermatogonia and also Type B spermatogonia.

Type B spermatogonia complete Meiosis I to form 2 secondary spermatocytes (now with only 23 chromosomes) which go on to complete Meiosis II to form 4 spermatids.

Spermatids undergo changes resulting in sperm (process is spermiogenesis)

Question 3

Define capacitation.

Answer 3

Sperm must be altered in female reproductive tract before capable of fertilization.

Question 4

Describe oogenesis

Answer 4

All primary oocytes still containing 46 chromosomes are formed by the 5th month of fetal life and remain dormant in prophase of meiosis I until puberty.

During an ovarian cycle a primary oocyte will complete Meiosis I to form a secondary oocyte containing 23 chromosomes.

This secondary oocyte then enters Meiosis II and remains arrested in metaphase until (and if) fertilization occurs. At fertilization the secondary oocyte completes Meiosis II to form a mature oocyte.

** Usually only one oocyte is discharged during an ovarian cycle. At ovulation the oocyte is in metaphase of the second meiotic division and is surrounded by the zona pellucida and outer corona radiata.**

Question 5

What are the three layers the sperm must penetrate to fertilize the egg?

Answer 5

** 1) corona radiata**

2) the zona pellucida

**3) oocyte cell membrane **

Question 6

What happens after the sperm enters the oocyte?

Answer 6

The oocyte finishes meiosis 2

** Zona pellucida becomes impenetrable to other spermatozoa**

** Head of the sperm separates from the tail, swells, and forms the male pronucleus**

Sperm and Egg cell membrane fuse and cleavage begins

Question 7

Where does fertilization normally occur?

Answer 7

In ampulla of uterine tube

Question 8

What happens after fertilization completes?

Answer 8

Mitotic divisions (cleavage) result in a smaller, very compact 16-cell stage group of blastomeres called a morula

A cavity begins to appear within the morula and is then called a blastocyst

** Composed of blastocyst cavity (blastocoele), inner cell mass (the embryo) and the outer cell mass (the trophoblast)**

Implants before Week 2

Question 9

What cells are necessary for implantation?

Answer 9

Trophoblast cells

Question 10

The trophoblast develops into what 2 layers?

Answer 10

**Cytotrophoblast **

Syncytiotrophoblast - implants to endometrium and releases hCG; becomes placenta

Question 11

The inner cell mass or embryo blast forms what 2 layers?

Answer 11

Epiblast

Hypoblast

Question 12

The extraembryonic mesoderm splits into what 2 layers?

Answer 12

The somatopleure and splanchnopleure

Question 13

What two cavities form during week 2?

Answer 13

The amniotic sace

The yolk sac

Question 14

How long does an ectopic embryo typically survive?

Answer 14

1-2 months

Question 15

Describe Gastrulation

Answer 15

A primitive streak is formed on the dorsal surface of the epiblast (composed of the primitive groove, primitive node and the primitive pit)

Epiblast cells migrate toward and invaginate into the primitive streak to form endoderm

** Some cells lie between the epiblast and the endoderm to form mesoderm**

The remaining cells form ectoderm

** This creates the trilaminar embryonic disc**

Question 16

Describe Notochord location/formation.

Answer 16

The notochord extends from the primitive node (in the middle of trilaminar disk) to oropharyngeal membrane during mesoderm formation

Has inductive role in organ formation

Persists as nucleus pulposus (center of vertebrae) in adults

Question 17

What 3 important orientations does gastrulation define?

Answer 17

**cranial : sacral **

**right : left **

** dorsal: ventral **

Question 18

Describe head and tail folding.

Answer 18

Embryo grows at head and tails, resulting in those two places folding down like a banana.

The caudal end continues gastrulation and grows while the cranial end differentiates

Question 19

Describe lateral folding

Answer 19

Flat embryo-disk folds into a tube and closes all body cavities

Question 20

Describe Neurulation

Answer 20

Lateral folding contributes to neurulation

First, the notochord induces the overlying ectoderm around the center of the embryo to thicken and form the neural plate

A groove forms in the neruoectoderm, deepens, and pinches off to form the neural tube (primitive spinal cord)

Neural crest cells migrate to needed areas

Question 21

List some derivatives of neural crest cells.

Answer 21

CNS: Schwann cells, PNS myelinated cells, dorsal root ganglia, cranial nerve ganglia, autonomic ganglia, gut intrinsic innervation-myenteric plexus and submucosal plexus, the adrenal medulla, meninges surrounding CNS,

Head and Neck: head mesenchyme, forming parts of facial bones, muscles, and ligaments, odontoblasts and the dentin producing cells of teeth,

Skin: pigment-producing cells, melanocytes

Question 22

Name 2 neural crest cell defects

Answer 22

DiGeorge Syndrome (deletion in #22)

**Neural Tube Defects: **spina bifida occulta, rachischisis, anencephaly

Question 23

Describe layers of mesoderm

Answer 23

Paraxial Mesoderm - close to the midline, organized into somites; also form myotome and Dermatome

Intermediate Mesoderm - will differentiate into urogenital structures.

Lateral plate mesoderm - parietal and visceral layers line the intraembryonic cavity and surround organs.

Question 24

What is a visual difference between the cytotrophoblast and syncytiotrophoblast?

Answer 24

Cytotrophoblast has distinct cell boundaries

Syncytiotrophoblast has blurry cell boundaries that grow into the endometrium

Question 25

What is the first step of gastrulation?

Answer 25

Formation of primitive streak.