EMBRYOLOGY & TISSUESEMBRYOLOGY & TISSUES

Sonya Schuh-Huerta, Ph.D.Sonya Schuh-Huerta, Ph.D.Human AnatomyHuman Anatomy

Human Fetus, 12 weeks

Basic Embryology, Ch 3

(Physiology, Silverthorn, 2000)

Fertilization & the Early Embryo

Embryology

• Embryology – study of the origin & development of a single individual

• Prenatal period– Embryonic period first 8 weeks– Fetal period remaining 30+ weeks

The Prenatal Period

Fertilization 1-weekconceptus

3-weekembryo(3 mm)

Embryo

Duration: Weeks 9–38 (or birth)Major events: Organs grow in size and complexity.

Embryonic period

Fetal period

12-week fetus(90 mm)

8-week embryo(22 mm)

5-week embryo(10 mm)

38weeks

Duration: First 8 weeks post-fertilizationMajor events: Organs form from 3 primary germ layers.

The basic body plan emerges.

The Basic Body Plan

• Skin dermis and epidermis • Outer body wall trunk muscles, ribs, vertebrae• Body cavity & digestive tube (inner tube)• Kidneys & gonads deep to body wall• Limbs

The Embryonic Period

• Week 1 from zygote to blastocyst– Fertilization – in lateral 3rd of uterine tube– Zygote (fertilized oocyte) – moves toward

uterus– Cleavage – daughter cells formed from zygote– Morula – solid cluster of 12–16 blastomeres– Blastocyst – fluid-filled ball of cells

• Inner cell mass forms embryo• Trophoblast forms placenta

(d)Early blastocyst(morula hollows out and fills with fluid).4 days

Blastocystcavity

Inner cell mass

Blastocyst cavity

Trophoblast

(e) Implanting Blastocyst(consists of a sphereof trophoblast cells andan eccentric cell clustercalled the inner cell mass). 7 days

Cavity ofuterus

Uterus

Endometrium

(a)Zygote(fertilized egg)

(b)4-cell stage2 days

Ovulation

Ovary

Fertilization(sperm meets and enters egg)

Uterine tube

Oocyte(egg)

Sperm

Week 1: Early Embryonic Development

hESCs

d3.5d3.0

A Closer Look at Week 1 of Human Embryo Development

Reijo Pera R, et al. 1999

Embryo Development in Action

Wong C, et al. 2010

(a) Day 5: Blastocyst floating in uterinecavity.

(b) Day 6: Blastocystadheres to uterine wall.

(c) Day 7: Implantationbegins as trophoblastinvades into uterine wall.

Trophoblast

Inner cell mass

Trophoblast

Wall ofuterus

Cavity of uterus

Inner cell mass

(d) Day 9: Implantationcontinues; innercell mass formsbilaminar disc.

(e) Day 11: Implantationcomplete; amnioticsac and yolk sacform.

Trophoblast

Amnioticsac cavity

Hypoblast

Amnioticsac cavity

Layers fromtrophoblast

Amnion

Bilaminarembryonicdisc

Yolk sac

Epiblast

Hypoblast

Epiblast

Implantation of the Embryo

Week 2: The 2-Layered Embryo

• Bilaminar embryonic disc inner cell mass divided into 2 sheets– Epiblast & hypoblast

• Together they make up the bilaminar embryonic disc

• Amniotic sac formed by an extension of epiblast

• Filled with amniotic fluid• Surrounds developing embryo/fetus

Week 2: The 2-Layered Embryo

• Yolk sac – formed by an extension of hypoblast– Digestive tube forms from yolk sac– NOT a major source of nutrients for

mammalian embryo– Tissues around yolk sac

• Give rise to earliest blood cells and blood vessels

Week 3: The 3-Layered Embryo

• Primitive streak = raised groove on the dorsal surface of the epiblast

• Gastrulation = a process of invagination of epiblast cells & gives rise to the germ layers– Begins at the primitive streak– Forms the 3 primary germ layers!

