Early Embryonic Development

Gametogenesis

Process by which diploid germ cells produce haploid gametes through meiosis and maturation

Gametogenesis consists of

Spermatogenesis and Oogenesis

GnRH

Hormone released from hypothalamus that stimulates FSH and LH secretion

FSH in males

Stimulates Sertoli cells and spermatogenesis

LH in males

Stimulates Leydig cells to produce testosterone

Testosterone

Essential hormone for spermatogenesis and male secondary sexual characteristics

Inhibin

Hormone from Sertoli and granulosa cells that inhibits FSH secretion

FSH in females

Stimulates growth and maturation of ovarian follicles

LH in females

Triggers ovulation and corpus luteum formation

Estrogen

Promotes proliferation of endometrium during proliferative phase

Progesterone

Maintains secretory endometrium and supports implantation

Site of spermatogenesis

Seminiferous tubules of testes

Spermatogonium

Ploidy 2n stem cell that initiates spermatogenesis

Primary spermatocyte

Cell formed from spermatogonium that enters meiosis I

Secondary spermatocyte

Haploid cell formed after meiosis I

Spermatid

Haploid cell formed after meiosis II

Spermatozoon

Mature male gamete formed after spermiogenesis

Spermiogenesis

Transformation of spermatids into mature spermatozoa without further cell division

Acrosome

Cap-like structure containing enzymes needed for fertilization

Middle piece of sperm

Contains mitochondria that provide ATP for motility

Tail of sperm

Flagellum responsible for sperm movement

One primary spermatocyte produces

Four spermatozoa

Oogenesis

Formation and maturation of female gametes in the ovary

Site of oogenesis

Ovary

Oogonium

Diploid germ cell that undergoes mitosis during fetal life

Primary oocyte

Cell arrested in prophase I from fetal life until ovulation

Primary oocyte arrest stage

Prophase I of meiosis

Secondary oocyte

Cell formed after meiosis I and ovulated from the ovary

Secondary oocyte arrest stage

Metaphase II of meiosis

Ovum

Female gamete formed only after fertilization completes meiosis II

Polar body

Small cell produced during unequal cytokinesis that usually degenerates

Primordial follicle

Primary oocyte surrounded by a single layer of flattened follicular cells

Primary follicle

Follicle in which zona pellucida begins to form

Secondary follicle

Follicle characterized by formation of an antrum

Graafian follicle

Mature follicle containing a secondary oocyte ready for ovulation

Corpus luteum

Structure formed after ovulation that secretes progesterone

Follicular phase

Ovarian cycle phase dominated by FSH and follicle development

Ovulation

Release of secondary oocyte from Graafian follicle triggered by LH surge

Luteal phase

Phase dominated by corpus luteum and progesterone secretion

Ovarian cycle phases

Follicular phase, Ovulation, Luteal phase

Menstrual phase

Days 1–5 characterized by shedding of endometrium

Proliferative phase

Days 6–14 characterized by estrogen-driven endometrial growth

Secretory phase

Days 15–28 characterized by progesterone-driven glandular activity

LH surge functions

Completes meiosis I, triggers ovulation, and forms corpus luteum

Start of spermatogenesis

Puberty

Start of oogenesis

Fetal life

End of spermatogenesis

Continues throughout life although declines with age

End of oogenesis

Menopause

Cytokinesis in spermatogenesis

Equal division producing four functional gametes

Cytokinesis in oogenesis

Unequal division producing one ovum and polar bodies

Number of gametes from one primary spermatocyte

Four spermatozoa

Number of gametes from one primary oocyte

One ovum

Arrests in oogenesis

Prophase I and Metaphase II

Arrests in spermatogenesis

None

Production pattern of sperm

Continuous

Production pattern of ova

Cyclic

Fertilization

Fusion of male and female gametes to form a zygote

Site of fertilization

Ampulla of uterine tube

First barrier encountered by sperm

Corona radiata

Second barrier encountered by sperm

Zona pellucida

Third barrier encountered by sperm

Oocyte plasma membrane

Phase 1 of fertilization

Penetration of corona radiata

Phase 2 of fertilization

Penetration of zona pellucida

Phase 3 of fertilization

Fusion of sperm and oocyte membranes

Acrosin

Acrosomal enzyme important for penetration of zona pellucida

Zona reaction

Mechanism preventing polyspermy after fertilization

Pronuclei

Male and female haploid nuclei formed before fusion in zygote

Zygote

First diploid cell formed after fertilization

Significance of fertilization

Restores diploid chromosome number

Significance of fertilization

Determines genetic sex of embryo

Significance of fertilization

Initiates cleavage divisions

Significance of fertilization

Produces genetic variation through mixing of parental genes

Cleavage

Rapid mitotic divisions of zygote without increase in overall size

Purpose of cleavage

Increase cell number while maintaining embryo size

Blastomeres

Cells produced during cleavage divisions

2-cell stage

First cleavage product of zygote

4-cell stage

Second cleavage stage

8-cell stage

Third cleavage stage where compaction begins

Morula

Solid ball of approximately 16 blastomeres formed by day 3–4

Compaction

Process in which blastomeres maximize contact with each other

Blastocyst

Fluid-filled structure formed from morula around day 5

Embryoblast

Inner cell mass that forms the embryo proper

Trophoblast

Outer cell mass that contributes to placenta formation

Blastocyst cavity

Fluid-filled cavity inside blastocyst

Day blastocyst forms

Day 5 after fertilization

Implantation begins

Day 6–7 after fertilization

Implantation

Attachment and embedding of blastocyst into endometrium

Normal implantation site

Posterior superior wall of uterine body near fundus

Most common ectopic implantation site

Ampulla of uterine tube

Other ectopic implantation sites

Isthmus of tube, ovary, cervix, abdominal cavity

Ectopic pregnancy

Implantation outside the uterine cavity

Placenta previa

Implantation near or over internal cervical os

Bilaminar germ disc

Embryonic disc composed of epiblast and hypoblast

Week of bilaminar disc formation

Second week of development

Epiblast

Upper layer of bilaminar disc that gives rise to all three germ layers

Hypoblast

Lower layer contributing mainly to yolk sac lining

Two layers of embryoblast

Epiblast and Hypoblast

Two layers of trophoblast

Cytotrophoblast and Syncytiotrophoblast

Two cavities in week 2

Amniotic cavity and yolk sac

Gastrulation

Process by which trilaminar germ disc is formed