Stages of Human Development

Ilse Pienaar

Lecture 4

Embryology

Developmental Periods Prenatal Period

Developmental changes before birth

SpermGr. sperma = seed

SpermatozoonL. spermatos = seed; zōon = animal

Zygote 1×sperm * 1×ovum = 1 cell

An embryo

EggOocyteOvum (s), Ova (p)

Stage 1

Developmental Periods Gametogenesis

MeiosisLLLL

Stage 1

2*Diploid46 X

2*Haploid23 X

Developmental Periods- Meiosis I Meiosis I

Interphase

Stage 1

Developmental Periods- Meiosis I Meiosis I

Prophase

Stage 1

Developmental Periods- Meiosis I Meiosis I

Metaphase

Stage 1

Developmental Periods- Meiosis I Meiosis I

Anaphase

Stage 1

Developmental Periods- Meiosis I Meiosis I

Telophase

Stage 1

Developmental Periods- Meiosis II Meiosis II

Prophase II

Stage 1

Developmental Periods- Meiosis II Meiosis II

Metaphase II

Stage 1

Developmental Periods- Meiosis II Meiosis II

Anaphase II

Stage 1

Developmental Periods- Meiosis II Meiosis II

Telophase II

Stage 1

Developmental Periods End stage of Meiosis I & II

23 X

Diploid46 X

23 X 23 X 23 X

Stage 1

Developmental Periods Zygote

Mitotic cell divisions / cleavage

Stage 2

Zygote Morula Early blastocyst

Stage 3

Late blastocyst

Developmental Periods

Stage 4

Implantation Gastrula

The Organogenic Period Week 4 – 8

The major external and internal structures are established

Minimal function

The Organogenic Period Week 4 – 8 Phases of Embryonic Development

Growth

Stage 2 Stage 3

Morphogenesis Differentiation

Stage 1

Germ Layer Derivatives Formed during gastrulation Pimordia of all tissues and organs

Germ Layer Derivatives Ectoderm

Central Nervous System (CNS) Sensory epithelial

Eye, Ear, Nose, Epidermis, Hair, & Nails

Mammary glands Pituitary gland Subcutaneous glands Teeth enamel

Ectoderm

Mesoderm

Endoderm

Germ Layer Derivatives NeuroEctoderm

Neural Crest Cells Spinal Cranial

V, VII, IX, X Autonomic ganglia Ensheathing cells of PNS Muscle Connective tissues Meninges (brain & spinal cord)

Ectoderm

Mesoderm

Endoderm

Germ Layer Derivatives Mesoderm

Connective tissue Cartilage Bone Muscle

Striated & smooth Heart Vessels

Blood & lymph

Ectoderm

Mesoderm

Endoderm

Germ Layer Derivatives Mesoderm

Kidneys Ovaries & testes Genital ducts Serous membranes

Line body cavities Spleen Cortex of suprarenal glands

Ectoderm

Mesoderm

Endoderm

Germ Layer Derivatives Endoderm

Epithelial lining Gastrointestinal, respiratory tracts, tympanic cavity & bladder

Parenchyma of the tonsils Thyroid and parathyroud glands Thymus Liver Pancreas

Ectoderm

Mesoderm

Endoderm

The Organogenic Period Embryonic Folding

From flat trilaminar embryonic disc Cylindrical embryo Median and Horizontal planes

Head Fold Tail Fold

Cranial

CaudalSi

de Sid

e

The Organogenic Period Median Plane Folding

The Head Fold Brain: Primordium Developing forebrain grows

beyond oropharyngeal membrane Heart: Ventral move – septum transversum,

primordial heart, pericardial coelom, oropharyngeal membrane

Foregut: Primordium of pharynx, esophagus… - lie between the brain and heart – develops into the central tendon of the diaphragm

Embryonic coelom – Primordium of body cavities

The Organogenic Period Medial Plane Folding

Tail fold Growth of the distal part of the neural tube –

primordium of the spinal cord Caudal eminence projects over the claucal membrane Part of the endodermal germ layer is incorporated into

the embryo to become the hindgut – primordium of descending colon

Terminal part of cloaca dilates – primordium of urinary bladder and rectum

The Organogenic Period Horisontal Plane Folding

The Tail Fold Right and left lateral folds Produced by the rapidly growing spinal cord and

somites Midgut: Primordium of small intestine – part of

the endoderm germ layer is incorporated into the embryo

Initial wide connection between the midgut and yolk sac

The Organogenic Period Horisontal Plane Folding

The Tail Fold After folding – connection reduced Reduce communication between intra- and

extraembryonic coelomic cavities