Week 3: The 3-Layered Embryo

• Three Germ Layers*– Endoderm – formed from migrating cells that

replace the hypoblast – Mesoderm – formed between epiblast and

endoderm– Ectoderm – formed from epiblast cells that

stay on dorsal surface

*All layers derived from epiblast cells

Yolk sac(cut edge)

Cut edgeof amnion

Primitivenode

Left Right

Primitivestreak

Head end

Tail end

(e) Bilayered embryonic disc, superior view

The Primitive Streak Stage

Primitive streak

Notochord

Amnion

Amniotic sacPlane of section

Head

EctodermMesoderm

Embryonicdisc

Endoderm

Yolk sac

Sections(b) and (c)

Amnion

EctodermRightRight Left Left

Invaginatingmesodermalcells

Notochord

MesodermEndoderm

Yolk sac

(a)

(b) Section through primitive streak (c) Section anterior to primitive streak

Tail

Yolksac

Epiblast cells that migrate through the primitive node form the notochord.

Epiblast cells that migrate through primitive streak form the mesoderm layer.

Primitivenode

Formation of the 3-Layered Embryo

The Notochord

• Primitive node = a swelling at one end of primitive streak– Notochord forms from primitive node &

endoderm• Notochord – defines the body axis

– Is the site of the future vertebral column– Appears on Day 16

Neurulation• Neurulation formation of the brain & spinal cord

from ectoderm– Neural plate = ectoderm in the dorsal midline thickens– Neural groove = ectoderm folds inward

Somite

Intermediatemesoderm

Lateral platemesoderm

Coelom

NeuralcrestNeural

groove

Neuralfold

Somite(covered byectoderm)

NeuralgrooveNeuralfold

Splanchnic mesoderm

Somatic mesoderm

(b) 20 days. The neural folds form by folding of the neural plateand then deepen, producing the neural groove. Neural foldcells migrate to form the neural crest. Three mesodermalaggregates form on each side of the notochord (somite,intermediate mesoderm, and lateral plate mesoderm). Lateral plate mesoderm splits. Coelom forms between the two layers.

Primitivestreak

Neurulation Neural tube – hollow tube pinches off into the body

• Cranial part of the neural tube becomes brain• Maternal folic acid deficiency causes neural tube defects!

Neuralfold

Somite

Neuralcrest

Somite

Surfaceectoderm

Neuraltube

Notochord

(c) 22 days. The neural folds have closed, forming the neural tubewhich has detached from the surface ectoderm and lies between the surface ectoderm and the notochord. Embryonic body is beginning to undercut.

Cutedge ofamnion

Head

Tail

Neurulation

• Neural crest– Cells originate from ectodermal cells– Forms sensory nerve cells

• Induction– Ability of one group of cells to influence

developmental direction (differentiation) of other cells

Week 4: The Body Takes Shape

• The embryo folds laterally & at the head & tail– Embryonic disc bulges; growing faster than yolk sac– “Tadpole shape” by Day 24 after conception– Primitive gut – encloses tubular part of the yolk sac

• Site of future digestive tube & respiratory structures

Tail

Amnion

Head

Yolk sac

Lateral fold

Future gut(digestive tube)

Somites(seen throughectoderm)

Cut edgeof amnion Cut edge

of amnion

Primitive gutForegutHindgut

Neural tubeNotochord

Yolk sac

Yolk sac

Ectoderm Mesoderm Endoderm

Trilaminarembryonic disc

Head fold

Tail fold

(a) (b)

(c) (d)

Folding of the Embryo

Week 4: The Body Takes Shape

• Derivatives of the germ layers– Ectoderm forms:

• Brain, spinal cord, & epidermis

– Endoderm forms:• Inner epithelial lining of the gut tube • Respiratory tubes, digestive organs, & bladder

- Mesoderm differentiates further and is more complex than the other 2 layers– Somites & intermediate mesoderm– Somatic & splanchnic mesoderm

The Mesoderm Begins to Differentiate

• Somites – our first body segments; 40 pairs– Paraxial mesoderm

• Intermediate mesoderm – begins as a continuous strip of tissue just lateral to the paraxial mesoderm– Each segment attached to a somite

