Carnegie stage 1

Embryology - 1 Dec 2023 Expand to Translate
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Introduction

Human early zygote showing male and female pronuclei

The early human zygote where the male and female pronuclei (centre of image) have not yet combined to form the single zygote nucleus. These pronuclei are the nuclei from the spermatozoa (sperm) and oocyte (egg) and contain all the nuclear genetic material (chromosomes, DNA, genes).

Two of the egg's polar bodies (right, 3 o'clock position of image) are shown at the edge of the egg cytoplasm. These polar bodies contain the excess DNA from the meiotic divisions of the egg.

The zona pellucida (edge of image) forms a specialised thick extracellular matrix layer around both the egg and the developing conceptus for the first week.

The mitochondria in the cytoplasm contain additional genes and in humans these mitochondrial genes are entirely derived from the mother (oocyte).