• Lateral plate – most lateral part of the mesoderm– Coelom – becomes serous body cavities

• Somatic mesoderm – next to the ectoderm• Splanchnic mesoderm – next to the endoderm

Mesoderm– Somites divide into:

• Sclerotome• Dermatome• Myotome

– Intermediate mesoderm forms:• Kidneys & gonads

– Splanchnic mesoderm forms:• Musculature, connective tissues, & serosa of the

digestive & respiratory structures• Heart & most blood vessels

– Somatic mesoderm forms: • Dermis of skin, bones, & ligaments

(a) Embryo, day 24

Future gut (digestivetube)

Somatic mesoderm

Splanchnic mesoderm

Lateral fold

Yolk sac

CoelomSomite

Tail

Intermediatemesoderm

Notochord

Ectoderm

Mesoderm

EndodermHead

The Germ Layers in Week 4

(b) Embryo, day 28

Kidney & gonads(intermediate mesoderm)

Gut lining(endoderm)

Parietal serosa

Limb bud

Somaticmesoderm

Dermis

Visceral serosa

Splanchnicmesoderm

Smooth muscle of gut

Epidermis (ectoderm)

Dermatome

Myotome

Sclerotome

Peritoneal cavity(coelom)

Neural tube (ectoderm)Somite

Ectoderm

Mesoderm

Endoderm

The Germ Layers End of Week 4

(c) Adult

Skin Epidermis Dermis

Lining of digestive tube

Muscle of digestive tube

Visceral serosa

Peritoneal cavity

Spinalcord

Vertebralcolumn

Rib

Kidney

Trunk muscles Parietal serosa

Outer body wall

Inner tube

Ectoderm

Mesoderm

Endoderm

Trunk

Germ Layers & Their Adult Derivatives

Major Derivatives of Germ Layers

Ear

Pharyngeal arches

Eye

Heart

Upper limb budTail

Lower limb bud

Somites (soon to give rise to myotomes)

(b)(a)

A 4-Week Embryo

Developing Fetus

Developmental Events of Fetal Period

Developmental Events of Fetal Period

Developmental Events of Fetal Period

THE TISSUES, Ch 4

Tissues

• Cells work together in functionally-related groups called tissues

• Tissue– A group of closely associated cells that perform related

functions & are similar in structure

4 Basic Tissue Types & Their Functions

• Epithelial tissue covering (Chs 4 & 5)• Connective tissue support (Chs 4, 5, 6, & 9)• Muscle tissue movement (Chs 10 & 11)• Nervous tissue control (Chs 12–16 & 25)

Epithelial Tissue

• Covers a body surface or

lines a body cavity • Forms parts of most glands• Functions of epithelia:

– Protection– Diffusion– Absorption, secretion, & ion transport– Filtration– Forms slippery surfaces

Special Characteristics of Epithelia

• Cellularity– Cells separated by minimal extracellular material

• Specialized contacts– Cells joined by special junctions

• Polarity– Cell regions of the apical surface differ from the basal

surface

Special Characteristics of Epithelia

• Support by connective tissue• Avascular, but innervated

– Epithelia receive nutrients from underlying connective tissue

• Regeneration– Lost cells are quickly replaced by rapidly

dividing cells; many stem cells

Special Characteristics of Epithelia

Epithelium

Connectivetissue

Apical region of an epithelial cell

Basal region

Tight junction

Adhesive belt

Gap junctionDesmosome

Basal laminaReticular fibers

BasementmembraneNerve ending

Capillary

Cilia Narrowextracellularspace Microvilli

Cell junctions

Classifications of Epithelia

• First name of tissue indicates number of cell layers– Simple one layer of cells– Stratified more than one layer of cells

Classifications of Epithelia

• Last name of tissue describes shape of cells– Squamous = cells are wider than tall (plate-like)

‘squashed’ = squamous

– Cuboidal = cells are as wide as tall (like cubes)– Columnar = cells are taller than they are wide

(like columns)

Classifications of Epithelia

Squamous

Cuboidal

Columnar

(b) Classification based on cell shape

Stratified

Simple

Apical surface

Basal surface

Apical surface

Basal surface

(a) Classification based on number of cell layers

Simple Squamous Epithelium

• Description: single layer; flat cells with disc-shaped nuclei

• Function:– Passage of materials by passive diffusion & filtration– Secretes lubricating substances in serosae

• Location: – Renal corpuscles– Alveoli of lungs – Lining of heart, blood, & lymphatic vessels

– Lining of ventral body cavity (serosae)

Simple Squamous Epithelium(a) Simple squamous epithelium

Description: Single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm; the simplest of the epithelia.

Function: Allows passage of materials by diffusion and filtration in sites where protection is not important; secretes lubricating substances in serosae.

Location: Kidney glomeruli; air sacs of lungs; lining of heart, blood vessels, and lymphatic vessels; lining of ventral body cavity (serosae).

Photomicrograph: Simple squamous epitheliumforming part of the alveolar (air sac) walls (200).

Air sacs oflung tissue

Nucleiof squamousepithelialcells

Simple Cuboidal Epithelium

• Description: – Single layer of cube-like cells with large, spherical

central nuclei

• Function:– Secretion & absorption

• Location:– Kidney tubules, secretory portions of small glands,

ovary surface

Simple Cuboidal Epithelium(b) Simple cuboidal epithelium

Description: Single layer of cubelike cells with large, spherical central nuclei.

Function: Secretion and absorption.

Location: Kidney tubules; ducts and secretory portions of small glands; ovary surface.

Photomicrograph: Simple cuboidal epitheliumin kidney tubules (430).

Basementmembrane

Connectivetissue

Simplecuboidalepithelialcells

Simple Columnar Epithelium

• Description: single layer of column-shaped (rectangular) cells with oval nuclei– Some have cilia at their apical surface– May contain goblet cells

• Function: – Absorption; secretion of mucus, enzymes, & other

substances– Ciliated type propels mucus or reproductive cells by

ciliary action

Simple Columnar Epithelium

• Location: – Non-ciliated form

• Lines digestive tract, gallbladder, ducts of some glands

– Ciliated form • Lines small bronchi, uterine tubes, & uterus

Simple Columnar Epithelium(c) Simple columnar epithelium

Description: Single layer of tall cells with round to oval nuclei; some cells bear cilia; layer may contain mucus-secreting unicellular glands (goblet cells).

Function: Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or reproductive cells) by ciliary action.

Location: Nonciliated type lines most of the digestive tract (stomach to anal canal), gallbladder, and excretory ducts of some glands; ciliated variety lines small bronchi, uterine tubes, and some regions of the uterus. Photomicrograph: Simple columnar epithelium

of the stomach mucosa (1150).

Simplecolumnarepithelialcell

Basementmembrane

Pseudostratified Columnar Epithelium

• Description:– All cells originate at basement membrane– Only tall cells reach the apical surface– May contain goblet cells & cilia– Nuclei lie at varying heights within cells

• Gives false impression of stratification!

Pseudostratified Columnar Epithelium

• Function: secretion of mucus; propulsion of mucus by cilia

• Locations:– Non-ciliated type

• Ducts of male reproductive tubes • Ducts of large glands

– Ciliated type • Lines trachea and most of upper respiratory tract

(d) Pseudostratified columnar epithelium

Description: Single layer of cells of differing heights, some not reaching the free surface; nuclei seen at different levels; may contain mucus-secreting goblet cells and bear cilia.

Function: Secretion, particularly of mucus; propulsion of mucus by ciliary action.

Location: Nonciliated type in male’ssperm-carrying ducts and ducts of large glands; ciliated variety lines the trachea, most of the upper respiratory tract.

Photomicrograph: Pseudostratified ciliatedcolumnar epithelium lining the human trachea (780).

Trachea

Cilia

Pseudo-stratifiedepitheliallayer

Basementmembrane

Mucus ofgoblet cell

Pseudostratified Ciliated Columnar Epithelium

Stratified Epithelia

• Properties– Contain 2 or more layers of cells– Regenerate from below (basal layer)– Major role is protection– Named according to shape of cells at apical

layer

Stratified Squamous Epithelium

• Description:– Many layers of cells are squamous in shape– Deeper layers of cells appear cuboidal or

columnar – Thickest epithelial tissue

• Adapted for protection from abrasion

Stratified Squamous Epithelium

• 2 types keratinized & non-keratinized• Keratinized

– Location: epidermis – Contains the protective protein keratin– Waterproof

– Surface cells are dead and full of keratin • Non-keratinized

– Forms moist lining of body openings

Stratified Squamous Epithelium

• Function: Protects underlying tissues in areas subject to abrasion

• Location: – Keratinized – forms epidermis– Non-keratinized – forms lining of mucous

membranes• Esophagus• Mouth• Anus• Vagina• Urethra

Stratified Squamous Epithelium(e) Stratified squamous epithelium

Description: Thick membrane composed of several cell layers; basal cells are cuboidal or columnar and metabolically active; surface cells are flattened (squamous); in the keratinized type, the surface cells are full of keratin and dead; basal cells are active in mitosis and produce the cells of the more superficial layers.

Function: Protects underlying tissues in areas subjected to abrasion.

Location: Nonkeratinized type forms themoist linings of the esophagus, mouth, and vagina; keratinized variety forms theepidermis of the skin, a dry membrane. Photomicrograph: Stratified squamous epithelium

lining the esophagus (430).

Stratifiedsquamousepithelium

Nuclei

Basementmembrane

Connectivetissue

Stratified Cuboidal Epithelium

• Description: generally 2 layers of cube-shaped cells

• Function: protection• Location:

– Ducts of:• Mammary glands• Salivary glands• Largest sweat glands

Stratified Cuboidal Epithelium(f) Stratified cuboidal epithelium

Description: Generally twolayers of cubelike cells.

Function: Protection

Location: Largest ducts of sweat glands, mammary glands, and salivary glands.

Photomicrograph: Stratified cuboidal epithelium forminga salivary gland duct (285).

Cuboidalepithelialcells

Basementmembrane

Ductlumen

Stratified Columnar Epithelium

• Description: several layers; basal cells usually cuboidal; superficial cells elongated

• Function: protection & secretion• Location:

– Rare tissue type– Found in male urethra & large ducts of some

glands

Stratified Columnar Epithelium(g) Stratified columnar epithelium

Location: Rare in the body; small amounts in male urethra and in large ducts of some glands.

Function: Protection; secretion.

Description: Several cell layers;basal cells usually cuboidal;superficial cells elongatedand columnar.

Urethra

Stratifiedcolumnarepithelium

Underlyingconnectivetissue

Basementmembrane

Photomicrograph: Stratified columnar epitheliumlining of the male urethra (315).

Transitional Epithelium

• Description: – Has characteristics of stratified cuboidal &

stratified squamous– Superficial cells dome-shaped when bladder

is relaxed, squamous when full• Function: permits distension of urinary organs

by contained urine and also expansion of uterus• Location: epithelium of urinary bladder, ureters,

proximal urethra, uterus

Transitional Epithelium(h) Transitional epithelium

Description: Resembles both stratified squamous and stratified cuboidal; basal cells cuboidal or columnar; surface cells dome shaped or squamous-like, depending ondegree of organ stretch.

Function: Stretches readily and permits distension of urinary organ by contained urine.

Location: Lines the ureters, bladder, and part of the urethra.

Photomicrograph: Transitional epithelium lining the bladder,relaxed state (390); note the bulbous, or rounded, appearanceof the cells at the surface; these cells flatten and becomeelongated when the bladder is filled with urine.

BasementmembraneConnectivetissue

Transitionalepithelium

Questions?

What’s Next?Lab: Embryos and Tissues Mon Lecture: Tissues cont.; SkinMon Lab: Tissues & Skin

Rhythm of LifeDave Henniker